SEDIMENTOLOGY AND MICROSTRATIGRAPHY OF A CINCINNATIAN EDRIOASTEROID OBRUTION DEPOSIT

The Manchester edrioasteroid pavement is a shell layer in mudrock from the Corryville Formation (U. Ordovician, Maysvillain). No previous studies have examined sediments that buried Cincinnatian edrioasteroid pavements. To address this problem, a 16 cm thick stratigraphic sample measuring 90 cm by 30 cm was collected with a plaster jacket, dried for 2 years, then encased in fiberglass for dry cutting into slabs 2-4 cm thick. These were hardened with epoxy and polished dry with sandpaper. A prepared slab was sent to Bruker AXS for analysis using the M4 Tornado µ-XRF.

Delicate colonies of bryozoans, embedded in mudrock were found below the block and collected separately. The sample contains three sedimentary units. The lowest unit (Unit 1) is based by a packstone with fragmental bryozoans, trilobites, and brachiopods (Zygospira and Rafinesquina). Gradationally overlying the packstone is a concretionary mudrock containing fossil fragments which grades upward into gray shale. This shale is laminated, bioturbated by pyrite-lined Chondrites, and contains silt-sized fossil fragments. The upper portion is a concretionary underbed. Unit 2 begins with a thin, discontinuous, calcisiltite disrupted by burrows. Next is a laminated shale with burrows similar to unit 1. This unit also contains silt-sized fossil fragments and pyrite-lined burrows. Unit 3 has a basal limestone containing bits of trilobite, columnals of Iocrinus, phosphatic brachiopod fragments, ostracods, and Rafinesquina. This horizon also preserves delicate fossils and associations, such as Ambonychia, Schizocrania, and edrioasteroids Streptaster and Carneyella attached to articulated, spar-filled Rafinesquina associated with concretionary zones.

Evidence that the shelly limestone sub-units represent periods of low siliciclastic input followed by rapid burial include: 1) Development of underbeds and concretionary zones which require extended time near the sediment-water interface to form; 2) phosphate enrichment from the accumulation of broken and corroded phosphatic brachiopod shells; 3) mixed taphonomy suggesting periods of exposure beyond the lifespan of a single cohort; 4) spar-filled, edrioasteroid encrusted, Rafinesquina shells and intact delicate bryozoan colonies indicating repeated rapid burial of live communities.