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Exposures of the Sego Sandstone, on the northern flank of Rangely dome, Colorado, represent a thick (50 m [165 ft]) sequence of shoreline sandstone. The Sego Sandstone generally represents a progradational sequence. However, the interplay between sediment supply and subsidence resulted in stabilization of shoreline position. Deltaic distributaries were not observed, suggesting interdeltaic deposition.
The initial progradational phase within this sandstone complex is represented by three facies. (1) Basal facies composed of bioturbated shale with occasional silt stringers. (2) Medial facies, averaging 5 m (16 ft) in thickness, grading upward from the underlying shales, and composed entirely of ripple-stratified fine sandstone. Starved ripples at the base of this sequence grade vertically into flaser and amalgamated ripples. Channels filled with clay and ripple-stratified sand oriented south-easterly are present. (3) Upper facies is composed of fine-grained sandstone exhibiting basal hummocky cross-bedding that grades vertically into small-scale troughs and planer stratification. This facies is capped by a thin (35 cm [14 in.]) coal. Maximum observed thickness for this facies is 7 m 23 ft), with thinning toward the northwest.
Facies 1 represents the marine shale deposits of the Buck Tongue. Facies 2 is an ebb delta system believed to have functioned concurrently with the prograding shoreface (facies 3).
Stabilized shoreline conditions are characterized by four facies. (1) The basal unit is composed of fine to medium-grained sandstone. Trough and tabular cross-bedding is abundant, as is ripple stratification. Individual beds are 10 to 50 cm (4 to 20 in.) thick and the unit thickness is 10 to 20 m (33 to 66 ft). No apparent vertical trends with respect to either grain size or sequence of sedimentary structures were observed. (2) The second facies is fine grained, predominantly ripple stratified with some tabular and low angle bimodal cross-beds. Sand-filled channels with Ophiomorpha are present. This facies is 9 m (29.5 ft) thick and onlaps with a portion of facies 1. (3) The third unit is fine to medium grained, and as with facies 1, contains abundant trough and tabular cross-bedding s well as abundant ripple stratification. Paleocurrent data suggests a northwesterly transport direction for facies 3 as opposed to a southwesterly direction for facies 1. Bedding is between 10 to 50 cm (4 to 20 in.) thick with unit thickness of from 0 to 12 m (0 to 39 ft). This unit thins northwestward. (4) The upper unit thickens northwestward and is composed of bioturbated shales, humates, and very fine-grained sandstone.
Facies 2 represents a sand tidal flat situated behind the coastal barrier (facies 1). The lack of well-developed trends either in grain size or sedimentary structures within facies 1 indicates that the bar formed from the amalgamation of diverse environments. Overlying both the bar and tidal flat facies is the spit (facies 3) and lagoonal muds and washovers (facies 4). This shoreline sequence is overlain by fluvial deposits of the Mesaverde Group.
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