Stratigraphy and Petrology of Buck Hill Quadrangle, Texas

BUREAU OF ECONOMIC GEOLOGY The University of Texas Austin 12,Texas

— Report of Investigations No. 6

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 volcanic 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 part of the Jordan Gap aregratefully acknowledged. quadrangleby Graham(1942) duringJune and A grant from the Penrose Bequest of The July 1941 under the supervision of Goldich. GeologicalSociety of America defrayedexpense 1136 GOLDICH AND ELMS— BUCK HILL QUADRANGLE, TEXAS of a part of the field work and of most of the of a thick accumulation of volcanic tuff with chemical work which was done in the Rock numerous intercalated lava flows. The sue- AnalysisLaboratory of the Universityof Mm- cession is pierced by many igneous masses.

— Figure 2. Sketch Map of a Part of Trans-Pecos Texas Showing Principal Physiographic Features and Location of Buck Hill Quadrangle

nesota by arrangement with Frank F. Grout Lateorogeny resultedinbroadopenfoldsinthe This generous co-operationis appreciated. Tertiary volcanicsandinfaults witha displace- Field work in 1946 and the preparation of ment of as much as 1000 feet, but the physio- themanuscript weredoneunder the auspicesof graphic developmentof the region was largely the Bureauof Economic Geology, The Univer- controlledby differential erosion. Greater re- sity of Texas, and valuable suggestions and sistanceofthe flowsandintrusives toerosionre- criticism were received from staff members. sultedin mesas and inprominentpeaks. Thispaperisoneof a seriesreportingresultsof a Some of the most picturesque mesas of the Trans-Pecos Texas project sponsored by the southern front of the Davis Mountains extend Bureau of Economic Geology. into the northern part of the Buck Hill quadrangle. In thenortheasterncorner (PI. 1) Geography is thesouthern tip ofElephantMountainwhich dominates the region withits great bulk and al- PhysicalFeatures titude of over 6000 feet above sea level. A The Davis Mountains (Fig. 2) in northern massive caprock of microsyenite accounts for BrewsterandPresidio counties werecarvedout this large erosional remnant. West of Ele- GEOGRAPHY 1137 phant Mountainis CrossenMesa capped by a Climateand Vegetation and in the northwestern thick flow of trachyte, The Buck Hill quadrangle with an average corner of the quadrangleisMitchellMesa with annual rainfall of less than 15 inches is in the of its rhyolite caprock rising the southern tip driest climaticprovince of Texas. Mostof the South of these 5351 feet above sea level. rainfall comesduringthehotsummermonthsas mesas are lower ones capped with basalt. thunder showers of short durationbut often of face Green Valley which Their southern scarps greatintensityand sometimes accompaniedby occupies the central and southwest partof the violentwinds. Temperaturesduringthedayin quadrangle. On southeast, Creta- the hills of the summer commonly are above 100°, but the ceous limestone rise above the alluvium of nightsarecool. The averagemeanannualtem- Valley. nearly horizontal Creta- Green The perature forBrewster County is approximately ceous stratacontinue eastwardinthe Santiago 63°. quadrangle wheretheyhave beensharply Peak The sparsevegetationincludesagreatvariety thrust-faulted in the Santiago upturned and ofyuccas,cacti,and otherhardyplantsadapted Mountain range. to the semiarid climate. The relative abun- Green Valley was formed during an earlier danceof certainplanttypesshows a response to erosion cycle, and the alluvium of the valley physical factors. On valley flats sagebushand now is being dissected and removed by the creosoteare abundant. growths of oco- of the quadrangle Dense streams. The westernpart tillo characterize upland flats of volcanic tuff is drainedbyTerlingua Creek, and the eastern and breccia and are common on weathered partby the westernmosttributaryofMara villas slopes of andesite,basalt,and certainlimestone Creek known here as Calamity Creek. The formations. Rough hills and slopes have con- drainagedividein thequadrangleis poorly de- siderablesotol and lechuguilla. Ravines with fined,and the surface water maypass from one better than average supply of water may be drainage system into the other; yet the two choked with catclaw, and occasionally cotton- streams join the Rio Grande apart 50 miles woodtreesarefound,as at CottonwoodSpring, (Fig. 1). The streams have cut deep steep- Bunch grass is plentifulon the mesas but rela- sided channelsin the alluviumof Green Valley tivelyscarce onmany of the valleyflats. and haverenewedattackon thebedrock. Inthe southernpart of thequadranglesyeni- Cretaceous System intrusives form StraddlebugMountain and tic GeneralFeatures Buck Hill. In the southeasterncorner prominent scarps are formedby the Cretaceouslime- The Cretaceous formations (Table 1) repre- stones. sented in the Buck Hill quadrangle are the Georgetownlimestone, of which onlythe upper ErosionalAgencies 127 feet is exposed,the Graysonclay(Del Rio), Thechief erosional agency is running water. the Buda limestone, and the Boquillas forma- Although theannualrainfallis small,muchof it tion,whichwaseroded in part prior to thedep- comes as torrential downpours. Sheet washis osition of the Tertiaryvolcanics. The Creta- effective on the steep slopes. Undermining, ceous formations in this part of Trans-Pecos sapping, and marginalfragmentationwear the Texas are readily differentiated lithologically, more resistantrocks such as the igneous flows but in comparison withnorth and south Texas which cap the mesas. In thisprocessthe verti- the section is not well known, and there are cal joints,which permit downward seepage of many stratigraphic andpaleontologicproblems. water favoring weatheringand spalling, are an TheGeorgetownformationis a thick-bedded, important factor in the wasting and recession commonly massive limestone. The Grayson of the cliffs. Although the wind appears to ismainlyclayandshale with afew thincalcare- blowconstantly andoccasionallyreacheshurri- ous sandstone beds near the top of the forma- cane velocity,itis of secondaryimportance;the tionwhich containabundantHaplostichetexana scanty vegetation of the region is much more (Conrad), a large foraminifer. The Buda is effective against wind than against water ero- chiefly thick-bedded,fine-grained,gray to white sion. limestone with a nodular marly middle unit 1138 GOLDICH AND ELMS— BUCK HILL QUADRANGLE, TEXAS dens< fresh- cal- lowei thinl; unconsoli- flows; porphyritic with in nodulai sandstom arkose, andesite. flags middle more remarks porphyry. basalt and Limestone with calcareous limestone; and (Pleistocene?) conglomerate. nonporphyritic. intercalated basalt, top.limestone thin part. gravel. rhyolite conglomerate, near and with Lithology older and and trachyte, eroded. limestone upper medium-grained shale in and silt pink limestone; shale unit. shale breccia, to breccia, of thick-bedded to amygdaloidal; surface part. marly and layers. Recent dated Gray Tuff, Fine- to Tuff, waterflows Upper careous Alternating Thick-bedded Clay Massive,bedded to Vi- equiva- part Square beds, volcanic etc. quadrangle in series, Ford Correlation eja series, Hill Probablylent Chisos Peak Austin Eagle Buda Grayson Georgetown Buck feet (?) the in in ± 150 formations Thickness 30 38- 1000-1400 50-325+ 900-1000 210-300 65-68 60-76 127+ Geologic— basalt flows:Canyon Boquillas rhyolite andesite 1. Spring SheepHill with Table Mesa intercalated formation tuff and trachyte,Potato included and Alluvium Mitchell Duff Cottonwood tuff and Buda Grayson Georgetown Group Pruett Crossen basalt, Undivided, Q_, to o en

Age Cretaceous Cretaceous uaternary (Oligocene?) ertiary (Eocene?) Upper Lower CRETACEOUS SYSTEM 1139

which commonly is fossiliferous. The lower Grayson formation, which grades upward to a part of the Boquillas is typically thin argilla- shell bed, 2-4 feet thick, composed almost en- ceous limestone flags separatedby shale. The tirely of the ramshorn oyster, Exogyra arietina upperpartislargelyfine-grained,evenlybedded Roemer, and of sandy marly clayin which this limestone withbeds of marlor calcareous shale fossil is abundant. In theupper 50 feet of the becoming thicker and more numerous near the GeorgetownnearStoneTank, andinparticular topof the formation. in the upper 38 feet of relatively thin-bedded limestone,fragmentaryfossilremains are abun- GeorgetownLimestone dant, but few forms are identifiable. The Thelargerof the twooutcropsof the George- brachiopodKingenawacoensis (Roemer) is com- upper feet and extends about15 town formationin the Buck Hillquadrangleis moninthe 38 Exogyra in the southeasterncorner (PL 1) just east of feet lower. arietina Roemer extends the Stone Tank and continuing eastward in the from the Grayson downabout 8-10 feet in Santiago Peak quadrangle. Here, along the Georgetown. Onespecimenof Trigoniasp.was Chalk Draw fault the Georgetown makes a found8 feetbelowthe top of the formation,and prominent northward-facingcliff 100 feet high broken specimens ofPecten and Gryphaea were of massive limestone, except for the upper 38 collectedintheupperzone about2miles eastof feet which is distinctly bedded. Near Stone Stone Tank. The upper part of the George- Tank amaximumexposedthicknessof 127 feet towncontains aMain Streetfauna,but lack of of theGeorgetownwasmeasured. The outcrop identifiable fossils in the lower thick-bedded is up to1mile wide,and the nonresistant Gray limestone precludes correlation with other fau- sonformationhasbeenstrippedfrommost ofits nalzones described by Adkins (1927, p. 50) in surface exceptfor asmallhillwhichisprotected the Fort Stockton section where alternating by a cap of the Buda limestone. The second limestoneand marlbedsof Georgetownequiva- exposure is on an east-west fault southeast of lent have a total thickness of 245 feet. In Elephant Mountain where a narrow strip of southern Trans-Pecos Texas limestonereplaces Georgetown limestone crops out. much of the marlof the Fort Stockton section, The Georgetown limestone is gray to and the resultant massive reeflike limestone brownish gray and weathers to a dirty gray or whichis inpart equivalentto the Georgetown white,rough,andetchedsurface. Potholesare formation (Washita group) and inpart to older common alongcreekbeds, andthe bareflatsur- formations,KiamichiandEdwards (Fredericks- faces are covered with tinajitas or etched pot- burggroup), was named the Devils Riverlime- holes (Udden, 1925). Beds in the lower part stone (Udden, 1907a). Recent work in areas of the exposed formation are 10-15 feet thick, east of theBuckHillquadrangleshows that the and these thick layers make the formationone fossiliferous gray to brown Kiamichi clay can of the mostresistantintheareaand formverti- be recognized,and the Devils River formation cal cliffs and overhangingledges. The upper can besubdivided. 38 feet of the limestone is nodular, thinner- Grayson Clay bedded, and fossiliferous. The texture ranges from fine- to coarse-grained. Fossil fragments The largest outcrops of the Grayson (Del which are numerous and scattered throughout Rio) formationare in the southeastern partof the limestonearerecognizableby their crystal- the quadrangle. A good exposure is readily line calcite. The lower 8 feet contains some accessible just east of the Alpine-Terlingua chertinthe vicinity of StoneTank. highway south of the junction with the road to Only the upper contact of the Georgetown Marathon. The Grayson is 76 feet thick here limestone is exposed,and near the baseof Ele- andis capped withthick beds of theBudalime- phantMountainthe contact with theoverlying stone. East of Stone Tank the formation is Grayson is gradational. A 4-foot thickness of about 70 feet thick,and its outcrop can befol- the Georgetownis overlainby nodular, marly, lowedeastward into the Santiago Peak quad- thin-beddedlimestone, the "Acanthoceras cun- rangle. The bedsdip less than2° to thesouth. ningtoni zone" (Adkins, 1932, p. 388), of the Southeast of Elephant Mountain the Grayson 1140 GOLDICH AND ELMS— BUCK HILL QUADRANGLE, TEXAS

is exposed in the scarp of a low southward- Texas Grayson fauna. Three fossil localities facing cuestacappedby Buda limestone. The are shown on the map: (1) near the base and outcrop is complicatedbynumerous faultsand southeast of Elephant Mountain, (2) in minorfolds,but the average dip is about3° to CalamityCreek,4|milessouthof theKokernot the northeast. In the channel of Calamity ranchhouse,(3) at thesmallhill(3970 feet) If Creek 4f miles south of the Kokernot ranch miles east of Stone Tank. The ammonite Tur- house there are small outcrops wheremuch of rilitesbrazoensis Roemer occursnear the baseof the Grayson clay is eroded from beneath the the formation. Exogyra arietina Roemer Buda limestoneleavingthe latter as an over- ranges 30 feet up from the base but was not hangingledge. Here the dip is approximately foundin theupperhalf of the formation. The 4° N. The Grayson is about 60 feet thick in arenaceousforaminiferHaplostichelexana(Con- the northernpartof thequadrangle. rad)is abundant throughout, especially in the The Grayson formation is dominantly clay- sandy flagstones. About 2 feet below the top shale but also contains thin flags of limestone of the formation at the Elephant Mountain and sandstone. The clays usually are light locality is a conspicuous shell bed, 4 inches buff,tan,orlightgreenishyellow,weatheringto thick, which contains abundant Exogyra cart- acharacteristictawnyyellowwithlocallyabun- ledgeiBose,GryphaeagraysonanaStanton,Exo- dant secondary gypsum and limonite. The gyra whitneyiBose,Ostrea sp., and Pectenfrag- flags are most abundant near the top of ments. Heteraster inflatus (Cragin) is abun- the formationandaredarktolightgray,usually dantintheupper part ofthe formation. very sandy, nodular and lenticular; a bed 6 A large collection of dwarfed limonitic am- inches thick may disappear laterally withina monites and echinoids was made at the Stone distanceof 50feet. Themiddleandlowerparts Tank locality,but this collectionhas not been are usually covered with slumped clay,so that studied. Most of the fossils listed were col- fresh exposures arerare. The detailed section lected in eitherthe lower6 feet orupper20 feet of the upper part measuredinCalamity Creek of the formation: (Appendix,SectionNo.1) is typical of the for- Haplostiche texana(Conrad) mationinthecentral and southernpartsof the Holectypus cf. limitis Bose quadrangle. At the foot of Elephant Moun- Heteraster inflatus (Cragin) tain (Appendix,Section No. 2) and elsewhere Protocardia texana(Conrad) Inoceramus spp. in the northern part of the Buck Hill quad- Pecten (Neithea) budensis Kniker rangle,the upper part of the Grayson contains Exogyra arietinaRoemer relatively cartledgeiBose moresand. whitneyiBose The contact of the Grayson with the under- Gryphaea graysonana Stanton lyingGeorgetownrarelyis exposed,butat Ele- Ostrea sp. "Acanthoceras" sp. aff. A. cunningtoni Bose phant Mountain thelower contact is conform- Turrilites brazoensis Roemer able and gradational. The contact with the Engonoceras sp. overlyingBuda is an undulatingsurface with a BudaLimestone sharpbreakfromclayof the Grayson tonodular limestone of the Buda. In the Buck Hill Theprincipaloutcrops of theBudalimestone quadrangle there are no marked indications of are along Calamity Creek north of the Walnut an unconformity,but elsewhereinTrans-Pecos Draw faultand inthe southeasternpart of the Texas the Grayson formation is variable in quadrangle south of the Chalk Draw fault* thickness and locally absent. East in Val The Budaalso isexposed southeast of Elephant VerdeCounty inroadcuts onU.S.Highway90, Mountain and on the northwestflank of Strad- 1.3 miles east of Comstock, L. W. Stephenson dlebug Mountain in the southwesternpart of found included in the lower beds of the Buda thequadrangle wheretheCretaceousrockshave limestone cobbles and boulders derived from been domedby a small syeniteintrusive. Ero- Exogyra arietina shell layers of the Grayson sion has stripped the less resistant flags of the formation. overlying Boquillas leaving wide flat surfaces Fossils collected from the Grayson in the of the white togray fine-grainednodular Buda Buck Hillquadrangleare typicalof the south limestone. Commonlya heaviergrowthofoco- CRETACEOUS SYSTEM 1141 tillo serves to differentiate the Buda outcrops Nautiloidsp. Turrilites sp. from the Grayson and Boquillas. Although in Budaiceras spp. placestheBudaresemblestheGeorgetownlime- Graptocarcinus texanus Roemer stone,itslightercolor andmoredistinct bedding generallyidentifyit. BoquillasFormation The Buda of this area commonly has three All the strata above the Budalimestone and lithologic facies:upper andlowerunitsof thick- below the basal Tertiary conglomerate in the bedded,light-gray, fine-grained to porcelaneous Buck Hill quadrangle are tentatively assigned limestone anda middleunitoflight-graytobuff to the Boquillas formation. Clayey and sandy marland thin-beddednodular limestone. The limestone flags alternating withbeds of shale upper and lowerunits withbeds 4-6 feet thick are typical of the lower part of the formation. form cliffs or steep slopes; the middle unit This succession is similar to the beds above the weatherstoagentleslope. AtElephantMoun- Buda limestone in southern Brewster County tain the Buda is 65 feet thick with a middle namedby Udden (1907b, p.29-32) the Boquil- marlyunit of 16 feet. InCalamityCreek the las flags. Theupperbeds aremainlylimestone middlemarly unit is 20 feet thick. Although with somemarlandshale whichmaybeequiva- thesection(Appendix,SectionNo.1) measured lenttopart ofUdden's (1907b,p. 33) Terlingua in Calamity Creek is incomplete,it is repre- beds. How much of the Boquillas is Eagle sentative of the Buda lithology. Two miles Ford and how much Austin equivalentcannot east of Stone Tank the formation is 68 feet be said; zoningor redefining of the Boquillas thick,and themarly unitmeasures18 feet. must await more detailedstudies. The Buda generally is fossiliferous with the Large areas in the eastern part of the quad- middlemarly unit affording the best material. rangle are covered by the Boquillasformation Good fossil localitiesalongCalamityCreek are (PL 1). South of the Chalk Draw fault the indicated on the map (PL 1). The echinoid lowerflaggyBoquillascaps theupthrownblock Eemiaster calviniClark is abundant and marks east of the Terlinguahighway. In the down- a horizon about 20 feet above the base of the thrownblock the Boquillasbeds dip south and formationinthelowerpart of themiddlemarly rest against the Georgetown,the Grayson,and unit. About 2 feet above the Eemiasterhori- the Buda formations. West of thehighwayin zonis a4-foot zone in which Pecten predomi- the vicinity of Buck Hillthe upper limestone nates. A third zone 2-4 feethigheris charac- beds cap the uplandsouth of the Chalk Draw terizedby Leiocidarishemigranous (Shumard). fault. North of ChalkDraw road anextensive Ammoniteswerecollected in all the zones,but outcrop of the Boquillas is cut off by a north- these have not been studied indetail;Budai- west-southeast fault west of Butcherknife Hill ceras spp. occur lowest in the section with where the Pruett tuff is downfaulted against HemiaslercalviniClark. the limestone. This outcrop, 3-4 miles wide, Two fossil crab specimens from the middle extends eastwardinto the SantiagoPeak quad- marly unit were identified as Graptocarcinus rangle. texanus Roemer by Stenzel. The species was In the northeastern part of the quadrangle first described (Roemer, 1887) from the Buda the Boquillasforms a southward-facing escarp- limestonein Austin,Texas,and redescribed by ment along the Walnut Draw fault extending Stenzel (1944). It had hitherto not been from apoint just east of Whirlwind Springbe- known outside of central Texas. yond the eastern boundary of the quadrangle. Fossils found in the Buda formation in the Most of the downthrown block south of the BuckHillquadrangleare: Boquillas escarpment is covered by alluvium, Hemiaslercalvini Clark but east of Calamity Creek upper Boquillas Heteraster cf.traski (Whitney) limestonecapped with a smallremnant of the Leiocidarishemigranosus (Shumard) Protocardia texana(Conrad) Preutt tuff is downthrown against the lower Saleniasp. flags. The Boquillas also crops out southeast Pecten (Neithea) texanus Roemer ofElephantMountain,and thelower flags form (Neithea) whitneyi Kniker (Neithea)subalpinus Bose a crescent-shaped outcrop around the syenite (Neithea)subalpinus var.linkiKniker core of StraddlebugMountain. 1142 GOLDICH AND ELMS— BUCK HILL QUADRANGLE, TEXAS

