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GEOLOGIC QUADRANGLE MAP NO. 50 Geology of the Spicewood Quadrangle, Blanco, Burnet, and Travis Counties, Texas

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GEOLOGIC QUADRANGLE MAP NO. 50 Geology of the Spicewood Quadrangle, Blanco, Burnet, and Travis Counties, Texas I a> () .. • ,.; CJ) P< Q) Cfl .,.; +> Q) s::: • ..c: ;:so +> oo0 0 • Ct-I .. 0 CJ) '¢ ~ .,.; N 0 >. > .. r-i b.O (1j r-i 0 0 i..t Q) r-i (~ • 0 g 'd ~ () 0 s:::m .,.; (1j r-1 s 0 s::: 0 () µ.i Ct-I 0 BUREAU OF ECONOMIC GEOLOGY ~ (].) THE UNIVERSITY OF TEXAS AT AUSTIN l-1 ;:::! AUSTIN, TEXAS 78712 r.q • >. FlsREB, +> W. L. Director .,.; CJ) i..t Q) .,.;> s::: l=' GEOLOGIC QUADRANGLE MAP NO. 50 • fl) • Cl) ~ Q) • 8 ~ Geology of the Spicewood Quadrangle, Blanco, Burnet, and Travis Counties, Texas N By cj VIRGILE. [/") BilNBI Q., ~ ..J 0 U.l 0 0 V) -~ O' -~ 0 ... '<t mlli"' r- _ V) -~ u -M 0 "' '<t -0-"' 0 November 1982 THE UNIVERSITY OF TEXAS AT AUSTIN TO ACCOMPANY MAP-GEOLOGIC BUREAU OF ECONOMIC GEOLOGY QUADRANGLE MAP NO. 50 GEOLOGY OF THE SPICEWOOD QUADRANGLE, BLANCO, BURNET, AND TRAVIS COUNTIES, TEXAS Virgil E. Barnes 1982 CONTENTS Page Page General setting 2 Hensell Sand Member 6 Geologic formations 2 Glen Rose Limestone Member 6 Paleozoic rocks 2 Fredericksburg Group ... .... ... 7 Ordovician System (Lower Ordovician) 2 Walnut Clay ....... ......... 7 Ellenburger Group 2 Comanche Peak Limestone ........ 7 Honeycut Formation 2 Edwards Limestone ............ 7 Devonian System ...... ... ... .. 3 Cenozoic rocks ...... ....... .... 7 Quaternary System ............ .. 7 Stribling Formation .. .. .. 3 Pleistocene Series 7 Mississippian-Devonian rocks .. ...... 3 Colluvium 7 Crack fillings . ...... 3 Recent Series 7 Houy Formation 3 Travertine 7 Ives Breccia Member ... ....... 3 Alluvium 7 Doublehorn Shale Member 4 Subsurface geology 8 Mississippian System 4 Mineral resources ... ....... ....... 9 Chappel Limestone 4 Construction materials .. .. ... ..... 9 Barnett Formation 4 Dimension stone 9 Pennsylvanian System . ... .. ... 4 Crushed stone 9 Marble Falls Limestone .... .. .. 4 Road material 9 Smithwick Formation 5 · Sand and gravel 10 Mesozoic rocks . 5 Water 10 Cretaceous System (Lower Cretaceous) 5 References 10 Trinity Group 5 Appendix 11 L-i Travis Peak Formation 5 Sample descriptions . ...... ... .... 11 Sycamore Sand 5 Al Belanger, Jr. No. 1 Nella T. Evans 11 YO~\ Cow Creek Beds 5 E. J. Joost, Jr. water well ... ...... 12 Lr-> Shingle Hills Formation ... .... ... 6 Parker No. 1 Fee .. ..... .. .. .. 13 ..,. '-\ bu Su c,f(J\, of' ( 2 Bureau of Economic Geology, The University of Texas at Austin GENERAL SETTING The geology of the Spicewood quadrangle is mouth of Little Cypress Creek are extensively shown on a U.S. Geological Survey 7.5-minute folded with dips as high as 70 degrees. These rocks topographic quadrangle map. This quadrangle map described by Barnes (1948) as Ouachita facies is the sixteenth geologic map in the Llano region to rocks are included in the "Foreland facies rocks" appear on a modern 1 :24,000-scale, 20-foot con­ of the Ouachita structural belt described by Flawn tour interval base. The relief in the quadrangle and others (1961). The relatively flat-lying is about 870 feet; elevations range from 681 feet Cretaceous rocks dip eastward about 25 feet per (normal pool elevation of Lake Travis) to about mile. 1,550 feet on Shovel Mountain. Discussions of stratigraphic, structural, economic, Spicewood quadrangle is in the eastern part of and geophysical problems are in cited references. the Llano region, high on the eastern side of the Almost two-thirds of the area within the Llano uplift. Paleozoic rocks occupy about 9 square Spicewood quadrangle drains to the Pedernales miles; the rest of the quadrangle is occupied by River arm of Lake Travis by way of Cypress Creek Cretaceous rocks and some alluvium. The Paleozoic and its tributary Wallace Branch, and by Fall Creek rocks generally dip less than 10 degrees in an and a few unnamed drainages. The northern eastward direction, and in the northwestern part of remaining part of the quadrangle drains directly to the quadrangle faults mostly trending northeast­ Lake Travis by way of Little Cypress Creek and its southwest are common. The Smithwick rocks branch Sycamore Creek, Alligator Creek, Double exposed near water level of Lake Travis east of the Horn Creek, and a few short unnamed drainages. GEOLOGIC FORMATIONS PALEOZOIC ROCKS microgranular dolomite, and the rest fine- to medium-grained dolomite. The middle unit is ORDOVICIAN SYSTEM (LOWER ORDOVICIAN) about two-thirds microgranular dolomite and the rest is fine- to medium-grained dolomite and Ellenburger Group aphanitic limestone. The upper unit is mostly Honeycut Formation aphanitic limestone. The limestone in the lower unit is mostly light The main area of outcrop of the Honeycut yellowish gray to light gray and medium gray; in Formation is in the northwestern part of the Spicewood quadrangle. A small area of the upper­ the upper unit the color range is wider with most part of the formation crops out along Cypress yellowish gray and brownish grays more common. Creek near Cypress Mill in