SHELL BED GENESIS IN MUDROCKS: SEDIMENTARY DYNAMICS AND TAPHONOMIC FEEDBACK IN A MIXED SILICICLASTIC-CARBONATE FORELAND BASIN SYSTEM (UPPER ORDOVICIAN, SOUTHWEST OHIO AND NORTHERN KENTUCKY)

Alternating mudstone- and carbonate-dominated successions are common in many cratonic successions. The sedimentological and taphonomic features of mudstones and interbedded carbonates and silts within both the mudstone-rich and shell-rich intervals in the Upper Ordovician (Caradocian; Edenian Stage) Kope Formation in northern Kentucky provide insights into the development of such successions. We summarize evidence for: a) sedimentologic processes/environmental energy and b) relative amounts of time recorded in the mudstones vs. shelly limestone. Both phases accumulated under generally low energy conditions, and both show evidence for episodic erosive storm-generated scouring and sediment reworking. However, such storm episodes are much easier to recognize in shelly gravels, which can exhibit bedforms, grading, cross-bedding and rip-up clasts. Our results also indicate that Kope limestones are relatively condensed, while mudstones accumulated as a series of abrupt episodic deposits up to several centimeters thick. Skeletal debris in widespread Kope limestones is parautochthonous rather than allochthonous. Three models have been proposed to explain the origin of the Kope mudstone-limestone cycles. The first two assume that the mudstones represent low energy/ deeper water conditions, while skeletal limestones are primarily the product of increased storm wave energy and winnowing, owing to one of two processes: a) relative sea level fall, which brought the seafloor into the reach of storm waves, or b) increased intensity/frequency of storms without significant water depth change. A third model proposes that, in addition to storm winnowing, limestones represent accumulation of time-averaged skeletal debris during prolonged periods of siliciclastic sediment starvation, possibly associated with minor base-level rise. Present evidence indicates that decimeter-scale Kope shell beds formed during millennial-scale periods of siliciclastic sediment starvation combined with taphonomic feedback and episodic storm-related reworking and winnowing. This explanation constitutes an alternative to exclusively storm winnowing models of shell bed genesis in this and many other mixed siliciclastic carbonate successions that is more in accord with taphonomic, sedimentologic and paleontologic evidence.