The gray to tan orbrownlimestoneflagsand SectionNo.3) southeast ofElephantMountain interbedded shale in the lower part of the includes64 feetof typicallowerflaggybedsand Boquillas formation are easily recognized. In about 53 feet of upper limestone beds. The thelower 50 feet, limestone flags, 2-4 inches upperpart of the section is covered, and prob- thick, and shalebeds, 3-4 feet thick, are evenly ably 20 feet of additional strata beneath the spaced, and because the shale is less resistant Pruett tuff can be assigned to the Boquillas it erodes in a "stairstep" topographyin which giving a total thickness oi 127 feet. The the flags form treads at 3- to 4-foot intervals. formationprobably is thickest in the hills east A sample of the flagsdissolvedin dilutehydro- of CalamityCreek southeast of the Kokernot chloric acid gavean insoluble residue of about ranch house. Although no measurement was 14 per cent by weight. Most of the insoluble made, the maximum thickness is thought to material is finely divided clay mineral of the beless than300 feet. montmorillonite group. The larger clastic The Boquillas and the underlyingBuda in grains are silicic plagioclase and microcline; the Buck Hillquadrangle are concordant, but quartz is rare. On many of the thinner flags there is a marked lithologic change from the there aremarkings whichUdden (1907b, p.32) uppermost light-gray, thick limestone bed of describedas fossilice crystals. the Buda to the lowermost dark-gray, sandy, The upper part of the Boquillas is mainly banded, thin limestone flags of the Boquillas. grayish-white to buff limestone weathering to Roberts (1918, p. 20) discusses the discon- light gray or white. The rock is fine-grained formablerelationsof the Boquillas to the Buda and uniform, locally with pyrite concretions inValVerde County. Inroadcuts alongU. S. which are commonlyalteredto limonite. The Highway 90 nearLangtry the lowermostEagle beds rangefrom 1inch to 4 feet thick, and the Ford bed is a sandy limestone. The contact thicker beds of light-colored limestone differ- withtheunderlyingBuda,likethatintheBuck entiate the upper part of the Boquillas from Hillregion, generallyis undulating,butlocally the lower brown flaggy unit. There are some in these road cuts the basal Eagle Ford lime- shale ormarlbreaks, however,and thedistinc- stoneis conglomeratic. tionbetweenlowerandupperunits onlithologic Theupper contact of the Boquillasformation characters is not made easily everywhere. intheBuckHillquadrangleis anerosionsurface Calcareous shale in the upper part of the unconformablyoverlainby the basalconglom- Boquillas beneath the basal Tertiary con- erate and conglomeratic sandstone of the glomerateis exposedjust southof theTerlingua Pruett formation. At some localities, as on road \ milesoutheast of thehighwaybridge on eithersideof the02 ranch road,1J miles north- Calamity Creek Wash and at Hill 3927 ap- east of Whirlwind Spring, the conglomeratic proximately 2\ miles southeast of the bridge sandstone of the Pruett rests on limestone. and 2\ miles northeast of Butcherknife Hill. At other places, such as Hill 3927, the basal The freshshale is gray, weathering to a yellow conglomerate lies on stratigraphically higher brown or tawny yellow. AtHill3927 the cal- bedsof calcareousshale. careous shale is 30 feet thick and resembles Identifiablefossils in the Boquillasare rare. Terlingua beds along Terlingua Creek south Imprints of Inoceramus are abundantbut can- of the Buck Hill region, but the outcrop is not be identified specifically. The thin sandy limited, and alluvium obscures the relations flags, 12-20 feet above trie base of the forma- withtheBoquillas limestone. tion, contain a poorly preserved ammonite Because of pre-Pruett as well as recent fauna which according to W. S. Adkins (per- erosion and because of faults and local thick sonal communication, 1936) appears to be an cover of alluvium, a satisfactory measurement unrecorded assemblage. Fragments of a fossil of the thickness of the Boquillas could not be fish from the upper limestone were examined made in the Buck Hill quadrangle. In the by the late Curtis J. Hesse (personal communi- southern part of the quadrangle beds assigned cation,1937) whowrote: Boquillas aggregate 210 feet. Near to the "Your specimen is veryincomplete,too muchso Stone Tank the lower flaggy beds are 130-140 for accurate determination. However, it appears feet thick. The measured section (Appendix, to be similar to Ferrifrons rugosus, a fish described CRETACEOUS SYSTEM 1143 by D. S. Jordan (Univ. of Kansas Sci. Bull, vol. limestone and intercalated lava flows ranging IS, No. 2, p. 228, pi. 18-19, 1924), from the Nio- rhyolite (Fig.3). brara Chalk of Kansas. This, of course,is a some- from basalt to The volcanic what meaningless name since Jordan could not tell series lies with angular unconformity on the what the fish was related to; but it is interesting Boquillas formation in the Buck Hill quad- that the two records should be of approximately the samestratigraphic position." rangle, and the base of the Pruettformation is marked by a limestone pebble conglomerate The calcareous shale below the basal Pruett and conglomeratic arkosic sandstone. In- conglomerate at Hill 3927, northeast of tercalatedwith thePruett bedsare the Crossen Butcherknife Hill,contains impressions of am- trachyte, the Sheep Canyon basalt, and the monitesandpelecypods,but theseareunidenti- Potato Hill andesite. These flows came into fiable. Foraminifera were noted in a disin- the Buck Hillregion from the north, and their tegratedand washedsample,anda study of the mapped boundaries probably represent their microfauna was generously undertaken by original southern limits fairly closely. The Dr. D. L. Frizzell and Miss Glenn G. Collins flows wereerodedinpart and weathered before of the Departmentof Geology,The University beingcovered withash. The Pruettformation of Texas. Frizzell (personal communication, is 900-1000 feet thick. Inthenorthernpart of 1947) wrote: the quadranglethe intercalatedlavas make up 300-400 feet of the totalthickness, but south "Foraminifera are moderately common but in small variety, only seventeen species having been of CrossenMesa the whole thicknessis tuff and found in the sample. Although the following list relatedsediments. of identifications is to a degree tentative, a cor- formation is placed relation with the lower part of the Austin group The top of the Pruett seems definite. at the base of the Cottonwood Spring basalt Trochammina sp. which are variableinnumber and thick- Lenticulina sp. aff. L. miinsteri (Roemer) flows Dentalina,2 spp. ness, ranging from 50 to 325 feet or more. Gilmbelina moremani Cushman The flows are overlain by the Duff formation G. pseudotesseraCushman chiefly G. sp. cf. striata (Ehrenberg) which is over 1000 feet thick and is Buliminafabilis Cushman and Parker rhyolitic tuff with some breccia and localcon- B. sp. cf. B.reussi Morrow glomerate beds. The top of the Duff tuff is Neobuliminairregularis Cushman and Parker rhyolite Valvulineria sp. thebase of the cappingMitchellMesa. Globigerina sp. West and southwest of the Buck Hill quad- Eastigerinella alexanderi Cushman rangle the Mitchell Mesa rhyolite is overlain H. watersi Cushman Globotruncana canaliculata (Reuss) by a thirdmajorunit of tuff, tuffaceous sand, G. fornicata Plummer and thick flowsof andesite-basalttotalingabout Planulina austiniana Cushman" 1000 feet, so that the Tertiary volcanic suc- Because of its stratigraphic position above cession in this part of Trans-Pecos Texas has a the Buda limestone the lower part of the minimumthickness of 3000 feet andmay be as BoquillasformationintheBuckHillquadrangle muchas 4000 feet thick.— is correlated with the Eagle Ford; the upper- Age andcorrelation. TheBuck Hillvolcanic most part, from the fossil data now available, series is oflowerTertiaryage and is equivalent, part at least, to is lower Austin. Much remains to be done in in volcanic successions in the study of the fauna of the Boquillasin the otherparts of Trans-Pecos Texas. The Pruett Buck Hill quadrangle and in Trans-Pecos formationis assigned tentativelyto theEocene; Texas. In study the Forminifera will theDuff, tothe Oligocene. this silicifLed gastropods from provide valuable information to supplement A collection of Pruett themegafossils. tuffaceous fresh-water limestone of the formation in Sheep Canyon in the Alpine Tertiary System quadrangle(Fossillocality,PL 6) wasexamined by Mr. Paul Bartsch of the U. S. National Buck HillVolcanic Series — Museum for Hardin (1942). A Physa, the Principalfeatures. The Buck Hill volcanic tips of Goniobasis,and the youthful form of series is chiefly tuff with subordinateconglom- Cochliopa wereidentified. Bartsch concluded erate, sandstone, breccia, and fresh-water that theGoniobasisspecimens maybelongtothe 1144 GOLDICH AND ELMS— BUCK HILL QUADRANGLE, TEXAS

group including tenera cafterii. Goniobasis the 02 ranch examinedby Junius Henderson tenera carterii Conrad is described from the (Adkins, 1932, p. 514) are identical with the lower Eocene of western Wyoming, western snail described by Cockerell (1914) as Helix Colorado,and northern New Mexico (Hender- hesperarche. Cockerell's specimen came from

Figure 3.— Columnar Section of the Buck Hill Volcanic Series

son, 1935, p. 227). A few ostracods and the Cope collection, and, although it was charophytes ininsoluble residues of the lime- labelled "either Puerco or Torrejon, probably stone were examined by Mr. R. E. Peck Torrejon",its originallocalitycannot beestab- (Hardin, 1942) who found the internal molds lished. The specimen may have come from of Metacypris, Cypris, and varieties of that the Buck Hill volcanic series of Texas rather genus, Paracypris and Bairdiocypris, etc.; and thanfrom the EoceneTorrejon of NewMexico unidentifiable charophytes. None of these (Henderson,1935,p.134). forms is diagnostic. Fresh-water limestone The Duff formationis assigned to the Oligo- beds between flows of the Sheep Canyon cene on the basis of a smaller gastropod asso- basalt on the southern slope of Elephant ciated with "Helix" hesperarche at Church Mountain contain a similar fauna. The as- Mountain and also found in other places in signment of the Pruett to the Eocene is on the the Jordan Gap quadrangle. It is similar to basis of abundant specimens of Goniobasis in a species occurring in the Oligocene Vieja fresh-water limestone lentils in the upper part volcanic series in the Tierra Vieja Mountains, of theformation. western Presidio County (Fig. 2). The vol- Low-spired fossil snails werecollected from canic succession of the TierraVieja Mountains, the lower Pruett tuff beds near the base of Rim Rock country, was first studied by Crossen Mesa about 1mile east of the Pruett Vaughan (1900), who assigned it to the Cre- ranchhouse and fromnear thebase of Elephant taceous. In1946 Mr.BryanPatterson of the Mountain. Similar large low-coiled snails, Chicago Natural History Museum collected 2-3 inches in diameter, were collected by vertebrates and gastropods from the lower Graham (1942) from the Duff tuff at Church Vieja tuff beds northwest of Porvenir (Fig. 2). Mountain in the southeastern part of the Patterson (personal communication, 1947) Jordan Gap quadrangle. Land snails from states that the tuff beds are earliest Oligocene TERTIARY SYSTEM 1145 witha fauna equivalent to thatof the Chadron were correlated with the flora of the Raton formation. The gastropods associated with and Denver formations (lower Eocene) by the vertebratefossils wereidentifiedasMesodon Berry (1919). From tuff beds within 200 {"Helix") leidyi (Hall and Meek) by Dr.L. S. feet of the base of the volcanic series on the Russell (personal communication, 1947) who Caseyranch 11mileseast ofBalmorhea(Fig.2) states that the Duff tuff specimens probably in the northeasternpart of the Davis Moun- arethe samespecies. tains, Baker (1934, p. 151) collected a fossil A brief reconnaissance of the Porvenir area toothwhich R. A. Stirtonidentified as that of wasmade by Goldich withPattersoninNovem- Eyracodon, a lower or middle Oligocene ber 1946. The basal conglomerate of the rhinoceras. Vieja series lies withmarked angular uncon- The Buck Hill volcanic series probably is formityonthe truncatedsteeplydippingUpper equivalentin part to the Chisos beds (Udden, Cretaceous (Navarro) sediments. The vol- 1907b) whichinclude about 2000 feet of thin- canics have been deformed into broad folds beddedtuffaceous sediments abovethe Tornillo withdips of 10°-15° onthe limbs. There is a clay in the Chisos Mountains. Udden con- clear-cut angular unconformity between the cluded that there is no break between the Vieja tuff beds and the over-lying "lake beds" Chisos beds and the Tornillo whichcontains a of Pliocene (?) age. Normal faults of con- large quantity of weathered volcanic material siderable magnitudedisplacing the Vieja series (bentonite), and he assigned the Chisos beds and older rocks have been described by Baker to the Cretaceous. Adkins (1932,p.508-514) (1941). The basal conglomerate contains considers the Tornillo as probably uppermost many igneouspebbles andboulders,some 3 feet Cretaceous. The Chisos beds are now gener- or more indiameter, in additionto limestone ally consideredTertiary. pebbles and cobbles. The conglomerate is 20-30 feet thick and is overlain by an equal PruettFormation thickness of tuff which is succeeded by thin — flows of perlite and of trachyte. Only 3-4 Definition and description. The Pruett for- feet of the perlite immediately below the mation, named for the Pruett ranch in the trachyte is exposed. The trachyte, averaging north-central part of the quadrangle,is prin- 25-30 feetthick,is overlainby 400-500 feet of cipally volcanic tuff butincludes conglomerate, tuff capped by a massiveigneous rock which tuffaceoussandstoneandbreccia,andtuffaceous Lord (1900) described as quartz pantellerite. fresh-waterlimestone. Interfingeringwith the Patterson's collections were made from the tuff areflows of trachyte, basalt, and andesite. tuffbedsabovethe trachyte. ThePruettbeds are widelydistributed inthe South of the Tierra Vieja Mountains in the Buck Hill quadrangle and adjacent areas. Chinati range (Fig.2) Tertiary extrusiveslie East of CalamityCreek inElephantMountain on Cretaceousand Permiansedimentary forma- the tuff is 500-600 feet thick. The volcanic tions (Ross,1943). IntheQuitmanMountains succession in the Elephant Mountain area is in Hudspeth County a succession of flows, described in a later section. South of the breccia,and tuff,3500feet thick,rests on folded mountaininthe eastern partof the quadrangle and eroded Eagle Ford and older Cretaceous there are small remnants of the basal Pruett rocks. Humngton (1943) named this succes- beds on the Boquillas. A thickness of 500 sion the Square Peak volcanic series and as- feet of tuff was measured (Appendix,Section signed it to the lower Tertiary, Eocene, or No. 4) in the first canyon north of the south- Oligocene. easternend of Crossen Mesa 2 miles southwest In the Barrilla Mountains, Jeff Davis and of the Kokernot ranch house. The base of Reeves counties, 60 miles north of the Buck this sectionis coveredby alluviumand maybe Hill quadrangle, Tertiary volcanics rest un- 100-200 feet above the Cretaceous basement, conformably on "clays of Pierre Cretaceous because there is a minimum thickness of 650 (Upson clay, Taylor marls) age" (Baker and feet of the Pruett in the southern scarp of the Bowman, 1917, p. 119). Fossil leaves col- mesa. On the mesa additional tuff beds with lectedby Baker from the basal rhyolitic tuffs intercalatedlavas rest on the trachyte,and this 1146 GOLDICH AND ELMS— BUCK HILL QUADRANGLE, TEXAS

succession to the base of the Cottonwood members: (1) conglomerate and sandstone Spring basalt is about 100 feet thick. If a which are widespreadat the baseof the forma- thicknessofISO feetis assumedfor thetrachyte, tion,(2) fresh-waterlimestone whichinCrossen a totalthickness of at least900 feet isindicated Mesa underlies the trachyte caprock, 500-600 for the Pruett formation in the northern part feet above the base of the formation, and (3) of thequadrangle. breccia-conglomerate which occurs 60 feet South of Crossen Mesa along the 02 ranch belowthe topof thePruettformation.— road the basal conglomeratic sandstone of the Basal conglomerate and sandstone. The con- Pruett rests on Boquillas limestone. West glomerateat the base of the Pruett formation of Whirlwind Spring the upper beds of the consists chiefly of well-rounded pebbles and formation capped by the Cottonwood Spring cobbles of dense, light-tan to dark grayish- basalt are exposed continuously, extending brownlimestone with some marble, quartzite, beyondthe westernboundaryof thequadrangle petrified wood, and variously colored chert into the Jordan Gap quadrangle. A 125-foot pebbles. It is characterized by flat pebbles thickness of tuff was measured 1 mile south offibrous calcitewhicharepieces ofInoceramus of Cottonwood Spring (Appendix, Section shells. Pebbles of igneous rock are rare, re- No. 5). Alluvium covers most of Green quiring careful search, in contrast with their Valley, but the Pruett formation is exposed abundance in the breccia-conglomeratemem- alongTerlinguaCreek, and along most of The ber near the top of the formation. The Ditch and Calamity Creek Wash. The basal conglomerategrades up to coarse,cross-bedded conglomerateandlowerPruettbeds rest on the conglomeraticsandstone. Boquillaslimestoneat BuckHilland in asmall The conglomerate, tuffaceous sandstone, graben1miletothe west. and relatedbeds are wellexposedin a number The Pruett tuff is grayish white, bluish of places in the eastern part of thequadrangle. gray, greenish gray, buff, brown, pink, and Along the 02 ranch road south of Crossen red. Fragments commonly differ in color Mesa where the conglomerate rests on thin- from the matrix,and speckled color combina- bedded Boquillas limestone, angular slabby tions result. Locallyhard,dense, andsilicified blocks of the limestone are contained in the layers grade to loose, friable ash, but on the conglomerate which ranges from 2 inches to whole the tuff is wellindurated. Much of the 1 foot in thickness and grades up to coarse- Pruett is massive and thick-bedded, but thin- grained tuffaceous grit and sandstone. Ap- bedded, well-stratified material is common. proximately 21 miles to the northwest in a Stratified, varicolored beds are strikingly small hill just below Crossen Mesa and three- exposed at Boat Mountain (PL 3, fig. 2), a fourths of a mile east of the Pruett ranch spurlike prominence protected from erosion house, the lower tuff beds include a sandstone by a thick bed of breccia-conglomerate. layer. The section measured in the smallhill Except where protected by the breccia-con- follows: glomerate or by the overlying flows of the Thickness easily Unit Description in feet Cottonwood basalt, the tuff is eroded 6. Tuff, hard, purplish with small white and forms low rounded hills. It weathers to grains 30 nodular forms, and slopesgenerallyarelittered 5. Tuff, hard, dense,pink topurplish... 1 with smallhardpellets and nodules which roll 4. Sandstone, tuffaceous,gray withafew easilymaking walkingdifficult. black grains; well indurated (Sample No. 872). Pebbly layer 1-2 inches There are few easily recognizable units in thick at base 7 for the Pruett formation which can be used 3. Tuff, gray,similar to unit No. 1 4 correlating separated sections, and faults are 2. Tuff, very hard, purplish with white difficult to recognize and follow in the tuff rock andcrystal fragments 2 unless one side of the fault is marked by an 1. Tuff, gray to white; weathers to a by Cretaceous strata. hard nodular gravel. Base covered igneous rock or There by alluvium 6 are, however, three distinctive lithologic facies. They are tentatively proposed as Total thickness measured 50 BULL. GEOL. SOC. AM., VOL. 60 Goldichand Elms,Pl. 2 rhyolite Mesa Mitchell by capped tuff Duffmesas; lower forms Quadranglebasalt HillSpring Buck

Cottonwoodby Northern capped of tuff Pruett Panorama shown. isBaker) L. thick C. feet by 2000 over(Photograph successionmesas. upper Volcanicforms BULL.GEOL. SOC. AM., VOL. 60 Goldic hand Elms ,Pl.3

Figure 1. Duff Tuff Upper part of the Duff tuff in Jordan Gap quadrangle,6 miles west of Mitchell Mesa. Clastic dike inleft foreground.

Figure 2. Pruett Tuff Boat Mountain showing200 feet of Pruett tuff capped by the upper breccia-conglomeratemember.

