Cj Geologic Quadrangle Map

BUREAU OF ECONOMIC GEOLOGY Ill OF TEXAS -;;;;;;;;;;;;;;; THE UNIVERSITY AT AUSTIN 0.. -;;;;;;;;;;;;;;; 0u AUSTIN, TEXAS 78712 (/) ;;;;;;;;;;;;;;; 0.. CJ <C W. L. FISHER, Director ====- :E ;;;;;;;;;;;;;;; .J] - LJ"J ..J ;;;;;;;;;;;;;;; 0 .-'I w co Cl LJ"J - LJ"J .... - rn .... GEOLOGIC QUADRANGLE MAP NO • .-'I a - n.J Cl L- .... Illu -;;;;;;;;;;;;;;;= .... Geology of the Dunman Mountain Quadrang ;;;;;;;;;;;;;;; (') - ....0 Cl Llano, Burnet, and Blanco Counties, Texas By VIRGILE. BARNES r-·-··-i : : I ! I~ \ June 1978 The University of Texas at Austin To Accompany Map-Geologic Bureau of Economic Geology Quadrangle Map No. 44 GEOLOGY OF THE DUNMAN MOUNTAIN QUADRANGLE, LLANO, BURNET, AND BLANCO COUNTIES, TEXAS VIRGIL E. BARNES 1978 CONTENTS Page Page Introduction . 2 Hickory Sandstone Member . 5 General setting . 2 Cap Mountain Limestone Member . 5 Geologic formations . 3 Ordovician System (Lower Ordovician) . 5 Precambrian rocks . 3 Ellenburger Group . 5 Metasedimentary rocks . 3 Tanyard Formation . 5 Packsaddle Schist . 3 Staendebach Member . 5 Honey Formation . 3 Gorman Formation . 6 Sandy Formation . 3 Honeycut Formation . 6 Rough Ridge Formation . 3 Cenozoic rocks . 6 Click Formation . 3 Quaternary System . 6 Metamorphosed igneous rocks . 4 Pleistocene Series . 6 Orthoamphibolite (meta-gabbro) ........ 4 Terrace deposits . 6 Mafic dikes . 4 Recent Series . 6 Red Mountain gneiss . 4 Alluvium ...................... 6 Intrusive igneous rocks . 4 Subsurface geology . 7 Melarhyolite dike . 4 Mineral resources . 7 Aplogranites, aplites, pegmatites, Construction materials . 7 and quartz veins . 4 Dimension stone . 8 Town Mountain Granite . 5 Crushed stone . 8 Paleozoic rocks . 5 Road material . 8 Cambrian System (Middle Chemical stone . 8 and Upper Cambrian) . 5 Feldspar ........................... 8 Moore Hollow Group . 5 Water ............................ 8 Ci Riley Formation . 5 References . 9 £103\ c.5 IL\ GQ~~ C-,EoL MAP.5 cor.s 2 Bureau of Economic Geology, The University of Texas at Austin INTRODUCTION The Paleozoic rocks of Packsaddle Mountain McGehee's report (MS.) pertains primarily to were mapped by Barnes in 1949-1950; those in the Precambrian rocks, especially the stratigraphy of the southeastern part of the Dunman Mountain quad Packsaddle Schist. Pertinent exposures of various rangle were mapped by Barnes, assisted by units of this formation are present in the Cap A.R. Palmer, during 1947-1948. Precambrian rocks Mountain, Click, Dunman Mountain, and Kingsland in the northern part of this quadrangle and in quadrangles. McGehee is shown as coauthor of these quadrangle maps as his compilation was found to portions of adjacent quadrangles were mapped by fit neatly into the area of these four quadrangles. Barnes and L.E. Warren during 1943 in connection Publication of his manuscript was delayed until that with a feldspar study. Additional mapping of compilation could be placed on 7 .5-minute topo Precambrian rocks in the Dunman Mountain quad graphic quadrangle maps which were completed by rangle was done by S.E. Clabaugh, R.E. Boyer, and the U.S. Geological Survey during 1969-1970, and students, Department of Geological Sciences, The until a generalized map of the Precambrian could University of Texas at Austin, from about 1955 to be prepared to accompany his report. 1963. R.V. McGehee (1963b) compiled this map Maps of the four named quadrangles containing ping, which also contains his dissertation mapping McGehee's compilation are individually issued as (1963a), on a planimetric base at a scale of Bureau of Economic Geology Geologic Quadrangle 1:35,125, and subsequently submitted a manuscript Maps; this text concerns the geology of the to the Bureau of Economic Geology. Dunman Mountain quadrangle. GENERAL SETTING Dunman Mountain quadrangle is in the south Lyndon B. Johnson in the Kingsland quadrangle. A eastern part of the Llano region. The geology of small area in the southeastern corner of the the quadrangle is shown on a U.S. Geological quadrangle is drained by Slickrock Creek into Lake Survey 7.5-minute topographic quadrangle map; Lyndon B. Johnson in the Marble Falls quadrangle this is the tenth geologic map in the Llano region to the east. Other streams which drain directly into to appear on a modern 1:24,000-scale, 20-foot Lake Lyndon B. Johnson include Arant Creek, contour interval base. The relief in the quadrangle Sandy Branch, Spring Branch, Deer Hollow, and, is 597 feet; elevations range from 825 feet on the on the Burnet County side of the lake, Station surface of Lake Lyndon B. Johnson to 1,422 feet Creek and Dry Creek. on Packsaddle Mountain. Dunman Mountain quadrangle is structurally All of the quadrangle drains directly into Lake high on the southeastern side of the Llano uplift. Lyndon B. Johnson-almost half by way of Sandy Precambrian rocks occupy about 95 percent of the Creek and its tributaries Upshaw Creek and its quadrangle. The rest is occupied by Paleozoic rocks branch Lime Hollow, Ship Hollow, White Creek and a small amount of Quaternary