CARBON ISOTOPE STRATIGRAPHY OF THE SILURIAN HIGH FALLS FORMATION, SOUTHEASTERN NY

The Silurian Period is marked by large positive carbon isotope excursions (CIEs). The largest excursion, the late-Ludfordian Lau Event, was the largest positive CIE of the Phanerozoic. Thus instead of having been a stable, transitional time as previously considered, the Silurian had a more dynamic ocean-climate system marked by large perturbations of the global climate cycle. Currently the cause(s) remain unknown though intensified carbonate weathering during glacially induced sea level lowstands and various changes in oceanic mixing and circulation have been proposed.

Isotopic profiles have been compiled and correlated from coeval marine carbonate sections from, among other locations, Sweden, Austria, and the mid-western U.S. A prominent CIE has also been identified in paleosols and shallow nearshore marine facies of the Bloomsburg and Wills Creek Formations in central PA, which is tentatively correlated to the globally recognized Lau event. In this project, coeval sections were studied from which 1) the PA results could be verified; and, 2) ideally, a CIE could be identified in facies other than those from which the PA CIE had been obtained.

This study focused on the High Falls and Binnewater Formations of southeastern NY. A section containing the formations was measured and sampled along the Rondout River near High Falls, NY. At this section a δ¹³C curve has been compiled for most of the High Falls Formation, including an anomalous limestone unit, called the Powerhouse Limestone, which is known only from the High Falls location. The preliminary results indicate that a major positive CIE (of nearly 4.5‰, from -4.8‰ to -0.5‰) exists within the paleosols of the High Falls Formation. This excursion is tentatively correlated to the CIE in the Bloomsburg Formation in central PA. Further correlation of patterns in the carbon isotope record of the paleosols in the High Falls and Bloomsburg Formations is demonstrable, as is tentative correlation between the Powerhouse Limestone and a prominent limestone in the transition between the Bloomsburg and Wills Creek Formations in PA. Previously, the High Falls Formation was age-constrained by relative stratigraphy with the overlying Rondout Formation of latest-Silurian age, but the new isotopic correlations can be used to assign the formation to the late-Ludfordian.