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. The east and and sedimentary rocks overlie Permian and Creta- northeast border of the Chihuahua trough was con- ceous rock in the Wylie, Van Horn, and Eagle trolled by the Diablo platform which was formed Mountains. Late Cenozoic bolsonfill is more than along the Precambrian boundary between stable 1,500feet thick in Eagle Flatand inSalt Basin. crustonthenorthandtheunstable VanHornmobile All of these rocks have been mapped and de- belt on the south. The Streeruwitz thrust fault scribed in detail by earlier workers. Much of this (fig. 3) marked this boundary in the Precambrian. mapping is summarized on the Van Horn— El Paso Eagle Flat also follows this boundary and not only Sheet of the Texas Geologic Atlas (Barnes, 1968). is the "type locality" of the Texas lineament The generalized geologic base for the present geo- (Albritton and Smith,1957,1965) butalsoincludes physical map is mostly from that Atlas sheetbut is the Hillside fault, cited by Moody and Hill(1956) also from the maps by Hay-Roe (1957), Twiss as a part of the Texas direction of wrench faults. (1959b), and Underwood (1963). Geologic Base Map Tectonic History Geologic data on the base map are generalized Thebasement of the Diablo platform (fig. 2) was into six map units. Rocks of LatePrecambrian age formed by Late Precambrian orogeny in the Van are shown as two map units. The olderPrecambrian Horn mobile belt and was intermittently active map unit (pCI) comprises the Carrizo Mountain during the Paleozoic Era (Flawn, in Flawn et al., Groupand itsassociated 1,250m.y.intrusive rocks. 1961, pp. 57, 145). Strong uplift of the platform The younger Precambrian map unit (pCu) consists occurred in Pennsylvanian time, probably at the of the Allamoore,Hazel,and VanHornFormations, same time as the Ouachita Orogeny in West Texas. allof whichare younger than the Carrizo Mountain Direct evidenceislacking,butprobably theCoahuila Group. All Paleozoic rocks are shown by a single platform evolved, at least in part, atthe same time map symbol (Pal); they consist of mostly Ordo- and to the south of the Diablo platform (fig. 2). vician and Silurian rocks in the Baylor and Beach Ordovician to Mississippian rocks deposited in Mountains and mostly lower Permian (Wolfcamp much of the area of the present Sierra Diablo and and Leonard Series) rocks elsewhere. Mesozoic 6 Geologic Quadrangle Map— No. 40 rocks (X) comprise Lower Cretaceous formations Formations and the little disturbed Van Horn and a minor amount of Upper Cretaceous rock in Sandstone. the northeast Eagle and central Van Horn Moun- Allamoore and Hazel Formations-The Alla- tains. Cenozoicrocks include Tertiary intrusive and Moore and Hazel Formations crop out west of extrusiverocks (T)and Quaternaryalluvium and fill Van Horn in a roughly 8-mile-wide belt between (Q). Tertiary andQuaternary graveland fillnear the the Texas andPacific Railway on thesouth and the mountain fronts are shown as Quaternary(Q). south-facing escarpment of Sierra Diablo on the Structure shown on the map is generalizedfrom north. The Allamoore consists of about 2,500 feet the source geologic maps; none of the structure of limestone and dolomite, but minor thicknesses shown is interpreted from the geophysical data. of conglomerate,talcose phyllite, and volcanic flow Faults shown include the Streeruwitz thrust fault and pyroclastic rock are included. The Hazel (fig. 3),Laramide thrustand tear faults,the Hillside consists of more than 5,000 feet of interbedded fault (fig. 3), Cenozoic basin-range faults, and most conglomerate and sandstone (King, 1965, pp. 23- other faults having a known throw inexcess of 500 -27). Both formations are complexly folded and feet. Enoughof theremaining faults havingsmaller faulted and seem to be mixed tectonically in the displacementare shown to suggest the faultpattern. southern part of their outcrop belt. Scattered ex- Fold axes are not shown. posures of the Streeruwitz thrust fault zone along the southern part of the Allamoore outcrop show Generalized Stratigraphy that the CarrizoMountain Group was thrustnorth- ward over the Allamoore and, although not de- Older Precambrian Rocks monstrable, presumably over the Hazel as well The metasedimentary and meta-igneousrocksof (King and Flawn, 1953). The deformation and the Carrizo Mountain Group are the oldest rocks mixing of the Allamoore and Hazel Formations known inthe VanHorn— SierraBlanca region. They probably accompanied the movement of the thrust crop out in the Carrizo Mountains,in small parts of fault. The Hazelis less deformed in the northern theEagle,VanHorn,and WylieMountains,probably half of the outcrop belt, thus suggesting that the underlie most of Eagle Flat, and have been en- present outcrop of theStreeruwitz thrust fault may countered in wells east and south of Van Horn be close to its original maximum northwardextent. (Hay-Roe,1957;Denison et al,1969). The Carrizo The Allamoore and HazelFormations have not Mountain Group consists of more than19,000 feet been dated isotopically but, because of their of metamorphosed quartzite and arkose as well as involvement in 1,000 m.y. thrust faulting,must be mica-schist, slate, talcose phyllite, and limestone. older than 1,000 m.y. Non-fault contacts between the Carrizo Mountain Group and the Allamoore About 2,100 feet of rhyolite intruded the group as sill-like bodies and accompanied tilting, folding, Formation are not known, but King (1965, p.22) concluded what he termed "tenuous evidence" and regional metamorphism about 1,250 m.y.b.p. on that the Allamoore and Hazel were younger than (Denisonand Hetherington,1969; Kingand Flawn, 1953). the CarrizoMountain Group. No contacts between the Hazel and the Carrizo Mountain Group are The Carrizo Mountain Group was transported known; in all known exposures of the Streeruwitz over the younger Precambrian Allamoore and Hazel thrust fault, the Carrizo Mountain Group overlies Formations along the Streeruwitz thrust fault. It Allamoore was during this orogeny, dated at 950 to 1,000 the Formation. Van Horn Sandstone-The Van Horn Sandstone m.y.b.p. by Denison and Hetherington (1969), is mapped with the and Hazel Forma- that about 5,600 feet of diorite intruded the Allamoore tions. It consists of 1,000 to 2,000 feet of cross- Carrizo Mountain Group as sills. Near the close bedded conglomerate and sandstone. It of this orogeny, retrograde metamorphism cata- coarse un- conformably exposedin clastically fractured the metarhyolite and converted overlies allolder rocks the Van Horn— Sierra region and is tilted and the diorite to amphibolite (King and Flawn,1953, Blanca p. 63). broadly arched but is not metamorphosed. King (1965, pp. 29-30) considered the age of the YoungerPrecambrianRocks (peu) unfossiliferous Van Horn Sandstone to be Pre- cambrian or Precambrian(?), but other workers The younger Precambrian map unit (pCu) in- have suggested that itis younger,possibly mid- or cludes the severely deformed Allamoore andHazel Late Cambrian (McGowenand Groat, 1971, p.7). Gravity, Magnetic, and Generalized Geologic Map, VanHorn— Sierra Blanca Region 7

PaleozoicRocks (Pal) the Baylor and Beach Mountains and in Sierra Diablo. Theyare probably presentbeneath most of Rocks ranging in age from Ordovician through SaltBasin and are encountered in wells farther east. Permian are combined on the map as Paleozoic Permian rocks of the Wolfcamp and Leonard (undifferentiated). The lower Permian (Wolfcamp) Series crop out in the Wylie and Baylor Mountains Hueco Limestone is the lowest Paleozoic rock ex- and in Sierra Diablo. Carbonate and clastic rocks posed or present in most of the mapped area, but of the Guadalupe Series (Permian) crop out in the 2,000 to 2,600 feet of mostly carbonate rocks of Apache and Delaware Mountains along the east Ordovician through Pennsylvanian age crop out in border of the mapped area (table 1). About

Table 1. Permianrock units, VanHorn— SierraBlancaarea. (From King, 1965; AlbrittonandSmith,1965; Barnes,l96B; and Wood, 1968.)

rr FORMATION THICKNESS (FEET) CO MESOZOIC AND CENOZOIC ROCK MAJOR UNCONFORMITY Capitan 900 + Reef and reef talus limestone

bJ Seven Rivers a. Formation 160 Limestone and dolomite; grades northward into Capitan _J Munn Formation 450 Dolomite and limestone in Apache Mountains < Sandstone tongue 150- 190 Sandstone in western Delaware basin grading eastward to of Cherry— Canyon limestone,dolomite,sandstone in Apache Mountains z z -Formation Brushy + Canyon Formation 1000 Sandstone and shale of Delaware basin UNCONFORMITY Briggs Formation Briggs Formation in Malone Mountains= gypsum, limestone, 630-1800 dolomite. Unnamed carbonate equivalent in Finlay Mountains and unnamed (to 1800 ft.). Both partly equivalent (?) to Cutoff Shale,Victorio Peak Limestone, and Bone Spring Limestone Q UNCONFORMITY 01 Shale, siltstone, limestone < Cutoff Shale 275 of Diablo platform— Delaware basin margin-, thins eastward and southward GRADATIONAL Victorio Peak Limestone and dolomite of Diablo platform— Delaware basin Limestone 900-1500 margin; replaced by Bone Spring Limestone in basin . GRADATIONAL 1050 Chiefly black limestone of Delaware basin with reefs on Victorio ? Bone Spring (Maximum) and Blabb flexures;thins north and south of Victorio flexure Limestone UNCONFORMITY "Main Body" 0-1400 Limestone of platform and dolomite of Diablo platform-Delaware si .basiDmargiiL GRADATIONAL IS|| Powwow Mem. 0-250 Conglomerate, arkose, shale, limestone MAJOR UNCONFORMITY LOWER PALEOZOIC AND PRECAMBRIAN ROCKS 8 GeologicQuadrangleMap— No. 40