PruettandDuffFormations TERTIARY SYSTEM 1147

The basal conglomerate is well exposed in on Buck Hill, a short distance west of the Hill 3927, 2\ miles northeast of Butcherknife locality of thepreceding section, 30 feet of the Hill, whereit is 6 feet thick and grades up to lowerPruett is exposed. The section follows: pebblysandstone, thick. tuffaceous 15-20 feet Thickness Grains of biotiteand other dark minerals and Unit Description in feet fragments of dark chert give the sandstone a 5. Syenite to top of hill 50 speckled appearance. Good exposures of the Pruettformation; basal conglomerate member: 4. ,redandyellow, fine-grained.... IS basal Pruettbeds alsoare foundin the vicinity Tuff of Butcherknife Hill. On the northwest side 3. Sandstone, tuffaceous,calcareous.... 10 conglomerate, sandstone, and 2. Conglomerate and conglomeratic of the hill the sandstone 5 breccia dip 25° NW. On the east side, the the Boquillas limestone: lower part of the section is covered, and 1. Limestone, massive beds at top with conglomerateisnot exposed,but a 10-foot bed thin, slabby limestone below. Cov- of sandstone is included in the calcareous tuff ered atbase for 82 feet to Cretaceous limestone flat south of Buck Hill. beds. The sectionfollows: Thickness including coveredpart.... 126 Thickness Unit Description infeet Total thicknessmeasured 206 5. Interval covered with float of dense basalt exposedat top ofButcherknife Hill. Inpart, Butcherknife basalt. . 77 In the Buck Hill quadranglethe basal con- Pruett tuff; basal conglomeratemember: glomerate and sandstone generallydo not ex- 4. Tuff, containing thin limestone beds ceed 15 feet in thickness. The overlying tuff andnodulesnear topgrading down to beds sandy and calcareous and commonly calcareous tuff andsandy tuff at base are of interval 17 include one or more sandstone layers. Field 3. Sandstone, light gray, very coarse- relations indicate that the beds of sandstone grained, friable, tuffaceous, and cal- in the tuff atButcherknife hillandin thesmall (SampleNo. 1231) 10.5 careous. hill south of Crossen Mesa are not many feet 2. Tuff, gray,sandy 5.5 above the base of the formation. Accordingly 1. Tuff, red, fine-grained, base covered they are included in the basal conglomerate by float and alluvium 2 member which is denned to include the con- sandy Total thicknessmeasured 112 glomerate, sandstone and arkose, and and calcareous tuff in the lower 40-50 feet of At Buck Hill the syenite which now caps thePruettformation. the hill intruded the lowerPruett beds. The Fresh-waterlimestone.— -Massivebedsoffresh- followingsection was measured on the south- waterlimestone occur in the Pruett formation easternsideof thehill: just below the Crossen trachyte. In Sheep Thickness Canyon in the Alpine quadrangle (PI. 6) these Unit Description in feet tuffaceous limestone and highly calcareous 6. Syenite to top of hill tuff bedsare 300 feet thick in the spur between Pruett formation; basal conglomerate member: Sheep Creek and Calamity Creek. The lime- 5. Tuff, hard red 5 stone ranges from grayish white to tan and 4. Sandstone, calcareous, with layers of tuffaceous limestone 10 chocolatebrown. Much ofit is in beds up to 10 feet thick, but locally beds a few inches 3. Conglomerate, limestonepebblesin a calcareous sandstone matrix 3 thick are interbedded with calcareous tuff Boquillas limestone: or are separatedby shalypartings. The lime- 2. Limestone, thin-bedded,fine-grained. 2 stone is fossiliferous, and the silicified gas- 1. Limestone, thin- to thick-bedded, tropods stand out on weathered surfaces. dense, black, with poorly preserved Algalstructuresarecommon. pelecypods. Base covered 8 In Sheep Canyon the limestone is overlain by the trachyte and by the Total thickness measured 28 Crossen Sheep Canyon basalt where the trachyte has been In a large gully south of Bench Mark 4203 eroded. In the eastern scarp of the mesa east 1148 GOLDICH AND ELMS— BUCK HILL QUADRANGLE, TEXAS of Calamity Creek and north of Elephant and tuff beds above it are found along Duff Mountain, the fresh-water limestonelies both Creek in the western part of the quadrangle below and above the Crossen trachyte where and can be traced westwardinto the Jordan the latter has been thinned, presumably by Gap quadrangle where the breccia-conglom- erosion. Beds of the tuffaceous limestone, erate memberis as much as 40 feet thick. In 40feet thick,are overlainby the SheepCanyon the Duff Creek area the memberis 2-6 feet basalt, and the three lower basalt flows are thick and is composed of angular to well- separatedby bedsof the fresh-waterlimestone roundedpieces many of whichare2-3 inches in (Appendix, Section No. 6). The limestone is diameter. The proportion of fragments to in part older than the trachyte, and in part tuff matrixislarge. The following sectionwas its deposition spans the interval of extrusion measuredonthe westslope ofHill4325 eastof of the Crossen trachyte and the Sheep Canyon Duff Creek: basaltlavas. — _ Thickness Upperbreccia-conglomerate. Tuffbrecciaand Unit.. Description in feet conglomerate in the upper part of the Pruett CottonwoodSpringBasalt: formation, approximately 8. Basalt, fine-grained, massive, dark- 60 feet below the green Cottonwood Spring basalt, are designated the Pruett formation: upper breccia-conglomeratemember. It may 7.Tuff, dense, hard,baked,dark-red. 3 be a single thick, well-cemented, and ... coherent 6. Ash, soft, earthy, weathered(?), with layer or it may consist of severallayers about noapparent stratification, red 30 1 foot thick separated by thin beds of coarse 5. Tuff, thick-bedded, red with white tuff. The breccia contains angular fragments specks 11 of a variety of igneous rocks with some pieces 4. Tuff, coarse, poorly bedded but in of chert and jasper. Generally the pieces are thick rough layers, mottled gray and pink less thanhalf an inchin greatestdimension and 7 3. Tuff, thin-bedded,pink exhibitstructures and textures such as spheru- andgray.... 11 2. Upper breccia-conglomerate member, litic,eutaxitic, fine-grained dense or vesicular, resistant bed of large volcanic frag- and porphyritic with smallcrystals of sanidine mentsina five-grainedred tuffmatrix 2 and quartz. Some dark-red vesicular frag- 1. Tuff to baseof slope ments suggest basic igneous rocks, but most are silicic: Locally some of the pieces are Total thicknessmeasured 64 rounded and attaina footor morein diameter. The breccia-conglomerate is cemented with The breccia-conglomerate member was orig- calcareous tuffaceous material and is resistant inally extensive, covering many square miles. toerosion forminga benchin the Pruettforma- It caps the Pruett in the faulted block south- tion below the basalt-capped hills and mesas west of thePruett ranch house. A thinbedof west of Whirlwind Spring and north of the breccia with small angular gray to reddish- 02 ranch road. At Whirlwind Spring the bed, brownfragmentsin a porouslight-grayish-pink 2-6 feet thick, is composed of angular igneous topink matrix caps 50 feet of tuff onthe tra- fragments which generally are less than half chyte in Hill 5202 on Crossen Mesa. This an inch across. The layer is much thicker at breccia occupies a position below the Cotton- Boat Mountain wherelarge slabby pieces have wood Spring basalt which stratigraphically is slumped off from the caprock ledge and cover analogous to that of the breccia-conglomerate thebase of theslope. Inthis slumpedmaterial in the downfaulted area southwest of the mesa alongthe road there are more or less rounded and is considered a remnant of the breccia- igneousbouldersasmuch as 3 feet in diameter. conglomeratemember. NorthwestofHill5202 West of BoatMountain andsouth of elevation on Crossen Mesa the breccia-conglomerate 4054 feet (Appendix, SectionNo.5) the breccia- wasnot found,and its positionin the succession conglomerate is 2-6 feet thick with a consider- is occupied by the Potato Hill andesite flow able rangeinsizeof fragments. (Fig.3). — Good exposures of the breccia-conglomerate Composition. The greaterpartof the Pruett TERTIARY SYSTEM 1149 formation is fine-grained tuff, commonly Chlorite and sphene are the most abundant calcareous, and locally silicified. The lower nonopaque minerals. The chlorite is in small tuff beds above the basal conglomerate-sand- rounded flakes; the sphene commonly shows stone member generally are calcareous and crystalfaces. A few grainsof apatite,aegirine- commonly contain thin layers and nodules of — analyses samples limestone. Part of the calcium carbonate Table 2. Chemical of oftufffrom erosion of the the Pruett and Duff formations probably was derived from the (S.S. Goldich,analyst) exposed Cretaceous formations, but a large part probably was precipitatedin the lakes and ponded water in which the volcanic ash SiO 69.73 70.57 was deposited. In the northern part of the 2 A12O3 12.06 12.19 Buck Hillquadrangle and in the Alpine quad- Fe2O3 1.06 1.29 rangle thick beds of fresh-water limestone in FeO .13 .09 the upperpart of the formation weredeposited MgO .88 .60 in ponds. Abundant algal structures, charo- CaO 2.16 1.43 phytes, and fossil gastropods suggest that Na2O .78 1.80 organic processes were important in the ac- K2O 4.17 5.09 cumulation of the limestone. The limestone H2O+- 5.05 3.36 is tuffaceousandgradestocalcareous tuff. H2O 3.31 2.84 The tuff beds in the uppermost part of the CO2 .01 .14 formation generally contain less calcium car- TiO2 .18 .22 P2P .04 .08 Thin sections 2O6 bonate than the lower beds. MnO .01 .01 glass, relict structures show norecognizable but BaO .13 .04 suggest devitrified shards. Fragments of SrO .09 n.d. quartzand feldspar are variablein abundance. s : .00 .00 A light-gray to cream-colored sample from poorly stratified tuff below the Potato Hill 99.79 99.75 andesite at Hill 4975 on Crossen Mesa was Sp-gr 2.368 2.421 analyzed (Table 2, No. 1). The tuff, 37 feet 1. from Pruett bel< thick at this locality,is overlainby 40 feet of Tuff the formation »w Potato andesite at 4975 on Crosi;en Mesa. by of tuff to the Hill Hill andesite succeeded 26 feet base Sp. gr. Spring 24°/4°. of the Cottonwood basalt (Appendix, 2. Tuff fromtheDuff formation, approxiinate]y100 Section No. 7), so that the analyzed sample feet below Mitchell Mesa rhyolite. Mitchell was taken 70 feet below the top of the Pruett Mesa. Sp.gr., 26.674°. formation. The sample is similar to an analyzed sample from the younger Duff tuff augite, rutile, biotite, muscovite, and zircon of Mitchell Mesa (Table 2, No. 2). Both werenoted. A sample of gray calcareous tuff samples are considerably altered as indicated from below the trachyte in the ravine south in the large contents of water. Despite the of Hill 5202 contains similar heavy minerals, greater alterationsuggestedby its larger water but fresh biotite, zircon, and apatiteare rela- content, the Pruett tuff sample has a larger tively more abundant. Very hard, dense, relative amount of lime which in the absence purple,silicified tuff in thin layers about half- of carbon dioxide suggests that the parent way up the southern scarp of Crossen Mesa is material of the Pruett tuff representedby the composed of shards of silicicvolcanicglass with analyzed sample probably was more basic minor quartz and feldspar, a few rock frag- thanthatof theDuff tuff. ments,andalittlecalcite. Part of the analyzed sample was crushed, The coarser clastic materials of the Pruett and theheavy mineralsseparated withbromo- formation include conglomerate, breccia, and form(D = 2.86). Theheavy-mineralfraction arkosic sandstone. The basal conglomerate is small and consists principally of magnetite consists chiefly of well-rounded pebbles and partially altered to hematite and limonite. cobbles of limestonewithmore angularpebbles 1150 GOLDICH AND ELMS— BUCK HILL QUADRANGLE, TEXAS of chert, jasper, and petrified wood. Pebbles formationfrom scatteredlocalitiesin thequad- of marble and quartzite are common, but rangle were treated with dilute hydrochloric igneous rock pieces are rare. The related acid to dissolve the calcareous cement. The sandstone contains abundant orthoclase and lossin weightin the acid ranged from 26 to61 per cent. The clay and silt fraction ranges from1.6 to 4.2 per cent, and the sand fraction from 37 to 71 per cent. Constants for the sand fractions of the samples from cumulative curves (Fig. 4) are given in Table 3. These include the median diameter (Md) and coeffi- cients of sorting (So) and of skewness (Sk). The samplesarecalcareous sandstonesinwhich ttie mediumto coarse sand is characterizedby a high degree of sorting. There is no ap- preciabledifference betweensampleNo. 8 from theupperpartofthe formation andthesamples from thebasal conglomeratemember. Sample No. 5 from the southwest slope of Butcher- knife Hill is an arkose containing abundant silicic feldspar, quartz, and rock fragments cemented by clear granular calcite. Some dense, darkercalcitewith sections of Foramini- fera and structures suggesting fossil shells appears to be pieces of fragments derived from the Cretaceous limestones. Small fragments of ophitic and trachytic rocks, chert, and chal- cedonyareminor,andtheprincipalconstituents — in addition to the calcite and limestone frag- Figure 4. Cumulative Curves Representing ments are abundant angular orthoclase, silicic Textural Analyses or Sandstone SamplesFrom plagioclase, quartz. Accessory the Pruett Formation and minerals are biotite, muscovite, tourmaline, and a silicic plagioclase. In addition to the lower radiating fibrous mineral which may be a beds there are lentils and thin layers of con- zeolite. glomerateand sandstone at higher levels. On Thin sections of the upper breccia-con- the northwest slope of Hill 4140 south of the glomeratefromlocalities south of Cottonwood 02 ranch house a bed of conglomerate6| feet Spring and in Duff Creek show fragments of thick is overlain by 5 feet of sandstone. The spherulitic, eutaxitic, dense, and porphyritic conglomerateis composedof roundedlimestone varietiesof rhyolite, and trachytic and ophitic and tuff pebbles. These beds occur in the volcanics. The ophite consists of minute tuff 70 feetabove thebase of thehilland30feet plagioclaselaths in an indeterminablereddish- below the base of the overlying basalt flows brown matrix. The fragments are cemented whichare assigned to the Cottonwood Spring with a selvage of fine-grained material con- basalt, so that the sandstone and conglomerate taininggrainsofquartzand— alkalicfeldspar. are near the top of the Pruett formation. Stratigraphic relations. -The Pruett forma- The beds appear to be localand could not be tion rests unconformably on the Boquillas traced in other parts of the hill. Three miles formation and is overlain by the Cottonwood south of Hill 4140 in the bed of Terlingua Spring basalt. Deposition of the tuff and Creek tuffaceous sandstone is exposed along related sediments was locally interrupted by an east-west fault which is the western exten- invasions of lavas which moved into the area sionof theChalkDraw fault. from the north. The Crossen trachyte, which Nine samples of sandstone from the Pruett rests on tuff and fresh-waterlimestone of the TERTIARY SYSTEM 1151 upper Pruett, was eroded and removed from Hill quadrangle is designated the Crossen large areas, so that the younger Sheep Canyon trachyte. It covers approximately 20 square basalt flows came to rest on the fresh-water miles on the mesa (PL 1) and also caps ex- limestone member and overrode the erosion- tensive areas to the north in the Alpine quad- — Table 3. Bulkcompositionandtexturalconstantsfor samplesof sandstonefrom the Pruettformation Weiightper :ent

Bulk composition: Loss indil. HCI 33.2 59.4 27.8 41.6 40.4 60.8 30.2 26.2 36.0 Clay andsilt 1.6 3.4 1.6 4.2 4.0 2.6 6.0 3.0 2.0 Sand 65.2 37.2 70.6 54.2 55.6 36.6 63.8 70.8 62.0

Constants: Md (mm.) 0.23 0.41 0.78 0.26 0.26 0.35 0.23 0.38 0.39 So 2.17 1.49 1.32 1.52 1.47 1.56 1.71 1.54 1.70 SK .79 1.01 .81 .87 .88 .73 .85 .87 .91

1. Aboveconglomerate insmallhill below CrossenMesa, 1mile east of Pruett ranchhouse. Cat.No. 872. 2. Abovebasal conglomerate, along02 ranchroad,\\ milesnortheastof Whirlwind Spring. Cat.No. 869. 3. From basalconglomerate member,east sideofButcherknife Hill. Cat.No.1231. 4. Same as3, west sideofButcherknifeHill. Cat.No. 830-A. 5. Same as 3, southwest slopeof ButcherknifeHill. Cat. No. 831. 6. Above basal conglomerate, alongroad to Sid Place,southwest sideof fault, approximately 2miles west of Butcherknife Hill. Cat. No. 1232. 7. From bedof Calamity Creek Wash,half a milenorthwest of sampleNo. 6. Cat.No.1230. 8. Aboveconglomerate bed,approximately70 feetabove base onnorthwest slope of hill half a mile south of 02 ranchhouse. Cat. No. 883. 9. From bedof Terlingua Creek, along faultwest of Straddlebug Mountain. Cat. No.1244. thinned trachyte (Fig. 3). A third invasion rangle (PI. 6). The flow came into the Buck of porphyriticandesitefollowed. In thenorth- Hill region from the north or northwest. No ern part of the quadrangle the basal Cotton- trace of the trachyte is found in Elephant wood Spring flow rests on deeply weathered Mountain east of Crossen Mesa, but it may Potato HOI andesite or on tuff beds on the have been removed by erosion. Northwest andesite, but south of the Walnut Draw fault of ElephantMountain (PL 6) the trachyte was the basal flow rests on soft, earthy, red ash eroded from a large area by an ancient (Eo- which probably represents deeply weathered cene?)stream. Pruett tuff. Breaks in the deposition of the The fresh trachyte is hard and brittle with Pruett, other than those caused by the lavas, an evento subconchoidalfracture. Abundant are difficult to determine. The upper breccia- stubby phenocrysts of glassy feldspar are conglomerate, 60 feet below the top of the evenly distributed in a fine-grained, grayish formation, marks a break, the magnitude of to reddish-brown groundmass. Near the which is not known,but in thismemberis the southern rim of Crossen Mesa the upper part first appearance of large igneous cobbles and of the flow is vesicular with many fillings of boulders. chalcedony. The vesicles are strongly elongated, generally northwest-southeast, ac- Crossen Trachyte centuating the flow lines and the alignment — of the phenocrysts. The vesicular top of the Definition and description. The massive flow locally was broken and recemented to. flow of porphyritic trachyte capping Crossen form a prominent flow breccia which is well! Mesa in the north-central part of the Buck exposedin the bed of GoatCreek on the mesa 1152 GOLDICH AND ELMS— BUCK HILL QUADRANGLE, TEXAS and in Walnut Draw on the western edge of fication. It consists principally of microlites CrossenMesa. of silicicfeldspar withminoramounts of altered The present surface of the Crossen trachyte ferromagnesianmineral, which originallyprob- is essentially a dip slope, dipping l°-2°, N. ably was aegirine-augite, and of magnetite 20° W., from which erosionhas removedmuch and apatite. Secondary iron oxides and a — chloritic mineral are the chief alteration prod- Table 4. Chemical analysis of Crossen trachyte ucts, and the red color of the rock, is derived From the upper part of the flow,head of first gullynorth of the southeastern end of Crossen Mesa from the abundant finely disseminated (S. S. Goldich, analyst) hematite.

SiO2 71.17 The feldspar phenocrysts are rounded and AljOi 12.64 irregularly corroded. They are chiefly anor- Fe2O3 4.28 thoclase with delicately developed quadrille FeO .11 twinning and moderately large 2V. Clear MgO .13 untwinnedfeldspar withsmall(-)2Vis sanidine. CaO 1.05 Thephenocrysts aremuchaltered to Na O carbonate, 2 3.91 hematite, and a chloritic mineral. These K2O 5.05 secondary products E in part were developed 2O+..- .23 at the expense of original ferromagnesian H2O .12 minerals included in the feldspar phenocrysts. CO2 .19 Carbonate no2 .56 pseudomorphswith outlines marked by films P2P2O5 .07 of hematite and with small veinlets MnO .14 of hematite suggest original olivine. A few BaO .08 larger grains of the original pyroxene largely srO n.d. altered to a dark-green mineral appear to be tr. aegirine-augite. Grains of granular quartz probably are secondary, and chalcedony was 99.73 introduced. 3p. gr. (2474°) 2.64! — Chemical analysis. The trachyte is characterized by an unusually high silica content of the brecciated vesicular top. In the cliff of 71per cent (Table4), largelyas a result of on the southern edge of the mesa the trachyte the introduction of secondary quartz and is about 120 feet thick, and a thickness of 115 chalcedony. The relatively large content of feet is estimated (Appendix, Section No. 4) ferric oxidereflects the oxidation of the iron- in the first gully north of the southeastern tip bearingmineralstohematite.— of the mesa. The flowis considerablythicker Stratigraphic relations. The Crossen tra- to thenorthwest,averaging150feet. chyte rests on tuff and fresh-water limestone The trachyte weathers to a rusty-brown of the Pruett formation. In the vicinity of pitted surface, the pits resulting from the PotatoHillit is overlainby tuff beds assigned weathering out of the phenocrysts. Some of to the upper part of the Pruett which aresuc- the weathered rock is bleached to a light ceeded by the PotatoHillandesite. Near the yellow or yellowish gray, and as this type is northern boundary of the quadranglea flow of found inplace beneathcalcareous tuff it may the Sheep Canyon basalt rests directly on the represent ancient (Eocene?) weathering. The trachyte, and farther north in the Alpine verticaljointingof the flow favors the develop- quadrangle (Hardin, 1942) the trachyte was ment of vertical walls or cliffs on the edge of eroded by an ancient stream, the channel of the mesa. Marginal fragmentation of the which subsequently was rilled with flows of trachyte yields abundant debris that covers the Sheep Canyon basalt. Small hills or the more gentle slopes of the less coherent erosional remnants of the Crossen trachyte underlyingtuff. were surrounded by the flows, and these now Petrography.— The fine-grained groundmass appear as inliers in the Sheep Canyon basalt is not readily resolvedevenunder highmagni- (PL 6). The stratigraphic relationships of the TERTIARY SYSTEM 1153

