COMPARATIVE SEQUENCE STRATIGRAPHY OF TWO UPPER ORDOVICIAN (MOHAWKIAN) SHELF-TO-BASIN RAMPS: RELATIVE INFLUENCES OF TECTONISM AND EUSTATIC CHANGE IN THE TACONIC BASIN

Five Upper Ordovician Black River (Turinian) and Trenton Group (Chatfieldian) decameter-scale (“third-order”) depositional sequences were compared along two ramp-to-basin transects in eastern Laurentia in order to investigate the sedimentary dynamics of the initiation of the Vermontian Tectophase of the Taconic Orogeny. These oppositely-oriented transects are located in widely separated regions: a NW-SE cross-section of the carbonate bank to Taconic foredeep in New York State, and a S-N cross-section from the Lexington Platform into the Sebree intracratonic trough in Kentucky and Ohio.

Comparisons reveal similar patterns in facies development, sequence stratigraphic architectures, and timing of basin subsidence in both areas. Depositional sequences and smaller-scale cycles in both the upper Black River and Trenton Groups are widely correlatable, suggesting that eustasy, played a major role in the development of depositional patterns during this time, despite tectonic alteration of the seafloor. Time-equivalent systems tracts and parasequences show similar synchronous water depth histories. For example, Turinian to early Chatfieldian cycles uniformly show gradual lateral changes in peritidal to shallow shelf carbonate facies. Conversely, those of the mid-to-late Chatfieldian record abrupt lateral facies change from shallow, peritidal carbonates into deeper water shelf-to-ramp nodular carbonates and shales suggesting lateral topographic change of the seafloor after the onset of tectonism. Perhaps the most striking similarity between the two transects is the apparently simultaneous collapse of the Sebree Trough, and the Taconic Foreland Basin.

There are also some dissimilarities between the regions. First, the relative thickness of time-equivalent sequences indicate a disparity in sedimentation rates suggesting a primary paleoceanographic difference that impacted the production/transport/deposition of carbonates across both ramp transects. Second, sequences deposited on [the most cratonward transect] the Lexington Platform, show uniform, along transect, thicknesses, while sequences deposited at the cratonic margin, on the Trenton Shelf, show lateral thickness variations, indicating localized variations in sediment accumulation and preservation.