Geologybeneath and around the West Potrillo basalts, DofraAna and Luna Counties, New Mexico byWillian R. SeageLEarth Sciences Dept., New Mexico State University, Las Cruces, NM 88003

o e ..4 -q -.c io 6 :: :o :c ri :c Introduction :o iJ .. o., :a :o The West Potrillo Mountains, located 30 t. 21 :l mi southwest of Las Cruces, New Mexico :o io .! D (Fig. 1), are known as one of the largest con- ",E'. el :a o centrationsof basalticvolcanoes in the United e :o o States. Lying near the southern end of the AD (l rift, the volcanoesare part of an tt Rio Grande o assemblageof young volcanic fields that dot

(9 the rift throughout its length. More than 100 cinder cones, small shield volcanoes, and maars are so nearly untouched by erosion that their youthful age is obvious. Radio- metric datesfrom 1.2Ma to 0.1Ma havebeen determined, but some volcanoes may be as old as late Plioceneor as young as 20,000 years (Seageret al., 1984; GIle,1987). Lava flows associated with the volcanoes cover Adon Hllls more than 200 mi'. Known as the West Po- trillo Basalt (Hoffer, 7976), these lavas have mostlv buried an array of older rocks and structures whose ages and geometries have been discussedonly briefly in previous pub- lications. The purpose of this paper is to a, c summarize current interpretations of these rojJ",-o i.i: rocks and structures. oAlo'.\,..'.,"\a i- The pre-Quaternary geology of the West u Aden volcano l s : ','"''=' Potrillo Mountains is revealedin three ways: o z;-: 1) by scatteredoutcrops of older rocks around ll< :,E o "i loz the perimeter of the basalt, 2) by gravity data 3zo (Lanceand Keller, 1981;DeAngelo and Keller, '.' j... "'.; + 1988),and 3) by two important oil tests, the ."1' ..1 ;: ,''i *ilr;i*\ l'1 1.,-r':ir'l -- ''r.:c) Sunray Mid-Continent No. 1 FederalR and 'l',;.,,,,,,, Kllbourne Hole the Skelly No. 1-A State C wells (Fig. 1). These data permit reconstruction of a strati- '',."":..'/,,,,,,_'\'",,$), graphic column for the area (Fig. 2) and an 1", 'bor ,Str'. O\, 1.. Noat '' 1":r'\r' ;.Mtl Hole interpretation of the main tectonic features Eagle '" \\Hunt'g rl/, t. : \\ tt" (Figs.3 and 4). Much of the outcrop data is ,-/-.444: ! '='? new, the result of mapping for the SWt/aof ..'bunrayMidlcontlnent:.,i ". Skellv No. l-A ,. No. 1 Fsderal R Las Cruces and NW El Paso 1ox 2o sheet Stats C + Important work *s,oee' (Seager,in press). previous 9,437' 1 ;'':." ,; on pre-Quaternary rocks of the areaincludes Prscambrian -Precambrian Hill Hoffer, 1975;Kottlowski et al., 1969;Hoffer ro coh and Sheffield,1981; Hoffer and Hoffer, 1981; To El Paso MEXTCO x\ Broderick, 1984;Lance and Keller, 1981; mel Mountaln MEXTCO l*Etru," r""t Thompson, 1982; and Kilburn et al., 1988. '' , '.'nj 'Qt>",a _e/,,,,..,.., SCALE 5 10 mllos Stratigraphy '. Assembling a stratigraphic column of pre- - 0 5 10 15 kllometors 2o -= - Quaternary rocks in the West Potrillo Moun- C'r).i tains area is an uncertain task becauseout- 9z =- crops are small and separatedby wide tracts 2: t of sand and/or basalt. Stratigraphic relation- ships from one outcrop to the next are sel- o'. dom clear, especiallyin the Tertiary section. on t.. -9.1. a stratigraphic order No. 1 Moyotes Nevertheless, general ba.'. Yts,ztz'.lPemex (Fig. 2) is apparent, based mostly on out- -o^... Prgcambrian '?,... crops from the western and northern margin of the basaltfield and the EastPotrillo Moun- tains, as well as on data from the two drill FIGURE 1-Location map of the West Potrillo Mountains and vicinity. holes.

Ntw Mexito Geology August 1989 Precambrian and Paleozoic rocks tively identified on the basisof fossil content Mack (in press)measured 1,900 ft of marine Precambrian gabbro and granite were and lithologic similarity with Permian rocks clasticand carbonateshelf depositsabove a drilled in the Sunray and Skelly wells, re- exposed in the Franklin, Robledo, and Ca- basal conglomerate, all of which thin south- spectively, beneath lower Paleozoicsedi- ballo Mountains and in the West Lime Hills ward. Thesesomewhat-arkosic clastic rocks mentary rocks (Thompson, 1982;Kottlowski of the Tres Hermanas Mountains. Appar- and limestones contain an Albian-Aptian et al., 1969).Bliss Sandstone,El PasoFor- ently these Permian strata are widespread fauna and correlate with the Hell-to-Finish mation, Simpson(?) Formation, Montoya acrossthe southern part of the West Potrillo and U-Bar Formations of southwestern New Formation, and Fusselman Dolomite were Mountains area in contrast to Devonian, Mexico. At EagleNest, approximately1,000 recognized in the Sunray well. Thicknesses Mississippian, and Pennsylvanian rocks, ft of marine and nonmarine Hell-to-Finish of several of these units shown in Figure 2 which were eroded during mid-Wolfcamp- and U-Bar strata are upside down beneath are not corrected for Tertiary igneous intru- ian time (Thompson,1982). Permian carbonates and a Laramide thrust sions or possible steep dips. In the Skelly One mile east of Eagle Nest the entire Pa- fault. Two coarse-grainedarkose tongues in well, Bliss and lower El Pasoare missing by leozoic section is missing, and Upper Cre- the Hell-to-FinishFormation suggesta nearby faulting and Simpson(?)and Fusselmanare taceousor lower Tertiary clasticstrata overlie granitic source, and one mile to the east a missing by erosion. Middle and upper Pa- Precambriangranite. Laramide, as well as fault block of Precambrian granite was pos- leozoic strata are absent from both wells be- possible Early Cretaceous and mid-Wolf- sibly exposedduring Early Cretaceoustime. causeof erosion (Thompson,1982). campian uplift and erosion may account for Upper Cretaceousto lower Tertiary rocks- Permian carbonate rocks crop out in widely the absenceof Paleozoicstrata there. One mile east of EagleNest the fault block scatteredhills from EagleNest (Fig. 1) south- named Granite Hill by Broderick (1984)also ward to the Mexico border and eastward to Mesozoicrocks exposesa seriesof arkosic siltstone, sand- the East Potrillo Mountains. Incomplete sec- Lower Cretaceousrocks-These rocks crop stone, conglomerate,and minor limestone tions of Hueco (Colina), Yeso(Epitaph), and out in the East Potrillo Mountains as well as beds that nonconformably overlie the San Andres Formations have been tenta- at EagleNest. In the former areaSeager and Precambrian granite. Conglomerate beds

