Title: Geologic Map of the Poncha Pass Area, Chaffee, Fremont, and Saguache Counties, Colorado By Scott A. Minor, Jonathan Saul Caine, Chester A. Ruleman, Christopher J. Fridrich, Christine F. Chan, Theodore R. Brandt, Christopher S. Holm-Denoma, Leah E. Morgan, Michael A. Cosca, and V.J.S. Grauch 2019 U.S. Geological Survey Scientific Investigations Map 3436 Data release available at https://doi.org/10.5066/P9GYYF4F ABSTRACT: This report presents a 1:24,000-scale geologic map, cross sections, and descriptive and interpretative text for the Poncha Pass area in central Colorado. The map area is irregular in shape, covering all of one 7.5' quadrangle (Poncha Pass) and parts of five others (Mount Ouray, Maysville, Salida West, Salida East, and Wellsville). The map boundaries were drawn to cover all of the "Poncha mountain block," our designation for the approximately 15-kilometer-long northwestern end of the Sangre de Cristo Mountains. The map conveys the areal distribution of: (1) Proterozoic basement rocks forming the core of the Poncha mountain block, (2) overlying Tertiary volcanic rocks, (3) Miocene and younger basin-fill deposits, (4) Quaternary surficial glacial and alluvial deposits, and (5) faults and folds affecting all of the above units. The Poncha mountain block, which lies within the Rio Grande rift, is topographically and geologically distinctive. Generally, the Rio Grande rift is internally characterized by subsided structural basins or grabens and subdued, low-relief topography rather than elevated mountain blocks. The intrarift, topographically high Poncha mountain block spans the axial part of the rift and separates the low-lying basins of the west-tilted Upper Arkansas River half graben and east-northeast-tilted San Luis half graben. These distinctive aspects of the Poncha mountain block were the primary motivations to conduct geologic mapping in the area. Important questions that can be addressed by geologic mapping and related studies in the Poncha Pass area include: (1) what were the structural controls and tectonic mechanism(s) that resulted in development of the Poncha mountain block in an intrarift environment; (2) did surface uplift of the Poncha block occur during rift development in the Neogene and Quaternary; (3) how was extensional strain accommodated and relayed across the Poncha block between the opposite-polarity rift basins and flanking mountain blocks; (4) is there a clear Laramide deformational signal in rocks of the map area; and (5) have earlier Laramide contractional structures, if they exist, influenced later rift-related extensional deformation through reactivation or strain localization. Prior to our mapping, the geology of much of the Poncha Pass area had only been mapped in reconnaissance fashion, reflecting the area’s poor bedrock exposures, poor access due to the rugged terrain, and geologic complexity. The map presented here provides new details of the geology of this difficult area and helps elucidate the development of the Poncha block and improves understanding of the geologic framework and geologic history of the area. DISCLAIMERS: This database, identified as SIM3436, 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. REPORT FILES: The map for this report is located at and can be downloaded via the web from the U.S. Geological Survey publications website at https://doi.org/10.3133/SIM3436. List of files: 00ReadMe_SIM3436.txt SIM3436_sheet1.pdf (geologic map, unit correlation chart, list of units, map explanation, accessory figures) SIM3436_sheet1_georeferenced.pdf (georeferenced geologic map, unit correlation chart, list of units, map explanation, accessory figures) SIM3436_sheet2.pdf (geologic cross sections, place-name figure) SIM3436_sheet3.pdf (description of map units, geologic summary, geochronologic data tables, field photos) How to Obtain the Digital Report