Abstract

The Cretaceous rocks of the Uinta Basin, Utah, are exposed along the south flank of the Uinta Mountain anticline and are overlain unconformably by Tertiary sediments throughout this area.

The Dakota (?) sandstone of this paper disconformably overlies the Jurassic Morrison and is overlain conformably by the Mancos shale. It probably represents rocks of both Upper and Lower Cretaceous age.

The Mancos shale, as mapped and studied, comprises only a lithologic unit since in the Vernal vicinity it is equivalent in age and stratigraphic position to the Mesaverde group sandstones of the Tabby Mountain region to the west. The Mancos is divided into the following members, all easily identified throughout the Basin: lower shale member, Aspen shale, middle shale member, Frontier sandstone, and upper shale member.

The Mesaverde group refers only to a lithologic facies since at the west end of the area it is predominantly Niobrara in age, whereas at the east it is completely post-Niobrara. In the Vernal region it is divided into three members: the Asphalt Ridge sandstone, the Rim Rock sandstone, and the Williams Fork formation (contains more than type Williams Fork). The lower members of the Mesaverde group intertongue with the upper portion of the Mancos shale and wedge out toward the east.

The name Currant Creek is proposed for a sequence of conglomerate beds unconformably overlying the Mesaverde group in the Tabby Mountain region. This new formation is tentatively referred to the Upper Cretaceous but it probably represents the transitional strata deposited from late Upper Cretaceous to Eocene during the early phases of the Laramide folding.

The Uinta Basin is an asymmetrical syncline with its steep side along the flank of the Uinta Mountain anticline to the north from which numerous small noses and supplementary folds plunge southward. The Basin's structural history involves the following movements: local uplift in the west end in early Montana time; folding, progressively from west to east, during late Cretaceous to early Eocene; first uplift during post-Green River (Eocene)—pre-Uinta (Eocene); second uplift during post-Uinta (Eocene)—pre-Duchesne River (Oligocene); regional tilting in post-Duchesne River (Oligocene)—pre-Bishop (Miocene?); subsidence in post Bishop (Miocene?)—pre-Quaternary; continental uplift in late Tertiary or early Quaternary.

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