Geologic map of the San Antonio Mountain area, northern New Mexico and southern Colorado

Turner, Kenzie J.- kturner@usgs.gov, Thompson, Ren A.-, Cosca, Michael A.- mcosca@usgs.gov, Shroba, Ralph R.- rshroba@usgs.gov · 2018 · Report

The geologic map of the San Antonio Mountain area in northern New Mexico and southern Colorado is located along the west-central part of the San Luis Valley.

This 2018 document, Geologic map of the San Antonio Mountain area, northern New Mexico and southern Colorado, is preserved in the Mountain Man Mining Library for research and reference. Original source: pubs.usgs.gov.

? ? ? ? ? ? ? ? ? ? ? ? ? ? Integrated Age (Ma)±error (2σ) MSWD Age (Ma)±error (2σ) MSWD n/(total) Age (Ma)±error (2σ) Age (Ma)±error (2σ) MSWD RGR-421 Tbx Xenocrystic basalt of Hill 8489 2.12±0.02 2.12±0.02 2.12±0.02 19/(19) 303±3 2.13±0.03 whole rock RGR-009 Tlx Xenocrystic trachyandesite 2.78±0.05 2.78±0.05 2.87±0.09 12/(13) 291±10 2.82±0.06 whole rock RGR-017 Trx Basaltic trachyandesite of Red Hill 2.81±0.04 2.81±0.04 2.84±0.05 15/(15) 294±15 2.83±0.04 whole rock RGR-012 Tsr Rio San Antonio member of Servilleta Basalt 2.94±0.12 2.94±0.12 2.9±0.5 12/(13) 300±30 2.95±0.15 whole rock RGR-010 (no. 1) Tau Upper dacite of San Antonio Mountain 2.9±0.3 3.0±0.2 2.9±0.3 11/(13) 299.6±1.7 3.0±0.2 whole rock RGR-010 (no. 2) Tau Upper dacite of San Antonio Mountain 2.9±0.2 14/(16) 298±3 2.84±0.16 whole rock RGR-285 Tau Upper dacite of San Antonio Mountain 3.14±0.08 13/(18) 286±8 3.06±0.11 whole rock RGR-035 Tau Upper dacite of San Antonio Mountain 3.13±0.09 14/(15) 280±30 3.08±0.07 whole rock RGR-231 Tan Lower trachyandesite of north San Antonio Mountain 2.91±0.07 3.45±0.15 13/(18) 272±7 2.91±0.07 whole rock RGR-133 Tas Lower trachyandesite of south San Antonio Mountain 3.0±0.04 3.02±0.06 15/(15) 290±20 3.0±0.04 whole rock RGR-235 Tsw Wissmath Craters member of Servilleta Basalt 3.20±0.05 3.20±0.05 3.37±0.12 18/(18) 295±4 3.29±0.11 whole rock RGR-013 Tsm Pinabetal Mesa member of Servilleta Basalt 3.33±0.14 3.33±0.14 3.5±0.3 10/(13) 288±18 3.33±0.14 whole rock RGR-374 Trl Basaltic andesite of Rio de los Piños 3.39±0.04 3.39±0.04 3.49±0.10 18/(18) 293±3 3.32±0.09 whole rock RGR-149 (no. 1)

Basaltic andesite of Rio de los Piños 3.47±0.09 3.3±0.3 9/(16) 307±19 3.39±0.09 whole rock RGR-149 (no. 2)

Basaltic andesite of Rio de los Piños 3.4±0.2 9/(16) 303±9 3.52±0.10 whole rock RGR-131 Tca Basaltic andesite of Cañada los Ranchos 3.29±0.05 3.16±0.12 6/(15) 304±4 3.28±0.09 whole rock RGR-221 Tca Basaltic andesite of Cañada los Ranchos 3.32±0.06 3.32±0.06 3.48±0.11 15/(17) 296±3 3.39±0.10 whole rock RGR-015 Tcx Basaltic trachyandesite of Los Cerritos de la Cruz 3.36±0.05 3.36±0.05 3.41±0.06 13/(15) 280±30 3.38±0.05 whole rock Tna Rhyolite of No Agua Peaks* 3.88±0.06 3.88±0.06 glass Tna Rhyolite of No Agua Peaks* 4.1±0.03 4.1±0.03 glass/feldspar RGR-226 Tcb Basalt of Chino Peak 4.24±0.10 4.3±0.7 18/(18) 295±8 4.0±0.3 whole rock RGR-014

Basalt of Chino Peak 4.31±0.07 4.31±0.07 4.1±0.4 14/(15) 304±18 4.27±0.09 whole rock RGR-146 Tb Basalt and basaltic andesite of Valdez Tank (basaltic andesite) 4.37±0.05 4.33±0.11 15/(15) 301±7 4.36±0.06 whole rock RGR-371 Tbc Basalt and basaltic andesite of Valdez Tank (basaltic andesite) 4.37±0.03 4.32±0.03 4.37±0.03 15/(17) 278±3 4.23±0.07 whole rock RGR-357 Tb Basalt and basaltic andesite of Valdez Tank (silicic basalt) 4.25±0.02 4.30±0.03 16/(17) 274±6 4.24±0.05 whole rock RGR-362 Tb Basalt and basaltic andesite of Valdez Tank (silicic basalt) 4.36±0.04 4.24±0.03 4.36±0.04 15/(17) 200±20 4.22±0.07 whole rock RGR-364 Tb Basalt and basaltic andesite of Valdez Tank (basalt) 4.36±0.05 4.36±0.07 17/(18) 298±5 4.36±0.05 whole rock RGR-036 Tlb Basalt of Lucero Lakes 4.55±0.07 4.55±0.07 4.60±0.11 15/(15) 295±4 4.54±0.08 whole rock RGR-378 Thl Hinsdale Formation, alkaline basalt to basaltic trachyandesite 20.61±0.07 20.61±0.07 20.94±0.24 whole rock RGR-148 Tht Hinsdale Formation, tholeiitic basalt 25.54±0.24 25.11±0.35 whole rock RGR-456 Tht Hinsdale Formation, tholeiitic basalt 25.67±0.20 25.67±0.20 25.51±0.49 whole rock RGR-033 Thb Hinsdale Formation, basaltic andesite 26.05±0.21 26.05±0.21 25.47±0.35 whole rock RGR-494 Thp Hinsdale Formation, porphyritic basalt 26.17±0.07 26.17±0.07 26.17±0.07 whole rock RGR-466 Ttc Chiquito Peak Tuff 28.94±0.31 28.94±0.31 sanidine ages reported by Dickens (2007) and represent weighted mean of multiple ages determined on grains of glass and (or) feldspar. Reported ages calculated relative to Fish Canyon Tuff neutron flux monitor age of 28.02 Ma. Ages shown here are adjusted by a factor of 1.006 (determined by 28.201/28.02 1.006) to account for different flux monitor age. indicates sample location outside map area. Material analyzed Plateau Isochron Single grain total fusion Sample ID Easting Northing Map Unit Symbol Map Unit Name Preferred unit age (Ma)±error (2σ) Table 2. Summary of 40Ar/39Ar geochronologic ages. [Table shows ages determined using plateau, inverse isochron, and integrated age calculation methods for samples analyzed by step-heating methods, and total fusion ages for samples measured by single crystal analysis. Ages interpreted and preferred by the authors are shown in bold for each sample. Preferred unit age indicates preferred age for unit when multiple samples were analyzed. Northing and Easting values in meters using North American Datum 1983 (NAD83), Universal Transverse Mercator (UTM) zone 13. MSWD, mean square of weight deviates; n, number of heating steps used in calculation of isochron age; Ma, million years; σ, sigma; λ, lamda; Ar, argon; K, potassium. Ages determined using Fish Canyon Tuff as a neutron flux monitor with an age of 28.201±0.023 Ma (1σ) (Kuiper and others, 2008), 40K decay constants of λ (5.463±0.214) × 10-10yr-1 and λ(e-) (0.580±0.007) ×10-10yr-1 (Min and others, 2000), and a 40Ar/36Ar ratio of trapped argon equal to atmospheric value of 298.56 (Lee and others, 2006). Analytical data for all samples can be accessed at ://doi.org/10.5066/F72N51M5 (Turner and others, 2018)] 40Ar/36Ar no plateau no plateau no plateau no plateau no plateau no plateau Insufficient dispersion of step-heating analyses to calculate Insufficient dispersion of step-heating analyses to calculate Insufficient dispersion of step-heating analyses to calculate Insufficient dispersion of step-heating analyses to calculate Insufficient dispersion of step-heating analyses to calculate Map Unit Tbx Tlx Trx Tsr Tau Tan Tas Tse Tsw Tsw Tls Tsm Tcx Tcx

