Named; origin of name not stated, and no type locality designated. This formation occurs in the Marathon region, Brewster Co, western TX in Permian basin, where it unconformably overlies the newly named Lower Mississippian? Santiago chert. At base of Tesnus is the Rough Creek shale member (first used), 865 ft thick, of hard, compact and brittle, dark-green, occasionally black shale. Above are sandstone, shale, chert, and a few lenticular layers of conglomerate. The sandstones are from soft to highly indurated, thin-bedded to massive, predominantly dark, dirty green or rusty brown. The shales are dark, dirty green or black, well laminated when more clayey, roughly jointed when more sandy. Dense, dull-lustered chert occurs in thin layers and in black or brownish black in color. In eastern portion of Marathon region there is much more sandstone in upper portion of the Tesnus. In the northwest portion of the Marathon region near the line of the Southern Pacific RR [Brewster Co], it is likely that the Rough Creek shale member has disappeared, and the formation consists of thin-bedded greenish sandstones alternating with greenish and blackish clays with a few beds of dark-brown, finely conglomeratic limestone containing marine fossils. The only fossils found in the eastern portion of the formation were a few poorly preserved remains of land plants. Tesnus is 3,370 ft thick. Geologic map. Pennsylvanian in age.

Source: GNU records (USGS DDS-6; Denver GNULEX).