Reports: UR8 49336-UR8: Testing Hypotheses of Black Shale Deposition in the Late Devonian Catskill Basin, Watkins Glen State Park, New York

Nan Crystal Arens, Hobart and William Smith Colleges

The black shales of the Devonian Catskill Basin have significant fractured gas potential.  The goal of this study is to understand the factors—within and outside of the basin—that influenced the deposition of organic-rich black shales.  My research group has undertaken a multi-proxy study that tests three hypotheses of Devonian black shale formation: (1) A tectonic model in which black shales accumulated in deep zones following basin down drop on regional faults.  (2) A climate model in which black shale deposition was triggered by global climate and was synchronous with similar deposition elsewhere.  And (3) a biotic model in which black shale deposition was stimulated by increased weathering and nutrient flux from land.  Data were gathered from a 95 m long, complete section, exposed in Watkins Glen State Park, New York.  During this reporting period, we have completed a cm-scale measurement and description of the stratigraphic section.  We noted ball and pillow structures at various horizons in the section suggestive of regional seismicity.  Individual beds were sampled for analysis, with an average sampling interval of approximately 20 cm.  Carbon isotope analyses on organic matter have been completed and percent organic content of the sediment has been calculated.  As of this report, we note several carbon isotope excursions that will aid in correlating this section to other sections world wide.  Preliminary analysis of this chemostratigraphic pattern suggests that organic-rich intervals in the Watkins Glen section are not well correlated with known black shale events reported in other basins, suggesting within-basin control for black shale deposition.  Palynological samples were taken at approximately meter intervals.  The purpose of palynological samples was two-fold:  (1) to constrain the age of the sediments, and (2) to assess the diversity of terrestrial plants in ecosystems that provided nutrients to this portion of the basin.  Although most samples were productive for palynology, almost all spores recovered were extensively damaged by in situ growth of marcasite crystals.  Interestingly, chitinozoans in the same samples were not damaged by marcasite growths.  The presence of this mineral suggests a bottom water or sediment pore water environment that was oxygen-poor and acidic.  This is consistent with an interpretation of low-oxygen or anoxia in bottom waters or sediment.  Although the presence of marcasite damage in palynomorphs has been extensively documented, the mechanisms and environment of crystal growth is poorly understood.  Consequently, we cannot determine whether the growth of mineral crystals occurred upon deposition or after burial, and thus whether reducing conditions prevailed in the bottom water or were only characteristic of sediment.  We did, however, recover a single sample near the base of the measured section, in which spores were largely undamaged.  This sample did not contain sufficient spores for diversity assessment, but did produce an assemblage including Grandispora gracilis, Geminospora lemurata and Auroraspora macra that suggest a Famennian age for this stratigraphic horizon.  This is a surprise because the region is conventially mapped as Frasnian and Givetian in age.  We had hoped to verify age assignments with conodonts, but none have been recovered from the section to date.  With conventional palynology proving of limited use, we have turned to palynofacies analysis to give us a perspective on the degree of terrestrial influence on these sediments.  As of this writing, approximately 300 samples have been processed for palynofacies analysis and categorization of organic materials is underway.

 
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