Smithwick Formation (TXPAsw;0)

State	Texas
Name	Smithwick Formation
Geologic age	Phanerozoic \| Paleozoic \| Carboniferous Pennsylvanian-Middle [Atoka]
Original map label	IPsw
Comments	(fr. Llano Sheet, Geol. Atlas of Texas, 1981) In type locality east of Marble Falls, claystone, siltst., and ss. Clayst. predom. laminated or burrowed, carbonaceous; illite with less than 15 percent quartz and muscovite silt; ss mostly contains rock frags. (sublitharenite of Folk) and minor but stratigraphically important sandy ls.-pebble conglomerate derived from the Marble Falls Limestone; sole marks and a wide variety of sedimentary structures present in sandstone; plant material abundant in ss and clayst.; fossils includesome plants, locally abdt. trilobites, brachiopods, and cephalopods, thickness about 300 ft. In Mason County, Smithwick consists of black shale with ls. beds in lower part, which is laterally gradational to Marble Falls Ls., lower boundary climbs stratigraphically westward.
Primary rock type	fine-grained mixed clastic
Secondary rock type	limestone
Other rock types	shale; sandstone; conglomerate
Lithologic constituents	Major Sedimentary > Clastic > Mixed-clastic > Siltstone-Mudstone (Bed) Incidental 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, 1981, Llano sheet, Geologic Atlas of Texas, Bureau of Economic Geology, University of Texas at Austin, scale 1:250,000. Cheney, M.G., 1940, Geology of north-central Texas: American Association of Petroleum Geologists Bulletin, v. 24, no. 1, p. 65-118. Plummer, F.B., and Moore, R.C., 1921, Stratigraphy of the Pennsylvanian formations of north-central Texas: University of Texas Bulletin 2132.
Geographic coverage	Burnet - Kimble - Lampasas - Llano - McCulloch - Mason - San Saba