Files: The map sheet can be obtained via the internet from the U.S. Geological Survey publications website. Go to the web page at https://doi.org/10.3133/SIM3436 and follow the directions to download the files. The main product are four Portable Document Format (.pdf) map sheets, which require Adobe Acrobat for viewing. These Portable Document Format (PDF) files can be downloaded, from which paper copies may be printed. Acrobat software runs on a variety of systems, and is available for download free of charge from Adobe at http://www.adobe.com. How to View Data for Features in the Georeferenced PDF File: To view map attribute data for map features in Adobe Acrobat, use the Object Data Tool. In the sim3436_sheet1.pdf and the sim3436_sheet1_georeferenced.pdf files, map layers containing "Symbols" have graphic features that are visually attributed and should be turned on when printing; map layers containing "Data" have graphic features with map attribute data. If an unattributed layer overlies a Data layer, turn off the unattributed layer to select the feature using the Object Data Tool. The Shaded Relief layer must be turned off in order to select features with the Object Data Tool. Suggested citation: Minor, S.A., Caine, J.S., Ruleman, C.A., Fridrich, C.J., Chan, C.F., Brandt, T.R., Holm-Denoma, C.S., Morgan, L.E., Cosca, M.A., and Grauch, V.J.S., 2019, Geologic map of the Poncha Pass area, Chaffee, Fremont, and Saguache Counties, Colorado: U.S. Geological Survey Scientific Investigations Map 3436, 3 sheets, scale 1:24,000, https://doi.org/10.3133/sim3436. DATA FILES: 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, NAIP imagery, and geophysical (gravity and aeromagnetic) data imagery. Geologic reviews of the map, cross sections, and accompanying text, figures, and data tables were conducted by Vincent Matthews (Leadville Geology LLC, Principal), and Karl S. Kellogg (U.S. Geological Survey, Emeritus). 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 SIM3436.met for detailed metadata documentation for this geospatial database. List of files: SIM3436.met ppsabasr.tif ppsabasr.tfw pphlsd.tif pphlsd.tfw SIM3436GIS.zip: e00 directory: poncgeo.e00 pondikes.e00 poncfolds.e00 ponclin.e00 ponsills.e00 ppsakung.e00 poncxc.e00 poncpnt.e00 ppslipg.e00 pupbloc.e00 parsloc.e00 pphlsd.e00 wpgcmykg.shd.e00 geol_sfo.lin.e00 geoscamp2.mrk.e00 import.aml shapefiles directory: poncgeo.shp (.dbf/.prj/.sbn/.sbx/.shx.xml/.shx) polygon poncgeol.shp (.dbf/.prj/.sbn/.sbx/.shx.xml/.shx) line pondikes.shp (.dbf/.prj/.sbn/.sbx/.shx.xml/.shx) line poncfolds.shp (.dbf/.prj/.sbn/.sbx/.shx.xml/.shx) line ponclin.shp (.dbf/.prj/.sbn/.sbx/.shx.xml/.shx) line ponsills.shp (.dbf/.prj/.sbn/.sbx/.shx.xml/.shx) line ppsakung.shp (.dbf/.prj/.sbn/.sbx/.shx.xml/.shx) line poncxc.shp (.dbf/.prj/.sbn/.sbx/.shx.xml/.shx) line poncpnt.shp (.dbf/.prj/.sbn/.sbx/.shx.xml/.shx) point ppslipg.shp (.dbf/.prj/.sbn/.sbx/.shx.xml/.shx) point pupbloc.shp (.dbf/.prj/.sbn/.sbx/.shx.xml/.shx) point parsloc.shp (.dbf/.prj/.sbn/.sbx/.shx.xml/.shx) point SIM3436GIS.gdb directory (contains GIS data as geodatabase feature classes) How to Obtain the Digital Data Files: The digital GIS files constituting the geologic map database of this report can be obtained from the U.S. Geological Survey via the Internet from ScienceBase. ScienceBase link: https://doi.org/10.5066/P9GYYF4F Suggested citation for the data release: Minor, S.A., Brandt, T.R., Caine, J.S., and Ruleman, C.A., 2019, Data release for geologic map of the Poncha Pass area, Chaffee, Fremont, and Saguache Counties, Colorado: U.S. Geological Survey data release, https://doi.org/10.5066/P9GYYF4F. ArcReader may be downloaded free of charge from http://www.esri.com/software/arcgis/arcreader/download for viewing additional files included in this report. This map 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.