Tca Tca Tna Tcb Tcb Tbc Tb Tb Tlb Thl Tht Thb Tha Thp Sample ID RGR-421 RGR-009 RGR-017 RGR-012 RGR-285 RGR-231 RGR-133 RGR-134 RGR-235 RGR-438 RGR-135 RGR-492 RGR-229 89T019 RGR-149 RGR-496 RGR-495 RGR-130 RGR-226 RGR-534 RGR-371 RGR-254 RGR-362 RGR-036 RGR-248 RGR-250 RGR-230 RGR-228 RGR-494 Easting Northing XRF Lab SiO2 TiO2 Al2O3 Fe2O3 MgO CaO Na2O K2O P2O5 MnO Cr2O3 n.m. n.m. n.m. n.m. n.m. n.d. n.m. n.m. n.m. n.m. n.m. n.m. n.m. n.m. n.m. n.m. n.m. n.m. n.m. n.m. n.d. n.m. Total LOI n.d. n.d. n.d. n.d. Miocene and Oligocene Table 1. Representative major element geochemical analyses for select samples from the map area [These are representative samples only and are not placed on the map. Northing and Easting values in UTM meters using North American Datum 1983 (NAD 83), Universal Transverse Mercator (UTM) zone 13. X-ray fluorescence (XRF) Laboratories: (1) U.S. Geological Survey, Denver, Colo.; (2) SGS, Ontario, Canada; (3) Washington State University, Pullman, Wash. Oxide abundance reported as weight percent oxide; abbreviations include: n.d., not detected; n.m., not measured; LOI, loss on ignition. All major element and trace element compositions for the complete set of samples can be found at ://doi.org/10.5066/F72N51M5 (Turner and others, 2018)] Pliocene Indicates sample location outside map area. Figure 2. Landsat 7 satellite image (30-m meter [m] band 7-4-2 merged with 15-m band 8) acquired on Oct. 14, 1999, of the area surrounding San Antonio Mountain (image clip from Sawyer and others, 2004). Image shows selected geographic names referred to in unit descriptions. Yellow line indicates the map boundary. Red asterisks indicate known or inferred (with query) vent locations associated with the Taos Plateau volcanic field. At least two additional vent locations are probably concealed beneath dacite lavas at San Antonio Mountain. Figure 1. Shaded-relief index map of the southern and central parts of the San Luis Basin and adjacent areas showing the outline of the San Antonio Mountain area map boundary and the outline of Rio Grande del Norte National Monument. Abbreviations for labeled physiographic and volcanic features include: CA, Cerro del Aire; CO, Cerro de la Olla; CC, Cerro Chiflo; CM, Cerro Montoso; CT, Cerro de los Taoses, GM, Guadalupe Mountain; LM, Los Mogotes; NA, No Agua Peaks; SM, San Antonio Mountain; TO, Tres Orejas; and UM, Ute Mountain. Figure 3. Total alkali-silica classification diagram of Le Bas and others (1986). Major element compositions are recalculated to volatile-free with ferric/ferrous iron ratios as proposed by Middlemost (1989). Grayed out field names did not have any rocks that fell within that respective category. Shaded-relief base generated from U.S. Geological Survey 10-m Digital Elevation Model (DEM) data from the National Elevation Dataset, accessed June 2016 at ://ned.usgs.gov/. Rio Grande del Norte National Monument boundary simplified from U.S. Bureau of Land Management at ://www.blm.gov/nm/st/en/prog/blm_special_areas/national_monuments/rio_grande_del_norte. on Aug. 12, 2013. EXPLANATION Xenocrystic basalt of Hill 8489 (Tbx) Xenocrystic trachyandesite (/Tlx) Basaltic trachyandesite of Red Hill (/Trx) Rio San Antonio member of Servilleta Basalt (Tsr) Upper dacite of San Antonio Mountain (Tauc/Tau) Lower trachycandesite of north San Antonio Mountain (Tan) Lower trachyandesite of south San Antonio Mountain (Tas) Pinabetal Mesa member of Servilleta Basalt (Tsm) Basaltic andesite of Rio de los Piños (/Trl) Basaltic andesite of Cañada los Ranchos (Tca) Section 35 member of Servilleta Basalt (Tse) Wissmath Craters member of Servilleta Basalt (Tsw) Xenocrystic basaltic andesite of La Segita Peaks (Tls) Basaltic trachyandesite of Los Cerritos de la Cruz (/Tcx) Rhyolite of No Agua Peaks (Tna) Basalt of Chino Peak (/Tcb) Basalt and basaltic andesite of Valdez Tank (Tbc/Tb) Basalt of Lucero Lakes (Tlb) Alkaline basalt to trachybasalt, Hinsdale Formation (Thl) Tholeiitic basalt, Hinsdale Formation (Tht) Trachyandesite, Hinsdale Formation (Tha) Basaltic andesite, Hinsdale Formation (Thb) Porphyritic basalt, Hinsdale Formation (Thp) Na2O + K2O (weight percent) SiO2 (weight percent) Basalt Picrobasalt Basaltic andesite Andesite Dacite Rhyolite Trachydacite Trachyte Trachyandesite Tephrite/Basanite Phonotephrite Tephriphonolite Phonolite Trachybasalt Basaltic trachyandesite 6 MILES 6 KILOMETERS Rio Rio San San Antonio Antonio Rio Rio Piños Piños San Antonio Mountain San Antonio Mountain San Antonio Mountain Wissmath Craters Wissmath Craters Wissmath Craters La Segita Peaks La Segita Peaks La Segita Peaks No Agua Peaks No Agua Peaks No Agua Peaks Broke Off Mountain Broke Off Mountain Broke Off Mountain Laguna Larga Laguna Larga Laguna Larga Pinabetal Mesa Pinabetal Mesa Pinabetal Mesa Lucero Lakes Lucero Lakes Los Cerritos de la Cruz Los Cerritos de la Cruz Los Cerritos de la Cruz Chino Peak Chino Peak Chino Peak Dike with cinders Dike with cinders Dike with cinders Red Hill Red Hill Red Hill Bighorn Peak Bighorn Peak Bighorn Peak Valdez Tank Valdez Tank COLORADO NEW MEXICO 106° 106°07'30" 37° 36°52'30" 36°45' Ortiz de los de los EXPLANATION Vent location—Identity or existance inferred where queried Base from Landsat 7 satellite image, October 14, 1999 30 meter band 7-4-2 merged with 15 meter band 8 37°00' 36°30' 106°00' 105°30' RIO GRANDE DEL NORTE NATIONAL MONUMENT River River River Grande Grande Grande Red Red Red Rio Rio Rio San San San Antonio Antonio Antonio Rio Rio Rio de los de los de los Pinos Pinos Pinos Rio Rio Rio San San San Antonio Antonio Antonio Rio Rio Rio Rio Rio Rio Grande Grande Taos Tres Piedras Questa San Luis Antonito UM UM CO CO CM CM CT CT TO TO GM GM SAN LUIS HILLS T U S A S M O U N TA I N S S O U T H E A S T S A N J U A N M O U N TA I N S SANGRE SAN LUIS BASIN Costilla Plain COLORADO NEW MEXICO San Pedro Mesa SM SM LM LM CA CA NA NA TAOS PLATEAU DE CRISTO MOUNTAINS 15 KILOMETERS 15 MILES Map area Red Hill. View to west of the quarry at Red Hill where cinder deposits of basaltic trachyandesite of Red Hill (unit ) are being quarried. Photograph by K. Turner, May 3, 2016. Dike. Looking west across the canyon of the Rio San Antonio where a dike cross-cuts flows of basaltic andesite of Cañada los Ranchos (unit Tca). Above the dike are flows of the Pinabetal Mesa member of the Servilleta Basalt (unit Tsm) that are not cut by the dike. The white line indicates approximate contact between the dike and surrounding flows. The dike is petrographically and compositionally similar to the basaltic andesite of Cañada los Ranchos and is surrounded by loose cinder deposits near the rim of the canyon. Photograph by K. Turner, September 19, 2012. Esquibel member. Cobble-rich facies of the Esquibel Member of the Los Piños Formation (unit Tle) taken north of the Rio de los Piños. These beds are locally dipping eastward up to 10 degrees. In upper right is the valley incised by the Rio de los Piños with the northern Tusas Mountains in the background. The backpack in the center of the photograph is about 0.75 meter in length. Photograph by K. Turner, June 12, 2013. Rio de los Piños. View toward the east looking down the Rio de los Piños. The lowest distinctive ledge above the trees is held up by tholeiitic basalt flows of the Hinsdale Formation (unit Tht). The flat, upper surface north of the river (right side of the photograph) is held up by basaltic andesite flows of Rio de los Piños (unit Trl) and Cañada los Ranchos (unit Tca). The slope forming unit intercalated with the lava flows is the Esquibel Member of the Los Piños Formation (unit Tle). Photograph by K. Turner, June 11, 2011. Lava tube. View into the collapsed ceiling of a lava tube in flows of the Pinebetal Mesa member of the Servilleta Basalt (Unit Tsm). Sediments and fossils are preserved in the lava tube that are older than 0.85 Million year (Rogers and others, 2000). Fossils were excavated from sediments preserved in the lava as early as the 1950s (Rogers and others, 2000). Photograph by K. Turner, July 21, GQ-1750 (Thompson and Lipman, 1994b) GQ-1749 (Thompson and Lipman, 1994a) MF-1451 (Manley, 1982a) SIM 3417 (Turner and others, 2018) 106° 106°7'30" 106°15' 105°52'30" 105°45' 37° 37°07'30" 36°45' 36°37'30" 36°52'30" COLORADO NEW MEXICO PINABETOSO PEAKS LA SEGITA PEAKS NE CERRO DE LA OLLA TRES PIEDRAS NE TRES PIEDRAS MULE CANYON BURNED MOUNTAIN KIOWA HILL LOBATOS ANTONITO FOX CREEK LA SEGITA PEAKS BROKE OFF MOUNTAIN LOS PINOS SAN ANTONIO MOUNTAIN BIGHORN PEAK Index map showing the map area in yellow with dashed red outline and adjacent 7.5' U.S. Geological Survey (USGS) quadrangles. For USGS publications, the publication number, author, and year published are shown. Gray area indicates geologic mapping by Eppler (1976). 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Sturchio, N.C., Pierce, K.L., Murrell, M.T., and Sorey, M.L., 1994, Uranium-series ages of travertines and timing of the last glaciation in the northern Yellowstone area, Wyoming-Montana: Quaternary Research, v. 41, p. 265−277. Thompson, R.A., and Lipman, P.W., 1994a, Geologic map of the San Antonio Mountain quadrangle, Rio Arriba County, New Mexico: U.S. Geological Survey Geologic Quadrangle Map GQ-1750, 1:24,000 scale. [Available at ://.usgs.gov/Prodesc/proddesc_1235.htm.] Thompson, R.A., and Lipman, P.W., 1994b, Geologic map of the Los Pinos quadrangle, Rio Arriba and Taos Counties, New Mexico, and Conejos County, Colorado: U.S. Geological Survey Geologic Quadrangle Map GQ-1749, 1:24,000 scale. [Available at ://.usgs.gov/Prodesc/proddesc_1234.htm.] Thompson, R.A., Shroba, R.R., Machette, M.N., Fridrich, C.J., Brandt, T.R., and Cosca, M.A., 2015, Geologic map of the Alamosa 30'×60' quadrangle, south-central, Colorado: U.S. Geological Survey Scientific Investigations Map 3342, 23 p., scale 1:100,000, accessed Jan. 14, 2016, at ://pubs.er.usgs.gov/publication/sim3342. Turner, K.J., Thompson, R.A., Cosca, M.A., Shroba, R.R., Chan, C.F., and Morgan, L.E., 2018, Data release of geospatial map database, argon geochronology and geochemistry data for geologic map of the San Antonio Mountain area, northern New Mexico and southern Colorado: U.S. Geological Survey data release, ://doi.org/10.5066/F72N51M5. U.S. Geological Survey Geologic Names Committee, 2018, Divisions of geologic time-Major chronostratigraphic and geochronologic units: U.S. Geological Survey Fact Sheet 2018-3054, 2 p., accessed September 17, 2018 at ://doi.org/10.3133/fs20183054. Whitson, D., 1982, Geology of the No Agua deposit at No Agua Peaks, New Mexico, in Austin, G.S., compiler, Industrial rocks and minerals of the Southwest: New Mexico Bureau of Mines and Mineral Resources, Circular 182, p. 89-95. Zimmerer, M.J., and McIntosh, W.C., 2012, The geochronology of volcanic and plutonic rocks at the Questa caldera: Constraints on the origin of caldera-related silicic magmas: Geological Society of America Bulletin, v. 124, p. 1394-1408. INTRODUCTION This map summarizes the geology for the San Antonio Mountain area of northern New Mexico and approximately 10 square kilometers (km2) of southern Colorado. This geologic investigation was carried out with support from the U.S. Geological Survey (USGS) National Cooperative Geologic Mapping Program between 2012 and 2016. The map area is physiographically on the western edge of the Taos Plateau within the southern San Luis Valley. The San Luis Valley is the geomorphic expression of the San Luis Basin, an extensional basin of the northern Rio Grande rift. Approximately half of the map area is within the Rio Grande del Norte National Monument (fig. 1). The map area encompasses two full 7.5' quadrangles (see quad index on sheet); the San Antonio Mountain and Los Pinos, and three partial 7.5' quadrangles: Bighorn Peak, Pinabetoso Peaks, and La Segita Peaks. The partial quadrangles (Pinabetoso Peaks and La Segita) are included to the east to fully encompass the San Antonio Mountain volcano, and in the northwest area (Bighorn Peak) to include Pliocene volcanic rocks related to the Taos Plateau volcanic field. Previous work in the area included small-scale maps by Bingler (1968), Butler (1971), and Manley and others (1987). More detailed mapping included the Bighorn Peak 7.5' quadrangle (Manley, 1982a), San Antonio Mountain area (Eppler, 1976); San Antonio Mountain 7.5' quadrangle (Thompson and Lipman, 1994b), and Los Pinos 7.5' quadrangle (Thompson and Lipman, 1994a). Deposits within the map area record volcanic, sedimentary, and tectonic processes over the last ~33 million years (m.y.). Oldest exposed deposits include Oligocene volcanic rocks associated with the southeast San Juan Mountains locus of volcanism within the Southern Rocky Mountains volcanic field. The Southern Rocky Mountains volcanic field is a composite mid-Tertiary volcanic field that covered parts of southern Colorado and northern New Mexico (Lipman, 2007), of which the San Juan Mountains are one locus of volcanism. Overlying deposits of the Southern Rocky Mountains volcanic field are volcaniclastic sedimentary rocks interbedded with predominantly basaltic lava flows of Oligocene to Miocene age. Basalt to rhyolite volcanic rocks of the Pliocene to Pleistocene Taos Plateau volcanic field unconformably overlie Oligocene to Miocene volcanic and sedimentary deposits. Superposed on the Tertiary deposits are Pleistocene to Holocene alluvial and colluvial deposits. North-to northwest-trending faults displace rocks within the map area. Faults are assumed to have normal, chiefly dip-slip, displacement but are identified with varying degrees of certainty as their recognition within the map area is complicated by generally small displacements in young, brittle volcanic deposits that often lack an obvious fault scarp or in the poorly indurated Los Pinos Formation deposits that rarely preserve a prominent fault scarp and may not be exposed sufficiently to document offset. Normal faults attributed as 'accurate' or 'approximately located' were observed displacing map units where 'approximately located' indicates a lower degree of spatial certainty. Normal faults attributed as 'inferred' are associated with distinct topographic lineaments and cutoff ledges but lack direct observation of displaced lava flows. Magnitude of deformation is broadly correlative with age of the deposits inasmuch as Oligocene to Miocene rocks display a greater degree of fault displacement and east tilting than Pliocene volcanic rocks. Bedding attitudes recorded during field investigation are sparse as a result of (1) poorly indurated and poorly exposed Los Pinos Formation; (2) lava flows that are sufficiently magnetic to deflect a compass arrow; and (3) lava flows that rarely preserve a measurable surface representative of the attitude of the flow. Within the map area, faults displace Oligocene to Miocene deposits 10-30 meters (m) with generally down-to-west offset, and the units dip eastward 3-7 degrees. However, west of the map area fault displacement is as much as 200 m down-to-west along the west side of Broke Off Mountain (fig. 2) (Manley, 1982b). Pliocene volcanic rocks exhibit shallower eastward dips inferred primarily from the slope of upper lava flow surfaces that dip eastward 1-3 degrees. However, proximal to vent areas, dips increase and dip in all directions away from the vent. Fault offset of Pliocene volcanic rocks is likely underrepresented on the map due to cover by unconsolidated surficial deposits, but exposed faults displace Pliocene lavas less than 5 m. The magnitude of post-emplacement tilting of Pliocene lava flows is difficult to constrain because the lavas are interpreted to have flowed predominantly down an east-dipping paleoslope. Flow directions shown on the map are inferred from vent locations and topographic expression, and display predominantly easterly flow directions. Geology was mapped using a combination of remotely sensed data and field investigation. Natural-color, 1-m resolution imagery gathered by the U.S. Department of Agriculture National Agricultural Imagery Program (NAIP) from 2009 and 2014 for Rio Arriba and Taos Counties, New Mexico, and 2009 for Conejos County, Colorado, was used as an interpretive layer for new mapping and to evaluate previous mapping (later acquisition years of NAIP available as quarter quadrangle downloads from ://nationalmap.gov). New mapping utilized argon-argon (40Ar/39Ar) geochronology, geochemical data, and thin-section petrography to identify, correlate, and constrain the stratigraphic position and ages of volcanic units. Volcanic rock names are based on the standard International Union of Geological Sciences (IUGS) classification scheme of Le Bas and others (1986) using major element compositions recalculated as volatile-free with ferric/ferrous iron ratios as proposed by Middlemost (1989); a result of this calculation can cause a slight shift to higher concentrations in the major element values plotted in figure 3 relative to the original data reported in table 1. Geochemical analyses of select representative samples are presented in table 1 and 40Ar/39Ar ages are reported in table 2. The complete datasets that support these tables are available through ScienceBase (://www.sciencebase.gov) as part of the data release that accompanies this report at ://doi.org/10.5066/F72N51M5 (Turner and others, 2018). Qa Tbx