Crossen trachyte, the Pruett beds, and the of the northern boundary of the quadrangle Sheep Canyon basaltflows are shownschemat- follows : ically inFigure3. Thickness Unit Description infeet Pruett Sheep CanyonBasalt formation: — 4. Potato Hill andesite intercalation and description. Basalt flows in- 3. Tuff,gray,wellstratified, interbedded Definition with purplish-red ash speckled with tercalated with fresh-water limestone and tuff whitefragments; upper f feetbaked, bedsof thePruettformationarereferred to the dense, red tuff 27 Sheep Canyon basalt. These are younger 2. Sheep Canyon basalt intercalations: than the Crossen trachyte and older than the b. Basalt flow, black fine-grained rock with vesicular top altered Potato Hill andesite. The flows are named green; weathers to flattish bis- for Sheep Canyon, a re-entrant of Calamity cuitlike forms. Red baked ash Creek valley in the Alpine quadrangle ap- included in thin flow breccia at baseof flow 34 proximately 3 miles north of the Buck Hill a. Basaltflow,uniformblack,fairly quadrangle(PL 6). coarse-grained but not porphy- The Sheep Canyon flows originally were ritic. Well-developed vesicular extensive in the northern part of the Buck top 21 Hill quadrangle but have been eroded from 1. Crossentrachyteintercalation:Redpor- phyritic trachyte breccia largeareas. The basaltis now found on Cros- south- sen Mesa and in the downfaulted block Total thickness measured 82 west of the mesa. The southernmost outcrop of the flows is along an east-west fault ap- Half a mile south of this hillthe upperflow proximately1mile south of the Pruett ranch only is present. It is 27 feet thick, coarse- house. The flows cover largeareas in the Al- grained and porphyritic. The vesicular base pine quadrangle where they are much thicker. of the flow rests on baked, dense, red tuff In Sheep Canyon at least four flows are sep- gradingdownto well-stratifiedgray tuff resting arated by thin beds of tuff and tuffaceous ontrachytebreccia. The tuff is 13 feet thick, limestone. The green vesicular top of the basalt flow is The basalt typically is dark greenish black, overlain by stratified gray ash capping the even-textured,fine- tomedium-grained,locally low hill. To the west this ash is overlainby containing large, glassy, green plagioclase the PotatoHillandesite. East of Goat Creek phenocrysts, many of which are 1 inch long. and southwest of elevation 5016 feet, the On relativelysteep slopes thebasalt commonly Sheep Canyon basaltis intercalated with ash develops a platy structure on weathering; on resting on the Crossen trachyte. At this gentle slopes and in the bottoms of gullies point the basaltis 30 feet thick but feathers spheroidal structures result. The vesicular out to the southeast, and on the southern tops of the flows contain abundant fillings of scarpbelow elevation4975 feet no trace of the quartz, chalcedony,and calciteand commonly SheepCanyonbasaltis found. are a distinctive green. Inmany places small In the spur west of Goat Creek two flows of flats and benches aredevelopedat the contact the SheepCanyon basaltrest on 11feet of tuff of twoflows. and areoverlainby 20 feet of tuff beneath the At the type locality in Sheep Canyon in Potato Hillandesite. The flows are of equal the section measured by Hardin (Appendix, thickness, totaling 65 feet, and have well- Section No. 6), the basalt flows are 234 feet developed vesicular tops. A prominent flat thick. The upper basalt flow in this section or bench on the southeastern end of the spur is overlaindirectlyby thePotatoHillandesite, roughlymarksthe top of thefirstflow. but on Crossen Mesa in the Buck Hillquad- South of Crossen Mesa the Sheep Canyon rangle a thickness of 20 feet or more of tuff basaltis exposed in gulliesin an area approxi- separates the Sheep Canyon basalt from the mately 1 mile square. This area is bounded PotatoHill andesite. A section measured on on the east and south by the Cottonwood alittlehillnorth of PotatoHilland just south Spring basalt which is downfaulted against 1154 GOLDICH AND ELMS— BUCK HILL QUADRANGLE, TEXAS

the Pruett tuff. At this locality a flow 30 feet netite and pyroxene in the feldspar pheno- thick lies in well-stratified,grayish-white tuff. crysts are common. Augite, olivine, and The top is amygdaloidal with many fillings magnetite are the other essential primary of chalcedony and calcite. The flow was minerals, and apatite is the chief accessory — mineral. Olivine occurs in about equal Table 5. Chemical analyses ofSheep Canyon basalt amounts in all the flows. Hardin (1942) fromBuckHillandAlpine quadrangles estimates the composition of the olivine from (S. S. Goldich, analyst) measurementsof the opticangle tobe approximately FO7O Fa3o. The olivine is considerably altered to antigorite which shows the poly- SiO 2 44.33 45.65 45.97 gonal outline of the olivine crystals, but there A1203 14.69 16.48 17.26 are some residual olivine grains. The amount Fe O 2 3 4.27 6.92 2.84 of augiteis variable. Magnetiteis abundant— FeO 8.68 6.40 9.18 10-15 per cent of the rock. Analcime,calcite, MgO 4.88 4.05 5.63 sericite, fibrous zeolites, and clay minerals CaO 7.00 7.48 7.87 occurinvaryingamounts. Na2O 3.46 3.38 4.33 K2O 2.15 1.42 1.00 Thin sections of the basalt from the Buck HO + Hillquadrangleshow the rock to be generally 2 - 2.15 2.10 1.57 H2O 1.09 1.39 .20 similar to the flows in Sheep Canyon, but a CO2 .07 .37 .00 few special features are worthy of mention. TiO2 3.64 3.21 2.87 Samples of the flowsnorth of PotatoHillshow P2P 2O6 2.99 .87 .96 zoning in the plagioclase which is the reverse MnO .23 .18 .17 of the normal zoning described above, and BaO .17 .09 .07 rounded central zones are more sodic than the SrO .14 .09 .10 .. outer ones. The zones commonly are marked S .00 .00 .00 by lines of euhedralcrystals of magnetite. A 99.94 100.08 100.02 silicic feldspar, probably orthoclase, in the Sp.gr 2.818 2.829 2.874 groundmass is interstitial and marginal to the plagioclase. The serpentinousalterationprod- 1. Basaltflow ingully, 14^50 feet N. 48° W'. of Hill ucts of olivine include olive-greenfibrous ma- 4321', south of CrosserlMesa. Cat. 1STo. 841. terialand a reddish-brownflaky mineral,pro- Sp. gr., 25.274°. bably bowlingite. Calcite is an abundant Porphyritic basalt, upp

Figure 1. Large Mesa Capped by Cottonwood Spring Basalt Southwest of Cottonwood Spring. Flat-toppedMcKinneyMountain, Jordan Gap quadrangle,in distance.

Figure 2. Upper Cretaceous BoquillasFormation Rounded hills of the Boquillasformation south of ElephantMountain.

Figure3. Lower Cretaceous Formations The lower scarp on the right is formed by the Georgetown limestone; upper scarp on the left, by the Grayson clay overlain by the Budalimestone. (AquaFria quadrangle).

Cottonwood Spring Basalt and Creta ceousFormations BULL. GEOL. SOC. AM.,VOL.60 Goldich andElms,Pl. 5

Figure 1. Potato Hill Andesite Figure 2. Syenite at Buck Hill Deeply weathered andesite breccia in Walnut Draw; Inclusions of thin-beddedBoquillaslimestonein syenite. man isstandingoncontact of Cottonwood Springbasalt with tuff bed on the andesite.

Figure 3. Sligkensides onFault Plane at Buck Hill Syenite onright is downthrown against basal conglomerate member of the Pruett formation on left.

Relationships of Igneo usRocks TERTIARY SYSTEM 1155

Elephant Mountain porphyritic basalt more relative thinness of the andesite at this point closely resembles the porphyritic basalt sampled may be the result of weathering rather than north of Potato Hill on Crossen Mesa. The original thinning of the flow because in the large percentage of titania in the analyzed Alpine quadrangle the andesite is up to 70 samplesisnoteworthy. feet thick with appreciable flow breccia. In a measuredsection (Appendix, SectionNo. 7) PotatoHillAndesite 4975, — in Hill 1? miles northwest of Potato Definition and description. The Potato Hill Hill the andesite is 40 feet thick, and this andesite, named for Potato Hill on Crossen thickness holds fairly constant to the north- Mesa, is a dark reddish-brown porphyritic west. In the spur on Crossen Mesa between flow and flow breccia in the upper part of the Goat Creek and Walnut Draw a small bench Pruett tuff. The andesite caps a small hill or flat marks the break between the massive 500 feet southeast of Potato Hill. North- and brecciated phases of the andesite. The west of Potato Hill the flow makes a narrow section measured on the southeastern end of ridge,andits outcropcontinues to thenorthern the spur follows: boundary of the quadrangle on either side of Thickness the basalt-capped hills that crown Crossen Unit Description infeet CottonwoodSpring basalt: Mesa. The andesite is exposed in the spur 7. Basalt flow; fine-grained, jointed; between Goat Creek and Walnut Draw and weathers toreddish-brownslabs. To in the faulted blocks which make Walnut top of hill 44 Draw grabenon the western edge of Crossen Pruett formation did 6. Tuff; red, fine- to medium-grained, Mesa, but apparently the flow not extend grading down to weathered andesite, beyond the southern limit of the Crossen contact approximate 10 trachyte,because it is not found in the down- 5. PotatoHill andesite intercalation: dark faultedblock south of the WalnutDraw fault reddish-brown vesicular andesitepor- phyry in lower 25 feet; flow breccia, zone. much weatheredinupper 20 feet. 45 The PotatoHillandesiteis easily recognized ... 4. Tuff; gray to red, fine- to medium- by its color and prominentplagioclase pheno- grained 20 crysts. The tabular crystals, up to 1 inch in 3. Sheep Canyon basalt intercalations: length, are fresh and glassy. They are con- b. Fine-grained, dark basalt flow tainedinafine-grained,reddish-brownground- with vesicular top 32 mass or in a coarser granular grayish-brown a. Fine-grained basalt flow with vesicular top 33 matrix. Locally the andesite is altered to a 2. Tuff, gray, medium-grained. greenish color. The lower part of the flow, fine- to . 11 although vesicular, is massive; the upper half 1. Crossen trachyte intercalation: porphyritic trachyte flow breccia,base of is a flow breccia. Amygdules of quartz, chal- section cedony,and calcite are abundant. The upper brecciated part weathers more readily than Total thicknessmeasured 195 the massive andesite, and small flats are developed at the change of structure in the In Walnut Draw graben the brecciated flow. The flow was deeply weathered after phase is wellexposed in the bed of the creek extrusion and before the succeeding ash was and in a fault slice that hangs on the trachyte deposited. forming the east wall of the graben. In this At PotatoHillthe andesiteis 31 feet thick. exposure 15-20 feet of andesitebrecciais over- It lies on approximately50 feet of tuff which lain by weatheredandesite grading up to tuff rests on the Crossen trachyte. The andesite which is overlain by the basal flow of the Cot- is overlainby 25 feet of hard, dense, red tuff. tonwoodSpring—basalt(PL5, fig.1). Below elevation4806 feet, 6000 feet north of Petrography. Fine-grained hematite devel- PotatoHill,the andesiteis only18 feet thick. oped by oxidation of the primary ferromag- It is overlainby 29 feet of well-stratifiedtuff nesian minerals obscures the minerals in the below the Cottonwood Spring basalt. The groundmass. The chief mineral is andesine 1156 GOLDICH AND ELMS— BUCK HILL QUADRANGLE, TEXAS

insmalllathlikecrystals, but there are smaller andesite, like the Crossen trachyte and the amounts of a moresilicic feldspar,apatite,and SheepCanyon basalt, is anintercalation in the magnetite-ilmenite. The large rounded and Pruett tuff. The flow was weathered before corroded phenocrysts form about 12 per cent being covered with volcanic ash, and as a — result the contact between the andesite and Table 6. ChemicalanalysisofPotato Hillandesite the overlying tuff is not sharp. This tuff FromHill 4975 on CrossenMesa (S. S. Goldich, analyst) ranges from a few feet to 30 feetthick andIS SiO not differentiated on the geologic map (PL 1) 2 61.46 which shows A1203 the Cottonwood Spring basalt 14.85 restingonthe Fe2O3 7.23 PotatoHillandesite. FeO : .68 MgO .74 Cottonwood SpringBasalt CaO 5.23 and description.— Na Definition Thebasalt flows 2O 3.75 lying on the Pruett KO formation and overlain 2 2.58 by the Duff tuff are named for Cottonwood H2O+ .22 Spring whichissues from the base of the H2O-. .17 flows in a deep gulch cut CO2 .10 into the basalt-capped mesa south of TiO2 1.52 Mitchell and Crossen mesasand north P2P2O8 1.13 of the 02 ranch road. In this area the MnO ■ .11 Cottonwood Spring basalt covers about 17 BaO .18 squaremiles. The flows continue westwardin S .00 the Jordan Gap quadrangle where they cover a somewhat smaller area. Remnants of the 99.95 flows cap small hills north and south of the Sp.gr. (2474°).. 2.76< 02 ranch house alongthe western boundary of the quadrangle. The basalt dips about 2° to of the rock. They are zonedand range from the northwestpassing under the Duff tuff, but Anss ininner parts to An4s in the outer zones. in the northern part of the quadrangle, on A large amount of secondary silica occurs as the upthrown side of the Walnut Draw fault granular quartz filling vesicles and as fibrous zone, theflows cap the hills thatrise above the chalcedony lining the walls of the cavities. Crossen trachyte and occurin the fault blocks Associated with the quartz and chalcedony on the western edge of Crossen Mesa. Orig- is afibrous mineral,probably celadonite,— char- inally the Cottonwood Spring basalt formed a acterized by strong pleochroism light yellow sheet coveringa large part of the quadrangle parallel to the length of the fibers and blue and adjacent regions to the north and west. greenat right angles. Thebrecciatedandesite Thenumber of flows varies, andlittlesignifi- is similar in composition to the massive phase cance is attached to this feature. Locally butis cementedwithsilicaor calcite. small fronts apparently pushed out ahead of Chemical analysis.— The analyzed specimen the main body of lava to be overriddenlater. (Table 6), from a point 1.4 miles northwest of The lower parts of the flows commonly are Potato Hill, represents the lower massive flow breccias; the upper parts are vesicular phase of the porphyritic andesite. The high and amygdaloidal with numerous fillings of silica content reflects the large amount of in- quartz,chalcedony, jasper,and calcite. Large troduced quartz and chalcedony, and the crystals of calcite showing unusual crystal calculated quartz is 21 per cent of the norm forms and the rarely developedbasal pinacoid (Table 10). The oxidation of the rock is re- were collected from cavities in the flows on vealed in the large percentage of ferric iron, the hill south of the 02 ranch house, but the andhematiteis 7per cent of thenorm. Apatite crystal faces are etched and are not suitable is 2.7 per cent of the normative minerals. In for measurement. Pahoehoe or ropy lava is otherrespectstheanalysisisnotunusual. found in many places. The massive dense Stratigraphic relations.— The Potato Hill phase of the basalt is grayish black to dark TERTIARY SYSTEM 1157 greenish black; the vesicular rock commonly gests that erosion has not removed much of is reddish gray to reddish brown. The flow the flow, but younger flows probably were breccia usually is composed of dense black to eroded, and the original thickness may have fine-grained vesicular basalt. Weathering ap- been greater than the 300 feet of basalt parently is controlled by topography. In measured. gullies,low places,and ongentleslopes spheroi- A good section of the Cottonwood Spring dal forms develop; whereas on steep slopes basaltis exposed onHill4631 (Fig.6) If miles and in cliffs a thin slabby structure results. northeast of Cottonwood Spring where nine The weathered basalt favors the growth of flows are 300 feet thick. The lower part of ocotillo. the section beneath a 7-foot layer of red tuff The Cottonwood Spring basalt flows are is 154 feet thick and consists of four flows of thickest in the hills and mesas west of Whirl- which thelower one makes up 83 feet. Above windSpring wherea maximumof 325 feet was the tuff are five flows which aggregate 148 measured. The original thickness is not feet. Southwest of Hill 4321 in the spur at known because in this area the Duff tuff has Whirlwind Spring a similar succession of been stripped. The flows are divided into basalt flows with separatingtuff bedis 325 feet lower and upper units by a thin bed of tuff. thick. The lower unit comprises two to six flows, At Potato Hill on Crossen Mesa the basal totaling 100-150 feet. The upper unit in Cottonwood Spring flow is 85 feet thick; a mostplaces is asinglemassive floworremnant total thickness of 166 feet includes six flows of a thick flow which southwest of Cotton- to the top of the hill. North of Potato Hill wood Spring is 164 feet thick, but in other in Hill 4806, only 67 feet of the basal flow places this thickness may represent five or remains. In Hill 4975 (Fig. 6) there are six more flows. The tuff bed breaking the suc- flows. Five flows beneath a 7-foot tuff bed cession of Cottonwood Spring basalt is 1-19 total 110 feet of which the basal flow makes feet thick. Generally it is less than 5 feet up 61 feet. The remnant of a thick flowabove thick and is baked, hard,dense, red rock. In the tuff is 55 feet thick making the succession, thicker beds the tuff away from the contacts including the tuff, 172 feet in total thickness. is gray and less indurated. Locally rounded Theupper flowofHill4975 alsocaps Hill5016 fragments of vesicular basalt are contained in three-fourths of a mile to the northwest where the lower part of the tuff bed, and inplaces 135 feet of this flow remains. Immediately the tuff was fragmented and incorporated in belowis a densered tuff, 8 feet thick, whichis thebaseof theoverriding flow. underlain by 92 feet of basalt in two flows. Inthe typesection(Appendix,Section No.5) The totalthicknessof 235 feet is the maximum measured up the south point of the basalt measured for the Cottonwood Spring basalt mesa southwest of Cottonwood Spring the inthenorthernpart of thequadrangle. flows aggregate 300 feet. The lowermost In Walnut Draw graben the Cottonwood flow resting on red tuff of the Pruett forma- Springflows are overlainby the Duff tuff. On tion is 40-50 feet thick. The contact is the south slope of the elongated hill in the markedby a resistant ledge,3-4 feet thick,of graben 175 feet of basalt was measured, and coarse-grained basalt. The upper 9 feet is on the basis of amygdaloidaltops there are vesicular with many fillings of quartz, chal- seven flows below a thin layer of red tuff. cedony,agate,andcalcite. Four flows ranging Above the tuff is 80 feet of basalt which caps from 6 to 15 feet thick and one 50 feet thick the hilland forms prominent cliffs. The base complete the lower unit of 130 feet of basalt of the section is covered by alluvium, but a in six flows beneatha tuff bed, 8-19 feet thick. short distance to the south the Potato Hill Above the tuff is 164 feetof basalt, apparently andesiteis exposedin thebed of WalnutDraw. a singlethick flow withaprominentflowbreccia Petrography.— Thin sections show a range of vesicular basalt at the base and a 20-foot in composition from trachyandesite or latite amygdaloidal top. This flow caps the mesa to olivine basalt, but no detailed studies of and makescliffs60 to80 feethighonthesouth- variations between or within the flows have ern scarp. The thickness of amygdaloidsug- been made. The flows typically are fine- to 1158 GOLDICH AND ELMS— BUCK HILL QUADRANGLE, TEXAS medium-grained, ophitic to diabasic, uniform in clear fresh grains together with granular and nonporphyriticor with few small pheno- aggregates of analcime interstitial to the crysts. Flow structure, fractured and broken plagioclaselaths. Olivine,20-25 per cent of crystals, and wavy extinction are charac- the rock, is largely altered to serpentinous — Table 7. Chemical analyses of the basal flow of the Cottonwood Spring basalt, Butcherknife basalt, and similar rocks

!iO2 46.90 48.08 48.53 49.80 46.87 44.81 U203 15.82 15.87 17.42 18.38 17.04 16.81 4.18 3.74 3.46 1.84 3.12 4.06 ieO 7.74 7.49 7.48 7.57 6.81 7.19 4.48 3.86 3.10 2.63 3.35 4.21 :ao 6.24 5.76 6.60 7.09 7.66 7.21 *Ta2O 3.80 4.50 5.13 4.94 5.13 4.65 £ 20. 2.32 2.43 2.38 2.60 2.39 1.97 w+- 1.93 2.17 1.68 1.73 2.99 3.70 120I2O 1.13 .97 .26 .27 .21 1.07 :o2 .02 .02 .02 rio2 2.79 2.68 2.53 2.30 2.96 3.27 J2J 2o5 1.97 1.81 1.10 .95 .93 1.04 tfnO .21 .23 .21 .19 3aO .23 .12 .06 !rO .10 .09 .13 ! .00 .00 .01

99.86 99.82 100.10 100.10 99.65 99.99 >P- gr 2.823 2.762 2.808

. CottonwoodSpring basalt, basal flow ats PotatoHill on Crosisen Mesa. Cat.No.817. Sp. gr., 26°/4°, ;. CottonwoodSpring basalt, basal flow,three-fourths of a mdie southwestof CottonwoodSpring. Cat, No. 840. Sp. gr.,24.5°/4°. .. Basalt at Butcherknife Hill. Cat. No. 830. Sp. gr., 23.4'74°. :.Analcite trachybasalt porphyry, Mouritain 3513, Terlingua-!Solitarioregion, Texas. i. Analcite syenogabbro, Cigar MountaUn, Terlingua-Solitarii0 region. northe; , i. Analcite trachydolerite, 1mile Ast of Mountain35135 Terlingua-Solitario region. Nos. 1, 2,and 3, S. S. Goldich, analystt. Nos. 4, 5,and6,R.B.Ellestad,analystt (Lonsdale, 1940,p... 11(320). teristic. Some of the reddish-brown flows are products that penetrate the near-by fractured highly oxidized, and the original ferromag- grainsofplagioclase. Pale-green fibrous chrys- nesian minerals including much of the mag- otile with weak birefringence, and dark-green netite have been altered to or obscured by to yellow and brown iddingsite or bowlingite hematite. Other secondaryminerals are anal- with strongerbirefringence, goodcleavage,and cime, serpentine, uralitic amphibole, chlorite, marked pleochroism are common. Pyroxene andcalcite. rarely exceeds 10 per cent of the rock and is Thin sections of the analyzed specimen normal augite, some with a purplish tinge. (Table 7, No. 2) from the basal flow at the Most of the augiteis fresh,but some is altered type locality southwest of Cottonwood Spring to long slender needles of actinolite and to contain 60-70 per cent of labradorite. The chlorite. Magnetiteand apatiteare abundant. plagioclase, rarely showing well-developed Analcime occurs in minor amounts in clear twinning bands, is zoned and commonly has grains between the labradorite laths and in a mantle of orthoclase. Orthoclasealso occurs dusty granularaggregates replacingthe plagio- TERTIARY SYSTEM 1159 clase. Its abundance is difficult to estimate, basalt mesa to the southwest the basal flow about 5 per cent of the rock. Likewise ortho- is relatively thick, 43-85 feet. In both areas clase is insufficient amount to justify modifica- the succession of flows with included tuff bed tion of the field namebasalt, and the analyzed is similar, and there is little doubt that the specimen is classified as analcime trachybasalt flows arethe sameandthat they wereoriginally following the usage of Lonsdale (1940) for poured out on a relatively flat and probably similar rocks in the Terlingua-Solitario region weatheredsurface of thePruett tuff (Figs. 3,6). tothesouth. — Chemical analysis. Analyses (Table 7) of DuffFormation samples of the basal flow of the Cottonwood — Spring basalt fromPotato Hill and from near Definition anddescription. The Duff forma- CottonwoodSpringaresimilarin mostrespects. tion, named for Duff Springs in the north- The samples also resemble analysis No. 3 of western part of the Buck Hill quadrangle,is the dense basalt from Butcherknife Hill and chiefly rhyolitic tuff with minor breccia and the analyzedanalcime-bearingrocks from the conglomerate. The tuff lies on the Cotton- Terlingua-Solitario region (Lonsdale, 1940, wood Spring basalt and is overlain by the p. 1620). Thereare minor differencesbetween Mitchell Mesa rhyolite. In addition to the the analyzed samples of the CottonwoodSpring outcrops in the Mitchell Mesa area there are basalt and the Sheep Canyon basalt flows two smalloutcrops on the smallelongatedhill (Table 5), but it is not likely that the two south of the 02ranch house. The Duff forma- groups of flows can be separated by chemical tion coversmost of theJordan Gap quadrangle analyses of scattered samples. Selected to the west. samples probably wouldshow a greater chem- The Duff tuff is predominantly light gray ical diversity within each group than between and in strong sunlight is dazzling white in them. — contrast to the darker color of the flows and Stratigraphic relations. On Crossen Mesa the duller somber hues of the Pruett tuff. the Cottonwood Spring basalt rests on tuff Light shades of red and yellow are common, deposited on the weathered surface of the and alternations of colors give some of the PotatoHillandesite. The tuff at PotatoHill Duff a variegated appearance. Most of the is 25 feet thick, andthe fossilsoilontheandesite material is fine-grained, well-indurated ash is thin;elsewherethe tuff is thinand the weath- grading locally to tuff-breccia and breccia- ered zone is as much as 20 feet thick. The conglomerate. Much of the tuff is massive; weatheredandesite grades upward to red tuff some is cross-bedded; and a large part is well baked to a dense hard rock at the time of stratified, ranging from thin laminations to extrusion of the basal Cottonwood Spring beds 2-6 inches thick, but beds 1 foot or more flow. South of Crossen Mesa beyond the thick are common. The tuff weathers to southernlimit of the andesite, the Cottonwood chalk-white nodular forms, and the gentler Spring basalt rests on the Pruett tuff which slopesarestrewn withpellets andsmallrounded immediatelybelow the basal flowis ahardred pebblesof thetuff. rock, but below this baked layer is 30 feet of Lenticular beds of conglomerate which soft, earthy, red material which probably generallydo notpersist for any great distance represents tuff weathered contemporaneously occur throughout the formation, but probably with the andesite. are more numerous in the lower part. Con- The Cottonwood Spring flows were partly glomerate beds are fairly persistent at two eroded before deposition of the Duff tuff, and levels, however. The lower level is approxi- in part this erosion accounts for the thinning mately 40 feetabove the base of the formation; of the flows from a thickness of over 300 feet the upper levelis 200-250 feet below the top. near WhirlwindSpring to less than 50 feet 10 These conglomerate beds attain 40 feet in miles west in the Jordan Gap quadrangle. thickness and are characterized by cross-bed- The flows arevariablein numberand in thick- ding andchanneling. They aregenerallydark ness, but both on Crossen Mesa and in the brownandstandout inthelight-gray tuff. 1160 GOLDICH AND ELMS— BUCK HILL QUADRANGLE, TEXAS

