RELATIONSHIPS BETWEEN MAGNETIC SUSCEPTIBILITY AND CARBONATE CONTENT IN METER-SCALE PERITIDAL RHYTHMS: UPPER ORDOVICIAN CROWN POINT FORMATION, NORTHEASTERN NEW YORK

The Upper Ordovician (Darriwilian-Sandbian) Crown Point Formation consists of more than 200 meters of argillaceous carbonates that contain some of the oldest metazoan-based bioherms associated with a diverse metazoan community. Inter-bioherm successions have not previously been extensively studied despite being thicker than coeval biohermal successions, but are currently being analyzed using magnetic susceptibility (MS) analysis in order to recognize patterns relevant to subtle fluctuations in detrital influx. A relatively complete section of the upper half of the Crown Point Formation, located west of Plattsburgh in Clinton County, northeastern New York, consists of roughly meter-scale rhythms representing deposition in peritidal to storm-influenced subtidal settings, and serves as the basis of the present study. Such rhythms are interpreted as parasequences comprising a third-order highstand systems tract with a superimposed higher-order transgression-regression cycle. Samples were collected at 10 cm intervals and were subjected to MS analysis as well as percent carbonate as determined by standard loss-on-ignition techniques. Percent carbonate values are loosely inverse to MS values, as would be expected if MS is controlled primarily by detrital clay content. However, there is no strong consistent relationship between MS and carbonate content within meter-scale rhythms, both in peritidal facies and shallow subtidal facies. Most surprisingly, there appears to be a corresponding decrease of both MS and carbonate content in certain portions of the curve, indicating that the processes that concentrate detrital paramagnetic clays and diamagnetic quartz control MS trends, with changes in carbonate content playing only a secondary role.