Tlx Qsw Qsa Qsp Qf Qc

Tcx

Trl Tca

Trx Tsr Tauc Tau Tan Tas Tse Tna Tsw Tls Tsm

Tcb Tbc Tb Tlb Tlc Tle Thl Tht Thb Thp Tt Ttc Tcf Tha DESCRIPTION OF MAP UNITS [Due to the addition of a shaded relief base, colors in the Correlation of Map Units and the Description of Map Units may not exactly match unit colors on the map] SURFICIAL DEPOSITS The surficial units on this map are informal allostratigraphic units of the North American Stratigraphic Code (North American Commission on Stratigraphic Nomenclature, 2005). Mapped surficial deposits are known or estimated to be at least 1 meter (m) thick. Ages of time boundaries are those of the U.S. Geological Survey Geologic Names Committee (2018) except those for the middle-early Pleistocene and late-middle Pleistocene boundaries, which are those of Gibbard and others (2010). Age assignments for surficial deposits are based chiefly on (1) the relative heights above modern streams or channels of ephemeral streams, (2) topographic relationships with other surficial deposits, and, to a lesser extent, (3) relative degree of erosional modification of original (depositional) surface morphology. Stages of secondary calcium carbonate morphology (referred to as stages I and II that are formed on the bottom of clasts) are from Gile and others (1966) and Machette (1985). In this report, the terms "alluvium" and "alluvial" refer to sediment transported by running water confined to channels (stream alluvium), whereas those deposited by running water not confined to channels are referred to as sheetwash. The terms "colluvium" and "colluvial" refer to sediment transported downslope chiefly by mass-movement (gravity-driven) processes—such as debris flow, rock fall, and near-surface creep—aided by running water not confined to channels (Hilgard, 1892; Merrill, 1897). Surficial map units that include debris-flow deposits probably also include hyperconcentrated flow deposits. These latter deposits are intermediate in character between stream-flow and debris-flow deposits. Eolian deposits are present in the map area but are isolated deposits that are too small to show at the scale of the map. Much of the silty sheetwash alluvium (Qsw) frequently observed on Pliocene lava flows and much of silty fine sand in the matrix of colluvium (Qc) probably is derived chiefly from silty eolian sand. The latter may have been derived in part from silty fine sand eroded from the Los Pinos Formation and was deposited on sparsely vegetated, actively aggrading floodplains of the Rio de los Piños and other major streams within and near the map area. The sparsely vegetated, silty fine sand on these floodplains probably was later deflated and transported by strong winds and deposited as eolian sand. Grain or particle sizes of surficial deposits are based on field estimates, using the modified Wentworth scale (American Geological Institute, 1982). In the descriptions of surficial map units, the term "clasts" refers to particles larger than 2 millimeters (mm) in diameter, whereas all finer material is called "matrix." Most of the clasts in fan deposits (Qf) within the map area are very angular, whereas most of those in alluvial deposits are very angular to subrounded. ALLUVIAL DEPOSITS Stream alluvium (Holocene and late Pleistocene)—Mostly sand and gravel in stream channels as well as sand, silty sand, and gravel underlying adjacent floodplains and minor low stream terraces. Stream-channel deposits along the Rio de los Piños are composed of slightly bouldery, cobbly pebble gravel. Top of unit Qa is about 1-2 m above modern stream level. Unit Qa locally includes small fan deposits (Qf) and narrow aprons of sheetwash alluvium (Qsw) along valley margins. Low-lying deposits are prone to periodic stream flooding. Estimated thickness 1-5 m along the Rio de los Piños, 1-3 m along other streams Sheetwash alluvium (Holocene and late Pleistocene)—Chiefly slightly pebbly to pebbly, slightly silty to silty sand in aprons that overlie gentle slopes on Pliocene lava flows near northeastern corner of the map area. Unit Qsw locally includes deposits of stream alluvium (Qa) that are too small to show at map scale. Much of the silt-to fine-sand-size fraction in these deposits is likely to be of eolian origin. Low-lying areas of unit Qsw are susceptible to sheet flooding due to unconfined overland flow. Estimated thickness is 1-5 m Sheetwash alluvium and stream alluvium, undivided (Holocene and late Pleistocene)—Chiefly silty and sandy sheetwash deposits (Qsw) on gentle slopes and undifferentiated sandy and pebbly stream alluvium (Qa) along and near ephemeral streams mainly on Pliocene lava flows and Los Pinos Formation in the eastern part of the map area. Low-lying areas of unit Qsa in and adjacent to stream channels may be subject to periodic stream flooding; adjacent slopes may be subject to periodic sheet flooding. Estimated thickness is 1-5 m ALLUVIAL AND POND DEPOSITS Sheetwash alluvium and pond deposits, undivided (Holocene and late Pleistocene)—Composed chiefly of silty fine sand that accumulated in circular depressions commonly on Pliocene lava flows in the central part of the map area. Much of the unit consists of eolian sediment that was redeposited by unconfined overland flow as sheetwash alluvium (Qsw) or as lacustrine sediment in small ephemeral water bodies. Unit locally may contain marsh deposits. Low-lying areas of unit may be subject to periodic inundation by sheet flooding and ponded water. Estimated thickness is 1-5 m, possibly locally as much as 10 m ALLUVIAL AND COLLUVIAL DEPOSIT Fan deposits (Holocene to middle? Pleistocene)—Unit forms an extensive sedimentary apron that was deposited by debris flows and sedimentcharged, ephemeral streams along the lower flanks of San Antonio Mountain. Unit commonly consists of clastand matrix-supported, locally bouldery, pebbly and cobbly gravel with a silty sand matrix. Near the lower part of the fan apron, deposits locally consist of pebbly and cobbly, slightly silty sand that contains gravel lenses. Rock fragments in deposits of unit Qf are composed of dacite (Tau) eroded from San Antonio Mountain. Eolian sediment may have played an important role in the genesis of debris-flow deposits (Shroba and others, 2007). Stage I and weak stage II carbonate morphology on the bottom of clasts in deposits of unit Qf suggests that these deposits accumulated during the Pinedale glaciation (see table 2 in Machette, 1985), about 12-30 ka (Nelson and others, 1979; Benson and others, 2004, 2005, and references cited therein). Some of the deposits could be as old as 40-47 ka (Cole and others, 2007), and may have been deposited during an early advance of Pinedale ice (Sturchio and others, 1994). Unit locally may include small fan deposits of middle Pleistocene age. Near the lower part of the fan apron, unmapped deposits of sheetwash alluvium (Qsw) and stream alluvium (Qa), too small to show at map scale, locally overlie or are inset into deposits of unit Qf. Low-lying areas adjacent to stream channels are prone to periodic stream flooding and debris-flow deposition. Estimated thickness is 1-10 m COLLUVIAL DEPOSIT Colluvium, undivided (Holocene to middle? Pleistocene)—Deposits of non-sorted and non-stratified, mostly matrix-supported, sandy sediment and rock debris on and near steep slopes. Deposits range in size from pebbly silty sand to cobbly and bouldery rubble with a sandy matrix. Unit Qc consists chiefly of debris-flow, rock-fall, and creep deposits, as defined by Cruden and Varnes (1996). Unit locally includes small talus deposits on the upper flanks of San Antonio Mountain, and mass-movement (periglacial?) deposits above an altitude of 9,957 feet (ft) near the head of a small valley on the southeast side of San Antonio Mountain near the summit. Maximum thickness possibly about 15 m DEPOSITS OF THE TAOS PLATEAU VOLCANIC FIELD The Taos Plateau volcanic field (Lipman and Mehnert, 1979) is a basaltic to rhyolitic, Pliocene to Pleistocene volcanic field underlying the Taos Plateau. The volcanic field is predominantly basaltic with lesser intermediate and rhyolitic compositions that erupted from no fewer than 55 distinct vents. The Taos plateau occupies the southern San Luis Basin in northern New Mexico and southern Colorado and is bordered on the east by the Sangre de Cristo Mountains and on the west by the Tusas Mountains and southeastern San Juan Mountains (fig. 1). Within the map area, rock compositions include basalt to rhyolite (fig. 3; table 1) and range in age from 4.55-2.12 Ma (table 2). At least 15 exposed vent areas are known, or inferred, that vary from low-relief shields to the steep-sided volcano at San Antonio Mountain, which is composed of glassy dacite lava flows and a summit cinder cone (fig. 2; Eppler, 1976; Thompson and Lipman, 1994a, b). Volcanic edifices composed of basaltic trachyandesite with vent areas concealed by overlying upper dacite of San Antonio Mountain (Tau) may represent cogenetic precursors to the dacite lavas; however, a more thorough investigation is needed to confirm this relationship. Olivine tholeiitic basalt of the Servilleta Basalt is the most voluminous volcanic rock in the Taos Plateau volcanic field. Ages throughout the Taos Plateau range from 5.3 Ma to about 1 Ma (Lipman and Mehnert, 1979; Appelt, 1998; Thompson and others, 2015). The low viscosity olivine tholeiitic basalts were emplaced as floods from low-relief shield volcanoes throughout the Taos Plateau and share general characteristics: flow thickness generally less than 10-12 m; olivine phyric; rarely xenocrystic; vesicular segregrations in the form of vertical pipes and (or) horizontal planes; and groundmass that is usually medium grained with moderate to well-developed diktitaxitic texture. Composition is dominantly tholeiitic basalt, but more complete sampling and compositional characterization of flows throughout the volcanic field reveal basaltic andesite to basaltic trachyandesite compositions that have been included in the Servilleta Basalt (Lipman and Mehnert, 1975, 1979; Dungan and others, 1986; Thompson and Lipman, 1994a). Within the map area the designation of Servilleta Basalt is restricted to lavas with characteristics described above and are classified as basalt or basaltic andesite on the IUGS total-alkalisilica classification diagram (LeBas and others, 1986). The Servilleta Basalt is subdivided into informal members based on age, known or inferred source vent, composition, and textural characteristics that, in some cases, are atypical of Servilleta Basalt elsewhere in the Taos Plateau. The Servilleta Basalt members included here are the Rio San Antonio (Tsr), Section 35 (Tse), Wissmath Craters (Tsw), and Pinebetal Mesa (Tsm) members. Xenocrystic lavas, less common in other parts of the Taos Plateau volcanic field, are prolific within the map area. Whole-rock compositions vary from basaltic to basaltic andesite but also include mildly alkaline compositions ranging from trachybasalt to basaltic trachyandesite (fig. 3). Xenocrysts are dominantly quartz and plagioclase but rare xenocrystic pyroxene glomerocrysts are also observed. Grains identified as xenocrysts generally demonstrate highly rounded or extensively embayed grain morphologies, and plagioclase