The rocks of the Pennsylvanian and Mississippian Tesnus Formation of the Marathon basin, Texas, have been attributed to deposition in a wide variety of environments, ranging from deep sea to subaerial, by various previous workers. Detailed studies of some strata in the middle part of the Upper Tesnus Formation in the southeastern part of the basin suggest that black carbonaceous shales and chert of the sequence represent deposition in a marine bay and that overlying carbonaceous shale, siltstone, and quartzose silty sandstone reflect bay fill-in deposits. Partially equivalent and partially overlying orthoquartzites and quartzose subgraywackes represent bar, back-bar, and fore-bar deposits. Graywackes are interpreted to be tidal-channel and tidal-delta sequence and associated minor subgraywackes represent local reworking of the graywackes by littoral drift and tidal currents.

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