10 miles west of Sierra Blanca,west of the mapped The overthickened succession of lower Permian area, folded carbonate and evaporite rocks of the rock in Hovey channel (Amsbury, 1957, p. 26) Briggs Formation (Leonard Series) crop out inthe marks approximately the Paleozoic boundary be- Malone Mountains. These rocks may be present tween the Diablo and Coahuila platforms (fig. 2) beneath Sierra Blanca but must pinch out eastward but the nature of the boundary or join is not because Cretaceous rocks unconformably overlie understood. the Hueco Limestone in the Streeruwitz Hills and Severalnorthwest-trendingstructuraladjustments in the Eagle Mountains between Sierra Blanca and occurred in or near the Van Horn— Sierra Blanca Van Horn. region during the uplift of the Permian Diablo plat- Insharp contrast to the Precambrian history, the form. Fromnorth to south, these include the Babb early part of the Paleozoic Era in northern Trans- and Victorio flexures, several broad, open, east> Pecos Texas was a time of quiet deposition and trending anticlines and synclines, and the Hillside minor uplift, tilting, and faulting. Farther south, fault. The reefs of the Leonard Series are wellde- in the Marathon region, the Ouachita geosyncline veloped along the Victorio and Babb flexures, began to form as early as Late Cambrian time. thereby implying that the flexures were forming Deformation of the Marathon segment of the during the Leonard Epoch. It is not known what Ouachita geosyncline began in Late Mississippian influence the flexures may have had on the later time and ended during the WoIfcamp Epoch Capitanreef. (Flawn,in Flawnet al., 1961,p.58). The Hillside fault is exposedalongthe Texas and In northern Trans-Pecos Texas, tectonic adjust- Pacific Railway between Allamoore and Van Horn ment that began in the Late Mississippian orEarly where it is the boundary between the Carrizo Pennsylvanian Epochs was probably related to the MountainGroup onthe southandyounger rockson Ouachita Orogeny in the Marathon region. The the north. The fault strikes about N.70° W. and differential warpingof theseadjustmentsformed the is nearly vertical. It is downthrown at least 1,000 Diablo and Central Basin platforms and the inter- feet but the displacement may be much more veningMidland andDelaware basins. These adjust- (King, 1965, p. 114). The most recent displace- ments mayalso haveformed ancestral counterparts ment on the Hillside fault waspost-Cretaceousbut of the Late Mesozoic Coahuila platform and King (1965, p. 105) showed that it had an earlier Chihuahua trough. Parts of the Diablo and history. The Powwow conglomerate member of Coahuila platforms and of the Chihuahua trough the lower Permian Hueco Limestone, in exposures and Delaware basin are in the Van Horn— Sierra up to about a mile north of the fault, contains a Blanca region (fig. 2). large proportion of angularmetarhyolite fragments North of the BaylorMountains,the present west derived from the CarrizoMountain Group south of side of Salt Basin roughly coincides with the west the fault. The metarhyolite fragments decrease in side of the Delaware basin and the east and north number and size away from (north of) the fault, sides of the Diablo platform. The southeast and thus suggestinga high-standingpre-Huecoscarp as a south sides of the Diablo platform are bounded by local source of the coarse fragments. King (1965, the Marathon segment of the Ouachita geosyncline p. 114) suggested that this wasafault scarp along a and the Paleozoic Hovey channel (fig. 1; and Late Pennsylvanian or Early Permian Hillside fault. DeFord, 1958a, p. 56; 1969). Amsbury (1957, Several authors have interpreted the Hillside fault fig. 5), Atwill (1960, pp. 21-22), Haenggi (1966, as a major element of transcontinental wrench fig. 8, p.136),L6pez-Ramos (1969), and Gries and faulting. The grading of the angular metarhyolite Haenggi(1970) showed that the Chihuahua trough fragments awayfrom their seemingly nearbysource formed the west and southwest boundaries of the shows, however, that wrench faulting could not Diablo platform. have occurred along the Hillside fault after Penn- TheCoahuilaplatformextends southintoMexico sylvanian time. about 350 miles from the Ouachita front at The estimated density of Paleozoic rocks in the Marathon (DeFord, 1958a, p. 56). Haenggi (1966, region ranges from about 2.4 to 2.7 gm/cm3;the pp. 136-137) and Lt>pez-Ramos (1969) suggested estimated average density is 2.63 gm/cm3. that an ancestralCoahuila platform was contempo- raneous with the Paleozoic Diablo platform and MesozoicRocks (K) that the Chihuahua trough had been a negative feature duringmuch of the Paleozoic Era. Only Cretaceous rocks represent the Mesozoic Gravity, Magnetic, and Generalized Geologic Map, Van Horn— Sierra Blanca Region 9 — Table 2. NomenclatureofMesozoicrocks, Van Horn SierraBlancaarea. a oo < g UJ a: rr 21 FORMATION THICKNESS Ljl! (FEET) if CO CENOZOIC OLCANIC A D SEDIMENTARY ROCKS UNCONFORMITY Chispa Summit and Chispa Summit: shale, marl, flaggy limestone; BoquillasFormations 200-840+ Delaware basin and margin Boquillas: flaggy limestone; Diablo platform Limestone with basal conglomeratein Apache tr Buda Limestone 122-240 Mountains, limestone elsewhere LjJ Q_ Shale, sandstone, limestone in Apache and Wylie Boracho Formation 180-428 Mountains; equivalent to Benevides, Espy-Loma Plata, and Eagle Mountain Formations Eagle Mountain Sandstone, shale, siltstone, sandy limestone; 80-300+ Sandstone south of Diablo Plateau Espy Limestone Limestone, marl, shale, and sandstone on Diablo (=Loma Plata 175-1094+ platform, chiefly limestone to south; formerly Limestone) widespread, now eroded from most of Diablo Plateau BenevidesForma- Sandstone with minor limestone, shale, and marl tion (=Kiamichi 84-235 Interbeds; chiefly south of Diablo Plateau Formation)

CO Chiefly limestone with minor sandstone and marl Finlay Limestone 40-800 interbeds; presentover most of region O Sandstone in north grading to 50% limestone and LU Cox Sandstone 400-1737 8O 50% sandstone in south; present over entire region if) LIJ % Yearwood Formation 160-225 Sandstone, shale; limestone;on Diabloplatform only; LU o not west of Van Horn Mountains o ' Campagrande Limestone, marl, shale, sandstone, conglomerate;not Formation 0-800 south of U.S. Highway 80.Lowest Cretaceous on platform I Bluff Sandy limestone with sandstone and shale interbeds; Formation 1000-I5OO(?) not south of U.S. Highway 80 east of Van Horn Mountains Interbedded limestone, shale, conglomerate grading Yucca Formation 0-5000 south to sandstone, shale, and conglomerate;absent on Diablo platform CONFORMABLE? Torcer Formation 400+ Limestone, sandstone and shale with basal quartzite; Malone and northwestern Quitman Mountains Limestone, siltstone,sandstone, minor Malone Formation 850-850-100010(oo conglomerate»and gypsum; Malone Mountains cr CONTACT CONCEALED 00 LU

Era in the mappedarea. Jurassic carbonate, clastic, into Mexico. Wilson and others (1968) showed and evaporite rocks crop out in theMalone Moun- that the age of the extrusive rocks of Sierra Vieja tains 10 to 12 miles west of SierraBlanca andpossi- ranges fromlateEocene tolateOligocene or early(?) blyunderlie Cretaceousrocks in theQuitmanMoun- Miocene. Although the extrusive rocks of the tains and Devil Ridge. The Lower Cretaceous for- region have not been correlated in detail from the mations from the Yucca up through the Espy (or Van Horn Mountains westward, field relations Loma Plata) crop out south and west of the suggest that extrusive rocks in the QuitmanMoun- Southern Pacific Railroad except in the Van Horn tains are probably atleast time-equivalent with the Mountains. Lower Cretaceous rocks north of the extrusive rock of the Sierra Vieja country (Al- Southern Pacific include Campagrande through britton and Smith,1965). Espy Formations (table 2). Upper Cretaceous Rhyolite laccoliths of possible Oligocene age rocks crop out in the Apache and Wylie Mountains (Albritton and Smith, 1965) intruded Lower Creta- and in parts of the Van Horn and eastern Eagle ceous rock near Sierra Blanca; the largest of these Mountains. Eagle Flat lies athwart the southeast forms Sierra Blanca Peak. Miocene(?) and younger and south-trending margin between the Mesozoic rhyolite and quartz monzonite stocks intruded Diablo platform on the north and the Chihuahua Tertiary volcanic and older rocks in the Eagle, trough on the south (fig. 2). North and east of Van Horn, and Wylie Mountains. A few scattered Eagle Flat, on the Diablo platform, Cretaceous late(?) Tertiary basalt plugs crop out where they carbonate, clastic, and argillaceous rocks are only intruded thin platform Mesozoic and Paleozoic about 2,000 feet thick and are thin platform or rocks north of Eagle Flat and west of Salt Basin shelf facies equivalents of the Bluff throughEspy (King,1965,pi.1). (Loma Plata) Formations. South of Eagle Flat, Late Tertiary and Holocene sedimentary rocks these units thicken abruptly, and in the Eagle and are present in much of the region. For the most Quitman Mountains more than 12,000 feet of part, these are poorly consolidated,poorly sorted Lower Cretaceous carbonate and clastic rocks alluvialfan and bolson-fill clastic deposits,but some are present. of the older basin-fill is better cemented and The rocks in the Chihuahua trough were thrust resistant. north and northeastward and were extensively The Eocene to Miocene(?) volcanism was folded during the Laramide Orogeny in northern followed by strong uplift andblock faulting inmid- Mexico and Trans-Pecos Texas. In contrast, the Tertiary time (DeFord and Bridges, 1959; Wilson, thin succession of Mesozoic rocks on the Diablo 1965; Dasch et al., 1969). During this episode of platform was little disturbed until broken by mid- faulting, the Rim Rock, Neal, and Mayfield faults Tertiary high-angle faults. (boundingthe VanHornMountains) were displaced The estimated average density of Cretaceous about 4,000 feet (Twiss, 1959b), and other major rocks in the mapped area is 2.51 gm/cm3. The faults in the region were displaced lesser amounts. range of estimated density is between 2.32and 2.63 Minor normal faulting, with seeming diminishing gm/cm3 with slightly higher values beingestimated severity, has continued into Pleistocene and Holo- for the platform facies rocks than for the trough cene time (King, 1965). faciesrocks. These estimates arebasedonestimated At least 600feet of verticaldisplacementoccurred mineralogical content in measured stratigraphic along the Hillside fault after the end of Cretaceous sectionsreported by earlier workers and are subject deposition on the Diablo platform, possibly during to considerable error. The method of estimation the mid-Tertiary block faulting. There is no evi- and sources of data are in Wiley (1970, table 7). dence of the displacement having any significant lateral component in the Cenozoic Era. Cenozoic Rocks (T, Q) The estimated average density of the early to mid-Tertiary igneous rocks is 2.65 gm/cm3. The Tertiary volcanic and intrusive rocks are wide- estimated average density of late Tertiary to Holo- spread south of Eagle Flat and southeast of cene sedimentary rocks is 2.05 gm/cm3. This Van Horn; some of the volcanic units can be traced figure was confirmed by gravimeter "density from the Wylie and VanHornMountains east to the profiles" as described by Nettleton (1940). Davis Mountains and south through Sierra Vieja Gravity, Magnetic, and Generalized Geologic Map, VanHorn— SierraBlanca Region 11

