TESTING THE REPRODUCIBILITY OF CYCLOSTRATIGRAPHIC SIGNALS ACROSS A FACIES GRADIENT (UPPER ORDOVICIAN KOPE FM, CINCINNATI REGION, USA)

Cyclostratigraphy is rapidly gaining recognition as a powerful chronostratigraphic tool. However, work remains to develop best practices. With much headway made on the development of robust statistical techniques, we turn our attention toward the processes that capture and preserve ancient climate forcing. Depositional and diagenetic processes can bias the record through damping or erasing real climate signals or introducing false information, or they may amplify a primary signal. Understanding how sedimentary processes change along onshore-offshore gradients with regard to their ability to preserve and potentially amplify climatic signals is critical to the broader application of cyclostratigraphy throughout the ancient marine record.

Here, we reevaluate the classical Upper Ordovician Kope Formation (early Katian – Edenian) of the Cincinnati Arch area, well-known for its meter-scale cycles. In outcrop, these lithological cycles consist of alternating intervals of shale and skeletal limestones, the latter pinching out in an offshore direction. We have applied numerical time-series analyses to high-resolution portable X-Ray Fluorescence (pXRF) and magnetic susceptibility (MS) data series from a transect of cores and outcrop sections. These carefully selected sections contain different sedimentary facies and represent a basinal to mid-ramp transect. This exercise evaluates how cyclicity is expressed in different depositional environments, which contributes to further understanding of the mechanisms behind the cycle’s genesis.

Despite the loss of limestones offshore, the Kope Fm retains a cyclic record in other aspects of its composition. A consistent cyclic pattern over this facies gradient argues in favor of an external (climate) forcing (e.g. Milankovitch-type) versus potential local processes. Constraining the duration of these cycles provides additional challenges.