ABSTRACT

Within a sedimentary basin, the analysis of sandstone petrofacies defined by detrital modes of framework grains is a powerful means to detect areal or temporal changes in provenance related to evolving paleotectonics and paleogeography. In the central Utah foreland, the regional tectonic transition from thin-skinned Sevier thrusting during the Late Cretaceous to deep-seated Laramide deformation during the Paleogene is recorded by four contrasting petrofacies in uppermost Cretaceous and Paleogene sandstones of fluvial and deltaic facies. Coeval subaqueous environments were marine during the Cretaceous and lacustrine during the Paleogene. The Cretaceous quartzose petrofacies (Qm93, F1, Lt6) of mainly Campanian age was derived from sedime tary sources in the active Sevier thrust belt, and transported into a broad adjacent foreland basin by transverse paleocurrents. The heterogeneous Cretaceous quartzolithic petrofacies (Qm40-70, F5-20, Lt25-40) of uppermost Campanian age was derived from the Sevier orogenic belt during the waning phases of Sevier tectonism, and was distributed by longitudinal paleocurrents within a foreland basin whose floor had begun to be deformed by incipient Laramide tectonism. The Paleogene feldspathic petrofacies (Qm50, F35, Lt15) in Paleocene and Eocene strata is composed of arkosic detritus that was derived principally from basement rocks then exposed in the extensive San Luis uplift of Laramide age in south-central Colorado. The feldspathic sand was transported to the Laramide Uinta Basin and vicinity in Utah by a trunk river which flowed generally northwestward between monoclinal Laramide uplifts of the Colorado Plateau. The Paleogene quartzolithic petrofacies (Qm75, F2, Lt23) of dominantly Eocene age was derived mainly from sedimentary sources in the Laramide Uinta uplift to the north, but also in part from similar sources in the dormant Sevier thrust belt to the west. In the Rocky Mountain and intermountain regions, more widespread and systematic use of petrofacies analysis, in combination with standard paleocurrent and lithofacies analyses, could lead to better definition of the timing of Laramide uplift and improved understandi g of the evolution of complex Laramide paleogeography.