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Stratigraphy and Petrology of Buck Hill Quadrangle, Texas

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Stratigraphy and Petrology of Buck Hill Quadrangle, Texas BUREAU OF ECONOMIC GEOLOGY The University of Texas Austin 12,Texas — Report of Investigations No. 6 Stratigraphy and Petrology of Buck Hill Quadrangle, Texas By Samuel S. Goldich and Morris A. Elms Reprinted fromBulletin of The GeologicalSociety of America, Vol.60, No.7,!Ju1y,.1949 July, 1949 BULLETIN OF THE GEOLOGICAL SOCIETY OF AMERICA VOL. 60, PP. 1133-1182 6 FIGS., 6 PLS. JULY 1949 Stratigraphy and Petrology of the Buck Hill Quadrangle, Texas By. Samuel S. Goldich and Morris A. Elms Contents Abstract 1134 Chemical analysis 1159 Introduction 1134 Stratigraphic relations 1159 Location 1134 Duff formation 1159 Previous work..-. 1134 Definition and description 1159 Scope of field work 1135 Composition 1160 Acknowledgments 1135 Stratigraphicrelations 1161 Geography : 1136 Mitchell Mesa rhyolite 1161 Physicalfeatures 1136 Definition and description 1161 Erosionalagencies 1137 Petrography 1161 Climate and vegetation 1137 Chemical analysis 1161 Cretaceous system 1137 Stratigraphic relations 1161 General features 1137 Volcanic section in Elephant Mountain. .. 1162 Georgetown limestone 1139 Principal features 1162 Grayson clay 1139 Pruett formation 1162 Buda limestone 1140 Sheep Canyon basalt 1162 Boquillasformation 1141 Stratigraphic relations 1163 Tertiary system 1143 Quaternaryalluvium 1163 Buck Hillvolcanic series 1143 Intrusive igneousrocks 1163 Principalfeatures 1143 General statement 1163 Age and correlation 1143 Microsyenite 1163 Pruett formation 1145 Syenite of Buck Hill 1163 Definition and description 1145 Syenite of Straddlebug 1164 Basal conglomerate andsandstone 1146 Petrography 1164 Fresh-water limestone 1147 Chemical analysis 1164 Upper breccia-conglomerate 1148 Age andcorrelation 1164 Composition 1148 Butcherknife basalt 1165 Stratigraphic relations 1150 General features 1165 Crossen trachyte 1151 Petrography 1168 Definition and description 1151 Chemical analysis 1168 Petrography IIT2 Age andcorrelation 1168 Chemical analysis 1152 Igneouspetrology 1168 Stratigraphicrelations 1152 Structural geology 1169 Sheep Canyonbasalt 1153 General features 1169 folding description Cenozoic 1170 Definition and 1153 faulting Petrography 1154 Cenozoic 1170 Chemical analysis 1154 Walnut Draw and related faults 1170 Faults of central area 1171 Potato Hill andesite 1155 Chalk Draw andrelated faults 1171 Definition anddescription 1155 Elephant Mountain faults 1172 Petrography 1155 Physiography 1172 Chemical analysis 1156 Summary geologic history 1173 Stratigraphic relations 1156 Economic geology 1175 Cottonwood Springbasalt 1156 References cited 1175 Definition and description 1156 Appendix 1176 Petrography 1157 Detailed measured sections 1176 Illustrations Figure Page Plate Facing page 1.Indexmap of apart of Trans-Pecos 1135 1. Geologicmap of BuckHill quadrangle. 1133 2. Sketch map of a part of Trans-Pecos Texas showing principal physiographic 2. Panorama of northernBuck Hillquad- features and location of Buck Hill rangle 1146 quadrangle 1136 3. PruettandDuff formations 1147 3. Columnar section of the Buck Hill vol- canic series 1144 4. Cottonwood Spring basalt and Creta- 4. Cumulative curves representingtextural ceous formations 1154 analyses of sandstone samples from the Pruett formation 1150 5. Relationshipsof igneous rocks 1155 5. Variationdiagram ofBuckHillandsimi- lar igneous rocks 1169 6. Geologic reconnaissance map of the Ele- 6. Columnar sections athills4631 feet and phant Mountainarea,Brewster County, 4975 feet ". 1171 Texas 1182 1133 1134 GOLDICH AND ELMS— BUCK HILL QUADRANGLE, TEXAS Tables 1. Geologic formations in the Buck Hill 7. Chemicalanalyses ofthe basalflow of the quadrangle 1138 CottonwoodSpring basalt,Butcherknife 2. Chemical analyses of samples of tuff from the PruettandDuff formations. ... 1149 basalt, and similar rocks 1158 3. Bulk compositionand texturalconstants 8. Chemical analyses of Mitchell Mesa for samplesof sandstone from the Pruett rhyolite and other rhyolitesfrom Trans- formation 1151 PecosTexas 1162 4. Chemical analysis of Crossen trachyte.. 1152 5. Chemical analyses of Sheep Canyon ba- 9. Chemical analyses of syenite from Buck salt from Buck Hill and Alpine quad- Hill and other localities in Trans-Pecos rangles 1154 Texas , 1165 6. Chemical analysis of Potato Hill an- 10. Chemical analyses and norms of Buck [desite 1156 Hill rocks 1166 Abstract a middle down-dropped segment. Differential hardness of the rocks of the Buck Hill volcanic Tertiary tuff beds and related sediments with series controls the topography. intercalatedlava flows, aggregating over 2000 feet, arenamed theBuck Hill volcanicseries for outcrops Introduction in the Buck Hill quadrangle, Brewster County, Location Texas. The volcanic series rests unconformably ontheUpperCretaceousBoquillasformation. The The Buck Hill quadrangle, west-central Lower Cretaceousisrepresentedby the Georgetown Brewster County,Trans-Pecos Texas,is onthe limestone, the Grayson clay, and the Buda lime- southernfront of theDavis Mountains (Figs.1, stone. 