Title: Geologic map of the upper Arkansas River Valley region, north-central Colorado By Karl S. Kellogg,1 Ralph R. Shroba,1 Chester A. Ruleman,1 Robert G. Bohannon,1 William C. McIntosh,2 Wayne R. Premo,1 Michael A. Cosca,1 Richard J. Moscati,1 and Theodore R. Brandt1 1 U.S. Geological Survey 2 New Mexico Institute of Mining and Technology, Socorro, NM 2017 U.S. Geological Survey Scientific Investigations Map 3382 U.S. Geological Survey data release https://doi.org/10.5066/F75B00XQ ABSTRACT: This 1:50,000-scale U.S. Geological Survey geologic map represents a compilation of the most recent geologic studies of the upper Arkansas River valley between Leadville and Salida, Colorado. The valley is structurally controlled by an extensional fault system that forms part of the prominent northern Rio Grande rift, an intra-continental region of crustal extension. This report also incorporates new detailed geologic mapping of previously poorly understood areas within the map area and reinterprets previously studied areas. The mapped region extends into the Proterozoic metamorphic and intrusive rocks in the Sawatch Range west of the valley and the Mosquito Range to the east. Paleozoic rocks are preserved along the crest of the Mosquito Range, but most of them have been eroded from the Sawatch Range. Numerous new isotopic ages better constrain the timing of both Proterozoic intrusive events, Late Cretaceous to early Tertiary intrusive events, and Eocene and Miocene volcanic episodes, including widespread ignimbrite eruptions. The U-Pb ages document extensive ~1,440-Ma granitic plutonism mostly north of Buena Vista that produced batholiths that intruded an older suite of ~1,760-Ma metamorphic rocks and ~1,700-Ma plutonic rocks. As a result of extension during the Neogene and possibly latest Paleogene, the graben underlying the valley is filled with thick basin-fill deposits (Dry Union Formation and older sediments), which occupy two sub-basins separated by a bedrock high near the town of Granite. The Dry Union Formation has undergone deep erosion since the late Miocene or early Pliocene. During the Pleistocene, ongoing steam incision by the Arkansas River and its major tributaries has been interrupted by periodic aggradation. From Leadville south to Salida as many as seven mapped alluvial depositional units, which range in age from early to late Pleistocene, record periodic aggradational events along these streams that are commonly associated with deposition of glacial outwash or bouldery glacial-flood deposits. Many previously unrecognized Neogene and Quaternary faults, some of the latter with possible Holocene displacement, have been identified on lidar (light detection and ranging) imagery which covers 59 percent of the map area. This imagery has also permitted more accurate remapping of glacial, fluvial, and mass-movement deposits and aided in the determination of their relative ages. Recently published 10 Beryllium (10Be) cosmogenic surface-exposure ages, coupled with our new geologic mapping, have revealed the timing and rates of late Pleistocene deglaciation. Glacial dams that impounded the Arkansas River at Clear Creek and possibly at Pine Creek failed at least three times during the middle and late Pleistocene, resulting in catastrophic floods and deposition of enormous boulders and bouldery alluvium downstream; at least two failures occurred during the late Pleistocene during the Pinedale glaciation. DISCLAIMERS: This database, identified as SIM3382, 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, pamphlet, and readme files for this report are located at and can be downloaded via the Web from at the U.S. Geological Survey publications website at https://doi.org/10.3133/sim3382 List of files: sim3382_00ReadMe.txt sim3382.pdf sim3382_sheet1_no_hillshade.pdf (map without a shaded relief base) SIM3382_sheet1_hillshade.pdf (map with shaded relief base added) sim3382_sheet1_georeferenced.pdf (georeferenced pdf) sim3382_sheet2.pdf DATA RELEASE FILES The GIS database (data release) for this report is located at and can be downloaded via the Web from ScienceBase at https://doi.org/10.5066/F75B00XQ ArcInfo export files, ArcView shapefiles, and an ArcGIS geodatabase may be extracted from a zip-compressed file (SIM3382GIS.zip). 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, 1:24,000 color aerial photographs, and NAIP imagery. Margaret E. Berry (U.S. Geological Survey), Vince Matthews (formerly of Colorado Geological Survey), and Joseph P. Colgan (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 SIM3382.met for detailed metadata documentation for this geospatial database. List of files: SIM3382.met uabase.tif (Upper Arkansas sim 3382 topographic base) uabase.tfw uahlsd.tif (Upper Arkansas sim 3382 shaded relief base) uahlsd.tfw SIM3382GIS.zip: e00 directory: uageo.e00 uadikes.e00 ualayers.e00 uafolds.e00 uaage.e00 uaash.e00 uapts.e00 uahlsd.e00 wpgcmykg.shd.e00 geol_sfo.lin.e00 geoscamp2.mrk.e00 import.aml shapefiles directory: uageo.shp (.dbf/.prj/.sbn/.sbx/.shx.xml/.shx) polygon uageol.shp (.dbf/.prj/.sbn/.sbx/.shx.xml/.shx) line uadikes.shp (.dbf/.prj/.sbn/.sbx/.shx.xml/.shx) line ualayers.shp (.dbf/.prj/.sbn/.sbx/.shx.xml/.shx) line uafolds.shp (.dbf/.prj/.sbn/.sbx/.shx.xml/.shx) line uaage.shp (.dbf/.prj/.sbn/.sbx/.shx.xml/.shx) point uaash.shp (.dbf/.prj/.sbn/.sbx/.shx.xml/.shx) point uapts.shp (.dbf/.prj/.sbn/.sbx/.shx.xml/.shx) point SIM3382GIS.gdb directory (contains GIS data as geodatabase feature classes). HOW TO OBTAIN THE DIGITAL MAP AND PAMPHLET 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/sim3382/ and follow the directions to download the files. The map product is a Portable Document Format (.pdf) map, which requires Adobe Acrobat for viewing. The Portable Document Format (PDF) file 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. Suggested citation: Kellogg, K.S., Shroba, R.R., Ruleman, C.A., Bohannon, R.G., McIntosh, W.C., Premo, W.R., Cosca, M.A., Moscati, R.J., and Brandt, T.R., 2017, Geologic map of the upper Arkansas River valley region, north-central Colorado: U.S. Geological Survey Scientific Investigations Map 3382, pamphlet 70 p., 2 sheets, scale 1:50,000, https://doi.org/10.3133/sim3382. HOW TO OBTAIN THE DIGITAL GIS 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 https://doi.org/10.5066/33823382 and follow the directions to download the files. Suggested citation: Kellogg, K.S., Shroba, R.R., Ruleman, C.A., Bohannon, R.G., McIntosh, W.C., Premo, W.R., Cosca, M.A., Moscati, R.J., and Brandt, T.R., 2017, Data release for geologic map of the upper Arkansas River valley region, north-central Colorado: U.S. Geological Survey data release, https://doi.org/10.5066/F75B00XQ. ArcReader may be downloaded free of charge from http://www.esri.com/software/arcgis/arcreader/download for viewing additional files included in this report. These maps are 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.