Abstract Graptolite faunas exhibited strong biogeographical differentiation during the Early Palaeozoic, particularly in the Ordovician. Skevington recognized two major faunal provinces, the high to mid palaeolatitude ‘Atlantic Province’ and the low-palaeolatitude ‘Pacific Province’. Subsequent workers have generally accepted this pattern of graptolite distribution, but the controls on this pattern have been the subject of considerable debate. Two competing models have emerged: a surface water temperature model and a depth stratification model. It is likely that the some of the physical and chemical oceanic factors that vary with latitude may also vary in a similar way along an onshore to offshore transect. Hence, it may be that both depth and surface temperature play an important role in biogeographical differentiation. Biogeography also played a critical role in the evolutionary history of graptoloids. Important examples include the origination of axonophorans in deep, offshore environments from isograptid and pseudisograptid ancestors and their subsequent migration into shallow water regions; the replacement of the Diplograptina by Neograptina in the low palaeolatitudes during the Late Ordovician extinction event; and the origination of expansograptids in the ‘Atlantic’ Province as shallow water endemics followed by their worldwide dispersal into the oceanic biofacies.

ABSTRACT The Cincinnati Arch region of Ohio, Kentucky, and Indiana is an icon of North American Paleozoic stratigraphy, as it exposes strata ranging from Ordovician to Pennsylvanian in age. In particular, the highly fossiliferous Ordovician, Silurian, and Middle Devonian successions have been extensively studied since the nineteenth century, and continue to serve as a crucial proving ground for new methods and models of biostratigraphy, chemostratigraphy, and sequence stratigraphy in mixed clasticcarbonate depositional settings. These strata are locally capped by Middle Devonian limestones with their own diverse fauna and unique depositional history. Outcrops near Louisville, Kentucky, provide an excellent opportunity to examine these strata firsthand and discuss sequence stratigraphy, chemostratigraphy, sedimentary environments, and paleoecology. A series of new roadcuts south of Mount Washington, Kentucky, exposes the lower to middle Richmondian Stage (Upper Ordovician, Cincinnatian) and presents a diverse suite of marine facies, from peritidal mudstones to offshore shoals, coral biostromes, and subtidal shales. These exposures are well suited for highlighting the revised sequence stratigraphy of the Cincinnatian Series, presented herein. Nearby outcrops also include much of the local Silurian succession, allowing an in-depth observation of Llandovery and Wenlock strata, including several chemostratigraphically important intervals that have improved regional and international correlation. Supplementary exposures east and north of Louisville provide context for subjacent and superjacent Ordovician-Silurian strata, as well as examples of lateral facies changes and unconformities. Additionally, the Falls of the Ohio at Clarksville, Indiana, features an exceptional outcrop of the overlying Middle Devonian succession, including an extensive and well-preserved biostrome of corals, sponges, and other marine fauna. These fossil beds, coupled with significant exposures in local quarries, are critical for understanding the paleoecology and stratigraphy of the Middle Devonian of the North American midcontinent.

ABSTRACT Aseismic ridge subduction is common along modern convergent margins. We enumerate six criteria that can be used to recognize aseismic ridge subduction in orogens, including a magmatic gap with uplift followed by bimodal volcanism, which commonly includes explosive, voluminous rhyodacitic volcanism that erupts far from the trench. Features temporally linked with the explosive volcanism include retroarc thrusts and consequent thrust-loaded retroarc foreland basin development. Using these criteria to examine features of the Taconic orogen, together with new stratigraphic and structural data from the Utica basin that constrain the basin subsidence architecture and thrust timing, we propose that at least the older units of the 456–435 Ma Oliverian Plutonic Suite in New England were generated during steepening of the downgoing slab after passage of a subducting aseismic ridge. Weakened crust from delamination and decompression melting promoted westerly directed thrusts (present-day coordinates) that loaded the Taconic retroarc foreland. The resulting Utica basin subsided rapidly and nearly synchronously over an ~150-km-wide region and contains interbedded 453–451 Ma ash layers from the Oliverian Plutonic Suite or coeval plutons to the south. This history of basin subsidence indicates that the major thrust loads that drove development of the Utica basin were emplaced over a similarly brief interval beginning ca. 455 Ma. Thus, the Taconic thrusts, the Utica basin, the volcanic ashes, and the early Oliverian felsic magmatic units could all be related to an aseismic ridge subduction event. Because of the ubiquity of seamount chains, we expect that aseismic ridge subduction affected other segments of the Taconic orogen.