characteristically displays moderate-to well-developed, sieve-textured interiors. Grains interpreted as primary phenocrysts generally display euhedral morphologies with sharp grain boundaries and solid interiors, but mildly embayed grain boundaries, particularly in olivine, are considered phenocrysts. In rocks with a continuous range in grain morphologies, rounded grains with similar characteristics to euhedral grains are taken into account in modal estimates of phenocrysts. This classification ignores complexities in the magmatic system where phenocrysts formed at higher temperature and pressure conditions can be resorbed as the magma ascends and cools, which may explain the continuous range in grain morphologies from euhedral to highly embayed as observed in some rocks. Nevertheless, without extensive investigations of mineral chemistry, this approach to classify xenocrysts is reasonable for our descriptive purposes. Glomerocrysts are frequently present in lava flows of the Taos Plateau volcanic field. Unit descriptions do not categorize glomerocrysts separately relative to their modal percentage. Rather, modal phenocryst percentages include contributions from glomerocrysts. Although glomerocrysts may represent remobilization of either crystals settled in a magma chamber or crystal accumulations along magma chamber walls, in general, phenocrysts that compose the glomerocrysts are petrographically indistinguishable from isolated grains. In cases where grains within glomerocrysts are distinct from isolated grains, the glomerocrysts are identified as xenocrystic. Xenocrystic basalt of Hill 8489 (early Pleistocene)—Black to dark gray, moderately vesicular and porphyritic basalt (51 weight [wt.] percent silicon dioxide [SiO2]; table 1) lava flows and near-vent pyroclastic deposits (not on map). Phenocrysts include 4-6 percent elongate to tabular plagioclase, 0.3- 2 mm in length; 4-6 percent euhedral to anhedral olivine with minor iddingsite replacement, up to 1.8 mm; and less than 1 percent anhedral orthopyroxene. Glomerocrysts of plagioclase, olivine, and orthopyroxene are common. Up to 2 percent xenocrysts that include clear quartz grains mostly 1-5 mm, although some grains are larger, and plagioclase 1-10 mm. Biotite is present but is likely a secondary mineral as grains are only in some small vesicles and voids in glomerocrysts and plagioclase xenocrysts. Groundmass is aphanitic to glassy and composed of glass, plagioclase, opaque and other mafic minerals. Flows are preserved in the southeast corner of the map and near-vent deposits are preserved east of the map boundary near the northeast flank of No Agua Peaks. Outflow was mostly east-northeast of vent area where it flowed over 13 kilometers (km) from the vent. Whole-rock 40Ar/39Ar age is 2.12±0.02 Ma (RGR-421; table 2). Xenocrystic trachyandesite (Pliocene)—Dark gray, sparsely porphyritic trachyandesite (55 wt. percent SiO2) lava flows and near-vent pyroclastic deposits. Up to 1 percent phenocrysts that include a combination of orthopyroxene that is weakly pleochroic from light green to light brown, 0.4-1.2 mm, and rare clinopyroxene and opaque mineral pseodomorphs after olivine. Opaque mineral pseudomorphs of euhedral and embayed olivine are up to 0.5 mm and associated with orthopyroxene grains in glomerocrysts. Xenocrysts are about 1 percent and include plagioclase, 0.4-8 mm, and quartz up to 2.5 mm. Fine-grained groundmass with trachytic and intersertal textures is composed of plagioclase, pyroxene, glass, olivine, and opaque minerals. Vent area is located on northeast flank of San Antonio Mountain. Lipman and Mehnart (1979) report a potassium-argon (K-Ar) age of 2.24±0.15 Ma. Whole-rock 40Ar/39Ar age from lava near the vent is 2.78±0.05 Ma (RGR-009; table 2) Near-vent pyroclastic deposits—Cinder and spatter agglutinate with minor interbedded lava flow material Lava flows—Massive lava flows 5-to 10-m-thick. A distinct flow lobe extends to northeast nearly 7 km from the vent area and may be a compound single flow. A second flow is more proximal to the vent area Basaltic trachyandesite of Red Hill (Pliocene)—Dark gray, vesicular to non-vesicular, porphyritic basaltic trachyandesite (51 wt. percent SiO2) lava flows and near-vent pyroclastic deposits. Phenocrysts include 5-6 percent tabular to highly elongate plagioclase up to 1 mm that is occasionally intergrown with olivine; 1 percent euhedral to anhedral olivine, 0.1-0.4 mm, with iddingsitized cores in euhedral grains and often highly skeletal grains with little or no iddingsite alteration in anhedral grains; less than 1 percent orthopyroxene, 0.2-0.4 mm, with slight pleochroism from tan to light pink, and reaction rims at the grain boundary composed entirely of iddingsitized grains that may have originally been olivine or clinopyroxene; and less than 1 percent tabular clinopyroxene, up to 0.6 mm, with only slightly rounded grain boundaries. Fine-grained to aphanitic groundmass with intergranular and weakly developed trachytic textures composed of plagioclase, opaque minerals, and pyroxene. Vent is marked by a large cinder cone at Red Hill about 5 km south of San Antonio Mountain. Whole-rock 40Ar/39Ar age for sample along U.S. 285 is 2.81±0.04 Ma (RGR-017; table 2) Near-vent pyroclastic deposits—Cinder and spatter agglutinate with minor interbedded lava flow material. Cinder cone is being quarried Lava flows—Massive lava flows 3-to 5-m-thick. At least two flows extend over 4 km from the vent area to south-southeast. At least three more proximal flows extend to the west and north Rio San Antonio member of Servilleta Basalt (Pliocene)—Black, moderately vesicular to non-vesicular, sparsely phyric basaltic andesite (51-52 wt. percent SiO2). Phenocrysts include 5-10 percent microphenocrysts of anhedral to euhedral olivine, 0.1-0.8 mm. Plagioclase is likely a phenocryst phase as well but a distinction cannot be made from groundmass based on inspection of thin section alone. Groundmass is medium grained with intersertal, intergranular, and occasionally weakly trachytic textures and composed of elongate plagioclase up to 2.5 mm in length, but equant to tabular grains not uncommon; mostly devitrified glass; and subophitic clinopyroxene, up to 1.5 mm. Tsr lavas may have erupted from a single vent or multiple vents approximately coeval as all lavas mapped as Tsr occupy the same stratigraphic position. One possible vent may be a circular depression northwest of San Antonio Mountain; however, flows are higher in elevation south of the depression, suggesting Tsr flows may have erupted from multiple vents or were tilted eastward after emplacement. Flow thickness 3-8 m. Whole-rock 40Ar/39Ar age for sample from northwest of San Antonio Mountain indicates an age of 2.94±0.12 Ma (RGR-012; table 2) Upper dacite of San Antonio Mountain (Pliocene)—Black to gray, non-vesicular to moderately vesicular, sparsely porphyritic dacite to trachydacite (63-65 wt. percent SiO2) lava flows and near-vent pyroclastic deposits. Phenocrysts include 2 percent euhedral to subhedral orthopyroxene, slightly pleochroic from light green to light brown and often with darker brown cores, 0.2-0.4 mm; and 1-2 percent elongate to tabular plagioclase up to 0.5 mm in long dimension. Orthopyroxene often occurs in glomerocrysts. Fine-grained to glassy groundmass with weakly trachytic to felty texture and composed of glass, plagioclase, pyroxene, and opaque minerals. Lipman and Mehnart (1979) report a whole-rock K-Ar age of 3.12±0.17 Ma. The preferred whole-rock 40Ar/39Ar age sampled from a flow on the north flank of San Antonio Mountain is 2.9±0.3 Ma (RGR-010 [no. 1]; table 2) Near-vent pyroclastic deposits—Cinders and spatter with interlayered black glass to highly oxidized red flow material. Marks late-stage vent area along the southern peak of San Antonio Mountain Lava flows—Lava flows up to 30-m-thick that radiate outward from a central vent area. Typical flow structure is black and glassy at base of flow; light gray and fine grained in the middle; and coarse breccia top with red, highly oxidized and black, glassy blocks. Eppler (1976) provides a more detailed discussion of the internal flow structure Lower trachyandesite of north San Antonio Mountain (Pliocene)—Black, fine-grained, and highly to moderately vesicular, sparsely porphyritic trachyandesite (63-64 wt. percent SiO2) lava flows. Phenocrysts include 1-3 percent subhedral orthopyroxene often with slightly rounded grain boundaries, slightly pleochroic from light green to light brown and often with a darker brown core, 0.4-0.8 mm; and less than 1 percent tabular plagioclase, mostly 0.5 mm. Orthopyroxene often occurs in glomerocrysts. Xenocrysts of quartz are rare but can be greater than 1 mm. Fine-grained to glassy groundmass with weakly trachytic to felty and intersertal textures common and composed of, in decreasing order of abundance, glass, plagioclase, pyroxene, and opaque minerals. Exposures limited to the north flank of San Antonio Mountain. Lavas may represent an earlier eruptive episode preceding the main dacitic volcano building event based on similar composition and nearly identical phenocryst content and character. The preferred whole-rock 40Ar/39Ar age is 2.91±0.07 Ma (RGR-231; table 2) Lower trachyandesite of south San Antonio Mountain (Pliocene)—Black, sparsely porphyritic trachyandesite (55-56 wt. percent SiO2) lava flows. Phenocrysts include 1-2 percent elongate to tabular and rarely equant plagioclase, 0.4-1.5 mm; 1 percent anhedral and embayed olivine, 0.1-0.7 mm; and rare tabular to irregular grains of clinopyroxene (much less than 1 percent), up to 0.5 mm. Occasionally, plagioclase phenocrysts are intergrown with olivine and clinopyroxene. Olivine preserved within plagioclase also exhibits embayed grain boundaries. Fine-grained to glassy groundmass where intersertal texture is dominant but weekly trachytic to felty textures present and composed of plagioclase, glass, pyroxene, and opaque minerals. Unit is exposed only on south and southeast flanks of San Antonio Mountain. Whole-rock 40Ar/39Ar geochronology indicates age of 3.00±0.04 Ma (RGR-133; table 2) Section 35 member of Servilleta Basalt (Pliocene)—Black, sparsely phyric basalt (50-51 wt. percent SiO2) lava flows. Basalt is equigranular and medium grained and as a result phenocryst determination other than olivine is unclear. Olivine phenocrysts 5-7 percent as euhedral to anhedral grains, 0.2-1 mm. Groundmass exhibits well-developed diktytaxitic, intergranular, and intersertal textures and is composed of elongate plagioclase, 0.2-2.5 mm; equant clinopyroxene, 0.1-0.5 mm; opaque minerals; and brown to completely devitrified glass. Source of lava flows is undetermined but is inferred to be buried beneath the eastern flank of San Antonio Mountain or the alluvial fans that extend east from San Antonio Mountain based on lava distribution. The lava flows overlie flows of the Wissmath Craters member of the Servilleta Basalt (Tsw) east of the map boundary. Lava flow thickness about 10 m Wissmath Craters member of Servilleta Basalt (Pliocene)—Black, sparsely vesicular, porphyritic basalt to basaltic andesite lava flows (48-52 wt. percent SiO2). Phenocrysts include 1-5 percent euhedral to irregular olivine, less than 2.8 mm, irregular