Absolute Time. Part 8 Geochronology and the Time Scale

Transcription

1 Absolute Time Part 8 Geochronology and the Time Scale Unless otherwise noted the artwork and photographs in this slide show are original and by Burt Carter. Permission is granted to use them for non-commercial, non-profit educational purposes provided that credit is given for their origin. Permission is not granted for any commercial or for-profit use, including use at for-profit educational facilities. Other copyrighted material is used under the fair use clause of the copyright law of the United States.

2 What to Look For: Boundary ages for all the units on the chronostratigraphic scale can be estimated by bracketing between the ages of two datable igneous rocks. One of the rocks must be demonstrably older than the boundary. This is established by either superposition, inclusions, or crosscutting, or some combination of those principles. Superposition is usually the easiest to apply. The other must be demonstrably younger than the boundary, and again using some combination of the same three principles. Crosscutting relations is usually the easiest to use. The reported age for the boundary is the half-way point between the two igneous ages (their average) and the implied error is half the range between them. As new igneous dates become available they can change the estimated age and error if they provide a tighter bracket than before.

3 We now have three things: 1. A hierarchical chronostratigraphic scale that subdivides Geologic Time into packages ( units ) of various sizes. 2. A way to determine the numerical ages of rocks by analyzing the radioisotopes and their daughters that the rocks contain. 3. A big problem to solve: The chronostratigraphic scale is made using sedimentary rocks, but sedimentary rocks are, for all intents and purposes, not datable using radioisotopes. Conversely, igneous rocks are the preferred subjects for radiodating, but they have no fossils that let us assign them to the chronostratigraphic scale. How do we tie the two things together? Erathems Systems Series Stages pc PALEOZOIC MESOZOIC CENOZOIC Neogene Paleogene Cretaceous Jurassic Triassic Permian Carboniferous (= Penn./ Miss. Devonian Silurian Ordovician Cambrian Holocene Pleistocene Pliocene Miocene Oligocene Eocene Paleocene Mesozoic Series Omitted Paleozoic Series Omitted... Jackson Claiborne Wilcox etc.. Mesozoic Stages Omitted Paleozoic Stages Omitted

4 Two Reasons for What Follows: This is the logic behind virtually all the numerical ages on all the boundaries on the geologic time (geochronologic) scale, so you need to know it for this topic. Provides a detailed review of relative time as well, giving you practice applying principles from earlier in the term that are likely to appear on the final.

5 SUPERPOSITION YOUNGER If you see two superposed packages of rocks in the field you should be able to tell that one is older and the other younger. That is, not only do you know the relative ages, you know they are not the same age. OLDER This works within individual beds in a Formation, with Formations in a group, with biozones, or with Systems, Series, and Stages. This principle (superposition) is embedded in the geochronologic scale by the order of names arranged vertically up the page.

6 Just for practice, before you move on, see if you can figure out which Erathems are in the section.??? ERATHEM??? ERATHEM

7 FOSSIL SUCCESSION MESOZOIC ERATHEM Furthermore, by studying the fossils you find in the two packages you can now recognize any difference in age between them. Actually, the age you recognize is still only an inference. You are so far only able to recognize their chronostratigraphic membership. PALEOZOIC ERATHEM We have been assuming (or hypothesizing) that age underlies chronostratigraphic membership. We are now in the middle of testing that hypothesis.

8 Though you didn t actually learn enough about fossils to do it yourself, you should be aware of the possibility of narrowing the chronostratigraphic membership to smaller and smaller categories. In the first slide we assigned the fossils (trilobites and ammonites) to the correct Erathem of rocks. ERATHEM SYSTEM SERIES STAGE ZONE TRIASSIC If we identified the fossils more precisely and knew their ranges well we could assign them to Systems instead of Erathems, as the Diagram shows. (The two fossils on the previous page are not Permian and Triassic, they are Cambrian and Cretaceous. This is a hypothetical example.) PERMIAN

9 ERATHEM SYSTEM SERIES STAGE ZONE Even more familiarity with fossils allows even more precise determination of chronostratigraphic membership. LOWER TRIASSIC In this case the fossils are assigned to Series rather than Systems as on the previous page. UPPER PERMIAN

10 ERATHEM SYSTEM SERIES STAGE ZONE and so on. Here we are familiar enough with the fossils to know they indicate Stage membership rather than Series membership as on the previous slide. INDUAN We could go one step further and break our age determination down to biostratigraphic zones, but I don t know any zones of this part of the scale, so we will stop here. ( ) ERATHEM SYSTEM SERIES STAGE ZONE GUADALUPIAN

