INTERACTION OF REGIONAL STRUCTURE, TECTONICS, PALEOGEOGRAPHY AND PALEOCLIMATE TO EXPLAIN RAPID CHANGE FROM WARM-WATER TO TEMPERATE-WATER CONDITIONS ACROSS EAST-CENTRAL LAURENTIA AT THE LATE ORDOVICIAN BLACKRIVERIAN–TRENTON (TURINIAN–CHATFIELDIAN; LATE SANDBIAN) TRANSITION

In early Late Ordovician time, east-central Laurentia lay in the southern subtropics and was remarkable for widespread, warm-water, pelletal carbonates across an area called the Blackriver carbonate platform. Abruptly across an unconformity at the Blackriverian–Trenton (Turinian–Chatfieldian) boundary, the warm-water regime changed to temperate-water sedimentary and faunal regimes while still located in the same subtropical setting. This abrupt change is well-known and commonly explained by the advent of Late Ordovician glaciation. The unconformity may be key in understanding another interpretation of the change, as it coincides with initiation of a major Taconian tectophase, during which the Blackriver Platform broke up along old basement structures into a series of smaller platforms and shelves, separated by a linear low called the Sebree Trough. These basement structures were apparently reactivated by far-field, foreland forces accompanying this major phase of the Taconian orogeny, and soon after the breakup, phosphatic temperate-water skeletal carbonates spread across the resulting shelves and platforms, more than 1000 km from the nearest, open continental margin. The presence of phosphate in these shallow-water carbonates suggests that upwelling deeper, open-ocean waters must have been involved. Breakup of the Blackriver Platform apparently created smaller structural highs like the Lexington, Galena and Trenton platforms, which acted as foundations for extensive buildups of temperate-water carbonates, while the Sebree Trough funneled cold, deep, nutrient-rich waters from the southern Laurentian margin into the center of the continent. With open-ocean contact, the paleogeographic and paleoclimatic setting on Laurentia at the time established a quasi-estuarine circulation that moved surface waters to the southwest and replaced them with deep, cold, nutrient-rich waters through upwelling. Upwelling easily explains the abrupt change to temperate-water conditions, the presence of phosphates and the rapid aggradation of skeletal carbonates on the platforms. Although glacial influence is still a possibility, it is only in the integration of structure, tectonics, paleogeography and paleoclimate that other possibilities for these paleoenvironmental changes emerge.