THEUNIVERSITYOF TEXAS TO ACCOMPANY MAP GEOLOGIC BUREAU OF ECONOMIC GEOLOGY QUADRANGLE MAP NO.30

Geology of Black Gap Area, Brewster County, Texas

BILL E. ST. JOHN1

Contents

Introduction 2 Reverse faults 13 Stratigraphy 2 BigBrushymonocline 13 Pre-Cretaceous rocks 2 Structure of theBlack Gap graben 13 Cretaceous rocks 3 Major faults 13 Comanchean Series '.. 3 Stillwell anticline 13 GlenRose Limestone 3 Black Gap syncline 14 Absence of Maxon Formation 4 DoveMountainRanch anticline ~ 14 Telephone Canyon Formation .. 4 Maravillas Canyonmonocline 14 Del CarmenLimestone 4 Maravillas Canyonthrustfault 14 SuePeaks Formation 5 Structure of Cupola Dome 14 Santa ElenaLimestone 5 Analysis of structural features 15 Del Rio Formation 6 Geologic history 16 Buda Limestone 6 MesozoicEra 16 Gulfian Series (Terlingua Group) 8 Late Cretaceous-Tertiary Periods 16 BoquillasFormation 8 Quaternary Period 16 ErnstMember 8 Economic geology 16 San Vicente Member 9 Water 16 Pen Clay 9 Soil 17 Tertiaryrocks 9 Barite - 17 Extrusiverocks 9 Semi-precious stones 17 Intrusive rocks 10 Fluorite 17 Agglomerate 10 Silver ._ 17 Sills 10 Petroleum .. 17 Dikes 10 References cited 17 Plugs 10 Summary of Tertiary rocks 11 Illustrations Tertiary-Quaternary bolson fill 11 Figures— Quaternaryrocks younger thanbolson fill _ 11 Older gravel 11 1. Isopachousmapof DelRio Formation,Black Gap area, County, 7 Pediment gravelandundifferentiated gravel 11 Brewster— Texas Landslide material _ 12 Plates (in envelope) Alluvium _. 12 I. Geologic map of Black Gap area, Brewster County, Structure 12 Texas. Structure of Sierra del Carmen 12 11. Structure contour mapof topof SantaElena Limestone Block faults 12 inBlack Gap area. BigBrushyCanyongraben 12 111. Composite stratigraphic section from Black Gap area.

1Department of Geology and Bureau of Economic Geology, The University of Texas, Austin. 2 BureauofEconomic Geology,The Universityof Texas Introduction

The wedge-shapedBlack Gap areain southern Brewster faults. The re-occurrence of northwestward thrusting of County, Texas, occupies approximately 450 square miles the underlying Paleozoic strata, possibly resulting from east of the Big Bend National Park. The southwestern sub-crustal activity, compressed the strata, forming con- boundary of the map area is the easternboundary of the jugate shear sets striking approximately N. 20° W. and Park; the southeastern boundary is the Rio Grande; the N. 75° W. in the massive Cretaceous limestones. Release north boundary is drawn at lat. 29°45' N. The arealies of compression resulted in block faulting along the zones in the Trans-Pecos province and throughout this arid to of weakness set up by the compression. A left-lateral rift semiarid regionthe climate, flora, and fauna are similar. movementaccompanied the normal faulting. The Black Gap region can be divided into four physio- R.T. Hill (1900) discussed generalphysiographic fea- graphic areas: (1) the topographically high and impos- tures of this areain his study of the physical geography ing Sierra del Carmen to the southwest, which is an area of Texas. Udden (1907) made the first stratigraphic of stepped plateaus separated by steep, northwest-trend- studies, and it is upon his work that current nomencla- ing scarps; (2) the Cupola Mountain highland in the ture and understanding of gross features arebased. Baker northeast corner of the map area; (3) a topographically and Bowman (1917) entered this region during their low area extending from the Rio Grande northwest study of the southeastern Front Range. King (1937) ac- through the middle of the Black Gap area and charac- complished the nextmajor work inthis areawithhis study terized by low, steep-sided hills capped with basalt; and of the Marathon Basin. Eifler (1943) and Graves (1954) (4) the gravel-coveredlowland to the northwest which is mapped 15-minute quadrangles to the north and north- an areaof low,roundedhills. west of the Black Gap area. Their work is referred to in The outcropping strata of the Black Gap area areprin- many places in this report. Numerous unpublished mas- cipally Cretaceous but include Tertiary volcanic rock and ter's theses for areas of varying shapes and sizes within Quaternary alluvium. No rocks older than the Cretaceous Brewster County are in the libraries of The University of Glen Rose Limestone crop out within the map area, al- Texas, Texas A&M University, and elsewhere (Brown, though Paleozoic strata of the Ouachita System are ex- 1963). Of these, Wilson's (1951) and Shambaugh's posed near thenorthwest corner. (1951) study of a part of the Black Gap area must be The area is on the plunging northwest end of the Ser- mentioned. Their maps served as a starting point. Inter- rania delBurroand astride the frontal andinterior zonesof national Boundary and Water Commission geologistshave the Ouachita System. To the north, northwest-trending mapped a strip along the Rio Grande extending about 3 monoclines and an east-trending igneous belt border the miles from either side of the river from Lajitas to Del Marathon Basin; to the south lies the block-faulted and Rio. The West Texas Geological Society guidebook for reverse-faulted eastern margin of the Big Bend structural the Big Bend National Park is of special interest (Lons- belt. dale et al.,1955).A comprehensive report on thegeology Basically, the areais alarge northwest-trending graben of the Big Bend National Park by Maxwell and others flanked by a stable block to the northeast and a series of (MS.) is in preparation by the Bureau of Economic Ge- tilted fault blocks to the southwest. The stable block and ology. The Park report redefines numerous stratigraphic graben are terminated to the north by alarge northeast- units and proposes the nomenclatural scheme used in this trending faulted anticline. The mosaic of structural fea- report. tures is the result of several events. Uplift of the Sierra The writer is grateful for the assistance of the follow- del Carmenfollowed by decollement to the northeast across ing: W. R. Muehlberger, R. K. DeFord, P. T. Flawn, the Black Gap area formed the asymmetric Stillwell anti- C. I.Smith, R.A.Maxwell,J. W.Macon,and D. F.Scran- cline and other folds. Rejuvenation of northeast-trending ton. K.P.Young identified thefossils. D. S.Barker made Paleozoic faults is probably the cause of the northeast- an X-ray analysis of a whole rock sample. The text is an striking Dove Mountain Ranch anticline and associated abridgment of a doctoral dissertation (St. John, 1965).

