GY 112 Lecture Notes
D. Haywick (2006)
1
GY 112 Lecture Notes
Significance of Fossils: Time
Lecture Goals:
A) Chronostratigraphy versus Biostratigraphy
B) Paleontological correlations (facies changes)
C) Index fossils
Textbook reference: Levin 7th edition (2003), Chapters, 3 & 4; Levin 8th edition (2006), Chapter 6)
A) Chronostratigraphy vs. biostratigraphy
Last time we talked about the history of geological thinking and how geological ideas
evolved. You will be glad to know that we are just about done with historical figures
(some of you have confessed that you are not all that interested in people and places.
“Bring us the rocks!” you have cried. I will do my best to satisfy your every wish;
however, I suspect that you will regret your words once we really get into the rocks and
fossils.
William (Fossil Bill) Smith was one of the first people to use fossils to age rocks (okay so
I lied, we will still occasionally talk about dead people). He used them to correlate strata
from one place to another over the English country side. Correlation of rock units is not
always easy. Take the following situation:
Being able to trace rocks units is vitally important both for geological mapping and for
stratigraphic correlation, but geologists are often faced with situations like that depicted
here. Which of the limestones exposed in Mountain 2 is equivalent to the limestone
exposed in Mountain 1? There are several ways that you can correlate these rock units.
The first is by using absolute dating techniques. If you were to radiometrically date the
limestones at each mountain, you could then match them up. This form of correlation is
called chronostratigraphy (chrono means time). Unfortunately, it is generally not
possible to use radiometrically dating on limestones. They just do not have enough
radioactive isotopes within them to conduct radiometric dating.
Another way to correlate the limestones is to match up their respective lithologies. True,
they are all limestone, but there are different types of limestone. There is fossiliferous
limestone, non-fossiliferous limestone, evaporitic limestone and oolitic limestone. If the
outcrop at Mountain 1 is an oolitic limestone, and there is an oolitic limestone at
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Mountain 2, then it is possible that the two are correlative. This form of matching is
called lithostratigraphy (after lithology). Beware! The two oolites may not be the same
time wise. Sedimentary rocks frequently grade laterally into other lithologies. This is
called a facies change and we will discuss it further shortly.
For limestone, the best means of correlation is through fossil content. Identically aged
limestones should contain fossils of the same age (but not necessarily the same ones –
think about it). This form of correlation is called biostratigraphy and it is an
exceptionally important component of geology. My paleontology colleague here at South,
Dr. Murlene Clark, using biostratigraphy in her research. Biostratigraphy is also
important in the petroleum industry. In many cases, it is the only way to trace rock units
underground from one oil well to another. There are however, some complications.
You must first know how widespread the fossils you are dealing with were when they
were alive. Here is an example of what I mean. Some freshwater fish are very
widespread. In the United States, you can pretty much go bass fishing in any lake you
find. Bass are considered cosmopolitan species at least within lakes in the US.
In addition, Alabama is home to a pretty rare variety of sturgeon. This fish is only found
in a few rivers in Alabama, so you would be out of luck trying to catch it in a lake in
Colorado. This sturgeon is an example of an endemic species. Fossils could be either
cosmopolitan or endemic. The best beasties for correlation purposes are those that were
cosmopolitan. Endemic beasties may be better for paleoenvironmental interpretations.
We’ll get to this next time.
B) Paleontological correlations (facies changes)
The best way to handle this topic is with a cartoon. Assume that you have identified
several cosmopolitan beasties within some strata. There are starfishes, snails, clam shells
and horn corals. They are all marine and with the exception of the snails, are found in all
sedimentary lithologies (shale, sandstone and limestone). The snails you have
encountered do not occur in shale. Many organisms have a preference for a particular
type of sedimentary substrate. They are cosmopolitan in nature, but live in specific
environments. The following is the cartoon that I eluded to earlier:
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and here is the best possible correlation of the rock units:
From a biostratigraphic point of view, the trick is to match up the rock layers on the basis
of their fossil content. When you do it, you will learn several things about their
stratigraphy.
•
Sandstone A thickens from section 1 to section 3.
• Limestone B thins from section 1 to section 3
(Lateral thickness changes are very common in sedimentary successions.
•
Shales C and E are pretty consistent thickness at sections 1 and 2, but merge together
at section 3
• Sandstone D thins from section 1 to section 2 and is absent from section 3.
(This is called a pinchout; they are also common in sedimentary successions)
• Limestone E (section1) passes laterally into a sandstone at section 2.
(This is called a facies change and they are also common in sedimentary successions)
•
The sandstone mentioned above probably undergoes yet another facies change into a
shale at section 3.
This latter correlation is made on the basis of the rugose corals alone. The snails are not
present in the shale at section 3 and unless you remember that the snails could not live in
the environment where shale was deposited, you might have concluded wrongly that no
correlation was possible between sections 2 and 3. Biostratigraphic correlations are not
always straight-forward. If they were, everyone could do them and Dr. Clark would be
out of a job.
C) Index fossils
All organisms eventually evolve into new forms. Human beings today (Homo sapiens
sapiens) replaced an earlier now extinct sub-species called Homo sapiens
neanderthalensis (Neanderthal Man). Most species have limited time ranges, generally
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less than 4 million years. One of the things that paleontologists try to do is to identify the
age at which a species first evolves (this is called the FAD; the First Appearance Datum)
and the age at which a species dies off (the LAD; last appearance datum). The time
between the FAD and the LAD of a species is considered its Biozone.
If a species is cosmopolitan and has a well constrained age span, it can be regarded as an
index fossil. Index fossils allow you to establish (relatively accurately) the age of a
sedimentary rock.
Another way to age rocks is to use FAD and LAD data from several fossils within a rock.
An example of this is shown in the cartoon below:
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A rock containing all 3 fossils is necessarily constrained to a narrow time range, in this
case the Early Jurassic. The boxed interval is considered to be a fossil assemblage zone.
Given the number of fossils that have been identified in the rock record, it is not
surprising that biostratigraphic and paleontological data have become quite voluminous.
Don’t believe me, check out the two next diagrams that deal with one type of beastie
(foraminifera) over one short interval of time (Cenozoic) from one small part of the
planet (New Zealand). Multiply this by a couple 100,000 times and you’ll get an idea
about the amount of stuff that is out there. Both figures are from Hornibrook, N. de et al,
1989. Manual of New Zealand Permian to Pleistocene Foraminiferal Biostratigraphy.
New Zealand Geological Survey Paleontology Bulletin 56, 175p
GY 112 Lecture Notes
D. Haywick (2006)
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GY 112 Lecture Notes
D. Haywick (2006)
Important terms/concepts from today’s lecture
(Google any terms that you are not familiar with)
Chronostratigraphy
Lithostratigraphy
Biostratigraphy
Cosmopolitan
Endemic
Pinchout
Facies change
Index fossils
FAD (first appearance datum)
LAD (last appearance datum)
Biozone
Assemblage zone
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