Abstract

The Woods Hollow Formation is a shale lithosome 400 to 500 feet thick that is part of the Ordovician sequence (2000 feet thick) in the Marathon geosyncline, Trans-Pecos, Texas. Estimated percentages of rock types in the formation are 65 percent shale, 16 percent quartzarenite, 15 percent calcarenite and 2 percent each of mudstone and conglomerate (including boulder beds).

The olive-gray illite clay-shale was intensely burrowed by soft-bodied organisms; the only skeletal remains are graptolites and sponge spicules. Shale was deposited in a marine basin below surf base, probably in water several hundred feet deep. Quartzarenite and calcarenite beds 1 to 6 inches thick are intercalated through the shale and are characterized by bedding structures (grading; laminations overlain by convolute laminations and current-ripple laminations) that indicate they were deposited by waning currents of initially high velocity; these layers are probably turbidites. About 20 percent of the beds have atypical turbidite structures and possibly were transported or reworked by other marine currents.

Siliciclastic and calcarenite debris (ostracods, pelmatozoans, oölites and pellets) were probably derived from Llanoria and a flanking carbonate platform to the east. Boulder-bearing shale and mudstone lenses contain clasts as much as 9 feet long. The coarse debris includes a variety of shallow-water and intertidal carbonate rock types and scarce sandstone, felsite porphyry, and schist. The boulder beds formed as plastic mass flows after the shelf to the west was uplifted and eroded to provide clasts that slid into the basins (Wilson, 1954a).

Several beds near the top of the formation contain dolomite of inferred deep-water origin.

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