RECIPROCAL TIME-AVERAGING AND TAPHONOMIC FEEDBACK IN A COMPOSITE BIVALVE-BRYOZOAN CONCENTRATION FROM THE UPPER ORDOVICIAN OF KENTUCKY

A ledge-forming grainstone in the Upper Ordovician (Richmondian; upper Katian) "Cyphotrypa shale" of the Bull Fork Formation crops out near Mt. Sterling, central Kentucky , exposing an unusual surface characterized by a dense concentration of the spheroidal to domal bryozoan Cyphotrypa cf. clarksvillensis, completely covering the upper bedding plane. This horizon has been traced over several outcrops, suggesting that it represents a condensed bed. Bryozoan colonies are in growth position and preserve their original substrates as impressions on their basal surfaces — most colonies bear the molds of now-dissolved modiolopsid bivalve shells. Thus, the bryozoans indirectly preserve an underlying, otherwise undetectable, bivalve shell pavement. Both bryozoans and bivalves exhibit varying degrees of corrasion, bioerosion, and surface feature quality, suggesting time-averaging of both pavements. In many cases well-preserved bivalves occur at the bases of corroded bryozoans and, conversely, poorly preserved bivalves formed substrates for the well-preserved bryozoan cohort. This contrast in taphonomic signature between the encrusted bivalves and their epibiotic bryozoans indicates that well preserved bivalves were preferentially encrusted and therefore protected from degradation, while exposed (un-encrusted) valves were subjected to physical and biological deterioration and encrusted later. The Cyphotrypa pavement offers a unique case study in what is essentially a dually time-averaged composite concentration bed. The initial concentration deposit, the underlying bivalve pavement, displays evidence for time-averaging; however, the extent of taphonomic degradation is under direct control of encrustation by encrusting bryozoans, which themselves are time-averaged. Hence, the two skeletal types are undergoing time-averaging at the same time, but to varying degrees. This study not only demonstrates the utility of this assemblage in reconstructing both taphonomic and sedimentologic histories, but also represents a new dimension of hard substrate taphonomy and a new perspective on time-averaging.