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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>G. A. Izett</dc:contributor>
  <dc:creator>J. Wright Horton, Jr.</dc:creator>
  <dc:date>2005</dc:date>
  <dc:description>&lt;p&gt;&lt;span&gt;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., was drilled 2000 as the first in a series of new coreholes drilled in 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;to gain a comprehensive understanding of its three-dimensional character. This understanding is important for assessing ground-water resources in the region, as well as for learning about marine impacts on Earth. We studied crystalline-rock&amp;nbsp;&lt;/span&gt;&lt;span class="ScopusTermHighlight"&gt;ejecta&lt;/span&gt;&lt;span&gt;&amp;nbsp;and shock-metamorphosed&amp;nbsp;&lt;/span&gt;&lt;span class="ScopusTermHighlight"&gt;minerals&lt;/span&gt;&lt;span&gt;&amp;nbsp;from the&amp;nbsp;&lt;/span&gt;&lt;span class="ScopusTermHighlight"&gt;Langley&lt;/span&gt;&lt;span&gt;&amp;nbsp;&lt;/span&gt;&lt;span class="ScopusTermHighlight"&gt;core&lt;/span&gt;&lt;span&gt;&amp;nbsp;to determine what they reveal about the geology of crystalline rocks beneath the Atlantic Coastal Plain and how those rocks were affected by the&amp;nbsp;&lt;/span&gt;&lt;span class="ScopusTermHighlight"&gt;impact&lt;/span&gt;&lt;span&gt;. An unusual polymict diamicton, informally called the Exmore beds (upper Eocene), is 33.8 meters (m; 110.9 feet (ft)) thick and lies at a depth of 269.4 to 235.65 m (884.0 to 773.12 ft) in the&amp;nbsp;&lt;/span&gt;&lt;span class="ScopusTermHighlight"&gt;core&lt;/span&gt;&lt;span&gt;. This matrix-supported sedimentary deposit contains clasts of Tertiary and Cretaceous sediment (ranging up to boulder size) and sparse pebbles of&amp;nbsp;&lt;/span&gt;&lt;span class="ScopusTermHighlight"&gt;crystalline rock&lt;/span&gt;&lt;span&gt;. The matrix consists of muddy sand that contains abundant quartz grains and minor glauconite and potassium feldspar. Significantly, the sandy matrix of the Exmore beds contains sparse quartz grains (0.1 to 0.3 millimeter (0.004 to 0.012 inch) in diameter) that contain multiple sets of intersecting planar deformation features formerly referred to as shock lamellae. As many as five different sets have been observed in some quartz grains. Planar deformation features also occur in quartz grains in reworked crystalline-rock clasts in the Exmore beds. Such grains are clearly of shock-metamorphic origin. The presence of these features indicates that the quartz grains have experienced pressures greater than 6 gigapascals (GPa) and strain rates greater than 10&lt;/span&gt;&lt;sup&gt;6&lt;/sup&gt;&lt;span&gt;/second. Thus, the shock-metamorphosed quartz grains, although rare, provide clear and convincing evidence that the Exmore beds are of hybrid&amp;nbsp;&lt;/span&gt;&lt;span class="ScopusTermHighlight"&gt;impact&lt;/span&gt;&lt;span&gt;&amp;nbsp;origin. Identification of&amp;nbsp;&lt;/span&gt;&lt;span class="ScopusTermHighlight"&gt;shocked&lt;/span&gt;&lt;span&gt;&amp;nbsp;quartz grains in the&amp;nbsp;&lt;/span&gt;&lt;span class="ScopusTermHighlight"&gt;Langley&lt;/span&gt;&lt;span&gt;&amp;nbsp;&lt;/span&gt;&lt;span class="ScopusTermHighlight"&gt;core&lt;/span&gt;&lt;span&gt;&amp;nbsp;adds to the number of sites in the&amp;nbsp;&lt;/span&gt;&lt;span class="ScopusTermHighlight"&gt;structure&lt;/span&gt;&lt;span&gt;&amp;nbsp;where their presence is confirmed. Most of the clasts of&amp;nbsp;&lt;/span&gt;&lt;span class="ScopusTermHighlight"&gt;crystalline rock&lt;/span&gt;&lt;span&gt;&amp;nbsp;that are in and just below the Exmore beds are rounded, detrital, and typical of coastal plain sediments. However, a few have angular shapes and consist of cataclastically deformed felsite having aphanitic-porphyritic to aphanitic texture and peraluminous rhyolite composition. Three of these clasts contain quartz grains that display two sets of planar deformation features of shock-metamorphic origin. Shock-metamorphosed quartz is an integral part of the cataclastic fabric in these three clasts, indicating that both the fabric and the&amp;nbsp;&lt;/span&gt;&lt;span class="ScopusTermHighlight"&gt;shocked&lt;/span&gt;&lt;span&gt;&amp;nbsp;quartz were produced by the same high-energy&amp;nbsp;&lt;/span&gt;&lt;span class="ScopusTermHighlight"&gt;impact&lt;/span&gt;&lt;span&gt;&amp;nbsp;event. Some felsite clasts have spherulitic textures that may be features either of an&amp;nbsp;&lt;/span&gt;&lt;span class="ScopusTermHighlight"&gt;impact&lt;/span&gt;&lt;span&gt;&amp;nbsp;melt or of preimpact volcanic rocks. A weighted-mean total-fusion&amp;nbsp;&lt;/span&gt;&lt;sup&gt;40&lt;/sup&gt;&lt;span&gt;Ar/&lt;/span&gt;&lt;sup&gt;39&lt;/sup&gt;&lt;span&gt;Ar&amp;nbsp;&lt;/span&gt;&lt;span class="ScopusTermHighlight"&gt;age&lt;/span&gt;&lt;span&gt;&amp;nbsp;of 35.3±0.1 Ma (±lσ) for 19 analyses of 4 North American tektites records the&amp;nbsp;&lt;/span&gt;&lt;span class="ScopusTermHighlight"&gt;age&lt;/span&gt;&lt;span&gt;&amp;nbsp;of 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;event.&lt;/span&gt;&lt;/p&gt;</dc:description>
  <dc:format>application/pdf</dc:format>
  <dc:identifier>10.3133/pp1688E</dc:identifier>
  <dc:language>en</dc:language>
  <dc:publisher>U.S. Geological Survey</dc:publisher>
  <dc:title>Crystalline-rock ejecta and shocked minerals of the Chesapeake Bay impact structure, USGS-NASA Langley core, Hampton, Virginia, with supplemental constraints on the age of impact</dc:title>
  <dc:type>reports</dc:type>
</oai_dc:dc>