HOW MANY SHELLS ARE IN A SHELL BED? MIXED TAPHONOMY AND SHELL DESTRUCTION IN A TIME-RICH STORM-DISTURBED CINCINNATIAN (ORDOVICIAN, KATIAN) SHELL BED

Ordovician strata of the Cincinnati, Ohio area include many shell beds whose origin has been the subject of debate. Two models have been proposed: 1) that shell beds originate through storm-winnowing of shell-rich mud, and 2) that shell beds originate during times of sediment starvation, when shells accumulate in place in clear-water conditions. The first model predicts that the majority of shells would be well-preserved, having been previously buried in sediment without disturbance. The second predicts that shells would exhibit a full range of taphonomy, having been reworked and reburied by multiple disturbance events passing over the sea floor.

We investigated a thin (6—8 cm) shell bed from the Fairview Formation near Cincinnati through serial thin-sectioning. The shell bed grades upward into a thin (1 cm) siltstone drape. Such packstone/siltstone couplets are generally interpreted as storm deposits, and the escape burrows through both units support this interpretation. Using both point counting and areal mapping we documented the range of taphonomic grade of the shells and shell fragments within the shelly component and quantified the abundance of shell material within four “taphonomic grades”: 1) whole shells, including live-buried (obruted) lingulid and Trematis brachiopods with Isotelus trilobites 2) disarticulated and lightly fragmented shells, 3) very fragmented but identifiable shells, and 4) unidentifiable generally millimeter-scale shell hash. Disarticulated shells appear to be the most abundant on the surface of the bed. However, the comminuted shell material is the most abundant by volume, despite being “carbonate sand”, below the size threshold generally counted as shells.

The range of taphonomic grades indicates that, before the final reworking and deposition, the shell bed had been disturbed by multiple high energy events. Shell accumulation ended as the bed was reworked, redeposited, and finally buried by a storm event. The final living community was preserved by obrution. This occurrence shows how the signs of obrution in the fossil record can be subtle, because the living community at any one time is small compared to the volume of shells, shell fragments, and carbonate sand left by previous communities, then mixed together by reworking.