The type section for this member is near the southeastern end of Fossil
Butte within the monument boundaries. In the type area the Fossil Butte
Member can be seen to consist of four lithologic units.
The lowermost unit is predominantly mudstone. It occurs in a sequence
about 45 ft thick and contains light gray, fine-grained, calcareous
mudstone and siltstone.
The next overlying unit is about 75 ft thick and is mainly a limestone
unit. It consists of tan to gray limestone, shaly limestone, siltstone,
and paper shale which weathers into thin, curled flakes. A yellow-brown
mudstone tops this unit.
The third unit in the sequence is 45 ft thick and mainly composed of
shales. These shales weather a buff color and are calcareous. Oil shale,
organic rich paper shale, and marlstone comprise the actual layers in
the unit. A few, thin ash beds are also present. This unit is the most
significant one for Fossil Butte National Monument. About 10 ft below
the top of this unit (about 155 ft above the Wasatch) is a bed of varved
shales, one foot thick, that contains the fossil fish for which the
monument was established.
The uppermost unit contains a number of beds of oil shale that are brown
on a fresh surface but weather a grayish-white. An
orange-yellow-weathering limestone caps the 40-ft thick upper unit. Ash
beds are common and traceable over a wide area of Fossil Basin.
South of the monument the Sandstone Tongue of the Wasatch Formation
wedges in between the lowermost unit and the overlying limestone unit of
the Fossil Butte Member.
In the Fossil Butte Member an interesting sequence of facies changes can
be seen which reflect lateral changes in the environment of Fossil Lake.
In those areas where the member represents a deep-water environment,
organically formed limestones and shales are predominant. Shoreward,
these rocks grade into ostracodal limestone, gastrapodal limestone, and,
closest to shore, algal limestone. Then a muddy, sandy beach facies is
encountered where the Green River and Wasatch formations intergrade.
The contact with the underlying Wasatch is conformable and sharp. It is
often marked by a bench or by slump blocks of Green River Formation.
The Fossil Butte Member may in some cases overlap the Wasatch Formation
and be deposited on Paleozoic and Mesozoic rocks. This is a reflection
of topographic relief during the Eocene.
The thickness of the Fossil Butte Member, as reported by Oriel and
Tracey (1970:37), is from 208 to 269 ft.