Patrick I. McLaughlin

SIGNATURES OF SEA-LEVEL RISE ON THE CARBONATE MARGIN OF A LATE ORDOVICIAN FORELAND BASIN: A CASE STUDY FROM THE CINCINNATI ARCH, USA

Abstract Analysis of the limestone-dominated Upper Ordovician (Chatfieldian-Edenian) Point Pleasant–Fulton interval provides detailed documentation of the internal composition of a transgressive systems tract within a mixed-carbonate siliciclastic foreland basin succession. This 14 m interval is divisible into 13 small-scale cycles that are widely traceable and record a lithofacies gradient from calcarenite-rich outcrops of central Kentucky into interbedded fine-grained grainstones and organic-rich shale in the subsurface of western Ohio. Hardgrounds and condensed beds are widespread and numerous and commonly cap the limestone hemicycle of small-scale cycles. Taphonomic- and faunal-gradient analyses of 233 bedding planes reveal both lateral and vertical gradients indicative of deepening from central Kentucky northward and a similar signature of deepening upward through the study interval. The deepening-upward trend within the Point Pleasant member is coincident with a decrease in the thickness of shale hemicycles, whereas the deepening-upward trend within the Fulton submember shows a slight increase in shale hemicycle thickness and quartz silt content. A polymictic intraformational conglomerate, the most complex discontinuity surface within the study interval, marks the contact of the Point Pleasant–Fulton members. The study interval, thus, is interpreted to represent distinctive early and late phases (Point Pleasant and Fulton members, respectively) of the transgressive systems tract separated by a maximum starvation surface. These patterns suggest that this widespread limestone-dominated interval formed primarily in response to basinwide, relative sea-level rise and siliciclastic sediment starvation, rather than simply through winnowing.

Anatomy of a compound sequence boundary: a basal Silurian karstic unconformity in the Cincinnati Arch region

The Ordovician–Silurian contact in parts of the Cincinnati Arch region is an irregularly pitted erosion surface reflecting karstic weathering during a lower-mid Silurian (Aeronian) lowstand, superimposed on the Hirnantian–Rhuddanian Cherokee Unconformity. This irregular unconformity is overlain by a unit termed the “golden Brassfield”, which grades laterally into strata identified as the “red Brassfield”, a probable equivalent of the Oldham Limestone to the east–southeast. These units are Aeronian or early Telychian, considerably younger than the Rhuddanian–early Aeronian Brassfield Formation sensu stricto. Preservation of erosional topography, resulting from rapid flooding and burial of karstic surfaces, is typical of unconformities representing moderate durations.

Correlation of the Waco Member of the Alger Shale Formation (Silurian; Llandovery; Telychian) in east-central Kentucky and south-central Ohio

New and published stratigraphic data are integrated herein to resolve the age and correlation of the Waco Member (Alger Shale Formation), a problematic Telychian lithostratigraphic unit exposed in east-central Kentucky (United States). This interval is correlated with strata presently assigned to the Dayton Member of the Drowning Creek Formation in south-central Ohio and north-central Kentucky, where recent conodont studies refer these beds to the Pterospathodus eopennatus Superzone, a conclusion that is reinforced by an interval of elevated stable carbon isotope (δ13C) values, consistent with the lower Telychian “Valgu” Excursion. This correlation is strengthened by observations of facies and new carbonate carbon isotope (δ13Ccarb) data generated from localities in south-central Ohio and, for the first time in publication, from the type area of the Waco in east-central Kentucky. It is now clear that both lithostratigraphic units are substantially older than the recently redefined Dayton Formation of western Ohio, and a formal revision of nomenclature is needed. A series of revised correlations are proposed: the basal carbonate bed of the Waco in its type area is correlated to the lower “white” division of the “Dayton Member” in southern Ohio; an interval of fossiliferous shales, found in typical exposures of the Waco, is correlated to the upper “orange” division of the “Dayton” in outcrops further north. Continued use of the term “Dayton Member of the Drowning Creek Formation” at sections south and east of Greene County, Ohio, should be abandoned in favour of “Waco Member of the Alger Shale”.

Widespread soft-sediment deformation horizons in Lower Silurian strata of the Appalachian basin: distal signature of orogeny

Abstract Sedimentology and stratigraphic mapping of soft-sediment deformed beds (ball and pillow and convolute bedding) in the Lower Silurian of eastern North America demonstrate that these event beds are extremely widespread and that their component sediment layers were not deformed during initial deposition, but slightly later, during shallow burial. Successions of laminated arenaceous beds with interbedded shale in regressive (falling stage) systems tracts of third order depositional sequences appear to have been prone to deformation. Deformed zones are likely the result of shear-induced liquefaction of thixotropic muds and foundering of overlying (carbonate and siliciclastic) silts and sands during very large-scale earthquakes. The distribution of deformed beds, together with increased subsidence, clastic influx, and K-bentonite horizons, provides a meter of intensity and timing of pulses of Silurian orogenesis.

Sequence boundaries and chronostratigraphic gaps in the Llandovery of Ohio and Kentucky: The record of early Silurian paleoceanographic events in east-central North America

New and published data are integrated herein to resolve the age and stratigraphic relationships for problematic strata of the Aeronian and Telychian (Llandovery; Silurian) in Ohio and Kentucky (USA). At least two major depositional sequences were traced along the eastern flank of the Cincinnati Arch; these are separated by a regionally angular unconformity with complex topography. Underlying units are progressively truncated to the northwest while overlying strata change facies, condense, and onlap in the same direction. The basal unit of the upper sequence is the Waco Member of the Alger Shale Formation in Kentucky and southern Ohio and the Dayton Formation in western Ohio. A persistent, positive carbonate carbon isotope (δ13Ccarb) excursion associated with the mid-Telychian Valgu Event is recognized in the upper subunit of the Waco Member; the absence of a comparable signal in the Dayton Formation corroborates interpretations that it is significantly younger. The correlations proposed here can be used to understand the nuanced depositional history and chronostratigraphic completeness of the lower Silurian in eastern North America. This framework can be used to characterize sea-level history and local conditions that prevailed during global paleoenvironmental events.