Alluvial-Fan Deposition and Tectonic Significance of Two Late Cretaceous-Paleocene Conglomerates in North-Central Utah Thrust Belt: ABSTRACT

Abstract

The synorogenic fluvial Echo Canyon and Evanston Conglomerates provide evidence for tectonic events and paleoenvironments near the end of the Sevier orogeny. The great thickness (950 m), coarseness, obscurity of stratification, polymodal texture, poor imbrication, and abundance of fluid-escape structures in the Echo Canyon suggest high discharge, large sediment load, rapid deposition, and steep gradient. Deposits identified as low-relief gravel bars, shallow channel fills, sieves, debris flows, and infills of abandoned channels are present. The Echo Canyon is interpreted as a shallow braided-stream deposit of the proximal part of a large, humid-climate alluvial fan, analogous to a glaciofluvial outwash fan. In contrast, the basal conglomerates of the Evanston show better sorting and rounding, stronger bimodality, overall finer clast sizes, well-developed imbrication, and better sandstone-conglomerate segregation. Recognized deposits include distinct traction and suspension loads, longitudinal and transverse gravel bars, side channels, and flood-plain (lacustrine?) deposits. Inferred deposition was by a graded, shallow braided river in a distal alluvial-plain setting. Coarsening and increase of less durable clasts, together with imbrication and cross-bedding measurements, indicate dispersal from sources 10 to 30 km to the northwest. Clast counts show that the Echo Canyon was derived from Jurassic to mid-Paleozoic quartzite, sandstone, and carbonate rocks. The Evanston was derived from distinctive Cambrian and Eocambrian quartzite and carbonate rock and minor Precambrian gneiss. This pronounced compositional contrast and an intervening unconformity provide evidence for major thrusting between deposition of the two formations. The Echo Canyon source involved deformed, parautochthonous rocks in advance of the Willard thrust sheet, whereas the Evanston was derived from the older rocks of the allochthon itself. End_of_Article - Last_Page 437------------