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<oai_dc:dc xmlns:dc="http://purl.org/dc/elements/1.1/" xmlns:oai_dc="http://www.openarchives.org/OAI/2.0/oai_dc/" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://www.openarchives.org/OAI/2.0/oai_dc/ http://www.openarchives.org/OAI/2.0/oai_dc.xsd">
  <dc:contributor>David S. Powars</dc:contributor>
  <dc:contributor>Gregory Gohn</dc:contributor>
  <dc:creator>J. Wright Horton, Jr.</dc:creator>
  <dc:date>2005</dc:date>
  <dc:description>&lt;p&gt;&lt;span&gt;The late Eocene&amp;nbsp;&lt;/span&gt;&lt;span class="ScopusTermHighlight"&gt;Chesapeake&lt;/span&gt;&lt;span&gt;&amp;nbsp;&lt;/span&gt;&lt;span class="ScopusTermHighlight"&gt;Bay&lt;/span&gt;&lt;span&gt;&amp;nbsp;&lt;/span&gt;&lt;span class="ScopusTermHighlight"&gt;impact&lt;/span&gt;&lt;span&gt;&amp;nbsp;&lt;/span&gt;&lt;span class="ScopusTermHighlight"&gt;structure&lt;/span&gt;&lt;span&gt;&amp;nbsp;on the Atlantic margin of&amp;nbsp;&lt;/span&gt;&lt;span class="ScopusTermHighlight"&gt;Virginia&lt;/span&gt;&lt;span&gt;&amp;nbsp;is the largest known&amp;nbsp;&lt;/span&gt;&lt;span class="ScopusTermHighlight"&gt;impact&lt;/span&gt;&lt;span&gt;&amp;nbsp;crater&amp;nbsp;&lt;/span&gt;&lt;span class="ScopusTermHighlight"&gt;in&lt;/span&gt;&lt;span&gt;&amp;nbsp;the United States, and it may be the Earth's best preserved example of a large&amp;nbsp;&lt;/span&gt;&lt;span class="ScopusTermHighlight"&gt;impact&lt;/span&gt;&lt;span&gt;&amp;nbsp;crater that formed on a predominantly siliciclastic continental shelf. The 85-kilometer-wide (53-milewide) crater also coincides with a region of saline ground water. It has a profound influence on ground-water quality and flow&amp;nbsp;&lt;/span&gt;&lt;span class="ScopusTermHighlight"&gt;in&lt;/span&gt;&lt;span&gt;&amp;nbsp;an area of urban growth. The USGS-NASA&amp;nbsp;&lt;/span&gt;&lt;span class="ScopusTermHighlight"&gt;Langley&lt;/span&gt;&lt;span&gt;&amp;nbsp;corehole at&amp;nbsp;&lt;/span&gt;&lt;span class="ScopusTermHighlight"&gt;Hampton&lt;/span&gt;&lt;span&gt;, Va., is the first&amp;nbsp;&lt;/span&gt;&lt;span class="ScopusTermHighlight"&gt;in&lt;/span&gt;&lt;span&gt;&amp;nbsp;a series of new coreholes being drilled&amp;nbsp;&lt;/span&gt;&lt;span class="ScopusTermHighlight"&gt;in&lt;/span&gt;&lt;span&gt;&amp;nbsp;the crater, and it is the first corehole to penetrate the entire crater-fill section and uppermost crystalline&amp;nbsp;&lt;/span&gt;&lt;span class="ScopusTermHighlight"&gt;basement&lt;/span&gt;&lt;span&gt;&amp;nbsp;rock. The&amp;nbsp;&lt;/span&gt;&lt;span class="ScopusTermHighlight"&gt;Langley&lt;/span&gt;&lt;span&gt;&amp;nbsp;corehole is located&amp;nbsp;&lt;/span&gt;&lt;span class="ScopusTermHighlight"&gt;in&lt;/span&gt;&lt;span&gt;&amp;nbsp;the southwestern part of the crater's annular trough. A comprehensive effort to understand the crater's materials, architecture, geologic&amp;nbsp;&lt;/span&gt;&lt;span class="ScopusTermHighlight"&gt;history&lt;/span&gt;&lt;span&gt;, and formative processes, as well as its influence on ground water, includes the drilling of coreholes accompanied by high-resolution seismic-reflection and seismic-refraction surveys, audio-magnetotelluric surveys, and related multidisciplinary research. The studies of the&amp;nbsp;&lt;/span&gt;&lt;span class="ScopusTermHighlight"&gt;core&lt;/span&gt;&lt;span&gt;&amp;nbsp;presented&amp;nbsp;&lt;/span&gt;&lt;span class="ScopusTermHighlight"&gt;in&lt;/span&gt;&lt;span&gt;&amp;nbsp;this volume provide detailed information on the outer part of the crater, including the crystalline&amp;nbsp;&lt;/span&gt;&lt;span class="ScopusTermHighlight"&gt;basement&lt;/span&gt;&lt;span&gt;, the overlying&amp;nbsp;&lt;/span&gt;&lt;span class="ScopusTermHighlight"&gt;impact&lt;/span&gt;&lt;span&gt;-modified