2). It is reached most easily from Alpine, 30 The lower 900-1000 feet of the Buck Hill vol- miles north,by thehighway toTerlinguawhich canic seriesis named the Pruett formation, and the traverses the quadrangle fromnorth to south. upper 1000-1400 feet, theDuff tuff. Lavaflowsup Alternateroutes are fromMarathonby wayof 325 feet thick,designated the Spring to Cottonwood Del Norte Gap in the Del Norte-Santiago basalt, separate these formations. A limestone Mountain range and from Marfa by the 02 pebble conglomerate and arkosic sandstone at the ranchroadinParadiseValley. base of the Pruett mark the Cretaceous-Tertiary unconformity. The Pruett tuff commonly is cal- Previous Work careous and grades to beds of fossiliferous fresh- water limestone. A massive layer of breccia-con- The older literature, including the pioneer glomerate occurs near the top of the formation, work of Yon Streeruwitz (1890), Vaughan approximately 60 feetbelow thebaseof the Cotton- (1900),Udden (1904; 1907b), and others, isre- woodSpring basalt. Lava flows intercalated with viewedin The geologyofTexas (Plummer,1932 ; the Pruett sediments, from oldest to youngest, are Baker, 1934). The first petrographic work on trachyte, Sheep Canyon basalt, the Crossen the the rocks of the Davis Mountains was the Potato The volcanic and Hillandesite. Duff formation by on specimens by is chiefly silicic tuff with minor breccia and a few Osann (1892) collected Yon (1900) made petrographic thick beds of stream conglomerate. Itis overlain Streeruwitz. Lord by the Mitchell Mesa rhyolite, the youngest rock examinations of specimens collected by of the volcanic seriesin the quadrangle. Vaughan in the Tierra Vie]a Mountains. Inthe southernpart of the quadrangleStraddle- Muchof thegeologic knowledgeofTrans-Pecos bugMountain and Buck Hillare small intrusives Texasis based on theextensivefield work of C. of syenite;similar syeniticintrusives cut the Creta- L.Baker (1927; 1928; 1934; 1941; Baker and ceous, thePruett tuff, and the Sheep Canyonbasalt Bowman, 1917). King (1937) mapped the in the ElephantMountainarea. The igneous rocks and Monument Spring quadrangles analyzed Marathon are alkalic and chemically resemble the (Fig. 1). Eifler (1943) mapped the geology of the Terlingua-Solitario region to the rocks from the Santiago quadrangle adjoins south. Peak which Themainstructuralfeatures are west-andnorth- theBuckHill quadrangle on theeast. Albrit- west-trending normal faults. Two major fault ton and Bryan (1939) studied the Quaternary zones divide the quadrangle into three blocks with deposits of theDavis Mountains,including the INTRODUCTION 1135 northern part of the Buck Hill quadrangle. Hardin's map of the Elephant Mountain area Lonsdale (1940) studied the igneous rocks of the is reproducedin Plate 6. The basemap is an Terlingua-Solitario region in southern Brew- enlargement of the U. S. Geological Survey- ster andPresidio counties. topographicmap of the Alpinesheet, a 30-min- — Figure 1. Index Map of a Part or Trans-Pecos Texas Showing Buck Hill quadrangle (1), Elephant Mountain area (1A), Paradise Valley area (1B), and related areas of published maps: (2) MonumentSpring andMarathon quadrangles, King(1937); (3) Ter- lingua-Solitario region Lonsdale (1940); (4) Santiago Peak quadrangle, Eifler (1943). Scopeof FieldWork ute quadrangle (1:125,000) surveyed in 1893. The detailof this oldmap didnotpermit close Goodexposures ofthe Tertiary volcanicrocks a tie withthe BuckHillquadrangle,especially and the underlying Cretaceous strata in the on rugged slopes ofElephantMountain. Ad- Buck Hill quadranglemake this area favorable the field work for detailedstudies of theTertiarystratigraphy ditional was done by Goldlich in 1946. andpetrologyof thesouthernDavis Mountains. This investigationwas startedby Elms (1937) cknowledgments in the summer of 1936 and was continuedand A expandedby the writersin thesummerof 1940. The writersareindebtedto CharlesLaurence It became apparent that additionalmapping Baker for his stimulating interest and helpful and tracing of the rock units recognizedin the criticism. Mr.J.E.Wilsonassisted in the field BuckHillquadranglein adjacent areas wasde- work in 1936; R. W. Graves, Jr., and G. C. sirable. Reconnaissance mapping of the Ele- Hardin, Jr., in 1940; Hardin and D. W. phantMountain area in the southeasternpart Graham, in1941; and C. L. Seward,in 1946. of the Alpine quadrangle was undertaken by The paleontologic determinations contributed Hardin (1942), and of the Paradise Valleyarea bya number of geologistsindicatedin the text in the northeastern
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