LARAMIDE TECTONICS AND HUMID ALLUVIAL-FAN SEDIMENTATION, NE UINTA UPLIFT, UTAH AND WYOMING

Laramide uplift and erosion of the Uinta Mountains are recorded in a 1 0-km-long outcrop of the Lower Eocene Wasatch Formation, on the Utah-W yoming border. This 750-m-thick package of interbedded sandstones and conglomerates is dominated by a coarsening then fining-upward megasequ ence 650 m thick that records the growth and abandonment of a humid al luvial fan system during a major cycle of uplift and unroofing of the thrust-bounded northern flank of the Uinta mountains. Grain size, thic kness, and lateral extent of channel-complex deposits increase upward in the lower 400 m of the sequence, reflecting construction and northw ard progradation of the fan. Grain size and channel-complex thickness decrease upward in the upper 250 m of the sequence, reflecting gradual reduction of both sediment yield and sediment caliber during postorog enic lowering of source-area relief. Within the megasequence, coarseni ng-up sequences 10-100 m thick built mainly of channel-complex deposit s reflect progradation of fan lobes, punctuated by periodic fan-head a vulsions. These medium-scale sequences in turn comprise small-scale fi ning-up cycles 1-10 m thick that reflect such fluvial processes on the fan as bar building, discrete flood events, and the filling and later al migration of braided-stream channels. Both the medium-scale and sma ll-scale sequences are commonly underlain by paleosols. Evidence of de bris flows or other mass-movement processes is conspicuously lacking. Above the main megasequence a second, thinner megasequence containing lacustrine mudstones and wave-reworked conglomerates indicates that th e fan persisted as a locus of coarse clastic deposition during the fir st of a series of lacustrine transgressions that began as ratios of se diment flux to subsidence rate decreased toward the end of the Early E ocene. Partial inundation of the fan during this transgression transfo rmed it into a fan delta.