Abstract

A detailed study of the west-central part of the Gunnison Plateau, Utah, has disclosed stratigraphic and structural relations important in the geological history of central Utah. The area mapped includes the eastern half of the Axtell No. 2 quadrangle, Manti area (U. S. Dept. of Agriculture, Soil Conservation Service).

The bedrock ranges in age from the Arapien shale (Upper Jurassic) to the Green River formation (Eocene). The North Horn formation (Cretaceous-Tertiary), the Flagstaff limestone (Paleocene-Eocene), and the Green River formation (Eocene) successively overlap the Arapien shale and the Indianola group in the northern part of the area. In the southern part of the area, the North Horn formation and the Flagstaff limestone successively overlap the Price River conglomerate with angular discordance. The latter relation establishes a post-Price River and pre-North Horn orogenic movement in central Utah.

A conspicuous west-dipping monocline, broken by many high-angle faults and a graben, extends along most of the western margin of the area. This structure is similar to the monocline and graben in the Wasatch Plateau to the east. At one point in the northern part of the area, the North Horn and the overlying Flagstaff oppose the Arapien shale across a high-angle fault. The Green River formation extends across the fault and overlaps the Arapien. This relation suggests faulting between Flagstaff and Green River time because the Colton formation, which normally occurs between the Flagstaff and Green River, cannot be differentiated in this immediate area.

Numerous small intrusive masses of monzonite porphyry of post-Upper Jurassic age occur in the Arapien shale.

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