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The Paleosol Rock Layer
by Tim Farnham

A primary focus of the Denver Basin Project is to understand the sediments deposited by the uplift of the Rocky Mountains. This helps us create an accurate timeline that helps us better date fossil plant and animal localities throughout the Denver Basin. The Denver Basin paleosol is crucial to our understanding of the timing of the uplift of the Rockies as well as to our understanding of the environment of the Denver area millions of years ago. Analysis of the paleosol is ongoing, including both field and lab work.

The focus of my paleosol research is to understand what kind of environment is represented by the paleosol and its age. There are several components of a paleosol that provide clues to its environment.

Identification of clay mineralogy and micromorphology, or small-scale features, are the two primary areas of research when attempting to reconstruct an environment. Both of these techniques require extensive fieldwork and sampling of the paleosol. Fieldwork on the paleosol has proceeded with the help of an undergraduate intern, Adam Soldinger, from the University of Colorado at Boulder.

With the help of Bob Raynolds, I identified seven outcrops of the paleosol around the Denver Basin. The outcrops are spread out, giving us as much variation as possible. This ensures that we analyze the paleosol throughout the basin instead of only a small portion of the Denver Basin. Fieldwork consists of stratigraphic-section measuring and sampling of every identifiable unit or horizon in the paleosol. Samples are then brought to the labs at the University of Colorado. Once in the labs, some portions of the samples are made into thin sections for microscopic analysis, and other portions are ground up and run through an X-ray diffraction (XRD) machine in order to identify types of clay minerals.

Adam and I have already made an excursion to a paleosol outcrop slightly north of Kiowa called the Navajo Pit and have run samples through the XRD machine at the University of Colorado. Preliminary results showed the presence of the clay mineral kaolinite in every sample, as well as smectite and vermiculite in one sample. Kaolinite is indicative of a highly weathered environment, like a tropical soil, and smectite and vermiculite are associated with seasonality in soils.

Based on fieldwork completed so far, the general appearance of the paleosol is tropical. The paleosol at the Navajo Pit is dark red with very large white mottles caused by root traces or the migration of water through the soil. In fact, several of the layers at the Navajo Pit have the fossil roots still intact with white mottles around them. In addition to the red beds, there are four separate horizons that are bright purple with white and yellow mottles, which creates a spectacular outcrop.

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