The Fossil Basin is a small, linear and structurally controlled basin in
the southeastern part of the Wyoming overthrust belt. This “overthrust
belt” is represented by a number of small mountain ranges and high
ridges formed by the “thrusting” of sedimentary rocks over other
sedimentary rocks. Topographically, the Fossil Basin is bounded by the
Crawford Mountains and Tunp Range on the west, by Oyster Ridge on the
east, and by the Uinta Mountains on the south. The Crawford Mountains,
Tunp Range, and Oyster Ridge (Fig. 2) are areas of high relief developed
upon southerly extended salient ridges of deformed Paleozoic and
Mesozoic strata. In the center of the Fossil Basin, these earlier rocks
are covered by a veneer of early Tertiary sediments. Superficially, the
Fossil Basin appears to be a broad syncline with tilted beds dipping
sharply or gently basinward from the basin margins. The Tertiary
sedimentary cover, however, partially obscures what is a more complex
structural history.
Following deposition of the Late Cretaceous Adaville Formation, the
Fossil Basin was included in a period of intense structural deformation.
This deformation was the result of compressional forces acting along a
more or less east-west alignment. The strain, or the resolution of these
forces, was developed along a north-south alignment or perpendicular to
the compressional forces. As the stress level became too great, the
rocks were first folded and then faulted. Initial faulting of the rocks
relieved some of the stress; however, with continued application of
compressive forces many stages of folding and faulting were generated.
Because the alignment of the compressive forces remained about the same
throughout deformation (i.e., east-west) and because the strongest
relative compressive forces were from the west, successively younger
folds and faults were generated in an eastward direction.
The structural evolution of the Fossil Butte area may be interpreted as
follows from the schematic diagrams:
1. The Rock Creek-Needles Anticline is developed at the western edge
of the Fossil Basin. The present topographic highs developed on this
structure are the Crawford Mountains and Tunp Range (Fig. 9-B).
2. With continued application of compressional forces, Paleozoic and
Mesozoic rocks to the east of the Rock Creek-Needles Anticline are
folded (Fig. 9-C).
3. The fold becomes sharply asymmetrical and when the rocks can no
longer accommodate the compressional forces by further folding, a
low-angle fault is developed (Absaroka thrust fault). This fault
probably began as a fracture parallel to bedding which rose to
topographic surface at a low angle at the point of greatest strain in
the fold. Most likely, this fracture was an eastward extension of the
fault underlying the Rock Creek-Needles Anticline, as depicted in Fig.
9-D.
4. The rocks to the west of the newly developed fault overrode those
to the east. This was accompanied by further downwarping in the
syncline to the east of the fault and the erosion of uplifted
sedimentary rocks on the west side of the fault (Fig. 9-E).
5. Another cycle of folding and faulting was initiated to the east of
the Absaroka fault. This post-Absaroka deformation instigated further
downwarping of the Fossil Basin area between the Tunp-Crawford
Mountains and Oyster Ridge. Tertiary sediments began to accumulate in
the Fossil Basin (Fig. 9-F).
[Illustration: Fig. 9. Structural development of the Fossil Butte
area.]
LEGEND
T—Tertiary
K₂, K₁—Cretaceous
J—Jurassic
TR—Triassic
Pz—Paleozoic
X—Position of Fossil Butte
←, →—Relative movement along faults
[Illustration: A. Pre-deformation: Paleozoic & Mesozoic Formations]
[Illustration: B. Pre-Absaroka Deformation]
[Illustration: C. Development of the Absaroka thrust fault: folding]
[Illustration: D. Development of the Absaroka thrust fault:
faulting]
[Illustration: E. Development of the Absaroka thrust fault:
faulting]
[Illustration: F. Post-Absaroka Thrust Deformation]
[Illustration: G. Simplified structure section through present-day
Fossil Basin
(Oyster Ridge is off the diagram to the East)
Structural interpretation by Dr. D.L. Blackstone]
In early Eocene times, the effects of continued downwarping allowed
Fossil Lake to form in the Fossil Basin. By the late Eocene, however,
basinal subsidence could no longer keep pace with deposition and the
dominantly lacustrine (lake) sedimentation was replaced by a fluviatile
(riverine) sedimentary regime. Deposition of fluviatile sediments
probably continued into the later Tertiary when, in the late Pliocene,
regional uplift of the Rocky Mountain interior reversed the sedimentary
cycle from depositional to erosional. The last 3 or 4 million years of
geologic history have witnessed the excavation of much of the Tertiary
sedimentary fill from the Fossil Basin. Fossil Butte and the Ham’s Fork
Plateau to the northeast of the butte are high erosional remnants of
this early Tertiary basin fill. The traces of the Absaroka and other
thrust faults are buried beneath these remaining Tertiary deposits in
the Fossil Basin.
While most of this complex structural history took place before the
Tertiary sediments were deposited, some deformation continued into the
early Eocene. Near Prow Point, for example, is a fault that developed
during early Eocene time. This can be noted in the very northwest corner
of the monument, just to the west of Prow Point (Fig. 6). Here, the
Lower Member of the Wasatch Formation on the west is faulted against the
Main Body on the east (USDI 1964). This fault also is responsible for
elevating the Thaynes Limestone so it is exposed (Fig. 10).
[Illustration: Fig. 10. Structural relation of older rocks to
Wasatch and Green River formations near Prow Point in the northwest
corner of the monument (from U.S. Department of Interior 1964).]