Stratigraphy

Pre-Cretaceous Rocks and rhyolite boulders of probably Precambrian age in the Haymond Haymond Station, No Precambrian rock is exposed in the Black Gap area boulder beds near east of and little is known about the Precambrian rocks of the Marathon, 25 miles north of the map area.Muehlberger surrounding region. Numerous writers (see Flawn et al., et al. (in preparation) reported two ages from a granite- 1961, p.53) have commented on the occurrence of granite gneiss boulder from near Housetop Mountain. They as- GeologyofBlack Gap Area,Brewster County, Texas 3 sumed that the total-rock Rb-Sr age of 570 million years The Glen Rose Limestone is the lowest Cretaceous for- (early Cambrian) is the age of theorigin of the rock and mation exposedin the map area.It rests with angular un- believed that the biotite K-Ar age of 360 million years conformity on underlying Paleozoic strata to the north representsthe age of the latest metamorphism of the rock. in the Hood Spring quadrangle (Graves, 1954, p. 16). Near Persimmon Gap in the northwest corner of the Exposures of Glen Rose are found where the Rio Grande map area is an outcrop of limestone, shale, novaculite, has deeply incised the Cretaceous rocks northeast from and ortho-quartzite. Udden (1907, p. 18) mentioned this the mouth of Maravillas Creek and in the Sierra del Car- outcrop but assigned no age other than including his dis- menin thesouthern partof themaparea. cussion of it in a section on Paleozoic rocks. Baker and MS-3 (see PL III) includes anincomplete thickness of Bowman (1917, p. 103) assigned the outcrop to the Tes- 733 feet of Glen Rose; the base is not exposed. At this nus Formation of Pennsylvanian age. Lonsdale et al. locality, the formation consists predominantly of bio- (1955, p. 56), who mapped the area, depicted it as an micrite withinterbeddedbiosparite and calcareous clay. A area of thrust faults involving Maravillas, Caballos, and foot-thick bed of quartz siltstone 161feet below the top is Tesnus Formations ranging in age from Ordovician to the only siliciclastic rock in MS-3.Near the mouth of Big Carboniferous. C. L. Baker discovered outcrops of schist Canyon in the northeast part of the map area, approxi- east of the village of Boquillas, Mexico (Bose, 1923, p. mately 25 feet of silty, fine-grained, ripple-marked sand- 113).Flawn et al. (1961, p.99) reportedages of 240 and stone occurs in the upper part of the Glen Rose. There is 370 million years from the schist. These ages indicate a14-foot caprinidbioherm nearthebaseofMS-3. thatmetamorphism of the rock occurred during thePaleo- Orbitolina cfr. texana Roemer is abundant throughout zoic Era but give no information regarding the age of the lower 566 feet of the Glen Rose at MS-3. The com- origin of the rock. King (1937) discussed the Paleozoic posite section (PI. Ill) records other fossils from the Glen strataof the Marathon Basinindetail,both as to stratigra- Rose. phy and structure. Flawn et al. (1961) extended this King (1937, pp. 112-113) reported different thick- study into the subsurface with available well data plus nesses: 312 feet at Housetop Mountain, 500 feet along exposuresinMexico. the south edge of the Marathon Basin,and 559 feet on the Post-Paleozoic erosion planed the deformed Paleozoic west side of the Basin in the Del Norte Mountains. He strata over an extensivearea.Hill (1901, p. 363) named mentioned the abundant occurrence of O. texana and the erosional surface the Wichita paleoplain. The only Exogyra quitmanensis. King also remarked that GlenRose pre-Cretaceous Mesozoic rock reported in this region is beds terminate against the flanks of the Glass Mountains the Bissett Conglomerate (King, 1927, p. 212) which and that the Maxon Sandstone overlaps them. In the crops out along the northwest flank of the Glass Moun- Cochran Mountains, Eifler (1943, p. 1620) reported a tains 45 miles north of the map area. Diastrophism and thickness of 458 feet with amarly zone in the upper 150 erosionindicated at the end of the Paleozoic Era possibly feet, the whole section characterized by O. texana and E. extended through the Triassic and Jurassic Periods. quitmanensis. Graves (1954, p. 16) measured a complete section 475 feet thick in the Hood Spring quadrangle. CretaceousRocks His section includes abasal conglomerate. Graves pointed out the abundance of O. texana and Porocystis globularis Unconformably overlying the deformed Paleozoic rocks in the uppermarly part of the section and rudistids in the are Comanchean strata. These marine deposits continue lower, thick-bedded limestone section. Freeman (1964) upward into the Gulf Series whose uppermost section, mapped Glen Rose Limestone to the northeast in the In- missing in the Black Gap area, is typified by acontinental dian Wells quadrangle but gave neither thickness nor clastic facies. fossil content. He termed it "gray limestone that crops Stratigraphic nomenclature of the Cretaceous rocks is out in bottom of Rio Grande Canyon." Santiago Charles- that set forth by Maxwell et al. (MS.) in their study of ton2 reported an incomplete Glen Rose thickness of 485 the geology of the adjoining Big Bend National Park. feet at Cerro el Barco, Mexico, across the Rio Grande Comanchean Series slightly downstream from the mouth of Maravillas Creek, Approximately 3,000 feet of Cretaceous rock is exposed lat. 29°36'43" N., long. 102°43'25" W. (letter from in the Black Gap area.Most of the sectionis massivelime- C.I.Smith,February 16,1965). stone of Comanchean age. A distinct lithologic change The Glen Rose probably thins northward through the between the Buda Limestone and Boquillas Formation Black Gap area toward the Glass Mountains. King (1937, marks the contact between the Comanchean and Gulfian p. 110) stated that the Glass Mountains stood as an asym- strata. metric ridge during the advance of the Cretaceous sea. An increase of silt and sand to the north in the upper Glen Glen Rose Limestone Rose supportsKing's idea. Maxwell et al. (MS.) used Glen Rose Limestone for the General lithology, ripple marks, and plant fragments thin-bedded rocks beneath the massive Del Carmen Lime- stone in the Big Bend National Park. a Geologist, Petroleos Mexicanos, Monterrey, Mexico. 4 Bureauof Economic Geology, TheUniversity of Texas in the Glen Rose Limestone indicate a shallow-water en- stratigraphic relations in the Big Bend region and used a vironment. The lower, rudistid-bearing limestone is a local name; other authors have used the term "Walnut- neritic facies, and the clayey, sandy upper section repre- Comanche Peak" for the same rocks, thus implying a sentslittoral,near-shore deposition. correlation with Central Texas rocks which cannot be Absence of Mason Formation demonstrated. Baker and Bowman (1917, pp. 113-114) discussed The Telephone Canyon is poorly exposed in the Black basal clastic Comanche beds in the Marathon region, de- Gap areabecause of talus cover; in MS-3,however, there is an interval of feet between the top of the Glen Rose scribing a 40-foot sandstone around the east margin of 86 the base MS-3 the the Basin and a cross-bedded sandstone beneath the "Ed- and of the Del Carmen Limestone. At Telephone wards" in thesoutheast corner of the Basin. They didnot Canyon Formation is mostly covered but with silty clay name this unit,but King (1930, p. 92) named it Maxon scattered exposures of calcareous and inter- Sandstone from Maxon Station, Brewster County, Texas. bedded biomicrite. Other exposures reveal a clayey lime- stone into Exogyra texana The writer did not recognize a Maxon unit in theBlack that weathers nodules. and Protocardia texana Conrad are present MS-3. Both the Gap area nor did R. A. Maxwell (oral communication, in 1965) in the Big Bend National Park. At King's type upper and lower contacts of the formation are conform- along locality 23 miles north of themap areathe sandstone over- able.It crops out the scarp northeast from themouth lies and overlaps the Glen Rose Limestone and underlies of Maravillas Creek and along the northwest-trending Typical the "Edwards." King described the sandstone as medium fault scarps of the Sierra del Carmen to the south. exposures outcrops west to coarse grained, cross-bedded, and broken by joints. He of the latter are the on the side of Margaret along found nofossils at the type locality.Eifler (1943,p.1622) Basinand HeathCreek. (1954, 22) described a 73-foot section in the north part of the Santi- Graves p. reported 57.5 feet of Telephone Canyon but with neither the nor ex- ago Peak quadrangle and 114 feet in the southeast corner. section top bottom He mentioned prominent cross-bedding and large, weath- posed. Freeman (1964) estimated a thickness of 60 feet for exposures ered joint blocks. Eifler correlated the Maxon with the in the Indian Wells quadrangle. Fredericksburg Group of Central Texas on the basis of Exposurespreviously referred to as "Walnut-Comanche Corbis and Volvulina dolium. Graves (1954, p. 19) meas- Peak" crop out in Brewster County. King (1937, p.114) assigned along ured sections of Maxon Sandstone 115 and 157 feet thick 50 feet of marl and thinlimestone beds the 10 miles northwest of the map area. He described it as a east side of the Marathon Basin to the "Walnut-Comanche recognized changes. (1943, gray to brown, fine- to medium-grained, friable to dense, Peak" but lateral facies Eifler cross-bedded sandstone with fossiliferous limestone lenses. p. 1626) assigned an equivalent unit to Member No. 1of (1964, Because of the gradational contact with the underlying the Devils River Limestone. Lozo and Smith p. 297) Glen Rose Limestone and lack of definitive fossils,Graves have suggested that equivalent beds be assigned to correlated the Maxon Sandstone with theTrinity Group of the West Nueces Formationin easternKinney and western Central Texas. Geologists of the International Boundary Uvalde counties. and Water Commission extended the Maxon Formation The unit is a shallow-water, neritic to littoral, trans- through the Black Gap area south into the Big Bend Na- gressive sequence. tional Park. They described it as a gray, thin-bedded marl Del Carmen Limestone and limestone with thin,cross-bedded sandstone members. The Del Carmen Limestone is named for the Sierra del compila- Freeman (1964) utilized their mapping in his Carmen of the Big Bend National Park adjacent to the unit tion of the Indian Wells quadrangle but termed the map area. Maxwell et al. (MS.) denned this unit on the "Maxon Limestone." Freeman described it as limestone basis of stratigraphic relations in theBigBend region and clayey and limestone with subordinate sandstone and silt- used a local name. Other authors have used the term "Ed- stone comprising about 10 percentof the unit.He gave an wards Limestone" for these rocks. estimated thickness of 250 feet. The Del Carmen Limestone is 445 feet thick at MS-3 The writer does not believe that the Maxon Formation and in conformable contact with the Telephone Canyon is present in the Black Gap area. The sandstone beneath below and the SuePeaksFormationabove. Telephone Canyon the Formation in thenortheast part of The limestone cropsout along the Rio Grande northeast themap areaconsists of lenses or tongueswithin the upper of Maravillas Creek and in the fault scarps of the Sierra Glen Rose Limestone. Possibly these are tongues that del Carmen and Stairway Mountain. It is also exposedin thicken and occur at progressively higher levels to the the core of the DoveMountain Ranch anticline. north. The formation is principally a biomicrite containing Telephone Canyon Formation chert and a various marine fauna, especially Toucasia. Of The Telephone Canyon Formation is named for Tele- two intervals containing bioherms, the lower has cherty phone Canyon where Heath Creek crosses the Sierra del biomicrite beds flanking the bioherms whereas the upper Carmen near the southern boundary of the map area. hassilty calcareous claybetween the bioherms. Maxwell et al. (MS.) defined this unit on the basis of Graves (1954, p.24) measured a section 309 feet thick. GeologyofBlack GapArea,Brewster County, Texas 5