Chronostratigraphic Symbols Thickness Units for ure4 LithostratioraohicUnits o - LatePliocene Pleistocene Qb (basalt) 300 Fanglomerate,eolian depo{s; !g.9!!- # QTa Miocene o-2,o00 SANTAFE GROUP, lower part; conglomerate and sandstone + 500 l UVASBASALTIC ANDESITE

BELLTOP FORMATION; latite and latiteporphyry flows at top; flow-banded Oligocene 1,500(?) rhyolite;felsite; rhyolite ash-flow tutf; andesiteflows and domes; silicicto intermediatecomposition intrusives

Late Eocene- 1,500(?) RUBIOPEAK FORMATION; intermediate-composition flows and volcaniclastic earlyOligocene rocks

Eocene 1,000 LOVE RANCH FORMATION; SANd- LOBOFORMATION; arkose, stone and conglomerate conglomerate,sihstone, limestone TI I Late Cretaceous- 780 I earlyTertiary

s00 U-BARFORMATION: limestone EarlyCretaceous KI 440 HELL-TO-FlNlSHFORMATION; conglomerate, arkose, sandstone, -ili tlt.P 400 SAN ANDRES FORMATION; I|MESIONE Ht sc Ht a V,It Permian P 800 YESO(EPITAPH) FORMATION; dolomitic limestone I' @ to 1,000(?) HUECO(COLINA) FORMATION; limestone, shale E

Silurian 6s0 FUSSELMANDOLOMITE o

818 MONTOYADOLOMITE 3 E Ordovician 376 SIMPSONFORMATION (?); Shaleor sandstone

1,s95 EL PASOFORMATION; limestone, dolomite withintrusives

Cambrian I /J BLISSFORMATION; sandstone and shale and Ordovician #

Precambrian p€

FIGURE 2-Cornposite stratigraphic column for the West Potrillo Mountains area.

August 1989 Nao Merico Geology contain clasts of Lower Cretaceouscarbon- in the hills 4 mi north of Eagle Nest; else- correlative with the Bell Top Formation. Al- ates (Broderick, 1984)and are therefore where, stratigraphic relationship of the silicic though undated, these plugs are probably younger than Early Cretaceous.Regional re- volcanics is unclear. They are probably cor- Oligocene in age becausethey intrude Love lationships and physical characteristicssug- relative with the Bell Top Formation of Oli- Ranch and Rubio Peak Formations and fur- gest correlation of these conglomeratesand gocene age of the Sierra de las Uvas area nished clasts to Miocene fanglomerate. arkosic strata with the Lobo Formation, which (Seagea1973).The Bell Top also consistspre- Plutonic or hypabyssal intrusives-Intru- is probably of either Late Cretaceousor early dominantly of silicic ash-flow tuffs and flow- sive rocks are coarse-grainedequigranular or Tertiary age (Clemons, in press; Seagerand banded rhyolite beneath basaltic andesite porphyritic masses that crop out at Provi- Mack (1986).The arkosic rocks seemingly in- flows (Uvas BasalticAndesite). Distal parts dencecone, in the hills west of Camel Moun- dicate uplift and erosion of the underlying of Bell Top tuffs 4 and 6, as well as Bell Top tain, and at Granite FIiX(Fig. 1). The intrusive and adjacent Precambrian granite during sedimentary rocks, crop out beneath Uvas at Providence cone is biotite latite porphyry Laramide time, but the Lower Cretaceousar- Basaltic Andesite flows along the northem whereas the intrusives in the other two areas kose in the Hell-to-Finish Formation of the margin of the West Potrillo basalt field. are equigranular diorite and hornblende nearby Eagle Nest outcrops also suggests Finally, the large andesitic plug or plugs monzonite porphyry. At Granite Hill, dike- uplift and erosion of the granite or some other of Mt. Riley and Mt. Cox probably are also like intrusives have invaded Upper Creta- nearby granite massin Early Cretaceoustime.

