GY 112 Lecture Notes
D. Haywick (2006)
1
GY 112 Lecture Notes
Significance of Fossils: Paleogeography Interpretations
Lecture Goals:
A) What is paleogeography?
B) How it works (Late Cambrian fantasy example)
C) Paleoclimate resolution
Textbook reference: Levin 7th edition (2003), Chapter4; Levin 8th edition (2006), Chapter 6
A) What is paleogeography?
The term paleogeography refers to the geographical and spatial orientation of
landmasses and oceans some time in the past. Through paleogeographic reconstruction,
you can visualize the orientation of
continents and sea ways and you can
determine the distribution of
paleoenvironments of deposition.
Wegener came up with one of the first
paleogeographic maps when he
conceived Pangaea (see adjacent
image). The real issue with
paleogeography is not the final product
(i.e., the maps), but the information
that you need to get there. You must
consider time, fossils, environments of
deposition, lateral variations and pretty
well every other aspect of geology.
This is not easy, and if you want
accuracy, it takes a lot of dedication,
time and patience.
B) How it works (Late Cambrian fantasy example)
I think that the best way to illustrate paleogeographical reconstructions is to give you an
example of how it it done. We’ll choose a specific time, say the Late Cambrian Period,
about 514 million years ago The image below is a reconstruction of the paleogeography
at that time according to Chris Scotese. Scotese is based in Texas and through various
means (e.g., paleomagnetism), he has produced the most widely accepted
paleogeographic maps for the Phanerozoic Era. He also produced a flip book showing the
drift of the continents over this period of time which is available for purchase from his
website. I have one if you want to see what it is like before you buy.
GY 112 Lecture Notes
D. Haywick (2006)
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From Scotese C.R., 1995. Phanerozoic plate tectonic reconstructions, PALEOMAP Progress Report 336,
Department of Geology, University of Texas at Arlington (http://www.scotese.com/newpage12.htm)
Let’s concentrate on North America. The sketch below is derived from Scotese’s
paleogeographic map. Note that at this time, North America was tilted almost 90 degrees
GY 112 Lecture Notes
D. Haywick (2006)
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relative to what it is today, there was a seaway running down through the western
Provinces and States and the equator ran right through the heart of the continent. At this
time, tropical reefs lined shallow marine intervals of the not-yet-formed Appalachian
Mountains (really! I studied some of these reef complexes as part of my Masters study in
Newfoundland). Assume for the moment that Hudson’s Bay (in central Canada) was
around back in the Late Cambrian (it wasn’t at least not in the form it is today). Since our
fantasy bay would have straddled the equator, it is a good bet that tropical reefs would
have occurred in the shallow marine portions (just like I found in Newfoundland). The
shoreline itself may have been lined in a beach (quartz arenite sand). Inland, lay the
terrestrial rocks that comprised the North American continent. These are the crystalline
rocks of what we today call the Canadian Shield (mostly granite). Even 514 million
years ago, the continent was old (about 2 billion years old near Hudson Bay). Seaward of
the reef, the water depths should have increased possibly to 100 metres or more. The
following cartoon summarizes what the distribution of these various rocks and
depositional environments might have looked like 514 million years ago in our fantasy
reconstruction:
Now let’s come forward in time. It is the present and you are a geologist working with
the Geological Survey of Canada (one of the World’s most respected geological
organizations). You are up near Hudson’s Bay studying Late Cambrian sedimentary
rocks (bring your blankets and bug repellent. Up here you either freeze [winter] or get
eaten alive by mosquitoes, black flies, noseeums, deer flies and horse flies [the remaining
3 months of the year]). If you measured sections at 4 locations (shown on the map) you
might encounter the following successions of sedimentary rocks. I’ve drawn them up as
stratigraphic sections:
GY 112 Lecture Notes
D. Haywick (2006)
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In this situation, you could simply correlate the rock units on the basis of their lithology
(i.e., lithostratigraphic correlations). But if you are interested in paleogeography, you also
need to correlate the rocks according to time (i.e., chronostratigraphy). You must
determine facies changes. They are what allow you to derive paleogeographic maps.
Here is how the environments match up between the 4 sections:
GY 112 Lecture Notes
D. Haywick (2006)
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For this class, it was necessary to do this example backwards so you could see how
everything fits together, but if you were doing this for real, you would start with studying
the rocks in the outcrops they are presently exposed in (e.g., establishing their ages,
paleoenvironments of deposition, lateral facies changes etc.). Then you would plot these
data on a map, rotate the map relative to the latitude and longitude at the time of
deposition (paleomagnetism would help here) and correlate the various environments of
deposition from one section to the next. The next thing you know, you have a genuine
paleogeographic map. This one of the Permian when Pangaea was around comes to you
via the Paleogeography Atlas Project (University of Chicago).
Ziegler, A.M., M.L. Hulver, and D.B. Rowley, 1997. Permian world topography and climate. In: Late
Glacial and Post-Glacial Environmental Changes - Quaternary, Carboniferous-Permian and Proterozoic,
I.P. Martini (ed.), pp. 111-146., New York: Oxford University Press.
GY 112 Lecture Notes
D. Haywick (2006)
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C) Paleoclimate
We have already discussed the importance of some fossils in determining paleoclimate.
Corals are mostly tropical, so if you find a bunch of corals in the rock record, it usually
means that you are dealing with a tropical situation. Here are a couple of other less
blatantly obvious examples.
1) My paleo-colleague Dr Clark is a micropaleontologist. She studies microfossils.
Some of the beasties that she examines are foraminifera and she has noticed that their
sizes vary at different geological times. She has concluded that the size variation is due to
food availability and water temperature. So you can determine water temperature (a
variant of climate) by looking at the size range of these microbeasties.
2) I also encountered a “cold water” indicator in my work in New Zealand. There is a
scallop shell (Chlamys delicatula) that today only occurs in sub-Antarctica waters (it is a
cold water beastie). But you periodically find it within Tertiary-aged sedimentary rocks
in eastern New Zealand. The rocks you find it in were deposited during sea level
lowstands which corresponds to glacial periods. When the ice melted (interglacials), the
climate warmed up and Chlamys delicatula retreated back toward the pole. Now this may
seem to be a blatantly predictable exercise, but this type of paleoclimatic interpretation
works for much older rocks as well. By using similar cold water indicators, glaciations
have been identified in Australia (Permian) and possibly Africa (Ordovician).
Important terms/concepts from today’s lecture
(Google any terms that you are not familiar with)
Paleogeography
Paleogeographic maps
Seaway
Canadian Shield
Paleoclimate
Micropaleontology