The Duff formationis 1015 feet thick in the dikes and appears toberelatedto water move- type sectionmeasuredfrom the outcrop of the ment along the dikes. The analyzed sample Cottonwood Spring basalt 1| milesnorthwest (Table 2, No. 2) of the Duff tuff is a well- ofDuff Springs up alargegullyonthesouthern indurated, light-gray, fine-grained, rhyolitic slope ofMitchellMesa to the rhyolitecaprock. ash in which numerous small flakes of biotite Along the line of section the lower conglom- are recognizablewith ahand lens. The sample erate, 6 feet thick, is 37 feetabove the base of was collected from the east scarp of Mitchell the formation. Two beds of conglomerate Mesa 100 feet below the top of the formation. nearthe top of the formationareseparatedby In most respects the analysis of the Duff 46 feet of tuff. The lower bed with igneous sample is similar to that of the Pruett tuff, boulders 1 foot or more in diameter is 8 feet but its relative content of alkalies is somewhat thick; the upper bed consisting mainly of greater,and its percentageof lime is consider- pebblesand cobblesis 10 feet thick and is 200 ably less. The Duff tuff, like the Mitchell feet below the base of the Mitchell Mesa Mesa rhyolite (Table 8, No. 1), is potassic. rhyolite. Graham (1942) measured the Duff The rhyolitic composition of the tuff also formation at Whirlwind Mesa in the Jordan is revealed in petrographic studies of 18 Gap quadrangle, approximately4 miles west samples collectedby Graham (1942) from the of MitchellMesa. At this locality the Cotton- formation atMcKinneyMountain. Theindex woodSpringbasaltis coveredbyalluvium,but, of refraction of the volcanic glass in samples including the lower conglomerate which is fairlyregularlyspaced frombottomto the top exposed, the Duff section is 1020 feet thick. of the formation is 1.492 ± .003, indicatinga The lower conglomerate is 31 feet thick, and silica content of approximately 73 per cent. an upper single bed of conglomerate,250 feet Heavy-liquid separations made on pulverized below the topof the formation,is 38 feet thick. samples, from which the very fine material Allowing40 feet for the coveredintervalbelow was removed by washing, yielded uniformly the conglomerate at the base of the section, small fractions of heavy minerals, rarely as the total thickness of the Duff formation is much as 0.5 per cent by weight of the washed 1060 feet. At McKinney Mountain 10 miles sample. The light fractions, in addition to south of the WhirlwindMesa section, Graham volcanic glass and its alteration products, (1942) measured a thickness of 1400 feet for contained abundant feldspar and quartz. the Duff formation. The tuff was deposited Magnetite-ilmenite and alteration products on an irregular,eroded surface of the Cotton- are the chief heavy minerals. Biotite is the wood Spring basalt, and a variable thickness most abundant nonopaque mineraland occurs is expected. The maximum thickness may in all samples. There are two varieties of exceed the measured thickness at McKinney biotite. One displays a yellowish-green to Mountain. dark olive-green pleochroism; the other is a Numerous dikelike masses cut the Duff tuff. deep reddish-brown variety. Apatite and These are clastic dikes composedof indurated zircon follow in order of abundance. The clay or brecciated tuff, and slickensides and apatite is chiefly in subrounded grains, the brecciated textures indicate that they are the zirconin well-formed crystals. No variation result of small movements probably related in relative abundance of biotite, apatite, or to the faulting. Water moving along the zircon with stratigraphic position was found. brecciated zones deposited crystals of analcime Less important than these minerals both in and calcite in fractures. The clastic dikes relative abundance and in frequency of occur- are generally vertical or at high angles, and rence are aegirine-augite,hornblende, basaltic becausetheyaremore resistantthanthenormal hornblende, epidote,sphene,rutile,and musco- tuff erosionleaves wall-like remnants or natural vite. Aegirine-augite is rare in the samples stone fences (PL3, fig.1). from the lower half of the formation but ap- Composition— Unlike the Pruett tuff, the pears to be common in the upper part. It is Duff rarely is calcareous. A small amount abundant in well-rounded grains in a sample of calcite commonly is found near the clastic takennearthe top of thesection. TERTIARY SYSTEM 1161

A variety of volcanic and sedimentary rocks Gap quadrangle,but Graham (1942) measured is representedin the conglomerate bedswhich 70 feet of rhyoliteon the rim of AntelopeMesa contain abundant dense and vesicular basaltic 10 miles southwest of Mitchell Mesa and 150 types and dense to porphyritic light-colored feet onMcKinney— Mountain. aphanites such as rhyolite and trachyte. Petrography. The fine-grained groundmass Locally pebbles of fine-grained syenite are consists of quartz, alkalic feldspar, and un- common. The sedimentarypebbles arelargely identified material which may be rhyolitic chert and fine-grained limestone. Coarse sand glass and its devitrification products. Mag- commonly is associated with the upper con- netite, zircon, and apatite are accessory glomerate. Sand and tuffaceous material ce- minerals. Alteration products include small ment the conglomerates. — amounts of limonite,hematite, and leucoxene. Stratigraphic relations. In the Buck Hill The rock owes its pink color to finely dis- quadrangle and in the northern part of the seminated hematite. There is some silica Jordan Gap quadrangle the Duff tuff is easily (chalcedony)of lateintroduction. The pheno- delimited by the Cottonwood Spring basalt crysts are quartz and sanidine, rounded and at the base and by the Mitchell Mesa rhyolite embayed by magmatic resorption. Optical at the top. In view of the thinning of the data for the feldspar area => 1.522, /3 = 1.528, = Cottonwood Spring flows, the Duff tuff in 7 1.528; all ± .002. (-) 2V is small. places may have been deposited directly on The optic planeis normal to 010, and Z = b; the Pruett tuff. Such a condition has not X A a = s° +. The dispersion— isr>v. been recognized to date in the areas studied. Chemical analysis. A chemical analysis of the Mitchell Mesa rhyolite and analyses of MitchellMesa Rhyolite other rhyoliticrocks from Trans-Pecos Texas — are giveninTable 8. The high percentageof Definitionanddescription. The thinsheetof potash (6.34 per cent K2O) in the analyzed rhyolite capping Mitchell Mesa in the north- sample of Mitchell Mesa rhyolite is note- western corner of the Buck Hill quadrangle worthy, because the analyses of rhyolitic is named for the mesa. The rhyolite resting rocks from Trans-Pecos Texas indicate these on the Duff tuff covers about 3 square miles rocks are sodi-potassic. The closest approach of MitchellMesaandis animportant lithologic to the composition of the Mitchell Mesa unit in this region as the caprock of the high rhyolite is an analysis of a potassic rhyolite mesas. from the Solitario (Table 8, No. 2). This The rhyoliteis easily recognizedby its pink analysis,however,appearsto beof a weathered prominent glassy rounded color and pheno- specimen, and because soda undergoes greater of quartz and opalescenttabular phenocrysts losses than potash by weathering the analysis crysts of sanidine. The phenocrysts, rarely may deviateconsiderablyfromthe composition exceeding 0.2 inch inlength areset in a dense 3 of the fresh rock. The analysesof rocks from pink or reddish-gray groundmass. The uni- near Shafter (Fig.2) and from west ofPaisano form texture of the rock is broken by light- Mountain (Table 8, Nos. 3, 4) typical gray vesiculated areas and by red inclusions are of of baked and silicified ash. The fracture is available analyses of rhyolitic rocks in this subconchoidal to irregular. The weathered partof Texas. rock is dark reddish gray (rusty) or black. Stratigraphic relations.— -The Mitchell Mesa Unlike the pitted surfaceof weatheredCrossen rhyolite is the youngest rock of the volcanic trachyte, the weathered surface of the rhyolite series in the Buck Hill quadrangle. In Goat commonly showsnumerous smallprotuberances Mountain, 6 miles north in the Alpine quad- ofthemoreresistantphenocrysts. rangle, and in the Jordan Gap quadrangle Therhyolite onthe southernedgeof Mitchell younger unnamed tuff beds and flows occur Mesa is 38 feet thick and probably does not above the rhyolite. Reconnaissance work in exceed 50 feetonthemesa. Asimilarthickness these areas indicates the Mitchell Mesa rhyo- caps Whirlwind Mesa just west in the Jordan liteisaflow. 1162 GOLDICH AND ELMS— BUCK HILL QUADRANGLE, TEXAS — Table 8. Chemical analyses of MitchellMesarhyo- caps Elephant Mountain in the Alpine quad- lite and other rhyolites from Trans-Pecos Texas rangle lies on both the basalt and the tuff (PL 6). In the Buck Hill quadrangle the southern end of the mountain is downthrown SiO2 78.83 76.23 75.12 72.86 alongnorthwest-trendingfaults. On thenorth- A12O3 9.95 12.11 10.94 11.74 east side of the larger fault the Cretaceous Fe2O3 1.40 2.88 2.71 formations are exposed, and the Boquillas is FeO .10 } .86 1.66 overlain by the Pruett. Southwest of the MgO .16 .46 .07 .06 fault the Cretaceous does not crop out, and CaO .35 .36 .20 .24 the lowerPruettbeds are coveredbyalluvium. Na2O 1.78 1.40 4.46 4.63 Pruett —The Pruett formation in K2O 6.34 4.93 4.54 4.92 formation. the Elephant Mountain H2O+ .41 1.47 .19 .40 area is 500-600 feet - thick, H2O .11 1.80 .18 .51 of which approximately300 feet is de- CO2 .27 .04 tr. scribedin thesection(Appendix,SectionNo. 9) TiO2 .20 .13 .20 .20 measured in the large gully on the southern P205 .03 .01 none tr. slope of the mountain. The basal beds are MnO .08 .08 .07 tuffaceous conglomerate and sandstone or BaO tr. none none light-yellow to gray sandy tuff which weathers SrO n.d. none none to a characteristic bright yellow. These beds LiO n.d. 2 tr. tr. are succeeded by varicolored, but dominantly Rare earths n.d. .04 .05 tuff. SO n.d. gray, Inthe lowerpart of the section the 3 none tuff is S tr. .05 tr. massive with indistinct stratification. It is welllithified ZrO2 n.d. .13 .28 and weathers topebblynodu- Ignition 1055.... .92 lar gravel. There are a few minor lenses of breccia and conglomerate. In the upper part 100.01 100.24 99.98 100.33 sorting and stratificationare better developed, Sp-gr 2.596 2.617 2.635 and thin layers of calcareous tuff and fresh- waterlimestoneinterfingerwith the tuff. 1. Rhyolite porphyry capping Mitchell Mesa. — Sheep Canyon basalt. There are at least Cat. No. 599. Sp. gr.,23.4°/4°. S. S. Goldich, thick analyst. three flows of the Sheep Canyon basalt in the southern 2. Potassic rhyolite, East Tank, Solitario. Pheno- slope of Elephant Mountain. crysts of quartz andorthoclase. Groundmassof Along the line of section (Appendix, Section alkali feldspar and quartz. R. B. Ellestad, No. 9) the lowest flow is 88 feet thick with an analyst (Lonsdale, 1940, p. 1620). amygdaloidaltop 25-30 feet thick. The lower 3. Rhyolite near Shafter, Shafter quadrangle. part of the flow is dense porphyritic basalt. "Liparosenearalaskose. Phenocrysts ofsanidine Light-gray to brown cherty limestone, 4-13 and quartz. Groundmass of alkali-feldspar, feet thick and containingsilicified fresh-water (?), aegirite. Spherulitic quartz, riebeckite and gastropods, lies between the first flow and a bands traverse the rock. Sp. gr., 2.617, 15.5°." middleporphyritic basalt flow 121 feet thick. W. F. Hillebrand, analyst (Clarke, 1904, p. 74). Plagioclase phenocrysts, as much as 1 inch 4. Rhyolite west of Paisano Mountain, Alpine quadrangle. "Liparose. Contains alkali-feld- long, are contained in a dark fine-grained spars, quartz, arfvedsonite, and aegirite. Sp. groundmass. The upper 20-30 feet of the gr., 2.635, 15.5°." W. F. Hillebrand, analyst flow is vesicular with numerous chalcedony, (Clarke, 1904,p. 74). jasper,and calcite fillings. Abedof calcareous tuff withlensesoflimestone generallyseparates Volcanic SectioninElephantMountain — the middlefrom the upper flow,but is missing Principal features. On the southern slope along the line of section. This calcareous of Elephant Mountaina considerable thickness bed usually is 5 feet and locally as much as 15 of the Pruett tuff is overlain by flows of the feet thick. The upper basalt flow is 171 feet Sheep Canyon basalt. The flows pinch out thick, but an interval of 50 feet above the to the north, and the massive syenite which highest exposure is covered, and the amyg- TERTIARY SYSTEM 1163 daloidal zone expected at the top of the flow basalt sequence west of Calamity Creek: was not found, so the thickness of 380 feet (1) The stratigraphic succession of tuff, cal- measuredfor the flowsprobablydoes notrepre- careous tuff, fresh-waterlimestone, and basalt sent the total thickness. A probablemaximum flows in two areas is similar. (2) Fossils in thickness is 450 feet. The Sheep Canyon the fresh-water limestone in the tuff and be- basalt flows thin rapidly northward, and on tween the basalt flows are similar. (3) The the east side of Elephant Mountain, 1| miles basalt flows inthe two areas are similar chemi- north of the Buck Hill quadrangle, the flows cally and are porphyritic; they are the only are absent (PL 6). A similar pinch-out occurs porphyritic basalts known in the region. on the westsideof themountain 2\ miles north (4) The flows in both areas occupy former of theBuckHillquadrangle. streamchannels which are part of the ancient An analysis of the porphyritic basalt from drainage system developedafter extrusion of Elephant Mountain (Table 5, No. 3) closely the Crossen trachyte and before extravasation resembles theporphyriticSheep Canyon basalt of theSheepCanyonbasalt. on Crossen Mesa. The Elephant Mountain sample was collectedin a large gully near the Quaternary Alluvium base of the basalt, 105 feet thick, overlain by 10 feet of light-gray fresh-water limestone Alluvial deposits cover large areas in the beneath the syenite. Heavy talus obscures Buck Hill quadrangle and range from clay much of the outcrop, and the field observations and silt to boulders. The deposits are chiefly are inconclusive as to which of the three Ele- basin fill which accumulated inGreen Valley phant Mountain flows is represented. The to a thickness of 30 feet or more. Calamity sample was selected for analysis because it Creek, The Ditch, and Terlingua Creek in represents the freshest material collected. many places are steep-walled arroyos which Approximately1000 feet north of this locality havecut throughthealluviumintothebedrock. the basalt feathers out, and the syenite rests Albritton and Bryan (1939) on the basis of on limestoneand tuff of —the Pruett formation. disconformities have divided the alluvial de- Stratigraphic relations. The tuff, conglom- positsin the valley flats of the southernDavis erate, sandstone, and fresh-water limestone Mountains into three formations, from oldest beds in Elephant Mountain are part of the to youngest: (1) Neville, (2) Calamity, and Pruettformation. Largelow-spiredgastropods (3) Kokernot. In the present investigation collected from the lowerpart of the section are thealluviumisnot differentiated. similar to those collected from the ash at the base of CrossenMesa west of CalamityCreek. Intrusive Igneous Rocks Fresh-water limestone beds in the tuff and between the basalt flows in the Elephant GeneralStatement closely resemble Mountain area thelimestonein Syeniticigneousrocks with definiteintrusive the Sheep Canyon west of area Calamity relationships form two prominent hills in the Creek (PL 6). In both areas Goniobasis is southernpart of thequadrangle. In thesouth- commonin the limestone. North ofElephant centralpartsyeniteforms the flat-topped Mountain the Buck tuff beds are overlain by the Hillfor which thequadrangleis named. West Crossen trachyte. C. L.Baker (personalcom- of Buck Hill a syenite plug is the core of munication, 1941) found tuff, calcareous tuff, StraddlebugMountain. and fresh-waterlimestone,500 feet thick capped Approximately4 miles north of Buck Hill a with a trachyte flow 120 feet thick,resting on smallmass of dense blackbasaltforms Butcher- the Cretaceouslimestone on the western flank knifeHill. of Mt. Ord 12 miles north of Elephant Mountain. Microsyenite Insummary the followingpoints support the — correlation of the volcanic rocks in Elephant Syenite of Buck Hill. The syenite of Buck Mountain with the Pruett-Sheep Canyon Hill is alight-gray, fine-grained,even-textured 1164 GOLDICH AND ELMS— BUCK HILL QUADRANGLE, TEXAS rock weathering to yellowish brown. The At the contact with the syenite the Boquillas tabular mass is cut by two normal faults that flags have been baked to dark, fine-grained divide it into three segments with the middle marble. On the southwestside of Straddlebug block downthrown. The largest and south- Mountain a sill of syenite not more than 6 western segment measures 4500 feet in length feet thick intrudes the Boquillas and appears in a northwest-southeast direction and 3000 tohave hadlittleeffect on thelimestone. The feet across at its widest part. The middle sillis estimatedto be about 50 feet above the segment is 2500 feet wide at its southeastern base of the Boquillas— formation. end, butsplits to the northwest into a number Petrography. The freshest specimens of of narrow offshoots. The northeastern seg- syenite fromBuck Hillshow some weathering, ment is about 2000 feetlong and 700 feet wide. and in samples from Straddlebug Mountain The greatest overall dimension of the syenite the weatheringis pronounced. The feldspars massis 6500feet alonganeast-westline. are dusty, and the ferromagnesian minerals The syenite is a small intrusive which ap- are altered to limonite. The texture is fine- parentlyspread sill-like at or near the contact grained, trachytic to granular. Extinction in of the thin-bedded Boquillas limestone and the feldspar laths is anomalous, and the twin- the overlying basal beds of the Pruett. The ning is indistinct. Large anhedral grains of mass averages 65 feet thick, but it may have feldspar appear to be a microperthitic inter- been thicker as its roof has been removed. growth. Nepheline was not identified, and In the northeastern segment thin-bedded zeoliticmaterialisrare. Inthe syeniteof Buck Boquillas limestone is sandwiched between Hillthere is some hornblende with deep olive- massive sheets of the syenite (PL 5, fig. 2). green to brown pleochroism and a small On the southeasternside of the hill the middle amount of biotite withlight-yellow toreddish- segment of the syeniteoverliesa thinbedof the brown pleochroism. Magnetite and apatite basal Pruett conglomerate which rests on the are accessory. The intrusives are alkalic Boquillas limestone. The conglomerate was microsyenite. — not found on the northwestern side of the Chemical analysis. A chemical analysis of a middlesegment wherenarrowdikes or offshoots sample of syenite from Buck Hill is given in from the main syenite mass cut the Boquillas. Table 9 in which three analyses of similar One of these apophysesis over 2000 feet long. syenites are included. Ferric and ferrous In the southwestern segment, whichis the up- oxides andsome of theminorconstituents were thrown side of the larger fault, the syenite determined on a sample of the Straddlebug lies on a considerable thickness, 14-30 feet, of syenite. The relativelylarge proportionof fer- the Pruett formation. Southeast of Bench riciron is indicative of the weatheringof these Mark 4203 there are xenoliths of the Pruett masses. The percentage of alkalies is large, sandstonein thesyenite. — and calculated normative nepheline is 2 per Syenite of Straddlebug. The syenite of cent of the Buck Hillrock (Table10). Straddlebug Mountain closely resembles that Age and correlation.— Because of similarity of Buck Hill,but it is more weathered. The incomposition and because both the Straddle- smallplug measuresless than1000 feet across, bug Mountain and Buck Hillintrusives areon but, unlike the Buck Hill intrusive, it resulted an east-west line of structural weakness, the in a pronounced deformation and doming of two intrusives may be related and of the same the sedimentary rocks. The oldest beds age. The syenite of Buck Hill is clearly brought to surface are Buda limestone; the younger than the basal conglomerate of the youngest arelowerbedsof theBoquillas. The Pruett formation. sedimentary rocks dip away from the hillon Manysyeniticintrusives inthisregion closely all sides except on the north where they are resemble the microsyenite of Buck Hill and covered by alluvium on the downthrown side Straddlebug Mountain. Two conspicuous of the Chalk Draw fault. Dips as high as 80° masses are the syenite onElephant Mountain wereobserved on the southwest side of Strad- and the Santiago Peak intrusive east of the dlebug, but usually they are considerablyless. quadrangle. The Elephant Mountain syenite INTRUSIVE IGNEOUS ROCKS 1165 Table 9.— Chemical analyses of syenite from Buck intrusive. The timeof intrusionis post-Sheep Hill and other localities in Trans-Pecos Texas Canyon basalt and probably post-Duff. In the Santiago Peak quadrangle Eifler (1943, p. 1635) found Cenozoicsandstones and conglomerates on the eroded Boquillas sur- SiO2 60.98 62.46 59.36 59.62 face which on the southern and western sides A12O3 17.35 17.10 18.20 17.89 Fe2O3 5.46 5.25 2.49 3.97 of Black Mountain are overlain by syenite FeO .49 .08 2.65} 619{ 1.83 capping the mountain. At Santiago Peak the MgO .33 .28 .15 .75 sandstone is overlain by white and lavender CaO 1.38 1.27 1.64 2.78 tuffs 800-900 feet thick. The syenite of Na20.... 5.98 6.84 5.99 6.20 SantiagoPeak towers 1200 to 1300 feet above 4.55 K2O 5.58 5.44 5.28 the highest level at which the tuff was found. HO + .81 .22 .71 2 - .49 These figures are significant because they in- H2O .26 .15 1.10 .38 dicate the Santiago Peak syenite probably TiO .50 .38 tr. .66 2 was intrudedinto a sequence with a minimum ZrO2 .16 .10 CO2 .32 tr.? thickness of the Buck Hill volcanic series. P2P2O5 .28 .10 .11 tr. .26 The sandstone and conglomerate may be the MnO .21 .18 .18 tr. equivalentof thebasal conglomeratememberof BaO .03 tr. none the Pruett formation, and the total thickness SrO n.d. none of sandstones and tuff corresponds closelyto Li2O n.d. tr. the thickness of the Pruett. Originally de- S .00 .00 none posits equivalentto thecombined thickness of Cl n.d. n.d. .31 the Pruett and the Duff probably covered the Rare earths n.d. .03 region, and into these deposits the syenitic 100.12 99.97 98.44 99.60 magma of SantiagoPeak was intruded. The syenite rocks of Santiago Peak and of 1. Alkalic microsyenite of Buck Hill. Sp. gr., Elephant Mountainaremineralogicallysimilar 25°/4°, 2.659. S. S. Goldich,analyst. and ona lineconnectingthese massesare other 2. Alkalic microsyenite of Straddlebug Mountain. large syenitic intrusives in the Santiago Peak S. S. Goldich, analyst. quadrangle. These intrusives, the syenite of 3. Pulaskite, Santiago Mountain, Santiago Peak BuckHilland of StraddlebugMountain,and a quadrangle. Sp. gr., 2.581, 25.5°.' W. F. large number of others in this regionprobably Hillebrand, analyst (Clarke, 1904, 75). p. are closely related genetically. 4. Alkalic syenite, Elephant Mountain, Alpine quadrangle. L.A. Mikeskaand VV. A. Felsing, Basalt analysts (Schoch, 1918, p. 185). Butcherknife 5. Analcitic microsyenite, Sawmill Mountain, Ter- Generalfeatures.— -The Butcherknife basalt lingua-Solitarioregion. R. B. Ellestad, analyst is a fresh,black, very fine-grained rock with a (Lonsdale, 1940,p. 1620). conchoidal fracture. Because the rock spalls readily,the upperslopes of thehillare covered rests on tuff and fresh-waterlimestone of the with basalt fragments, so the relations of the Pruett formation and on flows of the Sheep basalt to the sediments could not be deter- Canyon basalt. The ElephantMountain sye- mined. Near the base of the hill tuffaceous nite (Hardin, 1942) ranges from 800 to 1100 coarsesandstone,sandy andcalcareoustuff, and feet in thickness,but some of the upper part gray ash with thinbeds and nodules of lime- may have been removed by erosion. In the stone of the basal Pruett rest on Boquillas valley just north of Elephant Mountain on limestone. The Boquillas strata away from the Neville ranch three small intrusives of ButcherknifeHillareessentiallyflat-lying, but similar syenitepierce both the Boquillaslime- the Pruett beds closer to thebasalt show local stone and thebasalPruett tuff (PI. 6). These disturbance, and on the northwest side of the probably are the feeders for the Elephant hill dip up to 25° NW. The high dips in Mountain syenite which is considered to be the Pruett beds inthe vicinity of Butcherknife 1166 GOLDICH AND ELMS— BUCK HILL QUADRANGLE, TEXAS

