CHANGES ACROSS THE TRENTON-MAQUOKETA (SUB-SULPHUR WELL) UNCONFORMITY: STRATIGRAPHIC RESPONSES TO STRUCTURAL INVERSION AND INITIATION OF THE TANGLEWOOD BUILDUP, MIDDLE/UPPER ORDOVICIAN LEXINGTON LIMESTONE, CENTRAL KENTUCKY

Upper Middle and Upper Ordovician rocks in central Kentucky can be divided into two, fourth-order sequences by the Trenton-Maquoketa (sub-Sulphur Well) unconformity, a prominent surface related to development of the Sebree Trough and Lexington Platform in east-central United States. Before unconformity development, much of the platform in central Kentucky had been inundated by deeper waters represented by micrograined limestones and interbedded shales in the Brannon Member of the Lexington Limestone. Though widespread, the thickest Brannon occurs in a linear trend between two structural lineaments with probable basement precursors, suggesting synsedimentary subsidence in a graben-like basin. Brannon deposition in central Kentucky, however, ended during an episode of uplift, shallowing and truncation reflected by the succeeding sub-Sulphur Well unconformity. The Brannon was replaced by high-energy, shallow-water, shoal deposits of the middle tongue of the Tanglewood Member, on top of which a carbonate beach-back beach sequence (Devils Hollow Member) accreted. Thickening trends in the shoal-beach complex coincide with linear, thickening trends in the underlying Brannon, suggesting the importance of structural inversion in the rapid change from deeper water, subsiding, Brannon environments to shallow-water, uplifted Tanglewood shoals. Beyond central Kentucky, changes across the sub-Sulphur Well unconformity coincide with a late Shermanian episode of regional tilting and deepening, while in central Kentucky, the unconformity marks inception of a regressive, shoaling complex called the Tanglewood buildup. The rapid change to a shallowing, regressive system represented in the buildup was facilitated by structural inversion, which together with tilting and deepening elsewhere marks a major change in tectonic regime. Though tectonic regime largely reflected events in the Taconic orogen more than 500 km from central Kentucky, events in the orogen were still capable of eliciting foreland stratigraphic responses like those above through the distal transmission of stresses focused on pre-existing zones of basement structural weakness.