WHERE ARE THE FAIR-WEATHER SHALES OF THE CINCINNATIAN? TESTING THE STORM-WINNOWING MODEL FOR THE ORIGIN OF INTERBEDDED LIMESTONE AND SHALE

Storm depositional processes have explained the origin of the interbedded limestone and shale that is characteristic of the Upper Ordovician in the Cincinnati region. The storm-winnowing model postulates that, under calm-water conditions, continuous mud deposition on a continuously-occupied sea floor led to thick accumulations of fossiliferous shale. The resulting fair-weather shale was occasionally disturbed by storms. During the waning phase of these storms, the heavier carbonate skeletal remains settled out rapidly in place while the finer silt and mud was deposited later, either in place, or further offshore. Thus, winnowing of diffusely fossiliferous shale resulted in the formation of skeletal wackestone or packstone on one hand, and relative pure shale on the other.

This model explains many characteristics of Cincinnatian limestone, but the storm-winnowing model also makes predictions about shale. For example, as water depth increases, there are fewer storms able to winnow bottom sediments. This should result in deeper-water deposits containing more limestone beds that overlay remnants of the of the fair-weather shale from which they were winnowed. Fossiliferous shale is common in some parts of the section, but it is invariably associated with more limestone, bigger fossils, and more abrasion and fragmentation. These indicate shallow water. Deeper-water deposits are associated with unfossiliferous shales.

It is likely that quiet-water fair-weather shale was not very fossiliferous, and shale may not have been deposited during fair weather periods. An examination of shale in core samples from the Maysvillian strata of Indiana and Ohio reveals that this shale is in fact built of successive discrete event deposits. These are probably tempestites as well.

Many features of shale and limestone are explained by the storm model. However, if quiet-water fair-weather shale was not fossiliferous, or if it was rare, then it is unlikely that storm-winnowing could have created limestone from shale. Shell-dense layers accumulated over long periods in clear water before being reworked and smothered by storms. Other factors, such as colonization of dewatered substrates, faunal succession and taphonomic feedback, should be considered when explaining the origin of limestone beds.