BULLETIN 82 The Geology of the Jarilla Mountains, Otero County, New Mexico By Paul G. Schmidt and Campbell Craddock 1 9 6 4 STATE BUREAU OF MINES AND MINERAL RESOURCES NEW MEXICO INSTITUTE OF MINING & TECHNOLOGY CAMPUS STATION SOCORRO, NEW MEXICO NEW MEXICO INSTITUTE OF MINING & TECHNOLOGY E. J. Workman, President STATE BUREAU OF MINES AND MINERAL RESOURCES Alvin J. Thompson, Director THE REGENTS MEMBERS Ex OFFICIO The Honorable Jack M. Campbell ....................... Governor of New Mexico Leonard DeLayo ...................................... Superintendent of Public Instruction APPOINTED MEMBERS William G. Abbott ............................................................................. Hobbs Eugene L. Coulson, M.D. .............................................................. Socorro Thomas M. Cramer ........................................................................ Carlsbad Eva M. Larrazolo (Mrs. Paul F.) ........................................... Albuquerque Richard M. Zimmerly ...................................................................... Socorro For sale by the New Mexico Bureau of Mines and Mineral Resources Campus Station, Socorro, N. Mex.—Price $1.50 Contents Page ABSTRACT ............................................................................................................ 1 INTRODUCTION ............................................................................................... 2 Previous work .................................................................................................... 3 Acknowledgments ............................................................................................. 3 Physiography ...................................................................................................... 3 Climate and vegetation ..................................................................................... 4 Culture ................................................................................................................ 4 SEDIMENTARY ROCKS .................................................................................. 6 Pennsylvanian System ...................................................................................... 6 Lithology ......................................................................................................... 6 Fossils .............................................................................................................. 7 Correlations .................................................................................................... 8 Measured section of Pennsylvanian strata ............................................... 10 Permian System ............................................................................................... 12 Lithology ....................................................................................................... 13 Fossils ............................................................................................................ 14 Correlations ................................................................................................... 16 Measured section of Laborcita and Hueco formations ......................... 18 IGNEOUS ROCKS ........................................................................................... 22 Biotite syenodiorite ......................................................................................... 22 Leucorhyolite ................................................................................................... 22 Monzonite-adamellite ..................................................................................... 23 Orthoclase adamellite ..................................................................................... 26 Dikes ................................................................................................................. 27 Origin of the intrusive rocks ......................................................................... 28 Extrusive igneous rocks ................................................................................. 29 METAMORPHIC ROCKS .............................................................................. 31 Northern area .................................................................................................. 31 Southern area ................................................................................................... 35 STRUCTURAL GEOLOGY ............................................................................. 38 Emplacement of igneous rocks .................................................................... 38 Dikes ................................................................................................................. 40 Folds and faults in sedimentary rocks ........................................................... 40 Mechanics of deformation .............................................................................. 41 GEOMORPHOLOGY ...................................................................................... 43 ECONOMIC GEOLOGY ............................................................................... 43 Nonferrous metals, lode mines ...................................................................... 44 Iron mines .......................................................................................................... 46 Placer operations ............................................................................................... 47 Other ores .......................................................................................................... 48 Future of mining ............................................................................................... 48 Ground water .................................................................................................... 49 GEOLOGIC HISTORY ................................................................................... 51 Precambrian .................................................................................................. 51 Early Paleozoic ............................................................................................. 51 Late Paleozoic ............................................................................................... 52 Mesozoic ........................................................................................................ 52 Cenozoic ........................................................................................................ 53 REFERENCES ................................................................................................... 54 Illustrations FIGURES View of Jarilla Mountains .................................................................. Frontispiece 1. Index map showing location of Jarilla Mountains ................................ 2 2. Nodule of Chaetetes eximius? ....................................................................... 8 3. Thinsections of Chaetetes eximius? ............................................................. 9 4. Isopach map for Pennsylvanian ............................................................. 11 5. Specimens of limestone-pebble conglomerates ................................... 14 6. Silicified log of Dadoxylon in limestone .................................................. 15 7. Thinsection of biotite syenodiorite ........................................................ 23 8. Thinsection of monzonite-adamellite .................................................... 25 9. Thinsection of orthoclase adamellite ..................................................... 27 10. Thinsection of unmetamorphosed limestone ...................................... 32 11. Thinsection of metamorphosed limestone ........................................... 33 12. Reaction of chert nodule with limestone .............................................. 34 13. "Intrusive skarn" ....................................................................................... 37 PLATES 1. Geologic map of the Jarilla Mountains ....................................... In pocket 2. Tabulation of Igneous and Metamorphic Thin Section Data ..................................................................... In pocket Abstract The Jarilla Mountains are a small range of hills in southern New Mexico. The oldest exposed rocks are of Desmoinesian (Pennsylvanian) age; they consist of cherty gray limestone with minor sandstone and shale and are correlated with the Bug Scuffle Member of the Gobbler Formation. This unit is incompletely exposed but is 822 feet thick. Rocks of early to middle Wolfcampian (Permian) and probably latest Pennsylvanian age crop out in the northern and eastern parts of the range. This section is also incompletely exposed but consists of 1200 feet of interbedded thin limestone, siltstone, shale, and conglomerate correlated with the Laborcita Formation, and 1000 feet of gray lime- stone correlated with the Hueco Formation. The Laborcita Formation and possibly the Bug Scuffle Member and the Hueco Formation were deposited in the Orogrande basin, a local subsiding element on the marine shelf of late Pennsylvanian and early Permian time. Igneous stocks, dikes, and sills of probable Tertiary age were in- truded into the Paleozoic rocks. Three main stages of injection are recognized: a massive syenodiorite, followed by monzonite-adamellite of somewhat variable composition, then by adamellite with large ortho- clase phenocrysts. The monzonite-adamellite has in places undergone severe