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In the Uinta Basin of northeastern Utah, the Uinta and Duchesne River Formations are composed of diverse fluvial sedimentary rocks. The rock units overlie extensive lacustrine deposits of Lake Uinta and provide a complete record of late Laramide (latest Eocene) tectonic events in this part of the Rocky Mountains. The fluvial deposits consist of heterogeneous, laterally discontinuous sandstone lenses with varying amounts of conglomerate and poorly stratified fine-grained rocks. Sedimentary structures and facies relationships indicate that the Duchesne River Formation was deposited mainly by relatively small, rapidly aggrading, southward-flowing streams. Most stream channels probably were braided, with high gradients and high velocities of flow.

Uplift of the Uinta Mountains changed geographic conditions and drainage patterns in Uinta Basin and strongly influenced the characteristics of contemporaneous sedimentary deposits. Important features of the stratigraphic sequence are: 1) the oldest major body of sediment (early Duchesnean) produced during uplift of the Uinta Mountains is considerably younger than the youngest preserved deposits of Lake Uinta (middle Uintan) ; 2) lower (early Duchesnean) and upper (late Duchesnean) conglomeratic rock units record two major episodes of uplift, each composed of several smaller events; and 3) volcanic ash deposits (now altered) accumulated during the time (middle Duchesnean) between major uplifts. Composition of the clastic rocks reflects the abundance of sedimentary source rocks in the Uinta Mountains. Recycled sedimentary material includes fragments of chert, carbonate and clastic sedimentary rocks. Much of the red pigment in the deposit is also derived from sedimentary source rocks. Altered volcaniclastic rocks probably originated in eruptive centers on the west, in the area of the present Wasatch Mountains or Basin and Range province. Differential movement of the Uinta Mountains and Uinta Basin during Duchesnean time represents a very late stage of Laramide tectonism. Volcanism in the Basin and Range province on the west may have begun during the Duchesnean. The latest Eocene and earliest Oligocene( ?), ~ 36-40 my BP, was thus a time of active tectonism in northeastern Utah.

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