the southwestern part of The fine- and medium-grained dolomite is various the quadrangle. In the northwestern area three shades of medium to dark gray and some light subdivisions have been mapped: a lower unit of gray. The microgranular dolomite is in general alternating dolomite and limestone (upper part darker than the microgranular dolomite in the only cropping out); a middle unit, mostly dolomite; Gorman Formation. Lateral gradation between and an upper unit, mostly limestone. limestone and dolomite is fairly common. The Honeycut within the quadrangle is similar Chert is common in some beds of the Honeycut to its appearance in Honeycut Bend of Pedernales Formation, and a particularly distinctive type is River where Cloud and Barnes (1948, p. 314) the chalky "cannonball" chert, which mostly measured a section. Because that publication is out is fossiliferous and commonly is found associated of print, the Honeycut Bend section was reproduced with Archaeoscyphia. Chert elsewhere in the in the report on the "Geology of Pedernales Falls Honeycut is in part chalcedonic to subchalcedonic quadrangle, Blanco County, Texas" (Barnes, in and in part porcelaneous to semiporcelaneous. It is press b ). white to various shades of gray and brown. In the Honeycut Bend section, about half of the A fossil from locality 6-35A was identified by lower unit is aphanitic limestone, a quarter A. R. Palmer as Tritoechia? sp. Geology of the Spicewood Quadrangle, Blanco, Burnet, and Travis Counties, Texas 3 DEVONIAN SYSTEM Houy Formation Stribling Formation The following information concerning the Houy Formation is quoted from Cloud and others One small outcrop of the Stribling Formation is (1957): located about 500 feet south of State Highway 71 The lowest and presumably oldest deposits of the at a point 4,000 feet east of Double Horn Creek. Houy Formation are beds or pockets of siliceous limestone with chert fragments, or silty calcareous The outcrop is about 10 feet in breadth and not shale that occur locally beneath the Ives Breccia. more than 5 . feet of beds is exposed. Two units Next in order, and in most places the basal unit of are present: the upper one composed of cherty the formation, is the Ives Breccia Member, rarely as dolomite is unfossiliferous; the lower one is dolo­ much as 3 feet thick. The chert fragments or nodules mitic limestone which contains sand, glauconite, that constitute the bulk of the breccia at most places are locally derived, are freshly fractured or preserve phosphatic pellets, fish (?) spines, and conodonts the form of nodules, and are evidently little-moved (locality 6-44F). lag breccias that accumulated near or at the source of The chert is chalcedonic to subchalcedonic and supply in low places on the floor of the invading sea. is yellowish gray to light olive gray. A thin section The principal unit of the Houy Formation, the of the glauconitic basal Stribling from locality 6-44F Doublehorn Shale Member, is a black, fissile, radioactive (about 0.01 percent equivalent uranium), is dominantly dolomite of about 0.1 mm grain spore-bearing shale. size in a calcitic matrix that includes a few calcitic The upper or phosphoritic unit of the Houy fossil fragments. A few phosphatic fossil fragments Formation, 2 feet or less thick, contains fish bones and pellets are present. One pellet 1 mm in size that have been determined by Dunkle and Wilson contains a quartz grain and several dolomite (1952) to be Dinichthys cf. C. terrelli Newberry and an unidentifiable arthrodire. D. terrelli and its relatives rhombs. A diffuse area 1.3 mm across is composed seem to be restricted to the Ohio Shale of Late of abundant carbonate in an isotropic yellowish­ Devonian age and its equivalents. Devonian conodonts brown matrix which may be phosphatic. Silt is have been found also in some of the phosphatic scarce and glauconite is very scarce. Thin sections beds. The same or closely associated beds, however, from four other localities of glauconitic basal contain Sedenticellula aff. S. hamburgensis (Weller) and other brachiopods considered indicative of earliest Stribling north of the Spicewood quadrangle Mississippian (Cloud and Barnes, 1948, Pl. 44, are similar to the one described in that all are figs. 12-16; cf. Plummer, 1950, Pl. 5, figs. 13a-b) as dolomitic, phosphatic, silty, and glauconitic. The well as Early Mississippian conodonts. Thus the Stribling Formation of the Llano region and its phosphoritic beds also seem to be partly Devonian and correlation have been discussed by Barnes and partly Mississippian, or to include extraneous fossils from one or the other source. Some beds are others (1945, 1947a). radioactive. Although phosphoritic or highly phosphatic beds MISSISSIPPIAN-DEVONIAN ROCKS seem everywhere to be lower Kinderhook, uppermost Devonian, or both, scattered phosphatic pellets are Crack Fillings likely to occur anywhere in the Houy Formation, including the Ives Breccia Member. Crack and joint fillings, denoted by the symbol, The type section of the Houy Formation and +D(?), are common in the upper 20 feet of the the Doublehorn Shale Member is along Burnam Honeycut Formation.
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