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GEOL6379-Applied Biostratigraphy - Day7 09-16-2022
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00:00 So what? Yes I think it . Should have been you're not finding
00:18 one grain at a time. Um right. What's your email again?
00:46 four? There you are. Yeah. What you know her
01:01 Is that her? Yes. Oh 52 MB. Well I think we
01:17 still email that. What we use use mail drop. It'll work.
01:27 take a minute. No thank Yes thank you. Wonderful. Okay
01:59 megabytes. It was easy. Um that looks like that's done. So
02:07 got your notes we can get So we had finished last time And
02:20 like 2nd too many things here. . Mhm. Mhm. Yeah.
02:41 do I get rid of that? you. Okay so we talked last
03:08 about how we can use the tribal is to understand the tectonics and the
03:12 . How by looking at the uh time, the age of the
03:20 the edge of the stratum to the of the cooling age that came out
03:24 the out of the of the We're going to look at a couple
03:28 variations on that just now. So going to zoom in on this region
03:37 in the Himalayas. Just gonna pay to this this bit right here.
03:43 This one sample is from a the the mari Andy Marchionne di river
03:51 which covers all of these rivers are in to that one spot. And
03:56 the advantage the advantage of doing this the tribal stuff is we're going to
04:00 about all the tectonics of this region having to go and collect all these
04:09 now. The disadvantage of course is we don't know exactly where these samples
04:13 coming from. They're coming from the up here somewhere. But you know
04:17 that's the trade off. You get sample, you learn about a lot
04:20 information but not not not location Um And for this we're gonna be
04:27 at sort of more recent activity in to understand the variation of topography in
04:33 active mountain range like this. So we think of closure temperatures, think
04:41 death ah but as we've talked about when we when we vary the surface
04:49 the earth by having valleys and Um That will complicate things a little
04:55 ah at the very it's the very bit when we have tall mountains we
05:02 compare the ages at the top and bottom. Um And if we I
05:08 see the slides changing in the Okay, you can't see the
05:14 Can you see the slides changing? what are we doing wrong here,
05:19 back to how do I get I can't hear you. Okay so
05:44 stop share and start again. Let's . Uh huh. There it
06:08 Yes, I can see now. So the what the mineral we're gonna
06:19 looking at in this application is muscovite that's because muscovite is very common in
06:27 rocks that are common all over the . And that's nice because muscovites present
06:34 all on almost all the outcrops were be considering if we were going to
06:38 at some mineral like garnett or some mineral, you know, we can't
06:43 sure it's in every place, but this is, this is Himalayas,
06:48 everywhere. That's good. Another thing that because let me ask what's the
06:54 temperature of argon and muscovite? 400 about 400. Yes. Okay,
07:03 that means because it's so hot, don't have to worry too much about
07:07 variations in the mountains and the valleys for what the ice with the 400
07:13 icy therm is doing 400 degrees is line down there about 13 kilometers.
07:20 so when we see a mountain such this one, the sample at the
07:25 would have would have reached that ice ice a therm first and then the
07:30 afterwards, Just like that example we at with the bio types last
07:35 Um Well if we were to take sample from the river that was looking
07:44 these mountains and these valleys, we then plot a distribution of their ages
07:51 what we're gonna do is compare that of ages to the distribution of elevations
07:58 in a simple, let me see a simple situation, we ought to
08:01 able to take the hip summitry, is the graph of elevation where we
08:07 the fractional area versus elevation. That us that most of the elevations are
08:12 , but this is the lowest this is the highest elevation and this
08:15 the most common elevation. Um There to be a if if we have
08:21 simple age elevation relationship which we would , if there was like steady state
08:27 through this thing, then we should able to take this and and and
08:31 in with relation to this and get a diagram which was showing the uh
08:39 of ages for the Bedrock Valley uh . And so we could get a
08:49 probability diagram. I talked about those time. That's how we're gonna compare
08:53 things. And so to get a probability diagram for elevation, all we
08:59 to do is to have a digital model, right, that's available to
09:05 sorts of things all over the And so you can just say,
09:08 the lowest elevation is zero m and highest elevation is 2500 m. And
09:13 some relationship in between. You can the same thing with the ages that
09:19 got from the river. You gather sand from the river and you date
09:23 bunch of the samples from the river then the youngest one is this and
09:26 oldest one is this. And you draw a curve. But how are
09:30 going to compare these curves? How we compare things with different units on
09:35 we're going to compare an elevation curb an age curve. We're gonna have
09:40 find a way to relate these together the way we're going to do that
09:45 by non dimensional izing both curves. gonna plot not elevation but what we
09:51 Z. Star and not age but we call T. Star, Z
09:56 is the elevation in question minus the elevation divided by the range abilities.
10:04 it's basically the star is what percentage the way we are up the men
10:08 the range and T. Stars the thing. And so we're now we're
10:12 at these non dimensional ized values as as a function of how far they
10:17 above the range of our data from whatever the lowest ages to the highest
10:22 , whatever the lowest elevation is to highest elevation. And now then we
10:26 compare these two things on the same because they both are dimension lists.
10:31 so we can see are these cumulative diagrams equal to one another. But
10:37 let me just stop. Does that sense? Does he follow what I'm
10:40 here? Okay. Um but we have a problem because for the amount
10:49 for for ages the bread curve and going to be somewhere between 50 and
10:55 . I mean you know maybe you crazy and you date 1000 minerals.
10:58 would be a lot. But let's say we did 100 and 17 are
11:02 favorite number. How many elevator? many points are there for? For
11:08 digital elevation model. There are many of points. If you have a
11:12 good remote sensing information. Is it is it fair to compare a sample
11:20 many thousands to a sample of many . You have an issue there.
11:26 so the way we're going to do is that we're gonna take the age
11:32 whatever they are. Um In this the age in this sample on on
11:38 I hear the number the end of . Star equals 295. That means
11:45 we analyzed 295 grains. And that's red curve. What we're gonna do
11:51 Z. Star is randomly Sample the elevation model 295 times. So that
12:03 are comparing apples to apples. We're a sample of 295 to a sample
12:07 2 95. But we're gonna do . Random sampling of the elevation model
12:14 different times and plot them all on curve. And so the why the
12:21 blue lines are all of the individual . The dark blue line is the
12:25 of all of them. And so gives us a better sense of how
12:29 can compare the data. And so when we compare these two curves we
12:35 then test out various assumptions and these these are the things we're wondering about
12:40 . Star and Z. Star. two curves will correspond if the following
12:47 are true. If we have thermal topographic steady state, that is.
12:53 are. Well actually the most important is well, a topographic steady state
13:00 that we are not we're not changing The surface conditions in significant ways.
13:09 the second most important one. Oh thing about topographic steady state is that
13:14 don't have big false in our in region if we have a big fault
13:22 going to affect the distance that the had to travel from their cooling age
13:26 the surface. So that's what topographic states referring to. And that's also
13:31 to uniform erosion rates across the If everything is is eroding the same
13:37 . We should see that the elevation the age should be correlate herbal in
13:42 way we describe. And we also to worry that of all the places
13:47 there in the catchment, we're sampling of them. Um because we have
13:52 in just about every outcrop here. feel pretty good about that. And
13:58 um what we did here was look some uh Detroit old data from this
14:07 in the Himalayas. Um There's a of different rock types in the
14:13 Um But I'm not sure that's very right now. Let's see, I
14:17 what does that say? Oh we the uniform erosion rates is a tough
14:24 because, you know, you've you've got steep slopes and flat
14:27 so that might not be a very way to keep think of things we
14:32 think of representative sampling being okay because I said, muscovites are everywhere.
14:38 and so here are the data. comes from a variety of publications that
14:42 been put together and what we have is a catchment. Let me
14:47 I didn't show you exactly where that this, where's, here's a
14:51 here's 40 kilometers and we're looking at region that is about in here out
14:57 . And so that this this is black line here is the is the
15:02 catchment of this sample right here, . 02 17. Inside that river
15:10 are other smaller rivers that have smaller bits in there. And then the
15:16 , the age probability diagram for each is shown with that red line,
15:22 can see that some of these are broad, like M. O.
15:25 17. That that's good. That's sense because it's the one that's covering
15:29 most region. Some of these are covering very small up this one
15:34 like, like good or S. S. 40 or uh M.
15:40 . M. O. Yeah, 40 is a good one. They
15:46 very high up the river. And there's a very small amount of samples
15:50 collect from that. And so you'd there to have a more narrow
15:55 And so this is the result of of the various catchment areas that we
16:01 . And remember if these two things match up the red and the blue
16:08 , Those those assumptions we made are correct. And the biggest correct.
16:14 biggest assumptions are is that there have no major faults since closure and that
16:23 , that the samples were coming up least kind of straight together and that
16:28 rate of erosion hasn't changed much over . You'll notice that in some places
16:34 things fit together in some places they and it's it's a bit interesting in
16:38 these numbers here represent the drainage area the individual samples in square kilometers.
16:45 we have a drainage area of only square kilometers. And the data match
16:49 pretty well. Sometimes we have a great drainage area of 81,000 square kilometers
16:57 those actually show up not too Some of these others turned out to
17:02 pretty bad, like, like this here in the upper mar sandy,
17:06 quite different from the blue curve. one's bad. This one's bad.
17:12 You know, it varies. This pretty good. This one's quite
17:16 Uh This one is very good, that you can see that for for
17:21 places, the assumption about our drainage can be shown to be pretty good
17:28 questionable. It's interesting that the the that have the largest drainage areas show
17:34 pretty well on this diagram and that's counterintuitive, isn't it? Because the
17:39 the drainage area gets, the more it is, it's not going to
17:42 uniform and everything is gonna be the and we're not gonna have big
17:46 And I've always thought the only reason that's the case is those look good
17:49 that somehow you've got some sort of going on and where it's bad in
17:53 area, it's good in another and ends up looking good overall. Um
17:59 anyway, it is a it is illustration of how we can use modern
18:04 and dating of samples from rivers To us something about tectonics of the recent
18:11 not, you know from today, back to the time at which these
18:16 were closing to Argon loss, which back in some of these cases
18:21 20 million years. So we can modern topography to tectonics over a 10
18:26 year period. And so if you're in how the how the are the
18:32 of the mountains has changed how the of material from the mountain to the
18:38 has changed. This is one way understand it without analyzing samples from all
18:43 the place, you could just go one river scoop up some sand,
18:47 two or 300 grains. And that's lot less work than analyzing this sample
18:52 this sample and this sample. But downside is the spatial spatial information is
19:02 . Oh, I put green boxes the ones that really worked well and
19:06 red boxes around the ones that don't well. Yeah. Um Okay,
19:15 that was all in this region this Nar Eonni basin. Let's now
19:20 over here. We have another sample NAG 12 and it comes from the
19:28 River, which has a different brain over here. And let's compare the
19:34 from those two places. Here are age distributions for the muscovites from the
19:40 places. And you can see that pretty different. I think we have
19:45 on one diagram. No, we we don't go back. You can
19:50 they're quite different. The M. . to 17 samples are almost all
19:58 than 12 million years old, Whereas energy 12 samples are almost all older
20:04 12 million years old. Um and there's some some details you might want
20:10 look but and look at the look the end value, have an equal
20:14 and equal 177. This is beginning be enough samples to begin to say
20:19 we have a reasonably good characterization of we should see. You know,
20:23 is nice, it's not 1000 but 20. So what does this tell
20:30 about? So to now we're looking bigger scale information about the mountain range
20:34 we've got these big rivers now and we can say that this tells us
20:39 the tectonics of the eastern region has much more active in the last
20:45 10 million years than in the Western . Because we are sampling muscovites from
20:50 entire rested region And age is less 10 million years are quite, quite
20:57 and that wouldn't be what you'd expect you've been having a lot of tectonic
21:01 since that time. That's what you over here in the east, you
21:05 something else in the west. And , we learned that by not going
21:10 every outcrop in the west and we just got two samples of two
21:16 . Um And it makes sense if look at the geology of the
21:21 there's a big fault system that goes there. The western Nepal fault system
21:25 which uh you know, doesn't perfectly these regions by by by great river
21:33 . And here's the here's the boundary the river catchments and here's the boundary
21:37 the tectonic zones. But it's close . Um and it explains that on
21:42 side of the fault, we're getting action on this side, it's
21:45 it's it's older. So there's a a structural Explanation for why these two
21:51 are different. Um So now I to look at, Yeah, I
21:58 to look now just at the Karnali and what we have here, I've
22:02 showed you this sample in A. . 12. What we have is
22:10 addition to the muscovite data which are on this side of the diagram,
22:18 also have fishing track zircon data from or less the same place. This
22:23 from a different paper but they got got data from the bullets. The
22:28 place. We have muscovite data from modern river, we have appetite fishing
22:33 from a modern river. We also samples that go back in time.
22:37 sand stones that are in the foreland of the Himalayas are called a shopaholic
22:43 group. And we have a sample about seven million and a sample at
22:48 15 million. And what we have are distributions of Muscovite ages and fish
22:54 track Zircon ages or the two samples the for the three times license 15
23:01 , seven million. Modern. What's closure temperature of fishing track? Zircon
23:12 . Yeah, I would. Let's say around 200. Yeah, around
23:17 21275 is pretty darn high. But just go with 200. So what
23:22 got here are are places that 11 of data is telling us broadly when
23:29 region was coming to to temperatures of 400. And then the next state
23:33 data are telling us broadly when we're to temperatures of about 200. And
23:39 notice that they're um they're full of variation I think the next diagram.
23:44 what we're gonna do is consider a of three locations. We've got
23:52 B. And A. And we're talk about those places as zones that
23:59 in the crust, but deep deep the crust that are coming out.
24:03 so if we look at the 15 the that says 13, that says
24:09 , I don't know. Let's call 15, I think this might be
24:12 . Um the 15 million year old here, the zircons are about Focused
24:22 around 16 million and the Muscovites are older, 2020 million years and
24:30 And that's good. We're gonna call the green rocks that were coming to
24:33 surface Around 13 million years ago. that are the blue rocks and the
24:39 rocks. So 22 million years ago green rocks were passing through 400°. That's
24:50 telling us that's telling us this because most of these muscovites are 22
24:56 . So around then, based on information, the green rocks, the
25:01 that came to the surface 13 million ago and we say they came to
25:05 surface 13 million years ago because they're a 13 million year old sedimentary
25:09 we're assuming that the amount of time took for them to get to the
25:13 and then deposited in this basin is small. So We can say that
25:20 rocks were at 400° in this period million years. We'll call it
25:27 These same rocks based on the zircon track data, which are also from
25:32 same sandstone, they must have been around 200 degrees around 16 million years
25:40 . Based on that peak of samples there. So we can take these
25:44 rocks and they were at at Zone has has mostly muscovites around 20.
25:50 they were going to put it But this same zone has zircons In
25:55 in this 10-20 range. So that's put them here at around 16
26:03 So, we've taken that group of that that group of samples that made
26:07 to the surface at 13. We put it back and say where they
26:11 at 16 and where they were at depth wise, or temperature wise.
26:17 then 13 million years ago, we that this group of rocks came to
26:22 surface. That's not told to us any thermal chronology. That's told to
26:26 by this tree geography. These rocks in a great Sandstone. That's 13
26:32 . Okay, what about the other ? Well, we know that rocks
26:38 we call Group B rocks in blue , that excuse me, Group B
26:44 were coming to the surface right around million years for the muscovites. And
26:52 about the same time for the So, what we see here in
26:56 Yukon and the appetite and the fishing . Excuse me, in the Zircon
27:01 track and the argon muscovite. The are pretty close together. This suggests
27:07 we had an acceleration of uplift around time, so that we moved from
27:11 from from from, from muscovite ages appetite. Zircon ages pretty quickly.
27:18 so we can put zone B you know, near the surface About
27:24 million years and pretty much gone from surface by seven million because of these
27:28 things. And then finally, Zone , which was below all of those
27:32 , We know that it was Um the We go, if we
27:38 back zone C was at about 400° 16 million years ago. And that's
27:46 with this, right? We've got ages here that are about 16,
27:51 going back. We can put zone here. And then we put zone
27:57 through the fishing track. Zircon time at about 10 million. Excuse
28:02 at about 10 million. And then they made it to the surface.
