Two cores containing thin organic-rich phosphatic black shale members, the Excello and Little Osage shales, and transitional lithologies from Pennsylvania cyclothems of southeast Kansas and northeast Oklahoma were characterized by geochemical and organic petrographic methods by very detailed sampling. Variations in relative amount of primary organic material, i.e. liptinitic versus humic, exerts a major control on the nature and quality of the organic profiles. Organic petrography, Rock-Eval pyrolysis, n-alkane and isoprenoid distributions, and C$\sb{27}$/C$\sb{29}$-sterane ratios indicate that shallow-water, more oxic facies (e.g. fossiliferous shales and limestones) contain dominantly terrestrial organic matter while "deep" water phosphatic organic-rich black shales contain significant proportions of marine algal organic matter. Pyrolysis Transformation Ratios ${\rm (S\sb 1/S\sb 1 + S\sb 2)}$ were lower for facies with significant marine components sugesting differential kinetics of bitumen generation for marine versus terrestrially-derived kerogen. Vitrinite reflectance (%R$\sb{\rm o}$) values were suppressed in liptinite-rich anoxic black shale facies relative to the coal "benchmark."
Anoxia levels varied within the sequences and had significant influence on several geochemical parameters. Oxidized facies, although having low total organic carbon (TOC) contents, tended to have increased TOC-normalized bitumen and hydrocarbon contents and increased diasterane/normal sterane ratios. Anoxic facies had higher TOC and bitumen contents, contained larger proportions of non-hydrocarbon extractables, and had lower diasterane/normal sterane ratios than oxidized facies. Highly (negatively) fractionated $\delta\sp{34}$S values for pyrite (and polar bitumen) in organic-rich black shale facies indicates extensive open-system sulfate reduction operated during diagenesis. Enrichment of V and Ni, and especially an increasing V/Ni + V ratio, coincide with increased anoxia. A reversal of conventional "terrestrial lighter than marine organic matter" carbon isotope relationships in organic-rich black shales was linked to utilization of isotopically-light "recycled CO$\sb 2$", derived from organic matter decomposition.
Eustatic sea level rise and rapid transgression over the craton, with flooding of adjacent brackish-marine (peat) swamps and resultant influx of large supplies of nutrients and humic detritus, appears to be the key geologic control of black shale deposition. The nutrients fostered high algal productivity while terrestrially-derived organic matter provided a sink for oxygen which enhanced anoxia.
| Identifer | oai:union.ndltd.org:RICE/oai:scholarship.rice.edu:1911/19061 |
| Date | January 1987 |
| Creators | WENGER, LLOYD MILLER, JR. |
| Source Sets | Rice University |
| Language | English |
| Detected Language | English |
| Type | Thesis, Text |
| Format | application/pdf |
Page generated in 0.0021 seconds