The Frontier Formation in the Green River Basin of Wyoming, Utah,
and Colorado, consists of sandstone, siltstone, and shale, and
minor conglomerate, coal, and bentonite. These strata were
deposited in several marine and nonmarine environments during early
Late Cretaceous time. At north-trending outcrops along the eastern
edge of the overthrust belt, the Frontier is of Cenomanian,
Turonian, and early Coniacian age, and commonly is about 610 m
(2,000 ft) thick. The formation in that area conformably overlies
the Lower Cretaceous Aspen Shale and is divided into the following
members, in ascending order: Chalk Creek, Coalville, Allen Hollow,
Oyster Ridge Sandstone, and Dry Hollow. In west-trending outcrops
on the northern flank of the Uinta Mountains in Utah, the Frontier
is middle and late Turonian, and is about 60 m (200 ft) thick.
These strata disconformably overlie the Lower Cretaceous Mowry
Shale. In boreholes on the Moxa arch, the upper part of the
Frontier is of middle Turonian to early Coniacian age and
unconformably overlies the lower part of the formation, which is
early Cenomanian at the south end and probably Cenomanian to early
Turonian at the north end. The Frontier on the arch thickens
northward from less than 100 m (328 ft) to more than 300 m (984 ft)
and conformably overlies the Mowry. The marine and nonmarine
Frontier near the Uinta Mountains, marine and mnmarine beds in the
upper part of the formation on the Moxa arch and the largely
nonmarine Dry Hollow Member at the top of the Frontier in the
overthrust belt are similar in age. Older strata in the formation,
which are represented by the disconformable basal contact of the
Frontier near the Uinta Mountains, thicken northward along the Moxa
arch and westward between the arch and the overthrust belt. The
large changes in thickness of the Frontier in the Green River Basin
were caused mainly by differential uplift and truncation of the
lower part of the formation during the early to middle Turonian and
by the shoreward addition of progressively younger sandstone units
at the top of the formation during the late Turonian and early
Coniacian. The sandstone in cores of the Frontier, from boreholes
on the Moxa arch and the northern plunge of the Rock Springs
uplift, consists of very fine grained and fine-grained litharenites
and sublitharenites that were deposited in deltaic and
shallow-water marine environments. These rocks consist mainly of
quartz, chert, rock fragments, mixed-layer illite-smectite,
mica-illite, and chlorite. Samples of the sandstone have porosities
of 4.7 to 23.0 percent and permeabilities of 0.14 to 6.80
millidarcies, and seem to represent poor to fair reservoir beds for
oil and gas. The shale in cores of the Frontier Formation and the
overlying basal Hilliard Shale, from the Moxa arch, Rock Springs
uplift, and overthrust belt, was deposited in deltaic and
offshore-marine environments. Samples of the shale are composed
largely of quartz, micaillite, mixed-layer illite-smectite, kaolin,
and chlorite. They also contain from 0.27 to 4.42 percent organic
carbon, in humic and sapropelic organic matter. Most of the sampled
shale units are thermally mature, in terms of oil generation, and a
few probably are source rocks for oil and gas.
General
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