1_readme.txt Geologic Studies of the Platte River, South-Central Nebraska and Adjacent Areas - Geologic Maps, Subsurface Study, and Geologic History By Steven M. Condon 2005 Professional Paper 1706 U.S. Department of the Interior Gale Norton, Secretary U.S. Geological Survey P. Patrick Leahy, Acting Director Any use of trade, product, or firm names is for descriptive purposes only and does not imply endorsement by the U.S. Government. INTRODUCTION The Platte River of south-central Nebraska was studied at three scales to place the river in its geological context and to trace its evolution through geologic time. At the largest scale the Elm Creek West and the Newark 7.5 minute quadrangles were mapped. These quadrangles are located just west and just east of Kearney and serve to illustrate the main geomorphic elements of the present Platte River Valley. The central elements of the quadrangles are the Platte River channels, islands, and bottomlands, which are flanked by terraces that step up away from the river to the north and south. Significant other elements of the landscape are eolian sand and loess deposits. The geologic maps are supplemented by topographic profiles of the mapped terraces and graphical representations of subsurface units in test wells that occur within the quadrangles. An intermediate-scale study consisted of examining descriptions of well cuttings in a 17 county area in south-central Nebraska, which includes the Platte River Valley, and building a database of information about sediment lithology and thickness. The wells penetrated a sequence of gravel, sand, silt, and clay beds from the ground surface to the top of the subsurface Tertiary Ogallala Group or Cretaceous formations. The sequence consists of Pliocene-, Pleistocene-, and Holocene-age strata that document the deposition of a veneer of alluvium by late Tertiary and Quaternary streams intermixed with and overlain by wind-blown loess. Various isopleth and structure maps illustrate the distribution and alluvial architecture of the sedimentary sequence, and support the interpretation of former positions of the Platte River. A regional-scale study consisted of documenting the geologic history of the Front Range and adjacent mountains and depositional areas east of the mountains in Colorado, Wyoming, Kansas, and Nebraska from the end of the Cretaceous Period, about 65 million years ago, to the present. The structural and sedimentary history of the region is outlined, and a series of paleogeographic maps shows the development of the drainage system in the east- central Rocky Mountains and adjacent Plains. Ancestral South Platte, North Platte, and Laramie Rivers are recognized as early as the late Eocene, although the South Platte probably flowed to the southeast from the mountain front at that time. Deposits of the North Platte River are recognized on the west side of the Medicine Bow Mountains of Wyoming in the Miocene, and the presence of distinctive rock clasts indicates that the Laramie River flowed from the North Park area of Colorado northeast across a filled Laramie Basin and the Laramie Range of southeastern Wyoming in the Miocene. The present drainage system developed in the late Miocene to the Pliocene and included the capture and diversion of the South Platte River into its present channel. The combined North and South Platte Rivers deposited gravel and sand across Nebraska and flowed southeast from Kearney, Nebraska through the middle to late Pleistocene. Within the past 25,000 years the Platte River below Kearney was captured and diverted into its present course and confined there by bounding valley walls of loess. FILES AND FOLDERS: 1_readme.txt - this file. autohtml.exe, autorun.inf, index.html, showhtml.ini, and usgsid.ico - Windows and browser operating files. ElmCrkWestMap - data folder for the Elm Creek West quadrangle ElmCrkWestMap.txt - file list for the Elm Creek West folder NewarkMap - data folder for the Newark quadrangle NewarkMap.txt - file list for the Newark folder pp1706.pdf - text for Professional Paper 1706 as a 68-page PDF file pp1706_plate1.pdf - Plate 1: Geologic map and topographic profile of the Elm Creek West 7.5 minute quadrangle (38x36 inches; 7.4 MB) pp1706_plate2.pdf - Plate 2: Geologic map and topographic profile of the Newark 7.5 minute quadrangle (32x36 inches; 5.9 MB) - Report, appendices, and illustrations. version_history.txt - Citation and release information. Acrobat - Installer for Acrobat Reader for Macintosh and Windows. images - Images used for browser interface. Metadata are provided in the two map-data folders. SYSTEM REQUIREMENTS Windows - Intel Pentium or Xeon processor, 800 MHz minimum, 1.0GHz or higher recommended - Microsoft Windows XP, or Windows 2000, - 256 MB minimum of RAM, 512 MB or higher recommended - VGA color monitor that can display 256 colors (16.7 million colors recommended) UNIX - Most UNIX system-based, and Linux platforms should be capable of manipulating these files. All platforms - Web browser such as Firefox, Camino, Safari, Microsoft Internet Explorer, Netscape Browser version 7.0 or greater. Adobe Acrobat Reader 6.0 or higher. - GIS software capable of reading ESRI shapefile format is needed to take full advantage of the GIS- capabilities of this publication. This report is for sale on CDÑROM from: U.S. Geological Survey Information Services, National Mapping Division Box 25046 Denver Federal Center Denver, CO 80225-0046 Telephone: (888) ASK-USGS Current pricing information is available from http://mapping.usgs.gov/esic/prices/. ISBN: 0-411-30507-8 The bibliographical reference for this publication is: Condon, Steven M., 2005, Geologic studies of the Platte River, south-central Nebraska and adjacent areas - geologic maps, subsurface study, and geologic history: Professional Paper 1706 (CD-ROM). This report and any updates to it are available on line at http://pubs.usgs.gov/pp/pp1706/