deposits. and its branch Hogpen Hollow, and Walnut Creek Paleozoic rocks in the southern part of the and its branches West Walnut Creek, Larrimore quadrangle dip northward about 10 degrees, those Hollow, Buffalo Hollow, and Richards Branch. The in Packsaddle Mountain in three fault blocks dip next largest drainage area is that of Pecan Creek gently westward, and those in Dunman Mountain and its tributaries Cedar Branch and Dry Branch. dip southward about 5 degrees. The structure of The area in the vicinity of Packsaddle Mountain is the Precambrian rocks is discussed by McGehee drained by Moss Creek northward to Lake (MS.). Geology of the Dunman Mountain Quadrangle, Llano, Burnet, and Blanco Counties, Texas 3 GEOLOGIC FORMATIONS PRECAMBRIAN ROCKS years. This sample of Packsaddle Schist is indistin guishable in age from the granites of the Llano Paige (1911, 1912) was the first to establish a region. Field relations show that the Packsaddle is solid framework for the Precambrian geology of definitely older; therefore, the age shown by the the Llano region. In an excellent review, Clabaugh mica is related to the age of the igneous rocks and and McGehee (1962) outlined developments in the is not the actual age of the Packsaddle Schist. understanding of the Precambrian geology of the Llano region for the intervening 50 years. McGehee Sandy Formation (1963b) compiled student mapping in the south eastern part of the Llano region and gave The Sandy Formation, named and described by formation names to four subdivisions of the McGehee (MS.), is the second formation above the Packsaddle Schist established the previous year by base of the Packsaddle Schist. Except where Clabaugh and McGehee (1962). These names are interrupted by thrust faults in its southeastern formalized, and the geology of the Precambrian in part, the Sandy Formation generally strikes north the type area of the Packsaddle Schist is described westward and dips southwestward about 30 degrees. by McGehee (MS.). The following discussion of the Its outcrop width ranges from about 1.2 to Precambrian is mostly abridged from McGehee's 2.5 miles. The Sandy Formation is an alternation report. and intergradation of leptite and hornblende schist. In its type section the Sandy Formation is 2,290 feet thick. METASEDIMENTARY ROCKS Rough Ridge Formation Packsaddle Schist The Rough Ridge Formation, named and de Honey Formation scribed by McGehee (MS.), is the third formation from the base of the Packsaddle Schist. Its outcrop The upper three-quarters at least of the Honey width is about 1.6 miles; it strikes northwestward Formation crops out in the northeastern half of and dips about 30 degrees southwestward. The the Dunman Mountain quadrangle. The Honey Rough Ridge Formation is chiefly a gray, tough, Formation, the basal formation of the Packsaddle quartz-feldspar leptite and biotite-cordierite gneiss Schist named and described by McGehee (MS.), is of great uniformity. It also contains some muscovite on the nose of the southeastward-plunging Baby schist, biotite schist, and biotite-microcline gneiss. head anticline, and its structure is further com The leptite is distinctly different from the light plicated by overthrusting and by intrusion of the gray, light pink, and light brown leptite of the Granite Mountain pluton. Its outcrop width is underlying Sandy Formation. Excluding meta about 4 miles. igneous rock, the formation is 5,370 feet thick in The Honey Formation consists of hornblende its type section. schist, graphite schist (localities 3-12A, 3A-12A, 3A-14A), leptite, and marble (locality 3-3C) in its lower part; muscovite schist and some marble in Click Formation the middle; and graphite-hornblende schist and The Click Formation, named and described by some marble in its upper part. Other rock types McGehee (MS.), is the top formation of the present include biotite schist, calc-silicate rock Packsaddle Schist. Only the basal few hundred feet (locality 3-2B), and quartz-albite-muscovite schist of the formation, consisting of quartz-feldspar-mica containing garnets (locality 3-2A). In its type schist and hornblende schist, crops out in the section the Honey Formation is 8,290 feet thick. southwestern part of the Dunman Mountain quad One of the samples isotopically dated by Zartman rangle, but it is fully represented in the Click (1964) was collected from the Honey Formation quadrangle to the west. The Click Formation on within the Dunman Mountain quadrangle along the Click quadrangle is chiefly greenish-gray actino State Highway No. 71 just northwest of Sandy lite schist at its base overlain by light brown to Creek. The sample is a fine-grained, muscovite pink leptite and quartz-feldspar-mica schist followed quartz-feldspar schist in which the muscovite gives by hornblende schist. In its type section in the a rubidium-strontium age of 960± 35 million years Click quadrangle, exclusive of meta-igneous rock, and a potassium-argon age of 1040±10 million the lower part of the formation is 3,410 feet thick.

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