Title: Geologic map of the Alamosa 30’ x 60’ quadrangle, South-Central Colorado by Ren A. Thompson, Ralph R. Shroba, Michael N. Machette, Christopher J. Fridrich, Theodore R. Brandt, and Michael A. Cosca 2015 Scientific Investigations Map 3342 Note: This map supersedes earlier preliminary OFRs: Open-File Report 2005–1392 “Preliminary Geologic Map of the Northwestern Part of the Alamosa 30'× 60' Minute Quadrangle, Alamosa and Conejos Counties, Colorado” Open-File Report 2008–1124 “Preliminary Geologic Map of the North-Central Part of the Alamosa 30' × 60' Quadrangle, Alamosa, Conejos and Costilla Counties, Colorado” ABSTRACT: The Alamosa 30’x 60’quadrangle is located in the central San Luis Basin of southern Colorado and is bisected by the Rio Grande having headwaters in the San Juan Mountains of Colorado and ultimately discharging into the Gulf of Mexico > 3000 km downstream. Alluvial floodplains and associated deposits of the Rio Grande and east-draining tributaries, La Jara Creek and Conejos River, occupy the north central and northwestern part of the map area. Alluvial deposits of west-draining Rio Grande tributaries, Culebra and Costilla creeks, bound the Costilla Plain in the south central part of the map area. The San Luis Hills, a northeast trending series of flat-topped mesas and hills, dominate the landscape in the central and southwestern part of the map and preserve fault-bound Neogene basin surfaces and deposits. The Precambrian-cored Sangre de Cristo Mountains rise to an elevation of nearly 4300 meters, almost 2000 meters above the valley floor, in the eastern part of the map area. In total, the map area contains deposits that record surficial, tectonic sedimentary, volcanic, magmatic and metamorphic processes over the past 1.7 billion years. The mapped distribution of units is based primarily on interpretation of U.S. Geological Survey (USGS) 1:40,000-scale, black-and-white, aerial photographs; U.S. Department of Agriculture (http://www.fsa.usda.gov/FSA/apfoapp?area=home&subject=prog&topic=nai), color orthoimagery; 2012 GeoEye (http://www.digitalglobe.com/resources/satellite-information) satellite imagery; 1:24,000-scale topographic maps and associated shaded-relief imagery; and limited use of lidar imagery. This geologic map compilation is based, in part, on previous mapping and reflects varying degrees of new synthesis and reinterpretation. Mapped surficial deposits and descriptions reflect reinterpretation and reclassification of units and revisions to geographic extent, lithologic character and map unit representation, particularly with respect to mapping of tills in glaciated valleys on the east flanks of the Sangre de Cristo Mountains and Culebra Range. Bedrock mapping represents a regional synthesis of geologic data compiled and published over the past quarter century and necessarily includes both simplification and expansion of previously published data with respect to the adopted stratigraphic nomenclature and aerial depiction of map units and faults. Regional and local unit nomenclature is constrained by reference to recently published and previously unpublished 40Ar/39Ar age determinations on Neogene volcanic deposits, the primary Cenozoic stratigraphic markers in the map area. Map unit descriptions rely heavily on previously published data and are simplified to embody the dominant characteristics of map units, particularly where units depicted on large-scale maps were combined for inclusion in this 1:100,000-scale map. Fault depictions are largely simplified and reflect our interpretation of the most regionally significant and better-exposed structures. Previously mapped inferred faults, particularly those buried beneath younger surficial deposits in the San Luis Valley, typically have large locational uncertainties and are poorly dated. Consequently, depiction of these buried faults on this map compilation is minimized. This map provides geologic information suitable for regional lithologic, structure, tectonic, volcanic and surficial geologic interpretation