New Mexico Bureau of Mines & Mineral Resources A DIVISION OF NEW MEXICO INSTITUTE OF MINING & TECHNOLOGY Geology of East Potrillo Mountains and vicinity, Dona Ana County, New Mexico by William R. Seager and Greg H. Mack Department of Geological Sciences, New Mexico State University, Las Cruces, New Mexico 88003 SOCORRO 1994 ii NEW MEXICO INSTITUTE OF MINING & TECHNOLOGY Daniel H. Lopez, President NEW MEXICO BUREAU OF MINES & MINERAL RESOURCES Charles E. Chapin, Director and State Geologist BOARD OF REGENTS Ex Officio Bruce King, Governor of New Mexico Alan Morgan, Superintendent of Public Instruction Appointed Diane D. Denish, President, 1992-1997, Albuquerque J. Michael Kelly, Secretary/Treasurer, 1992-1997, Roswell Lt. Gen. Leo Marquez, 1989-1995, Albuquerque Steve Torres, 1991-1997, Albuquerque Charles Zimmerly, 1991-1997, Socorro BUREAU STAFF ROBERT W. EVELETH, Senior Mining Engineer FANG LUO, Research Associate/Petroleum Engineer ORIN J. ANDERSON, Senior Geologist NANCY S. GILSON, Assistant Editor WILLIAM mcintosh Volcanologist/Geochronologist RUBEN ARCHULETA, Metallurgical Lab. Tech. AUGUSTUS K. ARMSTRONG, USGS Geologist DEBBIE GOERING, Staff Secretary CHRISTOPHER G. MCKEE, X-ray Facility Manager IBRAHIM GUNDILER, Senior Metallurgist VIRGINIA T. MCLEMORE, Senior Economic Geologist GEORGE S. AUSTIN, Senior Industrial Minerals Geologist WILLIAM C. HANEBERG, Assistant Director, NORMA J. MEEKS, Director of Publications Office ALBERT BACA, Maintenance Carpenter II Engineering Geologist BARBARA R. POPP, Biotechnologist JAMES M. BARKER, Assistant Director, JOHN W. HAWLEY, Senior Environmental Geologist, MARSHALL A. REITER, Senior Geophysicist Senior Industrial Minerals Geologist Manager, Albuquerque Office JACQUES R. RENAULT, Senior Geologist PAUL W. BAUER, Field Economic Geologist MATT HEIZLER, Geochronologist CINDIE A. SALISBURY, Cartographic Drafter II LYNN A. BRANDVOLD, Senior Chemist RON BROADHEAD, Assistant Director, LYNNE HEMENWAY, Computer Pub./Graphics Spec. SANDRA SWARTZ, Chemical Lab. Technician Senior Petroleum Geologist, CAROL A. HJELLMING, Associate Editor TERRY TELLES, Technical Secretary Head, Petroleum Section GRETCHEN K. HOFFMAN, Senior Coal Geologist REBECCA J. TITUS, Cartographic Supervisor KATHRYN G. CAMPBELL, Cartographic Drafter II GLEN JONES, Computer Scientist/Geologist JUDY M. VAIZA, Business Serv. Coordinator RITA CASE, Administrative Secretary (Alb. Office) PHILIP KYLE, Geochemist/Petrologist MANUEL J. VASQUEZ, Mechanic II STEVEN M. GATHER, Field Economic Geologist CHAMBERLIN, Field Economic Geologist ANN LANNING, Executive Secretary SUSAN J. WELCH, Manager, Geologic Extension Service RICHARD ANNABELLE LOPEZ, Petroleum Records Clerk NEIL H. WHITEHEAD, III, Petroleum Geologist THERESA L. LOPEZ, Receptionist/Staff Secretary MICHAEL WHITWORTH, Hydrogeologist RICHARD It CHAVEZ, Assist. Head, Petroleum Section DAVID W. LOVE, Senior Environmental Geologist MICHAEL W. WOOLDRIDGE, Scientific Illustrator RUBEN A. CRESPIN, Garage Supervisor NELIA DUNBAR, Analytical Geochemist JANE A. CALVERT LOVE, Editor JIRI ZIDEK, Chief Editor/Senior Geologist VIRGIL LUETH, Mineralogist/Economic Geologist ROBERT A. BIEBERMAN, Emeritus