tuff Duff 70.5712.19 1.29 0.090.601.43 1.80 5.093.36 2.840.14 0.220.08 0.010.04 n.d. 0.00 n.d. 99.75 2.421 Volcanic Pruett 69.7312.06 1.06 0.130.88 2.160.78 4.175.05 3.310.010.18 0.040.010.130.090.00 n.d. 99.792.368 Mitchell rhyolite 78.83 9.951.40 0.10 0.16 0.351.78 6.34 0.41 0.11 0.270.20 0.03 0.08 tr. n.d. tr. n.d. 100.01 2.596 Crossentrachyte 71.1712.64 4.28 0.11 0.131.05 3.91 5.050.23 0.12 0.190.56 0.07 0.14 0.08 n.d. tr. n.d. 99.732.648 Hill Potatoandesite 61.4614.85 7.230.68 0.74 5.23 3.75 2.58 0.22 0.170.101.52 1.13 0.11 0.18 n.d. 0.00 n.d. 99.95 2.764 rocks of mvmi Hill ite v 17.35 5.46 0.490.331.38 5.98 5.58 0.81 0.26 0.320.50 0.28 0.21 0.03 n.d. 0.000.16 2.659 sitesWiiuck 60.98 100.12 Buckof 3.46 7.48 3.10 6.605.13 2.381.68 0.26 0.02 2.531.10 0.21 0.06 0.13 0.01 n.d. 2.808 analyst) Butcher- knifebasalt 48.5317.42 100.10 norms d and Spring °£Ig° tss 3.74 7.49 3.865.76 4.50 2.43 2.17 0.97 0.02 2.681.81 0.23 0.12 0.09 0.00 n.d. 2.762 Goldich, Cot Analyses 48.0815.87 99.82 analyses S. (S. basalt Hill Chemical— Cottonwood Potato 46.9015.82 4.18 7.74 4.48 6.24 3.80 2.321.93 1.13 0.02 2.791.97 0.21 0.23 0.100.00 n.d. 99.862.823 10. Jgg»£ 45.9717.26 2.84 9.18 5.63 7.87 4.331.00 1.57 0.20 0.00 2.870.96 0.17 0.07 0.100.00 n.d. 100.02 2.874 Table basalt I Canyon Hill 45.6516.48 6.92 6.40 4.057.48 3.381.42 2.101.39 0.37 3.21 0.87 0.18 0.090.09 0.00 n.d. 100.08 2.829 Sheep IPotato 4321 Hill 44.3314.69 4.27 8.68 4.887.00 3.46 2.152.151.09 0.073.64 2.99 0.230.170.14 0.00 n.d. 99.94 2.818

(t°/4°)

2 5 2 O2O2 2 Oi O O+ 2 O5O 2 2 2 O- 2 Total Sp.gr. SiO AlF^O,FeO MgO CaO Na XK HH HH CO TiO PP MnOBaO SrO S ZrOa INTRUSIVE IGNEOUS ROCKS 1167

44.0437.2515.20 0.20 0.40 1.44 0.30 0.08 0.070.60 27.66 30.02 33.011.95 0.22^ 0.20 4.32 0.46 0.98 0.17 0.50 20.9415.57 31.9615.85 1.85 7.201.67 1.57 2.690.20 33.36 54.49 1.99 1.73 0.35 0.93 4.80 0.91 0.67 0.70 0.18 14.46 33.2717.51 5.25 6.64 8.55 5.10 4.71 2.69 tr. Norms 14.46 38.2515.85 1.36 2.29 9.74 5.34 5.17 4.37 tr. 13.34 31.9618.35 0.41 12.82 4.85 6.03 5.32 4.70 tr. 6.12 29.87 25.023.41 6.80 15.23 4.18 5.47 2.35 1.92 8.34 28.82 25.58 3.27 9.76 9.98 6.08 2.02 0.90 12.79 29.3415.29 1.02 14.91 4.15 6.26 6.84 7.060.20

Q or ab an ne C di wo hy ol mt hm il tn ru ap cc Z 1168 GOLDICH AND ELMS— BUCK HILL QUADRANGLE, TEXAS

Hilland the freshness, theeven dense texture, rocks of the Terlingua-Solitario region de- the irregular jointing, and the absence of flow scribed by Lonsdale (1940), although certain breccia, flow structure, or vesiculation in the rock types from the southern region are not basalt suggest that the rock is not a flow but represented in the analyzed specimens from rather a small—intrusive. the Buck Hillarea, from whichnew types also Petrography. The Butcherknife basalt is are recorded. Most notable of the latter is holocrystalline but very fine-grained. The the MitchellMesa rhyolite, a potassic rock in textureisintergranularophiticwithsmallgrains contrast with the sodi-potassichigh-silicarocks of pyroxene, euhedral crystals of magnetite, of the Terlingua-Solitario region. A rough feldspar, alkalic and minor analcime inter- comparison of normativeminerals of the two stitial to the lathlike plagioclase crystals. groups follows: The largest plagioclase crystals measure 0.3 mm. in length,but the average is much less. No.of Analyses or ab an ne ol Magnetitemakesup 10-15percent of the rock. Buck Hill 10 6-37 29-55 0-26 0-5 0-15 Apatite also is in small prismatic Terlingua- 21 8-31 5-55 0-20 0-11 0-16 abundant Solitario crystals. The rock is classified as analcime trachybasalt. — The analyses from these areas together with Chemical analysis. The chemical analysis three older analysesplottedin usual silica- of the basalt (Table 7,No. 3) is the Butcherknife oxides variationdiagramgiverelativelysmooth by its relativelylarge content of characterized curves for the group of 34 samples (Fig. 5). alkalies. The analysis somewhat resembles Certain featuresof the variations are worthy of that of samples of the Cottonwood Spring comment. basalt and those of the Sheep Canyon basalt (1) Strong curvature of the variation lines (Table 5) and of basaltic rocks from the suggests strong fractionation of the liquids Terlingua-Solitario region. The Butcherknife from which the rocks were derived. basalt closely approaches the composition of (2) The downward curvature for K2O in analcime syenogabbro from Cigar Moun- an the high-silica range is notable. for tain. Points — the curve require little smoothing except Ageandcorrelation. TheButcherknifebasalt for samples of the Potato Hill andesite and the is younger than the lowermost Pruett. Al- MitchellMesarhyolite. The andesite analysis though the basalt cannot be Butcherknife is anomalousinits abnormallyhighsilica con- Sheep correlated definitely with either the tentwhichis theresultof theintroduced quartz Canyon basalt or the younger Cottonwood and chalcedony of the amygdules. By sub- Spring basalt,it is muchlike themincomposi- tracting silica equivalent to the normative tion. quartz and recalculating, the andesite can be Igneous Petrology plottedat 51.4per cent SiO2. Therecalculated K2O then is 3.3 per cent, and this value fits The chemical analyses of the igneous rocks the K2O curve well. Similarlypoints for other from the Buck Hill area and the calculated constituents are shifted to the left largely norms (Xable 10) bringout the alkalic affinities eliminating the apparent anomalous positions which characterize the Tertiary igneous rocks of andesitepoints plotted on Figure5, Extra- of Trans-Pecos Texas, The silica content of polationfrom themainK2Oline totheMitchell the analyzed samples ranges from 44.3 to 78.8 Mesa rhyolite gives aline trending upward in per cent, and total alkalies (Na2O and K2O) the high-silicarange whichis insharp contrast from 4.8 to 7.5 per cent. Relatively large with the plotted curve for the Trans-Pecos amounts of TiO2 and P206 mark the low-silica rocksbut more nearlylike the usual variations lavas. Notable quantities of SrO and BaO ofK2O inrock series. were determined in these samples; strontia (3) As a group, the analyzed lavas from the ranges from 0.09 to 0.14 per cent, and baria, Buck Hill area contain relatively more total from 0.06 to 0.23 per cent. The Buck Hill iron oxide(FesO3) than the Terlingua-Solitario rocks are similarin compositionto the igneous rocks. INTRUSIVE IGNEOUS ROCKS 1169

Solitario region comparing them to alkalic subprovincesat the front of the Rocky Moun- trains.

Structural Geology General Features The effects of two crustal disturbances can be seenin the BuckHillregion. TheLaramide revolution at the close of the Cretaceous resulted in board upwarping and erosion prior to the deposition of the TertiaryPruett formation. IntheBuckHillandElephantMountain areas there is a discordance between the dips intheCretaceouswhichnormally donotexceed 4° and in the overlyingbasal Pruett conglomerate and sandstone. Along the 02 ranch road southof Crossen Mesa dips in the Pruett are as high as 14°, but the beds flatten rapidly northward. Dips in the underlyingBoquillas limestone are 2°-3° N. South of the road at this locality the basal conglomerate occupies a swale or depressiononthe oldBoquillas surface. At Hill 3927 northeast of Butcherknife Hillthe basalPruettbeds dip 8° N. The high dips in these beds are intrepreted as initial dips off hills on the Boquillas erosion surface. Faults of late Tertiary or early Quaternary age are the outstanding structural features, but the Buck Hill volcanic series has been deformed into broad open folds. Thefaults areallof the normaltype andcut the youngest rock of the volcanic series and also the syenite intrusives. In the Buck Hillquadranglethere aretwomajorfaultsor fault zonesalongwhich a central block is depressed structurally. The eastern part of the quadrangle is structurally — high. Cretaceous rocks are not exposed west Figure 5. Variation Diagram of Buck Hill and Similar Igneous Rocks of Whirlwind Spring in the northern part of Circles represent analyzed samples from the areaand westof The Ditch in the southern Terlingua-Solitario region;analyses of rhyoliterocks the from Shafter andPaisano Mountain and of syenite part, except at Straddlebug where the Cre- fromSantiago Peak are included taceous strata are domed around the syenite plug. From Whirlwind Spring northwest in Broader regional studies of the Cenozoic the quadrangle and west in the Jordan Gap igneous rocksof Trans-Pecos Texasare now in quadrangle progressivelyyounger rocks of the progress,and a larger number of analyses may Buck Hillvolcanic series areexposed. show greater diversity and relations other Effects of older orogenies that produced than those suggested by the present analyses. the structure in the Paleozoic rocks in the Lonsdale (1940) has discussed the chemical Marathon Basin (King, 1937) to the northeast characteristics and origin of the Terlingua- and in the Solitario (Sellards, 1932, p. 55-131; 1170 GOLDICH AND ELMS— BUCK HILL QUADRANGLE, TEXAS

Baker, 1934, p. 177-180) to the south cannot luvium. From Whirlwind Spring the fault be seen as rocks older than the Georgetown continues to the east boundary of the quad- limestone do not crop out in the Buck Hill rangleand is marked by the southward-facing quadrangle. Likewise, little information con- Boquillas cuesta. The Walnut Draw fault cerning the Laramide deformation is available zone within the quadrange is 15 miles long from work in the Buck Hill area; therefore, but continues eastward in the SantiagoPeak only the Cenozoic structural features are here quadrangleand northwardin the Alpine quad- considered. rangle. The displacement along the Walnut Draw CenozoicFolding fault ranges from 300 feet at the eastern end, where the upper beds of the Boquillas forma- The Buck Hill volcanic series in the Buck tion are droppeddown against the lower part Hill and Jordan Gap quadrangles has been of the Boquillas and against the Buda lime- folded into a broadnorthwest-plunging nose. stone, toapproximately1000 feet at the north- In the northern part of the Buck Hill quad- westernend wherethe Duff tuff is downthrown rangle theregionaldip islessthan2° N.orNW. against the Cottonwood Spring basalt. Be- The scarps of the Pruett tuff beneath the tween these points the displacement on any Cottonwood Spring basalt and of the Duff tuff one fault of the zoneis small, but the cumula- beneaththe MitchellMesa rhyolite face south- tive resultis astratigraphic throw of approxi- ward toward Green Valley. In the western mately 1100 feet (Fig. 6), part of the quadrangleremnants of the Cotton- Walnut Draw is a graben formed by two woodSpring flows cap hills north and south of faults of the zone that are as much as half a the 02 ranch house, and west of thesehills in mile apart. The downthrownblock is tilted the Jordan Gap quadrangle the scarps of the and dips 9° NW., so the throw of the faults basalt-capped cuestas face southeastward to- increases in that direction. Flows above the ward Green Valley with a gradual shift from Crossen trachyte on the mesa occur 400-500 aneast-weststrike in theBuck Hillquadrangle feet lower in the graben, and additionaldis- toa north-southstrikeinthe JordanGapquad- placementup to severalhundredfeet is effected rangle. Similarly,the outcrop of the Mitchell bya third faultthatmakesa horstof thesouth- Mesa rhyolite swings southward in the high westernblock of the Walnut Drawgrabenand mesas of the Jordan Gap quadrangle and brings the Duff tuff downagainst the Cotton- describes a rough arc with a change from a wood Spring basalt. The latter fault is the northerly dip to a westerly and southwesterly master of the zone. Vesicular basalt on the dip in western and southwestern Jordan Gap southwestsideof this faultsouthofthetrachyte- quadrangle. These structural features indi- capped spur is mapped as Cottonwood Spring cateabroadfold whose axisplunges northwest basalt but may be Sheep Canyon basalt, in- inadirection roughlyparallelto the northwest- dicating a minimum displacement of 500 feet trendingfaults of the westernpart of the Buck with a probablemaximumof 850 feet. In the Hillquadrangle. fault zone west of the Pruett ranch house the displacementis taken up ina number of faults, CenozoicFaulting and slivers of the Crossen trachyte have a pronounced roll or dip to the southwest in- WalnutDraw andrelatedfaults.—The Walnut dicating that displacement in part was ac- Draw fault zone, named for WalnutDrawon complished by bending of the volcanics. the western edge of Crossen Mesa, consists of East of Whirlwind Spring the Boquillas roughly parallel and imbricate faults trend- limestone forms a southward-facing cuesta in ing southeast that set CrossenMesa off from the upthrown scarp along the eastward con- the structurally depressed block to the south- tinuation of the Walnut Draw fault zone. west. The fault zone can be traced continu- Most of the Pruett formation has been dis- ously to the vicinity of Whirlwind Spring, placed along the concealed fault between butsouth ofDark Canyon and east of Whirl- WhirlwindSpringand the Boquillasoutcrop to wind Spring the master fault is coveredby al- the east. This fault, covered by alluvium STRUCTURAL GEOLOGY 1171 between the02ranchroadandDark Canyon,is profound influence on the physiography. Al- the master fault. It probably parallels the though the fault is traced with difficulty in mapped faulton the westsideof the Whirlwind theincompetent beds of the Pruett tuff and in Spring spur,but the mapped fault is a minor part is covered by alluvium,it extends prac-

— Figure 6. Columnar Sections at Hills 4631 Feet and 4975 Feet Showing correlation of the Cottonwood Spring basalt flows andinterpretationof structure.

