SILT: THE OVERLOOKED BUT ENLIGHTENING WILDCARD OF A CYCLIC SEDIMENTARY SUCCESSION

The Upper Ordovician (lower Cincinnatian; Katian) Kope Formation of the Cincinnati region is renowned for its faunal preservation and stratigraphic cyclicity. Meter-scale alternations of shale and skeletal limestones in the Kope Fm. ostensibly reflect global climatic cycles, but the specific mechanics of cyclicity are still debated and may relate to fluctuations in storm intensity, changes in water depth, or eustatic/climatic changes in sediment supply. To date, research on Kope cyclicity has focused primarily on the sedimentology of shales and limestones, largely ignoring a third component of the system: siliceous siltstones. The stratigraphic distribution of silts may provide insight into the nature of the 4^th- or 5^th-order sequences that comprise the Cincinnatian. Siltstone beds are prevalent in some shale-dominated intervals and among some thicker (>10 cm), hummocky-laminated beds, including gutter fills that are widely traceable throughout the region, indicating extraordinary events during which large volumes of silt (>90000 m³) were abruptly deposited.

To document and quantify the stratigraphic occurrences of silt throughout the Kope, several sections have been measured at high resolution, noting all siltstone beds >1 cm thick, at several locations across the Cincinnati Arch, covering a distal-to-proximal gradient. Sections have been statistically analyzed to determine distributions of siltstones within each meter-scale cycle and to compare these distributions with occurrences of limestones, the abundances of key taxa, and reconstructed sea level curves. Results to date indicate that siltstones are preferentially distributed in distinct, sparsely fossiliferous parts of the formation associated with highstand intervals and relatively deep water. Transgressive, shallow water limestone bundles are largely silt-free. Furthermore, isopach and trace element provenance studies strongly suggest that Kope silts and clays had different source areas, in the northern and southern Taconics, respectively. Taken together, these results demonstrate that silt is an important, if often overlooked component of cyclic carbonate and siliclastic fancies, as it tracks changes in strength, type and provenance of sedimentation.