Fluvial Transitions and Paleogeography in Upper Part of Maroon Formation (Pennsylvanian and Permian), Northwestern Colorado: ABSTRACT

Abstract

Six detailed lateral profiles (as much as 7 m thick and 50 m long) of fluvial channel sandstone bodies were measured at different locations in the northern part of the Eagle basin, northwestern Colorado. Contrasts in the grain size, internal geometry, and architecture of the sandstone bodies reveal significant facies changes across the basin. Proximal bodies that formed closest to the basin-margin ancestral Uncompahgre and Front Range uplifts consists of massive cross-bedded and flat-bedded coarse-grained to conglomeratic sandstone deposited mainly channel scour and fill and by vertical and lateral accretion on sand-gravel bars in low-sinuosity rivers. Medial sandstone bodies consist mostly of cross-bedded and flat-bedded fine to coarse-grained sandstone that was also deposited in low-sinuosity rivers on sand flats and in sandy bars. Bankfull discharge in proximal and medial channels was probably about 400 to 1000 m/sup 3//sec. The most distal sandstone body is composed of flat- and ripple-laminated, very fine to fine-grained sandstone that probably was deposited in a distributary area, in part by sheet floods. Overall facies changes indicate down-system decreases in competence and depth of flow. Paleocurrent data indicate that Maroon rivers draining the ancestral Uncompahgre uplift flowed northeastward across most of the basin before merging withmore » rivers draining the ancestral Front Range uplift. The depositional axis of the basin was therefore probably strongly skewed to the northeast during late Maroon time.« less