Geologic Interpretationof Gravity and Magnetic Anomalies

Methods (Nagata, 1961, pp. 128-129; Lindsley et al., 1966, pp. 548-549). The estimated valuesused are broad map Gravity and magnetic data shown on the approximations based onan empirical equation were obtained in the field by the writer between k = 0.001 x Mp June 1966 and October 1968. The survey com- where k is susceptibility in c.g.s. units (accurate to prises 1,566 gravity and 1,703 magnetic control ±0.001 c.g.s.units) and Mpis the volume percentof points, approximately 1.3 points per square or magnetic minerals in the rock (expressed as an mile. The Bouguer gravity anomaly, contoured in integer number). This equationgives susceptibilities density of 2.67 blue, is reduced with a Bouguer about 50 percent larger than those suggested by 3 The red contours depict the vertical gm/cm . Nagata (1961, p. 129) and about equal to those component of magnetic intensity after removal of givenby Lindsley et al. (1966,fig. 25-3,p.548). It a regional gradient. Additional information regard- is safe to assume that the magnetic susceptibility of ingthegeophysicaldataand theirprobableaccuracy most sedimentary rocks younger than Precambrian and precisionis givenin the Appendix. is nearly zero, at least within acceptable ranges of Much of the discussion following is based on error. qualitative interpretation of thegeophysicalanoma- Both density and susceptibility of particular interpretation is lies. This means that a geologic rock units are assumed constant over the range of made several anomalies simultaneously with- over the interpretation. Except in the Carrizo Moun- resorting mathematical representations of out to tains, errors introduced by this assumption are their causative bodies. The result is a generalized probably smaller than errors in estimation of or schematic interpretation. density and susceptibility. Quantitativeinterpretationof severallargeanoma- lies yielded subsurface geologic data of regional AdditionalAssumptions RelatedtoMagneticAnomalies significance. These interpretations are based on the two-dimensional profile matching methods discussed by Nettleton (1940, 1942), Grant and The following assumptions are made to facilitiate West (1965), and many others. Both the methods interpretation of magnetic anomalies: of ideal geometric shapes (Nettleton, 1940, 1942) (1) All magnetic anomalies are associated with and of line integrals (Hubbert, 1948) were used as local magnetic fields induced inconcentrated mag- minerals; appropriate. netic (2) remnant magnetization is probably an im- Assumptions portant contributor to the observed anomalies but is assumednegligiblebecause of theaforementioned Density and Magnetic Susceptibility lack of rock samples suitable for measurement; (3) the external inducing magnetic field (earth Geologic interpretation of gravity and magnetic field) dips 60° N. 13° E. over the entire map area anomalies requires knowledge of density and mag- (Dccland Howe, 1948). netic susceptibility of the rocks involved in the interpretation. Directmeasurement of theseproper- RegionalGradients ties requires fresh, unweathered rock samples, preferably core plugs. Because suitable samples of Theregionalchange of the verticalcomponent of the rocks in the Van Horn— Sierra Blanca region magnetic intensity and of the Bouguer gravity were not available, these properties were estimated anomaly (the regional gradient) is,in most places, from mineralogical or gross petrological compo- the effect of large, deep-seatedstructural or strati- sition. The estimated densities and susceptibilities graphic features superimposed on the effects of are tabulated in Wiley (1970, pp. 83-87), and shallower features. The Bouguer gravity anomaly densities are probably within about ±0.15 gm/cm3 map of the United States (Woollard and Joesting, of the average true density. 1964) suggests that the regionalgradient of gravity Estimates of magnetic susceptibility of rocks are inthe regionisless than1mgalpermile and is much hazardous, because of the large range of suscepti- less than that in most of the region. Moreover, bility produced by weathering or by smallchanges complex structure and the resulting overlapping in the percentage of constituent magnetic minerals gravity anomalies inmost of the mappedarea made 12 Geologic QuadrangleMap— No. 40 it hazardous or impossible to choose a meaningful anomaly and magnetic values decrease from the regional gradient of gravity. Accordingly, the edges toward the center of Wildhorse and Lobo regionalBouguer anomaly was assumed not signifi- basins. These gradients arenotconstant butaverage cant for mostof theareasinterpretedquantitatively, about 3.5 mgal and 125 gammas per mile. and no attempt was made to evaluate or remove Hay-Roe (1957) and Wood (1968,p.5) used the such a regional anomaly. The regional vertical term "half-graben" to describe Salt Basin, thus component of magnetic intensity hasbeen removed implying a major bounding fault on only oneside. by reference to standard magnetic charts (Dccland The gravity data support this conclusion and also Howe,1948). suggest several small horsts and grabens within the broader basin. Regional Subdivision Hay-Roe (1957) showed that the Wylie Moun- tains were bounded on the east and west by large- The known geology of the region,supplemented displacement normal faults. He suggested that the by the trend and style of the gravity and magnetic north flank of the Wylie Mountains might also be anomalies in the map area, suggests subdivision of fault bounded but did not find convincing geologic the region into four tectonic blocks: (1) Salt Basin evidence. Geophysical evidence does not suggest grabenin the eastern third of the maphavingnorth- faulting at the north end of the Wylies but suggests trending surface structure and low amplitude instead that the Permian rocks of the Wylie Moun- anomalies; (2) the Diablo Plateauarea in the north- tains dip northward beneath the alluvium of Salt central part of the map having west and west- Basin without displacement. northwest-trending structure and anomalies of zero The northward-elongated gravity minimum in to low amplitude; (3) the Carrizo Mountains- Michigan Draw east of the Wylie Mountains is as- Eagle Flat area in the south-central partof the map sociated in part with the East Wylie fault which having moderate to high amplitude anomalies and drops Permian and older rocks into the Michigan diverse surface structure; and (4) the DevilRidge— Draw basin. The minimum may also indicate QuitmanMountains part of the Chihuahua tectonic locally thick basin fill or a syncline or narrow belt in the southwest part of the map, which has grabeninPaleozoicrocksbeneath the alluvium. west-northwest-trending surface structure and low Wildhorse and Lobobasins are twosmall drainage to medium amplitude anomalies. basins— Wildhorse on the northwest and Lobo on the south and southeast. They are separated by a Salt Basin Graben low northeast-trendingdrainage divide between the Van Horn and Wylie Mountains (fig. 1). Major Salt Basin trends about S. 10° E. for 130 miles faults mark the boundaries of the basins at the from about 30 miles north of the Texasboundary Wylie and Carrizo Mountains and these are repre- in Otero County, New Mexico, through Culberson sented in steep gravity and magnetic gradients and Jeff Davis counties, Texas, until it merges toward the center of the basins. Farther south, physiographically with the broad, south-southeast- Lobo basin is a half-graben, bounded on the west trending plain west of the Davis Mountains (fig.1). by the Neal-Mayfield fault system at the Van Horn Near Van Horn, Salt Basin splits into three south- Mountains (Twiss, 1959b) and on the east by trending basins— Michigan Draw basin on the east seemingly unbroken, gently basinward-dipping and the Wildhorse and Lobo basins on the west. Tertiary volcanic rocks (Hay-Roe,1957). A broad The Wylie Mountains,uplifted along faults, lie be- gravity maximum trends north-northeast from the tween the Michigan Draw and Wildhorse and Lobo north end of the VanHornMountains (near control basins. points 757-758) about parallel to the front of the The Bouguer gravity anomaly decreases from CarrizoMountains 3miles to the west. This anomaly east to west at about 0.8 mgal per mile across Salt suggests a north-northeast-trending horst that ex- Basinnorth of the WylieMountains and inMichigan tends as far north as Van Horn and which possibly Draw. Generally northward-trending anomalies of extends northward into Salt Basin near the east from 1to 6 mgal are superimposedon theregional front of the Baylor Mountains. anomaly. The total variation in the vertical mag- netic component is less than 200 gammas, and the Diablo Platform and Foothills amplitude of the few well-defined magnetic anoma- lies is less than 100 gammas. Bouguer gravity Sierra Diablo,the Diablo Plateau,and their foot- Gravity, Magnetic, and Generalized Geologic Map, VanHorn— Sierra BlancaRegion 13 hills, the Baylor and Beach Mountains and the North and northeast of the northern Quitman Millican and Streeruwitz Hills (fig. 1), compose a Mountains are the five Oligocene(?) laccoliths that convenient unit for discussion even though they support peaks and ridges collectively known as are not strictly related tectonically. In much of Sierra Blanca Peaks (Albritton and Smith, 1965). this area, Bouguer anomaly values decrease north- These laccoliths, although not included in the ward at an almost constant 1.5 mgal per mile. The geophysical survey, are of interest because the few recognizable exceptions to this gradient are large gravity andmagnetic anomaly 6 miles north- small areally and have amplitudes less than 2 mgal. northeast of the town of Sierra Blanca suggests a Magnetic anomalies range from 50 to about 300 similar, but completely covered, laccolith. The gammas in the Millican and Streeruwitz Hills where laccolith was probably intruded along a fault or meta-igneous rocks crop out. Farther north, on fault zone that trends about northwest. the Diablo Plateau, magnetic intensity is nearly constant, thus giving no information on the Ealge Flat and Carrizo Mountains geometry of the magnetic basement. Perhaps the magnetic basement is deeply buried. The gravity Eagle Flat and the Carrizo Mountains occupy contours describe an arcuate pattern, that is, they most of the central and west-central parts of the trend south-southwest in the Baylor Mountains, mapped area (fig. 1). Eagle Flat is about 40 miles southwest at Beach Mountain,west in the Millican long and 4 to 10 miles wide; it is bounded on the Hills, and west-northwest in the Streeruwitz Hills east and north by the Van Horn and CarrizoMoun- and Diablo Plateau. This patternmay be anindica- tains, the Millican and Streeruwitz Hills, and the tion of a broad, northwest-trending thickening of Diablo Plateau. Green River Bolson (Twiss,1959a, the sedimentary section. p. 127), a southward extension of Eagle Flat, is Most of the smaller gravity and magnetic anoma- about 15miles longand 5 miles wide (fig.1). A low lies are the effects of near surface structural com- drainage divide between the Eagle and Van Horn plexity. Gravity maxima are associated,in many Mountains separates them. Green River Bolson places, with outcrops of Allamoore and minima and Eagle Flat are bounded by the Indio and Eagle with adjacent outcrops of Hazel Formation. Vol- Mountains and Devil Ridge on the west and south- canic rocks in the Allamoore Formation produce west. large magnetic maxima (400 to 600 gammas) but Gravity and magnetic anomalies in the Carrizo the anomalies are small areally. Rocks of the Hazel Mountains are elongated approximately parallel to Formation seem to have low and uniform magnetic the northeast strike of the metamorphic rocks in susceptibility because, regardless of structural sim- the eastern part of the mountains. More or less plicity or complexity exhibited by these rocks, equidimensionaloutcrops of amphibolite and meta- magnetic intensity is nearly constant throughout rhyolite predominatein the west part of the moun- the outcrop belt of the formation. tains and are associated with gravity and magnetic anomalies that have similar shapes. Themaximum amplitude of gravity and magnetic anomalies is Devil Ridge, Quitman Mountains, and SierraBlanca Peaks about 6 mgal and 600gammas. Several small bodies of Carrizo Mountain meta- Thrust-faulted and folded Lower Cretaceous rhyolite,of Permian,and of Lower Cretaceous rock rocks in the Chihuahua tectonic belt trend north- are exposedthrough the bolson fill and alluvium of west from the central Eagle Mountains across the Eagle Flat, but direct knowledge of the deeper southwest part of the mapped area. These rocks parts of the basin is lacking. The largest gravity hold up Devil Ridge,Back Ridge,and the Quitman anomaly in the mapped area is 40 miles long and Mountains, each of which is the upper plate of a 2 to 3 miles wide; it is a roughly linear zone in major Laramide thrust fault (Underwood, 1963; Eagle Flatin which Bouguer gravity anomaly values Albritton and Smith, 1965, pi. 1). These thrust decrease southwestward as much as 22 mgals. plates are characterized by low amplitude gravity From about 6 miles north of Sierra Blanca, this and magnetic anomalies that trend northwest anomaly trends southeastward to the Streeruwitz approximately parallel to the northwest trend of Hills,along the south sideof theCarrizoMountains, the ranges. The geophysicalanomalies suggest that to the Van Horn Mountains near Scott's Crossing the maximum thickness of the thrust sheets is (fig. 1). It continues southward from Scott's probably less than 8,000 feet (possibly much less) Crossing along the west flank of the Van Horn and that the magnetic basement is deeply buried. Mountains into Jeff Davis and Presidio counties. 14 Geologic Quadrangle Map— No. 40