grains with sometimes highly embayed boundaries suggesting some grains may be xenocrystic; 2-7 percent plagioclase with sharp grain boundaries and solid interiors. Glomerocrysts of plagioclase and olivine present in some samples. Less than 1 percent xenocrysts that include plagioclase, up to 6 mm, mostly with highly developed sieve-textured interiors and a thin mantle, but also present as large solid-cored grains with highly embayed grain boundaries; and orthopyroxene with irregular grain boundaries rimmed by clinopyroxene (Dungan, 1987). Aphanitic to medium-grained groundmass with intergranular texture and patchy subophitic pyroxene that forms a mottled appearance in thin section. Groundmass composed of plagioclase, pyroxene, olivine, opaque minerals, and minor glass. Lavas are compositionally similar despite the range in SiO2 and are interpreted to have erupted from a central vent marked by the depression at Wissmath Craters located immediately east of the eastern map boundary (fig. 2). However, multiple source vents could explain the petrographic variability and range in SiO2 content. Additionally, exposed in a road cut along highway U.S. 285 northwest of No Agua Peaks (fig. 1), a thin sedimentary interbed separates compositionally similar lavas indicating a hiatus in eruption, which could indicate multiple sources. Lava flows are observed stratigraphically above xenocrystic basaltic andesite of La Segita Peaks (Tls) in the southern part of Wissmath Craters. Flows associated with Wissmath Craters shield within the map area are over 6 km southwest of the crater, but east of the map area, flows extend 10 km east of Wissmath Craters. Whole-rock 40Ar/39Ar age from sample in the southwest area of the map near U.S. 285 is 3.20±0.05 Ma (RGR-235; table 2) Xenocrystic basaltic andesite of La Segita Peaks (Pliocene)—Black, moderately vesicular, sparsely porphyritic basaltic andesite (53-55 wt. percent SiO2) lava flows and red, scoriaceous near-vent pyroclastic deposits (east of map area). Phenocrysts include 1-2 percent euhedral to subhedral olivine, 0.2-1.3 mm. Up to 2 percent xenocrysts that include plagioclase with solid cores surrounded by fine sieve texture and thin mantle at outer grain boundary; and quartz grains up to 6 mm. Groundmass exhibits intersertal to intergranular textures and is composed of plagioclase, pyroxene, olivine, glass, and opaque minerals. Vent area is inferred from thick accumulation of near-vent scoriaceous deposits and agglutinated spatter that underlie most of the high points at La Segita Peaks located immediately east of the eastern map boundary. Unit extends to the southwest but is mostly concealed below Wissmath Craters member of the Servilleta Basalt (Tsw). 40Ar/39Ar geochronologic analysis on a whole-rock sample resulted in discordant data and therefore is not reported in table 2. However, analytical results are reported in the data release that supports this report (sample RGR-135, Turner and others, 2018) Pinebetal Mesa member of Servilleta Basalt (Pliocene)—Gray, moderately vesicular, sparsely phyric to porphyritic basalt lava flows (49 wt. percent SiO2). Rock is generally coarse grained and equigranular rendering phenocryst determination in most samples difficult with the exception of a sample from the depression rim at the main shield, which is porphyritic with a fine-grained to aphanitic groundmass. Phenocryst percentages based on porphyritic sample include 10-15 percent elongate plagioclase, 0.2-5 mm, that are occasionally intergrown with olivine and other plagioclase; and 7-10 percent subhedral to anhedral olivine, 0.1-2 mm, occasionally with moderately embayed grain boundaries. Samples with coarse-grained groundmass exhibit diktytaxitic, intergranular, and intersertal textures and are composed of platy plagioclase, up to 8 mm; subophitic pyroxene; glass, usually devitrified; and opaque minerals. Vent area for some lava flows is a shield with a central depression located northwest of San Antonio Mountain on Pinabetal Mesa. Flows may also have erupted from north-aligned depressions west of the main shield. Some lavas flowed south from the main vent where they ponded in paleotopographic lows and formed flows 20-30 m thick where exposed along the Rio San Antonio. Most lavas flowed north and northeast of the vent area for at least 10 km, at which point they are concealed by overlying Rio San Antonio member of Servilleta Basalt (Tsr). One lava tube is known to exist on Pinabetal Mesa, accessed through a collapsed ceiling at Cisneros Mine (Rogers and others, 2000). Flows in the northeastern corner of the map are also distinctly coarse grained and occupy the same stratigraphic position so are included in the Pinabetal Mesa member of the Servilleta Basalt; although, it is unknown if these lavas erupted from the same vent. With exception of ponded lava flows, flow thickness generally 5-10 m. Whole-rock 40Ar/39Ar age is 3.33±0.14 Ma (RGR-013; table 2) on a sample from the rim of Rio San Antonio west of San Antonio Mountain Basaltic trachyandesite of Los Cerritos de la Cruz (Pliocene)—Gray to black, vesicular to nonvesicular, porphyritic basalt to trachyandesite (51-57 wt. percent SiO2) lava flows and near-vent pyroclastic deposits. Phenocrysts are difficult to distinguish because of high xenocryst content, but grains interpreted to be phenocrysts are cumulatively 2-3 percent and include tabular to elongate plagioclase, 0.5-1.5 mm, with either sharp or only slightly rounded grain boundaries; euhedral to anhedral grains and grain fragments of olivine, 0.2-1.5 mm, with moderately to highly embayed grain boundaries and resorbed interiors suggesting some olivine may be xenocrystic; and rare euhedral clinopyroxene, up to 1.5 mm. Xenocrysts are up to 7 percent of rock and include plagioclase and alkali feldspar 5 mm and larger; rounded quartz, 3 mm and larger (Thompson and Lipman, 1994b); orthopyroxene, 0.5-1 mm, with grain boundaries rimmed by clinopyroxene; and glomerocrysts usually consisting of 20 or more grains of 0.5 mm clinopyroxene. Very fine grained groundmass with intersertal to intergranular textures composed of plagioclase, pyroxene, olivine, and sparse glass. North and south cinder cones mark vent areas. Mapped as a single unit because stratigraphic order between outflow from north and south vents could not be established in the field and eruptive activity is inferred to be similar in age; however, aeromagnetic data indicate normal polarity for the south cone but reversed polarity for the north cone indicating some amount of elapsed time between eruptions of the north and south cones (Drenth and others, 2011). Whole-rock 40Ar/39Ar age on lava flow located between north and south cones is 3.36±0.05 Ma (RGR-015; table 2) Near-vent pyroclastic deposits—Cinder and spatter agglutinate and minor lava flow material preserved at two cones Lava flows—Multiple lava flows 3-8 m thick originating from both vent areas Basaltic andesite of Rio de los Piños (Pliocene)—Gray, sparsely vesicular, porphyritic basalt to basaltic andesite (50-53 wt. percent SiO2) lava flows and near-vent pyroclastic deposits. Phenocrysts include 2-4 percent tabular to elongate plagioclase up to 2 mm with occasionally rounded grain boundaries; and 1-3 percent anhedral to subhedral olivine, 0.1-1.2 mm, some grains exhibit skeletal morphologies or are slightly embayed, and iddingsite replacement varies from moderate to none. Glomerocrysts of plagioclase and olivine are ubiquitous. Less than 1 percent xenocrysts of plagioclase (2.5 mm) and quartz (3 mm). Fine-grained groundmass with intergranular, moderate-to well-developed trachytic texture, and patchy subophitic pyroxene results in a mottled appearance. Groundmass composed of plagioclase, subophitic pyroxene, olivine, opaque minerals, and sparse glass. Flows originated from a single vent area at an unnamed hill north of the Rio de los Piños. Lavas that flowed to the northeast extend over 10 km from the main vent, drape older Valdez Tank lava flows (Tb), and appear to have filled a paleochannel. Lavas that flowed east rest on basaltic andesite of Cañada los Ranchos (Tca) south of the Rio de los Piños. Lavas are difficult to differentiate from Valdez Tank lavas in hand sample but basalt of Rio de los Piños has fewer plagioclase xenocrysts, occasionally has quartz xenocrysts, and lacks pyroxene phenocrysts. Preferred whole-rock 40Ar/39Ar age is 3.39±0.04 Ma (RGR-374; table 2) Near-vent pyroclastic deposits—Cinder and spatter agglutinate with minor lava flow material Lava flows—Multiple lava flows from 5-to 10-m-thick Basaltic andesite of Cañada los Ranchos (Pliocene)—Black to dark gray, moderately vesicular to highly vesicular, porphyritic basaltic andesite (52-53 wt. percent SiO2) lava flows. Phenocrysts include 2-3 percent euhedral to anhedral olivine, 0.2-1 mm, skeletal grains are common and some grains exhibit embayed grain boundaries; 1-2 percent elongate plagioclase, 0.5-1 mm. Xenocrysts (less than 1 percent) include plagioclase up to 3 mm, quartz up to 0.6 mm and rare (only observed in one thin section) elongate orthopyroxene, 0.2-0.6 mm, with rounded grain boundaries and rimmed by clinopyroxene. Fine-grained to glassy groundmass with trachytic, intersertal and intergranular textures common and patchy subophitic pyroxenes result in a mottled appearance in thin section. Groundmass composed of plagioclase, pyroxene, glass, olivine, and opaque minerals. Main vent is located near western map boundary, north of Rio San Antonio, where a circular depression is at the summit of a low-angle shield. At least seven moderately oxidized, highly vesicular, 1-to 2-m-thick flows are exposed in the wall of the depression. Northeast of the main vent area along the Rio San Antonio gorge is a dike associated with minor cinder deposits interlayered with lava flows indicating a possible fissure eruption linked with the main vent. Flows extend at least 16 km to the northeast. Preferred whole-rock 40Ar/39Ar age is 3.32±0.06 Ma (RGR-221; table 2). Although the mean age is younger than overlying basaltic andesite of Rio de los Piños (Trl), the ages are indistinguishable within analytical error suggesting the units were erupted at about the same time Rhyolite of No Agua Peaks (Pliocene)—Gray to pale brown perlitized rhyolite (73 wt. percent SiO2) flows, tuffs, and breccias. Lava flows are commonly flow banded, include obsidian to microcrystalline groundmass, and have onionskin and perlitic fracture textures. Degree of vesiculation varies from 2-20 percent of rock, where less vesicular varieties are generally less perlitized (Neart and others, 1980). Phenocrysts (xenocrysts?) include alkali feldspar as euhedral grains and irregular fragments. Previous investigations suggest No Agua rhyolites erupted from 1-4 vents (Neart and others, 1980; Lipman and Mehnert, 1979; Whitson, 1982; Breese, 1984; Chamberlin and Barker, 1996). No Agua rhyolite is present in the southeastern corner of the map area with all inferred vents located southeast of the map boundary. Dickens (2007) determined 40Ar/39Ar ages based on laser fusion of glass and feldspar separates indicating temporally distinct eruptive episodes at 3.88±0.06 and 4.1±0.03 Ma. Some single-grain laser-fusion ages determined on alkali