The Geological History of Fossil Butte National Monument and Fossil Basin

Part 1

As the Nation’s principal conservation agency, the Department of the Interior has basic responsibilities for water, fish, wildlife, mineral, land, park, and recreational resources. Indian and Territorial affairs are other major concerns of America’s “Department of Natural Resources.” The Department works to assure the wisest choice in managing all our resources so each will make its full contribution to a better United States—now and in the future.

This publication is one in a series of research studies devoted to special topics which have been explored in connection with the various areas in the National Park System. It is printed at the Government Printing Office and may be purchased from the Superintendent of Documents, Government Printing Office, Washington, D.C. 20402.

Library of Congress Cataloging in Publication Data

McGrew, Paul Orman, 1909- The geological history of Fossil Butte National Monument and Fossil Basin. (National Park Service occasional paper; no. 3) Bibliography: p. Includes index. 1. Geology—Wyoming—Fossil Butte National Monument. 2. Geology, Stratigraphic—Eocene. 3. Paleontology—Eocene. 4. Fossil Butte National Monument, Wyo. I. Casilliano, Michael, joint author. II. Title. III. Series: United States. National Park Service. National Park Service occasional paper; no. 3. QE182.F67M3 557.87'82 75-17511

The Geological History of Fossil Butte National Monument and Fossil Basin

Paul O. McGrew Michael Casilliano Department of Geology, University of Wyoming

NATIONAL PARK SERVICE OCCASIONAL PAPER NUMBER THREE

Contents

INTRODUCTION 1 LOCATION 1 HISTORICAL BACKGROUND OF RESEARCH 1 STRATIGRAPHY 5 Thaynes Limestone 7 Evanston Formation 7 Wasatch Formation 7 Basal Conglomerate Member 9 Lower Member 9 Main Body 11 Sandstone Tongue 11 Mudstone Tongue 12 Bullpen Member 12 Tunp Member 12 Age of the Wasatch Formation 12 Depositional Environment 13 Green River Formation 13 Fossil Butte Member 13 Angelo Member 14 Age of the Green River Formation 14 Depositional Environment 14 Fowkes Formation 15 Sillem Member 15 Bulldog Hollow Member 15 Gooseberry Member 15 Age of the Fowkes Formation 15 Depositional Environment 15 QUATERNARY 15 THE GEOLOGIC STRUCTURE OF FOSSIL BASIN 15 PALEONTOLOGY 19 Flora 19 Invertebrates 20 Vertebrates 20 Fish 20 Amphibians 24 Reptiles 24 Birds 24 Mammals 25 PALEOECOLOGY AND TAPHONOMY 30 GLOSSARY 34 REFERENCES 35 BIBLIOGRAPHY 37 ACKNOWLEDGMENTS 37

INTRODUCTION

Fifty million years ago the southwestern corner of what is now Wyoming was part of a system of three freshwater lakes that covered that area plus adjacent parts of Colorado and Utah.

These lakes began with Lake Flagstaff and later became Lake Gosiute, Lake Uinta, and Fossil Lake (Fig. 1). These lakes developed in intermontane basins that were created as a result of the geologic events that formed the Rocky Mountains. The three lakes, collectively known as the Green River Lake System, had a long, complex history of expansion and contraction. Varying climatic and geologic conditions were responsible for the changes in lake size and distribution.

The shores surrounding the lakes were blanketed by a lush, green canopy of palm, cinnamon, maple, oak, and other familiar trees. Hazel and lilac covered the forest floor. Rushes and other aquatic plants lined the lake shore.

The air was humid and warm. Streams flowing down from the hills and mountains built up flood plains and fed sediment into the lake, where it was deposited in shallow water near the shore. Deposits formed by chemical processes settled to the bottom further from shore in deep, quiet water. From these processes were formed the rocks from which the past history of Green River Lake System is read.

In the forest and undergrowth lived the ancestors of modern mammal groups. Ancestral rodents and tiny insectivores lived a furtive existence in the brush and mold that carpeted the forest floor, or else they sought a livelihood among the branches of trees. Large, bizarre animals with strange names lumbered through the reed-covered streamsides eating soft, succulent plants. The earliest members of the horse family browsed on the soft vegetation. The trees overhead were alive with the chattering and antics of early primates. Carnivorous mammals preyed on their plant-eating neighbors and so maintained a balanced community. Crocodiles and turtles basked in the sun on the beach. Flamingos concentrated in large nesting grounds. Snakes and lizards crawled about in the undergrowth. Insects, many exceedingly similar to modern types, flew about in the warm air or crept about on the plants.

