SEDIMENTOLOGICAL VARIABILITY IN THE MARCELLUS SHALE, FINGER LAKES REGION, NEW YORK

The Devonian Marcellus Formation of the Appalachian Basin typifies black shales that are hydrocarbon source rocks. It has become the focus of great economic interest as a highly profitable unconventional-gas reservoir. Traditionally, the description of black shales, including the Marcellus, has emphasized their homogeneity, high organic matter content, and very fine particle size (clay size). Likewise, traditionally the interpretation has been that they were deposited in the deepest water part of a marine or lacustrine basin by settling of particles that had been suspended in the water column. However, recent studies show that these black shales are not homogenous, display a high degree of variability at a small scale, and show evidence of current-induced deposition.

In our ongoing study, we seek evidence of the variations in the lithofacies within the Marcellus in the Finger Lakes region of Central New York, and the relationships of those lithofacies variations to large-scale stratigraphic patterns. We are analyzing physical sedimentological properties including grain type, grain texture, and shale fabric, as well as the distribution and variations of trace fossils, body fossils, and TOC. Our observations span first from microscopic scale to outcrop scale, and later will span horizontal scales from hundreds of meters to tens of kilometers. Results to date emphasize the lower member of Marcellus Formation, the Union Springs member. Details are documented in 35 samples collected from fresh, unweathered surfaces of an active rock quarry (Seneca Stone) in Seneca County. The sample vertical spacing averages 10 cm. The samples capture all the beds, and sum to approximately 60-70 percent of the unit thickness. Data include outcrop observations and laboratory analyses of polished rock slabs and thin sections.

The preliminary results reveal that the Union Springs member has at least two lithofacies generated by different depositional mechanism. The first one is dominated by mm-scale intercalations of silt-sized and clay-sized grains, with hints of erosion at the bases of silt lamina. The other one, although also laminated, has more homogeneous clay-sized particles and is darker grey. In both lithofacies, the primary shale texture is locally disturbed by trace fossils.