INFLUENCE OF TECTONICS, PALEOGEOGRAPHY AND PALEOCLIMATE ON THE SEDIMENTARY RECORD OF EAST-CENTRAL LAURENTIA AT THE MIDDLE-LATE ORDOVICIAN TRANSITION

By late Middle Ordovician time, collapse of the Blackriverian carbonate platform across east-central Laurentia, coeval with and related to inception of the Taconic tectophase of the Taconian Orogeny, set the stage for sequential development of the Galena-Trenton Shelf, Lexington Platform and Sebree Trough. Accompanying bulge moveout reactivated basement structures across the old carbonate platform, generating a series of structural lows and highs. The highs formed foundations for the buildup of carbonates that would become the Galena-Trenton Shelf and Lexington Platform. Intervening lows were sufficiently depressed to make contact with open seas to the south, which in the existing paleogeographic and paleoclimatic setting promoted regional, quasi-estuarine circulation. This circulation not only funneled deep, cold, mineral-rich waters into lows between the platforms, supressing carbonate deposition there, but also promoted upwelling into adjacent, shallow, platform waters, resulting in proliferation of fauna and rapid upward accretion of the adjacent shelves and platforms. The elevated, aggrading platforms acted to deepen and canalize waters in intervening low areas, which would become the Sebree Trough, and formed a barrier to clastic influx from the east, insuring sediment starvation in the lows. By Middle-Late Ordovician (Shermanian-Edenian) time, far-field tilting and related deepening permited siliciclastics to overflow from the Taconic foreland basin across the platforms and into the Sebree Trough, where thicker accumulations developed. The resulting clastic inundation effectively halted major carbonate deposition on the platforms thereafter.

Hence, sediment starvation and corrosion in lower trough areas and the concomitant, rapid aggradation of skeletal carbonates on platform highs were both related to interactions between Taconic far-field tectonism and Laurentian paleogeography and paleoclimate. Not only did these interactions control Trenton and later stratigraphic differentiation of the old Blackriverian carbonate platform, but in the process - and probably unrelated to later glacial influence - they changed the subtropical, Blackriverian, faunal and sedimentary realm across eastern Laurentia to a temperate-water realm that endured for the rest of Ordovician time.