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In the Uinta basin of northeastern Utah, the Uinta and Duchesne River Formations are composed of extremely diverse fluvial sedimentary rocks. The rock units overlie extensive lacustrine deposits of Lake Uinta and provide a sensitive record of late Laramide (latest Eocene) tectonic events in this part of the Rocky Mountains. The fluvial deposits are dominantly heterogeneous, laterally discontinuous sandstone lenses and varied amounts of conglomerate and poorly stratified, fine-grained rocks.
Uplift of the Uinta Mountains changed geographic conditions and drainage patterns in the 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) thick volcanic ash deposits, now altered, accumulated during the quiescent period (middle Duchesnean) between major uplifts. This sequence of events imposes some constraints on inferred geomorp ic development of the region. The deposits also demonstrate that the latest Eocene (about 40 m.y. ago) was a time of major differential movement of the Uinta Mountains and the Uinta basin, and was not a time of tectonic quiescence in northeastern Utah.
Because of the great distance of the Rocky Mountains from the North American continental margins, detailed knowledge of local tectonic timing affords the best opportunity for plausible speculations relating plate tectonic mechanisms to Laramide events and to early development of the Colorado Plateau and the Basin-Range province.
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