Date of Award

1970

Document Type

Thesis

Degree Name

Bachelor of Science (BS)

Department

Geology

First Advisor

Dr. Frank R. Karner

Abstract

In southeastern North Dakota, northwestern South Dakota and southeastern Montana, a previously poorly studied tertiary siliceous rock occurs in place, as residual boulders and as a gravel constituent. This rock is found associated with outcrops of all formations from the Cretaceous Hell Creek Formation to the Eocene Golden Valley Formation, though it can only be definitely attributed to the latter. It varies from an arenaceous chert to a sandy chert-cemented siltstone, the defining factor being the chert to detrital grain ratio. Macroscopically, it is typically dense, hard and dark gray in addition to containing numerous plant fossils. Exceptional occurrences are concretion-like. The detrital grains are generally moderately well sorted, subangular and silt-size. Some exhibit secondary overgrowths. Chert and plant matter, replaced and unreplaced, compose the rest of the rock.

The siliceous rock seems to have been formed in a marsh or pond environment characterized by an extensive shallow basin with local depressions. Sediments deposited in these depressions were silicifed by the precipitation of ground water-transported silica. Precipitation of this silica, in the form of chert and secondary overgrowths, was facilitated by an acidic environment at the site of precipitation, a result of the introduction of CO2 and/or humic acids due to the decomposition of organic matter.

Limited evidence suggests a multiple-horizon source for the siliceous rock.

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