EagleNest-Granite Hillarea, Luna Gounty, New Mexico- a newlook at some old rocks byWilliam R. Seagerand Greg H. Mack,Earth Sciences Dept., New Mexico State University, Las Cruces, NM 88003 Introduction covered by seemingly endlessexpanses of ern Luna County approximately30 km (19 The western perimeter of the West Potrillo sand, and onlv here and there do isolated mi) northeastof Columbus (Fig. 1). Mountains is a remote, desolateland with hills of bedroci projectabove the sanddrifts. Perhapsbecause of their remoteness,these few roads, few visitors, and few outcrops. Two of the more prominent hills are Eagle hills are not well known, even though they Waterless and treeless, much of the area is Nest and Granite Hill. locatedin southeast- have been visited occasionallyby geologists for more than 80 years.Published accounts are varied, contradictory, and brief, al- though an excellentrecent M.S. thesis pre- senteddetails of the geologyof Granite Hill (Broderick, 1984).There seem to be long standing disagreementsamong previous Aden Hllls : !s J workers about the generalage and structure ,'t: \Provldence Cone' of rocks at EagleNest and Granite Hill. In 1915Darton portrayed EagleNest out- crops as Pennsylvanianin age, whereasthose at Granite Hill, one kilometer farther east, he consideredto be Tertiary "agglomerate" intruded by Tertiarygranite porphyry. Dane and Bachman(1965) concurred with the Ter- tiary-ageassignment of the graniteat Gran- Aden volcano INDEX MAP OF NEW MEXICO z l J ioTo .,:,,:l 1i + J1 Alsoin this issue ,,,i Indexto NMGS guidebook papers on hydrology and related topics p.8 TommyLee Finnell(1923-1989) p. 14 East Potllllo NMGS spring meeting p. 14 tinorintalns Noticeof transferof NMBMMRfossil cephalopodspecimens ro coDiF figured by RousseauH. W MEXICO To El Paso Flower p.15 Mountaln MEXTCO Potrlllo maar NMGS 1989 abstracts p.16 Service/News p.18 SCALE NMBMMRMineralMuseum 5 10 mlles Fund p. 19 15 kllometers I Staff notes p.20 ?,. nto Gomingsoon SugariteCanyon State Park Mineraland mineral-fuelproduction ]!Pemox No. 1 Moyotes Y 't6,217', activitiesin NewMexico during Prscambrian 1988 Stratigraphyand structure of the FIGURE l-Location map of Eagle Nest-Cranite Hill area (from Seager,1989) The area of Fig. 9 is KlondikeHills shown by the rectangle. FIGURE 2-Eagle Nest ridge capped by cliff-forming Permian carbonates FIGURE 3-Gastropods and nautiloid in Colina (uppermost Hueco) For- above overtumed Lower Cretaceousstrata. Laramide thrust truncates car- mation at EagleNest. bonates at base of cliff. View looks northwestward. ite Hill but consideredthe sedimentary rocks basement-coreduplift of the AncestralRock- cambrian basement of south-central New at both Granite Hill and Eagle Nest to be ies system,the Burro-Florida-Moyotes uplift Mexico. Recognizing this, Broderick (1984) Cretaceous.Hoffer (1976)agreed with Dane resulted in erosion of various thicknessesof and Broderick et al. (1986)assigned the gran- and Bachman's(1965) age assignments of the pre-mid-Wolfcampian strata and, perhaps ite a Precambrian age, a correlation with which rocks in both outcrops. However, in 1981 locally, nondeposition of Pennsylvanian we concur. Hoffer and Hoffer reported the sedimentary strata. According to Broderick (1984) the granite rocks at Granite Hill to be Early Cretaceous, In Early Cretaceous time the Eagle Nest contains 55-607o anhedral orthoclase micro- and in 1983Hoffer and Hoffer assigned the areawas part of the rift basin of Mack et al. perthite, and 35-40Vo anhedral quartz. Ser- entire section at EagleNest to the Early Cre- (1986; 1988) that filled with arkosic marine icite, pyrite, and hematite are common taceousand consideredit to be right sideup. and nonmarine sediments. Granitic rift secondary minerals. The granite contains ap- They recoveredLower Cretaceousfossils from shoulders along the northern and north- proximately 73VoSiO, (Broderick et al., 1986). part of the Eagle Nest section.Mack et al. eastern margins of the rift furnished much (1986)also believed that the Lower Creta- of the arkosicsediment. ceousstrata at EagleNest were right side up In Laramidetime (LateCretaceous to early but additionally recognizedthat an upper, Tertiary) both Eagle Nest and Granite Hill New AAexnc@ massive, cliff-forming limestone was sepa- were part of a zone of thrusting, basement- rated from lower beds by a low-angle fault. coreduplifts, and basinformation extending Broderick (1984)mapped Granite Hill and from west Texasnorthwestward into south- GEOLOGY considered the granite to be Precambrianin easternArizona. Controversycontinues over . SGaenGeand Service age, overlain by Lower Cretaceousarkosic the dominant style of deformation in this re- Volums 12, No. 1, February 1990 strata; this view was repeated by Broderick gion: