MAGNETIC SUSCEPTIBILITY OF INTER-MOUND SHALLOW-WATER CARBONATES IN THE UPPER ORDOVICIAN CROWN POINT FORMATION (CHAZY GROUP) OF NORTHEASTERN NEW YORK

The Upper Ordovician (Darriwilian-Sandbian) Chazy Group of northeastern New York state consists of, in ascending order, the Day Point, Crown Point, and Valcour formations and records tectonic quiescence immediately before the Vermontian tectophase of the Taconic Orogeny. Shallow-water carbonates, including bioherms developed within each component formation, comprise the dominant lithologic components of the Chazy Group. The Crown Point Formation includes the upper portion (highstand-falling stage systems tracts) of the M1A sequence and the lower portion (transgressive systems tract) of the M1B sequence. This interval remains relatively poorly understood from a sedimentological standpoint, owing largely to its mixed siliciclastic-carbonate composition, reflecting in situ precipitation of carbonate particles and the influx of clay-sized siliciclastic particles from uplifted, distal source areas further to the south. This resulted in lithologically challenging compositions that complicate observational and thin-section analyses (i.e., discrimination between very argillaceous carbonates ad highly calcareous mudrocks). Further, detailed studies of this interval have focused heavily on the bioherms, present in the upper half of the Crown Point Formation, with little attention paid to thicker, more complete inter-bioherm successions. To address this issue, a section of the upper portion of the Crown Point Formation, approximately 35 meters thick and lacking bioherms, was sampled at 10 cm intervals for magnetic susceptibility (MS) analysis. MS values are typical for Paleozoic marine sedimentary rocks, indicating that the MS signal is controlled primarily by siliciclastic influx, and MS shifts are most pronounced at the contact separating the Crown Point and Valcour formations and a zone of more open marine deposits within the upper Crown Point that is interpreted as a fourth- or fifth-order transgressive episode. These results indicate that MS analysis is effective at highlighting variations in the proportion of autochthonous and allochthonous sediment sources, making it valuable in reconstructing fine-scale fluctuations in paleoenvironmental processes.