28:06 , by looking at two thermal kilometers three strata, graphic levels, we
28:11 we can sort out the overall history this big region, you know,
28:16 the past. What we've done is to take a single sample and and
28:20 it by different ways and do the sort of thing, temperature time diagram
28:25 , we're looking more broadly, but the same idea. Um Okay,
28:32 more thing from this region and then move on something else. Finally,
28:39 this is the work one of my students did a couple of years ago
28:43 what she did was go back to eastern region. And we have,
28:52 make we can make a map of bedrock ages. There's lots of bedrock
28:56 . We were talking about the tribal before, but it just so happens
28:59 this region. A lot of that bedrock data is also available. And
29:05 you can make a map of of of those bedrock ages here and in
29:10 region here. And we're just looking this region here, which is the
29:15 of this river that we've talked about uh M. 02 17. And
29:21 you can see that based on we have a zone here right along
29:25 fault Called the main central thrust where agents are quiet, you know,
29:31 million. And then on either side ages get older, this pretty much
29:41 that assumption We had to make That for the big drainage basin,
29:45 coming up at the same rate because they're not. We've got some muscovite
29:49 that are four million. That means . And remember muscovite 400 degrees.
29:54 that means that they have come up 12 or 13 kilometers in just three
29:57 four million years. That's very whereas on either side, not so
30:04 . Um, but we also have sample right down here from the drainage
30:13 that, that samples all the the tribal ages. So we can do
30:19 couple of of things here, we calculate what's called an enrichment factor Where
30:24 gonna take a particular age range whatever range you want, say like 10
30:29 to 12 million. And we're going calculate the proportion of that age range
30:37 this to try to sample as shown , you know, we've got this
30:41 try to sample how much falls between this is 12-16 million that curve,
30:48 much, what proportion of that blue portion is this whole thing. That's
30:54 the dominator, the numerator and the of this of this quotient is the
31:02 sort of proportion. But how much we see of that age range in
31:08 bedrock math. And but if if , if the area of these two
31:15 were the same, then the proportion enrichment factor would be equal to
31:19 And that means that we'd have the in the, in the bedrock,
31:26 the distribution in the, in the , in the sand grades. Um
31:32 that helps. Um Okay, so where those things come from. Well
31:39 is a map of enrichment factor and see that we took the log of
31:44 enrichment factor because the enrichment factor actually quite a lot by taking the log
31:49 put the numbers back down But where , where the value is less than
31:54 . The enrichment factor is is excuse ? Where the log is greater than
31:59 , the enrichment factor is greater than and where the log is less than
32:03 so forth. And so what we here is the enrichment factor mimics very
32:09 the age math in that this zone here is represented much more in the
32:18 try to sample right down here. represented in the Detroit. It'll sample
32:23 more than it is represented in the of the attachment. Remember if
32:30 if all of these regions for every centimeter of this region was eroding at
32:34 same rate, we'd expect an enrichment to be one everywhere. But here
32:39 have a very different enrichment factor in middle which tells us that this fault
32:44 goes right through here is a place enhanced erosion. That place in the
32:50 ranges eroding much more than either place above it or below it or north
32:55 it or south of it. And can do that by comparing the bedrock
32:59 to the tribal data. Sometimes I've that sometimes you don't have the bedrock
33:05 to compare to and that's true. here we did. So we
33:10 Um And you can see that the of bedrock ages and the contours of
33:15 enrichment factor are pretty similar. Uh so again, by comparing that information
33:23 can really highlight. I mean there's fault here and it's not surprising that
33:27 are, you know, zones of and something going on. But by
33:31 the tribal data to the bedrock we can show that that fall has
33:35 been a focused point of erosion for long time, which then will lead
33:40 to all sorts of understanding about the history here. We have to come
33:45 with a structural story that squirts out from here but doesn't erode at high
33:50 or a low elevation. This is of moderate elevation. Um Okay,
33:58 more thing than about the Himalayas is one here? Um I I showed
34:07 the data earlier of the uh the and we talked a little bit about
34:11 , but this comes from um there's broader bunch of information that comes from
34:16 paper by sulks at all 2006. what they did was looked at a
34:23 of these symbolic sections in these in these different cola's uh the Karnali
34:29 , the samurai cola, the These are just little valleys that are
34:33 where they can look at this and collected samples and they did paleo Mag
34:38 this. So they know the age these things pretty well and then they
34:42 the dating of the Detroit. It'll lights and these are the results they
34:47 . And again, I'm plotting the age versus the deposition all age.
34:54 they they they made they made much the fact that in this diagram there
35:00 to be an increasing lag time. and indeed that's what we got doing
35:06 other way that that looks like that time that 16 million was a much
35:10 active time than than since then. it's unfortunate that they did it the
35:15 they did it because if you look these red numbers up here, that's
35:19 number of samples they analyzed in in each stratum. In some cases
35:25 analyzed as few as 13 grains. talked last time about how many you
35:31 need to feel good. 100 is better, right? 200 of course
35:35 better and 1000 is better, but is not good. Um And You
35:44 , even this one here with 40 , they are making a case that
35:49 here, there's an absence of grains . But you know, you always
35:54 to remember that the absence of evidence not the evidence of absence. And
35:58 when you've only analyzed 40 grains Um is the sample that that that they
36:04 is the distribution they have for these . You know, we're looking at
36:08 data in a different way. Here's age of of sedimentation, here's the
36:12 at about 12 million. And they saying, well that's excuse me,
36:18 the that's the lag time, But is equal to 40 here. Let's
36:24 back to this graph graph we saw . If N is equal to
36:28 that means the detection limit is is . We're we're we're likely to miss
36:36 that represents less than 12% of the . And so it's quite possible that
36:44 that's not here, It's not here we just didn't look hard. 12%
36:49 a pretty significant amount. Um And they measured in all of this study
36:57 grains. That's great. But I that should you ever want to embark
37:03 a study like this? Don't? and and we always have to remember
37:08 amount of that this big in here going to be a factor of time
37:12 money. These things, these things effort, They cost money, that's
37:17 . But if you have enough time money to measure 400 grains, don't
37:22 them out among 123456789, 10, different strata. Because then you end
37:30 with 13 grains there that are Well actually not so meaningless here and
37:34 we do have a grain right down . But these guys over here,
37:38 is the problem is this an increasing time or did we just not analyze
37:44 ? What if instead of analyzing these grains and these 13 grains and these
37:50 grains we took, we didn't you know, they analyzed strata every
37:56 million years or so. What if analyzed every two million years but had
38:02 each one of them 90 or 100 , then they would have much better
38:07 that if they saw a pattern like , it's probably real. So if
38:13 have enough money to analyze one analyze it, that's better than
38:18 But if you only have enough money analyze 100 grains, do it all
38:23 the same sample. If you have enough money to analyze 400 grains,
38:29 you don't want to split that up more about four or five samples because
38:32 you'll have four good samples instead of . Not very good samples. So
38:40 that's part of, you know, again, if all you have money
38:43 is is 40 samples. Well, great. Okay. Any questions about
38:52 that stuff to talk about provenance and in the product. All right,
39:01 gonna move on to another topic then dating the tribal minerals. And this
39:06 a big topic and a lot of nowadays. And it's using the dating
39:13 doctrinal minerals to tell us about the of deposition. And this comes about
39:20 um Well, why not just date rock itself? Well, plastic rocks
39:26 made up of pieces of older So we can't really date the rock
39:29 tell us about the rock itself. difficult. Um Chemical sedimentary rocks generally
39:35 sufficient concentrations of uranium. Fantastic. um the dating of the tribal
39:43 The disadvantages there's no true age, guarantee of the true age other ways
39:48 always better. But sometimes this is only way. But we talked about
39:53 in the beginning. What if we these two tribal strata? How are
39:56 going to figure out the age of or the age of one of
40:00 And you know, we've got the of superposition. We know one is
40:03 than the other one. That's a to start. Um But how many
40:07 ago was that? Well, remember A is to date inter bedded volcanic
40:12 . That's the best way we can volcanic rocks. Or even if we
40:15 find, you know, those volcanic right next to each other bam we've
40:19 we've got that thing sorted, but not always, that's always option.
40:24 maybe we're gonna use fossils. Um you know, Don has given you
40:29 bunch of lectures on how we can fossils and sometimes that's quite useful,
40:33 not always. Sometimes we'll have fossils are not uh particularly diagnostic. Sometimes
40:39 are. Um But what if neither of those options are there, maybe
40:44 have no inter bedded volcanic rocks and fossils you have aren't very helpful.
40:50 maybe you have no fossils because you're at strata which were deposited in a
40:54 energy environment, like a river and have good fossil preservation. If those
41:00 don't exist, you've got to go plan C. Some other alternative to
41:04 out how old these things are. you know, we've decided for some
41:08 on our geologic study that we wanted hold these sedimentary strata are That's a
41:12 problem. Right? Well, we have fossils and we don't have volcanic
41:16 . Plan C can take a variety approaches. You could do magneto strategic
41:22 . Um That's that works. That pretty well. If you have a
41:28 of layers that you can analyze, know, you gotta analyze 150 samples
41:34 . But if you've got a nice section that might work. You can
41:38 the orbital forcing where you look at Milan Covic cycles and look at variations
41:43 thickness and relate that to changes in in the the motion of the planets
41:49 been done to. But both of techniques involve many samples up and down
41:55 strata. Graphic column. But if all, you're interested in this one
41:59 , you don't want to collect a of them. You can just collect
42:01 sandstone and go to the plant. third version of Plan C. Which
42:05 to try to dating. Um And in that in that sample, you
42:11 , we've got we've got 11, simple thought that unit two must be
42:17 than the youngest grain observed, Okay, so um what has what
42:26 been done into tribal dating mostly is dating of Detroit als or cons.
42:32 has two reasons. One because tribal are quite common even in very mature
42:38 rocks, because they're so durable in sedimentary environment. Um They also are
42:45 likely to be disturbed by burial. mean, you know, closure temperature
42:51 so high that you're always going to looking uh back to the original
42:57 And one of the most influential papers this regard was the paper by these
43:02 guys at University of Arizona build their and George Garrel 9 2009. And
43:08 looked at a series of sand stones uh the Colorado plateau and adjacent areas
43:16 new Mexico and texas. And I these are all Mesozoic samples And they
43:24 zircons um around 100 maybe more. they analyzed zircons from these various
43:30 I believe there were 54 of And what we have here is plotted
43:35 comparison of the oldest end of the age that you know this,
43:41 this part of the of the uncertainty sigma uncertainty and the youngest end of
43:48 strata graphic age and the strata graphic in these samples either comes from well
43:53 fossils or from inter bedded volcanic rocks were dated by argon. Um And
43:59 you'll see is that on and this line is when those two things are
44:03 same and we never want to have that fall below that line because that
44:08 mean the zircon um existed before the was deposited. That's not right.
44:15 so we see that there's a good good news about this diagram is that
44:19 never really violate any of the rules . We never get floated down here
44:22 the forbidden cell. And you back in 2009 this was this was
44:27 a bit of news. And they at the conclusion of their paper,
44:31 said our analysis confirms the utility of the youngest uranium lead age for individual
44:37 zircon grains to constrain maximum times of . Our results are testament to the
44:43 , full record of the uranium lead preserved by the title Zircon grains.
44:49 sounds pretty good. And yet they stop there. They had another thing
44:55 did. They said, well we're to offer alternative ways in which to
45:00 the maximum deposition. They're going to this one technique that we've just
45:05 The youngest. Single grain seems pretty but they said, well what about
45:10 if what if we did some other ? They proposed several methods. The
45:15 most the two methods that become most in the literature since then are what
45:20 called Y C one sigma and Y two sigma. The first is the
45:26 cluster of grains that overlap by one and have at least two grains in
45:33 cluster. The other one is the young a cluster of at least three
45:39 that overlap in the two sigma uncertainty there's a bunch of others, people
45:45 have published other rules, other ideas how to do this. But I'm
45:50 going to concentrate on those two right now, just a spoiler alert,
45:57 not a big fan of this and I'm going to talk to you about
46:00 I don't think it's a good Um And let's start out with some
46:04 data and how you might approach This is some real data and you
46:09 see that there's a range in They're just they're just listed in rank
46:12 here. And the the problem from perspective of those clustering approaches is when
46:20 have one data out by itself, that one there it doesn't overlap at
46:25 sigma or one sigma, it's off itself. And so these techniques here
46:32 exclude that, great because it doesn't within the cluster. And so what
46:40 would do is say exclude that take the average of those grains,
46:44 us something like that, that would the and they would say then that
46:48 maximum de positional age for a sample this is not this youngest grain of
46:55 but the average of these four grains a significant difference. Um Now I
47:03 say that I cheated a little bit when I told you that these were
47:06 data, they are real data but units are not millions of years.
47:10 units are seconds and these are the of the data points And this is
47:17 picture of the date of where the was collected. These are the times
47:20 the 2009 World Championship 200 m sprint Berlin. And you see winning the
47:26 here is Usain bolt, famous sprinter he's winning the race by a whole
47:31 , right? But if we applied Y C two sigma three plus
47:38 if this was zircons, we, know, we would we would have
47:42 saying both disappear because their data approach these people have advocated is a top
47:48 itself can't be trusted. We'll look these other guys and I I would
47:53 would say that, you know, have to say what's going on because
47:55 purpose of these endeavors, whether it's whether it's a race around the track
48:01 whether it's a dating of zircons to the age of the sandstone. The
48:05 of these endeavors is to identify the that's the most important grade and by
48:11 by insisting that you average them. sometimes by insisting that you average and
48:17 you're not even just averaging grades You're throwing out the youngest one and
48:21 a few older ones. You're not what you need to do. So
48:28 an example. Excuse the double, , bit at the bottom. But
48:31 think you can read it. I'd that you saying Bolt would not approve
48:35 something like this. This is an of some data published by Schwartz at
48:39 . Now they didn't actually do but but their data is appropriate to
48:44 this is that you could take these , there's a one zircon out there
48:47 82 million and it's got a nice on it. But by the rules
48:53 clustering, Which a lot of papers that, you know, that's what
48:58 doing. You're gonna end up with with an age of 95.3 with a
49:04 worse uncertainty because some of these data not very good. But that's what
49:09 people feel more comfortable with these days saying that 95.3 is a better estimate
49:14 the age of the deposition than the , that's eight. No shoot me
49:19 million year difference By ignoring that one . Um So I have to say
49:26 would you do this? Because you , here's the here's the data.
49:29 you plot the youngest single grain from Dickinson and girls, you plot the
49:34 data with the Y C two sigma plus and you get a very different
49:40 course the data move up because you're going to be making things older this
49:44 if we take the difference of these techniques and subtract the one from the
49:49 , we get ages, sometimes Sometimes it doesn't matter very much because
49:53 got a bunch of brains right down together averaging a few won't matter.
49:57 sometimes you'll ignore that youngest one and average really old ones and the difference
50:01 be as much as 100 million years so, you know, two thirds
50:07 the time, the difference is more 10 million years. Um, the
50:12 point here is trying to figure out to do this. So why would
50:15 do this? Well, the reason some people would use any one of
50:19 other things instead of the youngest single . And people have argued that
50:24 these techniques are more conservative. They're precise, they're more statistically rigorous and
50:31 generally disquieted by the by the N one problem. Just having one an
50:38 . People don't like that. this paper by Jackson and all specifically
50:44 what they're worried about, They say of the numerous geologic factors such as
50:48 loss and laboratory contamination that may affect interpretation. A weighted mean average of
50:54 youngest population with an end value greater three is required to ensure precision.