Tertiary rocks A wide variety of rocks of probable Ter- tiary age crop out around the perimeter of the West Potrillo Basalt field. They can be divided into six groups: 1) Love RanchFor- mation, essentiallynonvolcanic clastic rocks, 2) Rubio Peak Formation, intermediate-com- position volcanic rocks, 3) Bell Top Forma- tion and related silicic volcanic rocks, 4) medium- to coarse-grainedintrusive rocks of intermediate composition, 5) Uvas Basaltic Andesite,basaltic andesite flows, 6) SantaFe Group, fanglomerate derived largely from volcanic rocks. Love Ranch Formation-The oldest Ter- tiary unit consists of conglomerate,sand- stone,and shalethat crop out along the flanks of the Mt. Riley-Mt. Cox plugs on the south- eastern flank of the West Potrillo Mountains (Fig. 1). Lowest exposuresare cobble-boulder conglomeratesconsisting of Lower Creta- ceous and Permian detritus similar to the outcropping Permian and Cretaceousrocks .', of the East Potrillo Mountains. Red mud- stone and siltstoneare interbedded,and there are no volcanicclasts. The sequenceresem- blesthe EoceneLove RanchFormation of the CaballeSan Andres Mountains area with which it was correlated by Seagerand Mack (E (in press).Higher parts of the sectioninclude tan to olive sandstoneand shaleas well as lo conglomerate, all of which have a significant component of andesite and rhyolite clasts. o Rubio Peak Formation-Andesitic flows E and breccia overlie the Love Ranch Forma- tion on the flanks of the Mt. Riley-Mt. Cox plugs, and the same rocks form- the Aden Hills (Fig. 1) at the northeasterncorner of MEXICO the WestPotrillo basaltfield. Theseandesitic SCALE rocks seemingly are correlative with the Ru- 10 2 0 mlles bio Peak and Palm Park Formation of late Eocene age in the Sierra de las UvarGood 10 20 30 kllomot€rs Sight Mountains-Florida Mountains area (Clemons,1977; L979; in press). EXPLANATION Bell Top Formation and related rocks- -.--t/ Notmalfault cuttlngOuaternary dsposits --C* Syncllnalaxls inferred from gravity Masses of flow-banded rhyolite, lithic - and -G- Normal f ault Inlerred trom gravlty vitric ash-flow tuff, and latite and latite por- phyry flows crop out in small, widely sep- -- -a-' Normal tault burisd by Holocsns(?)alluvlum ? Uirit of West Potrilloand related basalts arated hills on the Camel Mountain fault block .-)fi Svnctinataxis * Cinder cone (Figs. 1, 3). Similar volcanicsalso were pen- etrated in the Sunray Mid-Continent No. 1 A-A' Lins of cross sectlons tor Figurs 4 and Skelly 1-A wells, described bv Hoffer (1985).Inbutcrops, the latite and litite por- FIGURE 3-Tectonic map of West Potrillo Mountains area. Shaded areas are basins with known or phyry flows underly Uvas BasalticAndesite inferred thick basin fill.

