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Late Cretaceous Paleogeography of the Southern Sevier Foreland, Southwest Utah, Southern Nevada, and Northwest Arizona

Late Cretaceous sedimentary rocks in southwest Utah, southern Nevada, and northwest Arizona were deposited in a continental setting, in the proximal reaches of the southern sector of the Sevier foreland basin. These strata record deformation in the Sevier fold and thrust belt to the west, and provide insight into the evolution of this part of the foreland basin. Sedimentologic and compositional data indicate that basal conglomerate throughout the study area was deposited on a regional pediment surface of Jurassic rock by eastflowing, bedload-dominated, braided fluvial systems. Abundant quartzite clasts suggest a provenance in the Wheeler-Gass Peak thrust system in the south, and the Wah Wah thrust in the north. In the northern part of the study area, at Three Peaks, Utah, northflowing, mudflow-dominated fan deposits that interfinger with east-directed, quartzose fluvial deposits are interpreted to reflect incipient movement on the Iron Springs thrust. Claystone, bentonitic tuff, finegrained sandstone, and carbonate that overlie the basal conglomerate indicate a change to low-energy fluvial and paludal environments. The preservation of these deposits in a proximal foreland setting suggests continued subsidence and sediment starvation. The last phase of Cretaceous basin evolution is marked by the eastward advancement of the thrust belt, and a change in regional drainage patterns. The change in' drainage from an earlier east-flowing, transverse system t'o a north-northeast flowing system, parallel to the orogenic front, is attributed to increased subsidence caused by thrust loading. Compositional data from sandstone and conglomerate indicate erosion of Paleozoic carbonate rocks suggesting that eastward thrust migration was manifested by development of the SummitWillow Tank and Muddy Mountain thrusts in the south, and the Blue Mountain and Escalante thrusts in the north.