51:01 , I think if you speak some with that sentence, you'll see it
51:03 pretty poorly written and that's probably because have a pretty poor sense of what
51:07 up to. Um, but basically , this, this expresses what a
51:14 of people have in mind when they're this. So let's look at some
51:17 these things in turn, people will this is more conservative and I suppose
51:23 is If you have these this is from Jackson at all. They have
51:27 sample here at 202 And then all other ones that go to the next
51:32 one is to 12 and they range 2-12 to 23. This is a
51:38 of data which all overlap at two . And so they took the
51:43 the weighted mean average of all of and they got this to 16.8 plus
51:47 minus 0.4. They argue that this more conservative and I suppose it is
51:54 conservative means we're not wrong ah but does it doesn't maximize conservatism. If
52:03 2 16 is conservative, 3 16 more conservative and 4 16 is more
52:08 than that. And you know, you want to maximize your conservative nous
52:12 have to just say, well it's than four billion right conservative is not
52:17 what we and I should point out there's nothing wrong with this point is
52:21 go to if we plot that on weather will diagram to give us a
52:24 of what kind of data it It plots right there on Concordia what
52:29 expect from a good analysis, there's wrong with that data except it exists
52:35 itself, but they threw it away of the rules that somebody put down
52:40 them. Um But you know, the the engineers of this bridge might
52:49 been more conservative if they evaluated the height that might go under it rather
52:54 the average height that might go You get this problem if you try
52:57 pay attention to the averages rather than extremes. Um and so, you
53:04 , just because saying something is more doesn't mean it's good because what,
53:08 know, we didn't, we didn't up in the morning to say I'm
53:10 maximize conservatism in my approach is now trying to understand something about the geologic
53:15 here. And so let us let do that, which increases the chance
53:21 maximizing geologic insight. And that's not saying, well, it's more
53:27 Other people are are are happy because way to mean will get things that's
53:32 precise. Imagine a uh bunch of here that are all plus or -10
53:39 might range from 100-95. Like The weighted mean of those of all
53:45 points here is 100 pleasure -3. just how weighted means. Workout.
53:50 another example and the weighted mean of goes down to so you're always going
53:55 put the weighted mean, the weighted helps in that regard. But this
53:59 the sample I showed earlier. Here's example of where the grain that was
54:05 actually has a better precision than the of the grains which are not
54:10 Uh So it doesn't always guarantee that gonna get more precise data. But
54:15 , once again and again this this here falls directly on the weather will
54:20 . It's a perfectly concordant point. nothing wrong with it acceptance by
54:25 Um And so You know, this million year difference is not worth
54:34 Uh Because and and and and even this case we didn't improve the
54:39 Um Here's a here's the paper from at all. They looked at a
54:44 of sand stones in into china Tibet those are the here are the ages
54:52 different um different strata all stone The age from about 1 92 to
54:58 here. And they took those data and and and basically did this is
55:05 in each case almost each case they a point here, this kind of
55:09 itself and they ignored it in each to get the average of these other
55:15 here. And so if you look the youngest grain, you start at
55:19 11 to 10 209208222 oh two. all kind of go up as you
55:24 up section. But if you ignore youngest ones and you look at the
55:29 is you get to 16 to 15 14 to 14 to 16. They're
55:34 about the same. And they made a fuss about that in their paper
55:38 saying, oh look the deposition, age of this entire strata graphic section
55:42 very close together. Well, all can say even by their approach is
55:47 in every case it's less than 16. All of these could be
55:51 be 100 right, it's just gonna less than 2, 16. But
55:55 we assume that, you know, gonna have some zircons near the age
55:58 deposition, you might say that makes . But then you've ignored all these
56:03 grains. So uh you know, don't think that that, you
56:08 this is a better diagram if you it, if you do it their
56:12 , you know, you've got these that are all the same. If
56:15 do it by the youngest single grain , you get you get a less
56:19 uh result, you get ages that younger at the top and older at
56:24 bottom instead of all the same. so what are we trying to maximize
56:32 ? You know, the precision goes the precision gets better. All these
56:36 plus -1. All those are most those are plus -3. Um And
56:44 here's another example, we talked about example earlier, I think we did
56:48 , we talked about this example where talking about the analyzing the uh the
56:54 model for the logo formation where you know, we have two basins
56:57 one basin but we can also use data to tell us about the age
57:02 the sandstone just straight away. And we look at the uh the victorian
57:09 , We say that the youngest grain is 57 And then there's a cluster
57:14 grains, but not until you get here to about 100. If you
57:18 to ignore, it's all by by itself. But if you were
57:23 ignore that grain, then you've learned from the zircons because we surely knew
57:28 grains were less than 90 million But you have that one great.
57:33 by itself but it's quite useful. so by paying attention to that
57:38 which is again a perfectly good Um you shouldn't have any problem.
57:43 again, precision or geologic insight, are we trying to do? Um
57:49 should also point out that this technique introduced. This statistical technique is interesting
57:54 you if we were to this is data, R I C P
57:58 S. Data and we talked about saturday that the I. C.
58:02 . M. S. Is fast cheap but it's the data are not
58:08 . If you analyze these same zircons Tim's instead of my eye. I
58:14 A PMS. And if if we that the central age doesn't change,
58:18 we change the precision, we move a bunch of data points that look
58:22 this. And with Tim's precision of the problem from the perspective of the
58:30 approaches. Now none of these days data overlap because the precision got so
58:35 good. And so I would argue a technique that falls apart when the
58:41 gets better, it's not really a technique. They've they've they've they've they've
58:45 this technique to be look good when data aren't good when the data become
58:50 good, this technique falls apart. just consider that. Now the other
58:57 that people argue for doing this way it's more statistically rigorous. I've tried
59:02 find, you know, some some measure of statistical rigor but there's no
59:07 thing. People just people just do and I like to think of you
59:11 what counts as statistically rigorous. Um may not know. But there was
59:17 A famous Supreme Court case in uh was uh Jacobellis versus Ohio 1964.
59:24 it's famous because this guy down here the left corner was named Potter Stewart
59:30 Potter Stewart wrote the opinion here was pornography, he said is this work
59:36 and they couldn't decide how to define . But Potter Stewart said well I
59:41 when I see it and that's what people are doing with statistical rigor,
59:45 don't they don't have a definition for , but they just assert that this
59:49 statistically rigorous and this is not and paper by Sigman at all really.
59:54 know, they think they know it they see it. They specifically say
59:58 is the order of statistical rigor, first the youngest single grain that's a
60:03 you can do but it's not as as the weighted mean of within one
60:08 of at least two grains or the the even better would be Y C
60:13 sigma three plus the most rigorous because got more grains I guess. But
60:21 the extent to which they justified this this declaration of rigor. But if
60:28 look too, if you talk to mathematicians and not geologists, they'll tell
60:31 that there are some fundamental things about average. We need to keep in
60:36 first of all that the mean is S. The mean of of data
60:41 an estimation of some true value. that averaging helps to eliminate the noise
60:47 the data to better see this true . And logically this this would form
60:52 kind of statement that if A. B. If the mean is an
60:57 of trump value then yes the averaging to eliminate noise in the data to
61:01 see the true value. But it logically follows that if not A.
61:06 not be if the mean is not estimation of some true value then averaging
61:10 doesn't doesn't gather doesn't make you things . You have to start with an
61:15 that the mean is the estimation of true value and the mean is only
61:19 estimation of some true value. If you are sampling from the same
61:24 the zircons or the appetites of the is whatever you're looking at have to
61:29 come from the same rock. They come from the same rock and it's
61:32 it's not an estimation of some true . It's just an estimation of different
61:37 . Take for example, these there's some numbers that have some
61:42 We could then, you know, that it's statistically rigorous to get the
61:46 average. There's a weighted average, a better number than those three by
61:50 I suppose. But maybe you don't that's true when I tell you that
61:56 numbers are the number of Electoral College the republicans got in the 20th
62:01 the number of strikeouts Nolan Ryan had year in his baseball career or the
62:05 of calories in in Kellogg cereal. can take the average of those
62:11 but clearly nobody's gonna make anything of because they're different things. And
62:16 um, if not a then not because these because if the numbers don't
62:22 from the same place, then averaging make the noise go away. It
62:27 makes it worse in the sense that end up thinking there's something there,
62:30 you're averaging something that never, you know, that's not a
62:35 And so here's a great example. guys junkin and gans, they went
62:39 further than some of these other They didn't they didn't have anything to
62:43 . They just said, well, analyzed a bunch of grains in this
62:48 and we've decided to say that the maximum de positional age we're going to
62:55 as the average of all the grains age is less than 200 million.
63:00 was their rule. And they could said 190 million or 300 million they
63:05 , they picked 200 took the But the youngest great. And they
63:10 an age, an average of But the youngest grain is off here
63:14 159 and that's a 14 million year . And of these grains here that
63:21 averaged Magmatic systems don't exist for 14 years. These can't be zircons that
63:30 came from the same thing, same . So they're violating that idea that
63:36 is good if you're looking at the population, but they're not the same
63:40 . This this this uh average is meaningful. So it's a, you
63:47 , it's a, it's a there's, you know, it makes
63:50 thing, something happened at 165, this is just an average of things
63:54 happened to be sitting in the same together. Um One more example
63:59 of questionable choice of this sort of came from a study of some rocks
64:05 Wales and these were sand stones again they analyzed the zircons on the
64:11 We see the data without the uncertainties the right, we see the data
64:14 the uncertainties and they decided to these all these are all the tribal
64:20 Zircons from sandstone. But they did , they decided to treat them as
64:25 they came from one igneous rock. they plotted a discord a line which
64:33 an intercept of uh 537 million Based 110 of their subset of 140 from
64:44 . I should point out that their 142 zircons ranged from 477 all the
64:49 up to 2500. So clearly some these grains didn't come from the same
64:54 , but they have treated 110 of as if they did and they got
65:01 value there. Now it's it's interesting point out that this uh this was
65:08 to be a Cambrian rock, but all of those grains that plot down
65:15 on the other side of that red , they have ages which are less
65:20 the apparent boundary between the Cambrian and division. So either, You
65:28 we either have to treat all of grains as if they came from a
65:31 537 million year old rock, even they're sandstone. But another option would
65:37 that. Well, you take a at these guys, these guys are
65:41 . This is the deposition alleges the , but that requires you to revise
65:45 paleontology because that would require this rock be coordination Because those rocks, those
65:51 are 488 million 488 was not in Cambrian. Okay, so when we
66:02 at these Igneous sir, cons like are shown here, it's perfectly good
66:07 average them together. And that's what did for that study I showed you
66:11 week where we looked at the Royal figured out how the age of the
66:14 was average away. They all came Mariah Light, which has an understandable
66:20 . But when you have zircons that from sand stones and these are the
66:24 are guns, look how nice and they are, you don't have the
66:28 luxury to say, well just average because you know, they're, they're
66:31 zircons and they're all in the That's not enough. That doesn't make
66:37 the same. And because they could come from anywhere, this, you
66:42 , like we could get a sample texas that could have come from north
66:47 and the fact that this grain could come from one part of north America
66:51 this grain from another part of and so, oh, and it
66:59 also the case that, you sometimes people do this as we've
67:02 sometimes do this to try to stuff do both of these at the same
67:05 . They want to understand the paleo , the description of the provenance.
67:09 know, you know that there's an peak here that tells us that we're
67:13 grains from, from Wyoming. That's . But then they also try to
67:17 the maximum deposition at the same time that's fine. That's fine to do
67:20 of them. But it's not fine apply different foundations to the data.
67:26 you're doing the provenance stuff, you they came from different places and you
67:33 them that way. You never would look at the distribution like this and
67:36 would never say that's a variety of . We're not going to average them
67:41 . But when we're going to look the grades down here at the
67:45 we're gonna say this one here, a single source. And we can
67:48 averaging to get well, when do , when you make that transition
67:51 Clearly it's a single source too. it's a variety of sources. You
67:55 , that's not a statistically rigorous thing do. That's just making stuff up
67:59 you go along. Um Now here's example of where it really wasn't statistically
68:07 . They had these samples from china think. And they these are the
68:11 , these samples here. There's one and there's six here and then this
68:15 the, this is the way to . But these zircons had something that
68:19 done. Sometimes you can measure other geochemistry and zircon when you're measuring the
68:25 and sometimes they measure half the um and this epsilon half name is just
68:30 measure of the variety of athenaeum isotopes happening. Isotopes are another way of
68:36 crustal history of these zircons when things different half the um isotope compositions,
68:41 probably came from different ages of And so what we have here are
68:49 the um isotopes that vary from minus to plus 15. And I know
68:53 haven't discussed half the m isotopes, just let me tell you that's a
68:57 big range. That's not the same chamber. There's no way one magma
69:02 is gonna producer cons that have a the isotope of minus 20 and also
69:07 10. But they looked at these and they said well but they have
69:11 the same age. So let's just them together. But clearly they had
69:17 their own paper, they had demonstration these things have very different geochemistry.
69:22 know, here's an example where we demonstrate that the assumption that some other
69:27 make is truly false. In the case, we just have to say
69:30 an assumption that's unproven. Here's an which has been falsified. Um Getting
69:38 to this, this discussion by Jackson all. They're very worried about lead
69:42 and laboratory contamination. This all gets to the N equals one problem.
69:46 some people just take this quiet about to deal with one sample. That's
69:51 right? One is not as good two but and we have to keep
69:57 mind that in this example one is what we're after. The whole point
70:02 this endeavor is what are the extremes that means looking at the grains one
70:05 a time. Um Some people you know, they say well the
70:11 is is that with the reason we to add to average things together is
70:18 grains come from what might be, know, when you analyze something you're
70:23 to get some theoretical distribution of a curve, right? That's that's
70:28 And and if we had access to arai light, you know they all
70:32 grains wouldn't be exactly the same. be a range that's that's that's
70:38 And some so some people are worried what if that unequal one that we
70:42 at turned out to not be in troop near the true age. But
70:45 here like this. Well by definition of your distribution falls more than two
70:52 minus two sigma away from your your . And so people worry that as
70:57 push that down away from it, going to start interacting with you know
71:01 getting a problem with the age of the deposition. Well that's that's a
71:07 . And so that means that one of 40 times you'll you'll get,
71:11 get this out over here in that business. And here's some real
71:16 This is a riot a site in which was analyzed by Argon 4039 dating
71:22 Macintosh in Ferguson 1988. I I think 36 grains and they got
71:27 range in ages. And that if if you believe that it's a system
71:31 which you can average and it probably you get this average here in the
71:35 . But if you take the youngest of that, that's shown here in
71:40 , it's still overlaps with this. it's not, it doesn't overlap a
71:44 , but it's not. If this the actual deposition, all age of
71:47 sample, this was still overlap with . Um and we can think of
71:54 problem a little bit like this, that, you know, what are
71:56 worried about? We're worried about getting wrong. And so imagine we have
72:02 age of a deposition or rock sedimentary and an age of a volcanic rock
72:07 is shedding material into that sandstone. we have a situation like this where
72:13 green dot represents green dotted line represents deposition age of the sedimentary rock that
72:18 sampling, the volcanic rock is just little bit older. It's a
72:24 small problem. If that one out 40 times we would analyze in this
72:31 here and we would get an That's just a wee bit younger than
72:35 age of the sedimentary rock. Probably so much younger that it wouldn't overlap
72:40 uncertainty. But you could call that very small problem. Um This,
72:48 , will never be a problem as as this thing is a little bit
72:52 . You really like the idea of from this blue portion of the,
72:58 the Gaussian curve. That's the best to analyze it because it's pushing your
73:04 of mexican deposition further down. We want to analyze a 10 billion year
73:09 zircon every time. That doesn't tell anything we want to analyze the
73:13 that's right up next to it. this in equal one problem, they
73:17 , well what if it's what if two sigma outlier that's terrific. As
73:21 as it doesn't, as long as don't have this this issue,
73:25 That would be kind of. But have to keep in mind that we
73:31 chosen Plan C here, we didn't any fossils in this rock, we
73:36 have an inter bedded volcanic rock. when you when you're when you're,
73:40 you're embarking on Plan C, you to remember that sometimes there's going to
73:43 a price you have to pay. the price you have to say is
73:46 , Yeah, but this this all prices only paid here if you have
73:50 volcanic rock, which is the source the sediment that is just very,
73:56 close in age to the time it's erupted and then it's, it's eroded
74:02 that the difference between these two is small. Um, and and,
74:09 this will never be a problem if source that we're talking about is a
74:16 metamorphic or sedimentary rock, it's it's only going to trouble us if
74:21 have a volcanic rock which was erupted because if it's a plutonic metamorphic or
74:27 rock, there are always going to enough time. But these things have
74:30 be that seems to be brought up the surface and that's always going to
74:34 us lots of time to be. a difference between the age of this
74:37 and the age of the sedimentary The only time we have to worry
74:40 is if we made this igneous rock the surface of the rock on the
74:44 of the earth yesterday. So I a problem here. Uh, what
74:53 I do? I said that So yeah, worrying about the unequal
74:58 problem is not worth it. If end up changing the maximum deposition of
75:03 from 84 to 95. That seems me just not maximizing geologic understanding.