New Mexico Geology August 7989 ceousor lower Tertiary strata,and in the hills deformed basalt, fault blocks of the Fio Grande associated thrust faults in the East Potrillo west of Camel Mountain unidentified meta- rift dominate the structure of the West Po- Mountains involve Lower Cretaceous and morphosed limestone has been cut by the trillo Mountains beneath the basalt (Figs. 3 Permian rocks, trend N30'W and verge to- intrusion. Undated, the intrusives may be and 4). In addition, there is evidencefor three ward the northeast. Most thrust faults dip Laramide or, more likely, middle Tertiary in older periods of significantdeformation: mid- moderately wesfward and appear to origi- age. Becauseoutcrops in each area are con- Wolfcampian, Early Cretaceous, and Lar- nate in the cores of anticline-syncline pairs. fined to isolated hills, the extent of the plu- amide. Locally, however, Permian strata lie above tons is also unknown. Cretaceousrocks on thrust faults of possible regional extent. Structural relief due to both Uvas Basaltic Andesite-Basaltic andesite Mid-Wolf campian deformation flows in the West Potrillo Mountains areaare folding and faulting approaches2,000 ft. The The western flank of the West Potrillo probably correlative with the Uvas Basaltic deformation projects northwestward be- Mountains is positioned between two well- Andesite of the Sierra de las Uvas area. The neath the West Potrillo Basalt where similar documented uplifts of mid-Wolfcampianage: flows crop out along the northern edge of structures presumably exist in the subsur- the Florida islands of Kottlowski (1960)to the the West Potrillo basalt field and in the hills face.At EagleNest a thrust fault, which trends west and the Moyotes uplift of Navarro and 4 mi northeast of Eagle Nest. In these re- N20"W and dips moderately westward, has Tovar (1975)to the south. It is not surprising spective areasthe flows overlie Bell Top tuffs carried Permian carbonates northeastward then that evidence for the mid-Wolfcampian or sediment and latite porphyry flows. Is- above overturned Lower Cretaceous strata episode of uplift and erosion exists in the lands of basaltic andesite flows also are scat- €ig a). Both hanging-wall and footwall rocks West Potrillo Mountains. tered within the northern part of the West are truncated by the thrust, but stratigraphic Evidence for mid-Wolfcampian tectonism Potrillo basalt field. With an ageof 27-28Ma separation cannot be determined. The very is stratigraphic.Lower Permiansedimentary in the Sierra de las Uvas region (Clemons, coarsegrained, Upper Cretaceous-lower rocks (Hueco-Colina) apparently uncon- 1979),the basalticandesites are the youngest Tertiary arkosic strata and conglomerate at formably overlie lower Paleozoicrocks in at radiometrically dated pre-Quaternary vol- Granite Hill also suggest Laramide base- Ieastpart of the Camel Mountain fault block, canicrocks in the WestPohillo Mountain area. ment-cored uplift and erosion at that area, and they (Abo Formation(?))overlie Precam- However, undated massesof rhyolite, which only 1.mi east of the thrust faults and over- brian granite in the Pemex No. 1.Moyotes crop out near the axis of the Akela basin syn- turned folds at EagleNest. well in northern Chihuahua (Navarro and cline, appear to overlie the basaltic andesite Whether the Laramide thrusts and folds Tovat, 1975;Thompson et al., t978; Fig. 1). and may be younger. are examplesof regionaloverthrusts (Drewes, Regionalisopachs and faciesindicate that pre- 1982) are Santa Fe Group (lower)-Fanglomeratic 1978;Woodward and DuChene, or Permian strata, including rocks of Pennsyl- of basement-coredblocks strata consisting of conglomerate, conglom- marginal structures vanian, Mississippian, and Devonian age, (Seager Mack, 1985;Seager et al., 1986) eratic sandstone,and sandstonecompose the and were deposited across the region (Green- The general structural style lower part of the Santa Fe Group. These are is not known. wood et al., 1977),although the Florida uplift in terms of either model. tilted alluvial fan deposits exposedat Camel may be interpreted may have been emergent during at leastpart involved in at least some Mountain and along the western edge of the That basementwas of the Pennsylvanian (Kottlowski, t963). lt Laramide structures seemsclear from EastPotrillo Mountains horst. Clastsat Camel of the seems likely then that the absenceof pre- the arkosic strata at Granite Hill. Mountain consist mostly of felsite, flow- Permian strata in the Camel Mountain area banded rhyolite, and silicicash-flow tuffs with can be attributed to uplift and erosion in mid- Late Tertiary deformation a minor component of more mafic volcanic Wolfcampian time. The Florida-Moyotes faults-A system of debris. In exposures in the East Potrillo uplift trend apparently is a major, ancestral Early rift low-angle exposed in Mountains, a wide variety of volcanic clasts Rocky Mountain, basement-coreduplift that low-angle normal faults is also (Seagerand Mack, are mixed with Lower Cretaceousand Per- trends northwesterly and extendsfor at least the EastPotrillo Mountains a). Thesefaults offsetLaramide mian detritus as well as with an occasional 60 mi from northern Chihuahua to the Dem- in press;Fig. are younger; they Precambrian granite cobble. In both areas ing area and beyond, where it is called the structures and therefore range- clastsrange in size from boulders to pebbles; Burro uplift (Elston,1958; Thompson, 1982). are older than the modem high-angle, bedding includes both planar and cross- boundary faults. Seagerand Mack (in press) bedded types. Cementation is strong in both consider the faults to be of probable early areas,but the Camel Mountain strata are ex- Early Cretaceousdeformation Miocene age, formed during an early phase tensively silicified. Dips of strata are ap- Evidence for Early Cretaceous deforma- of extension in the Rio Grande rift. The sys- proximately 10 to 25 degrees. tion comesfrom the EagleNest area.Coarse- tem trends N30'W dips generally NE, and Although contactswith older rocks are not grained arkosein the Lower CretaceousHell- may be projectednorthwestward beneaththe exposed, the clast content of the fanglom- to-Finish Formation suggestsuplift and ero- West Potrillo basalts.Stratigraphic separa- erates indicates that they are younger than sion of nearby Precambriangranitic terrane, tion across the fault system, which consists the Eocene-Oligocenevolcanic section.They possibly the fault block at Granite Hill. This of three or four malor strands,is 1,650ft. clearly correlatewith strata of the lower part Precambrian terrane may have been part of This fault system has some of the char- of the Santa Fe Group of the Las Cruces- the Early Cretaceousrift shoulder of Mack acteristicsof a detachmentfault zone. Lo- Caballo area, specifically with the Hayner (1987;Mack et al., 1986)or possibly an in- cally, at least, hanging-wall strata are more Ranch and Rincon Vallev Formations of earlv trarift uplift. Mid-Wolfcampian erosion from broken and rotated above low-angle sole faults to middle Miocene age-(Seageret al., 797i; the Florida-Moyote uplift may also have than are footwall rocks. Also, long segments Seagerand Hawley, 1973).The fanglomerate causedremoval of someof the Paleozoicunits of the fault zone remain low angle (30' or exposures are probably uplifted parts of the from the Eagle Nest-Granite Hill area, less)after 25'of late Tertiary tilt is removed. early rift Malpais basin, which gravity data whereas Permian and perhaps older Paleo- These features suggest the fault zone may indicate underlies a broad part of the south- zoic strata may also have been eroded during have been initiated as a low-angle detach- ern West Potrillo Mountains area. The lower the Early Cretaceousdeformation. ment with tilted, lishic, fault-boundedblocks Santa Fe fanglomeratesare overlain by Qua- active in the hanging wall. However, no my- ternary basalt or sand. lonitic or metamorphic fabrics are present Laramide deformation either within or below the fault zone; brittle Evidence for Laramide deformation in the deformation characterizesboth hanging wall Tectonics West Potrillo Mountain region is based on and footwall. If the fault zone is a detach- Although they are scarcelynoticeable be- outcrops in the East Potrillo Mountains and ment, it representsan upper crustal segment cause of the great outpourings of nearly un- at Eagle Nest and Granite Hill. Folds and of comparatively small displacement.

