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 Pediment gravelandundifferentiated gravel 11 Gap area,Brewster County, Texas 7 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.