THE IMPORTANCE OF PERITIDAL MUDS AND SILICICLASTIC PHASE REVERSAL IN UNDERSTANDING CINCINNATIAN CYCLOGENESIS

The Cincinnatian Kope Formation (C1 sequence) is well studied, but the overlying Ordovician strata, C2-C6 have had less attention. A result is that the Kope Formation is seen as cyclic while overlying strata are not. Kope cycles include two phases; one dominated by shale, the other by grainstone ostensibly formed by storm winnowing.

Nevertheless, other parts of the Cincinnatian section are cyclic. For example, the Fairview-Miamitown-Bellevue transition (C2) in Cincinnati has meter-scale cycles characterized by a mudrock-rich phase alternating with a wavy-bedded shelly packstone and mudrock phase. This interval correlates in the subsurface of Indianapolis to an offshore Kope-like facies with Kope-like cycles. Further up section, the coral zones of the C5 show different cycles. Here coral-rich horizons are interlayered with thicker intervals of muddy dolomitic siltstone.

Autocyclicity is unlikely given the distances that cycles are correlated in the C1 and C2, but the imprint of an allocyclic forcing mechanism is not everywhere identical. The unifying theme is that subtidal cycles have a siliciclastic phase and a carbonate phase. If these cycles are part of the same depositional system, then storms could not have caused cyclicity; they cannot explain the growth of large coral heads. Fluctuations in siliciclastic sediment supply explain deep subtidal grainstones and shallow subtidal coral heads equally well.

Intertidal/supratidal facies are found in the south of the Cincinnatian outcrop belt. They include the laminated and mud-cracked dolomitic silty mudrocks of the Tate (C2), Terrill (C3), Rowland (C4) and Saluda (C5) units. Each is lithologically monotonous, lacking carbonate/siliciclastic cycles. The Tate near Richmond, KY, was deposited at the same time as cyclic sediments in Cincinnati and Indianapolis. Continuous flat partings in the Tate suggest omission surfaces.

One hypothesis is that Siliciclastics trapped in Tate deposits during rising stages might have cleared the water for growth of subtidal carbonates, while sediment bypassing the flats during falling stages, resulting in omission surfaces, would have smothered subtidal carbonates in mud. This “siliciclastic phase reversal” can be tested by high-resolution correlations through supratidal – subtidal facies transitions.