ISOTOPE CHEMOSTRATIGRAPHY AND CORRELATION IN AN UPPER ORDOVICIAN PLATFORM FACIES-MOSAIC: LEXINGTON LIMESTONE, CENTRAL KENTUCKY, U.S.A

Much of Early, Middle and early Late Ordovician time on central and eastern Laurentia was characterized by widespread, very shallow open-marine or peritidal sedimentation. In east-central and eastern Laurentia, this pattern abruptly changed with the advent of the Taconic tectophase at the Turinian-Chatfieldian transition in early Late Ordovician time (mid-Caradoc; Black River-Trenton transition). Widespread, Blackriverian, warm-water, peritidal carbonates abruptly gave way to a temperate-water, carbonate-facies mosaic across a subtle regional unconformity. Platform differentiation into various facies was apparently related largely to reactivation of basement structures by far-field forces produced during orogeny to the east. Different facies commonly developed on different fault blocks during at least two major phases of far-field structural differentiation in the area. Concomitant facies differentiation was especially intense across small areas in the central Kentucky region of east-central United States during deposition of the Lexington Limestone. Here, a structurally related Late Ordovician carbonate buildup, approximately 100-km across, formed, and as faults were periodically reactivated, a complex facies mosaic developed, which has largely defied attempts at finer-scale correlation.

Based on the example of the globally correlative Millbrig K-bentonite and the Guttenberg Isotope Carbon Excursion (GICE), 754 whole-rock, stable-isotope samples were analyzed from six sections in the buildup area. ä13C data were then plotted versus stratigraphy and elevation above a bentonite datum to form a chemostratigraphic profile, composed of apparently cyclic positive and negative shifts; each pair of shifts was interpreted to be an “isotope zone.” After data normalization for thickness, it was found that isotope zones correlated across buildup facies and that zonal boundaries effectively form time-plane horizons useful for cross-facies correlation. As a result, it has been possible to generate coeval facies maps for each zone. Although these zones are now interpreted to be regional in nature and are not as robust as the GICE, like the GICE, future work may show that some of them have global correlative value for Ordovician rocks beyond Laurentia.