BEDROCK GEOLOGIC MAP OF THE EL PASO MOUNTAINS IN THE .
U.S. DEPARTMENT OF THE INTERIORTO ACCOMPANY MAP 1-2389U.S. GEOLOGICAL SURVEYBEDROCK GEOLOGIC MAP OF THE EL PASO MOUNTAINS IN THE GARLOCK ANDEL PASO PEAKS 7-1/2' QUADRANGLES, KERN COUNTY, CALIFORNIABy Michael D. Carr, Robert L. Christiansen, Forrest G. Poole,and John W. Goodge1973; Carr and others, 1984) quartz diorite to quartzmonzodiorite pluton, also unaffected by any ductiledeformation, intrudes the metasedimentary and metavolcanic rocks in the easternmost El Paso Mountains.The stratigraphy and structure of the Paleozoicmetasedimentary and metavolcanic marine strata in theeastern part of the El Paso Mountains are the focusof this study. Hess (1909) and Hulin (1925) brieflydescribed these rocks, and Hess (1909, p. 30) reportedtwo poorly preserved fossils from the eastern part ofthe metamorphic rock assemblage as probably no youngerthan Carboniferous in age. Dibblee (1952, 1967)mapped the central part of the El Paso Mountains ata scale of 1:62,500 and formally divided the metasedimentary and metavolcanic rocks into the nowabandoned Mesquite Schist and the now-abandonedGarlock Formation (see Carr and others, 1984). Dibblee(1952) subdivided the Garlock Series (later renamedthe Garlock Formation by Dibblee, 196 7) into twentytwo members and reported Permian fusulinids near themiddle part (his Member 12) of the formation.Christiansen (1961) mapped the El Paso Mountainsfrom Red Rock Canyon eastward to the longitude ofthe village of Garlock at a scale of 1:24,000; his mappingof the area now covered by the Garlock 7 .5-minutequadrangle forms a partial basis for this compilation.Poole and others (1977, 1980) and Poole andChristiansen (1980) reported the results of additionalfossil discoveries from the western (structurally lower)part of the Garlock Formation, demonstrating thepresence of Ordovician and Devonian rocks in theformation. Poole (197 4) also tentatively correlated rocksnear the middle part of the Garlock Formation withMississippian rocks of the Antler foreland basin ofNevada. Building on this previous work, the remainder of the present compilation is based on 1:24,000scale geologic mapping and stratigraphic studies donebetween 1979 and 1982. The preliminary findingsof this study were presented by Carr and others (1980,1984) and include the results of additional fossil discoveries throughout much of the Garlock Formation.The metasedimentary and metavolcanic marine strataof the El Paso Mountains represent nearly every systemof the Paleozoic Era. There is no direct fossil evidencefor the Silurian, but Silurian to Early Devonian fossilshave been recovered from generally correlative rocksequences exposed in Pilot Knob Valley, about 50 kmeast of the El Paso Mountains, and some Silurian rockscould be present in the El Paso Mountains as well {Carrand others, 1992).GEOLOGIC SETTINGThe El Paso Mountains are a range of mountain peakstrending east-northeast along the north side of theGarlock Fault, approximately 25 km southwest of thecity of Ridgecrest in eastern Kern County, California.The south flank of the range is structurally controlledby the left-lateral Garlock Fault and descends abruptly.into Fremont Valley, an alluvial basin with a closeddrainage system that terminates in Koehn Lake playa(see fig. 1). Numerous surficial features that indicateyouthful activity along the left-lateral Garlock Fault arepresent along the south edge of the mountain range(Clark, 1973). The north flank of the range slopesmore gently and is bordered by a tableland terrainunderlain by gently dipping Tertiary sedimentary andvolcanic strata (Dibblee, 1952; Cox and Diggles, 1986).Numerous narrow canyons dissect these tablelands. Mostof these canyons drain northward and westward intoIndian Wells Valley, but several prominent drainagescut southward through deeply incised canyons that crossthe axis of the El Paso Mountains, exposing the preTertiary metamorphic and plutonic rocks that form thecore of the range. The west end of the El Paso Mountainsis separated from the south end of the Sierra Nevadaby low hills west of Red Rock Canyon; eastward theEl Paso Mountains merge into a hilly terrain underlain by Mesozoic granitic rocks.Most of the west half of the El Paso Mountains (westof Mesquite Canyon) is underlain by an intrusive suiteof Late Permian and Early Triassic age (table 1; appendix 1; Cox and Morton, 1980; Carr and others,1984) ranging in composition from gabbro to granite(Dibblee, 1952; Christiansen, 1961). [Note: Geologicages in this report are based on the time scale of Palmer,1983; ages cited from the literature have been converted as necessary to conform to decay constants recommended by Steiger and Jager, 1977.] Immediatelywest of Mesquite Canyon, the Late Permian gneiss ofWeiss Mountain and numerous smaller bodies of foliated felsic and mafic Late Permian intrusive rocks intrudethe diverse assemblage of Paleozoic metasedimentaryand metavolcanic marine strata that underlies most ofthe east half of the El Paso Mountains. The Permianintrusive rocks are gneissic, having a penetrative foliation that is coplanar with the foliation in the metamorphic rocks to the east. The Triassic plutons thatintrude the gneiss of Weiss Mountain on the west areundeformed; their intrusion postdates the ductiledeformation that affected the older rocks (Christiansen,1961). A Late Jurassic (table 2; Armstrong and Suppe,1
