Abstract

The Gunnison Plateau shares form, altitude, and part of its stratigraphy with other elements of the Colorado Plateau province, but its facies changes, folds, and many faults are more representative of the Great Basin. A series of 14 cross sections details the structure along the east front of the Gunnison Plateau, and different interpretations of stratigraphy and tectonic history are set out in the captions.

Any hypothesis of cause and succession of tectonic events affecting the Plateau and Sanpete Valley must accord with the relations in these sections.

Despite the variety of stratigraphic units involved and the different structural relations, the sections show that the rocks of the Sanpete Valley block pressed both westward and upward against the Gunnison Plateau block, and produced folds, high-angle reverse faults, and imbricate sheets of rock sheared from the valley block. These rocks are mostly Jurassic beds beneath the Late Cretaceous unconformity, which unconformity was rotated in the compressive and shearing process.

Traditional explanations of the tectonic history of the Gunnison front have evolved from simple compression to the extrusion of a great fan-fold of Jurassic rocks in the Sanpete Valley block, with consequent shearing and bending along its west margin, followed by dropping of the Valley block (or elevation of the Plateau block) along the Gunnison fault. The cross sections may also support the idea of a diapiric antiform of Jurassic rock under the Valley block, one which has bulged and collapsed episodically to account for: a) changes of thickness and of facies in the Late Cretaceous and Paleogene rocks, and b) the structures that mark the east flank of the Gunnison Plateau.

The Gunnison front is exposed by post-Crazy Hollow (early Oligocene?) normal faulting that dropped Sanpete Valley and raised Gunnison Plateau, mostly in its northern part. It may have formed by collapse of a linear diapir of evaporites and mudstone.