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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>Charles Sandberg</dc:contributor>
  <dc:contributor>K. Malkowski</dc:contributor>
  <dc:contributor>M.M. Joachimski</dc:contributor>
  <dc:creator>J. R. Morrow</dc:creator>
  <dc:date>2009</dc:date>
  <dc:description>&lt;p&gt;At Hancock Summit West, Nevada, western USA, uppermost Givetian (upper Middle Devonian) and lower and middle Frasnian (lower Upper Devonian) rocks of the lower Guilmette Formation include, in stratigraphic sequence, carbonate-platform facies of the conodont&lt;span&gt;&amp;nbsp;&lt;/span&gt;&lt;i&gt;falsiovalis&lt;/i&gt;,&lt;span&gt;&amp;nbsp;&lt;/span&gt;&lt;i&gt;transitans&lt;/i&gt;, and&lt;span&gt;&amp;nbsp;&lt;/span&gt;&lt;i&gt;punctata&lt;/i&gt;&lt;span&gt;&amp;nbsp;&lt;/span&gt;Zones; the type Alamo Breccia Member of the middle&lt;span&gt;&amp;nbsp;&lt;/span&gt;&lt;i&gt;punctata&lt;/i&gt;&lt;span&gt;&amp;nbsp;&lt;/span&gt;Zone; and slope facies of the&lt;span&gt;&amp;nbsp;&lt;/span&gt;&lt;i&gt;punctata&lt;/i&gt;&lt;span&gt;&amp;nbsp;&lt;/span&gt;and&lt;span&gt;&amp;nbsp;&lt;/span&gt;&lt;i&gt;hassi&lt;/i&gt;&lt;span&gt;&amp;nbsp;&lt;/span&gt;Zones. The catastrophically deposited Alamo Breccia and related phenomena record the ~&amp;nbsp;382&amp;nbsp;Ma Alamo event, produced by a km-scale bolide impact into a marine setting seaward of an extensive carbonate platform fringing western North America. Re-evaluation of conodonts from the lower Guilmette Formation and Alamo Breccia Member, together with regional sedimentologic and conodont biofacies comparisons, now firmly locates the onset of the Johnson et al. (1985) transgressive–regressive (T–R) cycle IIc, which occurred after the start of the&lt;span&gt;&amp;nbsp;&lt;/span&gt;&lt;i&gt;punctata&lt;/i&gt;&lt;span&gt;&amp;nbsp;&lt;/span&gt;Zone, within a parautochthonous megablock low in the Alamo Breccia.&lt;/p&gt;&lt;p&gt;Whole-rock carbon isotope analyses through the lower Guilmette Formation and Alamo Breccia Member reveal two positive&lt;span&gt;&amp;nbsp;&lt;/span&gt;&lt;i&gt;δ&lt;/i&gt;&lt;sup&gt;13&lt;/sup&gt;C&lt;sub&gt;carb&lt;/sub&gt;&lt;span&gt;&amp;nbsp;&lt;/span&gt;excursions: (1) a small, 3‰ excursion, which is possibly correlative with the&lt;span&gt;&amp;nbsp;&lt;/span&gt;&lt;i&gt;falsiovalis&lt;/i&gt;&lt;span&gt;&amp;nbsp;&lt;/span&gt;Event previously identified from sections in Western Europe and Australia, occurs below the breccia in the Upper&lt;span&gt;&amp;nbsp;&lt;/span&gt;&lt;i&gt;falsiovalis&lt;/i&gt;&lt;span&gt;&amp;nbsp;&lt;/span&gt;Zone to early part of the&lt;span&gt;&amp;nbsp;&lt;/span&gt;&lt;i&gt;transitans&lt;/i&gt;&lt;span&gt;&amp;nbsp;&lt;/span&gt;Zone; and (2) a large, multi-part excursion, dominated by a 6‰ positive shift, begins above the start of the&lt;span&gt;&amp;nbsp;&lt;/span&gt;&lt;i&gt;punctata&lt;/i&gt;&lt;span&gt;&amp;nbsp;&lt;/span&gt;Zone and onset of T–R cycle IIc and continues above the Alamo Breccia, ending near the&lt;span&gt;&amp;nbsp;&lt;/span&gt;&lt;i&gt;punctata&lt;/i&gt;–&lt;span&gt;&amp;nbsp;&lt;/span&gt;&lt;i&gt;hassi&lt;/i&gt;&lt;span&gt;&amp;nbsp;&lt;/span&gt;zonal boundary. This large excursion correlates with the&lt;span&gt;&amp;nbsp;&lt;/span&gt;&lt;i&gt;punctata&lt;/i&gt;&lt;span&gt;&amp;nbsp;&lt;/span&gt;Event, a major positive&lt;span&gt;&amp;nbsp;&lt;/span&gt;&lt;i&gt;δ&lt;/i&gt;&lt;sup&gt;13&lt;/sup&gt;C excursion previously recognized in eastern Laurussia and northern Gondwana. Consistent with previous studies, at Hancock Summit West the&lt;span&gt;&amp;nbsp;&lt;/span&gt;&lt;i&gt;punctata&lt;/i&gt;&lt;span&gt;&amp;nbsp;&lt;/span&gt;Event is apparently not associated with any regional extinctions or ecosystem reorganizations.&lt;/p&gt;&lt;p&gt;In the study area, onset of the main&lt;span&gt;&amp;nbsp;&lt;/span&gt;&lt;i&gt;punctata&lt;/i&gt;&lt;span&gt;&amp;nbsp;&lt;/span&gt;Event began after the start of both the&lt;span&gt;&amp;nbsp;&lt;/span&gt;&lt;i&gt;punctata&lt;/i&gt;&lt;span&gt;&amp;nbsp;&lt;/span&gt;Zone and T–R cycle IIc, and preceded the Alamo impact by less than 650&amp;nbsp;k.y., as inferred from conodont biochronologic and regional rock-accumulation rate estimates. Although complicated by the heterolithic, high-energy deposits of the Alamo Breccia, the carbon isotope record of the breccia and post-breccia beds does not indicate a major impact-correlative perturbation to the carbon cycle.&lt;/p&gt;&lt;p&gt;This study extends recognition of the&lt;span&gt;&amp;nbsp;&lt;/span&gt;&lt;i&gt;punctata&lt;/i&gt;&lt;span&gt;&amp;nbsp;&lt;/span&gt;Event to western Laurussia, further reinforcing the potential global scale of the event and its potential importance to understanding early to middle Frasnian marine geochemistry and palaeoenvironments. Based on previous models and our observations, increased tectonic activity, increased nutrient flux to oceans, increased marine bioproductivity, widespread anoxia, and increased organic carbon burial were all likely key factors in driving the&lt;span&gt;&amp;nbsp;&lt;/span&gt;&lt;i&gt;punctata&lt;/i&gt;&lt;span&gt;&amp;nbsp;&lt;/span&gt;Event excursion. Furthermore, periodic eustatic and regional relative sea-level rises may have played an important role in promoting organic carbon burial and in maintaining a link between the primary open-marine geochemical signal and that recorded on the shallow-marine, lower Guilmette carbonate platform.&lt;/p&gt;</dc:description>
  <dc:format>application/pdf</dc:format>
  <dc:identifier>10.1016/j.palaeo.2009.08.016</dc:identifier>
  <dc:language>en</dc:language>
  <dc:publisher>Elsevier</dc:publisher>
  <dc:title>Carbon isotope chemostratigraphy and precise dating of middle Frasnian (lower Upper Devonian) Alamo Breccia, Nevada, USA</dc:title>
  <dc:type>article</dc:type>
</oai_dc:dc>