75:12 then these guys also say, but what about loss of daughters or
75:16 of, you know, loss of or yeah, loss of daughters in
75:20 in the case of lead loss, , it's a bit of an
75:24 And as I quoted these guys before Reiners at all said, you
75:27 the greatest advances in the accuracy of led geo chronology has not come from
75:32 LED Los, but instead of learning to avoid it. And so these
75:37 represent lead loss or inheritance or it's to know. It's hard to
75:43 Um but for assignment of maximum de ages, we don't care unless the
75:51 loss occurred after the deposition in the . Take a take a look at
75:55 distribution here. Now there's a distribution and most people wouldn't give that distribution
76:01 thought. But but to this one them pause because we got some data
76:06 there by themselves. And they well, what about lead loss?
76:09 would have made it younger. that's a problem. Well, only
76:15 that led loss occurred after the age deposition. If it occurred before the
76:20 of deposition were happy. Because once , it's pushing the estimate even
76:26 The only time we worry about it when the age of deposition is
76:31 And and that led loss puts it the to the unhappy zone. But
76:36 only gonna happen if you disturb your . And if we're looking at uranium
76:42 , injure cons they love Zircons because don't get messed up. This is
76:47 unlikely if you're looking at an unmet , beautiful little sandstone when you see
76:52 off by itself. It's probably not it's less than the age of
76:56 it's just off by itself. And , and then the final thing about
77:00 loss and contamination is all right. you say that guy's been subject to
77:06 loss. Okay, well then we're get rid of it. What about
77:14 one? Now, it's off by . Well, blood loss and before
77:18 know it, you can apply that every single sample once you, once
77:23 quality question, a process that could worked on the youngest one. Why
77:27 you trust in the other ones? the same thing goes with,
77:32 and then you can you can do same thing for argon loss.
77:36 is a little bit. Well, the same thing for our gun
77:39 I'll just say, and then people about contamination, contamination is not a
77:44 problem. It's a laboratory problem. is when you mix your samples
77:48 don't do that. Of course. . You gotta be careful, clean
77:52 out when you're doing when you're doing . But if you, you
77:54 people have had contaminated samples before. contamination is the same problem. If
78:00 if you say, well, that's by itself because contamination, well,
78:06 , that grain is bad. But not that great, Why not?
78:11 you have evidence of contamination. I'm , but all that work that went
78:15 your sample, you gotta throw the thing away because contamination is not something
78:19 just happens to the youngest grain. when we worry about lead loss,
78:26 N1 contamination, all of these Um, the data from this paper
78:32 Dickinson and Garrels go back to the and look at, it doesn't seem
78:36 be a problem. The data pretty plot either on the line or above
78:40 . So after going through all that averaging always bad. Well,
78:46 but you can average stuff, but if it's clear the data came from
78:49 same population. If you're looking at and breakfast cereal, it's probably not
78:55 it to average things together. Um zircons, we have ways of testing
79:00 they have similarities. You could look the halftime isotopes or the fishing track
79:04 or the helium age. You could at the other geochemistry, like the
79:08 thorium ratio, the rare earth element , the shape of the zircon called
79:13 coupon typology in argon dating. You've other things we can look at that
79:18 us about the chemistry of these And if these things match up in
79:22 way, you might want to well they're the same, they're close
79:25 the same age and they have these characteristics that make them likely to be
79:30 together. Um And back in the 19 eighties, there was all this
79:37 about this thing called the plateau age argon data mentioned, but people stopped
79:42 attention to it because they realize that a lot of information in plateau information
79:47 variable age spectrum. Um and so , you know, you can look
79:51 the age spectrum like that and say got no plateau or you can say
79:55 has valuable information and I won't bother that second diagram. But um you
80:00 , the the thing to remember is this is Plan C. But don't
80:05 too upset about it. It's what have to deal with. And that
80:09 people, whether it's plateau ages or ages in in sand stones, people
80:17 comfort in rules, they've been told follow a rule, but a rule
80:21 not a substitute for thinking If you've this rule that says throw away your
80:26 grain and make your maximum de positional 50 million years, older. is
80:31 really a good rule? Now, rules are not constants of nature,
80:36 ideas from people. And so when see a situation like this, you
80:42 , your your I should naturally be to the winner of the race,
80:46 this cluster of things way back And the same thing should be true
80:49 zircons. Okay, one more thing can do in looking at sedimentary
80:55 then we'll take a break and then go to look at things more
80:58 Our last reason to look at the of sedimentary rocks is to look at
81:05 post deposition all thermal history of a . And remember if we're looking at
81:11 post deposition of thermal history, we're gonna be able to do these other
81:15 , because post deposition and thermal history that we have disturbed the signal
81:21 Whatever system we're looking at because of heating of the basis. Now we
81:29 look at, we can look at paleo geography. If we look at
81:32 lead zircon and we look at post , all thermal history. If we
81:36 at uranium, uh helium con, are two systems. But in
81:41 but in one system we can do or the other. We can't do
81:45 . Um And so again, advantages disadvantages. Well, of course,
81:51 we got a lot of different thermo is available to us and then we
81:55 pick the right one and we can , we can understand how hot things
82:00 , we can understand when they got . We may wish to add that
82:03 the organic methods of understanding basin evolution as victor night reflected for these organic
82:10 . Uh those those methods tell us little bit about peak temperature, but
82:15 don't tell us when this disadvantage is course, if we're doing any of
82:20 , we lose information about the but that's just, you know,
82:23 not our fault. Um And you know, these these sorts of
82:29 are made for sedimentary basins. Looking the history of the, of the
82:34 versus time. And and and then top of these, on top of
82:38 things may add maybe added a Uh Yes. Oh, jeez,
82:48 , thank you. Well, you what if that's Yeah, you're
82:55 We're at 10%. So maybe it's for us. This is this is
83:00 good place to stop. I'll go to my office and get my power
83:04 and then we'll we'll fix up. , thanks for mentioning that.
83:09 so we'll take a break now. I'm gonna stop sharing and they'll stop
83:18 recording. Okay, We're ready to again. Thanks. Very good.
83:28 , back to sharing our screen. I'm gonna start over. I'm gonna
84:23 Thank you. Very good. Um we were here talking about green is
84:35 and so we'll have to start Stop sharing, share again. Where
84:42 it? Oh, there we Okay, good. Doesn't move.
85:29 , it doesn't. Okay, so can you can you can see that
86:56 . Right, okay, let's see happens when I go to slide
87:02 Good. Okay, okay, so up. Still working. Okay,
87:45 we are interested in the post all thermal history, you know,
87:52 example, did the basin get hot to produce hydrocarbons? Um So,
87:59 want to make a model. This model. Um you know, would
88:03 just assume a a temperature based on static geothermal gradient question. Okay.
88:13 but you could do better about that by taking, you know, modeling
88:17 certain approaches. For example, here's a modeling of some fishing tracks
88:23 appetites and it tells us that this , these various samples were at about
88:29 degrees here at about 60 million, . All of these are getting to
88:36 degrees about 60 million years. The before 60 million. It's fair,
88:41 unconstrained because we're only really good at what happened to the sample since it's
88:46 at 100 degrees. But all of fishing track modeling and remember that,
88:51 is an approach that takes it and into account the number of fission tracks
88:56 their links. The distribution of Do we have all old long ones
89:00 there are long ones and short And this modeling then I would say
89:04 we were at 100 degrees 60 million ago and we cooled pretty rapidly ever
89:09 . That's, you know, that then be used to put into whatever
89:12 whatever basic model you wanted to sort . Um suppose you had um a
89:22 exposure of of some sample and you , that they went down and came
89:27 up, but, you know, was the geothermal gradient? Was it
89:32 high or very low? If it's high, we got two temperatures,
89:34 know, of 300 degrees, if was very low we got to temperatures
89:39 only 100 degrees. So not knowing geothermal gradient, then, you
89:44 doesn't tell, you know, the graffiti tells us that this sample got
89:47 than this one, but how much if you don't know the geothermal
89:51 you know, you're going to be off. Um and so you may
89:55 to try and and sort that out seeing which of your samples were reset
90:00 various, you know, various if have appetite helium, for example,
90:05 a geothermal gradient was 15 to 25 expect appetite helium to be in this
90:10 . An appetite fishing track to be this range, cooling versus depth.
90:15 cooling age versus depth would be a different group of samples if the geothermal
90:20 was 25 to 35. So by at some samples that are constrained in
90:27 their strata graphic location, you can model what the evolving geothermal gradient was
90:32 that's a huge, huge information to you because because often in talking about
90:40 models, perhaps the most important parameter can come up with is t max
90:46 was the maximum temperature of the basin ? The next, The next thing
90:50 want to know was, how long it experience that maximum temperature? Um
90:56 an understanding of geothermal gradient, strategic is not going to take you
91:01 Um Here's an example of how you be able to uh sort that out
91:06 argon. This comes from the uh basin and We have four feldspar is
91:17 look at here, they're all single fell spars and they've all been analyzed
91:22 the Argon 40 39 technique. Some these felt sparks come from this well
91:27 others from this well. And the between these two wells is their depth
91:31 , 10,001 is 13,000 and the 10,000 well was producing liquid hydrocarbon, Whereas
91:39 13,000 ft well was producing dry So a different hydrocarbon dry gasses a
91:46 thing, liquid oil is a lower thing. And what we have here
91:50 the age spectrum for these individual felt , the de positional age of the
91:57 in which these felt sparks were obtained and here at the boundary between the
92:04 and the pink. And you can that for these samples from wells that
92:10 producing liquid oil, the age Spectra to be just the kind of age
92:16 you'd get from a slowly cooled rock had an age of about a billion
92:21 and then was brought to the surface then deposited in this uh, devonian
92:27 , you know, as it's shown . But then that sample was never
92:32 . You see the apparent ages of of these steps are older than the
92:37 will age here, which is But compare that to the ages of
92:44 samples that come from the dry gas area. Dry gas means hotter.
92:51 clearly it was hotter because these they're both, they're all case else
92:55 . These two K sells cars have spectra that indicate that they were reheated
93:00 deposition because we're getting apparent ages that down there in the Mesozoic, not
93:05 the early paleozoic when the rock was . So this is a sort of
93:13 we were in a frontier area and didn't know that there was oil or
93:16 here. You would, you would at a sample like this and
93:20 aha, this area got pretty hot it is disturbed the argon obviously you
93:25 need to disturb the argon to make . This sample shows it, but
93:30 you and that might be fine, , maybe that's what you want to
93:34 are gone. Data from, from all fell spars that have all their
93:38 older than de positional age. That say oil is a possibility if you
93:43 getting gauges that are younger than the . All age best you could probably
93:47 for is gas. If if if and and of course if this age
93:52 , You know, this age spectrum a lot of gas that's older than
93:55 deposition allayed. But if you got age spectrum that was down here at
93:59 40 million, that would indicate that sample, the sample got really hot
94:05 and that's bad for oil. So checking and again, this is just
94:10 belts bars, they have a very closure temperature if you wanted to do
94:13 same sort of analysis for appetite fishing . You'd have, you know these
94:18 are probably completely reset from appetite fishing but not completely reset for our
94:27 That's the end of my Detroit. there's dating discussion. So that was
94:35 lot of detritus dating. Any questions to mute? You have to ask
94:47 I'm asking you? Oh, never . Um So no questions. Okay
94:54 then I think since we just had little break we'll we'll go on to
95:01 my my exercises, they're not So let's go change area exercises.
95:29 I email you the exercises? I that's fine. I want you to
95:33 through them with me. 1st share please share screen uh exercises.
95:55 Is that working now? You see ? Okay. Good. Alright so
96:00 have 20 different questions here. Are are you just moving the slides Because
96:06 only see the home page of I don't see the presentation.
96:10 so it's not working. Thank you benching that we're gonna start over.
96:19 let's try this again. Let's uh that here, scare screen exercises.
96:32 you see it now? Okay, can you see it move?
96:38 Okay great. So I've got a of little questions here about data.
96:45 me based on these two age spectra you know what you're what you're gonna
96:52 in here are some some ice a data of some sort and some geologic
96:58 . Sometimes the only geologic information might the kind of rock, but that's
97:03 . So interpret the history of this light. What can we say about
97:08 rye light that has these data? are the why are the by typing
97:32 case? Bar a specter different Well, they do have different closure
97:40 . But but but for a real , what would we expect them to
97:48 ? No, no, no. don't care for a wry light.
97:53 doesn't matter what the closure temperature Right? Because remember Riley lights or
97:57 rocks, they should always they should the same age, no matter what
98:01 closure temperature. I could say fishing . I could say uh uranium lead
98:08 lights cool rapidly. You know, go from a magnet on the surface
98:13 the earth in a day. It matter what system we use to date
98:19 . We should get the same Why are we not getting the same
98:44 ? What about you off there in cyber cyber world? Can you tell
98:49 why they're different? Probably not. you gonna do you have a good
99:10 ? Start of the closure temperatures It has something to do.
99:15 let's, let's, let's start What are the two closure temperatures in
99:19 in in in play here we're talking this is an argon diagram.
99:30 okay. Yeah. 302 100 Something like that. 39 or
99:38 Well, don't that is not that that that that that was just one
99:42 of one part of one feldspar. we can think in general that this
99:48 ranges from probably 2 50 to 1 something like that. But if that's
99:55 case, why aren't these? And is a highlight reel lights ought to
99:58 a very simple history. This is that maybe it's not so simple.
100:04 are they different? Yes, this these these these two minerals are from
100:15 same, same rock, same specimen? If this was a
100:27 would it be any, would would be any potential problem? I
100:32 this is telling us that the by clothes temperatures 300°. That means this rock
100:37 at 300°90 million years ago. But the closure temperature of case bar is
100:42 to 1 50 this says that 30 years later, it was at 100
100:46 50 degrees. How do we explain ? 30 million year difference? This
100:50 a highlight. They're supposed to cool an afternoon. Did this rock take
100:57 million years to cool? What's the option? The other option is that
101:12 reheated something happened to the rock to these two things. Not the
101:18 When did that reheating take place? , that's the first event, I
101:26 to know when the second reheating event place. The weathering Russian.
101:35 there might have been erosion, but importantly, there was probably burial or
101:38 an intrusion nearby. Something had to heat erosion doesn't add heat erosion would
101:43 the rocks off. So we have come up a reason to get this
101:47 hotter because this rock cooled down the it was erupted. Right, a
101:54 weeks after this realistic eruption, this is surface temperature. But yet these
102:00 , these these minerals are different. only that means that there's been a
102:04 in the system. The best way disturb it is to heat it
102:07 How do we heat rocks up But again, inside the burial is
102:14 way another way was to bring up igneous intrusion next door. Right,
102:17 either have you have the our burial you have contact metamorphose. Let's just
102:23 burial, simple burial. Can we when the maximum burial was, There's
102:30 60, right? That's the youngest down here. We know that this
102:34 was still at 150°60 million years And we know that it was probably
102:41 this reheating event probably didn't exceed 300° the by type. And the case
102:48 would have the same age here. we can say that this real
102:52 We know it's a real light. that means that you have a simple
102:55 . It doesn't these ages are This tells us this area has been
103:01 . If this was a real that was, you know, in
103:04 interesting sedimentary basin, that would be way to tell because you know,
103:10 just this is a simple test of of the model that the basin never
103:15 hot. This basin, if this a basin, it got hot,
103:20 got over 100 degrees hot because that's you got the case bar to
103:24 you know, now I'm sort of that the bio tied hasn't been,
103:29 been disturbed, that this eruption occurred million years ago. That's a good
103:35 , but it's not guaranteed. How we test that assumption? I'm saying
103:39 is a bright light. It got , it got erupted 90 million years
103:43 . It got buried such that 60 years ago. It got re heated
103:47 to about 100 or 200° How could , how could I be sure that
103:51 bright light was actually erupted 90 million ago? That's 100. Well,
104:04 keep it. Just look at this here. What else can we do
104:08 this rock? We just dated. talked about how many different dating
104:18 12 of them. There's two of . Can you think of another
104:22 We might want to do that would helpful. You're just all over the
104:37 , uranium lead in what kind of ? Yeah, in the right
104:42 Not in a theatrical value in the light itself. If we did uranium
104:46 on Zircon, that's gonna be the age of eruption. Right? You
104:52 have to worry about reheating. So we dated. So if if I
104:57 say this rock is a was a that was erupted 90 million years
105:01 What does that if I'm right, does your uranium lead zircon value
105:09 Know how many million years, It be 90 because that those two things
105:21 be the same. And the reheating has only affected the case from the
105:26 was sufficient to remove argon from the bar but not bother these other
105:32 And so we could say that the basin was buried to temperatures above
105:37 but below 250. And then you put that into your geologic model because
105:45 a really light. We've got we've got those assumptions that everything should
105:49 the same when it's not the that that gets interesting. They got
105:57 questions from you back there, you ? Okay, Yeah, you
106:06 You do have a question or no . Okay, good. Let's move
106:12 to number two. Here we have bunch of data from a granite.