To the west, Eifier measured 209 and 255 feet (1943, p. equivalents of both units, as indicated by fossils, are 1629). King (1937, p. 115) reported a maximum thick- present at MS-4. Eifler (1943, p. 1627) reported thick- nessof 200 feet. Both Eifler and Graves commented onthe nesses of 30 to70 feet of Member No. 3 of the DevilsRiver abundance of Toucasia. Freeman (1964) estimated a Limestone, stating that the member is "strikingly fossil- thickness of about 400 feet for exposures along the Rio iferous." Graves (1954, p.26) measured thicknesses of 35, Grande in the Indian Wells quadrangle. Santiago Charles- 46, and 57 feet in the Hood Spring quadrangle. He de- ton reporteda thickness of 138 meters (453 feet) at Cerro scribed thecoquina marker bedand statedthatit wasover- el Barco. Mexico, across and downstream from the mouth lain by massive limestone, whereas 10 to 12 feet of cal- of Maravillas Creek, lat. 29°36'43" N.,long. 102°43'25" careous siltstone separates these units in the Black Gap W. (letter fromC.I.Smith,February 16,1965). area. Oxytropidoceras sp., Gryphaea sp., and Enallaster The Del Carmen Limestone becomes thinner toward the texanus characterize the Sue Peaks Formation in Graves' GlassMountains. Itispossible that theMarathon area was area. Freeman (1964) mapped the "Kiamichi(?) For- uplifted during deposition of the Comanchean sediments. mation" in the Indian Wells quadrangle and estimated its The Del Carmen Limestone correlates with Udden's thickness at about 50 feet. Santiago Charleston reported a Member No. 2 of the Devils River Limestone and with the sectionof Sue Peaks Formation 24 meters (79 feet) thick EdwardsLimestone of Graves (1954).It isalso correlative atCanonCeferino, Coahuila,10miles eastof themaparea, withthe upperpart of the West NuecesFormation and the lat. 29°30'43" N.,long. 102°45'43" W. (letterfrom C.I. McKnight Formation of the southern Edwards Plateau Smith,February 16, 1965). and southwest Texas (Lozo and Smith, 1964, p. 290). The Sue Peaks Formation of the Black Gap area is cor- The Del Carmen Limestone of southern Brewster relative with the "Kiamichi" marl of other workers in County is the product of a neritic environment. The bio- Brewster County and probably includes the "Duck Creek" herms are shallow-water reefs. of King. The Sue Peaks Formation of the Black Gap area is not equivalent to the McKnight Formation in Uvalde Sue Peaks Formation and Kinney counties but rather to the lower part of the The Sue Peaks Formation is named from Sue Peaks in Salmon Peak Formation (letter from C. I. Smith, April the southeast part of the Big Bend National Park. Max- 29,1965). (MS.) in Bend well et al. defined this unit for use theBig The Sue Peaks Formation represents a shallow,neritic National Park. King (1930, p.96) and Graves (1954, p. environment and contains clastic material derived from a 25) used the term "Kiamichi Formation" and Eifler nearby positivelandmass. (1943, p. 1627) used "Member No. 3 of the Devils River Formation" for the same rocks. The implied correlation Santa Elena Limestone has not been demonstrated. The Santa Elena Limestone was named from Santa The Sue Peaks Formation is 86 feet thick at MS-3; Elena Canyon in the Big Bend National Park. Maxwell MS-4 includes an incomplete section. At Stairway Moun- et al. (MS.) defined the unit. The "Santa Elena Lime- tain west of theBlack Gapheadquartersit is 73 feet thick. stone" replaces the "Georgetown Limestone" of other The formation is easily eroded and mostly covered. Ithas authors. scattered exposuresof calcareous clayey sjltstone. Micrite Thesection at SantaElena Canyonis740 feet thick.The and biomicrite beds near the top contain an abundant writer measured a composite section at MS-4 for a total marine fauna. The composite section (units 25 and 26, thickness of 943 feet, and Santiago Charleston reported a PL III) gives alist of fossils collected from this interval thickness of 305 meters (1,000 feet) at Canon Ceferino, within the map area. Other fossils from the Sue Peaks Coahuila, lat. 29°30'43" N., long. 102°45'48" W., 10 Formation of the Black Gap areainclude Adkinites imlayi miles east of the map area (letter from C. I.Smith, Feb- from an exposurenear the thrust fault north of Maravillas ruary16, 1965). Canyon (PI. I,K-7) and from the Dove Mountain Ranch The Santa Elena Limestone of the Black Gap area is anticline (PI. I, H-l). The writer also collected an Ad- predominantly biomicrite with beds of micrite and bio- kinitesbravoensis (Bose) and a singleshark vertebra from sparite.There arerudistidbioherms midway inthe section, the latter locality and a Protocardia texana from an ex- and chert nodules are scattered throughout the section. posure on the road along Maravillas Canyon. The Sue Toucasia sp. is relatively abundant. The composite section Peaks Formation forms a distinctive light-colored, slope- (PI.Ill) lists other fossils.The writer found a single speci- forming, vegetation-free band on scarps throughout the men of Durnovarites sp. cfr. quadratus Spath in talus de- area. The change from the Del Carmen Limestone below rived from this limestone. and the Santa Elena Limestone above is abrupt, and the The Santa Elena Limestone crops out over approxi- contacts are sharp except where obscuredby talus. mately 75 percent of the Black Gap area.It constitutes the King (1930, p. 96) measured 62 feet of "Kiamichi" stripped structural surface of the northeast third of the marl containing Gryphaea navia overlain by 21 feet of map area and is the uppermost unit capping Sierra del "Duck Creek" nodular marly limestone in theGlass Moun- Carmen and Stairway Mountain in the southern half of tains. K.P. Young (oral communication, 1965) believed the area. It is also exposed near the intersection of the 6 BureauofEconomic Geology,TheUniversity of Texas graded road to the Black Gap headquarters and State The pale orangecolor results from weathering.No fossils Highway 385 where it forms northwest-trending ridges.R. were found in the lower part. Maxwell (oral communica- A. Maxwell (oral communication, 1965) found no com- tion, 1965) gave several thicknesses from within the Big plete section in the adjoining part of Big Bend National Bend National Park. These are shown on the isopachous Park. Exposure of the basal contact of the Santa Elena map (fig. 1). Santiago Charleston reported a section 3 Limestone is common, but the upper section at the same meters (10 feet) thick from Arroyo Hormiga, Coahuila,3 locality is usually eroded. The writer measured a complete miles southeast of Canon Ceferino at lat. 29°27'25" N., section at MS-4 near the mouth of Drift Canyon. A com- long. 102°43'25" W. (letter from C.I. Smith, February plete section could also be described at the Cupola Moun- 16, 1965).This location is 10 miles east of the map area. tain scarpbut minor faulting wouldhave tobe considered. The Del Rio is absent to the east along a line paralleling Udden (1907, p.22) estimated atotal thickness of 2,000 the Rio Grande (fig. 1). The isopachous map (fig. 1) feet of "Lower Cretaceous Limestones" in the Sierra del suggests that during deposition of the Del Rio Formation Carmen. Baker and Bowman (1917, p. 115) recognized a high existed in the areanow occupiedby the Rio Grande strata of "Georgetown age" in the Marathon areabut did and also along a trend extending to the northwest,north not measure any clearly defined sections. King (1937, p. of andparallel with thepresent courseof Maravillas Creek. 115) described "Georgetown Limestone" as "about 200 The Del Rio thins because of relief onthe depositional sur- feet thick" on the westsideof the Marathon Basinbut gave face rather than from erosion or post-depositional struc- a thickness of 175 feet on a chart (p. 20) showing forma- tural activity. Termination of lower beds and overlap by tions in the Marathon area.Eifler (1943, p.1628) gave a upper beds is demonstrable in the Black Gap area. The thicknessof 475 to 500 feet for Member No. 4of theDevils western deepbasin indicated by the isopachous mapcould River Limestone in the Santiago Peak quadrangle. He have resulted from continuous downwarp orfaulting along described the unit as massive andcontaining rudistids and the northwest-trending, down-to-the-east fault zone shown Gryphaea sp. Graves (1954, p. 27) reported incomplete on the geologic map (PL I,D-7). The structure-contour thicknesses of 385 and 393 feet and stated that the map (PL II) drawn on top of the Santa Elena Limestone "Georgetown"is a dense tosublithographic limestone with also suggests this. a marly upper section. Lozo and Smith (1964, p. 303) Haplostiche texana is the diagnostic fossil for the Del have redefined the "Georgetown" of Kinney and Uvalde Rio Formation in the Black Gap areaand appears in soil counties because of facies variation and proposed the term formed on the limestone interbeds in Del Rio Formation "Salmon Peak Formation." They also discussed the re- slopes. The composite section (PL III) lists other fossils gionalfacies variationsof thisunit. from this formation. Exogyra arietina characterizes this The Santa Elena Limestone appears to thin toward the formation in other areas but was not found in the Black Marathon Basin; this is added evidence for uplift during Gap area. the Comanchean. Udden (1907,p. 27) recognizedthe DelRio "zone" and The Santa Elena Limestone of theBlack Gap arearepre- stated that it was much "heavier" at Terlingua than east sents a neritic environment. It contains shallow-water of the Chisos Mountains; he believed that it thins or dis- reefs as evidenced by unit 29 of thecomposite section (PL appears to the east. Baker and Bowman (1917, p. 114) III). discussed a "yellowishmarly clay, not more than 100 feet thick" above the "Georgetown" and below the "Buda" Del Rio Formation along the western edge of the Marathon Basin about 25 The term "DelRio Clay" wasused Hill Vaughan by and miles north of the Black Gap area. King (1937, p. 115) (1898, p. 236) for greenish clay exposed near laminated reported a thickness of 15 or 20 feet from the west slope Del in Verde County, A. (oral Rio Val Texas.R. Maxwell of the Del Norte Mountains, 20 miles northwest of the communication, 1965) to apply term chose the "Del Rio" map area. Eifler (1943, p. 1630) reported 73 feet from in Big "Grayson" the Bend National Park rather than as the southwest part of the Santiago Peak quadrangle. preferred by the Geologic Names Committee of the U. S. Graves (1954, p.29) reportedit in the Hood Spring area Geological Survey. Maxwell (oral communication, 1965) but gave no thickness because of faulting. Each of these preferred to call this unit the Del Rio Formation because writers mentioned the occurrence of H. texana in these geographic proximity of type locality. of Lozo and Smith beds. Freeman (1964) reporteda thickening to the south- (1964, 290) p. concurred withMaxwell'susage. west within Indian quadrangle to in the Wells from 30 80 Outcrops the westernpart of the Black Gap areaare feet. MS-2, mostly covered, as at but scattered exposuresreveal The clayey, silty rock of the Del Rio Formation marks silty claystone to clayey traces grading siltstone with it as alittoraldeposit. of gypsum and limonite pseudomorphs after pyrite. The formation is usually deeply weathered and typified by a Buda Limestone very pale orange color. The greatest thickness of several The Buda Limestone was named by Vaughan (1900, p. measured is 185 feet in Big Brushy Canyon. The lower 18) to include hard, white,limestone beds. The type lo- partis relatively fresh and dark gray at the thick exposure. calityison ShoalCreek,Austin,Texas. Geology ofBlack GapArea,Brewster County, Texas 7

Fie. 1. Isopachous map of Del Rio Formation,Black Gap area,Brewster County, Texas. Thickness in feet.Isopachdashed whereinferred.

