The Cretaceous Record in a Northeast-Trending Transect, Northern Utah to East-Central Wyoming

Abstract

Cretaceous sedimentary rocks in the Laramide basins of the middle Rocky Mountains include 16,600 ft (5060 m) of predominantly siliciclastic strata in the thrust-belt of northern Utah and 7800 ft (2380 m) of mainly siliciclastic and calcareous strata near the craton in east central Wyoming. Regional changes in the thickness of the strata indicate that crustal subsidence during the Cretaceous was generally greatest in northern Utah and western Wyoming where it was associated with tectonic and sediment loading. However, the considerable thickness of uppermost Cretaceous nonmarine beds in several other areas reflects pronounced basin subsidence during early stages of the Laramide orogeny. In a transect from northern Utah to east-central Wyoming, based on outcrop sections, borehole logs, and chronostratigraphic data, Cretaceous rocks grade northeastward from mainly fluvial and nearshore marine synorogenic conglomerate, sandstone, mudstone, coal, and bentonite to mostly nearshore and offshore marine sandstone, mudstone, calcareous shale, and bentonite. Lateral changes in the lithofacies and in the extent of enclosed unconformities indicate marine transgressions and regressions that were effected by structural deformation, sedimentation, and eustatic events. Significant unconformities have been found at the base of the Cretaceous strata, at two horizons within beds of Albian age, at two horizons withinmore » rocks of Cenomanian and Turonian ages, at one horizon within Coniacian strata, and at two horizons within Campanian beds. Most of these unconformities are either flooding surfaces or sequence boundaries.« less