Nan Crystal Arens, PhD, 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. Previously, we 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. We noted several carbon isotope excursions that may be correlative with other sections worldwide. However detailed palynological analysis and a return to the section in search of conodonts that would provide time stratigraphic control have failed to yield material of sufficient quantity or quality to permit detailed global correlation. 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.
Analysis of carbon isotopic data show a statistically significant (r2 = 0.2, p < 0.001) relationship between carbon isotopic value of preserved organic material and organic carbon content of the sediment. Samples with high organic carbon content (black shales) tend to be depleted in 13C. This suggests that rather than being driven by changes in carbon source (secular variation in the isotopic composition of atmospheric CO2 or marine DOC), the relationship may reflect changes in the mixing ratio of reduced carbon sources, for example terrestrial versus marine organic carbon.
A palynofacies analysis was undertaken to assess the degree to which terrestrial sediment and organic matter dominated the basin and whether terrestrial input varied with the carbon isotope signature. This would allow preliminary testing of the third hypothesis outlined above, in which nutrient runoff from terrestrial sources stimulated marine productivity and contributed to bottom water anoxia. Or, alternatively, whether terrestrial input drove organic carbon preservation directly. Approximately 300 samples were processed for palynofacies analysis. Preliminary analysis midway through palynofacies data collection suggested that samples were irregularly contaminated. We revisited our laboratory protocols and learned that the dionized water used to prepare our samples was essentially tap water due to a failure in several stages of the filtering process. Reprocessing of the sample was completed in early 2012. Without additional student help, counting proceeded slowly in 2012 and the lab was closed for construction during summer 2012 so no additional students could be engaged to assist with this research. We hope to complete palynofacies analysis and the analysis of the entire data set in 2013.