one, and east of Hill 4631 the Cottonwood tically the breadth of the quadrangleand con- Springbasaltis downthrownagainst the upper tinues eastward in the Santiago Peak quad- breccia-conglomerate member of the Pruett rangle to Chalk Draw where it swings to the with a displacement of 60— feet. southeast. The fault was named by C.L. Faults ofthe central area. South ofMitchell Baker (1934, p. 209), and its upthrown scarp Mesa numerous northwest-southeast faults forms the west wallof ChalkDraw valley from cut the Cottonwood Spring basalt. The dis- the southern part of the Santiago Peak quad- placement on these faults usually is less than rangle to the Rosillos Mountains (Fig.2). 100 feet, but their net effect is an appreciable Although the southern upthrown block of lowering of the volcanics to the southwest. the Chalk Draw fault in the Buck Hill quad- The faults cannot be traced to the southeast rangleis structurally high,there is astructural because of the alluvium in Green Valley,but loweringfromeast towestaccomplishedmainly in the eastern and centralparts of the quad- by a set of northwest-southeast faults. These rangle three faults with a northwest trend are secondary faults divide theupliftedblock into mapped, and these also result in a structural three segments. The easternmost segment is lowering to the southwest. The large fault east of the Alpine-Terlinguahighway and is southwest of Butcherknife Hill swings east- cappedby thelower flaggy beds of the Boquil- wardandparallels the ChalkDrawfault,form- las formation whichalso form the downthrown ing a graben. Chalk Draw road follows this block onthenorthside of the ChalkDraw fault. alluvium-coveredstructural depression. Northeast of StoneTank thedownthrownblock Chalk Draw and relatedfaults.— -The east- dips at an angle of about7° towardthe fault west Chalk Draw fault structurally dominates plane and abuts against the Georgetownlime- the southernpart of the quadrangleand has a stone. The throw on the Chalk Draw fault 1172 GOLDICH AND ELMS— BUCK HILL QUADRANGLE, TEXAS

here is approximately225 feet, but westward Physiography the stratigraphic throw decreases, and the TheBuckHillquadrangleliesin theMexican Boquillas beds rest against the Grayson and Highlands Province (King, 1937, p. 2) and Buda formations. affords a markedcontrast betweenitsnorthern The middle segment is largely west of the part containingresidual mesas of the southern highway and is cappedby the upper limestone Davis Mountain front and its southern part beds of the Boquillas. Small grabens result occupiedlargelyby the northeastern end of an from the northwest-southeast faults in this extensive lowlands, Green Valley. East of segment; in one a blockof the syeniteat Buck thequadrangleis thefolded SantiagoMountain downfaulted,and in Hillis a second the basal range which is continued to the southeast in of tuff beds thePruett liein avalleyflanked by the Sierra del Carmen (Fig. 2). North and the Boquillas limestone. The downthrown south of the quadrangle the Alpine and Ter- side of the ChalkDraw fault in this segmentis linguaareas are important centers of Tertiary the Pruett tuff, and along the fault the tuff intrusive action, and in both these areas the beds are characterizedby dipsup to 40° NW. prominentphysiographicfeatures arethe peaks The high dips suggest that the throw along whichareigneousplugs uncoveredbyerosion. Chalk Draw fault may be great,althoughdis- The semiarid climate of Trans-Pecos Texas placementon the faultcannotbedetermined. brings out the resistant quality of the lime- The third segment in the southwest corner stones and igneous rocks whichformthe promi- of the quadrangle is composed of the Pruett nent topographic features. The mesas in tuff whichis downfaulted against the Boquillas the northern part of the Buck Hillquadrangle limestone south of Loy Place and also is ex- are of unequal height and altitude, because posed in The Ditch and in Terlingua Creek. different flows cap them and because faulting In this area the Chalk Draw fault cuts the has disturbed the normal relations of the vol- north slope of Straddlebug Mountain and is canic series. The mesa tops are essentially exposed to the west in Terlingua Creek where dip slopes on flows. MitchellMesa cappedby the tuff beds on the downthrown side of the rhyolite porphyry is the highest point in the fault dip 36° NW. The high dips in the quadrangle with an altitude of 5351 feet, and Cretaceous strata flanking the syeniteplug of betweenthis mesa and Green Valleythere is a Straddlebug Mountain are the result of the maximumrelief of 1700 feet. intrusive action. Crossen Mesa cappedby trachyteporphyry The structural lowering from east to west is several hundred feet lower than Mitchell along the Chalk Drawfault is inkeeping with Mesa. Hills rising above the trachyte cap- the increasing magnitude of structural de- rock are cappedby flows of the Cottonwood pression from east to west along the Walnut Springbasalt. PotatoHillis of this type, and Draw fault and asimilar loweringfrom north- is the highest point on Crossen Mesa (B. M. east to southwest on the faults in the central 5071 feet). The lower mesas southwest of part of the quadrangle. The northwest dip Crossen Mesa are also capped by the Cotton- of the Pruett tuff beds along the Chalk Draw wood Springbasalt and share the low regional fault may be related to this increasing dis- northwest dip. The highest points near their placement. southern scarps generally are less than 4500 Elephant Mountain faults— Two parallel feet above sealevel. The breccia-conglomer- faults trending northwest lower the Sheep ate, 60feet below the top of thePruett,makes Canyon basalt and Pruett tuff in the southern a prominentbench in the tuff below the Cot- tip of the mountain, The displacement on tonwood Spring basalt. The breccia-con- the larger northeastern fault is at least 550 glomeratecapsa number of spurs;a prominent feet. In this corner of the quadrangle the one is Boat Mountain (PI. 3, fig. 2). Cretaceous formations attain their greatest Small consequent streams on the mesas altitude,and the top of the Cretaceous is 4550 flownorthward on thedipslopes,but the master feet above sea level as compared to 4000 feet drainage is southward and opposed to the abovesealevelat BuckHill. regional dip. The northward-flowingstreams PHYSIOGRAPHY 1173

on Mitchell Mesa are captured in the Alpine town limestone to calcareous shale of Eagle quadrangle by Walnut Draw and Goat Creek Ford orlowerAustin age. South of the quad- and diverted to theTerlinguadrainageby way rangle, Cretaceous formations (Adkins, 1932, of Duff Creek and The Ditch. South of Duff p. 451, 505-512) younger than the Boquillas Springs a trellis drainage pattern controlled include the Terlingua, Aguja, and Tornillo by the northwest-southeast faults has been formations. The Terlinguaandthe lowerpart developed, and the tributaries to Duff Creek of the Aguja are marine,but the upper Aguja occupy small grabens. Two miles south of beds contain black carbonaceous and lignitic Duff Spring the valley is half a mile wide,but shales that mark a change from marine to the present stream flows in a narrow channel brackish-water and nonmarine deposition. 30 feetdeepcut inthealluvium. Remnants of the Terlinguaand Aguja forma- The central and southern partsof the quad- tions occur south and east of the Buck Hill rangle consistof rollinghillsand cuestas carved quadrangle. Calcareous shale at Hill 3927 inCretaceouslimestoneand shaleand of valley northeast of Butcherknife Hill contains lower flats which are alluvium-filled, structurally Austin Foraminiferaand maybe aremnant of low areas. The cuestas are fault scarps with the Terlingua (Austin) formation. the downthrown side commonly covered by Uplift inlate Cretaceous time initiating the alluvium. The most conspicuous scarp is Laramide revolutionresulted in erosion of the formed by the Georgetownlimestonealong the Terlingua,Aguja, and the Tornilloif thelatter ChalkDrawfault. The Boquillascuestaalong was deposited in the Buck Hill area. The the Walnut Draw fault east of Whirlwind basal Pruett conglomerate, which rests on the Springhas beenbreached by Calamity Creek. Boquillas limestone in the Buck Hill quad- North of the fault scarp on either side of rangle,lies on the Terlinguaclay in anumber CalamityCreek arelowroundedhills whichare of places in the Aqua Fria quadrangle and on typical of the Boquillas formation (PI. 4, the Aguja formation on either side of the fig. 2). Calamity Creek, like Duff Creek, has Alpine-Terlingua highway southeast of Hen cut a steep-walled channel in the alluviumof Egg Mountainin the northernpart of the Ter- Green Valley. Similar channels characterize lingua quadrangle,approximately 20 miles to muchof the courses of TheDitch andTerlingua the south. North of the quadrangle in the Creek hi the southwestern part of the quad- vicinity of Mount Ord, the volcanics rest with rangle, and these streams are now actively "well-marked unconformity" (King, 1930, eroding the Pruett tuff bedrock. p. 98) on rocks of Washita age. These ob- The active downcutting with renewal of servations indicate the widespread angular erosion of bedrock in this region is attributed unconformity between the Cretaceous strata byBaker (1934, p. 140) to recent rejuvenation. and the Tertiary volcanics, with the volcanic According to Baker the development of the series resting on progressively younger Cre- Rio Grande involved a number of local base taceous rocks as one goes south toward the levels whichhave left their recordin "erosion- Chisos Mountains. The persistence of the cut or 'pediment' surfaces of the bed rocks in basal Tertiary conglomerate suggests that a numerous places". Remnants of terraces or similar break might be expected between the benches approximately50 feet abovethepresent Tornillo formation, if it is of Cretaceous age, valley level can be seen west of the Alpine and the Chisos beds in the Chisos Mountain highway in the northern part of the quad- area. rangle and in a number of placesin the south- West of the Buck Hill region in western western part. These benches are erosion-cut Presidio County (Fig. 2) the effects of the surfaces on well-indurated Pruett tuff beds Laramide revolution aremore pronounced,and with a thin veneerofgravel. the Tertiary volcanics rest on Permian strata in the Shafter area and on folded Upper Cre- Summary Geologic History taceous formations in the Rim Rock country of the Sierra Tierra Vieja. In the latter area The Cretaceous record in the Buck Hill near Provenir vertebrate fossils date the Vieja quadrangle is limited; it ranges from George- volcanic seriesaslowermostOligocene. Abun- 1174 GOLDICH AND ELMS— BUCK HILL QUADRANGLE, TEXAS

dant igneous boulders in the basal Vieja con- mechanically derived from the exposed Cre- glomerate indicate Eocene or possibly late taceous formations, but the thick sequence of Cretaceous igneous activity with erosion of tuffaceous limestone inSheep Canyon and in intrusives and possibly flows to supply the the ElephantMountain area was precipitated. boulders. In contrast with the Vieja con- The algal structures and charophytes suggest glomerate the basal Pruett conglomerate in that muchof thelimestoneis of organicorigin. the Buck Hill region is composed of well- The Tertiary lakes probably were temporary roundedpebbles and cobbles of limestone and and shallow, and the lavas do not show struc- chert. Large angular pieces were derived tures or alteration commonly found in sub- from the formations immediately below the aqueous flows. The source of the volcanic conglomerate, but igneous pebbles are few or ash is problematical. The finermaterial could wanting. The first appearanceof largeigneous havecomefromgreatdistances,but thecoarser pebbles and boulders is in the upper breccia- detritus,such as thebreccia-conglomerateinthe conglomerate in the upper part of the Pruett upperPruettand theconglomeratesof theDuff, tuff, and at approximatelythis level the inter- probablywerederivedfromnear-by sources. calated flows appearin the sequence. In late Tertiary time syenitic and basaltic The Pruett is assigned tentatively to the magmasintruded the Cretaceous and Tertiary Eocene on the basis of fossil gastropodsin the formations. The syenite at Buck Hill in- fresh-water limestone beds that occur below truded the lower Pruett beds, and the Ele- and above the Crossen trachyte and between phant Mountain syenite invaded upper beds, flows of the Sheep Canyon basalt. Therefore, coming to rest on the Sheep Canyon basalt. these flows canbe dated tentativelyas Eocene. The syeniteof SantiagoPeak stands 1200 feet The younger PotatoHillandesite alsomay be above the highest level of Cenozoic tuff beds of Eocene age,but there is nodefinite evidence that encircle its base. In the southwestern for dating the time of its extrusion or of that part of the Jordan Gap quadrangle igneous of the overlying Cottonwood Spring basalt plugs intruded the Duff formation and the flows. The Duff formation is assigned tenta- younger tuff beds above the Mitchell Mesa tively to the Oligocene. It is not likely that rhyolite. These intrusives may not all be of deposition was continuous, and undoubtedly the same age,but theyindicate activity inlate materials were reworkedand locally were re- Tertiary time;however, theyantedate the late movedby streams. Ratherlargebreaks in the faulting. In the Marathon basin numerous tuffs could escape detection because of the small igneous intrusives commonly are closely nature of thematerials. related to the structure, and King (1937, p. The various flows in the Buck Hill volcanic 117) points out that the Tertiary intrusives series were all more or less weathered and "appear to have come up along the planes of eroded. The Crossen trachyte in Sheep thrust faults of Paleozoic age". The align- Canyon (PL 6) was deeply incised, and the ment of the syenite intrusivesatBuck Hilland younger Sheep Canyon basalt flows which StraddlebugMountain along the Chalk Draw filled the ancient stream channels rest on fault is interesting in this respect, and suggests fresh-water limestone and on trachyte. The that the fault along which movement took PotatoHill andesitealso was deeply weathered, place in late Tertiary time was an older zone of and the variable thickness of the Cottonwood weakness along which the syenite intrusives Springbasalt,inpart due to thinning or feath- wereemplaced. ering out of the flows, probably is also the The time of folding and faultingof the Buck result of erosion prior to deposition of the Hill volcanic series cannot be demonstrated Duff tuff. from the data at hand. It is at least late Stratification of the tuff and the fresh-water Tertiaryand older than the basin fill and the limestone beds indicate that much of the Pruett alluvial deposits of the region. Thestructural was deposited in water, probably in large history of Trans-Pecos Texas is complex, and lakes. The calcium carbonate, especially in a full understanding will be gained only with the lowerpart of the formation, may havebeen solution of some of the larger structural prob- SUMMARY GEOLOGIC HISTORY 1175

have been summarized by Baker Baker, C. L.(1928) Desertrange tectonics of Trans- lems which Pecos Texas,Pan-Am. GeoL,vol.50,p.341-373. (1934; 1941) and King (1935; 1937). (1934) Major structuralfeatures of Trans-Pecos Since the late Tertiary uplift, erosion has Texas, Part 2, The geology of Texas, vol. 2, economic geology, Univ., great thickness volcanic Structural and Texas removed a of the Bull. 3401, p. 137-214. series, formingGreen Valley and again expos- (1941) Rim Rock country of Texas, Pan-Am. strata. Terraces and al- GeoL, vol. 75, p. 81-90. ing the Cretaceous and Bowman, W. F. (1917) Geologic explora- luvial deposits indicate that the process has tion of the southeastern front range of Trans- been controlled by temporarybase levels, and Pecos Texas,TexasUniv.,Bull.1753,p. 61-172. present downcutting indicates re- Berry, E. W. (1919) An Eocene flora from Trans- the active Pecos Texas, U. S. Geol. Survey, Prof. Paper cent rejuvenation. 125-A, p. 1-9. Clarke, F. W. (1904) Analyses of rocks from the laboratory the United States Geological Sur- ■ of Economic Geology vey,U. S. Geol.Survey, Bull. 228. Cockerell, T. D. A. (1914) Tertiary Molluscafrom There are no ore deposits in the Buck Hill New Mexico and Wyoming, Am, Mus. Nat. islittlelocaldemandfor Hist., Bull, vol. 33, p. 104. quadrangle,and there Eifler, G. X.,Jr. (1943) Geology of the SantiagoPeak rock aggregateorbuildingstone. A numberof quadrangle, Texas, Geol. Soc. Am., Bull., vol. tests drilled in exploration for oil and gas in 54, p. 1613-1644. Elms, M. A. (1937) Geology of the Buck Hillquad- the region have been unsuccessful. Water rangle, Brewster County, Texas, M. S. Thesis, supply is a localproblemin part taken careof Agric. Mech. College of Texas. of tanks to store the surface Graham, D. W. (1942) The geology of Paradise by construction Valley, Trans-Pecos Texas,M. S. Thesis, Agric. run-off for watering cattle. The tanks are Mech.College of Texas. earthen or more rarely concrete dams thrown Hardin, G. C., Jr. (1942) The geology of Elephant Mountain and vicinity, Trans-Pecos Texas, Ph. across drainageways with the storage capacity M. Thesis,Univ. Wis. commonlyincreasedby scooping out a depres- Henderson, Junius (1935) Fossil non-marine Mol- sion. Water alsois obtainedfromwellsequip- lusca of North America, Geol. Soc. Am., Spec. Paper 3. ped with windmills. In the northwestern Huffington, R. M. (1943) Geology of the Northern part of the quadranglethe springs whichissue Quitman Mountains, Trans-Pecos Texas, Geol. from the flowsof the CottonwoodSpringbasalt Soc. Am.,Bull., vol. 54,p.987-1046. King, P. B. (1930) The geology of the Glass Moun- have been the main source of water which is tains, Texas,Part 1,DescriptiveGeology, Texas. piped to the southern part of the area. The Univ., Bull. 3038. Cottonwood Spring has (1935) Outline of structural development of water in the basalt Trans-Pecos Texas, Am. Assoc. Petrol. Geol., been developedrecentlyby the 02 ranch,and Bull., vol. 19, p. 221-261. a number of wellsup to 300 feet indepth were (1937) Geology of the Marathonregion, Texas, U. S. Geol. Survey, Prof. Paper completed in the area of the basalt south of 187. Lonsdale, J. T. (1940) Igneous rocks of the Ter- Mitchell and Crossen mesas. A good well at lingua-Solitario region, Texas, Geol. Soc. Am., the Pruettranch house obtains waterfrom the Bull., vol.51, p. 1529-1626. Lord, D. N. (1900) Report on igneous rocks from WalnutDrawfaultzone. Water at theKoker- the vicinity of San Carlos and Chispa, Texas, not ranch is drawn from wells in the alluvial U. S. Geol. Survey, Bull. 164, p. 88-95. gravel. Osann, C. A. (1892) Report on the rocks of Trans- Pecos Texas, Texas Geol. Survey, 4th Ann. Rept., p. 123-137. References Cited Plummer, F. B. (1932) Cenozoic systems in Texas, Part 3, Thegeology of Texas, vol.1,Stratigraphy, Adkins, W. S. (1927) The geology and mineral re- Texas Univ., Bull. 3232, p. 519-818. sources of the Fort Stockton quadrangle, Texas Roemer, Ferdinand (1887) Graptocarcinus texanus, Univ.,Bull. 2738. tin Brachyure aus der oberenKreide yon Texas, (1932) The Mesozoic systems in Texas, Part 2, Neves Jahrb. Bd. 1, p. 173-176. The geology of Texas, vol.1,Stratigraphy, Texas Roberts, J. R. (1918) A geological reconnaissance Univ., Bull. 3232, p. 239-518. of Val Verde County, Texas Univ., Bull. 1803. Albritton, C. C, Jr., and Bryan, Kirk (1939) Ross, C. P. (1943) Geology and ore deposits of the Quaternary stratigraphy in the DavisMountains, Shafter mining district, Presidio County, Texas, Trans-Pecos Texas,Geol. Soc. Am., Bull., vol. U. S. Geol.Survey, 8u11.928-B, p. 45-125. 50, p. 1423-1474. Schoch, E. P. (1918) Chemical analyses of Texas Baker, C. L. (1927) Exploratory geology of a part of rocks and minerals, Texas Univ., Bull. 184. southwestern Trans-Pecos Texas, Texas Univ., Sellards, E. H. (1932) The Pre-Paleozoic systems in Bull. 2745. Texas, Part 1, The geology of Texas, vol. 1, 1176 GOLDICH AND ELMS— BUCK HILL QUADRANGLE, TEXAS

Stratigraphy, Texas Univ., Bull. 3232, p. 15- country, Brewster County, Texas, Texas Univ., 238. Bull.93 (Sci. ser., Bull.11), 101p. Stenzel,H.B. (1944) A newCretaceouscrab,Grapto- (1925) Etched potholes, Texas Univ., Bull. carcinus muiri, from Mexico, Jour. Paleont., 2509, p. 5-9. vol.18, p. 550-551. Vaughan, T. W. (1900) Reconnaissance in the Rio Streeruwitz, W. H., yon (1890) Geology of Trans- Grande coalfields of Texas,U. S. Geol.Survey Pecos Texas, Texas Geol. Survey, Ist Ann. Bull. 164, p. 76-81. Rept, p. 217-235. Udden, J. A. (1904) The geology of theShafter silver Univ. Texas, Bur. Economic Geology, Austin, mine district, Presidio County, Texas, Texas Texas; Phillips Drilling Co., Milam Univ.,Bull. 24 (Mm. Surveyser.,Bull. 8), 60 p. Bldg., San Antonio, Texas. Manuscript (1907a) Report on a geological survey of the Received by the Secretary of the lands belonging to the New York and Texas Society,May 23, 1947. Land Company, Ltd., in the Upper Rio Grande Project Grant338-40. embayment in Texas, Augustana Library Pub., Published with Permission of the Directorof No. 6, p. 51-107. the Bureau of Economic Geology, The 1907b) A sketch of the geology of the Chisos University of Texas. Appendix DetailedMeasuredSections SectionNo. 1 Thiclkness Feet the limestone and the upper part Inches Section of Buda of ing tolight tawny-yellow tored- the Graysonclay dish buff; erosion leaves ledges Measured inthe westbankof Calamity Creek at of the limestoneflags 2 6 apoint4|miles southof theKokernotranchhouse. i. Marl, sandy, buff, fine-grained, Thickness soft, some thinlimestonelentils 3-6 Feet Inches . Clay, green to buff, compact or Budaformation: slightly shaly 17. Limestone, gray, porcelane- 6 ous, thick-bedded, brecciated; i. Shellbed;composed almost en- weathering to a dark-gray tirely ofHaplostichetexana with etched surface 25 sandy matrix, weathering to buff and tawny yellow 1-2 16. Limestoneand marl; limestone, gray,nodular,thin-bedded,fine- >. Sand, shaly, weathers to bright grained,fossiliferous;marl, buff yellow, dullblue andbuff; fine- togray,weathering tolight tan, grained, friable 2 Ammonites,echi- fossiliferous. ■. Clay, light-buff, greenish-yel- noids,etc 20 low, weatheringto tawnyyellow 15. Limestone, gray, thick-bedded, and variegated colors, forms brecciated;lowersurface undu- receding slope 2 2 lating 12 i. Limestone, flaggy, lenticular, Graysonformation: dark gray-blue, medium- 14. Clay, blue-green, brown, red; grained, sandy, weathering red- oxidizedalongjointsandcracks; dishbuff 2 some thin sandstonelayers.... 2 !. Shale, light greenish-yellow to 13. Clay, green to buff; variegated olive-green, weathering to very green, blue,buff, andyellow on light-buff, grading laterally to weathering; compact to lami- sandy limestone flags 3-4 nated,grading toshale 2 .. Limestone andclay 2 12. Limestone flags andmarl, similar to unit No.9 5 Total thicknessmeasured... 77 11. Marl and clay; weathers to Lower part of Grayson covered. tawny yellow 2 Section No. 2 10. Limestone and marl; limestone gray,weatheringwhite,nodular, ■ectionof the upper part of the Graysonclay %n eas\ contains Haplostiche texana; bank oflargefault gully at southeasternfoot clayey beds tawny yellow with ofElephant Mountain abundant echinoids and Exo- Thickness gyra 8-12 Feet Inches 9. Limestone and marl; limestone flags, nodular, dark fadaformation: gray-blue,sandylenticular^weathering to 13. Limestone, gray, fine-grained, reddish sandy surfaces; marl, nodular; lower surfaceuneven, sandy andclayey,buff, weather- undulating 3 APPENDIX 1177

