PROXIMALITY TRENDS IN METER-SCALE CYCLES ALONG A LATE ORDOVICIAN RAMP: IMPLICATIONS FOR CYCLE GENESIS

The Kope Formation (Upper Ordovician, Edenian Stage) of Ohio and Kentucky comprises 70-75m of sparsely fossiliferous, gray mudrock and thin skeletal limestones. Muds, derived from Taconic orogenic terranes to the NE, accumulated on a storm-dominated ramp bordering the Sebree Trough, a narrow NE-SW basin. Previous studies have documented meter-scale alternations of mudstones/thin HCS-laminated siltstones and compact decimeter-scale limestones, within Kope outcrops. The origin of such meter-scale cycles has been debated, largely based on outcrop features, without a complete understanding of their regional shelf to basin extent or pattern. However, our recent study of a series of cores drilled in south-central Ohio permits the assessment of variation of Kope cycles in the subsurface. Distinctive taphonomic, sedimentologic, and faunal markers (e.g. epiboles) have been used to trace individual dm-scale shell beds and intervening mudrocks with confidence over 10s of km in outcrop and through the drill cores, proving the regional persistence of meter-scale cycles. Preliminary analysis of these cycles reveals several proximality trends relevant to their interpretation. Mudrocks thicken slightly and exhibit gradients of litho- and biofacies reflecting increasingly deep and dysoxic conditions downramp. Shelly limestone beds thin and show increasing evidence (e.g. skeletal degradation, mineralization) of quiet water condensation distally, while still retaining pack- to grainstone fabrics. Skeletal components of these beds also show a faunal gradient parallel to that of the mudstones, indicating their formation by concentration of locally- derived material during episodes of siliciclastic starvation that affected the entire ramp gradient. Downslope thinning of shell beds reflects reduced skeletal production. Evidence of increased condensation in distal areas further suggests mud sequestering during minor episodes of sea-level rise. This contrasts with previous models that invoked lowering of sea-level or increased storm intensity to explain Kope shell beds, and with earlier notions of grainstones and mudrocks as products of rapid storm and slow background sedimentation, respectively.