Sr. Petroleum Geologist SAMUEL THOMPSON III, Emeritus Sr. Petroleum Geologist FRANK E. KOTTLOWSKI, Emeritus Director/State Geologist ROBERT H. WEBER, Emeritus Senior Geologist Research Associates WILLIAM L. CHENOWETH, Grand Junction, CO WILLIAM E. KING, NMSU GLENN R. OSBURN, Washington Univ. RUSSELL E. CLEMONS, NMSU BARRY S. KUES, UNM ALLAN R. SANFORD, NMT CHARLES A. FERGUSON, Univ. Alberta, CAN MICHAEL J. KUNK, USGS JOHN H. SCHILLING, Reno, NV JOHN W. GEISSMAN, UNM TIMOTHY F. LAWTON, NMSU WILLIAM R. SEAGER, NMSU LELAND H. GILE, Las Cruces DAVID V. LEMONE, UTEP EDWARD W. SMITH, Tesuque CAROL A. HILL, Albuquerque SPENCER G. LUCAS, NMMNH&S JOHN F. SUTTER, USGS BOB JULYAN, Albuquerque GREG H. MACK, NMSU RICHARD H. TEDFORD, Amer. Mus. Nat. Hist. SHARI A. KELLEY, SMU NANCY J. MCMILLAN, NMSU TOMMY B. THOMPSON, CSU HOWARD B. NICKELSON, Carlsbad BEIDI YuE, NMT Graduate Students ROBERT APPELT DAVID ENNIS TINA ORTIZ ULVI CETIN VENKATA GANTI DAVID J. SIVILS DAN DETMER JOHN GILLENTINE Plus about 30 undergraduate assistants Original Printing Published by Authority of State of New Mexico, NMSA 1953 Sec. 63- 1 - 4 Printed by University of New Mexico Printing Services, August 1994 Available from New Mexico Bureau of Mines & Mineral Resources, Socorro, NM 87801 Published as public domain, therefore reproducible without permission. Source credit requested. 111 Contents ABSTRACT 5 INTRODUCTION 5 LOCATION, ACCESS, AND PHYSIOGRAPHY5 PREVIOUS WORK 5 PRESENT STUDY 6 ACKNOWLEDGMENTS 6 STRATIGRAPHY 6 SEDIMENTARY ROCKS 6 Permian rocks 6 Lower Cretaceous stratigraphy 8 Upper Cretaceous and/or lower Tertiary rocks 15 Santa Fe Group 15 Upper Pleistocene deposits 17 Uppermost Pleistocene to Holocene deposits 17 IGNEOUS AND VOLCANICLASTIC ROCKS 17 Rubio Peak Formation 17 Fine-grained intermediate-composition volcanic rocks 17 Mt. Riley-Mt. Cox volcanic domes 17 Other intrusive rocks 17 West Potrillo Basalt 18 Potrillo maar rim deposits 18 STRUCTURAL GEOLOGY 18 LARAMIDE FAULTS AND FOLDS 18 Middle to late Tertiary rotation of Laramide structure 20 Laramide models 20 MIDDLE TERTIARY LOW-ANGLE NORMAL FAULTS 21 Present fault geometry 21 Original geometry and nature of fault system 21 Direction and amount of extension 23 Possible transverse shear zone and oppositely tilted blocks 23 LATE TERTIARY HIGH-ANGLE NORMAL FAULTS 23 ECONOMIC GEOLOGY 24 REFERENCES 24 Table 1—Detrital modes of Lower Cretaceous sandstones and one sandy oolitic limestone 12 Figures 1—Map of East Potrillo Mountains and surrounding area iv 2—Generalized columnar stratigraphic section 7 3—Conglomerate member of Hell-to-Finish Formation 8 4—Clast size versus stratigraphic position in conglomerate member of Hell-to- Finish Formation 8 5—Hummocky cross-stratification in mottled siltstone member of Hell-to- Finish Formation 9 6—Crossbeds in sandstone member of U-Bar Formation 9 7—Rudistid pelecypods in rudistid limestone member of U- Bar Formation 10 8—Correlation chart for Lower Cretaceous sedimentary rocks 11 9—Overturned syncline at northern end of East Potrillo Mountains 19 10—Thrust-faulted, overturned syncline at northern edge of East Potrillo Mountains 19 11—Thrust faults and drag folds in eastern limb of syncline 