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Description
Up to 600 m (1970 ft) of non-marine Williams Fork Formation sedimentary rocks are exposed along the north-south trending Douglas Creek arch on the western edge of the Piceance Creek basin near Rangely, Colorado. Williams Fork deposits conformably overlie deltaic Iles Formation strata and are unconformably overlain by Eocene Wasatch Formation fluvio-lacustrine deposits. Williams Fork and lIes strata are interpreted to represent a prograding clastic wedge that advanced east-ward from the late Jurassic to early Late Cretaceous Sevier orogenic front, and onto the western shore of the Western Interior Seaway. An interval of Williams Fork deposits about 250 m thick (820 ft) that contains nine major sandstone bodies was studied. Sedimentary lithofacies recognized include sandstone, interlayered siltstone and mudstone, gray-brown claystone, and carbonaceous shale and coal. Sandstones have erosional bases, and are up to 1200 m wide (3370 ft) and 20 m thick (63 ft). Basal lag deposits include mudstone and siltstone rip-up clasts, bone fragments, and silicified logs. Sandstone beds are composed of very fine-to medium-size grains and show fining-upward trends. Sedimentary structures include large-scale trough cross beds, micro-trough cross laminations, horizontal laminae, and convolute bedding. Sedimentary structures decrease in size upward through sandstone intervals. Sandstone deposits are interpreted as fluvial channel, point bar, and crevasse splay deposits. Interbedded siltstone and mudstone deposits showing syndepositional dips of 5° to 10° are interpreted as lateral-accretion bedding, whereas similar deposits that are approximately horizontal are interpreted as flood deposits laid down proximal to active channels. Gray-brown claystone beds are also interpreted as flood deposits, based on their lateral extents, sharp bases, and horizontal nature. Carbonaceous shale and coal beds, and associated sandy lenses containing abundant snail shell casts, are interpreted as swamp and pond deposits. Paleocurrent directions from nine major sandstone trends were measured on trough axes, trough cross beds, parting lineations, flute casts, and ripple marks. Paleocurrent data show a shift in paleoslope direction upsection, with highly variable flow trends toward the southeast in the lower part of the section, and distinct flow trends toward the northeast in the upper part of the section. This is similar to the trend reported from Williams Fork-equivalent strata exposed on the east side of the Piceance Creek basin. There, the shift has been associated with the Laramide orogeny. This suggests that a reorientation in the paleoslope toward the northeast may have been synchronous throughout the Piceance Creek basin area. Thin sections studied show Williams Fork sandstones to be moderately sorted, subangular to subrounded chert and argillite litharenites, with argillaceous matrices, calcareous cements, and low porosities (1% to 5%). Sandstone mineralogy suggests a source terrane that was probably a recycled orogenic belt, with detritus derived mainly from sedimentary rocks. No significant change in mineralogy was detected through the section studied. Paleocurrent data suggest that at least some Williams Fork channels were highly sinuous (sinuosities of 1.5 or greater). Williams Fork streams probably meandered across a low, wet, well-vegetated alluvial plain. Typical stream channels would have been about 60 m wide (200 ft), 4 m deep (13 ft), and would have had average annual discharge rates of 55 m^3/s (2000 ft^3/s).