feldspar yielded ages of 25.35±0.13 Ma, which Dickens (2007) suggests indicates incorporation of feldspar xenocrysts from an Amalia Tuff-related source Basalt of Chino Peak (Pliocene)—Gray to black, moderately vesicular and porphyritic basalt (49-50 wt. percent SiO2) lava flows and near-vent pyroclastic deposits. Although range in SiO2 content is narrow, lavas included in map unit have substantial variability in wt. percent magnesium oxide (MgO), potassium oxide (K2O), and numerous trace elements including rubidium (Rb), niobium (Nb), and lanthanum (La). Petrographically, rocks are broadly similar. Phenocrysts include 1-4 percent euhedral to anhedral, and commonly skeletal, olivine, 0.1 to 2.5 mm; 1 percent tabular to elongate plagioclase, 0.1 to 1 mm. Sparse xenocrysts include plagioclase and orthopyroxene where grain boundary is irregular and mantled by completely iddingsitized olivine(?) grains. Mediumgrained to aphanitic groundmass with intergranular to rarely intersertal texture composed of plagioclase, pyroxene, olivine, and opaque minerals. Micro enclaves of quenched melt inclusions that consist dominantly of brown glass, opaque minerals, elongate plagioclase and pyroxene are present in samples with greater than 8.0 wt. percent MgO (8.04-9.55) and wt. percent K2O less than 0.5 (0.43-0.49). Samples with wt. percent MgO less than 7.0 (5.95-6.77) and wt. percent K2O greater than 0.9 (0.9-1.02) do not contain enclaves of melt inclusions. Two compositionally transitional samples with 7.40-7.95 wt. percent MgO and 0.75-0.77 wt. percent K2O also contain enclaves of melt inclusions. These compositional and petrographic characteristics may indicate a magma mixing relationship. A single vent area is identified by a cinder cone at Chino Peak; however, highly oxidized cinders and agglutinate are located northwest of the cinder cone just east of a small fault. The preferred whole-rock 40Ar/39Ar age is 4.31±0.07 Ma (RGR-014; table 2) from a sample of lava with MgO less than 7.0 wt. percent that is interbedded within the cinder cone Near-vent pyroclastic deposits—Cinder and spatter agglutinate and minor lava flow material Lava flows—Multiple flows originating from vent area up to 18-m-thick where exposed along the Rio de los Piños Basalt and basaltic andesite of Valdez Tank (Pliocene)—Black to gray, weakly to moderately vesicular, porphyritic basalt to basaltic andesite lavas and near-vent pyroclastic deposits. Unit includes three compositional groups that form a broadly continuous progression in composition and possess similar petrographic characteristics; lava types include basalt (48-51 wt. percent SiO2), silicic basalt (50-51 wt. percent SiO2), and basaltic andesite-basaltic trachyandesite (51-55 wt. percent SiO2). Finegrained to aphanitic groundmass composed of plagioclase, clinopyroxene, opaque minerals, olivine, and sparse minor glass. Seriate texture common in low-silica basalt and shows continuous grain size from less than 0.05 mm up to 1.5 mm. Porphyritic, intergranular, and trachytic texture common in all types with groundmass up to 0.5 mm and phenocrysts up to 1.5 mm. Low-silica basalt phenocrysts include 1-2 percent plagioclase and 1-2 percent olivine; silicic basalt and basaltic andesite-basaltic trachyandesite groups include phenocrysts of 1-2 percent plagioclase, 1 percent clinopyroxene, 1 percent orthopyroxene, and up to 1 percent olivine. Glomerocrysts are common and consist of plagioclase only, plagioclase and both pyroxenes, and plagioclase and olivine with and without pyroxenes. All three compositional groups contain up to 1 percent xenocrysts of plagioclase (up to 3.5 mm). Multiple vent areas are identified: low-silica basalt correlates to a 1.5-m-wide dike approximately 2.5 km north of Bighorn Peak, north of the map boundary; basaltic andesite-basaltic trachyandesite correlates to a low-profile cinder cone along the north rim of Rio de los Piños and cinder deposits and lavas approximately 2 km to the north-northwest; and silicic basalt correlates to cinder deposits and lavas about 2 km north of Bighorn Peak just north of the map boundary. Lava types are grouped as a single map unit because differentiation in the field was not possible and the continuous compositional progression and similar eruption ages suggest they may be cogenetic. Preferred whole-rock 40Ar/39Ar ages for the distinct compositional types include ages of 4.36±0.05 Ma (RGR-364; table 2) for basalt; 4.36±0.04 Ma (RGR-362; table 2) for silicic basalt; and 4.37±0.03 Ma (RGR-371; table 2) for basaltic andesitebasaltic trachyandesite. Stratigraphically, basaltic andesite-basaltic trachyandesite lavas overlie both basalt types but no relative order was determined between basalt and silicic basalt types. If eruptions were coeval stratigraphic order may not be consistent Near-vent pyroclastic deposits—Cinder and spatter agglutinate and minor lava flow material Lava flows—Lava flows originating from multiple vent areas north of Rio de los Piños. Individual lava flows are 3-12 m thick, but, where tops are eroded erosional remnants can be less than 1 m Basalt of Lucero Lakes (Pliocene)—Light gray to gray, fine-grained, sparsely phyric basalt (49.5-50.1 wt. percent SiO2) lava flows. Microphenocrysts are difficult to differentiate from groundmass because of seriate texture but include 1-2 percent olivine, 0.1-0.25 mm with little or no iddingsite replacement, and 1-2 percent tabular to elongate pyroxene, 0.1-0.6 mm, dark green with moderate to extensive iddingsite replacement and some with well-developed skeletal texture. Sparse xenocrysts of quartz, plagioclase and pyroxene only observed in one sample. Fine-grained to aphanitic groundmass with intergranular and seriate textures and patchy subophitic pyroxene forms a mottled appearance in thin section. Groundmass composed of plagioclase, pyroxene, olivine, opaque minerals, and minor glass. Flows originated from Lucero Lakes area (fig. 1) and were channelized by paleovalleys with east-to southeast-directed flow. Lava flow thickness from 5-10 m. A northwest-trending, approximately 0.5-m-wide, dike that is sparsely phyric with rare green olivine phenocrysts protrudes above the surface in the southeast flank of Lamy Peak. Compositional data for the dike are not available; however, the physical characteristics of the dike are consistent with the basalt of Lucero Lakes rather than the Hinsdale Formation basaltic andesite (Thb) that caps Lamy Peak, and, therefore, the dike is considered part of the basalt of Lucero Lakes. Whole-rock 40Ar/39Ar age is 4.55±0.07 Ma (RGR-036; table 2) OLDER REGIONAL VOLCANIC AND VOLCANICLASTIC ROCKS Volcanic and volcaniclastic deposits that predate deposits of the Taos Plateau volcanic field record volcanism of the Southern Rocky Mountains volcanic field (SRMVF) and filling of early extensional basins. The SRMVF is a composite mid-Tertiary (38-23 Ma) volcanic field in northern New Mexico and Colorado (Steven, 1975; Lipman, 2007). Loci of volcanism of the SRMVF surrounding the map area include the San Juan Mountains, San Luis Hills, and the Latir volcanic locus near the town of Questa, New Mexico. Volcanic deposits associated with the southeastern San Juan Mountains are present in the northwestern part of the map area. Conejos Formation (Tcf) deposits represent the early magmatic system in the San Juan Mountains and are composed of mostly andesite to dacite lavas flows and breccias and derivative volcaniclastic rocks associated with central volcanoes. Eruption of intermediate-composition lavas was superseded by multi-cyclic caldera formation at the Platoro caldera complex associated with eruption of at least seven major ignimbrites (individual ignimbrite volumes range between 75-1,000 km3) between ~29-30 Ma that collectively form the Treasure Mountain Group (Lipman and others, 1996). The Chiquito Peak Tuff (Ttc) is the only distinct ignimbrite present in the map area, exposed along the Rio de los Piños in the northwest part of the map. Along the western margin of the San Luis Valley, the Chiquito Peak Tuff has previously been mapped as Masonic Park Tuff (Lipman,1975a, b; Manley, 1982a) and was inferred to have erupted from a buried source in the central San Juan Mountains (Lipman, 1975a). However, based on compositional, mineralogical, geochronologic, and stratigraphic evidence, Lipman and others (1996) determined previous usage of the Masonic Park Tuff combined two distinct ignimbrites and they renamed the ignimbrite that erupted from the Platoro caldera as the Chiquito Peak Tuff. The Chiquito Peak Tuff is the youngest widely distributed ignimbrite originating from the Platoro caldera complex and erupted at 28.94 Ma (RGR-466; table 2). Undivided Treasure Mountain Group deposits (Tt) below the Chiquito Peak Tuff are largely covered by rock-fall and colluvial deposits. The Los Pinos Formation is composed of redeposited volcanic material sourced from surrounding volcanic highlands (Manley, 1981). In the northern part of the map area, clasts within Los Pinos deposits are dominantly intermediate-composition volcanic rocks from the southeast San Juan Mountains and the San Luis Hills, which is diagnostic of the Esquibel Member (Tle). The base of the Esquibel Member is placed at the top of the Chiquito Peak Tuff (28.94 Ma). In the southern part of the map area, intermediatecomposition clasts become smaller and volumetrically minor compared to abundant rhyolitic clasts of the Amalia Tuff that characterize the Cordito Member (Tlc). Manley (1981) placed the base of the Cordito Member at the top of the Amalia Tuff (25.4 Ma; Zimmerer and McIntosh, 2012) where the units are in stratigraphic position south of the map area. The contact between the two members is time-transgressive as deposition of the Esquibel Member continued in the northern part of the map area coeval with deposition of Cordito Member. Interbedded within the Los Pinos Formation is the Hinsdale Formation, which includes basalt to rhyolite lava flows throughout its regional extent including the San Juan Mountains, San Luis Hills, and Sangre de Cristo Mountains adjacent to the southern San Luis Valley. Lipman and Mehnert (1975) suggested all basaltic lava flows interbedded within the Los Pinos Formation should be included in the Hinsdale Formation including some Pliocene lava flows correlative with flows within the map area. However, the Hinsdale Formation is here restricted to Oligocene and Miocene volcanic rocks due to the substantial age gap between the Miocene and Pliocene rocks and because the Pliocene volcanic rocks are compositionally and temporally related to the Taos Plateau volcanic field. Within the map area, deposits of the Hinsdale Formation consist of basalt to trachyandesite lava flows and are predominantly Oligocene in age. A single remnant of Miocene lava flow (Thl) is preserved capping Bighorn Peak in the northwest part of the map area. This lava flow is compositionally distinct from Pliocene and Oligocene lavas in the map area but is similar (fig. 3) to Miocene lavas more widely distributed to the north of the map area in southern Colorado (Lipman, 1975b). Los Pinos Formation (Miocene and Oligocene)—Beds and lenses of poorly sorted to moderately well sorted sandy pebble to slightly bouldery