Physical Geology lecture outline - Historical geology & the geological time scale.

Definition of historical geology: the science of unraveling the earth's history (which turns out to be long and at times quite dramatic).

Detectives, and mysteries provide a good metaphor for historical geology, and forensic science is a historical science (just a much shorter time frame than historical geology).


Unraveling local histories

Nicolaus Steno in 1600s: made several crucial contributions, mainly while living in Italy.

Photograph above is of UNO students on field trip down in the Grand Canyon. In the canyon wall behind is a series of stacked sedimentary rocks. Steno's law of superposition simply states that the older layers are below, and as you ascend through higher layers you are moving forward in geologic time. You might think of the layers as pages of a book, but where the cover page is face down. In addition, one can infer that this area, the Grand Canyon, is no longer characterized by deposition and accumulation of sediments that were turned into sedimentary rock, but instead by uplift and erosion that made the canyon that exposed these strata. So locally there are gaps in the geologic record.

James Hutton in the late 1700s (Edinburgh enlightenment):

Grand Canyon as an example. It is significant that there must be time for all this to happen.

Above is a view from the lip of the inner gorge (Bright Angel trail) in the Canyon from a UNO geology field trip. Note the distinct boundary that separates flat lying sedimentary rocks (sandstones the students are sitting on) above from much older metamorphic and plutonic rocks with almost vertical layering below. Some of the ligth colored layers in the lower sequence are sill intrusions of the Zoraster granite into the Vishnu schist. What type of unconformity is represented here?

In this view picture below of the Grand canyon wall you can see the layers in the lower portion are at small angle to and truncated by the layers above. What is this type of unconformity known as? The cliff forming sandstone above the unconformity is the same one as in the photo above and is of Cambrian age.

This is another angular unconformity, but in quite a different place and between very different rock units. This is a cliff side at Reva Gap in the Slim Buttes area of NW South Dakota, and the strata are the Arikaree Group above, and the White River Group strata below, with a time gap between them of something less than several million years. A group of UNO undergraduates have conducted research projects in this area.

With only Steno's laws and other common sense considerations, such as cross-cutting relationships, fairly complicated geolgoic histories can be worked out for areas from fairly simple geologic observations.

An example of reconstructing history: Below is a schematic cross section showing the relationships between various rock units. What is the history of formation of the various rock units? What events can be inferred to have occurred between or during the formation of the various rock units.

Imagine this sketch captures a large cliff side. Note what rock types are involved, and then infer the history that unfolded to create this set of rock and their relationships with each other. After you try on your own you can click through the series below and it will show you an answer.

Step 1 Step 2 Step 3 Step 4 Step 5 Step 6 Step 7 Step 8 Step 9 Step 10 Step 11 Step 12

You can step through the various geologic stages that could have led to the above diagram by clicking on the above small versions. You will need to hit the back arrow to return to this page to get to the next step.

You should practice on this diagram below. Using the above concepts a fairly detailed history can be inferred for this cross section, which is loosely based on areas in the SW part of the U.S.. Sometimes, portions of the history are not well constrained; i.e. multiple interpretations are possible. You are often asked to interpret a diagram like this on your second exam.


Regional correlation, the geologic time scale and global history

Abraham Gottlieb Werner and Universal Formations: a failed model for the history of rocks. The basic idea was that different rock types formed at different points during the earth's history.

William 'Strata' Smith and George Cuvier (late 1700s) and the law of faunal succession and the science of biostratigraphy (the stratigraphic distribution of fossils).

Charles Lyell in the early 1800s utilized index fossils, biostratigraphic regional correlation, and initial development of time scale.

Biostratigraphy: The unique history of life, full of extinctions and evolution provides a time scale. While basalts and sandstones have been formed here and there throughout most of the history of the earth, most species have only survived for a brief period and their remains in the sediments can be used to constrain the time the sediments were deposited. Hence, the age of the dinosaurs. Extinction is a critical marker and process in this science.

Image to the right is a mural from the halls of the Museum of Natural History in New York City, showing an artists reconstruction of some Pterosaurs, a group related to but separate from the dinosaurs, but which lived coevally with the dinosaurs. This variety had a wing span of 20-25 feet. Flight was an early adaptation within the Era known as the Mesozoic.

An index fossil is one that is useful for indicating at what point in earth's history the entombing sediments were deposited. What characteristics of a fossil species would make them a good index fossil?

On the left is a slab of preserved trilobites on display at the Houston Museum of Natural History (linked to a more detailed view). These are very important index fossils for the Paleozoic. They were abundant, wide spread, preserved well and speciated rapidly into many distinctive forms. In the middle and on the right is another common index fossil for the Paleozoic, crinoids, which were a form of echinoderm (like sea urchins and starfish). They were attached to the sea bottom by a long slender column, and then had a main body with tentacles that filter fed from the water. Both of these general types of fossils can be found in the Omaha area. The middle photo is from the Houston Museum of Natural History and the right photo is from the Confluence Museum in Lyon France.

Ammonites were marine organisms that were widespread, and evolved into a wide array of geologically short-lived forms and hence make good index fossils. The image to the left is from Rurikfjellet Formation in Spitsbergen, Norway. The two middle images are from the USGS web site http://esp.cr.usgs.gov/research/fossils/ammonites.html . The right most image (which is linked to a higher resolution version) is from the Confluence Museum in Lyon, France. Note the myryiad of forms displayed here.

These are three dinosaur skulls. The left one is now housed in the Chicago Field Museum, and the inner left (the well known Triceratops) is housed in the Smithsonian, and the inner left (Pachycephalosaurus) and very left (Maiasaurus egg) are from the Museum of the Rockies in Montana. Dinosaurs flourished in the same era as ammonites. While these are diagnostic of a certain portion of geologic history, a given species is more localized and hence in general dinosaurs are less useful as index fossils than are ammonites.

This is an early whale species on display in the Smithsonian museum in DC. Whales, as mammals, developed during the Cenozoic Era. Image is linked to a higher resolution version.

This is a very well preserved small croc (length about 8 inches) in some fine-grained sediment displayed in the Confluence Museum of Lyon, France. Crocs have been quite successful throughout a long span of geologic history, and so they don't make the best index fossils. Under the right conditions, often involving fine-grained sediments, exquisite details can be preserved in fossils (and sometimes DNA!).

Time scale units: Eras -> Periods -> Epochs -> Stages and Ages (larger to smaller).

USGS summary diagram of Geologic Time scale to right, from http://pubs.usgs.gov/gip/fossils/scale.html.

Widely used depiction of geologic history from USGS site: http://pubs.usgs.gov/gip/geotime/time.html .

Detailed replicas of fossil hominid skulls arranged in a postulated evolutionary sequence. From an exhibit in the Musuem of Natural History in New York City. As new hominid fossils are found and new data comes in (e.g. on partial DNA left within the hominid fossil) the details of our human lineage are being better and better understood, and models refined.


Good reference for further reading: Eicher, D. L., 1968, Geologic Time; Prentice Hall, 2nd ed., 150 p.
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