Abstract

Cenozoic stream systems east of the Laramie Range in Wyoming and Nebraska and their headwater areas in the Laramie and Front Ranges, the Hartville uplift, and North Park basin are better defined by stratigraphic and geographic distribution of sandstone petrofacies in the Cenozoic High Plains sequence than by stratigraphic parameters or paleocurrent measurements. Light and heavy minerals are grouped into four distinct populations: a volcaniclastic sandstone petrofacies transported to the plains from distant volcanic eruptions, a plagioclase sandstone petrofacies derived from the anorthosite complex in the Laramie Range, a feldspathic sandstone petrofacies eroded from granitic and metamorphic rocks in the Front and Laramie Ranges and the Hartville uplift, and a rhyolite-bearing feldspathic sandstone petrofacies derived from Cenozoic volcanic rocks in North Park basin and from granitic and metamorphic rocks in adjacent mountains. Reconstruction of stream systems and their evolution is possible for the High Plains sequence because of distinctive crystalline rock types in the headwater area, lack of diagenetic alteration of detrital minerals in the sandstones, and lack of mixing of distinctive suites of the channel deposits. The depositional system consists of two major components: (1) coarse-grained stream-channel deposits and (2) widespread fine-grained eolian, wash, and flood deposits made up largely of pyroclastic material. The nature, distribution, and abundance of these components were controlled by the rate of air-fall of pyroclastic material in relation to the supply of detritus from crystalline rocks in the Front and Laramie Ranges and the Hartville uplift and by regional uplift, persistent parallelism of stream channels away from the mountains, semiarid to arid climatic conditions, and the shape of the drainage basins.

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