THE UNIVERSITYOF TEXAS AT AUSTIN TO ACCOMPANY MAP— GEOLOGIC BUREAU OF ECONOMIC GEOLOGY QUADRANGLEMAP NO. 40 Gravity,Magnetic,andGeneralizedGeologicMapoftheVanHorn-SierraBlancaRegion,Trans-PecosTexas MICHAEL A.WILEY1 Contents Page Page Introduction 2 Regionalsubdivision 12 Purpose 2 Salt Basingraben 12 Location 3 DiabloPlatform and foothills 12 Previous investigations 3 Devil Ridge, QuitmanMountains, and Acknowledgments 3 SierraBlancapeaks 13 Regional geologic setting 4 Eagle Flat andCarrizoMountains 13 Tectonic framework 4 Rim Rock fault 14 Tectonic history 5 Interpretationof profiles 15 Geologicbase map 5 Streeruwitzthrust fault 17 Generalizedstratigraphy 6 CarrizoMountains 17 OlderPrecambrianrocks (pCI) 6 NorthernEagle Flat 18 YoungerPrecambrianrocks (p€u) .... 6 Hillside fault 18 AllamooreandHazelFormations ... 6 The Texas Lineament 19 VanHorn Sandstone 6 Conclusion 20 Paleozoicrocks (Pal) 7 References 21 Mesozoicrocks(X) 8 Appendix: Gravity and magnetic survey .... 23 Cenozoicrocks(T, Q) 9 Controlpoints andbases 23 Geologicinterpretationof gravity and magnetic Elevationandlocation 23 anomalies 11 Fieldprocedure 24 Methods 11 Dataadjustment andreduction 24 Assumptions 11 Driftcorrections 24 Density andmagnetic susceptibility ... 11 Reductionof gravity data 24 Additionalassumptionsrelatedto Reductionof magnetic data 25 magnetic anomalies 11 Precision andaccuracy 25 Regionalgradients 11 Illustrations Figures— Page 1. Map of part of Trans-Pecos Texasshowing physiographic featuresand study area 2 2. Tectonic frameworkof West Texasand northeasternMexico . 4 3. RimRock faultinTrans-PecosTexas 14 4. Bouguer anomaly profiles A-A,B-B 16 Tables Tables— Page 1. Permianrock units, Van Horn— SierraBlancaarea 7 2. NomenclatureofMesozoicrocks,Van Horn— SierraBlancaarea 10 3. Results of interpretationof profiles A-A' andB-B' .... 15 4. Probable accuracy of elevationcontrol 24 Atlantic RichfieldCompany,Dallas, Texas. Introduction Purpose gest the possibility of a transcontinental fracture zone passing near VanHorn andalong theboundary The Van Horn— Sierra Blancaregionlies athwart of the provinces. Later workers called Hill's frac- part of the boundary between two contrasting geo- ture zone the Texaslineament (Ransome, 1915) or logic provinces. The contrast between these two the Texas direction of wrench faulting (Moody and provinces, splendidly displayed by Holocene land- Hill,1956). Where it is exposedin the region, the forms in the present map area, has a history that Rim Rock fault (fig. 1) seems to form part of the dates back to at least Late Precambrian time. This boundary between the provinces. Numerous remarkable contrast led R. T. Hill (1902) to sug- workers have suggested a long-enduringhistory of Fig. 1. Mapof part of Trans-PecosTexas showingphysiographic features and study area. Gravity,Magnetic, and Generalized Geologic Map, VanHorn— Sierra Blanca Region 3 strike-slip movement along the Hillside fault (fig.1). Additional sources are cited in the text. King The Van Horn— Sierra Blanca region has been (1965) gave an extensive bibliography beginning mapped in detail geologically, but because nearly with the Perry Boundary Survey Report of 1857. half of the total area of the region is covered by R. T. HiU (1902, 1928), on the basis of his field late Cenozoic basin-fill and because subsurface mapping, suggested that a transcontinental fracture control is sparse, ithas not been possible to under- system might traverse northern Chihuahua. He stand the regionin detailin three dimensions. This also added, seemingly as an afterthought, that a report summarizes the methods and results of a similar fracture system might traverse Trans-Pecos groundmagnetometer and gravimeter survey under- Texas near Van Horn. This idea was givenaname, taken in an attempt to obtain a clearer understand- the Texas lineament, by Ransome (1915) and a ing of the tectonic history of the region and of the type locality, Eagle Flat, by Albritton and Smith structural significance of theHillside and RimRock (1957). Amplification and clarification of the idea faults. were givenby Baker (1927,1935) and Muehlberger (1965). Moody and Hill (1956) incorporated the Location ideainto their system of wrench fault tectonics and called it the Texas direction of world-wide wrench The Van Horn— Sierra Blanca region is in south- faulting. eastern Hudspeth and southwestern Culberson counties,Trans-Pecos Texas (fig. 1). The towns of Acknowledgments Van Horn, Allamoore, and Sierra Blanca are inthe mappedarea. Access to theregionis by Federaland The map and this report are based on a doctoral State highways, Farm-to-Market highways, and dissertation (Wiley, 1970). Professor William R. graded county roads (fig. 1). The principal Muehlberger supervised the dissertation and