No distinctive magnetic anomaly is associated with Rim Rock Fault the linear gravity anomaly, but 1to 3 miles to the north, a narrow, sinuous magnetic maximum, with amplitude exceeding 1,000 gammas, trends east- The Rim Rock fault crops out from the north southeast across northern Eagle Flat between the end of the Van HornMountains at the south edge Streeruwitz Hills and the CarrizoMountains. South of the mappedarea for 78miles southto theChinati f thelinear gravity anomaly, onemagnetic anomaly Mountains, 28 miles north of Presidio, Texas L is an amplitudelarger than250gammas; elsewhere (fig. 3). The fault strikes southto south-southeast in Eagle Flat, magnetic anomaly amplitude ranges and is downthrown to the west throughout its from zero (undetected) to about 100 gammas. length. Its stratigraphic separation exceeds 3,000

Fig. 3. RimRock fault inTrans-PecosTexas. Gravity,Magnetic,and Generalized Geologic Map, VanHorn— Sierra BlancaRegion 15

feet in the northern Van Horn Mountains and along the fault at Mica Mine, between Scott's possibly exceeds 4,000 feet (Twiss,1959a, p.110). Crossing and the reconnaissance gravity profile. The RimRock fault is the structural boundarybe- Estimates of displacement alongthe segment of the tween the Van HornMountains and the northpart faultbetweenMica Mineand the southernVanHorn of Green River Bolson and the south part of Mountains have not beenreported. Eagle Flat. The fault is not exposed in the Because no gravity measurements were made in alluvium-filled valleynorth of the Van HornMoun- the 18 miles between the aforementioned profile tains, but to the northwest, the high-standing es- and the south edge of the map, it is possible that carpment that forms the boundary between Eagle the gravity anomaly adjacent to the northern Flat and the Carrizo Mountains is indicative of Van Horn Mountains is not the same as that en- major faulting. Precambrian metamorphic and countered farther south at the trace of the exposed Permiansedimentaryrocks of the CarrizoMountains Rim Rock fault. It is probable, however, that the are abruptly truncated along this escarpment. two anomalies are one because in both places The aforementioned linear gravity anomaly that (1) an abrupt 14- to 16-mgal westward decrease of closely parallels the southwest side of the Carrizo Bouguer gravity anomaly occurs as astep-anomaly; Mountains turns southward near Scott's Crossing (2) the maximumgradient of gravityinboth places (fig.1)and closelyfollows the traceof the RimRock is high, 10 to 13 mgal per mile; (3) the maximum fault along the west side of the Van Horn Moun- andminimum Bougueranomaly values arethe same, tains. Gravity profiles,drawnapproximatelynormal -134 mgal and -149 mgal; and (4) the anomalies to the trend of the anomaly, exhibit the open trend south to south-southeast and are seemingly "S"-shaped, or step, curve that is characteristic of associated with the well-exposed Rim Rock fault. the gravity response to a high-angle fault (i.e., Therefore, because of marked geophysical simi- a large and abrupt horizontal change of rock larities and mapped geological continuity, the density). A reconnaissance gravity survey (outside RimRock fault seems the most probable cause of the mapped area) in the southern Van HornMoun- the linear gravity anomaly extending north and tains along the county road to Porvenir (Porvenir northwest from thenorthern VanHornMountains. mail route) crosses the outcrop of the Rim Rock Thus, the Rim Rock fault extends about 40 miles fault 20 miles south of Scott's Crossing. The fault northwest along the gravity anomaly from its last intersects this profile near the mid-point of a known exposure to the west end of the surveyed 14-mgal westward decrease in Bouguer gravity area north of Sierra Blanca, Texas. anomaly that takes place in about 2 miles. At the Interpretationof Profiles trace of thefault,the gradient of the gravity anoma- ly is 10.4 mgal per mile, the maximum along the The Rim Rock fault gravity anomaly trends profile. Because of structural complexity and N. 50° W. from Scott's Crossing 17 miles across sparse gravity control, a completely satisfactory Eagle Flat to the Eagle Flat section house on the quantitative interpretation of this profile was not Texas and Pacific Railway near control point 76. possible. The closest approximations suggested Gravity profiles A-A' and B-B' (fig. 4), each ap- density contrast between 0.4 and 0.5 gm/cm3 and proximately normal to the trend of the anomaly, minimum throw between 3,500 and 2,600 feet exhibit characteristics of the gravity response to (Wiley, 1970, fig. 11;p.119). The steep gradient high-angle faults similar to that already discussed. of the anomaly suggests that much of the anomaly Structural interpretation of these profiles was results from shallow density contrasts,perhaps less accomplished by the method of semi-infinite hori- than 6,000 feet deep. Twiss (1959b, cross-section zontal slabs. The results of these interpretations A-A') suggested displacement of about 4,000 feet are tabulated in table 3.

