Cretaceous Time and Paleogeography

The Cretaceous Timescale and Paleogeography

	The image at the left is the most current timescale for the Cretaceous Period. This interval of time spanned approximately 80 million years and is divided into two Epochs, Early and Late. The Cretaceous Period also is divided into 12 smaller units, called Stages. Click HERE to learn more about the official geologic timescale. The timescale at the left was produced using Timescale Creator (http://www.stratigraphy.org). Cretaceous rocks were deposited in Nebraska from the Albian until the Maastrichtian (~101-68 million years ago). The Albian-age rocks in Nebraska sit directly on 300 million year old late Paleozoic strata. This time gap (of ~200 million years) is what geologists call an unconformity or erosional hiatus. Another unconformity occurs between Cretaceous and Cenozoic strata in Nebraska. Different environments existed in Nebraska during the Cretaceous, and these environments varied throughout time depending on changes in global climate and sea levels. Click HERE to learn more about the rock types deposited in Nebraska during the Cretaceous Period. To learn more about global events during the Cretaceous, click HERE. Below the Cretaceous timescale are artistic reconstructions of what North America looked like during several time slices within the Cretaceous.





	Painting by Scott Eastman of a Plesiosaur based on the skeletal remains found near Niobrara State Park, Nebraska. Image from: http://www.museum.unl.edu/research/vertpaleo/Ples/index.html

Paleogeography

Paleogeographic reconstructions help scientists determine what the Earth looked like in the past. Clues from the geologic record can be used to determine the posistions of continents, as well as how sea level has changed throughout time. The images below are Cretaceous paleogeographic maps of North America (from Dr. Ron Blakey- http://jan.ucc.nau.edu/~rcb7/) that are based on observations in the geologic record. Click HERE to visit Dr. Blakey's website to learn more about paleogeography and how the continents have changed position throughout geologic time.

The map at the left is a reconstruction of North America from 75 million years ago (Campanian Stage). Global sea levels were much higher than today, and a large seaway disected North America into several large landmasses. This body of water has been named the Western Interior Seaway. Note that Nebraska was almost completely inundated by marine water during this time interval.

The map at the left is a reconstruction of North America from 100 million years ago (Albian Stage). Note the differences from the map above; the Western Interior Seaway did not yet connect the Arctic Ocean to the Gulf of Mexico and most of Nebraska was not inundated by marine waters.

Global sea levels continued to rise during the Cretaceous, which eventually led to the expansion of the Western Interior Seaway to the point where waters of the Arctic Ocean were connected to the Gulf of Mexico (see map above). These changes in water depths are recorded in the rocks of Nebraska. Click HERE to learn more.

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