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Paleography and Climate

Paleogeographic Effects

Geologic History of Pierre Shale

Depositional Environments and Rock Types

The Cretaceous was a long period and is divided into an Upper, or Later, and a Lower Cretaceous. During some of the period, a great part of North America was under water. The Gulf of Mexico overflowed and covered most of the southern United States, Cuba and Mexico. Texas, Oklahoma, Kansas and Wyoming gradually sank, and the waters crept across Canada and Alaska to join the Arctic Ocean. This great inland waterway known as the Colorado Sea, was more than a thousand miles across at its widest point, between Idaho and Wisconsin. Eastern Canada and the northeastern United States remained above sea level.

The formation of Shale and Sandstone at Pierre Shale

The Pierre Shale area was flat and wide with rock depth going up to 700 feet. When the convergent boundary near Pierre Shale subducted, sea water spread into the new depression in the North American continent and the land was over taken by the newly formed Cretaceous Sea making the climate got even warmer and more humid than before. Warm water could circulate heat across the land.

  • In the early part of the Cretaceous, Pierre Shale, Colorado was just about at sea level and topographic relief was minimal.
  • By 100 million years ago, shallow seas moved into the area, laying down a layer of beach sand and coastal sediments known as the Dakota Formation. Dinosaur footprints and plant fossils are often found there today because of this.
  • As the seas deepened, the western portion of the state received a thick deposit of marine mud, while the eastern portion of the state accumulated clay and mud.
  • Four million years before the end of the Cretaceous, the seas retreated and the Rocky Mountains began to take form.
  • The K-T (Cretaceous-Tertiary) asteroid impact devastated the Colorado landscape, killing off all the large animals and about half of the plant species.

Sandstone

Shale

Medium Low Energy

Low energy

beaches, rivers, sand dunes

marsh, swamps

mud, silt, clay

silt and sand

Paleoclimate: the climate of the entire globe was significantly warmer than the weather we have today during the Cretaceous Era. We can tell by the gastropod fossils found at our site that the Pierre Shale area was a under shallow water that never froze. I.E making the climate quite warm.

Fossils and their Ecosystems

Dating Methods

Small Gastropods

Radiolarians

The Marine Ecosystem

  • underwater during the Cretaceous Period
  • mostly open to shallow environments
  • productive environment
  • divided into three portions...

According to the Web of Science Database, I was able to determine that the age dating method at Pierre Shale was absolute.

In the basin which the authors focused on, the most common isotopes found were carbon, C-13 specifically. It was also apparent that the shell material that they were dealing with was very likely from the Upper Cretaceous Period.

Placenticeras meeki

deposited into open marine environment

deposited into shallow marine environment

lower-third

middle-third

Large Marine Lizards

deposited into open marine to very near shore environment

upper-third

Pierre Shale: Before and After

Past...

  • History of eustatic sea level changes and tectonism
  • originally formed after being deposited in a basin from subduction of Farallon Plate under North American Plate
  • Thrusting created features of the area

What Makes Pierre Shale Special?

During the Cretaceous Period, Pierre Shale was completely underwater due to the Western Interior Seaway that formed through the North American continent. The depositional environment represents the most significant aspect of Pierre Shale because during the Cretaceous Period, the entire area became submerged due to a rise in sea level across the globe (at this time, only 18% of the Earth was land)!

Present...

  • called "gumbo region" because plastic clay that weathers and forms around shale
  • layers of Pierre Shale are soft and easily eroded
  • weather into rounded hills

Pierre Shale, Colorado

Image shows weathered top of Cretaceous Pierre Shale

Jillian Brown

Pat Kelly

Joye Nettles

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