Title: Geologic map of the Weldona 7.5' quadrangle, Morgan County, Colorado by Margaret E. Berry, Emily M. Taylor, Janet L. Slate, James B. Paces, Paul R. Hanson, and Theodore R. Brandt 2018 U.S. Geological Survey Scientific Investigations Map 3396 ABSTRACT: The Weldona 7.5' quadrangle is located on the semiarid plains of northeastern Colorado, along the South Platte River corridor where the river has incised into Upper Cretaceous Pierre Shale. The Pierre Shale is largely covered by surficial deposits that formed from alluvial, eolian, and hillslope processes operating in concert with environmental changes from the Pleistocene to the present. The South Platte River, originating high in the Colorado Rocky Mountains, has played a major role in shaping surficial geology in the map area, which is several tens of kilometers downstream from where headwater tributaries join the river. Recurrent glaciation (and deglaciation) of basin headwaters has affected river discharge and sediment supply far downstream, influencing deposition of alluvium and river incision in the Weldona quadrangle. During the Pleistocene the course of the river within the map area shifted progressively southward as it incised, and by late middle Pleistocene the river was south of its present position, cutting and filling deep paleochannels now covered by younger alluvium. The river shifted back to the north during the late Pleistocene. Kiowa and Bijou Creeks are unglaciated tributaries originating in the Colorado Piedmont east of the Front Range that also have played a major role in shaping surficial geology of the map area. Periodically during the late Pleistocene, major flood events on these tributaries deposited large volumes of sediment at their confluences, forming a broad, low-gradient fan of sidestream alluvium that could have occasionally dammed the river for short periods of time. Eolian sand deposits of the Sterling (north of river) and Fort Morgan (south of river) dune fields cover much of the quadrangle and record past episodes of sand mobilization during times of prolonged drought. With the onset of irrigation and damming during historical times, the South Platte River has changed from a broad, shallow, and sandy braided river with highly variable seasonal discharge to a much narrower, deeper river with braided-meandering transition morphology and more uniform discharge. DISCLAIMERS: This database, identified as SIM3396, 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 map sheet report is located at and can be downloaded via the Web from the U.S. Geological Survey publications website at https://doi.org/10.3113/sim3396 The GIS database for this report is located at and can be downloaded via the Web from ScienceBase at https://doi.org/10.5066/F7610Z63 ArcInfo export files, ArcView shapefiles, and an ArcGIS geodatabase may be extracted from a zip-compressed file (SIM3396GIS.zip). In addition, an Adobe Portable Document Format (PDF) file (SIM3396 map.pdf), 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, color-infrared and historical black and white aerial photographs, and NAIP imagery. Ralph R. Shroba (U.S. Geological Survey), and William R. Page (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 SIM3396.met for detailed metadata documentation for this geospatial database. List of files: SIM3396.met 00ReadMe.txt welbasr.tif welbasr.tfw SIM3396GIS.zip: e00 directory: weldgeo.e00 wecrests.e00 wegchpts.e00 wegcsite.e00 wpgcmykg.shd.e00 geol_sfo.lin.e00 geoscamp2.mrk.e00 import.aml shapefiles directory: weldgeo.shp (.dbf/.prj/.sbn/.sbx/.shx.xml/.shx) polygon weldgeol.shp (.dbf/.prj/.sbn/.sbx/.shx.xml/.shx) line wecrests.shp (.dbf/.prj/.sbn/.sbx/.shx.xml/.shx) line wegchpts.shp (.dbf/.prj/.sbn/.sbx/.shx.xml/.shx) point wegcsite.shp (.dbf/.prj/.sbn/.sbx/.shx.xml/.shx) point SIM3396GIS.gdb directory (contains GIS data as geodatabase feature classes) HOW TO OBTAIN THE DIGITAL MAP 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://dx.doi.org/10.3133/sim3396/ and follow the directions to download the files. The main product is a Portable Document Format (.pdf) map, which requires Adobe Acrobat for viewing. This 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. Publication Reference: Berry, M.E., Taylor, E.M., Slate, J.L., Paces, J.B., Hanson, P.R., and Brandt, T.R., 2018, Geologic map of the Weldona 7.5' quadrangle, Morgan County, Colorado: U.S. Geological Survey Scientific Investigations Map 3396, 1 sheet, scale 1:24,000, https://doi.org/10.3133/sim3396. HOW TO OBTAIN THE DIGITAL GIS 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/F7610Z63 ScienceBase Reference: Berry, M.E., Taylor, E.M., Slate, J.L., Paces, J.B., Hanson, P.R., and Brandt, T.R., 2018, Data release for the geologic map of the Weldona 7.5' quadrangle, Morgan County, Colorado : U.S. Geological Survey data release, https://doi.org/10.5066/F7610Z63. 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.