106:17 want you to tell me about the history of this mountain range that it's
106:24 in. What's, what can we about it. Okay, let's go
106:38 and be bold and put a geologic associated with that. What what thing
106:43 then? Yeah. Well, I mean, what do you mean
106:54 ? Has it magma moving around? , the Magma could have been there
106:58 125. But the Zircon was created . Right? So I'm trying to
107:03 you to say that that was the of crystallization. Probably the whole
107:10 Why then? Is the horn blend a little bit younger? Different closure
107:17 . What's the closure temperature? So it took about three million years
107:23 this rock to go from 7 50 500. That's a lot of temperature
107:29 not a lot of time. What that tell us? But then it
107:34 a long time. Another 42 million to cool down to whatever that closure
107:40 is. Argon in bio tight. what temperature 200 300. 300.
107:51 how can we explain the fact that cooled pretty rapidly from 700 to 500
107:56 rather slowly from 500 to 300. cooled pretty slowly because of the veteran
108:12 the process of the what of the process? Just you mentioned the
108:19 Okay, so the erosion was slow ? Yeah but why did it cool
108:25 from 1 25 to 1 22. that fast erosion? I would say
108:34 probably wasn't fast erosion. This is example what we're talking about. Is
108:37 because we can we can say that granite was intruded at a depth that
108:42 temperature was less than 500 but more 300. Because it took a long
108:47 to get to 300. But it it took a relatively short time to
108:51 to 500. So it's probably intruded a depth of temperature four or 500°,
108:57 not shallow, because otherwise all three those ages would be the same.
109:02 of these. And look the bio and the fishing track and the helium
109:06 almost all the same. This is separate event over here. When we
109:11 that the bio tight and the fishing appetite and the uranium helium appetite are
109:16 the same. What does that This is the erosion you were talking
109:19 ? Right. We see an acceleration erosion from in the cretaceous here in
109:27 late cretaceous. In the early this rock was intruded, but in
109:32 lake rotations here, It experienced rapid and rapid cooling, which we can
109:39 to mean rapid erosion. Perhaps perhaps deposition in some basic if we're
109:45 to look for a complimentary base into mountain that this rock was found
109:49 We would look for ages of sediments 75 million or 70, something like
109:59 . Okay, move on to number . If there's no questions, it's
110:04 time. Is it three o'clock. in pretty good shape. All
110:09 This is a much simpler question. how would you interpret this diagram?
110:24 have a bunch of discordant points. is a granite. There are
110:35 Tell me how old the granite What do we do with dessert
110:55 with discordant points like this? Sometimes a line through them. Right.
111:04 line? What what does this line to you? Draw a line that
111:10 of went through here. Right. old is this granite? Well,
111:29 , Can you do better than Probably around About 4 40, something
111:34 that. Right. So I would the line through here. Upper intercept
111:40 going to be here around 4 44 something like that. This and
111:44 And I'm going to say that's the intercept is the age of crystallization because
111:48 the points are up here closer to . Hey, you know this,
111:52 , this, this actually looks like might have a negative age of lower
111:57 . We can ignore that doesn't So that's all I want to get
112:01 that diagram as we look at the values to draw a line for
112:05 Now, if I told you something about the geologic environment, you might
112:11 able to make a more nuanced But without me telling you what kind
112:16 rocks it's around as it is in contact is an in fault. Contact
112:21 an intrusive contact. Maybe that would the story. But in this
112:26 I would say that this means that rocks about 4 50 suppose we have
112:37 another rye a light here and we've a bunch of single crystal sanity ages
112:43 this rock? And we get all these points here? Why explain what's
112:51 age of this rye light and what uh what are the, what are
112:57 other, what's the whole distribution telling again, we would be looking at
113:08 Royal Light to data to understand to a chrono strata graphic marker in our
113:13 sequence. This might be our best to know the age of these sand
113:18 that don't have fossils in them. at this, what do you suppose
113:23 ages 32, maybe 32.5 ish, like that. What's, what's going
113:34 with these guys? It's 40 and and 50. Why are they
113:39 Why are you you seem to be them in determining the age. Um
113:49 have a geologic explanation for their Shouldn't a riled Light all give the
113:56 answer, but it doesn't what's going ? It isn't uplifted uplift. I
114:09 know what you mean, Like maybe the 40 and 44 um due to
114:17 anyone comic activity. Did it come the surface or something? Okay,
114:24 , I think probably, I mean want to think about these are all
114:27 in one rock. This is this a this is a volcanic rock.
114:30 not talking about a sandstone here. . So we're all looking at the
114:35 the products of a single eruption? not discuss that in terms of different
114:41 Or are you, are you saying these are from some older eruption?
114:48 did they get in this in this rock? I think you're on the
114:54 track, but we're just looking at single rock here. How does it
114:59 components of an older eruption? This back to the nature of explosive volcanism
115:09 . This wouldn't be found in a , but it might be found in
115:12 big eruption. Big blow up These are the pieces that might have
115:19 incorporated during that eruption that fell in the older rocks and the reason that
115:25 , and and because of that this is why we date these,
115:30 rye lights, wanted to top one at a time so that we can
115:35 these, these oddballs, these any , because if we have analyzed all
115:39 these grains at a time, we have gotten 32 a half, we
115:42 have gotten 33 or 34 not known difference, but having a 50 out
115:48 is gonna make a difference. Just one of those in and the whole
115:51 shifts a little bit and this thing being 32.5 and starts being 33.1 and
115:56 never know. Okay, um let's come back to that granite when
116:21 got too many igneous rocks for I want to talk about?
116:26 let's talk about this one. This good for now. I'm showing you
116:34 of ages. Let's say we have lot of ages, maybe 100 different
116:37 that have been analyzed and we have distribution of appetite. Fishing track ages
116:44 zircon uranium lead ages. And I you to give me three different interpretations
116:50 this one. If the sandstone if deposition will age is Eocene one of
116:58 Miocene and one of its unknown. with the thought that this is an
117:08 rock. When is when is the ? In millions of years? You
117:15 know you guys have been drilled in ? You know the answer to
117:21 Yes it would be between 55 and . So if this is an Eocene
117:33 explain the distribution of these two The fishing drugs of the appetite are
117:57 how decrease they're what the fission tracks the appetite are going a downfall in
118:04 region. Yes, I mean the . What I mean? I'm not
118:08 sure what I mean. The slope slope you mean that that there's a
118:15 narrow range of gray of ages? that's true. But that that's going
118:21 be true in all of these Why does it what does it matter
118:24 the rock is Eocene Miocene. What what how does that change our interpretation
118:30 the history? This is an Eocene , what does the appetite fishing track
118:35 tell us if the if the rock E. S. C. Let's
118:38 say let's let me let me not me not try and confuse you with
118:43 . What if I say the deposition age of this rock is 40 million
118:52 . What does that tell us about history of this rock? That
119:04 Okay, so how did how did happen? The rocks get hotter because
119:14 erosion cooling down but it's a I mean sand stones originated the
119:31 So we got a 40 million year sandstone with 80 million year old zircon
119:36 , lad and 20 million year old fishing track. Why? Why are
119:42 group of age is older than the and one group of age is younger
119:46 the deposition. This is the I didn't hear something dyke, but
120:10 mean you haven't speak in complete sentences . A dyke does what?
120:24 wait, wait a second. Are saying the zircons came from the
120:28 Now all of these zircons and all these appetites are in our sandstone.
120:33 you can you can call for you call in a dyke to do something
120:36 fine. I just want to make we're not we're not calling on the
120:39 to be from some other place there our sandstone. If you want to
120:43 a date in the story, that's . I mean, Before you start
120:52 on dykes or something, just just to me how broadly speaking, how
120:57 it be that the appetites are 20 years younger than the age deposition just
121:14 to heat them up. Maybe by by a dyke. That's a fancy
121:19 . There's a simpler way. How you heat sand stones up? Easy
121:33 . Yes, you do. Where you get oil from? How is
121:40 made? Excuse me. Did you erosion? Oh, carriage in is
121:53 source? Yeah, but how did that that that is Karajan has to
121:56 heated up. Right, You make by taking, yeah, thermal maturation
122:03 you get that by taking material that deposited at the surface and burying it
122:08 . Right? So we're talking about sort of burial here. That explains
122:15 the appetites are younger. Right? the closure temperature of fishing tracks in
122:24 Lower than that? About 100. 100. Um So what this is
122:34 us if the if the deposition all . Eocene, then this tells us
122:39 in the Miocene in 20 million years this rock was buried to temperatures in
122:45 of 100 degrees. Now, what the fact that these Zircon ages all
122:55 to be between 60 and 100 million ? Tell us if the age of
123:03 deposition is E. S. About 40 million, that's telling us
123:10 the provenance. We've got two different here and they're telling us about two
123:15 things. In one case we're learning where the grains came from. They
123:19 from a place that had predominantly cretaceous is just from 60 to 90.
123:25 very narrow range. But they but can't be reheated in the sedimentary
123:32 That's the uranium lead zircon closure temperature high. But the appetites can be
123:39 . That's only a close juncture of . So does that make sense?
123:47 because of burial? Now, what I told you now? Oh I
123:51 wrong. So in in in in a we've got this story in which
123:56 says there's this there's this there's a area of cretaceous blue thons and they
124:03 being eroded and they make a sedimentary in the Eocene which is subsequently buried
124:09 that 20 million years ago. Is above 100 degrees? That's interpretation.
124:14 But that assumes that a deposition in E. S. C. What
124:18 I now told you that the age deposition is 10 million years? It's
124:24 late Miocene. It's not that it's the middle of the EEC what what
124:30 are we going to put on this ? Now, give me the
124:34 Give me this burial history. The all history. The provenance history.
124:39 this rock is only 10 million years . The Sandstone Age of Deposition is
124:52 vs 40. We're just talking about . Now I've moved it to
125:09 Now we've got the appetite ages which older than the age of deposition right
125:14 they were younger. If the appetite track ages are older than the age
125:24 deposition, what does that tell us the depth of burial? The thermal
125:30 of this sandstone? Why did five is 100 and 10 degrees, maybe
125:49 . But don't think in terms of , just think in terms of
125:59 This rock was deposited 10 million years . And yet the appetites are much
126:04 , 2030, 40 million years How can that be a simple
126:16 We would go down to Galveston's this picked up some appetites. What ages
126:19 they be? Well, they'd be than zero. Right? Why?
126:28 ? And and if we were to that, why are they not younger
126:32 the date of deposition? Well, it's zero, it's kind of a
126:39 example. But um All right, stick with this example. Let's take
126:44 and add 10 million years. These are older, just like the grains
126:51 in Galveston's are older because they haven't buried yet. The grain. If
126:56 looked at appetite fishing track ages in Galveston's sand today, they would be
127:01 the fishing track ages are in the they came from from the llano uplift
127:05 the big bend region or whatever these in texas are coming from.
127:12 if we had ages of the sandstone are clearly older than the sedimentary
127:19 what does that tell us about how the sample could have gone Not very
127:27 . That's the answer I'm looking That's all there is to it.
127:30 very deep. Right? Because they been reheated. If this rock is
127:37 is if this rock is 40 million old, they've been heated above 100
127:41 . If this rock is 10 million a row, they haven't been two
127:45 different histories for this granite for this and two very different potential exploration targets
127:52 rock that's never been to 75 degrees a rock that's been 100 and 50
127:57 . It's very different potential for hydrocarbon . One has never been hot
128:02 Clearly, if you can't, if can't start reheating a few appetite fishing
128:08 , You might not be interested in $100 million dollars to drill down
128:14 But if you but if this rock eocene, you've clearly taken this rock
128:18 to above 100°. That might be good me. And in both cases,
128:34 Zircon uranium lead data aren't telling us about post deposition all history because they
128:41 do because the closure temperatures too The reserve con uranium lead data are
128:46 us about the provenance. Where did grains come from? We're gonna look
128:50 a place in our geologic understanding of that has grains that are that that
128:56 an age of 80 70 80 90 . Look for that place. And
129:02 where you can say there were rivers from that place to this place.
129:06 these rocks were being deposited, that's paleo geographic consideration, which again,
129:11 be very important in our understanding of history and the appetite fishing tracks.
129:18 so if But if the age of gray at the age of the Sandstone
129:22 10 million years, then the appetite track data are also telling us about
129:27 Geographic information. We are looking for place in which the zircons are
129:33 but the fishing tracks are 20. might be a different place than where
129:38 zircons are 80. And the fishing are 60. We've got a fingerprint
129:43 our of our uh, of our . If the if the Sandstone is
129:51 , We have a we have a complicated finger pinch of our provenance.
129:55 the Sandstone is 40, Yeah, always if the Sandstone is 40,
130:03 the appetites are telling us nothing about provenance. So it's not it's not
130:08 nuanced information. All we get is zircon information. The zircon ages are
130:14 . But if the if the if the if the if the deposition
130:18 ages 10 million, then we have more complicated story because we have to
130:22 90 million year old zircons and 20 year old. F more complicated,
130:28 what I meant because you know, good and this here where I'm
130:34 you know, I went back to saying when we talk about the chronology
130:37 a sandstone. If we look at system, we're either going to be
130:42 about the post de positional history of sample or the provenance never bolts at
130:46 same time. Because if it's hot to give us the first one,
130:50 too hot to give us the All right, here's a question.
131:03 need to know the age of a of fossil pour sand stones. We
131:08 they are paleozoic and contain some bentonite . You know what Ben night
131:16 Yeah, but what's the what's the the form? What's the genesis of
131:20 beds? Where did they come Bet night beds are generally thought to
131:25 altered volcanic units. Their mud mud. They're really quite altered.