Table 3. Results ofinterpretationofprofiles A-A'andB-B'. Distance N. 50 W. — Direction Fault trace Average density Throw from Scott's Crossing Profile from station at station contrast (feet) station 360 A-A'-l N.40° W. 4 0.50 gm/cm3 4,300 8.4miles 1470

b-b' N.44° W. 290 0.44 gm/cm3 3,000 13.2 miles Fig. 4. Bouguer anomaly profiles A-A andB-B. See figure 1for approximatelocationof profiles. Gravity,Magnetic, and Generalized Geologic Map, VanHorn— SierraBlanca Region 17

About 20miles westof VanHornalong theTexas map (1965, pi. 1) near the Hillside fault between and Pacific Railway, near the Eagle Flat section Van Horn and Allamoore. These scattered ex- house, the trend of the Rim Rock fault gravity posures and lineations in the metarhyolite suggest anomaly changes from N. 50° W. to N. 70° W. The that transport wasnorthwardbut yieldnoindication anomaly maintains this new direction for about of the true geometry of the fault. Gravity and mag- 6 miles along the south side of the Streeruwitz netic anomalies associated with the Carrizo Moun- Hills, but several small anomalies, probably indi- tain Group and the thrust fault provide a basis for cating faults, split off from the principal anomaly some further speculations,however. about midway of this distance and trend N. 50° W. Although Precambrian magnetic metamorphic West of the Streeruwitz Hills, the Rim Rock rock is juxtaposed with nonmagnetic Cenozoic fault gravity anomaly resumes its N. 50° W. trend bolson fill along the Rim Rock fault on the south- but is not as well defined as farther east. Near the west side of the Carrizo Mountains, magnetic west end of the StreeruwitzHills,the gravity survey anomalies that can be clearly associated with the suggests southward downthrow of about 2,000 Rim Rock fault are small areally, discontinuous, feet (using density contrast of 0.40 gm/cm3). The andhaveamplitudesgenerallyless than100gammas. gradient of gravity west of the Streeruwitz Hills The magnetic anomaly parallel to theHillside fault suggeststhe continuity of afault or,moreprobably, is large, about 400 gammas, but the associated a fault zone 1 to 2 miles across. The fault zone gravity anomaly is small, 2 to 5 mgal, depending may have displacement as much as 3,500 feet. The on location. This suggests that near the RimRock reason for the branching and abrupt changes in fault, the metamorphic rocks in the upperplate of strike of the RimRockfault isnot known. Possibly, the thrust are either thinner or that their magnetic the mid-Tertiary Rim Rock fault in part follows susceptibility is lower than at the Hillside fault, the N. 70° W.trends established duringPrecambrian 6 miles to the north. By assuming, withpossible deformation (Kingand Flawn,1953). large error, a constant susceptibility contrast of From the foregoing geological and geophysical 0.0025 cg.s. units between magnetic rocks of the evidence, it is apparent that the Rim Rock fault upper plate of the thrust and sedimentary(?), changes strike direction at the north end of the nonmagnetic rocks of thelower plate,the maximum Van Horn Mountains and extends atleast 40 miles thickness of the upper plate is about 4,500 feet. across Eagle Flat and that it divides Eagle Flat into This maximum occurs near the Hillside fault, about a structurally higher north part and lower south at the location of Interstate Highway 10. The part. The Rim Rock fault was active in Oligocene minimum thickness is about 1,500 feet and occurs and Miocene time (DeFord and Bridges, 1959, near the RimRock fault. pp. 292-293), but because of its closeproximity to The susceptibility of the rockbeneath the thrust the west and southwest edges of the much older cannot be accurately estimated because the rock Diablo and Coahuila platforms, it must have a type is not known. However, at every known ex- considerable earlier history. posure of the Streeruwitz thrust fault,metarhyolite of the Carrizo Mountain Group overlies carbonate StreeruwitzThrust Fault rock of the Allamoore Formation along a thrust fault contact (King and Flawn, 1953, pis. 1, 2, 3). Carrizo Mountains -The metamorphic rocks of Both the Allamoore and HazelFormations are com- the CarrizoMountain Group exposedin theCarrizo posedmostly of sedimentaryrock with low suscepti- Mountains and elsewhere in and around Eagle Flat bility, and either could directlyunderlie the thrust were thrust northward over younger Precambrian fault from place to place withoutmaterially altering sedimentary rocks alongtheStreeruwitz thrust fault the interpretation of the magnetic anomalies. (fig. 3). The thrust surface is nowhere wellexposed Metamorphic rocks of the Carrizo Mountain but where seen is a poorly exposedand weathered Group exposedin the EagleMountains are identical contact between metarhyolite of the CarrizoMoun- to and on strike with some of the map units of that tain Group and mostly carbonate rock of the group in the Carrizo Mountains. The magnetic Allamoore Formation north and northwest of anomaly associated with the metamorphic rocks in Allamoore and in the Streeruwitz Hills(King,1953, the Eagle Mountains suggests that only a few pp. 102-103). A similar,if somewhat speculative, hundred feet of Carrizo Mountain Group rock is contact was described as an emergence of the thrust present. This suggests that the Carrizo Mountain by King and Flawn (1953) and is shown on King's Group is in the upper plate of a thrust in the Eagle 18 Geologic Quadrangle Map— No. 40

Mountains,probably the Streeruwitz thrust fault. indicates that the upper plate is not more than Thus, the minimum horizontal displacement along about a mile thick beneath Eagle Flat. the Streeruwitz thrust fault is about 12 miles, the distance between the Eagle Mountains and ex- Hillside Fault posures of thefault in theMillican Hills. Northern Eagle Flat-The largest and most con- Gravity and magnetic anomalies related to the tinuous magnetic anomaly in the mapped area is a Hillside fault (figs. 3, 4) rangefrom 2 to 5 mgal and sinuous, high-amplitude magnetic maximum that 200 to 400 gammas. The anomalies are discon- trends northwest across northern Eagle Flat be- tinuous, difficult to delineate, and probably origi- tween the Carrizo Mountains and the southern nate in part from the Streeruwitz thrust fault, Streeruwitz Hills. This anomaly, nowhere more along the 9-mile length of the known or inferred than a mile wide, has an average closure of about trace of the Hillside fault between Van Horn and 300 gammas and local maxima with 900 to 1,200 Allamoore. Anomalies that suggest possible ex- gammas closure. It seems to be associated with out- tensions of the Hillside fault can be recognized as crops of amphibolite intruded into the Carrizo far as 8 miles east of Van Horn but not west of MountainGroupin boththe CarrizoMountains and Allamoore. the Streeruwitz Hills and probably indicates conti- King (1965, p. 114) estimated more than 1,000 nuity of the amphibolite beneath a thin alluvial feet of northward downthrow along the Hillside cover. This also suggeststhat the regional strike of fault near the Gifford-Hill quarry, 3 miles east of the Carrizo Mountain Group changes from west- Allamoore (near control points 135 and 1123). southwest in the western Carrizo Mountains to The north end of gravity profile A-A' (fig. 4) is northwest beneath Eagle Flat. The abrupt right- normal to the N. 70° W. trend of the Hillside fault lateral (northward looking west) offset of the gravity anomaly about a mile east of Allamoore anomaly about 3 miles west of Allamoore is not (anomaly A-A'-2). The anomaly is not longenough understood but may be related to younger (Ceno- to completely satisfy the requirements for two- zoic) faulting, to tear-faults associated with the dimensional interpretation, but such an interpreta- Streeruwitz thrust fault, or to crumpling, folding, tionprovides a reasonable estimateof displacement. and imbrication in the upper plate of the thrust. Using a density contrast of 0.20 gm/cm3,interpre- TheEagle Flatanomaly closelyresembles that of tation of anomaly A-A'-2 indicates about 2,000feet a long, thin prism having a finite depth range. of northward downthrow. Assuming a susceptibility contrast between 0.002 An east-trending, 6-mgal gravity anomaly a few and 0.003 c.g.s. units, a two-dimensional body miles east of Van Horn (control points 597-602) whose upper end is 300 feet below the surface suggests a possible eastward continuation of the would have to be 2,000 to 3,000 feet wide and Hillside fault. The large magnetic anomaly associ- would have to have its base 4,000 to 6,000 feet ated with theHillside fault in theCarrizoMountains below the surface, depending on location,inorder is missing and there is no obvious continuationbe- to satisfy the observed anomaly. The depth to the tween the two gravity anomalies. There is no com- top of the prism is arbitrary but is based onprox- pelling geologic evidence for a continuation of the imity to outcrops of the Carrizo Mountain Group Hillside fault,and, alternatively,the eastern anoma- and meager water well data. The width of the ly could be interpreted as part of a larger closed prism cannot be more than about half the width of anomaly associated withlatePaleozoic east-plunging the anomaly and the range given is, therefore, a folds as suggested by King's paleogeological map maximum. The depth to the base of the prism is (1965, fig. 2). Thus, the Hillside fault has,as King subject to errors, perhaps as large as 25 percent. (1965, p. 115) remarked, ". .. been ascribed an This is because of inherent insensitivity of the inordinate role in the tectonics of the region by magnetic method to the base of any regular Moody and Hill (1956, pp. 1223-1224)...," who parallelepiped. Ignoringpossible effects of remnant interpreted it as a major element in their "Texas magnetism and a poor knowledge of susceptibility direction of wrench faulting." TheHillside fault is contrasts as possible sources of error,and assuming a part of a local zone of adjustment between that the anomaly originates entirely within the tectonically active blocks; it has no regional or upper plate of the thrust fault, this anomaly also continental significance. Gravity, Magnetic, andGeneralized GeologicMap, VanHorn— Sierra Blanca Region 19