cobble conglomerate and friable, well stratified fine-grained to pebbly, coarse-grained, tuffaceous(?) sandstone. Much of the moderately well sorted conglomerate appears to be of fluvial origin, whereas the poorly sorted, sparsely bouldery cobble conglomerate may reflect debris flow deposits. Unit locally includes thin siltstone lenses and sandy-over-gravelly sheetflood(?) deposits. Unit locally may include lenses and thin beds of claystone derived from the alteration of volcanic ash. Sheetflood deposits consist of rhythmically bedded couplets consisting of alternating thin (5-40 centimeters [cm]), well-stratified layers composed of small-pebble, coarse-grained sandstone that commonly overlie slightly cobbly pebble conglomerate. Conglomerate clasts commonly are subrounded to well rounded and range in size from 1-2 cm up to 1.5 m in diameter. Clasts are dominantly volcanic and intrusive in origin shed from volcanic highs in the southeast San Juan Mountains, San Luis Hills, and the Latir volcanic locus. Locally, deposits include clasts of Proterozoic granitic gneiss and quartzite. Formation consists of the upper Cordito Member and the lower Esquibel Member (Manley, 1981) separated by a time transgressive boundary. Estimated thickness, as much as 400 m Cordito Member—Commonly contains clasts of Amalia Tuff and other volcanic and intrusive rocks from the Latir volcanic locus of the Southern Rocky Mountains volcanic field. Typically, the matrix of Cordito is lighter colored than that of the underlying Esquibel Member due chiefly to a more tuffaceous matrix and more abundant felsic clasts transported from the Latir volcanic locus along west-to northwest-directed paleodrainages that flowed into, and through, the Tusas Mountains Esquibel Member—Commonly contains abundant clasts of andesite, dacite, and basalt volcanic and intrusive rocks eroded from the southeast San Juan Mountains and the San Luis Hills. Clasts of welded ignimbrite erupted from the Platoro caldera complex in the southeast San Juan Mountains are also observed. North of the map area, in southern Colorado, early Miocene basaltic lava flows are interbedded with deposits of the Esquibel Member (Lipman and Mehnert, 1975). Miocene Hinsdale Formation lava flow that caps Bighorn Peak (Thl) suggests the Esquibel Member is as young as Miocene in the map area. Distinction from the Cordito Member is based chiefly on the absence of Amalia Tuff clasts. Clast provenance from the San Luis Hills and southeast San Juan Mountains indicates transport along paleodrainages that flowed south to southwest into and through the northern Tusas Mountains Hinsdale Formation (Miocene and Oligocene)—Includes Oligocene-aged, porphyritic basalt (Thp), trachyandesite (Tha) , basaltic andesite (Thb), and tholeiitic basalt (Tht), as well as a single isolated outcrop of Miocene basaltic lava flow (Thl) that caps Bighorn Peak. Tholeiitic basalt (Tht) and basaltic andesite (Thb) are most abundant Hinsdale Formation types within the map area whereas trachyandesite and porphyritic basalt types are more aerially restricted. Miocene basaltic lava is compositionally distinct from all other Hinsdale in the map area but is correlated to similar rocks to the north, which are about 20 Ma (table 2) Alkaline basalt to trachybasalt (Miocene)—Black to gray, mediumgrained porphyritic basalt lava flows (50 wt. percent SiO2). Phenocrysts include 5-7 percent euhedral to anhedral olivine, 0.2-0.5 mm, where some grain boundaries are embayed and have minor to extensive iddingsite replacement. Xenocrysts of plagioclase are rare. Fine-to medium-grained groundmass with intergranular and rarely intersertal textures composed of plagioclase, subophitic pyroxene, olivine, opaque minerals, and minor glass. Only one lava flow is preserved capping Bighorn Peak. Unit is more extensive north of map area (Lipman, 1975b). Whole-rock 40Ar/39Ar age from sample north of the map area is 20.61±0.07 Ma (RGR-378; table 2) Tholeiitic basalt (Oligocene)—Gray to dark-gray, moderately to nonvesicular, basalt lava flows (49-50 wt. percent SiO2). Phenocrysts include olivine and plagioclase, however, due to the medium-to coarse-grained groundmass, modal plagioclase is difficult to distinguish from groundmass grains. Euhedral to subhedral olivine is 7-15 percent of rock, 0.1-1 mm, and slightly to completely iddingsitized; larger grains often have slightly rounded to embayed grain boundaries. Groundmass is intergranular, intersertal, and weakly diktitaxitic and composed of plagioclase, 0.1-2.4 mm, some grains as large as 4 mm; clinopyroxene, 0.05 to 0.2 mm, individual equant grains and subophitic grains up to 1 mm; 3-5 percent opaque minerals; and 0-7 percent glass, partially devitrified with intergrown opaque minerals. Rare orthopyroxene grains intergrown with olivine have thick clinopyroxene overgrowths and are up to 2 mm. Plagioclase grains with sieve-textured interiors are sparse. Outcrop trends southwesterly from northeast part of map area along the Rio de los Piños, where 7-10 flows are present. Number of lava flows decreases to the southwest suggesting lavas erupted from a source to the northeast of the map area. Flow thickness 5-8 m. Preferred whole-rock 40Ar/39Ar age is 25.67±0.20 Ma (RGR-456; table 2) from a sample west of the map area, which is significantly older than a previously reported K-Ar age of 15.0±0.8 Ma (Lipman and Mehnert, 1975). Sample RGR-148 (25.54±0.24 Ma) was collected near the sample of Lipman and Mehert (1975), therefore the K-Ar age is considered incorrect Basaltic andesite (Oligocene)—Dark-gray to gray, sparsely phyric basaltic andesite lava flows (52-54 wt. percent SiO2). Phenocrysts include olivine and most likely plagioclase; however, plagioclase grains larger than the groundmass are not observed. Subhedral to anhedral olivine 5-7 percent of rock, 0.1-0.5 mm, with completely iddingsitized cores but rims of grains are generally unaltered. Medium-to fine-grained groundmass has intergranular to intersertal, and weakly developed diktytaxitic textures and is composed of plagioclase, 0.2-1.0 mm; glass, dark brown to black mostly devitrified; clinopyroxene, 0.1 to 0.5 mm, equant to tabular grains; and opaque minerals. One grain with orthopyroxene core and thick clinopyroxene rim was observed. A single lava flow observed in most localities but possibly up to three lava flows present on Broke Off Mountain. Source unknown but may be east of the map area and buried by Pliocene volcanic rocks. Whole-rock 40Ar/39Ar age is 26.05±0.21 Ma (RGR-33; table 2) on a sample west of the map area on Broke Off Mountain. Trachyandesite (Oligocene)—Black, sparsely phyric, trachyandesite lava flow (56-57 wt. percent SiO2). Microphenocrysts likely include olivine and plagioclase but there is no distinction in size from groundmass. Fine grained to glassy with intersertal texture and composed of 50-60 percent tabular plagioclase, up to 0.4 mm; 30-35 percent glass that varies from fresh and light-brown with little to no devitrification to dark brown and highly devitrified; 3-5 percent equant and elongate clinopyroxene, up to 1 mm; 3-5 percent opaque minerals that often fill areas between plagioclase grains or are equant cubic grains; and trace equant olivine, 0.1 mm, with minor to no iddingsite replacement, or, rarely, as phenocrysts up to 1 mm that are highly iddingsitized. Trachyandesite occurs as a single lava flow up to 8-m thick with highly stretched vesicles. Lava flow is overlain by tholeiitic basalt (Tht, RGR-456, 25.67±0.20 Ma; table 2), northwest of Laguna Larga, and basaltic andesite lavas (unit Thb, RGR-033, 26.05±0.21 Ma; table 2) in the area of Los Cerritos de la Cruz. Two vent areas are inferred. The first is northwest of Laguna Larga and west of western map boundary where sparsely phyric and highly oxidized scoriaceous deposits, inferred to represent near-vent pyroclastic deposits, are eroding from the slope below tholeiitic basalts. A second vent is inferred west of the northern cone of Los Cerritos de la Cruz where two intersecting dikes are sparsely phyric and petrographically similar to outflow lavas Porphyritic basalt to trachybasalt (Oligocene)—Black to dark gray, porphyritic alkaline basalt to trachybasalt (48 wt. percent SiO2) lava flow. Phenocrysts include 3-5 percent clinopyroxene, up to 1.5 mm; 3-5 percent euhedral to subhedral olivine, up to 2 mm; and 1 percent plagioclase, where phenocrysts are differentiated from groundmass grains based on more equant form and presence of mafic inclusions. Pyroxene is often in glomerocrysts with as many as 10 grains giving appearance in hand sample of larger phenocrysts. Groundmass is medium grained with intergranular texture and includes plagioclase, clinopyroxene, olivine, and opaque minerals. Groundmass plagioclase up to 2 mm in long dimension and groundmass clinopyroxene and olivine grains up to 0.5 mm. Lava occurs as a single flow up to 5-m thick. Source vent is unknown. Whole-rock 40Ar/39Ar age is 26.17±0.07 Ma (RGR-494; table 2) from a sample south of Rio San Antonio near the western map boundary Treasure Mountain Group, undivided (Oligocene)—Ignimbrite, air fall tuff, and volcaniclastic deposits associated with recurrent eruption and collapse of the Platoro caldera complex in the southeast San Juan Mountains. Within the map area, Treasure Mountain Group includes a distinct ledge of Chiquito Peak Tuff (Ttc) and a non-distinct, mostly covered slope stratigraphically between the Chiquito Peak Tuff and the underlying Conejos Formation. Undivided Treasure Mountain Group (Tt) includes predominantly volcaniclastic tuffaceous sedimentary deposits. Thickness of slope-forming unit below Chiquito Peak Tuff up to 60 m Chiquito Peak Tuff—Pink to white, moderately welded, low-silica rhyolite to trachydacite ignimbrite. In areas proximal to the Platoro caldera, phenocryst content is 40-50 percent, but within the map area, along the Rio de los Piños, phenocryst content varies from 10-20 percent. Phenocrysts include plagioclase, biotite, opaque minerals, and sanidine, ± augite (Lipman, 1975a; Lipman and others, 1996). Feldspar phenocrysts are equant to tablular and up to 1.5 mm. Biotite phenocrysts are up to 2 mm and are variably altered; grains with unaltered cores show pleochroism from light brown to greenish brown. Ignimbrite is only exposed along the Rio de los Piños in the northwestern part of map where it forms a single cooling unit up to 30 m thick. Single-grain, total-fusion age on sanidine from a sample collected along the Rio de los Piños is 28.94±0.31 Ma (RGR-466; table 2) Conejos Formation (Oligocene)—Chiefly composed of gray matrix-supported volcaniclastic debris flows. Clasts are predominantly dark-brown, porphyritic pyroxene-bearing andesitic and light-gray dacitic lavas. Clasts are angular to subangular and range from 1-75 cm. Unit exposed along the Rio de los Piños, but exposures are discontinuous in part because of colluvial debris from overlying poorly resistant Los Pinos Formation deposits. Thickness of unit in map area as much as 50 m but base is not exposed. Colucci and others (1991) report hornblende and biotite 40Ar/39Ar ages from Conejos Formation in the southeast San Juan Mountains between 33.5 and 29.5 Ma Qa Qsw Qsa Qsp Qf Qc Tbx Tau Tauc ? ? Tlx

