Upper Campanian nonmarine sedimentary rocks exposed between the Wasatch Plateau and the Green River in central Utah record a tectonic

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transition from thin-skinned deformation in the thrust belt to basement-cored uplift in the foreland region. Thick sections of the Mesaverde Group in the Wasatch Plateau on the west and the Book Cliffs on the east are separated by the San Rafael swell, a basement uplift across which the group is erosionally thinned. Strata in the west (Castlegate Sandstone and Price River Formation) were deposited by east to northeast-flowing braided rivers. Time-equivalent eastern sections comprise a lower sequence of mixed braided fluvial deposits (Castlegate Sandstone and Bluecastle Tongue of Castlegate, coastal swamp and meander-belt deposits (Neslen Formation), and nearshore marine deposits (Buck Tongue of Mancos Shale and Sego Sandstone), and an upper sequence that coarsens upward from meander-b lt deposits (Farrer Formation) into pebbly braided river deposits (Tuscher Formation). Paleocurrent data indicate that rivers of the lower sequence flowed east, while those of the upper sequence flowed northeast.

Figure

Sandstones within the section consist of two distinct compositional suites, a lower quartzose petrofacies and an upper lithic petrofacies. The compositional boundary occurs at the top of the Bluecastle Tongue and can be correlated across the San Rafael swell. The quartzose suite contains mostly compositional quartzarenites and sublitharenites; the lithic suite is composed of litharenites and feldspathic litharenites. Lithic grain populations of the upper petrofacies are dominated by sedimentary fragments in sections of the Wasatch Plateau and volcanic fragments in sections near the Green River. The sedimentary lithic grains were transported generally eastward from miogeoclinal strata uplifted within the thrust belt. The volcanic lithic grains of the Farrer and Tuscher Formations were erived from more distal arc sources to the southwest, and transported through the thrust belt somewhere west of the Kaiparowits region, where time-equivalent sedimentary rocks are also rich in volcanic lithic fragments. Disappearance of volcanic lithics and appearance of pebbles at the top of the Tuscher Formation is interpreted to reflect a latest Campanian reorganization of drainage patterns that marked initial growth of the San Rafael swell and similar basement uplifts to the south of the swell. Contemporaneous fluvial systems that deposited the uppermost part of the Price River Formation in the Wasatch Plateau were apparently unaffected by the uplift and continued to flow northeast. Depositional patterns thus indicate that initial growth of the San Rafael swell was probably concurren with late deformation in the thrust belt. Depositional onlap across the Mesaverde Group by a largely post-tectonic assemblage of fluvial and lacustrine strata (North Horn Formation) indicates a minimum late Paleocene age for growth of the San Rafael swell and deformation within the thrust belt.

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