|Name||Hickory Sandstone Member|
|Geologic age||Middle Cambrian|
Sedimentary > Clastic > Sandstone (Bed)
|Comments||The Hickory Sandstone Member is basal member of Riley Formation (of the Moore Hollow Group of middle(?) to late Cambrian age. Riley Formation in Llano uplift (in Moore Hollow area of Riley Mtns., central Texas) is about 600 to nearly 800 ft thck; thickest in southeast Llano Co., thinnest in northwest corner of uplift. The Riley ovelries precambrian rocks and underlies Wilberns Fm. The Hickory, in NW part of Llano area is described on the Llano Sheet (Geologic Atlas of Texas, 1981) as follows, Upper part--medium to coarse-grained sandstone, exceptionally well-rounded grains, hematite-cemented, iron content averages about 10 percent, dusky-red; a scarp slopes down to a cultivated bench that centers on the contact between the red and the silty parts of the unit; thickness about 90 ft. Middle part--mostly fine to medium-grained ss., argillaceous, silty, commonly thinly bedded and micaceous; forms a low hilly scarp, thickness about 270 ft. Lower part--mostly fine- to coarse-grained ss, poorly sorted grains rounded to subrd., light-yellowish-gray; granitic (and quartz) (Mutis-Duplat, 1982) pebble conglomerate locally as much as 75 ft thick at base (Paige, 1912); forms gently rolling land, mostly irrigated using water from this unit, thickness 100 +-ft, varies owing to relief on the surface on which the lower part was deposited. All the upper and most of the middle units grade laterally southeast to Cap Mountain Limestone. Thickness of Hickory Ss. Mbr. 275-470 ft, locally missing where monadnocks on underlying Precambrian rock are present.|
|Stratigraphic units||Moore Hollow Group, Riley Formation|
Bridge, J., Barnes, V.E., and Cloud, P.E., Jr., 1947, Stratigraphy of the Upper Cambrian, Llano uplift, Texas: Geological Society of America Bulletin v.56, no. 1, p. 109-123.
Paige, S., 1912, Llano-Burnet, Texas: U.S. Geological Survey, Geologic Atlas of the United States, Folio 183.
Mutis-Duplat, Emilio, 1982, Geology of the Purdy Hill quadrangle, Mason County, Texas: Bureau of Economic Geology, the University of Texas at Austin, Geologic Quadrangle Map 52, scale 1:24.000.
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.
|Counties||Blanco - Burnet - Gillespie - Llano - McCulloch - Mason - San Saba|