RECONSTRUCTING FLUVIAL MACROFORM ARCHITECTURE FROM 2-DIMENSIONAL OUTCROPS - EXAMPLES FROM THE CASTLEGATE SANDSTONE, BOOK CLIFFS, UTAH

The Castlegate Sandstone is part of the Upper Cretaceous (Campanian-Ma astrichtian) Mesaverde Group, a clastic wedge that prograded eastward from the Sevier Orogen of central Utah across the Western Interior Bas in. The unit comprises the lowstand part of a regional fourth-order st ratigraphic sequence. Earlier studies showed that the Castlegate Sands tone was deposited by braided rivers that deepened downstream from Pri ce to east of Green River. Outcrop mapping employing photomosaics to d ocument facies architecture, bounding surfaces, and paleocurrent infor mation 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 cha nnel orientation. Gradations between these end-member accretion geomet ries 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 counterpoi nt bar. Such bars are rare in braided systems. Accretion directions ar e determined relative to local paleoflow and are the basis for classif ication of bars as downstream- or laterally-accreted types. However, l ocal paleoflow may be oriented at a high angle to regional or total-ou tcrop mean flow direction, indicating locally high channel sinuosity. The results of this study indicate that megascopic heterogeneity in th e Castlegate Sandstone has a lateral dimension in the order of 100 m. It is suggested that this might be a suitable length dimension for sca ling of grid blocks in simulation models of petroleum production of ot her, similar sandy braided fluvial systems.