Judith L. Hannah
Holly J. Stein
Nicholas J. Geboy
Leslie F. Ruppert
Gyana Ranjan Tripathy
2016
<div class="abstract svAbstract " data-etype="ab">
<p id="sp0110">Coal deposits are integral to understanding the structural evolution and thermal history of sedimentary basins and correlating contemporeous estuarine and fluvial delatic strata with marine sections. While marine shales may readily lend themselves to Re–Os dating due to the dominance of hydrogenous Re and Os, the lack of a chronometer for near-shore sedimentary environments hampers basinwide correlations in absolute time. Here, we employ the Re–Os geochronometer, along with total organic carbon (TOC) and Rock–Eval data, to determine the timing and conditions of a marine incursion at the top of the Matewan coal bed, Kanawha Formation, Pottsville Group, West Virginia, USA. The observed range for hydrogen index (HI: 267–290 mg hydrocarbon/gram total organic carbon) for these coal samples suggests dominance of aliphatic hydrocarbons with low carbon (<C<sub>19</sub>) chain length. Average Re (<span id="mmlsi1" class="mathmlsrc"><span class="formulatext stixSupport mathImg" title="Click to view the MathML source" data-mathurl="/science?_ob=MathURL&_method=retrieve&_eid=1-s2.0-S0012821X15006056&_mathId=si1.gif&_user=111111111&_pii=S0012821X15006056&_rdoc=1&_issn=0012821X&md5=bc2944291302252512cfdefae42b865b">107.6±16.4 ng/g</span></span>) and Os (<span id="mmlsi2" class="mathmlsrc"><span class="formulatext stixSupport mathImg" title="Click to view the MathML source" data-mathurl="/science?_ob=MathURL&_method=retrieve&_eid=1-s2.0-S0012821X15006056&_mathId=si2.gif&_user=111111111&_pii=S0012821X15006056&_rdoc=1&_issn=0012821X&md5=8dec0d0a6e0498d3d756ca94a204da46">0.52±0.09 ng/g</span></span>) concentrations of the marine-influenced Matewan coal are higher by few orders of magnitude than published data for terrestrial coal. A Re–Os isochron for the Matewan coal provides an age of <span id="mmlsi3" class="mathmlsrc"><span class="formulatext stixSupport mathImg" title="Click to view the MathML source" data-mathurl="/science?_ob=MathURL&_method=retrieve&_eid=1-s2.0-S0012821X15006056&_mathId=si3.gif&_user=111111111&_pii=S0012821X15006056&_rdoc=1&_issn=0012821X&md5=c75685db623fd2b8eb6fe1a800af416d">325±14 Ma</span></span> (Model 3; MSWD = 12; <span id="mmlsi4" class="mathmlsrc"><span class="formulatext stixSupport mathImg" title="Click to view the MathML source" data-mathurl="/science?_ob=MathURL&_method=retrieve&_eid=1-s2.0-S0012821X15006056&_mathId=si4.gif&_user=111111111&_pii=S0012821X15006056&_rdoc=1&_issn=0012821X&md5=82cb9a0cbbb37111e82dde71ca23285d">n=19</span></span>; 2<i>σ </i>). This is the first Re–Os age derived from coal samples; the age overlaps a new composite Re–Os age of <span id="mmlsi26" class="mathmlsrc"><span class="formulatext stixSupport mathImg" title="Click to view the MathML source" data-mathurl="/science?_ob=MathURL&_method=retrieve&_eid=1-s2.0-S0012821X15006056&_mathId=si26.gif&_user=111111111&_pii=S0012821X15006056&_rdoc=1&_issn=0012821X&md5=11d71f446454cdf5c83161c90c00d583">317±2 Ma</span></span> for the immediately overlying Betsie Shale Member.</p>
<p id="sp0120">External precision for replicate Os analyses carried out for several Matewan coal samples shows a positive correlation with their HI. The HI, which is low in terrestrial organic matter, reflects the degree of marine influence. Thus, samples with the most profound marine influence also have the best analytical reproducibility. Equilibration of Os isotopes with seawater under marine conditions overwhelms variability inherited from terrestrial plant debris, decreasing scatter on the isochron. The <sup>187</sup>Re/<sup>188</sup>Os ratios of the Matewan coal (∼3300–5135) are higher than most of those previously published for Phanerozoic black shale (mostly <2000). Mass balance calculations based on Re/TOC and Os/TOC ratios for the Matewan coal indicate that both Re and Os are primarily marine in origin, and their high <sup>187</sup>Re/<sup>188</sup>Os ratios confirm efficient removal of both elements from a sulfidic water column into the coal. We show that Re–Os geochronology of marine-influenced coal can be a viable tool for constraining depositional ages.</p>
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application/pdf
10.1016/j.epsl.2015.09.030
en
North-Holland Pub. Co.
Radiometric dating of marine-influenced coal using Re–Os geochronology
article