19 12—Four models of thrusting for Laramide deformation 20 13—Cross sections of southern San Andres and southern Caballo Mountains 20 14—Map of low-angle normal faults in central part of East Potrillo Mountains in pocket 15—Normal-fault patterns after removal of west-southwest tilt in pocket 16—Diagram of speculative northeast-trending shear zone 23 Sheets 1—Geologic map of East Potrillo Mountains and vicinity in pocket 2—Geologic cross sections of East Potrillo Mountains and vicinity in pocket 3—Lower Cretaceous rocks of East Potrillo Mountains in pocket 5 Abstract—Situated just north of the Mexico border in south-central Dona Ana County, New Mexico, the East Potrillo Mountains area reveals important details about Laramide, middle Tertiary, and late Tertiary deformation in the region, as well as data on Permian, Lower Cretaceous, and Cenozoic stratigraphy. The oldest rocks exposed in the area are limestone, dolomitic limestone, and minor silty or sandy beds, at least 300 m thick, that are believed to be correlative with the middle Permian Yeso and San Andres Formations of south-central New Mexico. Disconformably above these, Lower Cretaceous shelf limestone, siltstone, sandstone, and conglomerate are approximately 570 m thick in the northern part of the range, thinning southward. These rocks are Aptian-Albian in age and correlative with the Hell-to-Finish and U-Bar Formations of southwestern New Mexico. Orthoquartzite, possibly correlative with the Sarten or Mojado Formations, crops out locally. Ce- nozoic rocks are poorly exposed around the margins of the East Potrillo Mountains and on the flanks of the Mt. Riley-Mt. Cox volcanic domes. They include Upper Cretaceous and/or lower Tertiary conglomerate and sandstone (possible correlatives of the Love Ranch-McRae Formations), middle Tertiary intermediate-composition lavas and domes (correlative in part with the Rubio Peak-Palm Park Formations), and upper Oligocene or lower to middle Miocene fanglomerate. The latter rocks are thought to have been deposited in a broad "early-rift" basin extending 25 km or more to the west of the East Potrillo Mountains. Quaternary alluvial fans, basin-floor deposits, maar-rim tuffs, basalt flows, and cinder cones mantle older bedrock across much of the map area. Altogether approximately 1,350 m of Permian, Lower Cretaceous, and Cenozoic rocks are exposed. Laramide, middle Tertiary, and late Tertiary structural features are exposed in the East Potrillo Mountains. Laramide thrust faults above overturned, thrust-faulted synclines trend N30°W ± 10°; drag folds indicate tectonic transport to the northeast. Erosion is not sufficiently deep to demonstrate whether thin-skinned overthrusting or rigid basement-block uplift has produced the observed shallow structure. Middle Tertiary deformation involved northeast or perhaps nearly north-south extension resulting in a system of closely spaced normal faults trending from N30°W to N60°W that cut Laramide structures. These faults presently dip from 0 to 50° (average approximately 30°) to the northeast or southwest. Original dips of major fault segments were as low as 20°-25° east and, this geometry as well as relationships