TheBuda Limestonehasbeen elsewhere subdividedinto layer 6 inches to a foot thick separatesthe basal member threemembers. In the Black Gap area,however, thereis a from the overlyingbeds. This same coquina layer can be fourth, basal member, of varying thickness. The basal traced from the top of the green-alga biosparite into the member is absent in some parts of the areaand is up to 22 overlyinggraylimestone sectionalong the northside of the feet thick in others. The three members overlying the saddle on the Airplane Tank road (PL I,H-5). At this basal unit maintain a uniform total thickness of 80 feet. place the coquinalayer also contains bored cobbles. The basal member is a green-alga biosparite and overlies The Buda Limestone above the green-alga biosparite is the DelRio Formation.It forms a prominent ledge above divisible into three members in the western part of the themoreeasilyeroded Del Rio Formation and is yellowish map area. A massive biomicrite 10 feet thick overlies the brown to yellowish orange,thereby forming a sharp color basal biosparite and forms a prominent ledge. In some contrast withtheoverlying graybiomicrite.Itisinteresting places the lower surface of this unit is burrowed. Above that the DelRio Formationand thegreen-alga biosparite of the biomicrite bed is an easily eroded clayey micrite 40 the Buda Limestone thicken, thin, and pinch-out at the feet thick devoid of vegetation. same localities, suggesting a similar history. The upper The uppermost member is a sublithographic, nodular- surface of the green-alga biosparite at MS-1 has borings weatheringbiomicrite 31feet thick. Theupper three mem- that indicate subaerial exposure and therefore a discon- bers are yellowish gray to grayish pink and distinctive on formity. In Big Brushy Canyon (PL I,E-6) a coquina aerial photographs. The Buda Limestone is 80 feet thick 8 BureauofEconomic Geology,TheUniversity of Texas at MS-4, but both the Del Rio Formation and the basal lower, thin-bedded, fossiliferous, flaggy limestone and an green-alga biosparite are missing and the Buda rests on overlying chalky limestone. Eifler (1943, p. 1632) rede- the Santa Elena Limestone. The Buda Limestone appears fined the "Boquillas Limestone" in the Santiago Peak thin to medium bedded at MS-4 and is not divisible into quadrangle to include limestone beds above the Buda members. Limestone and below the top part of the Inoceramus Budaiceras characterizes this formation and Pecten undulatoplicatus zone.Maxwell et al. (MS.) denned the roemeri occurs near the base. Other mollusks were also Boquillas Formation to include the flaggy,marly units and collected throughout the section. subdivided the Boquillas Formation into the Ernst Mem- Udden (1907,p. 27) reported thinning of the Buda in ber, which includes the lower flaggy beds, and the San the Big Bend NationalPark to the east of the ChisosMoun- Vicente Member, which includes the chalky limestone and tains.King (1937,p. 115) reportedabout 60 feet of Buda calcareous shale above the flaggy beds and below the Limestone on the west flank of the Del Norte Mountains, gypsiferous Pen Clay. 20 miles northwest of the map area, where it varies from This unit is not subdivided on the geologic map but is nodular to regularly bedded. Eifler (1943, p. 1632) lumped as the Boquillas Formation. MS-1 and MS-5 to- measured 75 feet of Buda Limestone in the southwestern gether giveacomposite thicknessof 551feet. part of the Santiago Peak quadrangle.Graves (1954) re- The Boquillas Formation crops out in the Black Gap ported three members within the Buda Limestone. Free- area within the northwest-trending structural and topo- man (1964) also mentioned the three members within the graphic low extending from Stillwell,Heath, and Horse Indian Wells quadrangle and reported thickening to the Canyons northwest through the Black Gap headquarters west from 78 to 90 feet. Santiago Charleston reported a to State Highway 385. The broadest area of outcrop is thickness of 30 meters (98 feet) in Arroyo Hormiga, betweenBlack Gap andMaravillas Canyon. Coahuila,lat. 29°27'25" N.,long. 102°43'25" W., 7miles Erosional remnants of the Boquillas Formation crop west of the map area (letter from C. I.Smith, February out at Cupola Mountain, Black Mountain, and west of 16, 1965). Dove Mountain. Other exposuresare in the low hills be- Lozo and Smith (1964,p. 290) pointed out that workers tween Big Brushy Canyon (PL I,E-7) and the Stillwell have demonstrated lithologic continuity of the Buda ranchhouse and along the gravel-covered scarp west of throughout southwest Texas. Bear Creek west of the Dove Mountain Ranch anticline The basal green-alga biosparite is a marginal basin (PL I,E-2). facies, and the abundance of sparry calcite suggests a rela- Ernst Member-Maxwell et al. (MS.) named the Ernst tively high-energy environment. A disconformity sepa- Member of the Boquillas Formation from Ernst Tank on rates the basal member of the Buda from the overlying the west side of the Sierra del Carmen. They defined the section, which represents a deeper water, low-energy en- Ernst as the basal member of the Boquillas Formation. vironment. The disconformity is indicated by a coquina The writermeasured 277 feet of silty, thin- to medium- zonecontaining bored cobbles. bedded, flaggy, brown to gray micrite and biomicrite at Gulfian Series (Terlingua Group) beds MS-1. A sharp lithologic change marks the contact In the Black Gap area the Gulfian Seriesis represented with the underlying Buda Formation and a topographic by the Boquillas and Pen Formations of the Terlingua and lithologic change marks the gradational contact with Group. the overlying San Vicente Member. Previous workers sug- gested Udden (1907, p. 33) used the term "Terlingua beds" that sinkholes in the Buda Formation containing for exposuresalong Terlingua Creek, Brewster County,35 flaggy Ernst beds are evidence of unconformity. These Gap miles west of Black Gap. He defined the Terlingua as same features occur in the Black areabut the writer chalky,marly clay that grades downwardinto theBoquillas is convinced these karst features are younger than the Flags and upward into the Rattlesnake beds (Aguja For- flaggy limestone beds.The flaggy beds in the sinkholes are brecciated, mation).Udden included those units he believed equiva- indicating collapse after lithification. Other lent to theAustin Chalk and Taylor Marlof CentralTexas. than the sharp lithologic change from massive, sublitho- Maxwell et al. (MS.) divided the rocks Udden mapped graphic limestone of the Buda to the overlying silty, thin- as Terlingua and Boquillas into three lithologic units: the bedded limestone of the Ernst Member, there is no physi- PenFormation and theErnst and SanVicente Members of cal evidence to suggest an unconformity between these the Boquillas (revised).They applied thename Terlingua units in the Black Gap area. Group to the two formations. The Cretaceous formations Fragments of Inoceramus labiatus are common in the above theTerlingua Groupin the Big Bend National Park Ernst Member, especially in the upper part. Other fossils arenotpresentinthe maparea. arepresent (PL III) but notabundant. Boquillas Formation Freeman (1964) described about 100 feet of "Boquillas Udden (1907,p. 29) originally used the term"Boquillas Flags" which belong to the Ernst Member. Maxwell cor- Flags" for rocks on Tornillo Creek, ChisosMountain quad- related the Ernst Member withEagle Ford Shale of Cen- rangle, Brewster County. Udden's Boquillas included a tral Texas. Geology ofBlack GapArea,Brewster County,Texas 9

The Ernst Member, as mapped in the Big Bend Na- trangressive movement cleared the water and the fauna tional Park and Black Gap area,includes the lower part flourished again, leaving the abundant fossils of the Ino- of Udden's original Boquillas Formation and the lower ceramus undulatoplicatus zone. (1937, part of Eifler's Boquillas Limestone. King p. 116) Pen Clay tentatively correlated equivalent beds near the Del Norte Maxwell et al. (MS.) named the Pen Clay from Chisos Mountains with the Eagle Ford Formation of Central Pen north of the Chisos Mountains in the Big Bend Na- Texas. tional Park. The Pen Clay is the same as the upper clay The occurrence of well-rounded and sorted quartz silt member of Udden's "Terlinguaclay." and sand grains indicates deposition in thelower beds near The Pen Clay cropsout only in thebasalt-capped center a coarse sand, or shoreline. Lack of gravel, coquinalayers part of the map area.No Cretaceous strata younger than plus well-defined bedding suggest a slow advance or re- Pen Clay remain in the Black Gap area. The highly re- withrelatively low-energy treat of the shoreline associated sistant basalt cover has protected the easily eroded Pen waveaction. Clay; otherwise it too wouldhave been removed long ago. (MS.) San Vicente Member-Maxwell et al. named the At MS-5 only 50 feet of Pen Clay is presentbeneath the San Member from the abandoned of San Vicente village basalt. From outcrop width,estimated dip, and the topo- in Vicente the BigBendNational Park. graphic map, the writercalculated a thickness of 500 feet Composite in thickness of the San Vicente Member the west of the Black Gap syncline and immediately north of Gap is 274 Black area feet. This member differs from the the Black Gap headquarters. Udden estimated a thickness underlying Member; it Ernst has thicker beds and weathers of 1,280 feet near Chisos Pen in the Big Bend National yellow gray. feet composed to The lower 125 is of thin- Park including, at the base, 100 feet equivalent to the medium-bedded labiatus. nodular micrite containing /. upper part of the San Vicente Member of the Black Gap micritelayer is Above this acovered interval66 feet thick area.Eifler (1943, p. 1634) reported approximately 200 easily clay. with scattered exposuresof eroded calcareous feet of graymarl overlying the BoquillasFormation along Without exposure the associated beds this clay of micrite the west side of YE Mesa 12 miles northwest of the map interval could not be differentiated from younger Pen the area. The graymarl probably is correlative with the Pen Clay. No fossils were found in the clay interval. A clayey Clay. clay is gradational micrite 83 feet thick overlies the and The Pen Clayis calcareous and gypsiferous in theBlack into the overlying Pen Clay. The contact between the San Gap area.Inoceramus grandis occurs nearBee Cave Tank Vicente Member and the overlying Pen Clay is drawn at 3 miles northwest of the Black Gap headquarters, and the top of the uppermost sp. limestone bed.Inoceramus is Inoceramus prisms are not uncommonin the Pen Clay. common throughout micrite, of this and a concentration The Pen Clay consists of strata formerly termed "Ter- Inoceramus undulatoplicatus 35 50 occurs to feet above lingua (restricted)" in the BigBend area.Maxwell et al. the base of the San Vicente Member. This is the Ino- (MS.) correlated it with the upper chalk member of the ceramus undulatoplicatus zone (Maxwelletal.,MS.;Eifler, Austin Chalk near Austin. 1943, p. 1634). Graves (1954, p. 30) reported an ex- Although younger Cretaceous rocks are missing in the posure in the Hood Spring quadrangle of chalky lime- Black Gap area, the Pen Clay is overlain by the Aguja stone thick, 15 feet which he called "Terlingua Forma- Formation in the Big Bend National Park. The clay itself tion." The strata he described belongnot in thePen Clay suggests sediments laid down by a regressing sea. The of this reportbut intheSanVicenteMember. occurrence of gypsum in the clay suggests shallow-water Maxwell et al. (MS.) correlated the San Vicente Mem- evaporitic conditions. ber with the Austin Chalk of Central Texas.They included Udden's lower flagstone member of the Boquillas Forma- Tertiary Rocks tion and the lower flagstone member and middle marl member of Udden's Terlingua Formation. Extrusive Rocks The upper and lower micrite beds containing Inocera- Tertiary extrusive basalt covers a large area near the Gap headquarters mus sp. are probably a result of deposition in water Black in the central part of the map deeper than that which received the sediments of the area. This outcrop is in the major northwest-trending graben between the Sierra del Carmen and the Cupola Ernst Member. As the sea transgressed, the sea bottom Mountain area. became deeper, the shoreline was farther from the area A conglomerate containing chert similar to that found deposition, of and the low-energy environment continued. in the Paleozoic rocks of the Marathon Basin occurs be- The calcareous clay suggests a seaward retreat of the neath the basalt in the Black Gap area (Wilson, 1951, p. shoreline and reworkingof the newly depositedcalcareous 68). sediments with a resulting transport of clay-size particles The basalt extrusion followed down-to-the-west faulting into clear water. The sudden abundance of clay particles which brought Pen Clay to the west of the Black Gap would have muddied the water and probably killed the syncline into contact withBoquillas Formation to the east. fauna, so that fossils are lacking. Re-establishment of the The basalt overlies Cretaceous rocks with slight angular 10 Bureau ofEconomic Geology,TheUniversity of Texas

