ORIGIN OF THE MIDDLE-LATE ORDOVICIAN SEBREE TROUGH, EAST-CENTRAL UNITED STATES

The Sebree Trough is a late Middle (Shermanian) to early Late (Edenian) Ordovician, subsurface, linear, trough-like thickening of shale and micrograined limestones that trends northeastwardly through western Kentucky, southeastern Indiana, and northwest-central Ohio and unconformably overlies the Lexington/Trenton sequence. The "trough" coincides with similarly trending Keweenawan basement faults, which separate the Galena Shelf to the northwest from the Lexington Platform to the southeast and has been ascribed to "valley erosion" or facies changes along a structural hinge. Recent analysis of regional stratigraphy, biostratigraphy, and structure suggests that the trough first appeared during collapse of the extensive Blackriverian carbonate platform due to bulge moveout near the Blackriverian-Rocklandian transition at the inception of the Taconic tectophase of the Taconian Orogeny. Reactivation of basement structures left a series of connected structural lows, which became the Sebree Trough, and adjacent highs, which formed the foundations for carbonate buildups that would become the Galena Shelf and Lexington Platform. The Lexington Platform formed a barrier to sedimentation from the east, and structurally low trough areas were sufficiently depressed to make contact with open seas on the Ouachita margin, which in the existing paleoclimatic and paleogeographic setting promoted the incursion deep, cold, mineral-rich waters, inimical to carbonate deposition, into the trough. The combination of barriers to sedimentation and cold-water corrosion generated a Lexington/Trenton surface of omission and corrosion in the structurally low areas that was expressed as a trough-like corridor of sediment starvation, now called the Sebree Trough. In latest Middle Ordovician (Shermanian) time, an apparent change in subduction polarity in the Taconic orogen resulted in major, regional deepening, far-field tilting, and structural reactivation that allowed clastics from the northeast to initially flood into the trough and eventually inundate both adjacent platforms. From start to finish, the origin and development of the Sebree Trough was apparently related to an interplay between far-field tectonic forces, paleogeography and paleoclimate.