Thickness Thickness Feet Inches Feet Inches Grayson formation: 12. Clay, and shale; alternating 36. Limestone, light-buff, fine- brown shale and white clayey grained, weathers buff 2 material, laminated; weathers 35. Limestone, fine-grained, dull tannish white 6-8 slightly nodular beds approxi- 11. Sandstone, fine-grained,stained mately1foot thick separatedby reddish 2 thin marly layers, contains Inoceramus 8 6 Shale, brown, 10. tan to weathering gray to lighter color,laminated 5 34. Limestone, thick bed, to light-buff or tan, very fine- 9. Sandstone and shale alternat- grained, contains small limo- ing, reddishsand in layers less nite particles, forms overhang- than 1inch thick;brownish-red ing ledge, contains Inoceramus shale 1 3 fragments 2 6 8. Sandstone andshalewithabun- 33. Limestone and marl alternat- dant fossils: Exogyracartledgei, ing in 4-inch beds, limestone Exogyra whitneyi, Ostrea sp., slightlynodular 2 Pecten commonly sp. Shells 32. Limestone, 1-foot sepa- broken; sand, fine, tan to red- beds dish brown 1 rated by thinmarl layers 5 6 31. Limestone, light-gray withbuff 7. Shale, sandy, brown, fine- grained, 1 3 tinge, fine-grained, weathers laminated buff to light yellow, contains 6. Limestone,gray,nodular,even- impressionsof Inoceramus 2 6 textured, slightly sandy near top 6-8 30. Limestone, and marl interbedded; limestone as in inter- 5. Shale and sandstone; thin- val above, averaging 8-10 bedded, brown, fine-grained, inches thick; marl slightly yel- weathers tan, friable 7-9 low, gray near top 3 4. Shell breccia with calcium car- 29. Limestone, light-gray to buff, bonate cement 2-3 fine-grained 1 3. Shale, some sand, brown, lam- 28. Limestone and marly shale, inated 8 alternating in beds up to 6 2. Sandstone, calcareous, fine- inches thick; limestone, gray; grained, well cemented 2 marl,buff; Inoceramuscasts. .. 6 1. Limestone and shale; alternat- 27. Limestone and marl inter- ing layers; nodular limestone, bedded in layers averaging 2 very fossiliferous near top with inches thick; marl beds more Inoceramus, echinoids, andgas- uniform in thickness; colors as tropods; weathers to tan and above 5 dark buff 4 26. Limestone flags and shalealter- nating, contacts undulating Total thickness measured 14 with thickening and thinning, ... shale weathers to platy structure;flags 2inches thick, except SectionNo. 3 for 4-inch flag toppinginterval; banding results from weather- Sectionof theBoquillasformation ing of flags. Many Inoceramus Measured in gully on south sideof Elephant casts 8 6 Mountain3 miles northeast of theKokernot ranch 25. Flags and marly shale, alter- house, beginning at small fault just north of water nating as above with 1-foot tank. limestone layer at top of inter- Thickness val, layer of Inoceramus Feet Inches shell fragmentsbeneath 8 BoquillasFormation: 24. Marly shale with interbedded 39. Covered interval, may be Bo- limestoneflags 1 6 quillasbeneathPruett tuff 20(?) 23. Flags and shale alternating, 38. Limestone, bluish-gray, fine- bluish-gray flagstones averag- grained, a little marl, weathers ing |inch thick; shale has pro- chalky white 5 nounced limonitic stains on 37. Limestone, light-gray to buff, bedding planes,average3inches fine-grained, in 1-foot beds thick; fossil ice markings on separated by marly layers upper shalelayers;flagsweather not over4 inches thick 21 darkgray,shalesbuff 2 1178 GOLDICH AND ELMS— BUCK HILL QUADRANGLE, TEXAS

1hickness Thickness Feet Inche, Feet Inches 22. Marly shale and flags; flags 8. Limestone flag, grayish-blue, average 1 inch thick, marly very fine-grained withlittle fine layers 3-4 inches; gray to blu- sand, some banding, weathers ish-gray flags weather gray to tosandy surface 5-7 buff, light-gray marl weathers 7. Shale and thin sandy limestone buff 7 layers alternating irregularly, 21. Marly shaleandlimestoneflags very thin-bedded 6 alternating, flags \ inch, to 1| inches thick, gray to dark blue- 6. Limestone flag, gray, very black weathering gray to buff; sandy, in places banding marly layers up to 2 inches brought out by stains of limo- thick,rather hard, fine-grained, nite 4 even-textured,weathering light 5. Shale, laminated, light-tan tanandgray 3 weathering grayish white; 20. Limestone flag, light bluish- minorgray to blue sandy lime- gray, very fine-grained, weath- stone 4 ering light tan to buff, forms 4. Limestone flag, gray, banded, overhanging ledge 4 verysandy 6 19. Flags and shaly marl alternat- 3. Shale,laminated,tan, sandy 4 ing, slightly nodular flags 1-2 ... inches thick, marl 1-3 inches 2. Limestone flag, gray, banded, thick, weathering buff with sandy, weathers dark gray with platy structures 4 rough sandysurface 6 18. Limestone flag,bluish-gray with Budaformation: \-inchdark blueandgreenband 1. Limestone, gray, massive, near top 4 weathers with rough etched surface, very fine-grained, por- 17. Limestone flags averaging 2 inches thick alternating with celaneous 38 laminated marl in beds 8-10 inches thick, weathering light Total thickness measured 155 tan to yellow with platy struc- 8 ture SectionNo. 4 16. Limestone flag, light-gray weatheringlight grayishbrown, Dartial section of the Pruetl formation below the inplaceslaminatedor banded.. 4 Crossen trachyte 15. Marl and shale with marl pre- dominating, light-gray white, Measuredin first gully north of the southeastern to :nd of the Crossen Mesa 2 miles southwest of the laminatedto thinly bedded.... 3 8 iokernot ranchhouse. 14. Limestoneflag, light-gray, very Thickness fine-grained, weathers to dark Feet gray 5 °ruett formation: 13. Covered,probably thin-bedded 15. Crossen trachyte intercalation:Dark limestone andshale 3 6 reddish-brown flow of trachyte porphyry conspicuous 12. Limestone flag, blue-gray, very with colum- fine-grained, sandy; weathers nar jointing 115 to sandy surface and banded 14. Covered, in part fresh-water lime- appearance 4 stone asindicated by float 20 11. Shale and shaly marl, gray 13. Tuff, fine-grained, dove-colored, to light bluish-gray; weathers banded 4 with platy structure to light 12. Tuff, white, fine-grained,forms re- gray or white 3 ceding slope 32 10. Flags alternating with shale 11. Tuff, coarse gray to white beds in and thinner limestone layers. finer-grained 81 Flagstones average about 4 lessindurated ash inches thick, shale with lime- 10. Tuff, gray,massive, formingan over- stone layers about 2 inches hanging bluff 30 contains abun- thick. Shale 9. Tuff, fine-grained, nodular, hard, dant Inoceramus prism frag- blue-gray 146 ments, commonly stained with limonite 2 6 8. Coveredinlineof section,tuff 76 9. Limestone flag, dark-gray to 7. Tuff, massive, weatheringto arough light-brown, sandy 2 nodulargravel 24 APPENDIX 1179

Thickness Thickness Feet Inches Feet 6. Coveredinline ofsection,tuff 20 flow breccia, 2 feet thick; middle part, denseand uniform 6-9 5. Tuff,massive,gray tobuff withlarge amount of white ash fragments, 10. Basalt flow No. 2; vesicular top hard,weathers toaporous surface. 18 with amygdules of chalcedony, cal- . cite,abundantredandyellow jasper. 6-15 4. Tuff, gray, fine-grained with small flakes of biotite,massive, weathers 9. Basaltflow No. 1; basalflow of the to anodular hard surface, forms an Cottonwood Spring basalt; upper 9 overhanging cliff 26 feet amygdaloidal reddish-brown, ropy (pahoehoe), with many fillings 3. Tuff, extremelyhard, dark grayish- of calcite, chalcedony, and agate. blue,dense, weathers tobluishgray. Green chloritelikecoatingon basalt 2. Tuff, similartounitNo.4 in cavities and around amygdules; fillings of agate commonly termi- 1. Tuff,gray towhite 10 natedinward withsmallquartzcrys- tals. Lower part of flow ismassive Total thicknessmeasured 613 and formsprominentbenchorledge 3-4 feet thick of coarse-grained, Base of sectioncoveredby alluvium, black basaltwithplagioclaseneedles approximatealtitude, 4200 feet. aligned in flow structure. Flow is badly weathered,but fresh samples Section No. 5 are available from heavy ledge and fromlargeboulders. Analyzedspec- Section of the CottonwoodSpring basalt flows and imenNo.840, Table 7,No. 2 42-50 of the upper beds of the Pruettformation in vicinity of elevation 4054 feet south of Cottonwood Pruettformation: Spring 8. Tuff, gray to reddish-brown with conspicuous light-gray to white Thickness mottling, even earthy fracture, Feet baked to hard, reddish-brown rock CottonwoodSpring basalt: with subconchoidal fracture imme- 16. Basalt flow No. 7; thick, massive diately below basalt flow; contact basalt capping mesa and forming with overlying basaltsharp and dis- high cliff insouthern scarp. Upper tinct 48 amygdaloidal part 20 feet thick 7. Tuff, gray topink,thin-beddedwith grades down to coarse-grained ba- upper beds somewhat thicker than salt. Fresh color reddish gray- lowerones,indurated 16 weathering to gray and yellowish brown. Lower part of flow forms 6b. Breccia-conglomerate: massive slope with only scattered outcrops ledge, s|-6 feet thick, corresponds and with abundant ocotillo. Vesic- to 6a below. Section was offset at ularflowbrecciaatbase 164 this point. Beds from6a and down weremeasuredbelowelevation4054; 15. Tuff, light-gray to red; red rock is beds from 6b and up were offset ap- hard, dense, baked tuff with sub- proximately 400 feet to the west conchoidal fracture. Basalt frag- mentsroughly roundedinlower 1-2 6a. Massive breccia and conglomerate feet of gray to pink tuff. Basaltis of igneous rock pebbles and frag- vesicular andinpiecesup to4inches ments capping spur at elevation in diameter 8-19 4054' south of Cottonwood Spring and continuing eastward to Whirl- 14. Basaltflow No.6;massive, weather- wind Spring and westward into the ing with slabby structure in upper JordanGap quadrangle. Fragments part and spheroidally inlower part; generally 0.1-0.2 inch across but upper part is 14-foot thick amygda- many larger up toboulders of 1foot loid 46-51 or more in diameter. Chiefly ig- 13. Basalt flow No. 5; upper 6 feet neous rock fragments with some amygdaloidal with fillings of calrite chert and banded red and yellow andchalcedony; lowerpartmassive, jasperin a gray to yellow matrix of uniform 10-12 volcanic ash, locally calcareous. Weathers to a porous nodular, yel- 12. Basalt flow No.4; amygdaloidal top low to dark-brown ledge commonly about 4 feet thick, reddish-gray; covered with ocotillo. Varies later- lowerpart,densebasalt 12-13 ally both in thickness andin sizeof 11. Basalt flow No. 3;amygdaloidal top component fragments,butisaprom- 2\ feet thick with abundant fillings inent ledge maker and horizon of chalcedony and of agate with marker 2 small quartz crystals in cavities at 5. Volcanic ash, gray to light-yellow, centers of fillings. Base of flow is roughly bedded,and essentially one 1180 GOLDICH AND ELMS— BUCK HILL QUADRANGLE, TEXAS

Thickness Thickness Feet Feet massive layer; fine-grained, with 1. Fresh-water limestone:dense, gray, rough uneven fracture, weathers to massive, tuffaceouslimestone dark yellowbrown 13 4. Tuff, purplish-gray, hard, brittle, Total thickness measured 455 5 banded 1 . 3. Ash, pink to reddish,mottled with white,weathers to a smoothslope... 23 SectionNo. 7 2. Ash, gray to red, poorly bedded,in Section atHill 4975 on CrossenMesa part well lithified. Beds from 6 inches to 1foot thick. Alternation Thickness ofindurated andsofter bedsproduce Feet astep-likeappearanceinoutcrop.... 7 Coltonwood Spring basaltflows: 11. Basalt flow No. 6; dense, black ba- 1. Ash,gray tored, weathers tosmooth salt with prominent flow breccia at slopeusuallycoveredwith debris 16 base 55 10. Tuff,red,dense,hard,baked 7 Total thickness measured 437 Thickness of the CottonwoodSpring 9. Basalt flow No. 5, amygdaloidal at basalt,300± feet. top 7 Base of section, 3992 feet abovesea 8. Basalt flow No. 4, amygdaloidal at level. top grading down to coarse-grained basalt, weathersto yellowishbrown. 16 Basalt flow No. 3, amygdaloidal SectionNo. 6 7. at top with blowholes inch or more Section the Sheep Canyonbasaltflows across, fillings of calcite and chal- of cedony 11 Measured by G. C. Hardin, Jr., in Sheep Can- 6. Basalt flow No. 2, amygdaloidal at yon, Alpine quadrangle. top2-3feetthick 15 Thickness Feet 5.Basalt flow No. 1, amygdaloidal at Spring topabout 14feet thick; fine-grained, Cottonwood basalt: somewhat vesicular flow breccia at 12.Basalt flow, amygdaloidal at top, oxidizedand badly weathered 33 baseabout 12feet thick 61 flow, amygdaloidal at top, Pruett formation: 11. Basalt 4. Tuff,grayish-white, fine- tomedium- thinflow brecciaat base 98 grained 26 Pruettformation: 3. Potato Hill andesile intercalation: 10. Tuff, pink at base and blood red 3 porphyritic andesiteflow, red resid- near top ual material and weathered vesicu- 9. Potato Hill andesite intercalation: lar andesiteat top,upper part offlow Massive porphyritic andesite near altered green, many amygdules of base;upper30-40feet flow breccia.. 72 chalcedony; gradesdown toreddish- brown vesicularandesiteflow breccia 8. Sheep Canyon basalt intercalations: whichweathers to greenish and yel- Porphyriticbasaltflow No.4 28 lowish-brown boulders. Massive an- 7. Tuff, light-pink grading to light desite atbaseof flow withnumerous green 4.5 plagioclasephenocrysts up to 1inch inlengthand |inch across. No.3;amyg- Ocotillo 6. Porphyritic basaltflow commonon weatheredslopes of an- daloidal near top; phenocrysts lo- desite. AnalyzedspecimenNo.812, cally segregated (glomeroporphy- Table 6 40 ritic) 42.5 2. Tuff, light yellowish-gray to green- 5. Fresh-waterlimestone, light-gray to ish-gray with numerous light-gray white,tuffaceous 6 fragments. AnalyzedspecimenNo. 4. Basalt flow No. 2; coarse even- 813, Table2, No. 1 37 grained with a few phenocrysts, 1. Crossen trachyte intercalation: por- amygdaloidalnear top 32 phyritic trachyte flow with many 3. Tuff with very thin lenticular beds stubby feldspar phenocrysts in a offresh-waterlimestone 5.5 dense red groundmass 2. Basalt flow No. 1; dense, even- grained, amygdaloidal near top. Total thicknessmeasured 275 Possibly more than one flow as sec- Base of measured section, 4699 feet tionis partially coveredwith talus.. 131 abovesealevel. APPENDIX 1181

SectionNo. 8 Thickness Feet Section of the Duff tuff measured in largest gully 9. Tuff and breccia, dark-buff with onsouthslopeof Mitchell Mesa small white ash flecks; forms a flat surface weathering 3 Thickness on Feet 8. Tuff, light-gray, fine-grained, uniform, massive, nodular on weather- Mitchell Mesarhyolite: ing 17 24. Rhyolite porphyry,phenocrysts of quartzand glassy sanidinein dense 7. Volcanic breccia,light- todark-buff, to vesicular grayish-pink ground- with fragments several millimeters mass 38 across in fine-grained ash matrix. ! ledge overhanging layer. Duff formation: Forms and 2—3 23. Coveredinlineof section,tuff 64 6. Tuff, light-gray to tan, in most white,massive,fine-grained places peppered with flakes of bio- 22. Tuff, ... 35 tite, weathers to nodular surface,in 21. Tuff, gray to buff and light-brown, placescut by clasticdikes 53 roughly bedded and cross-bedded, medium-grained 104 5. Conglomerate, generally stained brown, great range in size of com- 20. Conglomerate, well-roundedpebbles ponent pieces which are largely inmatrixofcoarsetuffaceoussand.... 8-10 volcanic igneous rocks, from small pebbles 6 19. Tuff, gray to light-buff, cross- to boulders feet across. bedded, fine-grained 46 Matrix commonly cross-bedded coarse sandstone. In the line of 18. Conglomerate,gray,roughly bedded section this conglomerate bed is and cross-bedded; pebbles,cobbles, relatively thin, but inplacesitis up andboulders up to 1foot indiame- to 30 feet thick 6 ter,mainly of igneous volcanicrocks 4. Tuff, dark-grayto buff, verycoarse, ranging fromrhyolite tobasalt, some roughly bedded and cross-bedded, of vesicular lava, commonly well grades places rounded. In places cobbles about tosandstonein 20 6 inchesin diameterare well sorted 3. Tuff, gray, fine-grained, massive, withlittleornomatrix material 6-8 weathers to a porous honeycombed 17. Tuff, light-buff to reddish-brown, surface. Forms vertical face, com- locally gray; medium-grained, monly undercut 9 bedded in layers 4-6 inches thick; 2. Coveredin line of section,tuff. ... 8 stands with verticalfront 105 CottonwoodSpring basalt: 16. Tuff, grayish-white with several 1. Basalt flow, fine-grained, very vesi- 6-inch beds of brownish-redstained cular at top 5 ash, fine- to medium-grained, massive 123 Totalthicknessmeasured 1058 15. Tuff, stained light brownish red, with small white spots, coarse- to medium-grained, forms aprominent Section No. 9 bench 4-6 Section of Sheep Canyon basalt 14. Tuff,gray with several 6-inch to I- flows andPruett tuff footreddish-buffbeds near top,fine- to medium-grained, massive and Ingully west of firstlarge fault northeastof the holdsa steep face onscarp; weathers Kokernot ranch house on the southern slope of to small nodular forms. Many Elephant Mountain. thin clastic dikes of dark-gray Thickness to buff-colored sandy ash cut this Feet unit 238 12. Covered interval, may possibly be 13. Tuff, buff to light-brown, medium- basalt 50 grained 2-4 11. Sheep Canyon basalt intercalation: basalt porphyry flow,black, weath- 12. Tuff, gray, fine-grained, massive, ering brown. gray to rusty Vesicular cut by many small brown, and with openings inpart filledwithcal- varicoloredclastic dikes 29 cite, vesicles elongated in direction 11. Tuff, buff, coarse-grained; forms approximatelyN.65° W 171 benches; grades laterally to a thin 10.'Sheep Canyon basalt: porphyritic breccia 3 flow, upper 20-30 feet vesicular and 10. Tuff, light- to dark-gray, fine- amygdaloidal with fillings of jasper, grained, weathers to a nodular, chalcedony,and calcite. Abundant concretionary surface; stands in glassy, green, plagioclase pheno- steep faces andformsbenches 125 crystsup to1inchinlength. Lower 1182 GOLDICH AND ELMS— BUCK HILL QUADRANGLE, TEXAS

Thickness Thickness Feet Feet part of flow is massive, weather- 6. Tuff, hard, well-indurated, gray to ing spheroidally. Gray, medium- white, fine-grained, maintains verti- grained tuffoccursbetweenthis flow calfaces 70 andthe upperbasaltinsomeplaces 121 . 5. Tuff,indurated,light-gray,medium- 9. Fresh-water limestone, light-gray, grained 10 fine-grained, very in beds 1-2 feet 4. Tuff,hard,graytolight-yellow 32 thick. Weathers to chalky white, forms prominent ledges. Gastro- 3. Conglomerate, chert and hard tuff pods and shell fragments in part pebbles 4.5 Thickness variable;may silicified. Tuff,hard,gray,uniform 8 be asmuch as13 feet 4 2. Sheep Canyon porphyritic 1. Tuff, gray to pink or purplish,mas- 8. basalt: sive, weathers to nodular, pebbly flow, vesicular in upper half with surface. Some pebbles of dense many calcite amygdules. Lime- gray limestone. Molds oflow-spired stone aboveflow contains pebblesof basalt and extends down filling fresh-water snails 84 crevices and openings inupper part of flow 88 Total thickness measured 725. 5 j 7. Coveredinterval,probably tuff 83 Baseof sectioncoveredby alluvium.