The Texas Lineament

The existence of the Texas lineament was sug- Thus, the topographic corridor of EagleFlat,and gested by R. T. Hill in 1902. Ransome (1915), probably the topographic corridor of Hueco Bolson Baker (1927, 1935), and Hill (1928) discussed and as well, satisfies the definition of a lineament: amplified the conceptof a possible transcontinental alined structural or stratigraphic anomalies that zone of fracture between the Transverse Ranges of produce topographic lines. Moreover, this zone of California and Trans-Pecos Texas. structural and stratigraphic discontinuity is the In their discussions of the Texas lineament,Hill, Texaslineament as originally proposed. Theincon- Ransome, and Baker clearly were referring to a trovertible fact of the Texas lineament in Trans- linear anomaly between alined structural andstrati- Pecos Texas and the popular hypothesis of its graphic anomalies of regionalsignificance that were origin by wrenchfaulting along the Texas direction detectable on the ground and on the small-scale of Moody and Hill(1956) or alongany other direc- regionalgeologic andtopographic maps available to tion must not be confused. them. They variously called the Texas lineament Moody and Hill (1956) included the Texas linea- a line of faulting, a linear zone of faulting or ment in their world-wide system of wrench faults. fracturing, and a linear zone between contrasting They showed the Hillside fault as a short segment geologic provinces, but they never suggested or of a transcontinentalleft-lateral wrench fault of the implied strike-slip movement as the origin of the Texas direction. Muehlberger (1965), on the basis lineament. of offset Precambrian isochrons,postulated 150 to Albritton and Smith (1957), ina review of the 200miles of right-lateralstrike-slip movementalong subject, restated Baker's (1935) concept of the the Texas lineament and Hillside fault during late lineament as a broad band of faulting rather than a Paleozoic time. line and suggestedthat itmight bea broad eastward In the Van Horn— Sierra Blanca region, neither continuationof the thenrecently discoveredMurray geologic nor geophysical evidence supports wrench fracture zone. Their illustrations show a 60- to faulting. Two through-going fault trends are de- 90-mile-wide zone extending eastward from the monstrable, viz., the north-trending (exposed) part Transverse Ranges along the southern base of the of the Rim Rock fault and Salt Basin boundary Colorado Plateau to Trans-Pecos Texas as consti- faults and the northwest-trending segment of the tuting the lineament. In addition,in accord with Rim Rock fault. These are Cenozoic structural stratigraphic practice, they proposeda type locality features that are more or less superposed on parts for the lineament: "... the segment of about of the boundaries of the Diablo platform of 55 miles in length which runs along the corridor of Permian and Cretaceous time. There isno evidence Eagle Flat." This topographic corridor extends for Cenozoic strike-slip motion alongthe lineament from a pointabout 10miles southwest of VanHorn, and much evidence that such movement, if any, Texas, N. 60° W. through Sierra Blanca and for is much older. another 20 miles beyond toward El Paso. It King(1965) has shown that strike-slip movement separates two regions having strongly contrasting along the Hillside fault could not have occurred geologic history: the Diablo platform to the north after the beginning of Permian time. Moreover, and theChihuahua troughto the south. Permian reef complexesintheregion are not offset Along the north side of Eagle Flat, the Carrizo in the vicinity of the lineament or in the vicinity of Mountain Group, in the upper plate of the Streeru- the aforementioned fault trends. Thus, it is most witz thrust fault, was thrust northward over the improbable that major strike-slip displacement younger Allamoore and Hazel Formations, which occurred in West Texas after the beginning of were deformed and tectonically mixed duringLate Permian time. If the lineament has an old history Precambrian deformation. The Diablo platform of wrench faulting, the evidence necessary to was defined along this old boundary late in Paleo- demonstrate itmust be inrocks older thanPermian. zoic time and again late in Mesozoic time. The Much of thisrock is missinginTrans-Pecos Texas, southern borders of Eagle Flat and Hueco Bolson either because of nondeposition or erosion, and mark approximately the maximum northward ex- much of that remaining is covered. Lucia (1968), tent of a Permian or older seaway (L6pez-Ramos, working with this meager data, was able to map (1969) and of thick Cretaceous sedimentation in marine, shoreline, and tidal flat facies in Lower the Chihuahua trough. Ordovician rocks in New Mexico and Trans-Pecos 20 Geologic QuadrangleMap— No. 40

Texas. The facies boundaries trend north to ries. northwest. They define a 50- to 80-mile-wide Thequestion of the origin of theTexaslineament shoreline facies that is not seemingly offset where is stillan enigma. Nevertheless,the several more or it crosses the Texas lineament. If the origin of the less superposed tectonic disturbances in the lineament is related to strike-slip faulting, such region, ranging from Precambrian deformation and movement must have occurred inCambrian orPre- metamorphism toCeno zoicblock faulting,not only cambrian time. Denison and Hetherington (1969) suggesta fundamental crustaldiscontinuity but also did not suggest Precambrian wrench faulting in tend toobscure evidenceof itsoriginwith structural their synthesis of Precambrian terranes of the complexity. With thepresently available data,itis region which was based on isotopic age determina- possible to state only that this lineament is a broad tions andpetrographic examinationof core samples. zonethat trends about N. 60° W. across Trans-Pecos They did not specifically exclude thepossibility of Texas, that the Van Horn— Sierra Blanca regionlies wrench faulting, and it is not difficult to imagine athwart this zone, and that the length and remarka- wrench faults along some of their terrane bounda- ble straightnessof the zone suggest wrenchfaulting.

Conclusion

Gravity data show that the Rim Rock fault The Texas lineament is a broad zone of tectonic follows the southern and western boundaries of the discontinuity that may reflect a zone of funda- Paleozoic andMesozoic Diablo platform and that it mental crustal weakness. Geological and geo- forms oneboundary of EagleFlat. Gravity andmag- physical evidence suggests that the lineament may netic data show that the Hillside fault does not have formed in Late Precambrian time. This evi- extend much east of Van Horn nor west of Alla- dence does not require that the lineament be moore. East of Van Horn,gravity anomalies trend associatedwith orindicate wrench faulting of trans- in a northerly direction, and west of Allamoore continental significance, but neither does it ex- they merge with or trend parallelto theRimRock clusively deny such anhypothesis. The distribution fault anomaly. Magnetic and geologic data suggest of Paleozoic and Mesozoic rocks in the region is that the maximum northward extent of Late Pre- incompatible with major wrench faulting since cambrian deformation is in or slightly north of Late Cambrian or Ordovician time. Later, pre- Eagle Flat. Thus,the Texaslineament observed by dominantly vertical movement alongthe lineament Hill (1901, 1928), Ransome (1915), and Baker helped to define the boundaries of the LatePaleo- (1927, 1935) is a Cenozoic landform superposed zoic and Late Mesozoic basins and platforms of on tectonic features dating from at least Late Pre- Trans-Pecos Texas andnorthern Chihuahua. cambrian to at least mid-Tertiary. Gravity, Magnetic, and Generalized GeologicMap, VanHorn— SierraBlanca Region 21 References

Albritton, D., Jr., D. and Smith, J. F., Jr. (1957) The printed as Univ. Texas, Geology Texas Bur. Econ. Rept.Inv lineament: Internat. Geol. Cong., 20th, Mexico No. 9. City, 1956, sec. 5, Relaciones entre la tectonica y la (1953) Carrizo Mountains, in King,P. 8., and sedimentacion,pp. 501-518 [1961], Flawn,P.T., Geology and mineral deposits of Pre- and (1965) Geology of theSierraBlanca cambrian rocks of the Van area, area, Hudspeth Horn Texas: Univ. County, Texas: U. S. Geol. Survey Texas Pub. 5301,pp. 51-69. Prof.Paper479, pp. 131 , Goldstein, August, Jr., King, R, AMSBURY, (1957) P. and D.L Geology of thePinto Canyon area, Weaver, C E.(1961) The Ouachita System: Presidio Univ. County,Trans-PecosTexas: Univ. Texas,Austin, TexasPub. 6120, 401pp. Ph.D.dissertation,203 pp. (unpublished). Grant,F.SL, and West,GF. (1965)Interpretationtheory Atwill,ER., IV (1960) Stratigraphic nomenclaturein in applied geophysics: McGraw-Hill Book Co., New Sierra Pilares, Chihuahua,Mexico: Univ. Texas,Austin, York, xvii,583 pp. M.A. thesis, 90 pp.(unpublished). Gries, J.C. (1970) Geology of Sierra de la Parra area, Baker,C. L. (1927) Exploratory geology of a part of northeastern Chihuahua, Mexico: Univ. Texas,Austin, southwesternTrans-PecosTexas: Univ.TexasBulL 2745 Ph.D. dissertation, xiii, 151 pp. (unpublished). 70pp. ,andHAENGGL W. T. (1970) Structural evolution (1935) Major structural features of Trans-Pecos of the eastern Chihuahua tectonic belt (abst.), in Texas, in The geology of Texas, Vol. 11, Structural and Seewald,Ken, and Sundeen,DAN(eds.), The geo- economicgeology: Univ.TexasBull.3401 (Jan.1, 1934), logic frameworkof the Chihuahua tectonicbelt, A sym- pp. 223-402. posium: West TexasGeol.Soc, Midland,November4-6 Barnes, V.E (ProjectDirector) (1968) VanHorn— ElPaso 1970, pp. 45-49. Sheet (with text): Univ. Texas, Bur. Econ. Geology Haenggi, W. T. (1966) Geology of El Cuervo area, north- Geol.Atlas of Texas. eastern Chihuahua,Mexico: Univ. 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(1958a) Cretaceous platform and geosyn- vicinity, Culberson and Jeff Davis counties, Texas: cline, Culberson and Hudspeth counties, Trans-Pecos Univ. Texas, Bur. Econ. Geology Geol. Quad. Map Texas: Soc. Econ.Paleont.Mineral.,PermianBasinSec, No. 21(withtext). Guidebook 1958 Field Trip, April 10-12, 1958, Van Hill, RT. (1902) The geographic and geologic features, Horn,Texas: 90 pp. and their relationshiptothe mineralproductsofMexico: (1958b)Tertiaryformationsof RimRock Country, Trans. Amer.Inst.Mm.Engrs., voL 32, pp. 163-178. Presidio County, Trans-Pecos Texas: Texas Jour. Sci., (1928) Transcontinental structural digression vol.10, pp. 1-37. Reprinted as Univ. Texas,Bur. Econ. (abst.): Bull.Geol.Soc. America,voL 39, p. 265. GeologyRept.Inv.No. 36. Hubbert,M.K. (1948) A line-integralmethodof computing (1969) Some keys to the geology of northern the gravimetric effects of two-dimensionalmasses: Geo- Chihuahua: New Mexico Geol. Soc, Field Trip Guide- physics, vol.13, pp. 215-225. book,20th, pp. 61-65. Jones, B.R», andReaser,D.F. (1970) Geology ofsouthern ,and Bridges, L. W. (1959) Tarantula Gravel, Quitman Mountains, Hudspeth County, Texas: Univ. northern RimRock Country, Trans-Pecos Texas: Texas Texas, Bur. Econ. Geology GeoL Quad. Map No. 39 Jour. Sci., vol. 11, pp. 286-295. (with text). Denison, R.E., and Hetherington,E.A.,Jr. (1969) Base- King, P.B. (1949) Regionalgeologicmap ofpartsofCulber- ment rocks in far West Texas and south-central New son and Hudspeth counties, Texas: U. S. GeoL Survey Mexico,in Kottlowski,F.E., andLeMone,D. V. (eds.), Oil andGasInv.Prelim.Map90. Border stratigraphy symposium: NewMexico State Bur. (1965) Geology of the Sierra Diablo region, Mines and Mm.Res., Socorro,Circ. 104,pp. 1-16. Texas: U. S. Geol.SurveyProf. Paper480, 185 pp. ,Kenny, G S., Burke, W. H, Jr., and Hether- ,andFlawn,P.T. (1953) Geology and mineral ington,EA, Jr. (1969) Isotopic age of igneous and deposits of Precambrian rocks of the Van Horn area, metamorphic boulders from the Haymond Formation Texas: Univ. TexasPub. 5301, 218 pp. (Pennsylvanian), Marathon Basin, Texas, and their sig- Lindsley, D. H, Andreasen, G. E., and Balsley, J. R nificance: Bull.Geol.Soc.America, vol.80, pp. 245-256. (1966) Magnetic properties of rocks and minerals, m DOBRIN,MB. (1960) Introductionto geophysicalprospect- Clark,S.P., Jr. (cd.), Handbook of physical constants ing: McGraw-HillBook Co., New York, ix,446 pp. (revised cd., sec 25): Geol. Soc. America Mem. 97, Flawn,P. T. (1951) Pegmatites of the Van Horn Moun- pp. 543-552. tains, Texas: Econ. Geol., vol. 46, pp. 163-192. Re- LSpez-Ramos,Ernesto (1969) Marine Paleozoicrocksof 22 Geologic QuadrangleMap— No. 40

