New Mexico Geological Society Downloaded from: http://nmgs.nmt.edu/publications/guidebooks/55 Subsurface Stratigraphy in the Southern San Luis Basin, New Mexico P. Drakos, J. Lazarus, J. Riesterer, B. White, C. Banet, M. Hodgins, and J. Sandoval, 2004, pp. 374-382 in: Geology of the Taos Region, Brister, Brian; Bauer, Paul W.; Read, Adam S.; Lueth, Virgil W.; [eds.], New Mexico Geological Society 55th Annual Fall Field Conference Guidebook, 440 p. This is one of many related papers that were included in the 2004 NMGS Fall Field Conference Guidebook. Annual NMGS Fall Field Conference Guidebooks Every fall since 1950, the New Mexico Geological Society (NMGS) has held an annual Fall Field Conference that explores some region of New Mexico (or surrounding states). Always well attended, these conferences provide a guidebook to participants. Besides detailed road logs, the guidebooks contain many well written, edited, and peer-reviewed geoscience papers. 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No material from the NMGS website, or printed and electronic publications, may be reprinted or redistributed without NMGS permission. Contact us for permission to reprint portions of any of our publications. One printed copy of any materials from the NMGS website or our print and electronic publications may be made for individual use without our permission. Teachers and students may make unlimited copies for educational use. Any other use of these materials requires explicit permission. This page is intentionally left blank to maintain order of facing pages. 374 New Mexico Geological Society Guidebook, 55th Field Conference, Geology of the Taos Region, 2004, p. 374-382. SUBSURFACE STRATIGRAPHY IN THE SOUTHERN SAN LUIS BASIN, NEW MEXICO PAUL DRAKOS1, JAY LAZARUS1, JIM RIESTERER1, BILL WHITE2, CHRIS BANET2, MEGHAN HODGINS1, AND JOHN SANDOVAL2 1Glorieta Geoscience Inc. (GGI), PO Box 5727, Santa Fe, NM 87502 2US Bureau of Indian Affairs (BIA), Southwest Regional Office, 615 First St., Albuquerque, NM 87102 ABSTRACT.—Subsurface lithologic and geophysical data from a series of municipal, exploratory, subdivision, and domestic wells are used to delineate variations in thickness and extent of Tertiary through Quaternary sediments and Pliocene basalt flow sequences in the southern San Luis Basin. Servilleta basalts increase in thickness from south to north within the Taos Valley; they are absent in BOR1, present as a single thin flow in BOR3, and present as three multiple-flow sequences at BOR4, K3, RP2500, and the Airport well. The Ojo Caliente Sandstone Member varies in apparent thickness from thin or absent at BOR1 and BOR2/3 to greater than 1000 ft (300 m) at RP2500 to 340 ft (100 m) at the Airport well. The apparent thickness of the Ojo Caliente is greatest along the Rio Pueblo, suggesting deposition in a depression, possibly an ancestral Rio Hondo or Rio Pueblo drainage, during mid-to-late-Miocene time. The Ojo Caliente sand may have been derived from the west, accumulating against Chama-El Rito alluvial fans building off the eastern mountain front. Data from the drilling program are also used to identify and constrain offset on several intrabasin faults and a mountain front graben bounded on the west by the Seco Fault, across which the Servilleta Formation is offset 950 ft (290 m), and on the east by the Town Yard fault. The average subsidence rate within the graben is estimated as approximately 0.1mm/yr (0.3 ft/1000yr). Down-to-the west offset across the Town Yard Fault is > 1390 ft (420 m). INTRODUCTION per), are used to identify lithologic and geophysical signatures of the basin-fill units. These data are utilized to determine which Subsurface data collected from drilling programs conducted stratigraphic unit each well is completed into, and are used to during the past 15 years are utilized to evaluate the subsurface explore the stratigraphic framework and shallow structures of the stratigraphy in the southern San Luis Basin. Of particular impor- Taos Valley. tance are a series of exploratory wells drilled to depths ranging from 1000 ft (300 m) to 2990 ft (910 m) below ground surface GEOLOGIC SETTING (bgs), as part of a cooperative exploratory drilling program con- ducted by the Town of Taos and Taos Pueblo, with funding from The study area is situated in the Taos Valley within the south- the US Bureau of Reclamation (BOR). Data from the drilling ern San Luis Basin (Fig. 1). The San Luis Basin is situated in the program were compiled, and preliminary correlations were made northern Rio Grande rift, a generally north-south trending series between deep wells to provide a stratigraphic framework for an of fault-bounded basins extending from