TECTONIC AND SEDIMENTOLOGIC EVOLUTION OF THE UTAH FORELAND BASIN

Abstract

The Late Cretaceous foreland basin in central Utah developed adjacent to the Cordilleran fold and thrust belt between Albian and latest Campanian time. Subsidence resulted from the lithospheric 'oad of coeval thrust sheets to the west. Compositional trends of foreland-basin sandstones record unroofing of stratigraphic sequences above ramp-style detachment thrusts until the middle Campanian, when folding above a frontal blind thrust system caused recycling of previously deposited foreland basin detritus. Basement uplifts within the foreland basin terminated subsidence in latest Campanian time. Thrust loading created a westward-thickening basin in which the sedimentary wedge fines eastward. Coarse-grained synorogenic strata along the western edge of the basin are included in the Indianola Group, which consists of a lower marine-dominated sequence and an upper fluvial sequence. The marine sequence correlates with the marine Mancos Shale farther east, while the upper fluvial sequence is equivalent to the Mesaverde Group. Individual lithostratigraphic units are time-transgressive, becoming younger eastward. Eight distinct depositional facies are recognized in the Indianola Group: alluvial fan conglomerate, braided fluvial conglomerate, braided fluvial pebbly sandstone, meanderbelt fluvial sandstone and siltstone, delta distributary sandstone, lagoonal sandstone, siltstone, and mudstone, nearshore marine sandstone, and open marine mudstone and siltstone. The Mesaverde Group was deposited mostly by sandy to pebbly braided and meandering rivers which transported detritus eastward from the thrust belt. Facies in the basin combine to form an offlapping sequence of eastward-fining clastic wedges. Sandstones of the basin are quartzarenites, sublitharenites, and litharenites derived from the sedimentary source terrane of the thrust belt. Detrital carbonate grains are an important fraction of the sedimentary rock fragments that dominate the lithic population of the sandstones. Feldspathic litharenites high in eastern exposures of the Mesaverde Group were derived from an arc terrane lying beyond the thrust belt. Linear petrographic trends shown by triangular QtFL and QpLsLv plots resulted from mixing of detritus from multiple sources. The age of synorogenic deposits and their succession by a Maastrichtian to Paleocene overlap assemblage indicate that foreland basin subsidence and major thrust faulting were continuous from late Albian through late Campanian time in central Utah.