ABSTRACT

The Upper Ordovician Kope and Fairview Formations and the Bellevue Tongue of the Grant Lake Limestone exposed in the Cincinnati, Ohio region, represent storm-dominated sedimentation on a prograding, intracratonic ramp. The upper Kopelower Fairview transition zone (25 m thick) is an outer to intermediate ramp lithofacies and is composed of numerous (1-3 m thick) shallowing-upward subtidal cycles. Bedding cycles are composed of two depth-controlled facies: 1) a shale-dominated distal facies and 2) a fossiliferous proximal facies. The distal facies records low-energy, largely single-storm events and consists of thin graded shale beds, nearly in situ fragile-fossil accumulations, thin parautochthonous packstones, and quartzose siltstone beds. Siltstones contain abunda t storm-depositional indicators: tool-marked soles, gutter casts, mud-filled scours, hummocky lamination, and hummocky bedforms. The proximal facies represents deposition in an energetic, storm-wave-swept environment and consists of amalgamated sets of thick-bedded, fine- to coarse-grained skeletal grainstones and packstones with low-angle cross-lamination, megaripples, and shale lithoclasts. Sedimentary structures and paleocurrent patterns from both facies record deposition under a combined flow regime with the unidirectional current vector oriented in the offshore direction.

Cycles are laterally continuous across the 35 km by 32 km study area. Cycle boundaries (flooding surfaces) are parallel to a unique storm-generated marker bed, implying that these surfaces are effective time lines. Their widespread character and relatively deep subtidal-facies assemblage argues against autocyclic processes in cycle formation. We propose that the alternation of proximal and distal facies was tied to fluctuations in storm-wave base driven by short-term glacio-eustasy.

Three scales or orders of sedimentary cyclicity are recognized in the study area. Meter-scale bedding cycles are considered fifth-order parasequences (20-100 Ka duration) and stack into genetically related parasequence sets. These fourth-order units have been previously described as member-scale lithostratigraphic tongues. Analysis of Fischer plots suggest that cycle thickness and subfacies composition was controlled by both accommodation and siliciclastic sediment supply. These higher-order stratal units are super-imposed onto the highstand portion of the Kope to Bellevue depositional sequence and give insight into the high-frequency controls on the progradation of the ramp complex.