11 You also know about unconformities and some ways to recognize them. Angular unconformities are easy because of the difference in orientation above and below them. Nonconformities are easy because of the igneous or metamorphic rocks beneath them. Disconformities are tougher, but we did talk about (and you examined in a lab exercise) some ways of identifying them. If there is a clear erosional surface at the contact then there is clearly a disconformity. In lab you saw another way. If one or more biostratigraphic or chronostratigraphic units are not present then there must be a disconformity. In this example the Guadalupian Stage is missing, therefore the contact is a disconformity. INDUAN (TRIASSIC) LEONARDIAN (PERMIAN) (GUADALUPIAN)

12 Why is this important? INDUAN (TRIASSIC) Because the numerical age of this unconformity is approximately the same as the numerical age of the PZ/MZ boundary! Make very sure you see why before proceeding. The unconformity is above (younger) than known Permian rocks and below (older) than known Triassic rocks. LEONARDIAN (PERMIAN)

13 Let s put this into a more familiar format. INDUAN (TRIASSIC) Early on you learned to outline the history of a place by placing the events you see there in the correct order and numbering them, starting from the bottom. The cross section at left records three events, and we can do a little better than put then in order. We can place them within and age chronostratigraphic framework. Here is the correct sequence of events: LEONARDIAN (PERMIAN) This sentence will stay in green because this is the target we are trying to date. 3. Deposition of Induan rocks very early in the Triassic, perhaps right at the start. 2. Uplift and erosion either very late in the Permian or very early in the Triassic, probably the former. This produced the disconformity corresponding approximately to the PZ/MZ boundary. 1. Deposition of the Leonardian rocks late in the Permian, but not too close to the end of it.

14 INDUAN (TRIASSIC) Without further information we cannot add any ages to this scheme because there s probably nothing to date radiometrically. So far all we ve seen are sedimentary rocks. Any 14 C or U/Pb intermediates are long gone from Permian and Triassic rocks, so that s no help, and even if we did find some glauconite the chance of getting a reliable date out of it is pretty small. LEONARDIAN (PERMIAN) But suppose that as we explored further, or studied the sediments more carefully we began to find some igneous rocks. Suppose more mapping turns up a section with a volcanic ash bed right near the top of the Permian rocks. What does this do for us?

15 INDUAN (TRIASSIC) Now we can add a step to our history and that step is based on a rock that we CAN potentially get a numerical date from an igneous rock. That rock is clearly older than the PZ/MZ boundary, giving us a maximum age for the boundary. Here is the sequence now: LEONARDIAN (PERMIAN) 5. Deposition of Induan rocks very early in the Triassic, perhaps right at the start. 4. Uplift and erosion either very late in the Permian or very early in the Triassic, probably the former. This produced the disconformity corresponding approximately to the PZ/MZ boundary. 3. Deposition of the remaining Leonardian. 2. olcanic eruption and deposition of ash bed. 1. Deposition of some of the Leonardian rocks.

16 From what you ve already memorized you should already have an idea of the age of this ash. It must be older than the PZ/MZ boundary, which you have memorized at ~245 my. We send the ash to a lab for age analysis and get back a result of 250 my +/- 1.8 my. 1.8 is less than 2% of 250, so we accept this age for the ash. From the sequence we have determined this means that the Paleozoic ended and the Mesozoic started less than 250 my ago! 5. Deposition of Induan rocks very early in the Triassic, perhaps right at the start. 4. Uplift and erosion either very late in the Permian or very early in the Triassic, probably the former. This produced the disconformity corresponding approximately to the PZ/MZ boundary. 3. Deposition of the remaining Leonardian. 2. olcanic eruption and deposition of ash bed approximately 250 my ago! 1. Deposition of some of the Leonardian rocks.

17 Let s look briefly at another possibility. INDUAN (TRIASSIC) Suppose the 250 my igneous rock we find by further mapping is not an ash bed or flow, but an intrusion. This is an even simpler scenario, but the logical result is the same: the boundary we care about is younger than 250 my. Remember that there are actually three ways we can tell this: LEONARDIAN (PERMIAN) 1. The Induan rocks overlie it at point A. (Superposition the unconformity is a nonconformity right there.) 2. The intrusion cuts the Permian but not the Triassic rocks. (Cross-cutting) 3. There are eroded pieces of the intrusion in the Triassic. (Inclusions)

