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 series of 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.