Basal Mesaverde deposits exposed near Rangely Dome in northwestern Colorado indicate a progradation of deltaic sandstones over the marine Mancos Shale. Overlying these deposits is a thick nonmarine sequence containing four major facies: (1) extensive organic-rich siltstones with laterally discontinuous coals, (2) elongate lenticular cross-bedded sandstones, (3) thin (0.3 to 2.5 m) ripple-stratified, sheetlike fine sandstones, and (4) thick (maximum 11 m) fine-grained sandstones with large-scale foresets and syndepositional slumping.

Two fluvial facies models were developed on the basis of study of over 80 measured sections. Variations in the models are seen in the types of cross-bedded sandstones. Type 1 forms belts 80 to 130 m wide and up to 21 m thick. Major erosion surfaces delineate nearly symmetrical units 3 to 8 m thick. In contrast, type 2 lenticular sandstones belts are broader (600 m) and thinner (3 to 7 m). The component lenses are fewer in number, have a greater width to depth ratio, and commonly show evidence of lateral accretion. Individual units in type 1 belts show little vertical variation in grain size or scale of structures. Type 2 units commonly fine upward in both respects. The fine-grained sandstones flank the major belts and have paleocurrent indicators at high angles to the transport direct on of the coarser lenses.

The coarse-grained sandstones were major channels and the fine-grained sandstones were crevasse splays. The thin splays were deposited along levees or in shallow flood basins. The thick splays with foresets are associated only with type 1 belts. They were Gilbert deltas which prograded into interfluvial lakes. The type 1 belts, major interfluvial lakes, and Gilbert splays were associated with greater subsidence rates. This model is comparable to recently described anastomosing fluvial systems. Our other model is similar to a low-sinuosity fluvial system.

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