131:40 that's what we've got to go We need to know how old these
131:43 stones are for our geologic history We're studying a new region and we
131:49 to know some things about it. pretty sure they're paleozoic, but we
131:53 to do better than that fossils aren't . Are we going to figure out
131:57 age of this sequence? Well, on what? Yeah, the bed
132:15 of volcanic rock. So how are gonna date it? I'm saying that
132:21 can know the age of the sandstone just looking at the volcanic rock just
132:25 to it. Okay. But I , and I have suggested that these
132:28 tonight's came from volcanic rocks, but not, I mean it's not good
132:34 to say look at it's not good to say argon you need to give
132:39 a that's still not good enough. when you're dating something, you have
132:45 say the system and the mineral. what system and what mineral will we
132:51 to? What rock? Well, tonight means that it's really altered a
133:07 like corn blend or case bar. are really just now. Almost all
133:13 . So they probably wouldn't they probably buy type same. No, that's
133:21 the only one left. Well, the only one day. That's only
133:24 for argon. But you've got other besides argon uranium uranium uranium. What
133:32 got three kinds of uranium dating uranium , uranium fission track or uranium helium
133:43 is right. But you've got three here. You did you say uranium
133:52 ? I didn't hear uranium lead. think that's good on what material you're
133:58 to apply this uranium lead system. . Yeah, americans are probably still
134:03 to be available in a bentonite. is a very altered bentonite are usually
134:08 when a rile of a realistic deposit a distal deposit is deposited in water
134:14 that gets real altered. And all all the glass turns to clay,
134:19 the felt sparks turns to clay the tight and armband go away but there
134:24 cons are forever. Right. in this case we're picking Zircon because
134:36 it's resistant to chemical and physical Um We don't know, I mean
134:43 also nice that the closure temperature is , but it's probably the only mineral
134:49 has any chance. It's also got really high closure temperatures. And that's
134:54 we, you know, when, trying to figure out the age of
134:58 volcanic rock in a sequence of sand , your first choice is always going
135:03 be, is there a real Are they're zircons? That's number
135:07 that's always going to be the best thing because maybe, you know,
135:12 say these rocks are paleozoic, they've around a long time. That's what
135:17 said, it's it's good for dating for the whole history of the
135:26 But the event that we might be in this case is the burial of
135:29 rock. We don't, we want know the age of the deposition,
135:34 the only way we're gonna get the of the of the deposition of this
135:38 by using potassium argon. And by way, you shouldn't say potassium
135:42 we've moved on from that. It's 40 39. Right? So if
135:46 if we could date by argon that be fine. But that assumes that
135:50 rocks have never been buried or We've said their bench nights that means
135:55 , but maybe they're nice spent two with feldspar. Sin Zircon is always
136:01 than feldspar if you don't worry about cost and you don't worry about other
136:06 . Um Zircon is better because it's resistant to weathering and it's got a
136:11 closure temperature. These rocks are paleozoic think, which means they've had 400
136:17 years to be buried and then uplifted if they were buried below about 200°,
136:24 argon systems are going to be But the but the the zircon uranium
136:30 zircon system will not be reset. really, I mean if you take
136:38 home thing from you take one thing with you from these lectures is you
136:43 , your first question is can we zircons Zircons in a really light?
136:48 your if your problem is, I you've got two problems, right?
136:53 you've got your your your your going these sorts of things. These isotopes
136:58 the fossils aren't aren't good enough and know, that's the fossils may be
137:04 or they may be not diagnostic. fossils might tell you these are paley's
137:09 rocks that's not good enough. You want to know early early permian
137:13 late permian, not just paleozoic. now we have to do this isotope
137:19 . The best choice for dating just age of rocks is always going to
137:24 a zircon. Then the next we to all these other methods, then
137:29 whole then our second question for sedimentary might have nothing to do with what
137:34 age of deposition is. We know age of deposition. The fossils tell
137:38 the deposition is you know is cena or something like that, hup hup
137:45 vary it. So one of those of the of the of the Jurassic
137:48 something. We know that that's good . But now we want to know
137:52 happened to them since then. That's second bunch of stuff is when we
137:56 to say, well did these rocks hot enough to make oil or metamorphose
138:01 whatever you want? Whatever you're interested . Then you go to a different
138:05 of things. But for this we're interested in the age of sedimentation.
138:12 zircons. If you can't find a you hope you can find a feldspar
138:16 something else. What if there were . Okay but what if this bentonite
138:21 so unbelievably altered that you can't use ? And it didn't have any zircons
138:26 it? Is there any hope in the see the age of sediment the
138:32 sequence basically. I'm saying we don't fossils and we don't have volcanic rocks
138:38 we still think they're probably paleozoic and like to do the best we
138:44 Is there anything else we can There's no zircons in the bentonite?
139:00 if there was air cons in the stones. Yeah we could but why
139:11 that of all your, you gotta we got three ways today to
139:16 Right. Why did you choose that . 00 comes in the sandstone.
139:30 do you want to do with Why? He said fishing tracks
139:36 Why do you guys pick fishing We're trying to figure out the age
139:39 the deposition. Is that the best ? Uh Well, they could be
139:56 than the sandstone they're in. If been reheated, you're you're picking,
140:04 picking a system that is prone to reset. It's a palace or
140:10 If you picked fishing track in if the rock got heated up to
140:14 160 degrees, you might lose some those fishing tracks. 150°. isn't very
140:23 . This rock's been around for 400 years. What's so so you could
140:28 that? And you know, it be interesting. It probably wouldn't be
140:32 first choice if the job was to the age of deposition. What if
140:37 looked at the zircons and looked at Iranian lead ages? What would that
140:42 us? I just spent an hour about how people do things wrong in
140:49 ? You should do it this do it this way. You got
140:57 bunch of zircons in a in a And I want it. And and
141:03 that have, can that help me the deposition? All age of the
141:06 at all. What if I dated zircons. And I had a range
141:22 ages from, let's say 395 - . That's my range of ages of
141:36 100 Jerkens 395-1000. When was the deposited? I know for certain that
141:46 was deposited in the paleozoic because I a fossil that's I got a trilobite
141:51 tells me tells me that it's paleozoic nothing else. But I gotta zircons
141:58 go from 390 to 1000. When the sandstone deposited? Yeah. And
142:07 than Older than 2 45. Based the fossil the fossil the trial by
142:16 it has to be paleozoic. Right that narrows it down. It has
142:20 be younger than the youngest Circon and example I gave that's 3 90 the
142:26 sells this has to be paleozoic but not a good trial bike because it's
142:30 know it's got a whole range of paleozoic. I assume there are such
142:35 . Maybe there aren't but there are fossils that are not good that
142:39 Right so the next step remember dating try those or cons for age determination
142:51 not our first choice. First choice go to a real life. Maybe
142:56 right light didn't have any zircons. choice we go to our fossils that's
143:02 better than than our third choice. here this fossil was only telling me
143:08 I need to do better than just . So now I'm going to my
143:12 plan plan C let's just date all zircons and what this told us is
143:18 than 3 90. How much younger 3 90? We don't know younger
143:22 three nights younger than the youngest That's all we can say. But
143:27 how you would go through the plan . My first plan you say there's
143:32 bent night. That might be a bentonite. Let's find uh see if
143:36 can have any minerals in there. only mineral we're likely to find zircon
143:41 the others are gonna be too Didn't find any. Go to Plan
143:47 . Which will go to Plan The the fossils. That's not helpful
143:53 C. Now if it turns out we know it's paleozoic and we get
143:59 youngest grain is not 390 but You , 2 90, then we've narrowed
144:06 down between 2 92 45. What the youngest grain is 2 50?
144:10 now we know these rocks are late . You see that's why sometimes you
144:16 you go and you date 100 grains you still get 1000 million years and
144:20 didn't learn anything Francis dating of the . Zircons is the third choice in
144:28 and trying to figure out the age a sedimentary union dating an inter bedded
144:33 rock is always number one. If don't have that, then the fossils
144:37 usually tell you, okay. But the fossils are bad either they don't
144:43 because you're deposited in a high energy or the fossils are not diagnostic.
144:49 know, some fossils just have a long range, right? Some fossils
144:53 just be only diagnostic of the paleozoic . Some fossils might not be only
144:58 of the fan or iso if they change the whole time. That's the
145:02 they looked in the Devonian. That's way they look in the cretaceous.
145:05 fossils are like that and there they be excellent fossils for environment of deposition
145:12 they're terrible fossils for age of So if that's all you've got,
145:17 go to the third choice is to dating to tribal minerals. The problem
145:21 that is that it will only tell about whatever the youngest one is.
145:25 you happen to find a young that's the same age as your
145:28 You got it. But you'll never that. You just know that the
145:32 must be younger than that. Let's what else we got here.
145:44 we just basically discussed that one. let's see. Alright, let's look
145:57 , let's look at this. This . We've got a strata graphic sequence
146:03 . We've got a granite shale, sandstone and highlight. I want to
146:10 the deposition all age of unit B sandstone and there are no fossils to
146:21 us. I'm not gonna allow you use any of the ice a
146:29 Systems we have discussed in these three . We've been together. Um You
146:35 use more than one. You have you gotta $1 million dollar budget.
146:40 worry about that. Um How many systems. How many different rocks?
146:46 many what minerals in what rocks give a plan? I'm gonna expect more
146:52 one thing to be done here. lead. Okay, uranium leads or
147:06 on the rye like. Okay, good. That'll tell us the age
147:09 the highlight. So that means the has to be older still. Fair
147:16 . That's a start. Can we anything else? Yes. You don't
147:25 inventory. Um, uranium thorium. you say? Say it again
147:31 Yeah. Say it again mandatory. And the helium uranium thorium helium dating
147:42 what minerals in what rock? Well, um if you dated the
147:55 light by uranium lead Zircon, you tell the deposition all age of
147:59 Rye light. As good as you buy dating the uranium helium on the
148:04 rock. You would certainly, you almost certainly get either the same age
148:08 a younger age. Well, I the helium has a lower closure
148:15 So you're either going to get the age because this is simple. Relight
148:19 cooled rapidly or you're gonna get a age for the helium than you got
148:24 the lead in the in in the of learning about the deposition? All
148:29 of unit B. That won't help much because we've already got the the
148:35 oldest age we can get from the light from the uh uranium lead.
148:41 , uranium lead the uranium helium Well, tell us something about that
148:46 light. But all it will tell is if that real light has been
148:50 since then it won't help us understand age of the sandstone. Because all
148:55 we already know that the sandstone is than the age of the uranium
148:59 Zircon in the reelect. So, not sure. I would recommend
149:04 I mean, unless we are, that's part of a much larger geologic
149:09 , it's not going to tell it's not going to tell us when
149:11 sandstone was deposited if we already have the data which was already suggested.
149:18 , I'm gonna say probably not. is there anything we can do with
149:23 of these other rocks? The tribal on the sandstone? He says,
149:30 me more. The tribal dating of ? By which method? Which minerals
149:48 the sandstone? The quartz. you got to say which mineral,
149:53 can't say the tribal dating is the part then you have to say which
149:57 then you have to say on which ? That's okay. Yeah. Now
150:03 now you're thinking should I say zircon time. And the the answer is
150:07 , you should probably say zircon every . Okay, we got to know
150:11 all these answers, but Zircon is the best thing to choose.
150:16 yeah, okay, now, I there are other. So so zircon
150:21 the sandstone, uranium leads? You uranium lead. Okay. Zircon on
150:26 sandstone? Fine. What's that gonna us? And by and by how
150:31 zircons you wanna date? 117, . Okay, if you had a
150:35 budget, you might go for you're gonna date up, you're gonna
150:39 dozens of them. Alright. And of those grains is going to be
150:43 important to you? Which of those is going to be most important to
150:50 ? The youngest one. Very So when we get the youngest
150:53 we will know that the sandstone is than the youngest one. So it's
150:58 so we're going to be young. sandstone is going to be younger than
151:01 youngest Zircon in the sandstone, it's to be older than the zircons in
151:06 Royal. I so we're now we're it down. Okay. Is there
151:12 we can do with the shale? is something but we just talked about
151:20 very briefly last friday, Yeah, talked about rainy um osmium dating that
151:27 an ice a topic technique that can shales directly and that might work in
151:33 case you'd have, that might be on how good the detritus dating of
151:38 sandstone is If you date those zircons uranium lead, you know, you
151:41 get lucky and get the same age you got for the real light in
151:45 you've you've narrowed it? You've got figured. But you know, let's
151:49 say for the rye light, we an age of 100 million. And
151:54 if the youngest zircon in the sandstone a billion years old? That didn't
152:01 at all, did it? So now we know that the sandstone
152:07 between 100 and a billion years Is there anything else? Is there
152:13 else we can do? We can the uranium osmium. Fair enough.
152:18 say the radium osmium doesn't work anything we can do. We're focused on
152:27 method. We can learn about the of deposition of unit B Any method
152:32 all. But I've told you that dated the sandstone zircons and we didn't
152:36 anything younger than a billion. Which us that we've got some pre Cambrian
152:41 out there, which tells us something the paleo geography at the time.
152:46 it doesn't help us figure out the of deposition so far. All we've
152:50 down the highlights 100 million. And sandstone is sourcing an area from billion
152:55 old rocks. The shale. We the Renea mausoleum. It didn't
153:02 What would the OK. And how you date the granite uranium would
153:11 Okay. Um And what would that you then? Suppose? We got
153:16 I got an age on that granite 400 million. What does that tell
153:21 about? The agent sandstone? This is 400 right. Lights 100.
153:35 granite is 400. So now, do we know about the Sandstone?
153:42 sorry. Say again. It will between the 100 and sweet. Very
153:46 . Can we do a better, we can we narrow it down and
153:49 more? Yes, but let's say we've we've tried to get a nice
153:58 age on the shale and we keep . It's not working. Is there
154:01 else we can do? We've got uranium lead age on the granite.
154:05 got some uranium lead age on the . We've got uranium lead age on
154:09 sand stones and granite. Excuse We have a uranium lead on the
154:14 in the sandstone. But they weren't . They were all too old.
154:18 there anything else we can do to granite? What we got from the
154:25 was the crystallization age? Right. we know that this granite at some
154:30 was at the surface of the earth it has a shale on top of
154:34 ? Right. That's a deposition contact I've drawn it. That's what I
154:38 it to be. When did that get to the surface of the
154:48 Was it the same time that the in that granite crystallized? That was
154:54 long time before the zircons crystallized down . Right. Probably Appetite Which not
155:08 appetites. Appetites don't have potassium in . Iranian lead in appetite could be
155:17 thinking of maybe trying something like argon the feldspar. Mm Because that will
155:26 us an age. That is when when the granite was coming towards the
155:32 . But it's not such a low temperature that you see this granite has
155:36 subsequently been buried by a shale sandstone O'Reilly, we don't know exactly what
155:40 thickness of these things are and or much other things were deposited on
155:44 But we know this granite has has been intruded, it was uplifted brought
155:50 the surface, the shale was deposited top of it. Then some other
155:54 were deposited on top of it. I don't want to go to the
155:57 lowest closure temperature. Do I let think if we went to if we
156:03 this thing by appetite fishing track? , if we got an appetite fishing
156:10 age that was older than 100 that would be very helpful because if
156:15 older than 100 million, that means it didn't get reset by the rye
156:20 deposition. If it's younger than 100 , then then there was a lot
156:25 burial up here. But but I'm saying that in order to figure out
156:30 age of the sandstone, we want temperature that brings the temp the granite
156:34 to the sandstone uranium lead. Zircon great. It's unambiguous, but it's
156:40 a long way away from the temperature which Sandstone was being deposited. We
156:45 we could get an idea of when granite was at the surface, when
156:48 shale was being deposited. So what I did a 4039 age on a
156:53 that gave an age of 140 million ? Even though that granite is not
157:00 contact with sandstone, it's strata graphically the same basic story here. So
157:06 the granite cooled to 200 degrees 140 years ago and this rye light was
157:13 at the surface 100 million years Now we've narrowed the whole thing down
157:17 100 and 40 and 100 and That's better than the narrowing down that
157:23 were describing between 400 and 100. that's how I would go through the
157:30 here. You know the plan because I mean, our goal here is
157:34 , we need another deposition all age unity, but unit B and units
157:40 difficult to date directly. We can dating to tribal minerals. We can't
157:45 we if we date the D triple and they come up unhelpful because they're
157:49 super old that they didn't tell us . We didn't already know, then
157:52 try and date the other rocks nearby maybe we'll come into something. I
157:57 this next this next story really is kind of version of that. We've
158:05 unit A B. D and I don't know why we don't have
158:09 unit. Oh yeah. So assume we have, I want you to
158:15 with this problem for a few Look at these things. Unit A
158:21 top of unit A sits unit On top of unit D. On
158:26 of BSD on top of Dsc. are this is strata graphic column.
158:31 might go find someplace, granite, , sandstone, Riley in there.
158:38 have these various values of data. want you to spend the next 55
158:46 minimum. I'm just gonna go down hall and get a drink of water
158:49 just take a break. I want guys to look at these data carefully
158:54 think about the geologic complications that come if we've got these data and this
159:00 graffiti, I want you to interpret whole history of this sequence. Considering
159:05 it means to be a conglomerate and and a wry light. Notice that
159:09 conglomerate has grenade class in it. important. Uh So remember the closure
159:18 . Think about how you can make sequence go. I'm gonna give you
159:24 at least five minutes to sit and about it. Make a little,
159:27 some notes. Think about a Uh what this means. We've we've
159:32 from, we've gone from this example we didn't know any of this
159:36 What should we do? This is example of, we went and did
159:40 and got these data. Now I you to interpret them. So take
159:46 few minutes with that. Yes. the first. Thank you. Thank
165:11 . Okay the each of the unity some alterations come back to B and
165:52 . It should be in between of B and unity. Um You mean
165:59 you? No we're just gonna I know why the letters are like
166:03 I mean are you saying where what their unit C. Is there?