Mexico: BulL Amer. Assoc. Petrol. Geol., vol. 53, Trans-Pecos Texas: Univ. Texas, Austin, Ph.D. disserta- pp. 2399-2417. tion, 234 pp. (unpublished). Lucia,F. J.(1968) Sedimentation and paleogeography of (1959b) Geology of Van Horn Mountains, Texas: the El Paso Group, in Delaware Basin exploration: Univ. Texas, Bur. Econ. Geology GeoL Quad. MapNo. West Texas Geol. Soc. Field Trip Guidebook 68-5sa, 23 (with text). pp. 61-75. Underwood,J.R,Jr. (1962) Geology of Eagle Mountains McGowen,J. H., and GROAT, GG. (1971) Van Horn Sand- and vicinity, Trans-Pecos Texas: Univ. Texas, Austin, stone, West Texas: An alluvial fan model for mineral Ph.D. dissertation, 559 pp. (unpublished). exploration: Univ. Texas, Bur. Econ. Geology Rept. (1963) Geology of Eagle Mountains and vicinity, Inv.No. 72, 57pp. Hudspeth County, Texas: Univ. Texas, Bur. Econ. Moody,J.D.,and Hill,MF. (1956) Wrench fault tec- GeologyGeoL Quad. MapNo.24 (withtext). tonics: Bull. Geol. Soc. America, voL 67, pp. 1207- Wiley, MA. (1970) Correlation of geology with gravity -1246. and magnetic anomalies,Van Horn— SierraBlancaregion, MUEHLBERGER, W.R (1965) Late Paleozoic movement Trans-Pecos Texas: Univ. Texas, Austin, Ph.D. disserta- along the Texas lineament: Trans.New York Acad. Sci., tion,xix,331pp. (unpublished). ser. 11, vol. 27, no. 4, pp. 385-392. Wilson, J.A (1965) Cenozoic history of the Big Bend NAGATA, Takesi (1961) Rock magnetism (revised cd.): area,West Texas,in Geology of the BigBendarea,Texas: MaruzenCo., Ltd., Tokyo, 350 pp. West Texas Geol. Soc. Field Trip Guidebook 65-51, Nettleton,LL (1940) Geophysical prospecting for oil: Midland,pp. 34-36. McGraw-HillBook Co., New York, 444pp. , Twiss, P. C, DeFord, R X, and Clabaugh, (1942) Gravity and magnetic calculations: Geo- S.E. (1968) Stratigraphic succession, potassium-argon physics, vol. 7, pp. 293-310. dates, and vertebrate faunas, Vieja Group, Rim Rock Powell,J.D. (1965) LateCretaceousplatform-basinfacies, Country, Trans-Pecos Texas: Amer.Jour. ScL, vol.266, northern Mexico and adjacent Texas: Bull. Amer. pp.590-604. Assoc.Petrol. Geol., vol.49, pp. 511-525. Wood, J.W. (1968) Geology of ApacheMountains, Trans- Ransome,F.L. (1915) The Tertiary orogeny in the North Pecos Texas: Univ. Texas,Bur. Econ. Geology GeoL American Cordillera and its problems, in Rice, W. N., Quad. MapNo. 35 (with text). et aL, Problems of American geology: Yale Univ. Woollard,G.P.,and Joesting,HR (compilers) (1964) Press,New Haven,pp. 287-376. Bouguer gravity anomaly map of the United States: Twiss,P.G (1959a) Geology of Van Horn Mountains, AmericanGeophys. Union,Washington, D.C. Gravity, Magnetic, and Generalized GeologicMap, VanHorn— SierraBlanca Region 23

Appendix: Gravity and Magnetic Survey

Control Points and Bases Magneticbases were set inthesamemanner as the gravity bases. Magnetic bases A, C,F,H, J, N,and The survey consists of 1,545 gravity and 1,683 P andsupplementary magneticbases1045 and1466 magnetic control points ("stations") plus21gravity are not adjacent to gravity bases. There is no and 20 magnetic base, or reference,points. Inmost gravity base 1045 or 1466 and there is no magnetic of the area, gravity andmagneticpointsare adjacent base AA. and have the same identifying number. It was Inaddition toestablished(markedandnumbered) necessary to move some magnetic stations away gravity stations, magnetometerreadings weremade from the correspondinggravity location inorder to at 142 locations to obtain better detail in areas of avoid local magnetic interference; where practical, steep magnetic gradient. Most of these are not these points are shown on the map and are labeled shown on the map but were honored incontouring with "M" as a prefix, e.g., magnetic stationM872 the data. Where shown, these are indicated by the corresponds to gravity station 872 but is about number of anearby establishedstationand a suffix, 1,100 feet to the south. Base stations are desig- e.g., 1466G. nated by capital letters, although some were first Two Sharpe Instruments vertical component established as routine observation stations, givena fluxgate magnetometers were used in the survey. number, then resurveyed as a base, and given dual Six bases and 139 stations were also read with a letter and number nomenclature. For example, Ruska vertical component (Schmidt balance type) base Lis also station 478. Stations were numbered magnetometer. About 150 stations and threebases consecutively as established or occupied by the were read with Sharpe magnetometer number gravimeter. It was not possible to keep station 510167 in August 1966. All bases were reset and numbers insequenceby areas. more than 95percent of the stations were read with When the survey was begun in June 1966, Sharpenumber 704281in1968. The Sharpemagne- 13 gravity base stations were set and tied together tometers read directly in gammas and can usually by the base-loop method described by Nettleton be read to ±2 gammas on the most sensitive scale if (1940, pp. 38-42). After a base-loop is closed, magnetic intensity changes at rates less than about the differences in values around the loop are made 500 gammas per mile and wind velocity is below to add to zero by small adjustments in each legof 15 miles per hour. the loop. Eventually, 21 gravity bases were set. Of these, bases X, AA, and AC were not tied into a Elevation and Locations loop and adjusted because of low meter drift rates and short distances between bases. Additionally, The geographic position of 1,732 control points gravity bases P, AB,804,and 1017 were not looped was established byreference to landmarks on topo- because they were intended as secondary reference graphic maps,by automobile odometer alongroads points in lower accuracy reconnaissance lines and or trails,by plane-table surveying, and by pace-and- because of excessive travel time required to obtain compass traverse. The elevation of 1,573 of these a loop. was established by benchmarks,plane-tablesurvey, Four gravimeters were used in the survey: reference to large-scale topographic maps, and Worden Prospector model gravimeters number 64 aneroid altimeter. and 152; LaCoste and Romberg number 96; and The location and elevation of 1,417 stations LaCoste and Romberg geodetic meter G-135. It were determined from preliminary, field-checked would have been desirable to complete the entire editions of 16 new U. S. Geological Survey survey with one gravimeter,but prior commitments 1:24,000-scale topographic maps. Thirteen of these for the various instruments made this impossible. maps werecontouredat 10-foot or smaller intervals, The calibration of Worden-64, stated by the manu- two were contoured at a 20-foot interval,and one facturer as 0.08232 mgalper dial division,was used was at a 40-foot interval The field-established as a standard throughout the survey. All other elevation of many landmarks,such as fence corners, instruments were recalibrated in the field to tie to road junctions, and arroyo crossings, is shown on Worden-64 by re-running base-loop B-C-E-D-B in these maps to the nearest foot. Stations were eastern Eagle Flat. Thelargest difference in calibra- established on 529 of these "usefulelevations" and tion constants determined was -0.00042 mgal per on 172 benchmarks. The elevation of 94 stations dial division for Worden-152. was established by plane table and alidade. The 24 Geologic Quadrangle Map— No. 40 location of 53 of these was also established by Data Adjustment and Reduction plane table, but the remaining 41 were located by reference to landmarks on l:48,000-scale plani- Drift Corrections American Paulin System metric geologic maps. An As used here, drift is the difference between altimeter, calibrated in 2-foot intervals, surveying readings at bases at the beginningand at the end of elevation of 23 stations was used to establish the two-hour or shorter intervals of observing gravity established by reference to whose location was or magnetic values. The largest part of gravimeter 1:63,360 planimetric maps. The landmarks on drift is instrumental and isinduced by temperature remaining 760 stations were located by reference changes, slack in the reading dial gear-train, and odometer; their to landmarks and by automobile nonlinearity of theinstrument (whennot otherwise established reference contours elevations were by to accounted for). The gravity effect of earth-tides is on the large-scale topographic maps. Elevations small in atwo-hour intervalandis includedasinstru- were not determined at 159 stations that were mental drift. Magnetometer drift is caused mostly occupied only by the magnetometer. Table 4 by diurnal variations in magnetic intensity, but shows probable accuracy of elevation control and some instrumental drift is induced by sudden and the probable effect on the gravity survey. large changes inambient temperature and by vibra- tion during transport. Drift was corrected on a Field Procedure linear time scale between observations at bases. For example, for gravity, 0.01 mgal per minute Field procedure consisted of first reading the was added to the observed readings to correct for gravimeter or magnetometer at a convenient base, a1.00mgal downward drift in100minutes. Magne- reading at several stations,and finally returning to tometer drift was corrected in the same manner. a base. Exceptinrare instances when it was wholly impractical, theinstruments werereturned to a base Reductionof Gravity Data everytwohours. Bysodoing,earth tidesand instru- mental drift can be treated together and adjusted Adjusted observed gravity data were reduced to out of the gravity data. This procedure also mini- Bouguer gravity anomaly by four adjustments or mizes the possibility of reading the magnetometer corrections: (1) free air correction; (2) Bouguer during a reversal in diurnal drift. As anadditional correction; (3) terrain correction; and (4) adjust- check for abnormal drift, about 20 percent of all ment for theoretical gravity. The free-air and stations were re-read to check drift and repeata- Bouguer corrections were combined into a single bility. Because most of the survey was conducted correction because both are dependent on altitude by automobile or jeep alongroadsandtrails,10 to above (or beneath) an arbitrary datum, usually 20 new stations and 2 to 4 check stations were mean sea-level. In this survey, the combined occupiedinatwo-hour traverse between bases. correction was 0.06 mgal per foot altitude above

