Valley Spring Gneiss

Valley Spring Gneiss
State Texas
Name Valley Spring Gneiss
Geologic age preCambrian (Proterozoic)
Lithologic constituents
Major
Metamorphic > Metasedimentary > Paragneissparagneiss; dominantly light-colored acidic, contains some altered ls. and bands of basic dark material. Grades, by transition zone, into overlying darker basic Packsaddle Schist. (in east part of Llano 1:250,000 sheet) top down-pink quartz-feldspar gneiss, well-foliated, augen gneiss near top, thickness 130 ft; gray quartz-feldspar-biotite gneiss, 250 ft thick; pink quartz-feldspar gneiss, moderately to poorly foliated, some marble-wollastonite layers west and south of Llano, thickness 8,000 ft, base not exposed. In west part of Llano 1:250,000 sheet (Purdy Hill quadrangle) at least 15,000 feet of Valley Spring Gneiss composed of, from top down, 1) quartz-muscovite schist, 150 ft; 2)biotite-rich quartz-feldspar schist and gneiss 700 ft thick; 3) quartz-feldspar gneisswith mica or amphibole, locally migmatite, 1,500 to 2,500 ft thick; 4) epidote-rich quartz-feldspar gneiss with amphibole or biotite, 800 ft thick; 5) quartz-feldspar- muscovite gneiss with alternating layers of quartzite, thickness 1,400 ft; 6) quartz-feldspar-mica gneiss, as thick as 4,200 ft where other facies absent; 7) biotite- or hornblende-rich quartz-feldspar gneiss and schist with some layers of marble, calc-silicate gneiss, and amphobolite, thickness as much as 4,200 ft where other facies absent. Thickness of units 4 through 7 inclusive about 4,200 ft. Additional units beneath those in Purdy Hill quadrangle (units 4 through 70 are exposed in Fly Gap quadrangle: pink and green gneiss characterized by abundant green amphibole, thickness as much as 2.200 ft; fine-grained quartz-feldspar gneiss, thickness as much as 4,600 ft.; gneiss with lenses of green acicular chlorite in a matrix of quartz and albite, 250 ft thick.; quartz-muscovite-hematite schist, 350 ft thick, a basal assemblage of schists, gneisses, and migmatites with abundant biotite and iron-titanium oxides, thickness 2,200 base not exposed.
Minor
Metamorphic > Metasedimentary > Metacarbonate > Marblesome marble-wollastonite layers west and south of Llano
Comments Llano Series (from the Wilmarth Lexicon, USGS Bull. 896); Grenville Series Central Texas (Geol. Map of Texas, 1992, Bur. Econ. Geol.) Lower fm. of Llano series, also called Llano Group (Mutis-Duplat, 1982) in Llano Mason, Gillespie, and Burnet Counties.
References

Wilmarth, M.G., 1938, Lexicon of geologic names of the United States: U.S. Geological Survey Bulletin 896, 2396 p.

https://pubs.er.usgs.gov/publication/b896

Paige, S., 1911, (Title unknown): U. S. Geological Survey Bulletin 450, p. 14-23.

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

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.

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