The waters of the lakes teemed with many types of fish. Relatives of the perch, herring, and sting ray swam in the warm lake water. Occasionally, large-scale mortalities of the fish occurred. As the fish died, they sank to the bottom of the lake, and were preserved in the lake sediments.

Today, the lakes are gone and where once there were lush tropical forests there is now a semi-desert covered with sagebrush and greasewood. The history of these lakes, forests, and animal life can be read in the thick sediments deposited so long ago.

This report concerns itself with one of these lakes, the smallest of the three, Fossil Lake. Famous for its fossil fish beds, part of this ancient lake is now Fossil Butte National Monument. It is the purpose of this report to relate the history of Fossil Lake and its now fossilized inhabitants, thereby, hopefully presenting the order of events in a landscape obscured by 50 million years of time.

LOCATION

Fossil Butte National Monument is near the geographical center of Fossil Basin. The basin itself is located in the extreme southwestern part of Wyoming in Uinta and Lincoln counties, near the Utah-Idaho border (Fig. 2).

The monument is situated about 10 miles west of Kemmerer, Lincoln County, Wyoming (Fig. 3). U.S. Highway 30N runs just south of the monument as does the Union Pacific Railroad line to Oregon. The abandoned town of Fossil is also immediately south of the monument.

The Kemmerer and Sage quadrangles of the United States Geological Survey cover the entire monument and the surrounding area.

HISTORICAL BACKGROUND OF RESEARCH

The principal rocks involved at Fossil Butte are the Green River and Wasatch formations. The first published notice of these two rock units was by Hayden (1869). Although short, Hayden’s descriptions were the basis for later refinement of the stratigraphy of the Green River and Fossil basins. Hayden also mentioned the quantities of fossil fish from the Green River Formation.

The work done by Hayden was conducted under the auspices of the U.S. Department of the Interior, Geological and Geographical Survey of the Territories. The survey lasted from 1867 to 1878, during which time Hayden and his associates published annual reports of their findings.

In the 1870 report, Hayden (1871) mentioned the discovery of the Petrified Fish Cut. This cut is located on the main line of the Union Pacific Railroad about 2 miles west of Green River, Wyoming. A. W. Hilliard and L. E. Rickseeker, employees of the Union Pacific Railroad, discovered the cut and obtained many fossil fish which they turned over to Hayden.

Previous to Hayden, rocks now known as the Green River and Wasatch formations were occasionally mentioned in the various diaries, journals, and reports of early missionaries like those of S. A. Parker or those of explorers like Fremont (Knight 1955).

Hayden’s report (1871:425-437) included a description of the fish fossils from Petrified Fish Cut by E. D. Cope, a vertebrate paleontologist. Several years earlier Leidy (1856), another paleontologist, described a fish from an unknown locality in the Green River Formation.

Part 2

Hayden divided this survey area into several districts. The Green River district was put in charge of A. C. Peale. Peale’s (1879:535) report contained the first geologic description of Fossil Butte itself, as well as a short discussion of the fish fossils obtained there. The description of the butte is quoted here:

... In the lower part of the bluff from which these specimens are taken, the bright coloured beds of the Wasatch are seen outcropping, although the entire section cannot be seen, as their softness causes them to weather so that the debris conceals the strata. The fossils are found at several horizons in the shales. Near the top of the bluff is a band of hard, bituminous, or oily shale, which burns rather freely with a strong bituminous odor.... It is brownish-black in color and on the weathered surfaces a bluish white....

The next major publication dealing with Fossil Basin was that by Veatch (1907). He mapped the rocks in certain areas of the basin in an attempt to bring a semblance of order to Hayden’s (1869) broad stratigraphic descriptions. His work resulted in the designation of most of the rock units in Fossil Basin from Jurassic to Recent.

Schultz (1914), an associate of Veatch, published a paper dealing with the structures to the north and east of Fossil Basin. This helped to delineate the features surrounding and forming the basin.

Subsequent work on the geology of southwest Wyoming mainly has been concentrated on the Green River Basin just east of Fossil Basin. Significant work on Fossil Basin has been done by Rubey et al. (1968a, b) who mapped the northern part of the basin, and by Oriel and Tracey (1970) who have published results of the latest study on the stratigraphy and age of the rock units in Fossil Basin. This latest work is the culmination of many years of study and incorporates and refines data presented in shorter, earlier papers.

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