ESTABLISHING PATTERNS IN SCLEROBIONT DISTRIBUTION IN A LATE ORDOVICIAN (CINCINNATIAN) DEPTH GRADIENT: TOWARD A SCLEROBIOFACIES MODEL

ABSTRACT: The stratigraphy, paleobathymetry, and paleontology of the Cincinnatian strata have been studied extensively, providing an excellent framework within which to explore spatial and temporal variations in encrusting sclerobiont assemblages. Recently, the relative paleobathymetry of Upper Ordovician Cincinnatian strata has been assessed using assemblages of light-sensitive microendolithic ichnotaxa, resulting in the application of a zonation based on light intensity: shallow euphotic, deep euphotic, and dysphotic–aphotic. This paper assesses differences in sclerobiont communities observed on brachiopod shells (primarily Rafinesquina) among shallow euphotic-, deep euphotic-, and dysphotic-zones from the Upper Ordovician strata of the Cincinnati Arch region. Sclerobiont assemblages can thus successfully define a sclerobiofacies sensitive to interpreted paleobathymetric photic zonation. Ordovician sclerobionts show predictable declines in sample richness, frequency of encrustation of host shells, and areal coverage of host with relative depth and/or photic zonation. Bryozoans predominate in all environments, although notably sheetlike trepostome bryozoans and inarticulate brachiopods were particularly dominant in the shallow euphotic zone, and paleotubuliporid bryozoans, microconchids, and cornulitids were more successful in the deep euphotic zone. If sclerobiont assemblages can be correlated with relative light intensity zonation throughout the Phanerozoic, this work may be an important first step in delineating depth-related sclerobiofacies that could provide an important tool for paleobathymetric reconstruction throughout geologic time