18 Here s a summary of the history in this diagram: 4. Deposition of Induan rocks very early in the Triassic, perhaps right at the start. 3. Uplift and erosion either very late in the Permian or very early in the Triassic, probably the former. This produced the disconformity corresponding approximately to the PZ/MZ boundary. INDUAN (TRIASSIC) 2. Intrusion of an igneous dike approximately 250 my ago! 1. Deposition of the Leonardian rocks. Notice that the conclusion stands: the PZ/MZ boundary is less than 250 my old. Now that we ve established a maximum possible age for the boundary, all that remains is to set a minimum age for it. You can probably already see how that works. LEONARDIAN (PERMIAN)

19 More mapping reveals another intrusion which cross-cuts both the Permian and Triassic and is therefore younger than both. We sample it, send the samples for age analysis and receive a result of 240 my +/- 1.5 my, making it a reliable estimate. we might also find Triassic xenoliths (inclusions) in the younger dike. Here is how the history stands now: TR INDUAN (TRIASSIC) TR 5. Intrusion of an igneous dike approximately 240 my ago! 4. Deposition of Induan rocks very early in the Triassic, perhaps right at the start. 3. Uplift and erosion either very late in the Permian or very early in the Triassic, probably the former. This produced the disconformity corresponding approximately to the PZ/MZ boundary. 2. Intrusion of an igneous dike approximately 250 my ago! LEONARDIAN (PERMIAN) The boundary we care about is now bracketed by two known numerical ages and 240 my ago! 1. Deposition of the Leonardian rocks.

20 INDUAN (TRIASSIC) In the previous slide the younger dike was shown to be younger than the boundary and the Triassic by cross-cutting (which is generally the easiest thing to spot) and/or by inclusions of Triassic rocks within it. LEONARDIAN (PERMIAN) If the younger of the bracketing igneous rocks is an ash, it is shown to be younger than the boundary by its superpositional position above the boundary.

21 Don t get the idea that we can just run out and find datable igneous rocks any time we like. They are where they are. Fortunately there are enough of them that all the chronostratigraphic boundaries can be bracketed between two of them. Each and every date on the geochronologic scale the true time scale was determined in this way. They were all bracketed between two datable rocks, one demonstrably older and the other demonstrably younger than the boundary. The boundary age is given as the average of the ages of the two radiometric dates closest in age to each other near a boundary. In our example the age assigned would be 245 my. There is an implied error, though not the same as a radiometric error. The error in our example would be +/- 5 my because the boundary could be as old as 250 or as young as 240 (plus the radiometric errors). The errors range from a few tens of thousands for the younger Epochs and Ages to a few million years for the older Systems. For rocks back into the Jurassic the ages are very well constrained because good examples sit above a unique nonconformity on the sea-floor. The continuous production of crust at the ridge assures us of precise, reliable dates for those units. Older oceanic crust has all been subducted so there is generally more error in Triassic and older boundary ages.

22 When I was a student in Geology II I memorized the Precambrian/Cambrian boundary age as 600my. It wasn t obvious on my intro time scale, but there was an error of 100 my in that estimate in other words a 16.7% error. The problem was that there was (and is) a known igneous date of ~500 my within the Cambrian and, at the time, a known igneous age within the Precambrian of 700 my. That was the bracket my. The reported age was therefore 600 my +/- 100 my. In the early to mid 1990 s some newly discovered Precambrian igneous rocks were mapped on what had been highly restricted Soviet missile bases in eastern Siberia. The dates on the newly accessible rocks shrank the age bracket for the boundary and changed the average of that range. Initially the values went to 500 (Cambrian) and 584 (Precambrian) providing an age estimate of 542 my +/- 42 my. The age continues to bee tweaked with each new igneous rock discovered, and each time the bracket gets smaller and the date a little more tightly constrained and closer to certain. There are a couple of approaches to choosing and assessing which dates to use in the boundary age estimate, but in general if a new rock does not make the bracket smaller it is either ignored or has little influence in the decision. Only dates that improve the estimate carry much weight.

23 Take-Home Message: Boundary ages for all the units on the chronostratigraphic scale can be dated by bracketing between the ages of two datable igneous rocks. One of the rocks must be demonstrably older than the boundary. This is established by either superposition, inclusions, or crosscutting, or some combination of those principles. Superposition is usually the easiest to apply. The other must be demonstrably younger than the boundary, and again using some combination of the same three principles. Crosscutting relations is usually the easiest to use. The reported age for the boundary is the half-way point between the two igneous ages (their average) and the implied error is half the range between them. As new igneous dates become available they can change the estimated age and error if they provide a tighter bracket than before.