unconformity. Near the mouth of Maravillas Canyon, the quarters. The areaof exposure is small and an extensive basalt rests on SantaElena Limestone. gravelmantle masks the true dimensions. Thisagglomerate Other exposuresof extrusivebasalt occur at Dove and probably marks a volcanic vent. The agglomerate con- Black Mountains and Cupola Mountain peak to the north. tains basalt boulders as much as 4 feet in diameter. The The edges of an outcrop of extrusivebasalt in the gravel- basalt is dark gray to pale brown, vesicular and scori- covered lowland of the northwest part of the map area aceous. The matrix material is fine grained, extremely (PI. I, C-2) are covered by alluvium. It is not definitely weathered, and light olive gray to pale brown. Pebbles of known which rockunitunderlies thebasalt. limestone and clay are also scattered through the matrix. The thickness of the basalt depends on the amount of This was the only agglomerate observed in the Black Gap post-basalt erosionand onthepre-basalt topography.Maxi- area. mum thickness of 450 feet occurs on the east side of Still- Sills-There are two sills in the Black Gap area. A 68) wellMountain (Wilson,1951,p. . basalt sill intruded into Glen Rose Limestone is exposed The thick basalt section is divisible into two units. The along the Rio Grande in the northeast corner of the map lower unit is dark gray and weathers to a smooth slope. area (PI.I,P-3).An abandoned mine shaft exposesa sec- The upper unit is reddish brown, weathers into massive tion 12 feet thick. Debris from the Glen Rose Limestone Petro- angular blocks, and forms near-vertical scarps. masks the outcrop and makes it difficult to define the graphic studies by Wilson (1951) show that the two units lateral limits. The basalt is nonporphyritic, fine-grained similar, identical, are if not in thin section. Both basalt holocrystalline and is composed of 49 percent plagioclase units consist of several flows separatedby vesicular zones. plus biotite, titanaugite, amphibole, iddingsite, augite, The vesicles are elongated horizontally and filled with magnetite, and analcime plus traces of apatite,calcite,and chalcedony or calcite. Both units are nonporphyritic and leucoxene. fine grained. Both units exhibit flow structure and each is An analcime gabbro sill intruded into the Ernst Mem- (labradorite), composed of plagioclase titaniferous augite, ber extends overmuch of the west-central part of the map olivine, iddingsite, and minor accessory minerals. area.It is exposed in the low hills west and northwest of No onehas attempted to date the seriesof flows in these the Black Gap headquarters, where it is 112 feet thick. It outcrops. is also exposed in Big Brushy Canyon, in the valley be- ranch, Pinto Tank, on the Dove Mountain is the site of hind Guliher's house (PL I,D-5), and in the low gravel- intrusive in core a collapse feature. an plug the of Olivine capped hills 5 miles west of Stillwell Mountain. The sillis basalthas extruded alongthe faults bordering the collapse also exposedon thesides of Stillwell Mountain and beside area.The olivine basalt is holocrystalline,nonporphyritic, theroad justeastof Black Gap.Thesillisdeeply weathered and trachytoid. The rock was originally composed of 67 wherever exposed and has a dark reddish-brown mottled olivine, percent plagioclase, 31 percent and 2 percent appearance. It weathers into subspheroidal blocks. Petro- iddingsite magnetite. The olivine has altered to so that graphic examination reveals that it is fine- to medium- the rock now contains 24 percentiddingsitepseudomorphs holocrystalline and nonporphyritic. The rock is 76percent after olivine and 7 percent olivine. Also, the plagioclase plagioclase,plus analcime, aegirine-augite,and ironoxide. partly is altered tosericite. Lesser amounts of accessory minerals are present. The Volcanic activity resulting in the extrusion of the basalt analcime is late- or post-magmatic and fills void spaces in faulting occurred prior to much of the great folding and the feldspar. The aegirine-augite crystals are zoned. now evident. A good example of thisis Black Gap syncline where the basalt forms the upper surface of this feature. Dikes-Along the fault trace on the eastern edge of Several small plugs in the Black Gap area and other Dove Mountain Ranch anticline are several exposures of nearby plugs may account for the mass of lava which igneousrock.Petrographic examinationindicates a fine- to blanketed the areaat one time. medium-holocrystalline, porphyritic olivine basalt. Flow Lonsdale et al. (1955, p.47) reported extrusive olivine structure is manifest and euhedral olivine crystals have basalt between the Chisos Mountains and the Rio Grande altered completely to iddingsite. The rock also contains to the west of the Black Gap areabut also stated that "this abundant augite. Samples from an unusual rock at this lava has not been found on the east side of the Chisos same location reveal anintrusive basalt breccia. The rock Mountains." J.A.Wilson (oral communication,1965) be- is merocrystalline and porphyritic.Micro-xenoliths or vol- lieved the flow to be of Middle or Late Eocene age. He canic-rock fragments up to 6.5 mm long are basalt and has collected Middle Eocene fauna from the continental exhibit vesicles and oriented plagioclase laths. This is the only dike the writer found in the Black Gap area.— sedimentary rocks beneath the basalt. J. A. Wilson also x reportedLate Eocene fauna from tuffs overlying the basalt Plugs-Numerous small plugs seldom exceeding 50 at Castolon on the west side of the Chisos Mountains. feet in diameter occur in the Black Gap area. The plugs are concentrated west and south from Dove Mountain. Intrusive Rocks A basalt plug (PL I,H-2) east of the Dove Mountain Agglomerate-An agglomerate is exposed beside the Ranch anticline is composed of holocrystalline, porphy- graded road 2 miles northwest of the Black Gap head- ritic, and trachytic rock. The basalt is 74 percent plagio- GeologyofBlack GapArea,Brewster County,Texas 11

clase. The plagioclase crystals are rounded and partially ranch inthe extreme south endof themap area.Two miles resorbed, suggesting crystallization at depth followed by south of the turnoff to Stillwell Canyon, terrace levels are intrusion with temperature-pressure changes. The rock cut into deposits of pinkish-orange silty clay. The clay also has partly resorbed augite phenocrysts and augite extends south to the Rio Grande. No bones have been with leucoxene coronas surrounded by hematite rims. found in thebolson deposits; however,an extensivesearch West of Cupola Mountain peak (PI. I,K-3) is a plug was not made. Possibly the bolson was deposited during exposingunusually fresh rock. The rock is holocrystalline, both Quaternaryand Tertiary periods. nonporphyritic, fine- to medium-grained basalt.It is com- The bolson fill is predominantly silty clay with inter- posed of 62 percent plagioclase plus pyroxene,iddingsite, bedded gravel. The gravellayers are 2 to 5 feet thick and and magnetite. Iddingsite has completely replacedolivine. are composed of coarse sand,pebbles,and cobbles of lime- The pyroxene in this plug is not the same as that in the stoneand igneousand volcanic rock.Nochert or novaculite sills mentioned earlier; the difference suggests different wasnoted inthese gravels. sources or atleast different times of intrusion. The bolson extends south into Mexico anundetermined At Pinto Tank, Dove Mountain ranch, is abasalt plug distance. Terracelevels are visible on the Mexican side of associated with a collapse feature. The magma of the plug the river which arenotpresentontheTexasside. wasprobably very hot and "wet" (D.S.Barker, oral com- munication, 1965). The plug is in contact at the surface with theBuda Limestone and theDelRio Formation. Sur- Quaternary Younger rounding plug is rim resulting Rocks thanBolsonFill the a resistant of skarn from Older Gravel metamorphism. The resistantrimis about 10 feet wide and surrounded by marble for a distance of 50 feet. No trace Certain gravels in the Black Gap area are depositsfrom of fossils or bedding appears in the marble, which con- older erosion cycles and can be mapped separately. Most sists of equantgrainsaveraging 3 mmdiameter. The skarn of these older gravels are composedprincipally of igneous rim contains an abundance of chalcedony and hydro- rocks uncommoninlater deposits. grossularite garnet (D. S. Barker, oral communication, A mile south of Maravillas Creek along the dirt road 1965) plus significant percentagesof calcite, epidote, and from State Highway 385 to the Black Gap headquarters is aegirine-augite. The basalt plug is porphyritic with a an extensive, topographically high, gravel deposit. The cryptocrystalline matrix composing 42 percentof therock. gravel consists of pebbles and cobbles of white, black, X-ray analysis indicates the matrix is composed of anal- green,and red chert from the Marathon region.Abundant cime, plagioclase, pyroxene, biotite, and amphibole. igneous rock typesincluding cobbles of agateare also pres- Associated with this plug is basalt extruded along a ent. Similar gravel deposits occur to the west along the fault trace. Perhaps at some unknown depth the intrusive edge of the mountain range bordering the Big Bend Na- plug left a void, which resulted in the "collapse" of the tional Park. Maxwell et al. (MS.) mapped these deposits overlying sedimentary rock along faults. Basalt later as "older gravel." The writer has extended this unit into passed through these fault zones,extruding on thesurface theBlack Gap area. and metamorphosing the Santa Elena Limestone on either At the mouth of Big Canyon,in the northeast cornerof side of the fault trace for a distance of 5 to 10 feet. the map area, is a remnant of an older gravel composed Southeast of Dove Mountain and northeast from Black entirely of limestone cobbles and boulders cemented with Mountain is a circular rim of intrusive basalt (PL I,K-l) calcite. The remnant exhibits massive, torrential cross- several hundred feet in diameter. The surrounding Santa bedding and is not related to the surrounding uncemented Elena Limestone ismetamorphosed for adistance of 10 to terrace gravelandalluvium. not to 20 feet. The limestone around the plug does seem be Pediment Gravel and Undifferentiated Gravel tilted to any degree.The crater is breached to the north- west and the interior is covered with alluvium. Probably Pediment gravels of mappable extent occur alongMara- it is a remnant of a vent and the rim representsmore re- villas Canyon near the Rio Grande and along the Rio sistant rock near the contact zone. Grande northeast from the mouth of Maravillas Creek. These gravels contain subangular to rounded cobbles and Summary of Tertiary-QuaternaryBolson Fill pebbles of locally derivedlimestone plus thecoloredcherts The Tertiary igneous and volcanic rocks of the Black indicative of Marathon Basin Paleozoic rocks. Different Gap area are part of an alkalic kindred. Differences in levels aremapped where identifiable. In some areas these mineral composition are not so great as to require differ- gravels arecemented with caliche. ent sources; such differences as exist could be a result of About 100 square miles of the northwest corner of the pre-intrusive differentiation. map area is mantled by gravel. The gravel is composed mainly of limestone pebbles and cobbles plus a significant Tertiary-Quaternary Bolson Fill amount of chert from Paleozoic rocks of the Marathon Basin and a few pebbles of igneous rock. The gravel de- A partiallydissected bolson fill is located on the Adams posits appear to be nowhere more than 30 feet thick and 12 Bureauof Economic Geology,The Universityof Texas the entire gravel mantle may have been deposited by a Where the dip is nearly vertical and the scarp has not yet meandering Maravillas Creek. For this reason,the deposit retreated througherosion, thefault trace must separate the in this areahas been mapped as "undifferentiated gravel" limestone from the landslide material whatever the thick- and not as bolson fill. nessof thematerial.

