A NEODYMIUM ISOTOPE INVESTIGATION OF SEDIMENT SOURCES FOR THE MIDDLE DEVONIAN MARCELLUS FORMATION, PENNSYLVANIA, USA

The Middle Devonian Marcellus Formation in the eastern U.S. is the focus of an intense level of exploration and exploitation for natural gas. Determining local and regional sediment sources and depositional environments in the Appalachian Basin during its formation, as well as understanding post-depositional processes, are important for our understanding of the evolution of the basin and subsequent generation of natural gas. To approach these issues and minimize effects of weathering, we are conducting an isotopic study of cores and drill cuttings that span the Marcellus Formation.

Preliminary neodymium (Nd) isotope data were obtained on a set of samples from a core extracted in Greene County, southwestern Pennsylvania, that spans a depth range of ~25 m (2374-2398 m). The samples range from marly limestone to calcareous shale to organic-rich black shale. Shales within the Marcellus Formation can have high calcium carbonate contents that can reflect post-depositional/diagenetic precipitation. To isolate the isotope systematics and understand provenance of the silicate and organic portions of the shale, carbonate was removed using acetic acid. The acetic acid-soluble portions of shales in this section of the core ranged from 15 to 39%. The residues, consisting primarily of clastic silicate minerals and organic matter, yield ε_Nd(T) values within a tight range of -7.4±0.4 when corrected to 390 Ma (Middle Devonian). Depleted mantle model ages for the residue samples range from 1.4 to 1.6 Ga, consistent with an earlier analysis of a Marcellus outcrop sample from New York State (Caeser et al., 2010, GSA NE/SE Sect. Abstr. 111-3), and with a sediment source dominated by orogenic highlands to the east (Patchett et al., 1999, Science 283, 671-673). Ashes within this section contain inherited zircon cores which have U-Pb ages of about 1 Ga, suggesting a contribution from Grenville basement. Dy/Yb ratios between 1.5 and 1.8 are consistent with a similar source material for these layers. The tight clustering of ε_Nd(T) values and Sm/Nd ratios suggest that post-depositional disturbance to the organic and clastic portion of the shale lithologies from this core has been minimal. Additional work is focusing on the Sr/Nd isotope contributions from exchangeable sites and carbonate, and their relationship to Marcellus produced water.