GEOLOGIC SETTING AND STRATIGRAPHY

During the Late Ordovician, the Cincinnati region was located nearly 20 degrees south of the equator and rotated 45 degrees clockwise from its present orientation (Figure 2) (Scotese and McKerrow 1991). The Cincinnati region occupied a distal position in the Taconic foreland basin, developed from the collision of an island arc system with the eastern seaboard of Laurentia (Brett and Algeo 2001). The study area assumed a ramp structure which gently dipped from the modern southeast to northwest and was covered by a shallow epieric sea up to 30 m deep (Holland 2008).

Strata of the type Cincinnatian Series are divided into six depositional sequences, C1 to C6 (Figure 3) (Holland 1993; Holland and Patzkowsky 1996). Each sequence spans between approximately three quarters of a million years to two million years. Thin transgressive systems track deposits occur at the base of the sequences, but the majority of the sequences represent highstand deposition. Each sequence records an overall shallowing upward progression, and the sequences shallow upward overall due to infilling of the basin.

Four primary depositional environments were arrayed along the ramp: offshore, deep subtidal, shallow subtidal, and peritidal (Holland 1993, 2001). Sedimentology varies with paleoenvironment, but in general, Cincinnatian sediments represent storm-derived deposition (Brett and Algeo 2001). Layers alternate between terrigenous mudstones comprising sediments sourced from the weathering Taconic highlands and bioclastic packstones to grainstones, which represent the shelly storm lag (Jennette and Pryor 1993). In deeper-water paleoenvironments, mudstones comprise a high percentage of the section as only the largest storms were able to disturb the seafloor sediments, whereas shallower water deposits are dominated by limestone layers because the fine-grained particles were almost entirely winnowed away by storms or waves (Jennette and Pryor 1993; Holland 2001). Although storm processes were pervasive in the Cincinnatian system, lateral transport of skeletal debris was limited (several meters) as evidenced by the ability to resolve detailed paleoecological changes within limestone units at the outcrop scale (Frey 1987; Barbour 2001).

In general, Cincinnatian limestones lack features associated with tropical deposition (e.g., ooids, peloids, and significant micrite) and exhibit characteristics of cool-water carbonates. Because these layers were deposited in the paleotropics, they are interpreted to represent a high-nutrient system rather than truly cool-water deposition (Holland and Patzkowsky 1996). This is particularly the case for the C1 to C4 units. These units contain phosphate grains and crusts indicative of high-nutrient conditions (Holland 2008). In the C5 and C6 sequences, phosphate deposition decreases, nutrient levels appear to drop, and micrite becomes more abundant. This shift in nutrient load has been interpreted as a shift in paleooceanographic conditions (Holland and Patzkowsky 1996). This shift appears to coincide with the Richmondian transgression and the influx of invader taxa into the basin, as discussed more fully below.