LandslideMaterial Alluvium The term "landslide material" is virtually self-explana- tory and, as expected, the material occurs on the slopes Unconsolidated,relatively fine-grained clastic material beneath fault and erosion scarps. The landslide deposits, is depositedalong thebanks of the Rio Grande andMara- however,present a problemof presentationon the geologic villas Creek plus innumerable small drainage creeks. map; for example, on the southwest side of Stillwell Can- Two levels are discernible along Maravillas Creek be- yon (K-10). Tothe north, the landslide deposit covers the tween State Highway 385 and the Santa Elena Limestone fault trace and laps onto the Santa Elena Limestone, scarp east of the Stillwell ranch. Considering the vast whereas to the south the fault marks separation of the amounts of runoff handled by this creek, however, and the limestone beds from thelandslide material. This is a func- volume of coarse material that can be moved in a short tionof the dipof thefault plane. Where thedip is inclined time by such torrential flooding, these alluvial levels have from the vertical,the fault traceis partly coveredby debris. not been differentiated. They may not be there tomorrow.

Structure

The Black Gap area is at the junction of several struc- Block Faults tural provinces.It lies astride the boundary between the Numerous faults trending approximately N. 20° W. frontal and interior zones of the Ouachita System. The with hanging wall down-to-the-east form ahost complex Serrania del Burro Arch of Mexico terminates in the area. of difficult access terrain in the southern part of the Sierra Immediately to the north is the hinge line of the upwarp del Carmen. The dip of these faults appears to deviate of the Marathon Dome marked by an east-trendingbelt of from verticalby less than 10°. igneous plugs marking a line of vents. Black Hills and A seeminglyanomalous feature inthe Sierra del Carmen East Black Hills onthe DoveMountain quadrangle are the is the bending of the N. 20° W. fault traces to approxi- easternmost of these. To the west along the same trend are mately N. 75° W. This is readily observed west of Red unnamed plugs in the Hood Spring quadrangle, Santiago House Tank, at the north end of Stairway Mountain,and Peak plug in the Santiago Peak quadrangle, and the at the entrance to Big Brushy Canyon. Butcherknife Hill plug in the Buck Hill quadrangle. The Throw within the Black Gap area is at a maximum in northwest trend of faulting shown in the Cretaceous and the Sierra del Carmen. The major faults in this area aver- younger rocks on either side of this belt becomes west- age1,700 to 2,000 feet throw with a maximum of approxi- northwest within the belt. The block- and reverse-faulted mately 2,300 feet. The throw is graphically illustrated on eastern margin of the Big Bend structural block forms the the cross sections (PL I) and on the structure-contourmap southwest boundary of the map area. (PI. II) withthe inset indicating relative displacement. Near-vertical northwest-trending fault blocks dominate The mostprominent fault scarp inthe Sierra delCarmen the structure, which is further complicated by small re- extends from the northeast side of Stairway Mountain verse faults and minor strike-slip movement along faults, south to the Rio Grande along the southwest side of Still- tight narrow anticlinal and synclinal folds, plus faulted well Canyon. The fault is not the one with the most throw monoclines. but itmarks the easternedgeof the Sierra del Carmen and forms an imposing scarp overlooking the topographically Gap graben. Structure of Sierra del Carmen low Black Massive block faulting has occurred in the topographi- Big Brushy Canyon Graben cally high Sierra del Carmen. Most of the fault blocks are Big Brushy Canyon (PI.I,D-7) isin a graben whereon folded and tilted to thesouthwest (cross sections C and D, rocks of the Del Rio,Buda, and Boquillas Formations are PI.I). exposed. Flanking the graben are structurally and topo- Near Persimmon Gap are several reverse faults over- graphically high blocks of Santa Elena Limestone. The thrust to the southwest. The reverse faults extend south isopachous mapof the Del Rio Formation (fig. 1) indi- as far as Dog Canyon. A monoclinal fold occurs farther cates maximum thickness in the graben areaand suggests south along the same trend but has reversed the sense of downwarp of this area during deposition of the Del Rio movement and is down-to-the-northeast. Formation. GeologyofBlack Gap Area,Brewster County,Texas 13

The uplifted block of Santa Elena Limestone on the is exposed in a Avater gap. Dip of the fault is 62° W. This east flank of the graben is domed and reveals a sharp roll- is the only normal fault in the area definitely not near- over into the fault bordering the graben. The roll-over vertical. maybe the result of drag-folding or moreprobably drap- A mile south of Black Gap a near-vertical normal fault ing over faulted basement blocks. The northwest-trending down-to-the-east terminates at the older Black Gap fault. fault forming the southwest edge of the graben is nearly The fault extends southeast to the Rio Grande and forms vertical. Throw along each side of the graben is approxi- the northeast edge of the fault block southwest of Horse mately 600 feet. Canyon. The fault has approximately 600 feet of throw. The total displacement of normal faults separating the Reverse Faults Black Gap graben and the Cupola Dome is down-to-the- Northwest-trending reverse faults near Persimmon Gap southwest a maximum of 1,200 feet. At the head of have brought Paleozoic rock (Ordovician to Pennsylvan- Whiskey Canyon (PI. I,K-5) a fault exposes Glen Rose ian) to the northeast into contact with Cretaceous Limestone to the northeast in contact with Santa Elena (Glen Rose Limestone) rocks to the southwest. Six Limestone to the southwest. miles north of Persimmon Gap where State Highway 335 There are several northeast-trending faults in the map crosses Maravillas Creek is another areaof reversefaulting area, but only two are large enough to be significant. where only Cretaceous rocks are exposed. Strike of the re- Whiskey Canyon, northeast of Maravillas Canyon, and verse vaults is the same at both locations but in opposite Dog Canyon (PI. I, J-7) to the southwest are located directions. At Persimmon Gap the fault surface dips to along the trace of a northeast-trending fault down-to-the- thenortheast whereas atMaravillas Creek thefault surfaces northwest with less than 200 feet of throw. The fault is dip to thesouthwest.Everett (1964) has shown thatreverse significant because of its length. faults near Del Norte Gap, north of Persimmon Gap, be- The northeast-trending fault which marks the eastern come vertical atdepth.Probably the same situationoccurs flank of Dove Mountain Ranch anticline extends more in theBlack Gap area. than 12 miles and terminates the tight fold of Stillwell anticline. This suggests that the faulting is older than Big Brushy Monocline folding in the area.The fault also terminatesthe Serrania northwest-trending A monocline down-to-the-east 2miles del Burro Arch. Throw is variable along this fault and is west of Big Brushy Canyon ontrend wththe Persimmon actually reversesnear the intersection with Stillwell anti- Gap faults, movement reverse but the sense of is reversed. cline. The throw is at a maximumof 1,400 feet near the begins Dog Canyon The monocline just south of and ex- north boundary of the maparea. tends south to the fault scarpnorth of Stuart Peak. Maxi- mum structuralrelief along the monocline is 500 feet. The Stillwell Anticline axis of the monocline has the same strike as the major Stillwell anticline is probably themost impressivesingle fault trend,N.20° W. structural feature inthe Black Gap area.Itis asymmetrical A major northwest-trending fault terminates the mono- withitsaxialplane inclined to the northeast.Althoughless cline to the south,provingthat thefoldingis olderthan the than a quarter of amile wide, it is nearly 8 miles long. faulting. Structural relief along the anticline is approximately 300 feet. Structure of the Black Gap Graben All strata younger than Santa Elena Limestone have The northwest-trendingBlack Gap grabenextends from been stripped from the anticline leaving the competent, the Rio Grande northwest to State Highway 385. It is resistant limestone core. This feature is a decollement bounded on the southwest by the Sierra del Carmen horst which slid along the underlying incompetent Sue Peaks complex and on the northeast by the fault scarp marking Formation over the massive Del Carmen Limestone. In- the Cupola Dome. It encompassesan areacontaining nor- clination of the axialplane to the northeast suggests over- mal and near-vertical northwest-trending faults, the nar- thrust movement from the southwest. There are similarly row and tight Stillwell anticline plus monoclinal and syn- shaped folds in the lower Ernst Member flaggy limestone clinal folds associated with faults,plus a reversefault and beds. acomplex collapse structure. The asymmetrical folding was caused by uplift of the Sierra del Carmen areafollowed by decollement sliding of Major Faults massive limestone over incompetent shale and silty clay- A northwest-trending down-to-the-west fault hereafter stone to the northeast across the Black Gap area.Vertical referred to as the Black Gap fault occurs along the north- movement of basement blocks possibly initiated uplift of east flank of the Black Gap syncline.More than500 feet of the Sierra del Carmen. Gravity moved the rocks downdip PenClay to the westof the fault andits absence eastof the away from the Sierra del Carmen high and toward the fault is evidence that the faulting antedated extrusion of northeast. Shortening of the crust by folding resulted in the basalt. It is probably the oldest fault in the area. A folds with axes normal to direction of movement, such as mile west of the Big Horn sheep pasture the fault surface the Stillwell anticline with an axis trendingN.40° W. 14 BureauofEconomic Geology,TheUniversity ofTexas