regional overthrusting or foreland-style Editor:Carol A. Hiellming Draftingassistance: Rebecca Titus et al. (1986). basement-blockuplifts (e.9. Drewes, 1978; Publishedquarterly by Our interpretations of Granite Hill and Davis, 7979;Seager and Mack, 1985). New Mexico Bureauof Mines and Mineral Resources Eagle Nest differ significantly from previ- Finally, both Eagle Nest and Granite Hill a division of New Mqir Irutitute of Mining & llthnd%y ously published accounts.In this report we BOARD OF REGENTS are part of the late Tertiary CamelMountain Ex Officio presentevidence that Lower Cretaceousstrata fault block (Seager,1989), one of the many Garrey Canuthers, Gopernorof Nrut Menco at EagleNest are upsidedown and that they fault-block uplifts in the southern part of the Alan Morgan, Sulvilltendent of Publiclnstruclnn Appointed are overlain by a thrust sheetof Permian car- Rio Granderift. Tilted eastward15 to 20 de- Lenton Malry, Pra ,1985-1991,Albuquerque bonates.Right-side-up sedimentary strata at grees, the Camel Mountain fault block is Robert O Andemo^, S{ lTrcas,1 7-1993, Roswell Lt Cen Leo Marquez, 1989-1995,Alhuquerquc Granite Hill, which nonconformably overlie nearly buried by Quaternary sand and ba- Carol A Rymer, M D 1989-1995,Albuqueryuc Precambrian granite and are intruded by salt. Stmcturally high hills such as EagleNest Steve Torres, 1967-1991,Allruqileryut middle Tertiary(?) dikes, are latest New Mexico lnstitute of Mining & Technology Creta- and Granite Hill, as well as the prominent Pr$idqt laurence H littman ceousor early Tertiary in age.The two hills, Quaternary fault scarp along the westem New Mexico Bureau of Mines & Mineral Resures Eagle Nest and Granite Hill, are separated margin of the block (Camel Mountain es- Directornnd StntcGeolosist Frank E Kottlowski M. Robe?tson by a major fault zone whose geometry and carpment,Fig 1.), the geo- Aswiate Dircctol James constitutesome of Subrriptions: lssued quarterlt February, Mat AuSust, history of movement are somewhat specu- logic evidencefor the presenceand extentof November; subsription prie $6 0O/elendar year lative. These conclusionswere reported by the fault block. Editorialmtla: Articles submitted for publication should Seager(1989; in press). be in the editor's hands a minimum of five (5) months before date of publication (February,Mat August, or Stratigraphy November) and should be no longer than 20 tyPewrit- Regional setting ten, double-spacedpages All rientific papere will be reviewed by at least two people in the appropriate field The EagleNest-Granite Hill areais part of Precambrian rocks of study AddressinquiriG to CarolA Hiellming, Editor ot Nru Mexia Geology,New Mexico Bureau of Mines & four tectonic elements of different ages. It Brown to grayish-orangegranite crops out Mineral Resources,Saorc, NM 87801 may be within the Burro-Florida-Moyotes along the western base of Granite Hill. Al- Publshed as public tloruifl, ther4ore reprcdrciblewithout pa- uplift of mid-Wolfcampanian age (Kottlow- though interpretedas Tertiary in ageby some mission Sourcecredit requ5ted ski 1950; 1963; Navarro and Tovar, 1975; previous workers (Darton, 1916;Dane and Circulation:|,ffi Greenwoodet al.,1977;Wilson and fordan, Bachman, 1965; Hoffer, 1976), the coarse- Prirfer: University of New Meico Printing Plant 1988;Thompson et al. 1978;Seageg1989). A grained granite is typical of much of the Pre- 2 February 1989 Nal Mexico Geology FIGURE4-Nautiloid in Colina (uppermost Hueco) Formation at EagleNest. FIGURE S-Cephalopods in Colina (uppermost Hueco) Formation at Eagle Nest (below scale). The mineral composition, silica content, with the Yeso Formation and uppermost beds cates a Paleozoicage for the carbonate sec- and very coarse grained texture is consistent of the Hueco Formation of south-central New tion (D. V. LeMone, personalcommunication, with other Precambrian granitic rocks of Mexico (e.g., Kottlowski et al., 1956)and with 1989).The strata have none of the lithologic south-central New Mexico. It is strikingly the Epitaph and Colina Formations of south- characteristicsof lower to middle Paleozoic dissimilar in terms of texture and compo-si- western New Mexico (Zeller, 1965). strata in the region and are almost certainly tion to Tertiary-age stocks in the Tres Her- Cliff-forming, thick-bedded limestone of Permian. Rock types and fauna are most manas, Organ and |arilla Mountains, and the Colina (uppermost Hueco) Formation nearly like those seen in uppermost Hueco Cooke's Range. It also is dissimilar to the overlies the thrust fault. These massive, dark- strata in the El Paso area (D. V. LeMone, lower Paleozoic syenitic stocks of the Florida gray to blue-gray limestone beds are at least personal communication) and like those re- Mountains (Evans and Clemons, 1988).