Table 4. Probableaccuracy of elevation control

Typeof Probable maximum Probable effeci elevation control Maximum error erroringravity survey ongravitydata (feet) (feet) (mgal) Topographic maps— 40 ft.C.I. ±20 ±10 ±0.60 20 ft.C.I. ±10 ± 7.5 ±0.45 10 ft.C.I. ± 5 ± 4 ±0.24 5 ft.C.I. ± 2.5 ± 2 ±0.12

Benchmarks ± 0.1 ± 0.1 ±0.0006

'Usefulelevations" ± 1.0 ± 1.0 ±0.06

Plane-tablesurveys ± 1.0 ± 1.0 ±0.06

Altimeter ± 5.0(?) ± 5.0(?) ±0.30(?) Gravity, Magnetic, and Generalized Geologic Map, VanHorn— Sierra BlancaRegion 25 mean sea-leveL This corresponds to an average Reductionof MagneticData density of surficial rock, the "Bouguer density," of 2.67 gm/cm3. Although this density assignment Observed vertical magnetic intensity data are is open to some question, many noncommercial adjusted for drift in much the same way as surveys inthe Western Hemisphere arereduced with described for gravity data. Driftrates ranged from this value. By using this possibly high value for zero to more than 100gammas per hour. When the Bouguer reduction, this present survey may be drift rateexceeded 30gammas per hour,all affected tied to and compared with the extant government stations were re-run unless corrections could be work intheregion. made using check stations. Theoretical gravity accounts for the increase Regional magnetic intensity changes from place of gravity from equator to pole. One method of to place in broad trends. Dccl and Howe (1948) making corrections for theoretical gravity is to published charts showing the regional variation of contour the northward increase of gravity on the vertical intensity for 1945. This variation is sus- base map on which the control points are plotted. ceptible to contouring and treatment in the same It is then a simple and satisfactorily precise pro- manner as theoretical gravity. Various procedures cedure to interpolate between the contours and to for determining the regional gradient of magnetic determine the correctionfor eachstation. Nettleton intensity havebeenused by other workers inrecent (1940, pp. 51-53) described the method and years, for example, the departure of the observed (pp. 137-143) provided tables of theoretical gravity magnetic intensity "surface" from a fitted "nth" from which these contours maybe plotted. order mathematical surface. This procedure is TheBouguer and free-air corrections bothassume useful in some surveys, but the data of thispresent that the ground surface at an observation point is survey donot warrant suchsophisticated treatment. flat. If, on a 1,300-foot radius surrounding the In this survey, the interpolated value from the station, the average surface elevation differs more chartof Dccl and Howe(1948) was subtracted from than about 100 feet from the station elevation, the adjusted observed magnetic intensity at each terrain correction is necessary. Approximately station in order to produce a map value above or 70 percent of the gravity stations in this survey below an arbitrary datum. Station elevation has no were corrected for terrain irregularity through significant effect on ground magnetometer data, zone J (4.1miles = 6.6 km) by the chartand overlay and terrain effects, if any, can be neglected(Grant methods of Hammer (1939). This was a slow and and West, 1965, pp. 306-308). tedious process but quite necessary and worth- while because some corrections amounted to more Precisionand Accuracy than 2.0 mgal and many were in the range 0.5 to Gravity and readings 1.0 mgal. The first four zones (zones B-E) were magnetometer have the range of precision repeatable. completed in the field when the station was to which they are occupied. The elevation differences of the outer Duringthe courseof the survey,248 (16 percent)of the 1,556 nonbase gravity stations routinely zones were determined from the large-scale topo- were re-read check the graphic maps or from enlargements of the small- as stations. In magnetometer 392 (23 percent) of 1,703 nonbase scale topographic maps. survey, the stations were re-read as check stations, and an ThefinalBouguergravityanomaly,gz,isfoundby + + additional 158 stations (9.3 percent) were re-run gz=go E-gt T because of excessive drift or for other reasons. where go is adjustedobservedgravity,Eis the eleva- The average repeatability of the gravity survey is tion correction (combined Bouguer and free-air within ±0.04 mgal, close to the 0.01mgalresolving corrections),g^.is theoretical gravity,andTis terrain power of the Worden gravimeter (on a givenscale). correction. Note that T is always positive (see Similar results were obtained with the LaCoste and Nettleton, 1940, pp. 56-57) and gt is always Romberggravimeters. The extreme range of gravity subtracted. repeatability is +0.16 to -0.13 mgal, and only 23 The survey was tied to 15 benchmark-gravity stations (9.3 percent of those repeated or 1.5 per- stations of the U.S.Coast and Geodetic Survey and cent of all stations) failed to repeat within ±0.06 Air Force Chart andInformation Center. The final mgal. tie was accomplished by linear adjustment of the The repeatability of the magnetometer survey is entire survey to obtain a best fit of the 15 prime about 11.5 gammas but the range and spread are stations. large, ranging from +35 to -37 gammas. About 26 Geologic Quadrangle Map— No. 40

20 percent of the stations repeatedfailed to repeat 2.67 gm/cm3 is necessary inorder to tie thesurvey by +20 gammas. The repeatability is about to earlier work by federal agencies and others. 2.6 percent of the 300 scale, the magnetometer Using ±5.0 feet for elevation precision, 1.14 mgal scale most frequentlyused. per mile for the average south-to-northgradient of The accuracy of the survey describes how truth- gravity (Nettleton,1940), and readingprecision of fully it represents the actual distribution of the ±0.04 mgal, the gravity survey is accurate to about gravity and magnetic anomalies. Thisis inherently ±0.40 mgal excluding terrain corrections. The difficult to testand evaluate but, empirically, if the accuracy of terrain corrections is difficult, if not data are contourable and if the contours seem impossible, to assess. Considering the amount of reasonableinview of thesurface geology, the survey data that is incorporated into them, the terrain must be assumed tobe reasonablyaccurate. Assum- corrections probably are not inerror by more than ing the aforementioned precision of reading, the 15percent;the average error probably is about 5 to accuracy of the survey dependson the precision of 10percent. geographic location, choice of regional gradient, The accuracy of the magnetometer survey de- if any, and faithfulness of the contours to the data. pends on the precision of reading, horizontal The accuracy of the gravity survey also depends on positioning, and contouring of the resulting map the accuracy of the elevations, terrain corrections, values. Assumingthat the error of location does not and the choice of the Bouguer density. exceed ±200 feet,the accuracy of the uncontoured The precision of horizontal positioning is about data is dependent solely on the readingprecision ±200 feet throughout the survey. Elevations were (as stated earlier, about ±11.5 gammas, or about determined with an average precision of ±5.0 feet ±2 percent of the maximumrange of the survey). or less. The choice of Bouguer density equal to