Black Gap Syncline a maximum stress directed along a northeast-southwest Parallel with and 2 miles to the west of Stillwell anti- line. Outside of these monoclines, there is no evidence for cline is the Black Gap syncline. This feature is arelatively such a stress. Indeed,such a stress is inconsistent with the tight fold in the areaof the Black Gap headquartersbut regional structural pattern. More likely, the monoclines of opens outinto thebroad alluvium-filled valley to thenorth- the area formed as a result of draping over basement west (Pis. Iand II).Negative movement in this areahad blocks. If so, a series of faults down-to-the-southwest must begunprior to vulcanism. extend from the northbeyond Bullis Gap southward to the Continued downwarp and faulting occurred after the fault marking the northeast boundary of the Black Gap basalt cover was extruded. The basalt layers now dip ap- graben. Stratigraphic separation dies out to the west and proximately 35° toward thesynclinal axis alongboth sides northwest along eachof the monoclines. of thesyncline. Maravillas Canyon Thrust Fault Dove Mountain Ranch Anticline Five miles northwest from the Rio Grande and 1mile In the north-central part of the map areais the broad, northeast of Maravillas Canyon is a spoon-shaped thrust asymmetrical, north-northeast-trending Dove Mountain fault terminated at its lower end by anormal fault. Exact Ranch anticline. The anticline has been breached along displacement is unknown but approximately 100 feet of its crest by erosion and the Del Carmen Limestone is ex- reverse throw can be demonstrated. The upper part of the posed in the center. The Santa Elena Limestone is ex- Del Carmen Limestone in the hanging wall is in contact posed alongtheflanks and onthesouthward-plunging nose. with the lower part of the Santa Elena Limestone in the The anticlinal axis of the fold is less than half a mile footwall. west of the termination of the southeast flank against the The east trace of the thrust fault is well exposed where northeast-trending fault. East of the anticlinal axis the the fault surface dips 25° to the west. The fault can be average dip is 25° SE. Immediately west of the axis the traced along a narrow ridge to the north and along the averagedip is10° to 15° W., but 3 miles west the average top of the mountain to the west. At its northernmost limit, dip is 5° W. The westward-dipping west flank of the dip of the fault surface is approximately 20° to the south. anticline also is the northeast flank of the alluvium-filled The fault can be traced to the west where it enters a topo- part of the Black Gap syncline (Pis. Iand II).The fold graphic trough extending to the southwest; the trough is plunges southward immediately north of Maravillas Creek littered with debris and caliche. Numerous joints in the (PL I, G-3). The anticline follows an arcuate course trough areadip 20° to 25° to the southeast. The southern through the Dove Mountain quadrangle and opens out to extremity of the trough is the same down-to-the-south nor- thenorthintotheMarathonDome. malfault that terminatesthe easternfault trace. A major fault striking approximately N. 30° E. marks Thus, the fault trace surrounds an area approximately the eastern edge of the anticline. The interior part of the half amilein diameter and it dips toward the center. This fold is faulted (Pis. Iand II) and throw isreversed along feature is additional evidence for northeastern movement some of these faults. The anticline and associated faults awayfrom theupliftedSierra delCarmen. have the same trend as do the Paleozoic structures to the in suggests north the Marathon Basin. This that the anti- Structure of the Cupola Dome cline is a result of rejuvenation along Paleozoic fault trends and probably marks the border between the frontal In the northeast corner of the map area is a broad, andinterior zonesof theOuachitaSystem. domed area representing the northwest extension of the Serrania del Burro. An arcuate fault zone with maximum Maravillas Canyon Monocline throw of 1,200 feet (Pis. Iand II),extending northwest A monocline extending northwest from the mouth of from the Rio Grande to Cupola Mountain and north to the Maravillas Canyon has lowered the section on the south- west of Dove Mountain, separates the uplifted block from west side of the canyon approximately 200 feet relative to the Black Gapgraben to thesouthwest. the northeast side. Maravillas Canyon occupies the zone Cupola Dome (PL II) is elongated along a northwest of flexure and the monocline is not obvious.Nevertheless, trend andhas approximately 200 feet of closure. Erosional there is a distinct roll-over of beds in Maravillas Canyon remnants of younger Cretaceous rocks capped with Ter- 5milesfrom the RioGrande. tiary extrusive basalt remain at Cupola Mountain peak, The Maravillas Canyon monocline has a sense of mo- Black Mountain, and Dove Mountain, thus dating the tion opposite to that of the Big Brushymonocline. Relative marginal faulting as post-basalt. The dome itself is un- stratigraphic separation in Maravillas Canyon is down- faulted except for very minor north-northwest-trending to-the-southwest, whereas in the Big Brushy monocline it faults onthe east flank. is down-to-the-northeast. Big Canyon and Rio Grande Canyon are to the east of To the northeast, near Bullis Gap, is a series of mono- Cupola Dome. A narrow ridge cut by minor northwest- clinal flexures, each down-to-the-southwest. For these to trending faults separates Big Canyon and Rio Grande have resulted from horizontal compression would require Canyon. GeologyofBlack GapArea,Brewster County,Texas 15 Analysis of Structural Features by King (1937), show faults trending dominantly north- west in Paleozoic, Cretaceous, and Tertiary strata. The The fracture and fold pattern of the Black Gap area northwest trend of fractures observed throughout the re- indicates several structural events, distinct yet related. gionis probably related to the same source.Late Paleozoic Dove Mountain Ranch anticline lies athwart the trend northwest overthrusting can be demonstrated. It is pos- of all other structures in the area.If it isprojected south- sible that the Paleozoic orogeny wascaused by sub-crustal west along trend to the Sierra del Carmen, it is found to movement. A recurrence of sub-crustal activity following divide the reverse faulted portion to the north from the Tertiary vulcanism explains the northwest-trending frac- asymmetric folding to the south. This difference is in- ture patternin the Cretaceous and younger rocks.The sub- ferred to reflect a change in rock types underlying the crustal movement caused compression of the overlying Cretaceous from Paleozoic sedimentary rocks such as are crystalline and sedimentary rocks, setting up conjugate exposed in the Marathon Basin immediately to the north shear sets.Itis possible that theshear sets developedin the to metamorphic rocks such as are exposednearBoquillas, crystalline and overlying sedimentary rock and even the Mexico, immediately southwest of the area. The Dove extrusive Tertiary basalt. Strike-slip movement along the Mountain Ranch anticline thus lies above the inferred shear sets or, more probably, cessation of the sub-crustal boundary between the frontal and interior zones of the activity effected a release of horizontal compression which Ouachita System as used by Flawn et al. (1961, p. 169). was followed by block faulting involving vertical move- Stillwell anticline, Big Brushy monocline, and the ment of basement blocks. Maravillas Canyon thrust fault are believed to have re- King (1937, p. 140) inferred that the latest uplift of sulted from decollement to the northeast from anuplifted the Marathon Dome to the north accompanied by sub- Sierra del Carmen. The Sierra del Carmen strikes ap- sidiary folding along the flanks occurred in Late Eocene proximately N. 20° W.; therefore, movement away from or Early Oligocene at which time dip-slip movementprob- the uplift would be in aN. 70° E. direction. This is ap- ably occurred along the older near-vertical shear sets. proximately the direction of movement of the thrust fault, Everett (1964) has shown that the thrust faults of and both Big Brushy monocline and Stillwell anticline Del Norte Gap, north of Persimmon Gap,become vertical strike normal to the proposed direction of movement. Big faults at depth. The same situation probably is true at Brushy monocline is down-to-the-east and Stillwell anti- Persimmon Gap and Dog Canyon, thus explaining the so- cline is asymmetrical to the east. These relations are ad- called thrust faults. ditional evidence for movement to the northeast. Each of A mile east of Black Gap along the south bank of the these features is cut by younger near-vertical normal main drainage course is a near-vertical fault down to the faults. west approximately 20 feet. The fault strikes N. 24° W. A rhombic pattern of joints and faults in two principal On the east side of the fault are drag folds in the Ernst sets isfound in the Sierra del Carmen area.The dominant Member of the Boquillas Formation. The folds in the set strikes approximately N.20° W. and the minor set ap- flaggy limestone beds strike N. 06° E. The folds are in proximately N. 75° W. Throw suggests that movement beds 2 inches thick and have an amplitude of 6inches and was contemporaneousalong both sets. The sets have es- a period of 24 inches. The folds make an angle of 30° sentially vertical dips and anintersection angle of approxi- with the fault, indicating left-lateral strike-slip movement. mately 55°. This suggests an origin as conjugate strike-slip Preferential movement along the N. 20° W. shear of the faults havingmaximum principal stress trending approxi- conjugate shear-set would cause the required stress. mately N. 47° W. and minimum principal stress striking From the evidence presentedit appears the sequence of approximately N.43° E. structural events was as follows: The stress pattern requires maximum horizontal com- 1. Uplift of the Sierra del Carmen and Dove Mountain pression acting along aN.47° W. trend. This is thedirec- Ranch anticline followed by decollement to the north- tionof motionof the latePaleozoic thrustsof the Marathon east across the Black Gap areamade the tight asym- Basin (King, 1937,pi. 15).The frontal edge of the thrust metric fold of Stillwell anticline. sheet is believed to extend through the Black Gap area 2.Vulcanism covered the area with a layerof basalt. from the Dove Mountain Ranch anticline in the north 3.Northwestward movement of the underlying thrust to the vicinity of Dog Canyon in thesouthwest. Rejuvena- sheet,accompaniedby slightrejuvenation of Paleozoic tion of northwestward thrusting of the underlying sheet structural features,resulted inhorizontal compression sets up compression in the overlying Cretaceous massive in the massiveCretaceous limestones. Conjugateshear limestone. Horizontal movement of 100 feet or less was sets striking N. 20° W. and N. 75° W. formed but probably sufficient. little or nomovementoccurred. The northwest trend of fractures found in the Creta- 4.Reduction of horizontal compression permitted block ceous limestones of the Black Gap areas is not restricted faulting along the previously formed shear sets. either to the map area or to Cretaceous rocks. Geologic 5.Left-lateral strike-slip movement occurred along the maps of nearby areas, in other words, Hood Spring and N. 20° W. set contemporaneouswith or immediately Santiago Peak quadrangles plus the Marathon Basin map subsequent to block faulting. 16 BureauofEconomic Geology,TheUniversity of Texas