August 1989 NatsMexico Geology Latest Tertiary and Quaternary fault fractures in the zone during its ascentto the fill within the graben, uplifted and exposed blocks surface. fanglomerate crops out on both the western Major late Tertiary and Quaternary fault Akela basin-Revealed both by gravity data and eastern bordering uplifts, at Camel blocks of the West Potrillo Mountains are and by the synclinal structure of exposedvol- Mountain and along the western edge of the shown on the tectonic map in Figure 3 and canic rocks, the Akela structural basin trends EastPotrillo horst, respectively.The deposits in the cross sections in Figure 4. The blocks northwesterly, oblique to the West Robledo are tilted and faulted basin-fill deposits, were identified by their gravity signature fault (Fig. 3). The central,deepest part of the probably Miocenein age, derived largelyfrom (DeAngelo and Keller, 1988),by exposures basin appears to be a half graben bounded Tertiary volcanic rocks. Exposedthickness in around the perimeter of the West Potrillo on the northeast by the Good Sight fault, both areas is several hundred feet. Presum- basalt field, or by a combination of the two. which delineates the southwestern margin ably the samedeposits, possibly much thicker, RobledeEast Potrillo uplift-The Rob- of the Good Sight-Sierra de las Uvas horst occur in the deeper,buried parts of the Mal- ledo-East Potrillo uplift is the buried con- (Fig. 3). To the southeast,the basinbecomes pais basin. The older depositswithin the gra- nection between the Robledo uplift to the shallower and synclinal in form as evidenced ben, together with exposed sectionsof basin north and the EastPotrillo uplift to the south by increasing Bouguer gravity values and by fill on adjacent uplifts, may constitute the fill (Fig. 3). Whereasboth of the exposeduplifts synclinaldips in Uvas BasalticAndesite. The of an early rift basin whose original extent are horsts, their buried connection appar- synclinalpart of the basinterminates against was greater than the modern Malpais gra- ently is bounded only on the west by a major and may be truncatedby the West Robledo ben. fault zone.This fault, the WestRobledo fault, fault. Deepest parts of the basin probably Camel Mountain uplift-The structurally separatesthe uplift from the adjacent Akela contain thick basinfill, but the synclinalpart high block that emerges from beneath the and Malpais basinsto the west. The fault is is probably surfacedby Tertiaryvolcanic rocks, western margin of the West Potrillo basalt is downthrown to the west and, at leastalong some of which crop out along the hinge. Most the Camel Mountain uplift (Figs. 3 and 4). It its exposednorthernmost extent in the Rob- of the southeasternsynclinal part of the ba- is bounded on the west, north, and eastby ledo Mountains,has stratigraphic separation sin is buried beneath Quaternary sand or the Mesquite Lake, Akela, and Malpais ba- of approximately 2,500 ft. It can be traced basaltflows. sins, respectively. The uplift has relatively from the westem margin of the Robledouplift Malpais basin-Like the Akela basin, the low topographic relief and is widely covered southwestwardand then southerly acrossthe Malpais structuralbasin has little or no sur- by sand, so that the surfaceexpression of the West Potrillo Mountains for a total distance face expression.It is essentiallyhidden by uplift passesimperceptibly into the Akela and of nearly 60 mi. Acrossthe West Potrillo ba- sand and basalt. Malpais basins. Only a few bedrock islands saltsthe fault's position is markedby widely The basin is clearlyrevealed by gravity data project through the mantle of sand,and these, scatteredscarps and, more clearly,by a steep (DeAngelo and Keller, 1988).It is a north- together with drill-hole data, were utilized gravity gradient of 30 mgal relief. The fault trending graben, approximately 12 mi wide, to construct the stratigraphic column (Fig. 2). passesalmost through the middle of the West that is bounded on the eastby the WestRob- A block tilted eastward throughout much of Potrillo basaltfield, and half or more of the ledo fault and on the westby a steepBouguer its extent, the Camel Mountain uplift is a Quaternary cinder conesin the field lie on gravity gradientinterpreted to be a fault, the horst in its southern part. or adiacent to the fault. The scarpsin the Malpais fault zone (Figs.3 and 4). The basin The main boundary fault of the Camel basaltclearly indicate Quaternary movement has more than 30 mgl and 20 mgl relief on Mountain uplift is the Camel Mountain fault. on the fault, and the position of the cinder the eastand west sides,respectively. Downthrown to the west, the fault limits the conessuggests that basalticmagma utilized Although there are no exposuresof basin western edge of the uplift as well as the east-

WEST Camel EAST A Mesquite A' Lak Elev.(f t.) e Mt. Riley East Potrillo 5,000 basin lault r Mesillabasin QTa P Tv 2 0 QTa -5,000 ob I cob -10,00 i

Camel Mountain West PotrilloMountains B Malpais fautt East Potrillog, Mesquitell f ault West Elev. Lake Camel Mountain Malpaisbasin basin 5,000 ault block I ob QTa n QTa Pa7 ay

FIGURE 4-Sections across the West Potrillo Mountains showing, somewhat diagrammatically, interpretation of major subsurface structural features. Symbols also used in Figure 2; see Figure 3 for location of sections. Q, Quaternary alluvium; Qb, Quaternary basalt flows and cinder cones; QTa, Quaternary and Tertiary basin fill; Tv, Tertiary volcanics and intrusives; Tl, Upper Cretaceous and lower fertiary Love Ranch Formation, Lobo Formation, and related rocks; Kl, Lower Cretaceous rocks; B Permian carbonates; Pal, Paleozoic rocks, undifferenliated; p€, Precambrian granite and related rocks.