Colorado to Mexico evaluation of the hydrology of the basin fill aquifer system (see (Hawley, 1978; Chapin and Cather, 1994). Rifting in the San Drakos et al., 2004, this volume). The area of this study includes Luis Basin dates from approximately 30-27 Ma and has resulted the region between the Sangre de Cristo mountain front on the in approximately 8-12% extension, primarily along north-south east and the Rio Grande on the west, the Rio Hondo on the north trending, down to the west normal faults (Tweto, 1979; Brister and the Rio Grande-Rio Pueblo de Taos confluence on the south and Gries, 1994; Chapin and Cather, 1994; Kluth and Schafte- (Fig. 1). naar, 1994). The southern San Luis Basin in the vicinity of Taos is a deep graben with predominant down-to-the-west faulting along METHODOLOGY the east margin (Bauer and Kelson, 2004, this volume). In the San Luis Basin near Alamosa, Tertiary and Quaternary sedimentary Numerous studies (e.g. Baldridge et al., 1984; Dungan et al., deposits exhibit average dips of 0° to 12° to the east (Chapin and 1984; Lipman and Mehnert, 1979) have defined syn-rift Ter- Cather, 1994). tiary sedimentary and volcanic units in the general vicinity of the study area. Well log data from this study, particularly from DESCRIPTION OF STRATIGRAPHIC UNITS several deep exploratory and production well borings, are used to constrain the subsurface extent of and relationships between From oldest to youngest, the units underlying the basin dis- rift-related sedimentary and volcanic units. Because the subtle cussed in this study are: 1) Pennsylvanian Alamitos Formation, differences between basin-fill units can be difficult to discern in 2) Tertiary Picuris Formation, 3) Tertiary Santa Fe Group, 4) Ter- drill cuttings and due to the relative paucity of data points, the tiary Servilleta Formation, and 5) Quaternary Alluvium. Galusha stratigraphic inferences and subsurface correlations presented in and Blick (1971) subdivided the Santa Fe Group in the Española this paper are preliminary. Data from thirteen wells greater than Basin into the Tesuque Formation and the overlying Chamita 1000 ft (300 m) deep, including lithologic logs of cuttings and Formation. The Tesuque Formation is further subdivided into the geophysical logs (electric logs, neutron, natural gamma, and cali- Chama-El Rito Member and the overlying Ojo Caliente Sandstone SUBSURFACE STRATIGRAPHY IN THE SOUTHERN SAN LUIS BASIN 375 Symbol Explanation Colorado Plateau SL R Major stream or river io Hondo Study Area 1 0 1 Miles Jemez y Mountains Volcanic 1 0 1 Kilometers Field . 150 E State Rd. 68 Hwy Southern Rock A US Hwy 64 Jemez Lineament Great Plains S Mogollon - Datil Volcanic Field US Hwy 64 aos de T io Grande Rift blo P R ue Rio Grande P Town of io R Taos M 68 State Rd. Sangre de Cristo Mountains Kilometers Study Area Detail FIGURE 1. Location map – schematic map of New Mexico showing study area and the approximate limits of various physiographic provinces and geographic features. Major basins in the Rio Grande rift from north to south are: SL=San Luis, E = Española, A = Albuquerque, S = Socorro, P = Palo- mas, M = Mimbres. (state map modified from Sanford et al., 1995 and Keller and Cather, 1994). Member (Fig. 2; Galusha and Blick, 1971). Although extending tan, white, and brown fine-grained sandstone beds and shale beds, this Santa Fe Group stratigraphic nomenclature into the south- with limestone beds representing approximately 50% of the pen- ern San Luis Basin may be problematic, it is used as an initial etrated section. The shallow depth at which the Paleozoic section framework for this investigation. Some revision of stratigraphic was encountered in the Town Yard well is attributed to offset on units may be required as additional surface and subsurface data the Town Yard fault (Figs. 3 and 4; Drakos and Lazarus, unpubl. become available. report to the Town of Taos 1997; Bauer et al., 1999; Drakos et Along the basin margin on the east side of the study area, al., 2001). the Tertiary deposits are in fault and depositional contact with older sedimentary and crystalline rocks within the Sangre de Picuris Formation (Tp) Cristo Mountains. While not discussed in detail in this paper, the crystalline rocks include Proterozoic plutonic and gneissic The oldest Tertiary unit encountered is the Oligocene to Mio- complexes in the Taos Range in the northern part of the study cene Picuris Formation, which is penetrated in the BOR2 deep area, and metasedimentary rocks (quartzite, schist, and phyllite) piezometer and the BOR3 production well (located at the same in the northern Picuris Mountains located in the southern part of site, 36 ft [11 m] from one another) (Fig.