con- businesses of the region are cattle ranching and tributed time, ideas, and enthusiasm. Professors talc mining, and many roads and trails in the Ronald K. DeFord and Peter T. Flawn, The Uni- region are privately maintained to serve these versity of Texas at Austin, contributed freely of businesses and are on private land. Many of the their wide knowledge of the region. Professor more interesting geological features of the region James E. Case, The University of Missouri at are accessible only by private road. The property Columbia, assisted in the structural interpretation rights of the private landowners must be observed, of the geophysical data. The writer expresses and permission to enter should be obtained from appreciation for their aid and encouragement. the owner. This project would havebeen impossible without the cooperation of the ranchers and landowners in Previous Investigations the region. Almost without exception,they allowed free access to their land, and their hospitality and The most comprehensive geological work in the helpare hereby acknowledged. region is that of King and Flawn (1953), King This project was supported inpart by National (1965), and Albritton and Smith (1965). Signifi- Science Foundation Grant GA-1581 to William R. cant contributions to the stratigraphic andtectonic Muehlbergerbetween September 1968 and January knowledge of the region have also been made by 1970. The Bureau of Economic Geology of The Hay-Roe (1957), DeFord (1958a, 1969), Twiss University of Texas at Austin provided a field (1959b), Underwood (1963), Haenggi(1966), vehicle during parts of 1966, 1967, and1968. Barnes (1968), and Haenggi and Gries (1970). 4 Geologic Quadrangle Map— No. 40 Regional Geologic Setting Tectonic Framework tectonic elements, these provinces are the Diablo platform on the north and northeast and the The VanHorn— SierraBlanca region straddles the Chihuahua trough— Chihuahua tectonic belt on the boundarybetween two geologic provincesthat have south (fig. 2). As topographic entities, the blocky had markedly dissimilar geologic histories. As Diablo Plateau and Sierra Diablo are on the north, Fig. 2. Tectonic framework of West Texas and northeasternMexico. (Modified from DeFord, 1969; Haenggi, 1966; King, 1965; andAlbrittonand Smith, 1965.) Gravity, Magnetic, and Generalized GeologicMap, VanHorn— SierraBlancaRegion 5 whereas linear ranges, e.g., Indio, Eagle, and Hueco Mountains and eastward in the Permian Quitman Mountains, are on the south. The Basin were faulted and gently warpedduringPenn- boundary between the provinces is beneath Eagle sylvanian uplift of the Diablo platform. By early Flat, an elongate structural and topographic de- Wolfcamp (Permian) time, the Delaware basin was pression that trends west-northwest from south of a strong negative area northeast of the Diablo plat- VanHorn to north of SierraBlanca (fig. 1). form; probably the Chihuahua trough was also At least 2,500 feet of pre-Permian Paleozoic evolving at the same time to the southwest. rock was deposited in the Van Horn— Sierra Blanca The Delaware basin filled during Permian time region near the site of the Diablo platform of and received less than 2,000 feet of sediments Permian time. About 7,000 feet of Permian rock during the Mesozoic Era. The Chihuahua trough, was deposited on the platform, but because of however, was strongly negative by Late Jurassic regressive offlap, probably much less than half this time andreceivedmore than 20,000feet of Jurassic amount was ever deposited at any one place. and Cretaceous sediment. Laramide folding and Possibly as much as 2,500 feet of Mesozoic rock, thrusting rammed this thick succession of rock mostly Lower Cretaceous, was deposited on the against the stableDiabloplatformin theVanHorn- platform. Of the total of perhaps 15,000 feet of Sierra Blanca region. Tertiary volcanism, mid- Paleozoic and Mesozoic rock deposited on the Tertiary block faulting, and late Tertiary to Holo- platform, much has been eroded and less than cene normal faulting closed the tectonic history of 8,000 feet remains. More than 18,000 feet of the region. Cretaceous rock plus an unknown thickness of The present topography of the VanHorn— Sierra olderMesozoic and Paleozoic rock was depositedin Blancaregionmost stronglyreflects themid-Tertiary the Chihuahua trough or ancestral depositional tectonic history. However, the northwest trend of basins. Throughout the region, these rocks are the Quitman and Malone Mountains and Devil underlain by Precambrian rocks that include about Ridge (fig. 1) reflects the structural and, in part, 35,000 feet of metasedimentary and meta-igneous the stratigraphic strike of the Mesozoic Chihuahua rock. Several thousand feet of Cenozoic volcanic trough— Chihuahua tectonic belt.