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The coal-bearing upper carbonaceous member of the Wepo Formation (Upper Cretaceous) at Black Mesa in northeastern Arizona represents the landward segment of a regressive-transgressive sequence deposited along the southwestern margin of the Western Interior foreland basin. A detailed field investigation of the unit near the northeastern escarpment of Black Mesa resulted in the recognition of six lithofacies (Facies A through F). Facies A includes scour-based, predominantly trough-crossbedded sandstones that are interpreted as distributary, fluvial, and crevasse channel deposits. Facies B consists of rooted, heterolithic sandstone, siltstone, and mudstone sequences that are interpreted as levee deposits. Facies C includes sharp-based, predominantly ripple-laminated sandstones and siltstones that are interpreted as crevasse splay deposits. Facies D consists of rooted mudrocks that are interpreted as well-drained marsh or backswamp deposits. Facies E includes coals, carbonaceous shales, and organic mudrocks that are interpreted as poorly drained marsh or backswamp deposits. Facies F consists of burrowed sandstones and mudrocks that are interpreted as fluvial or interfluvial pond, interfluvial lake, or interdistributary bay deposits.

Vertical trends in the distributions and spatial relations of depositional subenvironments suggest that the upper carbonaceous member can be vertically subdivided into lower, middle, and upper stratigraphic intervals, representing deposition on a deltaic plain, backswamp-dominated alluvial plain, and lake-dominated alluvial plain to marginal marine coastal plain, respectively. The vertical succession of depositional environments records a deltaic to fluvial sequence produced by regression of the shoreline followed by a fluvial to marginal marine sequence produced by a rising base level associated with transgression. The depositional model developed from the vertical succession assists in defining depositional controls on both coal occurrence and geometry and suggests that the thickest and most laterally extensive coal deposits formed on a backswamp-dominated alluvial plain during regression and early transgression and possibly during a stillstand of the shoreline, with the proliferation of vegetation in protected, poorly drained interfluvial backswamps.

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