|Name||Barnett Formation, Chappel Limestone, Houy, Zesch, Bear Spring, and Stribling Formations, and Pillar Bluff Limestone, undivided|
|Geologic age||Late; Middle; and Early Mississippian and Devonian|
Sedimentary > Carbonate > Limestone (Bed)unit includes 7 formations, most of which are limestone, but some significant shale thickness in one formation (Barnett Fm.) and minor dolomite and phosphorite. Common chert in most units.
Sedimentary > Clastic > Mudstone > Shale > Black-shale (Bed)most of the shale appears to be found in the Barnett Fm., is eastern part of Llano 1:250,000 sheet
Sedimentary > Chemical > Phosphorite (Bed)
Sedimentary > Chemical > Chert (Bed)
|Comments||Description from Llano Sheet (1981) and Brownwood Sheet (1976), Geologic Atlas of Texas, from oldest to youngest, Pillar Bluff Limestone (Lower Devonina), upper part a coquinite of brachiopods, in fine- to coarse-grained matrix, yellowish-gray to ivory-colored, resting on fine- to very fine grained ls., gray, dull-yellow, or grayish-olive , sl. dolomitic, contains phosphatic granules, quartz, glauconite, and a few conodonts; occurs in small patches in an area of about 50 sq.ft., near Pillar Bluff Creek 5 mi SW of Lampasas. Stribling Formation (Middle to Lower Devonian)in area of type section chiefly limestone and chert, ls. microgranular, smooth fracturing, irregularly bedded, basal inch or two sandy, medium-gray to grayish-yellow, sl. fossiliferous; chert mostly along beds and in cross-cutting masses, may be absent for a few ft along strike; thickness 11 ft in type section. In Burnet County, in part dolomite, v. fine grained, chert similar to that in type area; patchy occurrence, thickness probably in no place over 20 ft. Bear Spring Fm.--granular, brownish- to grayish-yellow fossiliferous ls., cherty ls., and white to cream, fine- to coarse-grained ls., in part fossilif.; several tens of feet present in a collapse structure, present under Zech Fm. (lower Middle Devonian). Zesch Fm.--thin, highly siliceous ls., and leached silica rock, containsangular fragments of chert, brownish to yellowish, appears to greade upward to Ives Breccia, up to 2 ft thick; only positive ident. outcrop is in BearSpring area SW of Mason (Upper? Devonian). Houy Fm.--upper member, thin, brown phosphorite containing bone frags. locally in eastern part of Llano area; middle member Doublehorn Shale, black, fissil, pyritiferous, spore-bearing contined to eastern area; lower unit Ives Breccia, coarse angular chert breccia and fractured chert, with siliceous and some calcareous and phosphatic matrix, widely spread; all members, none, or any mbr. or combination of mbers may be present in any one place (mostly Upper Devonian, some highest Middle? Devonian, locally tothe west the transgressive Ives Breccia is Mississippian). Chappel Limestone. inequigranular with scattered pelmatozoan columnals, gray to brownish gray to east becoming coarser and pink to west (in Brownwood Sheet of Geo. Atlas of Texas), seldom more than a foot or two thick except in sinks where 20 ft or so may be trapped, absent in many places (Mississippian). Barnett Formation mostly shale, petroliferous, locally asphaltic, phosphatic beds contain sm. amt. uranium, dk. brown to blk., gen. recessive slope at foot of Marble Falls scarp, poorly exposed; some ls., includes thin beds of brachiopod-goniatite conquinite and thin beds and concretions up to 9 ft in diam. of microgranular ls., dk. brownish gray weathers brn., petrolif. odor when freshly broken, free oil occas. in interior cracks of concretions, goniatites common; thickness up to 40 ft,thins westward, locally absent or too thin to map (Mississippian).|
|Stratigraphic units||Doublehorn Shale, Ives Breccia (members of Houy Fm.)|
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.
|Counties||Brewster - Burnet - Lampasas - McCulloch - Mason - Presidio - San Saba|