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In the Powder River basin of Wyoming, Upper Cretaceous Shannon Sandstone shelf sand ridges are formed along the crest of a broad, subtle, southwest to northeast-trending paleoarch. During Shannon deposition, relief on the arch was great enough to alter shelf energies and cause sand ridges to develop within a predominantly silty shale interval.
Possible recurrent movement in the Salt Creek anticline created a paleohigh which strongly localized development of thick sand ridge complexes in the Shannon Sandstone. During Shannon deposition, relief on the paleohigh apparently was strong enough to cause ridges to build laterally as well as vertically. Shannon ridge complexes at Salt Creek are more oblate, bigger, thicker, and more closely spaced than most central Powder River basin ridges. Also, there are two vertically stacked ridge systems developed within the Shannon Sandstone. While the lower ridge system is coeval with the Shannon ridge system in the central basin, the upper ridge system is only developed locally and, we believe, is related to active growth on the paleohigh during Shannon deposition. At no time, however, did he paleohigh cause ridges to be subaerially exposed.
Eleven Shannon shelf ridge and ridge-associated facies were defined in outcrops on the Salt Creek anticline. Vertical and lateral changes in facies are relatively abrupt where observed in closely spaced outcrop sections and, in general, facies are stacked in coarsening-upward sequences with central bar facies commonly immediately overlying interbar sandstone facies. Porous and permeable potential reservoir facies include: central bar facies, a clean, cross-bedded sandstone; bar margin facies (Type 1), a highly glauconitic, cross-bedded sandstone containing abundant shale and limonite (after siderite) rip-up clasts and lenses; and bar margin facies (Type 2), a cross-bedded to rippled sandstone. These facies were formed by sediment transported and deposited in the form of medium to larg -scale troughs and sand waves on and across the tops of ridges by moderate to high-energy shelf currents.
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