117"45'NavalAir lANATIONSurficial deposits (Quaternary)Sedimentary and volcanic deposits (fertiary)El Paso MountainsGranitoid pluton (Jurassic)Granitoid pluton \friassic and Permian)Bond Buyer sequenceGarlock assemblageFigure 1 .Sierra NevadaSierra Nevada batholithMetasedimentary rocksRand MountainsGranitoid plutonRand SchistJohannesburg Gneiss of Hulin (1925)33'ContactFault-Dashed where inferred or approximatelylocated; dotted where concealedGeneralized geologic map of El Paso Mountains and surrounding area .Jennings and others (1962) and Carr and others (1984) .2AREA OF MAPModified from
Two distinct sequences of Upper Cambrian and Ordovician rocks are present in the El Paso Mountains section.Upper Cambrian and Ordovician strata in the Garlock7. 5-minute quadrangle (metasedimentary rocks of Colorado Camp) are mostly meta-argillite and argilliticmetachert belonging to an outer continental-margin facies.Upper Cambrian and Ordovician rocks in the El PasoPeaks 7 .5-minute quadrangle (metasedimentary rocksof El Paso Peaks) are lithologically more diverse, includingmeta-argillite, marble (detrital metalimestone), graptoliticslate, and orthoquartzite. These rocks represent faciestransitional between the outer and inner continentalmargin.The Devonian rocks in the Garlock and parts of theEl Paso Peaks quadrangles (metasedimentary rocks ofGerbracht Camp) also represent outer continental-marginfacies. The metasedimentary rocks of Gerbracht Campcomprise a sequence of metachert, metatuff, sandy marble(detrital metalimestone), and meta-argillite with finegrained quartzite (distal turbidites). Greenstone thatwe tentatively assign to the Devonian and infer to liestratigraphically below the metasedimentary rocks ofGerbracht Camp is present, as well, in both the Garlockand El Paso Peaks quadrangles. Calcsilicate hornfelsand subordinate marble mapped as the upper member of the metasedimentary rocks of El Paso Peaks couldalso be, in part, Silurian(?) and Devonian(?) judging fromtheir apparent stratigraphic position. These rocks, likethe lower members of the metasedimentary rocks ofEl Paso Peaks, probably represent a facies transitionalbetween the outer and inner continental margin.Lower Mississippian rocks assigned to the RobbersMountain Formation (Carr and others, 1992) rest unconformably on the metasedimentary rocks of GerbrachtCamp in the Garlock quadrangle. The lower member of the Robbers Mountain Formation consists of metaconglomerate containing clasts derived from theunderlying Devonian section, whereas the upper memberconsists of meta-argillite, signifying return to a lowenergy depositional regime. Nevertheless, the Robbers Mountain Formation marks the end of passivecontinental-margin sedimentation and the beginning ofa new episode in the geologic history of the region.The Robbers Mountain Formation was interpreted byCarr and others (1992) as a syntectonic deposit,corresponding in age with the Antler orogeny. During the Late Devonian to Early Mississippian Antlerorogeny, as it is understood from the geology in Nevada,oceanic strata deposited along the western outercontinental margin of early and middle Paleozoic NorthAmerica were emplaced eastward over coeval innercontinental-margin strata (Burchfiel and Davis, 1972,1975), disrupting sedimentation patterns and sheddingan apron of siliciclastic debris eastward into a latePaleozoic foreland basin (Poole, 1974).A Late Mississippian age is inferred for intercalatedmeta-argillite, quartzite, and sparse metaconglomeratethat form a turbidite sequence exposed near the ApacheMine in the Garlock quadrangle (metasedimentary rocksof Apache Mine). Strata conformably overlying themetasedimentary rocks of Apache Mine have yieldedPennsylvanian conodonts. Poole (197 4) provisionallycorrelated the rocks comprising the Apache Mine unitwith Mississippian rocks in the Great Basin interpretedas foreland basin deposits shed from the Antler orogenic highland. He interpreted the basal (western) contactof these rocks (metasedimentary rocks of Apache Mine)as a faulted unconformity (see Poole, 197 4, Carr andothers, 1980).The Pennsylvanian strata (metasedimentary rocks ofBenson Well) that conformably overlie the meta-sedimentary· rocks of Apache Mine also are a turbiditesequence (slate and quartzite member). In the El PasoPeaks quadrangle, however, metalimestone representinga shallower water environment forms the basal member (metalimestone member) of the metasedimentaryrocks of Benson Well, and the Pennsylvanian sectionrests unconformably on rocks of inferred Devonian age.The