The Time-Transgressive Deposition of the Utica Shale in New York Revealed Using Carbon Isotope Stratigraphy
Metzger, John G.; Fike, David; Smith, Taury
Carbon isotope ratios of dissolved inorganic carbon in the surface ocean are well-mixed and change through time. These trends are recorded in marine carbonates and carbonate cements (δ13Ccarb) and provide a means for high-resolution chronostratigraphic correlation in carbonates and carbonate-bearing siliclastic sequences. In contrast to absolute chronostratigraphy, which utilizes absolute ages obtained from radiogenic isotopes (e.g. U-Pb ages from volcanic ashes) a relative chronostratigrapy can be constructed for sections using δ13Ccarb, as this record is time-dependent and should vary synchronously between sections. The resulting level of temporal correlation (down to 104 yrs) meets or exceeds the analytical precision of current radiogenic isotope based dating methods as well as biostratigraphic methods. Well-resolved δ13Ccarb trends reveal the time-transgressive relationship between the Utica Shale in eastern New York and the Trenton Limestone of western and central New York. Calculated sedimentation rates suggest that the onset of the carbonate-rich Utica Shale in the east falls chronologically earlier (by 105 - 106 yrs) than the carbonate-poor Utica in the west. Correlations show the Utica contact is increasingly younger across New York, consistent with a migrating depositional facies. This work, carried out using samples from well cuttings and cores, supports previous stratigraphic relationships proposed for the Utica-Trenton outcrop belt using detailed biostratigraphic and K-bentonite data. The successful demonstration of δ13Ccarb in producing robust correlations through the subsurface highlights the potential that δ13Ccarb chemostratigraphy may have for answering other high-resolution stratigraphic questions in areas where outcrops are stratigraphically incomplete, scarce, or absent.
AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013