ABSTRACT

The Southwest Sego Canyon 7½-minute quadrangle is located in the Book Cliffs of eastern Utah and contains an exposed sequence of Upper Cretaceous through Eocene rocks. Exposed units include rocks from the upper Mancos Shale through the undifferentiated Wasatch Formation and were deposited during the final regressive phase of the Mancos sea. They represent shallow open-marine, wave-dominated deltas, and lower through upper flood-plain depositional environments. Coal has been produced from seams up to 8 ft (2.5 m) thick, but reserves are largely undeveloped. Hydrocarbons have been produced from adjacent areas, and similar structural and stratigraphic traps may exist in the quadrangle.

The area is crossed by low-profile, north-northwest-trending folds. The Thompson anticline is a faulted, salt-movement produced fold. The Cisco dome is caused by minor Laramide adjustment on the Uncompahgre fault. The quadrangle overlies the Paradox basin margin and may have deeper Paleozoic related traps.

Evidence of several structural events exists within the stratigraphic sequence. The Farrer Formation thins over the nose of the Cisco dome and documents Campanian movement on the Uncompahgre fault. Tuscher Formation current directions shift from east to northeast, indicating initiation of uplift on the San Rafael swell. Overlying conglomerate beds follow a 15-m.y. erosional hiatus and show sufficient uplift on the Uncompahgre fault to expose Mississippian rocks. Ratios of sandstone to shale in Wasatch Formation show derivation from the Uncompahgre uplift and a gradual reduction of the Uncompahgre highland.

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