Nan Mexico Geology August 1989 ern margin of the complementary Mesquite ing and thrusting has been inferred to the ozoic paleogeography of west-central United States: Lake basin. The surfacebxpressionofthe iault southeast in the juarez Mountains on strike Societyof EconomicPaleontologists and Mineralogists, is the Camel Mountain eicarpment (Fig. 1), with the West Potrillo deformation (Lovejoy, Rocky Mountain Section, pp. 173-"188. a prominent late Broderick, J. C., 1984,The geology of Granite Hill, Luna Quaternary scarp that ex- 7972, 1980;Drewes, 1978). County, New Mexico: Unpublished M. S. thesis, Uni- tends at least55 miles southward inlo Mexico During the late Eocene, Laramide uplifts versity of Texas(El Paso),89 pp. (Reeves,1969). Stratigraphic separation across were eroded and large volumes of coarse-to Clemons, R. E., En, Geology of west half Conalitos the fault, estimated from the structural ele- fine-grained detritus Ranch quadrangle: New Mexico Bureau of Mines and were transported into ,14. vation of the top of Precambrianrocks in the complementary Mineral Resources,Geologic Map basins.The volcaniccontent Cfemons, R. E., 1,979,Geology of Goodsight Mountains Sunray and Skelly oil tests, which are on of these deposits increasesupward, herald- and Uvas Valley, southwest New Mexico: New Mexico opposite sides of the fault, is approximately ing the onset of major andesitic and silicic Bureau of Mines and Mineral Resources,Circular 169, 2,200ft. volcanism during Oligocene time. Emplace- 31 PP. Mesquite Lake basin-The Mesquite Lake ment of andesitic Clemons, R. 8., in press, Geology of the Florida Moun- and silicicdomes, andesitic tains, Luna County, New Mexico: New Mexico Bureau structural basin lies to the west of the Camel lahars, silicic ash-flow tuffs, and andesitic to of Mines and Mineral Resoutces,Memoir. Mountain uplift on the downthrown side of Iatitic lava flows was widespread, all accom- DeAngelo, M. V., and Keller, G. R., 1988,Geophysical the Camel Mountain fault (Figs. 3 and 4). panied by emplacementof associatedplu- anomalies in southwestern New Mexico: New Mexico North trending, the basin extends south- tonic rocks. By late Oligocene time, 27-28 Geological Society, Guidebook to 39th Field Confer- ward into Mexico and westward ence,pp.71-75. to the vi- Ma, widespreadbasaltic andesite flows, lo- Drewes, H., 1.978,The Cordilleran orogenicbelt between cinity of Columbus, New Mexico. To the cally interbedded with thick fanglomerate, Nevada and Chihuahua: GeologicalSociety ofAmerica, north, it is separated from the Akela basin suggestthat an active extensionalstress re- Builetin,v. 89, pp. U1-657. by a low gravity ridge. From available data, gime had been established. Elston, W. E., 1958, Buno uplift, northeastem Iirnit of the Mesquite Lake basin appearsto be a gra- In sedimentarybasins of southwesternNew Mexicoand early to middle Miocene time, during southeasternArizona: American Association of Peho- ben. Basin fill as much as 5,000 ft thick was an early stageof extension in the Rio Grande leum Geologists, Bulletin, v. 42, pp. 2513-:?517 . penetrated in the Skelly well (Thompson, rift, thick sequencesof fanglomerate and as- Gile, L. H., 1987,A pedogenic chronology for Kilbourne 1982)on the easternedge of the basin ad- sociated rocks filled the Malpais basin lo- Hole, southem New Mexicell,Time of the explosions jacent to the Camel Mountain fault. This fig- cated along the southwestern edge of the and soil events before the explosions: Soil Society of ure may be representative America, Journal, v. 57, pp.752-7ffi. of the depth of West Potrillo Mountains. Low-angle faults, Greenwood, E., Kottlowski, F. E., and Thompson, Sam, the MesquiteLake basin. possibly detachment faults of modest dis- I1I, 1.977,Petroleum potential and srratigraphy of Ped- placement, probably are the sameage as the regosa Basin-comparison with Permian and Oro- Summary of tectonic history basin, and although they are grande Basins:American Association Petroleum now exposed Geologists, Bulletin, v. 51, no. 9, pp.7M8-7469. pa- in pre-basin rocks, they may have been After deposition of lower and middle as- Hotrer, J. M., 1976,Geology of Potrillo basaltfield, south- leozoic shelf carbonates, uplift and erosion sociated with development of the basin. cenhal New Mexico: New Mexico Bureauof Mines and along a northwest-trendingaxis took place l,atestTertiary and Quatemary fault blocks Mineral Resources,Circular 749,30 pp. in mid-Wolfcampian time (Thompson, t9SZ;. of the southern Rio Grande rift evolved dur- Hoffer, f . M., 1986, Subsurfacevolcanic stratigraphy of ing south-centralNew Mexico; inHofrer, J. M. (ed.), Geol- Known as the Burro-Florida-Movotes unlift the latest Miocene after 10 Ma. Maior ogy of south-central New Mexico: El Paso Geological (Thompson, 1982),it was part of the unies- faults such as the East Potrillo fault, West Society,Field Trip Guidebook, pp.719-122. tral Rocky Mountain system and probably Robledofault, Good Sight fault, Malpais fault, Hoffer, J. M., and Hoffer, R. L., eds., 1981,Geology of separated and CamelMountain fault border the maior the border, southern New Mexicenorthern Chihua- the Pedregosibasin on the southi- hua: structural blocks of the West Potrillo Moun- El PasoGeological Sociery,95 pp. west from the Robledo shelf and Orogrande Hoffer, J. M., and Sheffield,T. M., 1987,Geology of West basin on the east. Lower to middle Permian tains area. Quaternary movement is indi- Potrillo Mountains, south-centralNew Mexico; in Hof- clasticsand carbonateseventually buried the catedby scarcelyeroded scarpsin Quatemary fer, J. M., and Hoffer, R. L. (eds.), Geology of the bor- uplift. deposits on all except the Good Sight and der, southem New Mexico-northem Chihuahua: El Paso Geological Society,pp. 79-82. During Early Malpais faults. The rise of basaltic magmas Cretaceous time, a major Kilburn, J. E., Stoeser,D. B., Zimbelman,D. R., Hanna, northwest-trendingrift extendedacross fhe along the West Robledofault zone during the W F., and Gese, D., 1988, Mineral resourcesof the area from southwestern New Mexico into Quaternary produced an impressive align- West Potrillo MountainsMount Riley and the Aden southeasternArizona (Bilodeau and Lind- ment of cinder conesalong the fault, and the lava flow WildernessStudy Areas, Dofla Ana and Luna associated Counties, New Mexico: U. S. GeologicalSuruey, Bul- berg, 1983;Mack et al., 1986).Deposition of West Potrillo Basalt flows buried letin 1735-8, pp. 1.-1.4. Lower Cretaceousmarine and nonmarine a vast area of the underlying fault-block Kottlowski, F. E., 7960,Summary of Pennsylvaniansec- clastics and carbonatesfilled the rift. Clastic structure. tions in southwestern New Mexico and southeastern componentswere derived, at least in part, The cinder cones and lava flows today are Arizona: New Mexico Bureau of Mines and Mineral known as the West Potrillo Resources,Bulletin 55, 187 pp. from the raised rim of Precambrianrocks and Mountains, but Kottlowski, F- E.,1963, Paleozoicand Mesozoic strata of Paleozoicsediments located along the north- those volcanics are only a thin rock mantle southwestern and south-central New Mexico: New ern and northeastemmargin of the rift (Mack that concealsa varied and complex rock re- Mexico Bureau Mines and Mineral Resources,Bulletin et al., 1986).An exposure of Precambrian cord of volcanism, sedimentation,and re- 79, 700 pp. peatedtectonism datingback Kottlowski,F. E., Foster,R. W., and Wengerd,,S.A.,7969, granite in this rim, or an intrarift fault block, to Precambrian Key oil testsand stratigaphic sectionsin southwestern in the Granite Hill areawas possibly a source time. New Mexico: New Mexico Geological Society,Guide- of coarsearkosic debris deposited in adjacent Ac

