Title: Geologic Map of the Homestake Reservoir 7.5' Quadrangle, Eagle, Lake, and Pitkin Counties, Colorado By Chester A. Ruleman, Michael G. Frothingham, Theodore R. Brandt, Colin A. Shaw, Marc W. Caffee, Keith A. Brugger and Brent M. Goehring 2020 U.S. Geological Survey Scientific Investigations Map 3451 Data release available at https://doi.org/10.5066/P9ON6QBE ABSTRACT: The Homestake Reservoir 7.5' quadrangle lies at the northwestern end of the Upper Arkansas Valley, and headwaters of the Arkansas River, and the Roaring Fork, Fryingpan, and Eagle Rivers of the Colorado River system. The quadrangle lies within tectonic provinces of the 1.4 giga-annum (Ga) Picuris orogeny and includes the late Paleozoic Ancestral Rockies, Late Cretaceous-Paleocene Laramide orogeny, Oligo- cene-to-Miocene and Pliocene(?) volcanism, and Miocene to the present Rio Grande rift extensional tectonics. In the eastern half of the quad- rangle, high-angle, east-dipping, Neogene normal faults displace Proterozoic rocks, and locally Miocene-to-Pliocene(?) volcanic rocks. Many quartz veins and hydrothermally altered zones are exposed along the eastern flank of the quadrangle, indicative of the multiple tectonic episodes the region has experienced. The main intent of the map is to unravel the structural complexity by partitioning the structures and volcanism within the appropriate geologic interval. This ultimately permits accurate identification of geomorphic features suitable for chronologies related to landscape evolution studies, seismic and other natural hazard identification, ground and surface water modeling, and paleoclimatic studies. Within the western half of the quadrangle, Mesoproterozoic and Paleoproterozoic igneous and metamorphic rocks of 1.4 Ga St. Kevin Granite and 1.8–1.7 Ga Biotite gneiss and schist, respectively, are uplifted along the generally east- dipping, high-angle Sawatch fault system. In the northwest portion of the quadrangle, strands of the Homestake shear zone have been mapped, dated and assigned to the 1.4 Ga Picuris orogeny of northern New Mexico. 10Be and 26Al cosmogenic nuclide ages of the youngest glacial deposits indicate a last glacial maximum age of about 22–21 kilo-annum (ka) and complete deglaciation by about 14 kilo-annum, supported by chronologic studies in adjacent drainages. The Turquoise Lake impounding lateral and terminal moraine complex was deposited during late Pleistocene glacial maximum ~22–21 ka. No late Pleistocene tectonic activity is apparent within the quadrangle. DISCLAIMERS: This database, identified as SIM3451, 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/sim3451. List of files: 00ReadMe_sim3451.txt sim3451.pdf (print quality) sim3451_hillshade_base.pdf (map with the shaded relief base) sim3451_georeferenced.pdf (interactive georeferenced map) 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/sim3451 and follow the directions to download the files. The main product is a Portable Document Format (.pdf) map, which requires 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. That allows for measuring, viewing coordinates, and selecting features (points, lines, and polygons) to obtain attributes. Polygon attributes may be blocked by the "Topographic Basemap" layer; turning off this layer will allow polygon selection. In the sim3451_georeferenced.pdf file, 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. Suggested citation: Ruleman, C.A., Frothingham, M.G., Brandt, T.R., Shaw, C.A., Caffee, M.W., Brugger, K.A., and, Goehring, B.M., 2020, Geologic map of the Homestake Reservoir 7.5' quadrangle, Eagle, Lake, and Pitkin Counties, Colorado: U.S. Geological Survey Scientific Investigations Map 3451, 1:24,000, https://doi.org/10.3133/sim3451. 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 and NAIP imagery. Donald W. Sweetkind (U.S. Geological Survey), and Joseph L. Allen (Concord University, Athens, WV) reviewed the geologic map and accompanying text. The geospatial database that supports this work is formatted with the GeMS schema in ArcGIS and is zipped in a folder. A non-GeMS geodata- base version is also available. Please refer to the file SIM3451.met.txt for detailed metadata documentation for this geospatial database. List of files: HomestakeReservoir_GeMs_metadata.xml SIM3451.met.txt homebasr.tif homebasr.tfw HomestakeReservoir_GeMs.gdb.zip HR_MapUnitLabels_Final_Non_GeMS.zip hrhlsd.zip hrhlsd.tif 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/P9ON6QBE Suggested citation: Ruleman, C.A., Brandt, T.R., Frothingham, M.G., Shaw, C.A., Caffee, M.W., Brugger, K.A., and, Goehring, B.M., 2020, Data release for Geologic map of the Homestake Reservoir 7.5' quadrangle, Eagle, Lake, and Pitkin Counties, Colorado: U.S. Geological Survey data release, https://doi.org/10.5066/P9ON6QBE. 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.