Evidence and implications of possible far-field responses to Taconian Orogeny: Middle-late ordovician Lexington platform and sebree trough, East-Central United States

Stratigraphic and structural differentiation of the shallow-water Blackriverian carbonate platform across east-central United States near the Blackriverian-Rocklandian (early Caradoc) transition coincides with inception of the Taconic tectophase of the Taconian Orogeny. Collapse of the Blackriverian carbonate platform and sequential development of the Galena Shelf, Lexington Platform, and Sebree Trough followed. The distribution of resulting facies parallels similarly trending basement structures with little or no surface expression, suggesting that facies changes reflect reactivation of basement structures at depth by far-field processes. Bulge moveout at tectophase inception apparently reactivated old structures across the Black River Platform, leaving a series of structural lows and highs. The highs acted as foundations for extensive builup of carbonates that would become the Galena Shelf and Lexington Platform. Intervening, linear low areas were sufficiently depressed to make contact with open seas to the south, which in the existing paleogeographic and paleoclimatic setting promoted quasi-estuarine circulation. This circulation funneled deep, cold, mineral-rich waters, inimical to carbonate deposition, from the southern margin of Laurentia into structural lows between the platforms, generating a Trenton surface of omission and corrosion there, expressed as a trough-like corridor of sediment starvation called the Sebree Trough. The resulting influx of cold water also changed broad sedimentary and faunal patterns during three million years of mid-Kirkfieldian to late Shermanian time. More local responses across the Lexington Platform include development of a carbonate buildup, facies changes related to structural trends, and repeated horizons of widespread, seismically induced liquefaction. The diversity of responses across wide areas during narrow intervals of time, as well as coincidence and repetition of responses along basement structures, reflect the likely significance of far-field effects during cratonmargin orogeny. Literature review suggests that stratigraphic responses like those above are typical of orogenic complexes and probably reflect the distal transmission of stresses, focused on pre-existing zones of foreland, basement weakness. Hence, understanding basement structural framework and the paleogeographic and paleoclimatic settings in which it occurs, even in allegedly quiescent, distal, settings, can be critical in deciphering stratigraphic, sedimentary, and faunal relations of the foreland.