Geologic History

The lack of surface exposures of pre-Cretaceous rocks Tamaulipas Platforms were inundated. The Comanchean within the map area precludes interpretation on a local sea deposited the Glen Rose Limestone upon the eroded basis for that era.Consideration of regional geology, based surface. The massiveDel Carmen and Santa Elena Lime- on the work of others,does, however, furnish information stones thin toward the Glass Mountains, indicating for the interpretation of pre-Cretaceous geologic history. incipient uplift of the Marathon area. Minor folding The following pre-Cretaceous history briefly summarizes occurred before or during deposition of the Del Rio previous work. Formation. Cambrian,Ordovician,andDevonian rocksof theMara- Comanchean deposits are predominantly shallow-water thon region are transitional from a predominantly clastic limestones whereas the occurrence of silt and clay in the facies to the south and southeast and a carbonate foreland Gulfian rocks suggests retreat of the shoreline and emer- shelf facies to the northand northwest. TheTesnus Forma- genceof theMarathonDome. tion of Mississippian and Pennsylvanian age is a south- ward-thickening wedge of flysch deposits, which marks Late Cretaceous-Tertiary Periods the beginning of amajor tectonic event. The axis of the geosyncline which had previously been located to the Late Cretaceous or Early Tertiary uplift of the Sierra southeast shifted northwestward, and previously formed del Carmen and Dove Mountain Ranch anticline and as- rocks to thesoutheast became source rocks for clastic sedi- sociated decollement to the northeast formed the asym- probably ments. Overthrusting from the south and southeast during metric Stillwell anticline. This activity was re- the Pennsylvanian formed northeast-trending structures, lated to theLaramide Orogeny. which in part determine the present outcrop pattern. An- Igneous intrusive and extrusive rocks are possibly of other northward shift of the geosynclinal axis occurred LateEocene orEarlyOligocene age. near the end of the Pennsylvanian. Permian carbonate Rejuvenation of northwestward thrusting of the under- rocks overlying Pennsylvanian strata form theGlassMoun- lying Paleozoic thrust sheetcaused compression and set up tains, whereas older Permian clastic rocks are present to conjugate shear sets in the massive Cretaceous limestones thenorthin the Val VerdeBasin. and possibly in the Tertiary extrusive basalt. The north- thrusting Flawn etal. (1961,p. 168),onthe basisof petrographic westward of the Paleozoic thrust sheet may have studies, mapped two main structural units within the been related to sub-crustal activity which would also have Ouachita System, a frontal zone containing unmetamor- affected theunderlyingcrystalline rocks. phosed to weakly metamorphosed sedimentary rocks and Release of compression allowed block faulting along an interior zone containing sedimentary rocks showing the conjugate shear sets. The block faulting involved ver- weak to low-grade metamorphism and having shear fea- tical movement of basement blocks. The block faulting is tures. probably younger than Oligocene and possibly related to Overthrusting carried rocks of the interior zone north- Basin andRange events. west over rocks of the frontal zone along a northeast- trending front extending through the Black Gap area. Quaternary Period was deposited during Mesozoic Era Coarse clastic continental debris the Quaternary. Deposition of bolson fill in the Adams Post-Permian uplift and erosion deformed and planed ranch area probably began during the latter part of the the Paleozoic strata. The Mesozoic sea filled the negative Tertiary andcontinued intotheQuaternary.Pediment and Mexican Geosyncline and Sabinas Basin to the south and undifferentiated gravel plus landslide material and al- continued its northward advance until the Coahuila and luvium composethe remaining Quaternary strata.

Economic Geology

Water area,although severalman-made earthen tankshold water The paucity of water accounts for thescarcity of settlers and serve as gatheringplaces for wildlife. Water wells in 1,000 in this country. Most of the rain falls rapidly as thunder- the areafurnish good water at depths from 300 to showers and runs off into the intermittent streams which feet. The aquifers are without exception fractured, mas- empty into the Rio Grande. Rainfall is infrequent and sive limestones. torrential. Flash-floods are common during the rainy sea- Several valleys are filled with Quaternary alluvial ma- son. There are no natural bodies of standing water in the terial but attempts to develop water in them have notbeen GeologyofBlack GapArea,Brewster County,Texas 17 successful. Numerous dug wells have proven that the Daugherty (1962) determined that the fluorite of the gravel- and silt-filled valleys lack aquifers.Either the clay La Linda deposit is a contact-metasomatic deposit asso- fraction in such deposits has resulted in low permeability ciated with intrusion of rhyolite into the massive Cre- or else fractures in the underlying limestone have drained taceousSanta ElenaLimestone. reservoirs.Numerous sinkholes inthe BudaLimestone and The presence of intrusive plugs and sills in the Black caves in theother massivelimestones attest tosolution. Gap areaplus the presence of the massive Del Carmen and Santa Elena Limestones make the occurrence of fluorite Soil possible,yet nonehasbeen found. It is possible that Cupola Dome was causedby intrusion. Attempts to irrigate the alluvial soil have been made If a petrologically favorable intrusion exists at depth, Stillwell ranch and along Maravillas Creek between the mineralization might be present in limestone of the Del Highway 385 well in State as as similar soil Stillwell Can- Carmen and Santa Elena. Depth of apossible host might yon and along the Rio Grande near the Adams ranch. also affect themineralization. Lack of an adequate watersupply plus the presenceof salt inthe soilhave made these efforts onlypartly successful. Silver Barite A prospect pit in a calcite vein within the Glen Rose Limestone reportedly was a silver prospect. The pit loca- A barite deposite west of Big Brushy Canyonhas been tion is shown on the geologic map (PL I,M,N-6). Big Brushy Canyon mine. mined. A road leads from to the A spectrographic analysis (Schofield, 1965) of samples occurs as a joint filling the fractured Santa The barite in from the prospectindicated onlyatrace of silver. Elena Limestone. No information on amount of oremined isavailable. Petroleum Semi-Precious Stones Traces of dead oil stain (?) occur in the Glen Rose The deposits of Quaternary older gravel contain chert, Limestone,and freshly broken surfaces of Glen RoseLime- chalcedony, and an abundance of the so-called "moss- stone and limestone beds in theSue Peaks Formationhave petroliferous reportedly developed agate." Local rock shops sell the stone and it is used for a odor. An oil scum on inexpensive jewelry. cuttings from a well drilled near DoveMountain. An odor of sulfide was also present. Fluorite Source rocks, reservoir rocks, and structural traps are presentin the area, yet little or no petroleum exploration Numerous fluorite deposits occur in nearby areas. East has been attempted. The prospective Cretaceous section is of the Black Gap areain Mexico is the La Linda fluorite exposed at the surface, thereby eliminating it from consid- deposit operated by Dow Chemical Company. Also, a de- eration.The underlyingPaleozoic sectionis considered to posit of commercial quality and quantity reportedly oc- be of unfavorable facies in addition to being complexly cursto the westin theBigBendNationalPark. faulted.

References Cited

Baker,C. L., and Bowman, W. F. (1917) Geologic exploration of rangle, Terrell andBrewster counties, Texas: U. S. Geol. Sur- the southeastern Front Range of Trans-Pecos Texas: Univ. vey Misc.Geol.Inv.Map1-395. TexasBull. 1753,pp. 60-177. Graves, R. W., Jr. (1954) Geology of Hood Spring quadrangle, B6se,Emil (1923) Vestiges of an ancient continent in northeast Brewster County, Texas: Univ. Texas, Bur. Econ. Geology Mexico: Amer. Jour. Sci., sth ser., vol. 206, pp. 127-136, 196- Rept. Inv.No.21, 51pp. -214, 310-337. Hill,R. T. (1900) Physical geography of the Texas region: U. S. Brown, T. E. (1963) Index to areal geologic maps in Texas,1891- Geol.Survey Topo.Atlas (folioNo. 3),12pp. -196 1: Univ.Texas,Bur. Econ. Geology,20 pp.,map. (1901) Geography and geology of Black and Grand Daugherty, F. W. (1962) Geology of the Pico Etereo area, mu- Prairies: U. S. Geol.Survey 21st Ann. Rept., pt. 7, 666 pp. nicipio de Acufia, Coahuila, Mexico: Univ. Texas Ph.D. dis- ,and Vaughan, T. W. (1898) Geology of the Edwards sertation, 154 pp. Plateau and Rio Grande Plain adjacent to Austin and San Eifler, G. X., Jr. (1943) Geology of the Santiago Peak quad- Antonio, Texas, with reference to the occurrence of under- rangle, Texas: Bull. Geol. Soc. Amer., vol. 54, pp. 1613-1644. ground waters: U. S. Geol. Survey 18th Ann. Rept., pt. 2, pp. Everett, J. R. (1964) Post-Cretaceous structural geology near 193-321. Del Norte Gap, Brewster County, Texas: Univ. Texas M.A. International Boundary and Water Commission (1950-1955) thesis, 61pp. Geologic strip maps, scale 1:50,000, covering an area about Flawn, P. T., Goldstein, August, Jr., King, P. 8., and Weaver, three miles on each side of the Rio Grande from Lajitas, C. E. (1961) The Ouachita System: Univ. Texas Pub. 6120, Brewster County, to Del Rio, Val Verde County: on file at 401pp. Bur. Econ. Geology. Freeman, V. L. (1964) Geologic map of the Indian Wells quad- King, P. B. (1927) The Bissett formation, a new stratigraphicunit 18 BureauofEconomic Geology,TheUniversity of Texas

in the Permian of west Texas: Amer. Jour. Sci., vol. 14, pp. Muehlberger, W. R., Denison, R. E., and Hedge, C. E. (in prep- 212-221. aration) Geochronology of the mid-continent region, United (1930) Geology of the Glass Mountains, Texas, Part States, Part111, Southernarea. I, Descriptive geology: Univ. Texas Bull. 3038, 167 pp. (Janu- Schofield, D. A. (1965) Quantitative spectrographic analysis of ary 1931.) samples 65014 and 65015: Univ. Texas, Bur. Econ. Geology (1937) Geology of the Marathonregion, Texas: U. S. Mm. Studies Lab. report. Geol.Survey Prof.Paper 187, 148 pp. (August 10, 1938.) St. John, B. E. (1965) Structural geology of Black Gap area, Lonsdale, J. T., Maxwell, R. A., Wilson, J. A., and Hazzard, Brewster County, Texas: Univ. Texas Ph.D. dissertation, 132 R. T. (1955) Big Bend National Park, Texas: West Texas pp. Geol. Soc, Field Trip, Guidebook, March 18-19, 142 pp. Shambaugh, J. S. (1951) Structure of the Black Gap area,Brew- Lozo, E., and Smith, C. I. (1964) Revision of Cre- ster County, Texas: Univ. Texas M.A. thesis, 60 pp. F. Comanche geology coun- stratigraphicnomenclature, Plateau, Udden, J. A. (1907) A sketch of the of the Chisos taceous southernEdwards try, Brewster County, Texas: Univ. Texas Bull. 93 (Sci. Ser. southwest Texas: Proc, Gulf Coast Assoc. Geol.Soc, vol. 14, 11), 101 pp. pp. 285-307. Vaughan, T. W. (1900) Reconnaissance in the Rio Grande coal Maxwell,R. A., Lonsdale, J. T., Hazzard, R. T., and Wilson, fieldsof Texas:U.S.Geol.Survey Bull.164,88 pp. J. A. (MS.) Geology of Big Bend National Park, Brewster Wilson, D. C. 0., Jr. (1951) Stratigraphy of Black Gap area, County, Texas: Univ. Texas,Bur. Econ. Geology. Brewster County, Texas: Univ. Texas M.A. thesis, 98 pp.