August 1989 Nao Mexico Geology Mexico:American Journal of Science,v.286, pp.309- Abstracts within the labechiid stromatolites of the Phylum JJI. Porilera. Samples for this studv which were taken, Navarro, A. and Tovar, J., 1975, Stratigraphy and tecton- in part, from'the McKelligon'Canyon stratotyPe, ics of the state of Chihuahua; ln Hills, J. M. (ed.), Ex- New Mexico GeologicalSociety indude a selection from Lechugui.lla Mound where ploration from the mountains to the basin: El Paso Toomey (1967) collected the holotype (USNM Geological Society, pp. 23-27 . The New Mexico Geological Society annual 155300) and fourteen These Early Or- Reeves, C. C., lr., 1969, Pluvial Lake Palomas, north- ParatyPes. spring meeting was held at New MexicoIn- sPonge bioherms are ob- western Chihuahua, Mexico: New Mexico Geological dovician stromatoporoid Society, Guidebook to 20th Field Conference, pp.1,43 stitute of Mining and Technology(Socorro) served to occur in nineteen cycles at the stratotyPe lv. on April 7, 1989.Following are abstractsfrom in the southem Franklin Mountains, El Paso County, Seager, W. R., 1973, Resurgent volcano-tectonic depres- sessionsgiven at that meeting.Abshacts from Texas. The organism acts most effectively as a sion of Oligocene age, south-central New Mexico: Geo- other sessionswere printed in the May issue dominant, laminated binder (bindstone-bound- logical Society of America, Bulletin, v. 84, pp. 3511 and will conclude in the November issue of stone) in the stressed climax stage. Similar occur- 3626. rences of Pulchrilamiru are recorded by Toomey Seager,W. R., in press, Geology of the southwest quarter New MexicoGeology. and Hamm 0967\ in the Kindblade Formation ol of Las Cruces and northwest El Paso 1ox 2o sheets: New Arbuckle Group Pre-Cha- Mexico Bureau of Mines and Mineral Resources, Ceo- the Early Ordovician logic Map. Keynote speech zvan-White Rock. between the Pulchrilamiw oc- Seager, W. R., andHawley,J.W.,1.973, Geologyof Rincon .tr..".r."" and the well-developed Middle and quadrangle, New Mexico: New Mexico Bureau of Mines Mto-Tnnlanv voLcANrsM, EXTENSIoN,AND MINER- Upper Ordovician stromatoporoid, and are re- and Mineral Resources, Bulletin 101, 42 pp. ALZATIONN NEW MEXICO:WHAT LTAVE WE LEARNED ported from North China and Malaysia. Seager, W. R., Hawley, J. W, and Clemons, R. E., 7971,, rHESEpAsr 40 YEARS?by W. E. Elston, Department DeBo CoNooovr BtosrRATIcRApHy oF THE KH-lv LtvE- Geology of San Mountain area, DoflaAna County, of Geology, University of New Mexico, Albu- New Mexico: New Mexico Bureau of Mines and Mineral sroNE (MrssrssrPPlAN),CENTRAI- NEw Mlxtco, by querque/ NM 87131 Resources, Bulletin 97, 38 pp. S.T. Krukowski, Departmentof Geoscience,New Seager, W. R., and Mack, G. H., 1986, Laramide paleo- Forty years ago, mid-Tertiary volcanic rocks were Mexico Institute of Mining and Technology, So- tectonics of southern New Mexico; in Peterson, J. A. regarded as mere overburden on Laramide ore de- corro, NM 87801 (ed.), Paleotectonics and sedimentation in the Rocky posits. Beginning in 1950, the NMBMMR spon- The Kelly Limestone (Mississippian) of central Mountain reqion: American Association of Petroleum sored systematic studies which built a stratiSraPhic New Mexico is located in widespread outcrops along Geologists, Memoir 41, pp. 569-685. framework not yet complete. The Director, Eugene Seager, W. R., and Mack, G. H., in press, Geology of the the western margins of the Rio Grande rift. It non- Callaghan, recognized the dominance of ignim- East Potrillo Mountains and vicinity, Dofra Ana County, conformably overlies Precambrian igneous and brites, a surprise because granitic magmas were New Mexico: New Mexico Bureau of Mines and Mineral metamorphic rocks and is unconformably overlain then unpopular. The 1960's brought federal money, Resources, Bulletin. by the Pennsylvanian Sandia Formation. The lower workers from the USGS and many universities, Seager, W. R., Mack, G. H., Raimonde, M. S., and Ryan, Caloso Member consists of sandstones, shales, and R. G., 1985, Laramide basement-cored uplift and basins Oligocene K-Ar dates, resurgent cauldrons, con- various limestones. The Ladron Member discon- in souih-central New Mexico: New Meiico Ceoloeical nections between siliceous Dlutonism and volcan- formably overlies the Caloso and is composed Society, Guidebook to 37th Field Conference, pp.1Zl ism, and plate tectonics. fhe tgZ0's and 1980's mostly of crinoidal grainstones. The Caloso Mem- 130. introduced petrogenetic models based on miner- Seager, W. R., Shafiqullah, M., Hawley, W., and Mar- ber consists of a lower clastic facies, a middle silty J. alogical, and isotoPic studies, con- vin, R., 1984, New K-Ar dates from basalts