Abstract

The Arikaree Group is an important constituent of the Tertiary sequence in the northern High Plains of the western United States. The "Upper Harrison" beds form a distinct formation-rank unit within the Arikaree Group. The bulk of the unit consists of air-fall pyroclastic debris; epiclastic detritus is only a subordinate part of the sediment. In the Agate Fossil Beds area, the "Upper Harrison" beds constitute a valley-fill sequence along the edge of a broad, shallow southeast-trending paleovalley. The valley fill has fluvial and eolian components. The fluvial component represents in-channel deposition within a sandy braided system. Large-scale trough cross-strata are the most common sedimentary structure in the fluvial deposits. The eolian component consists of unreworked ash-fall material. This sediment lacks current-produced sedimentary structures. In the Agate area, the fluvial and eolian components of the "Upper Harrison" beds differ from those in the subjacent Harrison Formation. Parallel laminae predominate in stream-channel deposits of the Harrison Formation and are associated with stream marginal dunes. The temporal change in fluvial style and in the character of associated eolian deposits observed between the Harrison Formation and the "Upper Harrison" beds suggests an increase in stream discharge with the passage of time and development of an effective vegetation cover. The latter may have resulted from the establishment of prairie grasses in the region. The absence of thick vertical accretion deposits is used to assign the fluvial facies of the "Upper Harrison" beds a braided-stream origin. The stratification sequence contains large-scale trough cross-strata but not large-scale planar-tabular cross-strata. The absence of planar-tabular cross-strata is atypical of stratification sequences of sandy braided systems.

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