Date of Award

1987

Document Type

Thesis

Degree Name

Master of Science (MS)

Department

Geology

First Advisor

R.D. LeFever

Abstract

The Bullion Creek Formation (Paleocene) in North Dakota is part of an Upper Cretaceous to Eocene wedge of siliciclastic rocks formed during uplift associated with the Laramide Orogeny. The Bullion Creek Formation consists predominantly of mudrocks , with fine-grained sandstone, lignite, and rare limestone lenses . Previous workers have postulated combinations of fluvial and lacustrine depositional environments for the rocks.

The study area covers 15 square kilometres in the badlands of the Little Missouri River in western Billings County, North Dakota, where approximately the upper 75 metres of the Bullion Creek Formation ls exposed. Twenty nine stratigraphic sections were measured and five lithologies distinguished: sandstone, mudrocks, lignite, limestone, and marl stone. Because the sandstones contain the most features that are environmentally sensitive, special emphasis was placed on their description.

On the basis of sedimentary structures and external morphology, five types (A-E) of sandstone bodies were recognized. Type A sandstone bodies are relatively thick and lenticular in cross-section. These bodies represent channel-fill deposits of low-sinuosity streams. Type B sandstone bodies are thin and tabular, and were probably deposited as crevasse splays, presumably from the channels responsible for the type A sandstone bodies, one type C sandstone body. This body There is only was probably deposited as a delta that filled in a small lake. There is only one type D sandstone body; it is tabular and continuous over the entire study area. This sandstone body is believed to represent a broad shallow delta, deposited in to a large swamp or shallow lake. Type E sandstone bodies include all sandstone bodies that are too poorly indurated to be ascribed to one of the other types.

Mudrocks of both fluvial and lacustrine origin were recognized as well as lignites that are believed to have formed in association with fluvial systems. Lower portions of the exposure in the study area are characterized by thin, broad, beach deposits, formed as a large lake regressed. Following this, fluvial systems migrated over the exposed lake plain, forming broad backswamps that contained abundant small lakes and ponds. Considerable reworking of deposits occurred during this time, as the fluvial systems migrated back and forth across a very low relief setting. The end of time represented by the Bullion Creek Formation was characterized by vast swamps and shallow lakes.

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