Abstract

The Castlegate Sandstone is part of the Upper Cretaceous (Campanian-Maastrichtian) Mesaverde Group, a clastic wedge that prograded eastward from the Sevier Orogen of central Utah across the Western Interior Basin. The unit comprises the lowstand part of a regional fourth-order stratigraphic sequence. Earlier studies showed that the Castlegate Sandstone was deposited by braided rivers that deepened downstream from Price to east of Green River. Outcrop mapping employing photomosaics to document facies architecture, bounding surfaces, and paleocurrent information has been used to reconstruct the geometry of bars (macroforms) and bar complexes in the Castlegate rivers. The most common bar types are those that accreted laterally and downstream relative to local channel orientation. Gradations between these end-member accretion geometries may be present within the same depositional element. In one case, curved accretion surfaces are concave in map view, with the concavity oriented in a downdip direction, suggesting an origin as a counterpoint bar. Such bars are rare in braided systems. Accretion directions are determined relative to local paleoflow and are the basis for classification of bars as downstream- or laterally-accreted types. However, local paleoflow may be oriented at a high angle to regional or total-outcrop mean flow direction, indicating locally high channel sinuosity. The results of this study indicate that megascopic heterogeneity in the Castlegate Sandstone has a lateral dimension in the order of 100 m. It is suggested that this might be a suitable length dimension for scaling of grid blocks in simulation models of petroleum production of other, similar sandy braided fluvial systems.

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