and the Beochemical lime mudstone facies, and an upper wackestone ceDts of ductile extension. and {Ar/3eAr dates. We evolution of the southern Rio Grande rift: Geological and packstone facies. The Ladron Member is also now recognize dozens of ignimbrite cauldrons and Society of America, Bulletin, v. 95, pp. 87-99. composed of three facies in ascending order: the 1982, gas andesite stratovolcanoes, results of partial melting Thompson, S., IJl, Oil and exploration wells in crinoidal grainstone facies separated into upper southwestern New Mexico; In Powers, R. B. (ed.), Geo- of, respectively, lower crust and upper mantle, in and lower parts bv the second facies, the "silver logic studies of the Cordilleran thrust belt: Rocky an extending lithosphere. Many mid-Tertiary ore pipe," a aigillaceous dolomitic mudstone named Mountain Association of Geologists, pp. 521-536. deposits are structurally controlled by cauldron after its association with lead and silver ore de- Thompson, S., III, Tovar R., J. C., and Conley, r. N., 1978, ring fractures (e.g., Mogollon), as are some mod- Oil and gas exploration in the Pedregosa basin: New Dosits; and the third facies, the BrYozoan - Pen- ern geothermal systems (e.g., Lightning Dock). Mexico Geological Society, Guidebook to 29th Field iremitesrcnoideus zone named after i conspicuous Some ore deposits are also genetically controlled Conference, pp. 337-342. fossil fauna. Basal limestones of the Caloso Mem- by cauldrons (e.g., Locations of volcanic Woodward, L. A., and DuChene, H. R., 1982, Tectonics Questa). ber have produced conodonts assigned to Patrog' and hydrocarbon potential the centers are critical for evaluating hydrocarbon po- of thrust-fold belt of nathus aariabilis, Polygnathus inornatus, ar.d southwestern New Mexico; in Powers, R. B. (ed.), Ceo- tential of certain sedimentary basins (e.g., Ped- Bispathodusaculeatus plumulus indicating upper sll- logic studies of the Cordilleran thrust belt: Rocky re8osa basin). Oligocene volcanism in New Mexico cata Zo^e (early Kinderhookian). The top of the Mountain Association of Geologists, pp. 4W-479. was a small part of an "ignimbrite flareup" that Caloso and lower Ladron Member have yielded buried .'1 million km'? of western Mexico and diagnostic conodont faunas from upper typicus to southwestern USA under hundred to thousands lower anchoralis-latusZones (early Osagean): of meters of ash and lava. It occurred during an Gruthodus cuneiformis, Gnathodus delicatus, Gnath' "extensional orogeny" that may have doubled the odus typicus M7 and M2. Polygnathus communis com- width of the Basin and Range province. It is a munis and Pseudopolygnathus oxypageous Ml and major geological phenomenon of a type now rec- M3. The Ladron Member produced conodonts ognized in many continents and periods (e.g., Per- throughout its thickness except for the "silver pipe" mian of Eurasia, of Sumatra and New Quaternary facies. The Ladron conodont fauna above the silver Zealand). pipe consists of elements diagnostic of upper fer- anus Zone (upper Osagean\: Cloghergnathus spp., Gruthodus texanus, " Spathognathodus"deflexus, and Sedimentary stratigraphy, and Taphrogrnthus aarians. The top of the Ladron is as- paleontologysession signed a latest Osagean-Meramecian age because it contains specimens belonging to the genus Ca- Putcupttaumt Eanlv OnoovrclAN LABECHIIDsrRo- auspnathus. The Caloso Member was determined MAToPoROIDAND ITS MouNDS, by D. V. LeMone, to be Kinderhookian by early workers based on Department of Geological Sciences, The Uni- brachiopod faunas. It was later reassigned a mid- versity of Texas at El Paso, El Paso, TX 79968 dle Osagean age based on the endothyrids Latien- Pulchrilamina is a principal component of the dothyra, Medioendothyra, and Tuberendothyra. The bioherms of the Early Ordovician McKelligon Can- Ladron Member was assigned to the late Osagean yon Formation of the El Paso Group. It was first based on brachiopod and blastoid assemblages, assigned in the late forties to the stromatolitic al- and the first occurrences of the foraminifers Pri gae. Later workers not only excluded it from the ylln, Pseudotaxis,andTbtrataxis. Results of this study stromatolitic algae but also from the stromatopo- have shown that the Caloso Member is early Kin- roids. In the late sixties it was classified as an un- derhookian at its base and early Osagean at the certain affinity organism assiSned to the hydrozoans top. The lower crinoidal facies of the Ladron Mem- of the coelenterate Dhvlum. Webbv, in a recent ber is early Osagean at its base to early Merame- excellent reanalysis of [h" tutott, piaces it clearly cian at the top. The disconformity between the two

Nm Mexico Geology Augrst 1989 59