and&amp;nbsp;&lt;/span&gt;&lt;span class="ScopusTermHighlight"&gt;impact&lt;/span&gt;&lt;span&gt;-generated sediments (physical geology, paleontology, shocked minerals, and crystalline ejecta), and the upper Eocene to Quaternary postimpact sedimentary section (stratigraphy, paleontology, and paleoenvironments). The USGS-NASA&amp;nbsp;&lt;/span&gt;&lt;span class="ScopusTermHighlight"&gt;Langley&lt;/span&gt;&lt;span&gt;&amp;nbsp;corehole has a total depth below land surface of 635.1 meters (m; 2,083.8 feet (ft)). The deepest unit&amp;nbsp;&lt;/span&gt;&lt;span class="ScopusTermHighlight"&gt;in&lt;/span&gt;&lt;span&gt;&amp;nbsp;the corehole is the Neoproterozoic&amp;nbsp;&lt;/span&gt;&lt;span class="ScopusTermHighlight"&gt;Langley&lt;/span&gt;&lt;span&gt;&amp;nbsp;Granite. The top of this granite at 626.3 m (2,054.7 ft) depth is overlain by 390.6 m (1,281.6 ft) of&amp;nbsp;&lt;/span&gt;&lt;span class="ScopusTermHighlight"&gt;impact&lt;/span&gt;&lt;span&gt;-modified and&amp;nbsp;&lt;/span&gt;&lt;span class="ScopusTermHighlight"&gt;impact&lt;/span&gt;&lt;span&gt;-generated siliciclastic sediments. These crater-fill materials are preserved beneath a 235.6-m-thick (773.12-ft-thick) blanket of postimpact sediments. A high-resolution seismic-reflection and seismic-refraction profile that crosses the&amp;nbsp;&lt;/span&gt;&lt;span class="ScopusTermHighlight"&gt;Langley&lt;/span&gt;&lt;span&gt;&amp;nbsp;drill site is tied to the&amp;nbsp;&lt;/span&gt;&lt;span class="ScopusTermHighlight"&gt;core&lt;/span&gt;&lt;span&gt;&amp;nbsp;by borehole geophysical logs, and it reveals the details of extensional collapse structures&amp;nbsp;&lt;/span&gt;&lt;span class="ScopusTermHighlight"&gt;in&lt;/span&gt;&lt;span&gt;&amp;nbsp;the western annular trough. Electrical cross sections based on audio-magnetotelluric (AMT) soundings image a nearly vertical zone of high resistivity at the outer margin of the annular trough, possibly indicating fresh ground water at that location, and they show impedance trends that match the curvature of the&amp;nbsp;&lt;/span&gt;&lt;span class="ScopusTermHighlight"&gt;structure&lt;/span&gt;&lt;span&gt;. They also image the subsurface contact between conductive sediments and resistive crystalline&amp;nbsp;&lt;/span&gt;&lt;span class="ScopusTermHighlight"&gt;basement&lt;/span&gt;&lt;span&gt;, showing that the depth to crystalline&amp;nbsp;&lt;/span&gt;&lt;span class="ScopusTermHighlight"&gt;basement&lt;/span&gt;&lt;span&gt;&amp;nbsp;is relatively constant&amp;nbsp;&lt;/span&gt;&lt;span class="ScopusTermHighlight"&gt;in&lt;/span&gt;&lt;span&gt;&amp;nbsp;the western part of the annular trough. Chemical and isotopic data indicate that saline ground water of the&amp;nbsp;&lt;/span&gt;&lt;span class="ScopusTermHighlight"&gt;Virginia&lt;/span&gt;&lt;span&gt;&amp;nbsp;inland saltwater wedge or bulge is a mixture of freshwater and seawater, and evidence for a mixing zone at the crater's outer margin supports the concept of differential flushing of residual seawater to create the bulge. Ground-water brine&amp;nbsp;&lt;/span&gt;&lt;span class="ScopusTermHighlight"&gt;in&lt;/span&gt;&lt;span&gt;&amp;nbsp;the central part of the crater was produced by evaporation, and brine production from the heat of the&amp;nbsp;&lt;/span&gt;&lt;span class="ScopusTermHighlight"&gt;impact&lt;/span&gt;&lt;span&gt;&amp;nbsp;is at least theoretically possible.&lt;/span&gt;&lt;/p&gt;</dc:description>
  <dc:format>application/pdf</dc:format>
  <dc:identifier>10.3133/pp1688A</dc:identifier>
  <dc:language>en</dc:language>
  <dc:publisher>U.S. Geological Survey</dc:publisher>
  <dc:title>Studies of the Chesapeake Bay impact structure - Introduction and discussion</dc:title>
  <dc:type>reports</dc:type>
</oai_dc:dc>