The Last Stand Of The Great American Carbonate Bank

The Upper Ordovician (450-460 Ma) Chazy, Black River and Trenton groups of eastern North America record the tectonic activation of a passive carbonate platform. Associated with tectonic collision and development of a peripheral-type foreland basin, the tectonic history is complicated by the occurrence of two basin-forming episodes. Although considered analogous by previous authors, a growing list of observations indicate that the stratigraphic record from both foreland basins and their coeval Great American Carbonate Bank (GACB) contain somewhat different, non-analogous, signatures. A number of key issues arise when comparing both tectonic episodes. These include: 1) non-analogous spatial-temporal scales whereby the northern (Vermontian) tectophase is much larger and of longer duration compared to the southern (Blountian) tectophase; 2) sedimentary provenance analyses show a more mafic contribution in the northern basin fill compared to that of the southern basin; 3) the position of K-bentonite swarms relative to basin filling phases is non-analogous; and 4) the location and timing of Ordovician volcanism/plutonism shows a pronounced change after the first tectophase. Thus, important research questions for this study include: A) How does the architecture of the foreland basin complex and adjacent GACB change spatially and temporally during each distinct tectophase?; B) Can provenance differences between tectophases be explained relative to tectonic events in the orogen?; C) What is the timing and significance of K-bentonite position and timing of plutonism/volcanism relative to foreland basin fill episodes?; 4) What inferences are gained from theoretical modeling of load geometries and foreland basin evolution when considered with empirical data from the Taconic Orogeny; and 5) Is a new model for the Taconic Orogeny needed to explain the growing list of incongruities? In order to investigate these questions, a refined, high-resolution, sequence stratigraphic framework has been constructed and utilized to re-calibrate Upper Ordovician strata during the last stand of the GACB. The refined framework was constructed using the occurrence of sequence boundaries, erosional surfaces, transgressive, highstand, and regressive systems tracts, hardgrounds and flooding surfaces, laterally extensive and unique lithofacies, highly repetitive sub-meter scale cyclic intervals, updated macrofaunal and microfaunal biostratigraphies, as well as known chemostratigraphic and event stratigraphic data (Nd, C, & Sr isotopic excursions, K-bentonite horizons, seismite horizons, etc.). Also defined for the first time are a number of time-restricted facies including widespread siliciclastic events, chert-rich intervals, and pronounced calcification events. As such, an improved temporal and spatial chronology of thirteen time-slices (~ 1 million year duration) has been produced for the Ashbyan, Mohawkian, to earliest Cincinnatian interval. This integrated stratigraphic model is used to link the sedimentary record of the foreland basin complex to climatic changes, sea-level oscillation, and specific tectonic events in the orogen. These data provide an understanding of the evolution of the foreland basin and its sub-components including the backbulge, forebulge, and foredeep basins. Moreover, coupled with newly calibrated strontium isotopic curves, this study has allowed for the development of a new model for the Taconic Orogeny that helps to explain differences in the timing and spatial relationships between the southern and northern tectophases.