Abstract: Stratigraphic Evidence for Early Cretaceous Normal Faulting Over the Cache Valley Salt Structure, Paradox Basin, Utah

Brann Johnson, W. M. Aubrey

The Cache Valley anticline in Arches National Park is a salt-cored fold involving rocks as young as Late Cretaceous. The fold is breached axially by two overlapping half grabens of opposite polarity that have bounding faults with displacements up to 1000 m. Prior workers inferred faulting initiated in early Tertiary time, but local and regional stratigraphic studies of the Lower Cretaceous Cedar Mountain Formation suggest a possible earlier inception.

Compared with regional patterns, the Cedar Mountain Formation over the Cache Valley salt high is anomalously thick and contains several atypical, laterally continuous, lithofacies. Thicknesses of nine Cedar Mountain sections along the 7-km Cache Valley outcrop belt range from 72 to 101 m, averaging 89 m. These sections are two to three times thicker than the nearest Cedar Mountain sections on the flanks of the salt structure.

Systematic variations of thickness of the Cedar Mountain suggest a syndepositional structural sag existed over the Cache Valley salt high during Early Cretaceous. Subsidence due to salt dissolution seems unlikely, because a 700-m overburden covered the salt high formed during regional extension. Most notably, prominent stratigraphic thickening occurs in syndepositional half grabens developed over single-sided piercement diapirs, where the half-graben bounding fault merges with a flank of the diapir. The single-sided piercement model provides an explanation both for the Lower Cretaceous isopach patterns and for subsequent Tertiary fault geometries, which we suggest reflect reactivation and propagation of the earlier faults into the overlying Cretaceous strata. The possible Early Cretac ous regional extension implied by the single-sided piercement diapir model could reflect plate flexure induced by the Seiver Orogeny to the NW.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994