ABSTRACT: Upper Cretaceous Calico Bed, Southwestern Utah; Sedimentology, Sea Level and Paleotectonics

Margaret C. Bobb, Thomas A. Ryer

The Calico bed of the Straight Cliffs Formation is a coarse-grained fluvial deposit that spread across southwestern Utah during late Turonian-early Coniacian time. The sheet-like Calico has an average thickness of 30 ft, is composed of coarse-grained sandstone and sandy conglomerate, and rests with a sharp contact on fluvial mudstones and claystones. Paleocurrents flowed to the northeast. In the northeast area of its outcrop, where subsidence was greater, the Calico is up to 120 ft thick and consists of multilateral, multistoried sheet-sandstone complexes. An upward decrease in grain size, different suites of sedimentary structures, and different alluvial architectures in the complexes record rising base level during Calico deposition, the end of which is marked by a tran gressive lag. This rising base level is probably correlative with a regionally rising sea level in the Cretaceous seaway during late Turonian-Coniacian time. The regional geometry of the Calico suggests basinal rebound/uplift, or cessation of subsidence, and broad fluvial reworking of the surface as sediment bypassed to the northeast. The internal sedimentology of the Calico records the influence of sea level. The system shows many similarities with the Vail et al. sequence stratigraphy model. However, the base of the Calico lacks the relief of an incised valley associated with either a Type I or Type II unconformity. Rather, the distribution of the Calico shows a strong control by tectonics in the foreland basin.

AAPG Search and Discovery Article #91002©1990 AAPG Rocky Mountain Section Meeting, Denver, Colorado, September 16-19, 1990