Abstract

Three distinct rock-stratigraphic units in the main body of the Wasatch Formation, below the lowest tongue of the Green River Formation, have been recognized and mapped near La Barge, Wyoming. These strata were formerly assigned to the Almy and Knight formations, which are not used here for rock in the Green River Basin. The basal unit, the Chappo Member, is unconformably overlain by the La Barge Member. The La Barge Member grades into and intertongues westward with an unnamed conglomerate member parts of which are also stratigraphically equivalent to higher parts of the Wasatch Formation. The Chappo Member rests directly on Paleozoic and Mesozoic rocks at all exposures examined along the margin of the basin. Vertebrates and mollusks from the Chappo Member are of latest Paleocene (Clarkfork) and earliest Eocene (Graybull) age. The La Barge Member of the Wasatch Formation directly underlies the Fontenelle Tongue of the Green River Formation and is separated from the Chappo Member below by an angular unconformity. Both members are similar in composition but they are separable locally by minor lithologic differences, as well as by the unconformity and structural attitudes. The La Barge Member is of Lysite(?) and Lost Cabin or middle(?) and late early Eocene age. The conglomerate member of the Wasatch Formation, which includes diamictite, sandstone, and mudstone, as well as conglomerate, is present only along the periphery of the Green River basin. It is laterally equivalent to the La Barge Member and to higher stratigraphic units. The conglomerate member is mainly late early Eocene (Lost Cabin), although parts may be older and younger. One inference drawn from these relations is that deformation, resulting in faulting and hydrocarbon migration, occurred in the La Barge, Hogsback, and Tip Top fields as late as early Eocene time.

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