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Pen Formation

Pen Formation
State Texas
Name Pen Formation
Geologic age Phanerozoic | Mesozoic | Cretaceous-Late
Comments Equivalent to upper part of Austin Chalk. mostly clay, gray, yellow, calcareous, and sandstone beds in upper part. lower 50 ft contains inch-thick chalk beds, concretions throughout, marine fossils throughout, Exogyra ponderosa common; 219-700 ft thick in Big Bend N.P., 1,000 ft thick in Terlingua area, mapped in south Brewster County.
Primary rock type clay or mud
Secondary rock type sandstone
Other rock types
Lithologic constituents
Sedimentary > Clastic > Mudstone > Claystone (Bed)
Sedimentary > Clastic > Sandstone (Bed)
Map references
Bureau of Economic Geology, 1992, Geologic Map of Texas: University of Texas at Austin, Virgil E. Barnes, project supervisor, Hartmann, B.M. and Scranton, D.F., cartography, scale 1:500,000
Unit references
Bureau of Economic Geology, 1979, Emory Peak--Presidio Sheet, Geologic Atlas of Texas, University of Texas at Austin.
Counties Brewster - Presidio