RELATIONSHIPS BETWEEN WHOLE-ROCK, STABLE-ISOTOPE COMPOSITION AND LITHOLOGIC UNITS IN THE UPPER ORDOVICIAN LEXINGTON LIMESTONE, CENTRAL KENTUCKY

The Lexington Limestone is an Upper Ordovician (Chatfieldian-early Edenian) unit exposed within the central Bluegrass Region of Kentucky. Overall, the Lexington Limestone of central Kentucky appears to be a structurally controlled carbonate buildup, the stratigraphy and environmental interpretations of which have been revised several times. Lower Lexington Limestone members are more or less tabular and fairly well understood, but interrelationships among the upper members are complex and poorly understood because much of the structurally controlled buildup is developed in this part of the formation.

In order to correlate complex parts of the Lexington Limestone with established, regionally and globally constrained time-boundaries, we have developed two isotope curves attempted to relate them to lithology, as well as to bio- and event stratigraphy. Our major focus here is the preliminary stable-isotope data. Whole-rock samples were obtained from two local sections (10-foot and 2-foot intervals) for two trial runs, using the isotope-ratio mass spectrometer (IRMS) and inductively coupled plasma spectrometer (ICP). Both trial runs showed two, well-defined, lithologically controlled, positive deflections in carbon-isotope composition (d¹³C), punctuating an overall decreasing trend (ranging +2.3 to –2.3 per mil V-PDB) that extends to the top of the of the formation. d¹⁸O evidence (ranging from –5.9 to –4.4 per mil V-PDB) indicates highly variable conditions during the deposition of the lower Lexington Limestone and largely invariant conditions during deposition of member in upper parts of the formation. Results for both stable isotopes fall within the range of published d¹³C and d¹⁸O Ordovician curves, suggesting that correlation of the Lexington Limestone with regional and global records are possible, especially relative to the Gutenburg (GICE) d¹³C excursion. Trace-element analysis suggests minor diagenetic alteration based upon published chemical criteria. Additional work will concentrate on collecting and analyzing low-Mg brachiopods. Brachiopods are abundant in the Lexington Limestone, but emphases will be on psedo-punctate brachiopod species, Dalmanella, Hebertella and Platystrophia, based upon their homogenous shell density that implies minimal diagenetic overprinting.