Trx

Tsr Tle Tht Thb Tsw Tha Thp Tls Tlc Thl

Tcb Tbc Tb Tan Tas Tsm Tlb

Tcx Tca

Tse Tna SURFICIAL DEPOSITS OLDER REGIONAL VOLCANIC AND VOLCANICLASTIC ROCKS DEPOSITS OF THE TAOS PLATEAU VOLCANIC FIELD Tcf late middle early 11.7 ka 126 ka 781 ka 2.58 Ma 5.33 Ma 23.03 Ma 34.09 Ma ALLUVIAL DEPOSITS ALLUVIAL AND POND DEPOSIT ALLUVIAL AND COLLUVIAL DEPOSIT COLLUVIAL DEPOSIT CORRELATION OF MAP UNITS Holocene Pleistocene QUATERNARY TERTIARY Pliocene Oligocene Miocene Trl Tt Ttc SCALE 1:50 000 1/ 2 4 MILES 21000 FEET 5 KILOMETERS EXPLANATION OF MAP SYMBOLS Contact—Solid where location is accurate; dashed where location is approximate Contact separating individual lava flows within same map unit—Identity and existence certain, location accurate Rim of volcanic crater—Solid where location is accurate, dashed where location approximate; queried where identity or existence is questionable. Hachures point into crater Lava flowline—Arrow indicates direction of lava flow, as inferred from vent locations and topography Normal fault—Solid where location is accurate; long dashed where location is approximate, short dashed where location is inferred, and dotted where location is concealed. Queried where identity or existence questionable. Ball and bar on downthrown block PLANAR POINT FEATURES Strike and dip of bedding Inclined Inclined, approximate 40Ar/39Ar geochronologic age sample location—With sample ID, preferred sample age shown in millions of years (Ma), and plus or minus (±) error (2 sigma [2σ]). See table 2 AREAL FEATURE Quarry RGR-012 2.92±0.12 Ma RGR-362 4.36±0.04 Ma RGR-364 4.36±0.05 Ma RGR-466 28.94±0.31 Ma RGR-146 4.37±0.05 Ma RGR-148 25.54±0.24 Ma RGR-149 (no.1) 3.39±0.09 Ma RGR-371 4.37±0.03 Ma RGR-226 4.24±0.10 Ma RGR-014 4.31±0.07 Ma RGR-374 3.39±0.04 Ma RGR-221 3.32±0.06 Ma RGR-012 2.94±0.12 Ma RGR-010 (no. 1) 2.9±0.3 Ma RGR-285 3.06±0.11 Ma RGR-035 3.08±0.07 Ma RGR-133 3.0±0.04 Ma RGR-235 3.2±0.05 Ma RGR-15 3.36±0.05 Ma RGR-036 4.55±0.07 Ma RGR-494 26.17±0.07 Ma RGR-231 2.91±0.07 Ma RGR-013 3.33±0.14 Ma RGR-09 2.78±0.05 Ma Quarry Tca Tca (dike) Tca Tca Tca Tsm Tsr Tsr Tsr Tsr Tsm Tsm Tsm Tsr Tsm Qa Qa Tcf Tcf Ttc? Dike (unit unidentified) Ttc? Ttc Ttc Ttc Qsw Tb Tb Tb Tb Tht Tht Tht Tht Tcb

Tcb Tb Tb Thl Tle Tle Trl Trl Trl Trl Trl

Tle Tle Tle Tle Tle Tle Tle Tle Tt Tt Tt Tle Tle Tle Tle Tbc Tbc Tbc Qsw Qsw Qsw Qsw Qsw Tlx

Qsw Qsw Qsw Qsa Qsa Qsa Qsa Qsp Qsp Qsp Tca Tca Tca Tle Tle Tle Tle Tle Tle Tle Tle Qsp Qsp Tha Tle Qsp Qsp Qsa Qsa Qa Qa Qf Tan Tse Tan Tan Qf Qf Qf Qsa Qsa Qsa Qsa Qsa Qf Qf Qa Qc Qc Qsp Qsw Qsp Qsp Qc Qc Tbx Tls Tls Qc Qa Qc Tau Tsw Tsw Tsw Tau Tau Tauc Tau Tau Qc Qc Qc Tas Tcx Tcx

Tcx Trx Trx

Tas Qc Qa Qa Qa Qa Tha Thp Thp Thp Thp Thp Thb Thb Thb Tlb Tlb Tlb Tlb Tlb Thb Thb Tle Tlc Thb Tlc Tlc Tls Tlc Tna Tlc Tlc Tlc Tlc Tlc Tle Tlc Thb Tlb? (dikes) Thb Tha Tha (dike) ? K.J. Turner and R.A. Thompson carried out mapping of bedrock units and structural features; M.A. Cosca and L.E. Morgan carried out 40Ar/39Ar geochronologic analysis; R.R. Shroba carried out mapping of unconsolidated surficial deposits and characterized Tertiary sedimentary units, and C.F. Chan assisted in field work and preparation for analytical work. GIS database and digital cartography by K.J. Turner Publishing support provided by the Science Publishing Network, Denver Publishing Service Center Edit and digital layout by L. Binder Manuscript approved for publication September 12, 2018 U.S. Department of the Interior U.S. Geological Survey Scientific Investigations Map 3417 NEW MEXICO MAP LOCATION COLORADO Geologic Map of the San Antonio Mountain Area, Northern New Mexico and Southern Colorado By Kenzie J. Turner, Ren A. Thompson, Michael A. Cosca, Ralph R. Shroba, Christine F. Chan, and Leah E. Morgan Base modified from U.S. Geological Survey US Topo 7.5-minute quadrangles, 2017: San Antonio Mountain, N. Mex.; Los Pinos, N. Mex.-Colo.; Bighorn Peak, N. Mex.-Colo.; Pinabetoso Peaks, N. Mex.-Colo; and La Segita Peaks, N. Mex., from The National Map, accessed August 24, 2017, at ://viewer.nationalmap.gov/launch/ North American Datum of 1983 (NAD 83) Projection and 1,000-meter grid: Universal Transverse Mercator, zone 13N Boundary of the Río Grande del Norte National Monument accessed on May 30, 2018 at ://navigator.blm.gov/data?keyword=rio%20grande%20del%20norte%20national%20monument CONTOUR INTERVAL 100 FEET NATIONAL GEODETIC VERTICAL DATUM OF 1988 APPROXIMATE MEAN DECLINATION, 2018 TRUE NORTH MAGNETIC NORTH 8½° Divisions of Quaternary, Neogene, and Paleogene Time Used in this Report1 Period or subperiod Epoch Age Quaternary Holocene 0-11.7 ka late 11.7-126 ka Pleistocene middle 126-781 ka early Neogene Pliocene Miocene Oligocene Paleogene Eocene Paleocene 1Ages of time boundaries are those of the U.S. Geological Survey Geologic Names Committee (2018) except those for the late-middle Pleistocene boundary and middle-early Pleistocene boundary, which are those of Gibbard and others (2010). Ages are expressed in ka for kilo-annum (thousand years) and Ma for mega-annum (million years). 781 ka-2.58 Ma 2.59-5.33 Ma 5.33-23.03 Ma 23.03-34.09 Ma 34.09-55.8 Ma 55.8-65.5 Ma To learn about the USGS and its information products visit ://www.usgs.gov/ 1-888-ASK-USGS For product and ordering information visit: ://store.usgs.gov/ ISSN 2329-132X (online) ://doi.org/10.3133/sim3417 Any use of trade, product, or firm names in this publication is for descriptive purposes only and does not imply endorsement by the U.S. Government This map or plate is offered as an online-only, digital publication. Users should be aware that, because of differences in rendering processes and pixel resolution, some slight distortion of scale may occur when viewing it on a computer screen or when printing it on an electronic plotter, even when it is viewed or printed at its intended publication scale Conversion Factors Multiply By To obtain Length centimeter (cm) inch meter (m) foot (ft) kilometer (km) mile (mi) Area square kilometer (km2) square mile (mi2) Volume cubic kilometer (km3) cubic mile (mi2) SI to Inch/Pound ? ? RIO GRANDE DEL NORTE NATIONAL MONUMENT Suggested citation: Turner, K.J., Thompson, R.A., Cosca, M.A., Shroba, R.R., Chan, C.F., and Morgan, L.E., 2018, Geologic map of the San Antonio Mountain area, northern New Mexico and southern Colorado: U.S. Geological Survey Scientific Investigations Map 3417, scale 1:50,000, ://doi.org/10.3133/sim3417. For more information concerning the research in this report, contact the Center Director, USGS Geosciences and Environmental Change Science Center Box 25046, Mail Stop 980 Denver, CO 80225 (303) 236-5344 Or visit the Geosciences and Environmental Change Science Center website at ://www.usgs.gov/centers/gecsc Digital files available at ://doi.org/10.3133/sim3417 ScienceBase data release GIS and other files that support this report are available at ://doi.org/10.5066/F72N51M5 Altitude, as used in this report, refers to distance above the vertical datum. COLORADO NEW MEXICO Map area COLORADO NEW MEXICO Map area

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