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Upper Jurassic (Oxfordian) sediments of Montana and Wyoming were deposited on a broad, shallow-marine shelf. Facies relations demonstrate that much of this shelf was characterized by a series of sand ridges separated by muddy swales. Ridges attain a thickness of about 12 m, an axial length of at least several kilometers, and a width measured perpendicular to crest less than 1 km. The crests of individual ridges are not parallel with one another or with the paleoshoreline. Analyses of directional features reveal multidirectional current vectors within individual ridges. The vertical sequence of internal sedimentary structures reveals that the ridges were constructed in distinct episodes, during which current flow was at least in the upper low-flow regime. Each episode is r presented in the rocks by a unique style, scale, and vector of cross stratification. Storms are considered to be the main process responsible for the buildup of ridges, individual storms being responsible for each of the sedimentation units of which the ridges are composed. As sand supply, storm intensity, and wind direction varied from storm to storm, so the style, scale, and vectors of the resultant cross stratification varied. Fair-weather processes were minor factors in the development of the ridge and swale system.
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