166:06 , no. I'm saying that the day age is 90. That's what
166:10 showing. Right. It should be 3 58 and wait but wait a
166:15 . That's the uranium helium age on appetite. What's the closure temperature of
166:20 system? That's I'm glad you noticed because that's an important thing here.
166:33 it's it's if you're saying that there's mistake there is not that's real.
166:38 can be that way. Because notice you're comparing different systems now, you
166:44 it should you know why is why nine You you thought that that unit
166:48 should be somewhere between 3 58 and 60. Those aren't the same.
166:54 got uranium lead zircon uranium healing appetite uranium helium zircon so those don't have
167:01 follow a standard 123 order because they're different. Does that help?
167:09 think about that mm. So you to go through it now. You
167:59 to start what can you tell Yeah that's what I mean. Clearly
168:10 are not quite the right way to it because you've got closure temperature issues
168:16 . It's young. You know the helium age of appetite of D.
168:20 younger but younger than the uranium lead of zircon. Let's start with you
168:28 . What can we tell about And B. Let's worry about dND
168:31 . What can we say about And B. What's the closure temperature
168:38 Latins of lead in spain? four or 500. That's fine.
168:45 the closure temperature of Argon and Horn ? Maybe 500. Yeah.
168:53 So what does that tell us about granite? Right, well not 500
169:04 . But I mean what we can is that this rock was that this
169:07 was at 500 degrees about 5 450 460 million years ago. Without any
169:15 information. You want to call that crystallization time. Okay, that's
169:20 So the granite started At 4 Let's say what is the feldspar?
169:29 us 200 - 3 80. It down to maybe 100 and 50 degrees
169:37 3 80. Right? So pretty to the surface. We know it
169:47 to the surface because there's a conglomerate top of it. And the class
169:52 that conglomerate have some uranium helium zircon . And their 3 60. Does
170:03 tell you where the granite class came ? It came from A.
170:09 it came from you today because that's probably exactly what you'd expect the helium
170:14 ages to be from you today? didn't if we were to actually do
170:17 helium zircon on unit A We'd probably something a little bit younger than 3-3
170:24 because the closure temperature of feldspar goes to about 201 50. The closure
170:31 of helium in Zircon Goes from like 80 - 100, something like
170:38 So that makes perfect sense that that was derived from the underlying granite.
170:44 . And so we've had erosion and that that granite was crystallized at 4
170:50 . It was brought to the surface 3 60 A little bit after
170:57 Right, What is the rest? now let's look at the rest of
171:03 story. What does unity tell Yeah, but I mean, You
171:15 move beyond the 700 and 800° business you're told what kind of rock it
171:20 . When did go ahead? I'm saying the driver volcanic rocks,
171:28 rocks. Absolutely. So we know this volcanic rock was deposited at the
171:35 3 58. Right? We know the granted in the Columbia glamorous were
171:40 to the surface just before that, somewhere around 360. So we've
171:48 we've got a granite, a conglomerate and highlight the conglomerate was at the
171:54 at 3 60. The Rye light at the surface at 3 50.
171:59 can we 3 58. Excuse How can we explain that Sandstone with
172:03 uranium helium appetite age of 90-90 million . That's the only thing that's yet
172:14 be explained. And you've both noticed , but I think you're still both
172:18 little puzzled by it. Or do have an answer now, burial?
172:31 . How much burial win? What's closure temperature of helium and appetite?
172:54 70 200. Yeah. Call call it 75. So how much
173:03 and when do you feel on the or something? I didn't hear that
173:15 part, 3 km3 km. let's let's not say it in terms
173:23 temperature. So we don't have to about the geothermal gradient. Just how
173:26 was it buried 75. Above Let's say we could have been above
173:35 . Yeah, well, we know was more than 70 but less than
173:40 than 100 and 50 maybe. So probably buried you around 100°. And when
173:48 that take place? Well, we that it cooled below 100 degrees 90
173:55 years ago. The burial, the burial was going to be at some
173:58 before 90, it cooled down below by 90. So sometime between 3:58
174:09 90. This entire package was You know, you've got a granite
174:15 came up and then was covered And then this whole thing was covered
174:19 and brought down to temperatures in excess 75° before 90 million years ago.
174:26 90 million years. But it was down to temperatures in excess of
174:31 but probably not much more than And then that whole thing was cooled
174:38 below 70 after 90. So from information, we can just, if
174:44 just look at the granite, we say that it went, it was
174:48 , it went up, it went , it went up again. We
174:51 we can, you know, if wanted to draw a history of that
174:55 , you know, you could draw , it would go up down up
174:59 then the other rocks would would move it depending on whether they existed yet
175:04 not. This is a good a Exercise that. # 13, I
175:14 it. Um Here's okay. We take a break. Yeah, go
175:25 . We'll take a quick break. , well, well, he's in
175:29 restroom um Madonna, you may wish Consider # 14. It's a similar
175:44 . So we'll do 14 in just few minutes. But it's but keeping
175:52 this 14 and 13 are are similar . We're going to look at the
175:57 and downs of of this based on data we have. So look at
176:01 various closure temperatures and the strategic see if we can sort that
176:08 Mhm. Oh that last one I . So it'll uh I'm not clear
178:15 we interpret that? We come up an interpretation of what that very
178:20 Well, no, you need a need a geologic map and some other
178:24 . All we can say from the on that slide is that it got
178:29 . I mean, obviously if you these were real samples, we'd have
178:33 have we'd have structure and strategic graffiti the whole, you know, who
178:36 this? Are these rocks? Where these rocks? You know, these
178:40 from florida or from kevin checker. mean, obviously you don't know
178:44 you know, you don't have a map. You don't have obviously that
178:48 be additional information. But but without doubt, we could say that these
178:52 were this granite crystallized at 450 came to the surface by 360 was back
178:58 by 90 was back up. Since . That much, we can say
179:03 a geologic map without a tectonic We can't say why, but we
179:09 simply say clearly it happened. We're now, we're working on the how
179:13 and when the next step is to to the, you know, to
179:18 this in the context of the broad setting the tectonic setting, the structural
179:25 and you know, but all of then has to be has to be
179:29 to the what and when, how and when that these data give us
179:34 datas tell us how hot and when the job is to interpret that in
179:39 of, you know, I'm a chronology ist. I work on the
179:42 , how hot and wind part. not a great structural geologist. So
179:47 I have to figure out the I go talk to my buddy mike
179:50 he and I work it out together I can explain to him well now
179:53 means the rock was at this hot hot this time. And he
179:57 well, okay then we gotta put fold here or a fault here or
180:00 we do. But no, you you can't figure out. Don't don't
180:05 bad that you haven't figured out why don't have any information. The next
180:09 is to work it out in terms the broad geologic context, the
180:14 the structure, that stuff. If were working on a real problem,
180:19 would know that stuff. So let's this number 14 question. We've got
180:27 shift As our basement here. It's uranium leads Orton age of 24 50
180:36 39 Muscovite of 4 60 40 bio tied at 455. And the
180:43 helium zircon of 300 sitting on top that. She just, It's a
180:51 with the uranium lead zircons that range from 325, 24 50. And
181:00 top of that we have a real with 4039 of feldspar at 250.
181:09 what can we say about how this about? First of all, we
181:18 say it's a shift. So that it's a metamorphosed rocket came from something
181:24 . Let's say those were shales. really can't shells or sand stones.
181:31 we say anything about those shells or stones that were metamorphosed to become the
181:43 ? We can. There's one thing bit of information we have about them
181:51 from here. This is a metamorphic , our basement rock in this
181:58 But we know that the the pro lift of the shift had really old
182:05 in it. Right. We had that were as old as 24:50.
182:12 , the shift contains rocks that are are key in late early protozoa.
182:20 all we can say about the total of the shift it contains this oldest
182:26 . But then what then then, else can we say about the shift
182:29 metamorphic rock? When was it Excuse me. 300. Um Why
182:50 ? Why do you say 300? , but what's the closure temperature of
182:58 helium zircon? It's quite variable, it's probably 200° and lower.
183:11 That's not the temperature at which rocks metamorphoses it? It's hotter than
183:17 So, we've got muscovite and bio those are those sound like kind of
183:21 minerals to me in a shift. . So I'd go with the metamorphosis
183:27 being in that sort of 460 Because that's when the bio types and
183:32 cooled off, the metamorphic ISM is than 4 60. Right? How
183:39 older? We don't know. It be somewhere between 4 60 24
183:46 We've got his chest, that's got in it And those Mike has cooled
183:51 350. Around 450. Oops. mean to do that. Um Are
184:21 following this at home Madonna, Did get that? Yes, that's
184:29 That's good. So, I'm gonna you to consider I'm gonna go through
184:33 again, the shift As a metamorphic . What what was metamorphosed some sort
184:40 sedimentary rocks, sand stones or those had material in them that were
184:47 as old as 2450. When was metamorphosed sometime between that Prada with age
184:56 these biker ages of 4 50 4 60. What is the uranium helium
185:03 tell us when the rock cooled down 200° And that's it. That's after
185:10 around 300. We have a sandstone sits on top of that. That
185:16 zircons in it. So you have lead ages that go somewhere between 3
185:21 And 24 50. What does that us about the age of the
185:40 Younger than 3 25. Now. fact, we already knew that because
185:46 uranium helium zircon in the schist is . So the sandstone has to be
185:52 than 300. Right? To the didn't tell us anything. The zircons
185:59 the sandstone didn't tell us anything that shift didn't already tell us. The
186:05 tells us that the Well, probably. Yeah. Yeah. The
186:12 is telling us that the that that was near the surface by 300.
186:18 so the sandstone, Well, you , the Sandstone is younger, the
186:22 is younger than 300. Um And now, so what then the
186:29 light sits on top of that? get a 4039 age of for the
186:35 of 250. So is this a , do we have a strong constraint
186:44 the age of the sandstone? Or ? Just tell me what is the
186:48 range in which we know the sandstone it could have been deposited in 2
186:56 - 300. That's correct. That's we got. If we had a
187:01 , that would help, that'd be . But without a fossil. And
187:05 dated all those zircons in the sandstone they didn't help Because we already had
187:10 300 from the underlying basement. So is old. And and notice that
187:18 sandstone has ages from 3 25 to 50 that 24 fifties the same age
187:23 the shift. Right. So some those zircons probably came from the underlying
187:27 , but just when Exactly, it's to say we know that the helium
187:33 age says that that shift came close the surface by 300. Doesn't guarantee
187:38 it got all the way to the . Uh it must have gotten to
187:42 surface before 250. That's all we say. Alright, let's see what
187:51 we got. We're running a little on time, but not terrible.
187:55 our next? I know I've got 20 of these 15 We're going
188:01 we did 13, 14, interpret the history of this sequence unit
188:09 is a granite Circon Age of By a tight age of 1400.
188:19 Fishing Track Age of 500. Then have a Triassic sandstone sits on top
188:26 that. And on top of we have a cretaceous sandstone With appetite
188:34 ages of 40 million. All tell me what happened. Sure.
189:27 , clearly the granite is old. . What is that? What what
189:34 the appetite fishing track age? Tell about the granite. That after the
189:41 age it got hotter, got Well, what's okay, certain what
189:46 age like the fishing temperature gives the , whether it got hotter and the
189:55 will disappear. Well, okay, mean, but um We've got this
190:01 . We tell the granite 1750, when it crystallized. Yeah, and
190:05 1400. That's when it got you , it was still hot 1400 million
190:10 ago. It was at a temperature say 300°. Yes. And then by
190:16 million years ago, we know, that it was at what temperature?
190:19 temperatures? Appetite fishing track? 100 maybe so far in the Cambrian.
190:27 rock was 100°. What temperature was this in the Triassic? More than
190:50 Well, we've got a sandstone deposited Triassic right, deposited on top of
190:55 granite. That means the granite was the surface in the Triassic, The
191:03 is at 100°. I don't want to in that surface Purpose would be 20°.
191:14 between 500 degree 500 million years ago it was at 100 and the Triassic
191:20 was I mean, we know that rock was never hotter Than 100° since
191:30 million. Yes. So The granite near the surface 500 million years
191:40 And then then what happened by the ? It probably didn't it didn't go
191:47 , right, because if it got , we know that this rock has
191:52 been hotter than 100 degrees since It made it to the surface in
191:58 Triassic temperature of 10° or 20°. And on top of that there's this
192:05 a much younger Sandstone that has appetite ages of 40°. What's the closure temperature
192:11 appetite helium About 75°. Right? So there's a cretaceous Sandstone, 40 million
192:22 old. Appetite healing ages. What that tell us? It was buried
192:31 to what temperature? Maybe not that , maybe only 80. In
192:38 we know it's not 100° we know not 100 degrees because if if the
192:45 sandstone got up to 100 degrees, temperature is the granite below? It
192:49 be at even harder? And we it never and we know that rock
192:55 got above 100° because it has an fishing track age of 500 million
193:02 75, And given this information, thick would you, would you predict
193:11 Triassic sandstone to be, Would you it would be thick or thin?
193:22 gonna give you two choices. Three of five m 100 m 1000 m
193:38 m. Because the difference between both 25 cents ius I'm thinking it should
193:44 around one kilometer thick, but if was if you're thinking the sandstone should
193:51 1000 m thick. This, the rock unit b you're saying that's 1000
194:00 , I think it has to be . Because if the if the appetite
194:04 age in unit C is 40 million , that means that it got heated
194:09 after the cretaceous to a temperature in of 70 degrees. If if the
194:16 sandstone is 1000 m, that means the granite is going to be another
194:22 m below that, and it would easy for that granite to have been
194:28 to a temperature above 1000 degree above degrees. But if the if the
194:34 Sandstone is nice and thin, five then we can reheat the the helium
194:41 the appetite, in the cretaceous sandstone changing the fission tracks in the appetite
194:50 the pre Cambrian granite. Which because if the pre Cambrian granite is only
194:55 m beneath the cretaceous sandstone, that's , basically in the same place,
195:02 if it's 1000 m below, then going to be at different temperatures.
195:05 granite is going to be hotter. if the if the caucasus sample got
195:09 enough to read to move helium around an appetite, that's going to be
195:14 enough in the one kilometer below to changes in the fishing tracks and these
195:20 tracks are very old there 500. I would predict that the sandstone have
195:26 be quite ship, quite thin, you'd start to see big differences in
195:31 ages or similarities in those ages, we see differences. Does that make
195:39 to you? Why the 1000 m not the right answer. Okay,
195:43 got that good. I would say . Um Okay, what else we
195:55 ? This looks similar. What's the here, granite with a zircon age
195:59 3 50 by tight age of 3 appetite. Fishing track of 200 we've
196:06 a Triassic sandstone again, we've got cretaceous sandstone again. But here we
196:12 appetite helium ages from 200 to So this is the same units.
196:20 now we give them different ages. are we going to change our
196:36 Oh so the zircon uranium lead zircon of the granite tells us that it's
196:48 pre Cambrian anymore. It's uh it's . So it's a younger. And
196:55 the fact that the uranium lead zircon the and the Argon 40 39 bio
197:01 are the same. What does that us? The youngest? Welcome.
197:12 you were you were talking about granite . So we're not we're assuming all
197:18 zircons are the same. This is a not a not a not a
197:21 where we have a spread of Although zircons are 350 And the by
197:27 is also 350. They have different temperatures. How did they get to
197:32 the same. The uranium lead zircon 3 50. The argon 40 39
197:42 tide is also 3 50. Yeah have different closure temperatures. Yes.
197:46 . Yeah we get the same It's not a highlight. So we
197:52 have the simple interpretation of everything cools the same afternoon. But this goes
197:58 to what we talked about a lot saturday is about depth of intrusion.
