DESCENT INTO DARKNESS: A REVIEW OF LATE ORDOVICIAN BASIN RESPONSES TO EXTENSION, COMPRESSION AND BASEMENT REACTIVATION IN THE CENTRAL TACONIC FORELAND

The Late Ordovician foreland basin, host to the Utica black shale, most likely developed in response to loading of the Laurentian crust.. The precise causal connection to events in the hinterland remains uncertain, however. Similarly, although it has been clear since the work of Ruedemann and Cushing more than a century ago that the Utica was deposited adjacent to a carbonate platform located to the west and that black shales oversteped that platform and extended into the craton during a series of pulses, the role of tectonic and eustatic contributions to this history has proven more difficult to discern.

Recent work from PA to Quebec indicates that fault controlled changes in accommodation space were a principal driver of basin growth and internal facies distribution. Contrary to previous models, accommodation space change regularly took place simultaneously across large geographic areas, leading to widespread, abrupt facies shifts. For instance: 1) faults caused local variation in the facies and thickness of Black River and Trenton group strata, including inliers of complete absence or erosion; 2) the base of the Utica does not gradually young to the west, but rather is effectively synchronous (latest C. bicornis Zone age; ~453.0 MA) from Albany to Herkimer, NY; 3) extensional structures such as the Little Falls, Noses, and Hoffmans faults exhibit growth fault geometries with striking differences in facies that appear to control the location of the Trenton-Utica platform boundary.

Overstep of the Dolgeville carbonate fan system at the margin of the Trenton platform by the Indian Castle Shale in early D. spiniferus Zone (~451.7 MA) occurred in response to a series of fault block rotations and platform collapse events (the Thruway Discontinuity). This pulse was contemporaneous with graptolite faunal turnover (including an epibole of the Panthalassic Amplexograptus tardus) and an abrupt shift in eye size of Triarthrus beckii. These events suggest tectonic driving of a regional basin reorganization and predict a driving compressional deformation along the hinterland margin of the foreland basin. New discoveries confirm this prediction (Jacobi et al. this volume). The rapidity and synchroneity of subsidence across the basin may reflect zones of crustal weakness imparted by pre-existing Iapetan opening faults.