metalimestone member of the Benson Well unitgrades upward into a sequence of very fine grainedturbidites (slate and quartzite member), suggesting basinsubsidence.West of Mormon Flat, Permian strata (metasedimentary rocks of Holland Camp, in part) rest unconformably on Upper Cambrian or Lower Ordovician rocks.There, the lowermost part (member A) of the Permian section is a lithologically heterogeneous unit of slatecontaining beds of arkosic quartzite, metaconglomerate,metadolostone, and metalimestone. The quartzite, metaconglomerate, and beds of metamorphosed carbonaterocks are mostly debris-flow and turbidite deposits. EarlyPermian fusulinids are present in the lower part of thesection. Member B is slate containing turbidite bedsof coarse-grained calcarenitic metalimestone. East ofGoler Gulch, most of the Permian section consists ofrocks assigned to member B (in part). There, member B apparently has a greater age range, possibly asold as Pennsylvanian. Deposition in this section couldhave been continuous from the metasedimentary rocksof Benson Well to the metasedimentary rocks of Holland Camp, but the contacts of both units with an intervening unit of poorly dated chert and marble arefaulted, obscuring the original stratigraphic relations.In the section exposed south of Mormon Flat, the turbiditesequence comprising member B is gradationally overlainby member C, which contains beds of tuffaceousmetasandstone and metaconglomerate. The influx ofvolcanogenic material in this section heralds the initiation of a magmatic arc along the western continentalmargin of Paleozoic North America. The Lower Permianmetasedimentary succession is overlain conformably byUpper Permian andesitic metavolcanic flows (andesite of Goler Gulch).Most of the Paleozoic strata of the El Paso Mountains dip moderately eastward, giving the appearanceof a thick homoclinal section. However, the entirePaleozoic section is deformed into tight to isoclinal,westward-vergent folds with subhorizontal axes. Fewof the largest map-scale folds close within the range.Faults that are nearly parallel to layering truncate manyof the fold limbs and juxtapose contrasting rock sequences, many of which are inverted. Penetrativefoliation and (or) spaced cleavage are present in manyof the rocks. Penetrative foliation is particularly strong3
near the west end of the Paleozoic section, where itis in ductile-fault contact with the gneiss of WeissMountain. A zone of blastomylonite marks this faultzone. Foliations in the Paleozoic rocks, the mylonitezone, and the gneiss are coplanar.Low-grade regional dynamothermal metamorphismaltered most of the metasedimentary and metavolcanicrocks of the El Paso Mountains. In most areas, relicsedimentary structures and textures are preserved. Ina zone 1 to 2 km wide adjacent to the gneiss of WeissMountain, however, there is a progressive westwardincrease in grade of the metamorphic rocks. Mineralassemblages indicate a progression through the albiteepidote-hornfels to the hornblende-hornfels facies ofcontact metamorphism and from quartz-albite-muscovite-chlorite subfacies to almandine-amphibolite faciesin a dynamothermal metamorphic progression. Formation of penetrative foliation in all of the metamorphic rocks accompanied metamorphism. Thesemetamorphic relations, together with the structuralrelations between the metamorphic rocks and the gneissof Weiss Mountain, led Christiansen (1961) to conclude that intrusion of the pluton (comprising the gneissof Weiss Mountain) was simultaneous with both ductile deformation and regional metamorphism of thePaleozoic metasedimentary and metavolcanic rocks. Thethermal aureole of the pluton apparently caused anoverprint of higher temperature contact anddynamothermal metamorphic mineral assemblages onan otherwise-typical, penetratively deformed, low-grademetamorphic terrane. Local retrograde metamorphismin the contact aureole of the gneissic pluton may haveresulted from later static contact metamorphis
the axis of the El Paso Mountains, exposing the pre Tertiary metamorphic and plutonic rocks that form the core of the range. The west end of the El Paso Mountains is separated from the south end of the Sierra Nevada by low hills west of Red Rock Canyon; eastward the El Paso Mountains merge into a hilly terrain under
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Lewis, R.S., 1998, Geologic map of the Butte 1̊ x 2̊ quadrangle: Montana Bureau of Mines and Geology Open-File Report MBMG 363, 16 p., scale 1:250,000. Reynolds, M.W. (compiler), 2000, Geologic setting and a generalized bedrock geologic map in
Topographic map of the bedrock surface beneath the area near Ashumet Pond as interpreted mostly from geologic borings, western Cape Cod, Massachusetts . John W. Lane, Jr., Emily B. Voytek, and Denis R. LeBlanc: Introduction: The bedrock surface of western Cape Cod defines the : lower boundary of groundwater models that are used to predict the .
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