198:03 . What does this tell us about this granite was intruded deep or shallow
198:09 to be shallow has to mean that closure the country rocks were at a
198:14 less than 300. How much less don't know, but we know that
198:19 appetite fishing trek ages are 200. by 200 this rock was at a
198:25 of about 100 degrees 200 million years . What what time, what time
198:35 is that? So, uh Early Mesozoic 200 200. Is that
198:56 that Jurassic or Triassic? I can't . It's uh I think it's
199:06 So we've got 200 million year fishing age and a Triassic sandstone. What
199:13 that tell us? That tells us granite was was uplifted to the surface
199:19 then to cover it up pretty This this Triassic sandstone is covering up
199:25 fission tracks in the granite. And there's a cretaceous sandstone on top of
199:33 . And the appetite helium ages in thing have a broad range from 200
199:37 100. When you have a broad , that's more indication of provenance than
199:47 . Right. If you were to a bunch of appetites, we would
199:52 down to Galveston today and gather up appetites from the sand. Would they
199:56 be the same age? Because they coming from different places? Right.
200:01 be different ages. But if we that sand in the Galveston's down to
200:07 They'd all end up being the same , whatever the burial time wants.
200:11 this range in ages. 200 - . What does that tell us about
200:16 depth of burial maximum depth of burial that cretaceous sandstone. How hot does
200:38 get to need to? How hot you need to get a sandstone to
200:41 the helium in the appetites? that's helium. You're talking about helium
200:49 be about 70. Remember we just that in the last example. So
200:53 tells us that this cretaceous Sandstone has been buried past 70° because we have
200:59 big range. All older. excuse me, we have No,
201:04 well, we have a range. this probably means this cretaceous sandstone was
201:10 after 100 million years ago And that 200 million is telling us about the
201:16 . This granite, this is where helium is coming from in this
201:20 Right, so is this a good to search for oil? Why?
201:29 should be the temperature will be around . But wait a second. Which
201:38 ? Which rocks had gotten to All of these once are just some
201:42 them, some of them which what the maximum temperature that this cretaceous sandstone
201:57 ? The helium appetites ranged from 200 100. With a big range like
202:05 . That suggests to me they have been reheated. If they were
202:10 they would look more like that with narrow range of that red. That
202:17 curve tells us it's a narrow range been reheated maybe. But this has
202:21 wide range. 200 - 100. don't think they've been reheated. They
202:29 been reheated, they haven't been heated a temperature of above 70°. Cretaceous sandstone
202:36 been to 70. Of course, rocks beneath it are going to be
202:39 because they're beneath it and we don't how thick the cretaceous. Let's say
202:42 , The Triassic sandstone is 100 m . That's a thick sandstone, But
202:49 still only 100 m. Is this good place to look for oil?
202:59 would say. No, it does look like the it doesn't look like
203:04 fishing track ages on the granite are same ages as some of the of
203:08 sandstone in here. This whole region look like it's been in buried since
203:15 cretaceous And hasn't been buried to temperatures 100° certainly not above certain. Maybe
203:22 even above 70°. So we're just barely at all we may not we not
203:29 be in the oil window at all . Is that you follow that?
203:36 entirely Go back to this range of 200-100. If we if we if
203:46 is the easiest. This is the closure temperature system. We've talked about
203:51 and alien. It's really easy to rid of the helium and if you
203:56 rid of the helium in a you would expect all the ages to
204:00 up being about the same. This a large rate. I mean,
204:05 are assuming simplistically, it's actually it's a little more complicated than this,
204:09 we are assuming simplistically that all all the appetites have the same closure
204:14 . That's fine. Let's just do . If they do then the range
204:20 make sense for reheating. This range is telling us about provenance, not
204:26 post deposition, all thermal history. , we can learn about one or
204:31 other, but we don't learn about at the same time. And
204:33 you know, it all depends on deeply the rock has been buried.
204:37 which bit of information we're getting here we have a range. I'd say
204:42 wasn't very deeply, Not even to that seems a little bit cold to
204:48 start making oil? Certainly too If you start making gas 17,
205:01 need to know the deposition all age the arcos. What do you do
205:07 time is critical? I'm in a hurry. There's a there's an un
205:14 . There's a basalt below, there's arcos. There's an un conformity,
205:19 a wry light above and above that a sandstone with tyrannosaurus rex. Did
205:25 know that tyrannosaurus rex is an index . Transfer respects is only found in
205:32 district and part of the cretaceous. very last part of the cretaceous.
205:36 didn't learn that until recently, but you see tyrannosaurus rex here in the
205:41 restriction is from 71 to 66. the whole thing is older than 71
205:47 of the T rex. Now I'm give you two questions. One is
205:53 to do if time and money is no concern and what to do if
205:57 in a big big hurry, the and money is no concern we're gonna
206:01 , we're gonna do everything okay, what's the first thing we need to
206:05 if we're really quite in a big to know what the arco stages,
206:10 is, what's in Arcos? Arcos a kind of sandstone, a feldspar
206:15 sandstone. No, you can do the corn. Yeah. Yeah I
206:27 think that's what I do the that's put the that'll put a age
206:32 It must be younger than that of because there's an un conformity there,
206:37 could be a big time missing We don't know. But yeah,
206:40 think I would agree that if I had if you can you can get
206:44 done quickly. Um one quick analysis will tell you that right that our
206:49 is older than that. Okay. now that's a quick thing. What
206:57 you're gonna do your whole PhD on region and you really want to figure
207:00 exactly you've got a lot of money you've got a couple of years to
207:04 this out. What else are you do? Ryan uranium lead dirk on
207:09 the right like that's a good start we know that that highlight better.
207:13 us an age older than 66 because got tyrannosaurus rex up there. That
207:19 rex is a late cretaceous fossil. But now we're gonna go into more
207:26 what else we're gonna do because all learned so far is the is the
207:30 coast is older than the highlight. . How can we date vessels?
207:55 probably not a lot of bio tied the basalt. Maybe feldspar.
208:13 you could probably try and data feldspar the basil. Um The assaults I
208:17 you last weekend were mostly dated by the ground mass. Just not worrying
208:22 the minerals. Just maybe even dating whole rock because remember I said,
208:28 is the most potassium? And we're we're gonna try and date it by
208:32 methods. You could also maybe try do a rubidium, strontium, isotope
208:36 . Or you could try and you could try and find delight in
208:40 like that. Uh Remember zirconium the kind of thing where zirconium goes
208:47 there's no silica to go with But uh you might try to date
208:53 basalt by argon methods. Either basalt mass or maybe there's some flash
208:59 We know this rock is at least million years old. So that's
209:03 That's good, you know, we have to worry about being too
209:06 right? If we didn't have that rex there. You know we
209:09 You know this rock could be two years old. But we know the
209:12 are older than 65. So I would date. That would probably
209:16 my second thing would be to date assault by one by by maybe as
209:22 , you know, dated by argon either on the feldspar. If we
209:26 feldspar, you know, not all salts have big plastic places in
209:30 but that could be a thing or do the or just do the uh
209:34 ground mass. That might work. of course that would tell us that
209:38 basalt is older than the Arcos. else. Well, the problem is
209:59 our coast is a coarse grain sandstone made of made of minerals that came
210:03 somewhere else. How are we gonna when you say date the our
210:07 That makes it sound easy, but not What are we gonna date?
210:14 zircons. And that would again give another chance. Uh It might not
210:20 us a much. This is not as an un conformity between the coast
210:24 the basil. So we could presume there's not a lot of time difference
210:28 the two. So that's why I the dating the basalt is the second
210:34 . And and dating the tribal minerals the arcos is third choice because they're
210:39 to be older than the basalt. telling us anything in terms of
210:44 Now we always like to date the minerals because they tell us about paleo
210:50 . But I would say that you in terms of if your if your
210:53 job is to figure out when this , this our coast was deposited,
210:58 want to date the I mean, you've got you've got what is in
211:02 ways an ideal situation. You've got volcanic rock above and a volcanic rock
211:07 . The only problem is that there's nonconformity above the Arcos which puts an
211:11 time in there. But you there are many places in the world
211:16 we don't know the exact story. could be one of them. Um
211:25 we've done that. We're running Oh we did that already. Wait
211:30 second. Oh we've got different ones . We've got eight. Getting
211:36 17 and 18 are sandstone right? our coast basil since 19 is
211:43 We've just changed, okay, we it around a little bit, we're
211:45 interested in the Arcos, but now have a sandstone with trilobites beneath that
211:50 a shale beneath that is an our and then there's an un conformity and
211:55 there's a granite. We're still interested the archos. So it's trilobites.
212:03 that means, what time period are looking at? Did you talk about
212:15 with Dawn? Yeah. Do you when trilobites were found, said trilobites
212:26 paleozoic. They're mostly young. They're , most trilobites are like Cambrian or
212:33 , but they could be permanent. we know these rocks are older.
212:39 permian are older. So what if do is time is critical. We
212:45 to know something about this our coast away. What's the first thing we
212:48 do in which rock? Yeah, probably good. Iranian leads are not
213:08 the granite. The only problem is there's an un conformity there and the
213:12 formed at depths. So there's a time between the crystallization of the granite
213:17 the deposition of the archives. You want to try and narrow that time
213:21 by looking at say, argon 40 of the feldspar in the granite,
213:27 would that would be the temp. would be the time in which the
213:29 was closer to the surface. That really narrow it. You know,
213:33 that granite is you know, if trilobites are Cambrian, let's say This
213:39 this granite is our key in. could take this rock and get three
213:45 years, that wouldn't be very But if that same three billion year
213:49 rock had a felt car and that 500 million years, 600 million
213:56 Now we've got it somewhere between, know, 604 50. So I
214:02 know, I mean I might go 40 argon dating of the feldspar in
214:07 granite first of all. And then have the question what if time and
214:13 is of no concern, then I probably go to the uranium lead zircon
214:18 the granite because I mean, it still, you know, I
214:21 there's no guarantee the thing is a , you know, five billion years
214:28 . Um Then we could try the , the uranium osmium on the
214:33 you know, that works sometimes. we could try and do you know
214:39 tribal dating of the Arcos? Look the zircons in the Arcos, maybe
214:43 at the felts bars in the arcos see what their ages are. This
214:47 a, you know, not knowing about the geologic environment because this is
214:51 totally made up example. Um this this, this presents a challenge because
214:57 a big un there's a nonconformity. don't know how big there's a nonconformity
215:02 the sense the sedimentary rocks and the that's easily most easily dated. And
215:09 also have, we also know these are at least 250 million years
215:15 which means they've had a long time be buried. So, some of
215:19 low temperature information that we might get be complicated by post de positional
215:25 So, um, this is one the more challenging situations because I haven't
215:31 you very many rocks that are easy date and they're really old. So
215:37 just how that works. Um, not gonna do that one.
215:44 I guess we've gone through them. are those are those are my,
215:50 I'm good on on Wednesday, I'm ask you questions that are kind of
215:54 these, I may ask you some details, some specific questions about,
216:00 know, how do we figure out temperature of a mineral or what you
216:03 ? But I mean general, I'm ask you, I'm gonna, I'm
216:06 just gonna ask you, you multiple choice questions like what the closure
216:10 of argon in feldspar is a 300 400. So I'm just gonna
216:15 you know these things you've got, got about 12 different systems we talked
216:20 , you got, you got the , you got the argon, you
216:23 the fishing track, you got the , you've got minerals like zircon
216:27 horn blend, bio tight muscovite There's a bunch of closure temperatures and
216:36 I will ask you some questions like that provide you some data and then
216:43 your understanding of the closure temperatures and bit of understanding about geothermal gradients and
216:53 like that, Try and figure out history of the, the little,
217:00 little story that I've come together is exam going to be online? Not
217:06 potion, I thought that was question , is the exam gonna be online
217:18 in person? I don't really care does Don want, we did his
217:38 online. The bios right, first , Don wants to be it
217:43 No, we didn't have exam online it It is usually between 6-9 and
217:50 gets pretty late. So yeah, would vote for online because I have
217:58 travel late at that. Well. okay. I can give you um
218:13 if we're gonna do it online then gonna do it on blackboard.
218:18 I can give you test on blackboard and you just take it there.
218:29 what time will that be? 6:00 Wednesday? Okay. I will set
218:37 a test that that becomes available on at 6:00 on Wednesday. You'll have
218:42 certain number of minutes to take I don't know how many yet,
218:45 won't be three hours. Uh What may end up doing. You know
218:53 , you know, the downside of online test is that, you
218:56 you're not proctor, you've got the , you've got your book. Um
219:02 way to sort of determine whether you , you know, whether that whether
219:07 answers come largely from you or from ability to google things is to make
219:13 time period not very long, But mean I will I will understand that
219:21 given you only 15 minutes to answer question. You know, I won't
219:25 you a question that takes two hours figure out if you know the closure
219:28 of these things. None of these should take more than 10 minutes to
219:31 . If you know the thing, you don't if you don't know the
219:34 , there's no point in taking two , right? So um but I
219:40 the best way to do an online is to give you questions one at
219:43 time and no backsies, you finished question one and you know I'm
219:49 give you, I don't know five and each question you're gonna have say
219:54 minutes to answer and there will be what I'll do actually. I think
220:02 black boards not that sophisticated. I they can give I can you can
220:07 like a whole hour to do the but I can I can put it
220:11 that once you answer a question you go backwards um and I can also
220:16 them to you in random order so you won't be answering the same question
220:21 the same time. So there's no each other up and saying, you
220:25 what you get on number one, don't know, I haven't done number
220:27 yet. Leave me alone. So how I like to do the online
220:32 . You'll have you'll have a you a say an hour to do the
220:36 . Um you'll have four or five . Um You'll get them in random
220:42 and there'll be no returning once you you answer a question you can't go
220:48 , how does that sound? That's . Usually Howard did with dr down
220:54 that he asked us to be in zoom meeting, he showed us a
220:59 of like three questions per slide and used to email him the answer instantly
221:06 the time frame. That works Yeah. Okay. I think I'm
221:13 with doing it that way. So mean my way that we'll do it
221:17 blackboard and so you know, I've I've done blackboard tests for other classes
221:21 I'm familiar. It works okay. I guess that's what we'll do.
221:26 it'll just be on blackboard and so don't have to give me anything or
221:30 there. You're you're off the Um So that's what we'll do.
221:36 will put together a test. It take between. I mean I mean
221:41 probably give you at least 45 maybe an hour and a half depending
221:44 how many questions and how hard they . Um but you know, you
221:49 to study the closure temperatures and think , you know, think about,
221:54 through these examples we just talked about trying to make sure you understand why
221:59 means that we heating and this means means this or that. So that's
222:04 we'll do. Do you have any between now and Wednesday? Just send
222:08 an email And I guess if you have any more questions we can call
222:16 . It's almost 5:00. So we call it done for today any
222:25 Thank you. Well I mean you certainly know that. I mean I
222:37 expect you to know that every I the Triassic Jurassic boundary. I don't
222:44 that very well. But I know Mesozoic paleozoic boundary. So to you
222:49 , 5 45 to 45 65 those , that should be something you already
222:55 . Ah um Yeah, I mean would be nice if you knew that
223:03 eocene was around 50 million things like . I mean, I don't feel
223:11 I'm too much of an ogre to , you know, the geologic time
223:14 . You're in graduate school now. yeah, yeah, at least at
223:19 the order of approximate ages. I , again, I won't ask you
223:23 question that I don't know. I what is the what is the
223:27 The Devonian mississippian boundary. I don't , but I can tell you it's
223:32 around 33 54 100 something like that you know it well, you
223:39 you damn well better not think it's . Okay, 400 will probably get
223:45 there and if you know, and should know that, you know ESPN
223:48 after police. See that kind of . But yeah, it is
223:55 you know, just just as it's for, you know, when I
223:59 about the metamorphic rocks with students, know, I want them to know
224:03 the chemistry of the minerals, the of the rocks, the mineralogy of
224:07 rocks, they all go back and . So the numbers of the
224:11 The names of the ages that you . Yeah, you should know a
224:14 bit about that. And um that's I got any questions from from
224:26 You're good. Okay, you're All right. I've just got to
224:32 a test and have it ready by on Wednesday. I can do
224:39 Send me an email if you have questions. All right. That's the
224:42 of
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