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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

    [Illustration: Excavations in progress]




                                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).

    [Illustration: Fig. 1. Paleocene-Eocene lake system (after Schaeffer
    and Mangun 1965).]

    [Illustration: Fig. 2. Geographic features of southwest Wyoming and
    adjacent areas (from Oriel and Tracey 1970).]

    [Illustration: Fig. 3. Physiography of the Fossil Butte National
    Monument region (from U.S. Department of the Interior 1964).]

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.

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  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.




                              STRATIGRAPHY


The complete geologic history of the Fossil Basin involves many
stratigraphic units. Most of these are not exposed within the monument
itself but are well known from outcrops outside the boundaries of the
monument and from deep wells (Fig. 4).

At the base of the stratigraphic section are rocks of Precambrian age.
These have not been penetrated by wells within the Fossil Basin but
should occur as much as 25,000 ft beneath the surface. They probably
consist of metamorphic and intrusive rocks. Beginning about 600 million
years ago, at the beginning of Cambrian time, sedimentary deposits
accumulated. These are both marine and continental in origin and range
in age from early Cambrian (600 million years old) to early Tertiary (50
million years old). Some 34 geologic formations are recognized in this
thick section, a list of which follows:

  Eocene                Fowkes Formation
                        Wasatch-Green River Formations
  Paleocene             Evanston Formation (upper part)
  Upper Cretaceous      Evanston Formation (lower part)
                        Adaville Formation
                        Lazeart Sandstone
                        Hilliard Shale
                        Frontier Formation
  Lower Cretaceous      Aspen Shale
                        Bear River Formation
                        Gannett Group
  Jurassic              Stump Sandstone
                        Preuss Sandstone
                        Twin Creek Limestone
                        Nugget Sandstone (upper part)
  Triassic              Nugget Sandstone (lower part)
                        Thaynes Limestone
                        Woodside Formation
                        Dinwoody Formation
  Permian               Phosphoria Formation
  Pennsylvanian         Wells Formation
  Mississippian         Brazer Limestone
                        Madison Limestone (upper part)
  Devonian              Madison Limestone (lower part)
                        Darby Formation
  Silurian              Not Present
  Ordovician            Leigh Dolomite
                        Bighorn Dolomite
  Cambrian              Gallatin Limestone
                        Gros Ventre Formation
  Precambrian           Complex of metamorphic and intrusive rocks

Within the monument itself the oldest rocks exposed are those of the
Thaynes Limestone near Prow Point. As will be seen in the discussion of
the structure of the Fossil Basin, all of the rocks beneath the Evanston
Formation are folded and faulted and within the monument are buried by
deposits of Tertiary age. For this reason detailed discussion of
stratigraphic units will be confined to those rocks that can be seen
within the boundaries of the monument or closely adjacent to it (Fig. 5
). The distribution of the various formations within the monument is
illustrated in Figs. 7 and 8.

    [Illustration: Fig. 4. Geologic time scale.]

  Era/Period/Epoch             Age  Major Biological & Geologic Events
                         (Millions
                        of years)
  CENOZOIC
    Quaternary                      Ice Age begins and evolution of
                                    modern mammals; man reaches N. A.;
                                    general uplift of continent.
      Holocene (Recent)       .01
      Pleistocene                3
    Tertiary                        Evolution and Development of major
                                    mammal groups; Cascadian Orogeny in
                                    northwestern United States; in latest
                                    Cenozoic, Eocene lake system.
      Pliocene                 5.5
      Miocene                 25.5
      Oligocene                 36
      Eocene                    54
      Paleocene                 65
  MESOZOIC
    Cretaceous                130   Extinction of dinosaurs; flowering
                                    plants flourish; height of
                                    Cordilleran Orogeny.
    Jurassic                  185   First birds, dinosaurs and mammals
                                    continue to evolve; beginning of
                                    Cordilleran Orogeny which formed the
                                    Rocky Mountains.
    Triassic                  230   Conifers evolved; first dinosaurs;
                                    first mammals; Appalachian Orogeny;
                                    some orogeny in Cordillera.
  PALEOZOIC
    Permian                   265   Archaic reptiles and mammal-like
                                    reptiles evolve. Appalachian Orogeny.
    Pennsylvanian             310   First reptiles, coal swamps formed;
                                    beginning of the Appalachian Orogeny.
    Mississippian             355   Expansion of amphibians; much
                                    carbonate deposition; Antler Orogeny
                                    in Cordilleran Mobile Belt (Rocky
                                    Mountain area).
    Devonian                  413   First land plants; first jawed
                                    fishes, sharks and bony fish;
                                    amphibians appear; Acadian Orogeny in
                                    the Appalachian belt.
    Silurian                  425   Jawless fishes evolved; much reef
                                    development; evaporite mineral
                                    development.
    Ordovician                475   First vertebrate remains known from
                                    this time; Taconic Orogeny in
                                    Appalachian belt.
    Cambrian                  600   First shelled invertebrates; major
                                    flooding of North America.


  Notes:
    Reformatted table to combine ‘Era’, ‘Period’, and ‘Epoch’ columns
          into one column in outline form.
    Beneath heading ‘Epoch’: “(Cenozoic only)”; beside other periods,
          “Epochs for these periods are not definitely established”
    As the last line, “Precambrian time began 5 billion years ago.”


                           Thaynes Limestone

At the very northeastern corner of Fossil Butte National Monument can be
seen an outcrop of the Thaynes Limestone of Triassic age, a marine unit
deposited some 200 million years ago. This formation is noted for its
rich and varied assemblage of marine fossils, mostly forms related to
oysters and clams. The Thaynes Limestone consists predominantly of
sandy, gray limestone and green-gray limey siltstones. The formation
weathers to a dark brown in the lower part and a grayish-yellow in the
upper. The fact that the Thaynes Limestone is exposed in the monument at
all is because of the faulting that took place just prior to the
deposition of the Evanston Formation of latest Cretaceous and earliest
Tertiary age (refer to section on structure) and the erosion of the
Tertiary rocks that once covered it.


                           Evanston Formation

Although not seen within the boundaries of the Fossil Butte National
Monument, the Evanston Formation is exposed just south of Highway 30N,
1.5 miles southeast of the southeast corner of the monument. The
Evanston was not involved in the complex folding and faulting but it is
somewhat disturbed and rests under the Wasatch Formation with angular
uncomformity. The Evanston Formation bridges the time boundary between
the Cretaceous and Tertiary periods. In the lower part of the unit are
found many fossil leaves, pollen, and spores