and modeling in support of land-use decision making, subsurface water resource evaluation and determination of regional geologic history. DISCLAIMERS: This database, identified as SIM3342, has been approved for release by the U.S. Geological Survey (USGS). Although this database has been subjected to rigorous review and is substantially complete, the USGS reserves the right to revise the data pursuant to further analysis and review. Furthermore, it is released on condition that neither the USGS nor the United States Government may be held liable for any damages resulting from its authorized or unauthorized use. Although these data have been processed successfully on a computer system at the U.S. Geological Survey, no warranty, expressed or implied is made regarding the display or utility of the data on any other system, or for general or scientific purposes, nor shall the act of distribution constitute such warranty. The USGS or the U.S. Government shall not be held liable for improper or incorrect use of the data described and/or contained herein. Any use of trade, product or firm names is for descriptive purposes only and does not imply endorsement by the U.S. Geological Survey. Although this information product, for the most part, is in the public domain, it also may contain copyrighted material as noted in the text. Permission to reproduce copyrighted items for other than personal use must be secured from the copyright owner. DATA FILES: The database can be downloaded via the Web from http://dx.doi.org/10.3133/sim3342. ArcInfo export files, ArcView shapefiles, and an ArcGIS geodatabase may be extracted from a zip-compressed file (SIM3342GIS.zip). In addition, an Adobe Portable Document Format (PDF) file (SIM3342 map.pdf, or SIM3342_map_hillshade) can be downloaded, from which paper copies may be printed. The database contact is: Theodore R. Brandt 303-236-1901 tbrandt@usgs.gov U.S. Geological Survey Box 25046, Mail Stop 980 Denver, CO 80225 The data were compiled from field mapping, interpretation of lidar data, and NAIP imagery. Paul E. Carrara (U.S. Geological Survey), and David J. Lidke (U.S. Geological Survey), reviewed the geologic map and accompanying text. This geospatial database consists of ArcInfo export files (.e00), ArcView shapefiles, and ArcGIS geodatabase files contained in a zip archive. See the list below. Please refer to the file SIM3342.met for detailed metadata documentation for this geospatial database. List of files: 00ReadMe.txt SIM3342pamphlet.pdf SIM3342_map.pdf SIM3342_map_hillshade.pdf (map with shaded relief base added) SIM3342.met alambasr.tif alambasr.tfw alamhlsd.tif alamhlsd.tfw SIM3342GIS.zip: e00 directory: amgeo.e00 amdikes.e00 amfolds.e00 amarage.e00 ampts.e00 amash.e00 amdrill.e00 alamhlsd.e00 wpgcmykg.shd.e00 geol_sfo.lin.e00 geoscamp2.mrk.e00 import.aml shapefiles directory: amgeo.shp (.dbf/.prj/.sbn/.sbx/.shx.xml/.shx) polygon amgeol.shp (.dbf/.prj/.sbn/.sbx/.shx.xml/.shx) line amdikes.shp (.dbf/.prj/.sbn/.sbx/.shx.xml/.shx) line amfolds.shp (.dbf/.prj/.sbn/.sbx/.shx.xml/.shx) line amarage.shp (.dbf/.prj/.sbn/.sbx/.shx.xml/.shx) point ampts.shp (.dbf/.prj/.sbn/.sbx/.shx.xml/.shx) point amash.shp (.dbf/.prj/.sbn/.sbx/.shx.xml/.shx) point amdrill.shp (.dbf/.prj/.sbn/.sbx/.shx.xml/.shx) point SIM3342GIS.gdb.zip directory (contains GIS data as geodatabase feature classes) HOW TO OBTAIN THE DIGITAL FILES: The digital files constituting the geologic map database of this report can be obtained via the Internet from the U.S. Geological Survey publications website. Go to the web page at http://dx.doi.org/10.3133/sim3342/ and follow the directions to download the files. The main product is a Portable Document Format (.pdf) map, which requires Adobe Acrobat for viewing. Acrobat software runs on a variety of systems, and is available for download free of charge from Adobe at http://www.adobe.com. ArcReader may be downloaded free of charge from http://www.esri.com/software/arcgis/arcreader/download.htm for viewing additional files included in this report.