Abstract

During Eocene time sediment accumulated in Fossil Lake, in what developed to be a small linear and structurally controlled basin. Fossil Lake was one of several lakes into which the Green River Formation was deposited in Wyoming, Utah and Colorado.

Detailed stratigraphic analysis of the Lower Unit of the Fossil Butte Member revealed a well developed lacustrine sequence south of Fossil Butte, and indicates four major depositional facies: (1) open lacustrine, (2) marginal lacustrine, (3) carbonate mudflat, and (4) marginal fluvio-deltaic. The open lacustrine facies is characterized by kerogen rich to kerogen poor finely laminated micrites , that consist mainly of calcite and very little dolomite. These carbonates contain well preserved fossil fish, ostracodes, molluscs and the kerogen that was produced mainly by algae. These rocks grade towards the margins into micrites that become more bioturbated closer to the margin of the lake, as well as into ostracodal and gastropodal limestones. Nearshore carbonates consist mostly of calcite and usually are well bioturbated. Typical fossils include molluscs and ostracodes. In some localized areas limestones can become oolitic, contain some typical nearshore plant remains and even some rare beach lag deposits of vertebrate bones. The carbonate mudflat facies is mainly restricted to the eastern margin where sediments were subaerially exposed and conditions favored precipitation of dolomite as indicated by several units with mudcracks. Sudden transgressions of the lake produced higher energy conditions which resulted in carbonate being ripped-up on the mudflat and subsequently deposited over scoured surfaces. Although fluvial events occurred through the life of the lake, towards the end of Lower Unit time fluvial activity increased. A Gilbert-type delta developed from the southwest prograding into the lake and virtually filling the whole lake, culminating Lower Unit time. Deltaic sediments consist of siliciclastic sandstones, siltstones and mudstones. Deltaic foresets characterize these sandstones, and fossil reptiles, mammals, fish, molluscs and ostracodes also occur.

The lateral and vertical relationship of Lower Unit lithofacies reflect the dynamic nature of Fossil Lake, where the interplay of a combination of factors such as distance from depocenter to margin, changes in depth, oxygenation, siliciclastic and carbonate sediment influx, and productivity resulted in a typical lithofacies succession from basin depocenter to margin: kerogen rich laminated micrites to kerogen poor laminated micrite to micrites and limestones to mudstones, siltstones and sandstones. This relationship also occurs vertically, but sudden variations of diverse factors resulted in cycles of two or more of these lithofacies, as well as abrupt changes in lithofacies deposition.

This study has further documented the importance of fluvial influences on the distribution of kerogen and the deposition of facies sequences within ancient lake complexes. It has provided a detailed and unique insight into fluvial-lacustrine transitions and relationships, only seen in core in other Green River Formation basins.

LLU Discipline

Geology

Department

Geology

School

Graduate School

First Advisor

H. Paul Buchheim

Second Advisor

Leonard Brand

Third Advisor

Clyde Webster

Degree Name

Master of Science (MS)

Degree Level

M.S.

Year Degree Awarded

1989

Date (Title Page)

6-1989

Language

English

Library of Congress/MESH Subject Headings

Geology; Stratigraphic -- Eocene; Paleoecology -- Wyoming; Paleolimnology -- Wyoming.

Subject - Local

Geological Sciences program -- Dissertations

Type

Thesis

Page Count

x; 134

Digital Format

PDF

Digital Publisher

Loma Linda University Libraries

Usage Rights

This title appears here courtesy of the author, who has granted Loma Linda University a limited, non-exclusive right to make this publication available to the public. The author retains all other copyrights.

Collection

Loma Linda University Electronic Theses and Dissertations

Collection Website

http://scholarsrepository.llu.edu/etd/

Repository

Loma Linda University. Del E. Webb Memorial Library. University Archives

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