Riley Formation showing Lion Mountain Sandstone and Cap Mountain Limestone Members, undivided

Riley Formation showing Lion Mountain Sandstone and Cap Mountain Limestone Members, undivided
State Texas
Name Riley Formation showing Lion Mountain Sandstone and Cap Mountain Limestone Members, undivided
Geologic age Middle Cambrian
Lithologic constituents
Major
Sedimentary > Carbonate > Limestone (Bed)Cap Mountain Limestone Mbr. (of Riley Fm.) is about 80 percent of the map unit
Minor
Sedimentary > Clastic > Sandstone (Bed)Lion Mtn. Sandstone Mbr (of Riley Fm.) is about 15 percent of the map unit.
Incidental
Sedimentary > Clastic > Siltstone (Bed)in Cap Mtn. Limestone Mbr. (of Riley Fm.)
Comments Upper two members of Riley Formation, Llano uplift, central Texas. Lion Mountain Sandstone and Cap Mountain Limestone, undivided. Lion Mountain Sandstone Member--quartzose greensand, glauconitic quartz-sandstone, impure limestone beds containing phosphatic brachiopods, and minor shale and siltstone; dusky-green to grayish-olive-green studded with white to yellowish-gray crossbeds of trilobite coquinite; thickness about 30 to 80 ft. Cap Mountain Limestone Member--in SE area fr. top down, limestone, siltstone and silty limestone, and mottled ls. grading to calcareous ss. at base; ls. is thick bedded, glauconitic, in part oolitic, various shades of light-olive-gray, moderate- to dark-yellowish-brown, yellowish-gray, and greenish-gray, mottles are dk-yellowish-orange; joints in lower ls. widened by solution causes distinctive pattern on aerial photographs; thickness about 175 to 500 ft, feathers out NW-ward by lateral gradation in lower part to Hickory Sandstone Mbr. In Cap Mtn. Ls. exceeds 500 ft to south in Kerr Co., south Texas. To NW across Llano uplift, the lower sandy unit of Cap Mtn. is replaced laterally by underlying Hickory Sandstone Mbr. of Riley Fm.
Stratigraphic units Lion Mountain Sandstone Member, Cap Mountain Limestone Member are upper two members of Riley Fm. of the Moore Hollow Group (new) (Barnes and Bell, 1977). The Moore Hollow Grp includes the Riley Fm. below and Wilberns Fm. above.
References

Barnes, V.E., and Bell, W.C., 1977, The Moore Hollow Group of central Texas: University of Texas at Austin, Bureau of Economic Geology Report of Investigations 88.

Bureau of Economic Geology, 1981, Llano sheet, Geologic Atlas of Texas, Bureau of Economic Geology, University of Texas at Austin, scale 1:250,000.

Bureau of Economic Geology, 1976, Brownwood sheet, Geologic Atlas of Texas, Bureau of Economic Geology, University of Texas at Austin, scale 1:250,000.

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.

NGMDB product
Counties Blanco - Burnet - Gillespie - Llano - McCulloch - Mason - San Saba