Benthic Community Development in Limestone Beds of the Waynesville (Upper Dillsboro) Formation (Cincinnatian Series, Upper Ordovician) of Southeastern Indiana

Abstract

ABSTRACT Vertical and lateral changes in composition occur within 13 individual limestone beds of the Waynesville (Upper Dillsboro) Formation (Upper Ordovician) near Brookville, Indiana. Fossil allochems formed approximately 40%. and carbonate ooze 60% of the original carbonate sediment, with later conversion of most of the ooze to microspar and pseudospar. The relative abundances of individual constituents, however, vary between the top, center, bottom, and margins of each limestone bed. The centers of the beds are distinguished from the tops, bottoms, and margins of beds using certain variables and discriminant function analysis. Variables important in separation include neomorphic sparry calcite and echinoderms within the bed centers, and micrite, bryozoans, and brachiopods within the tops, bottoms, and margins of beds. An increase in faunal diversity from the bottoms to the centers of beds indicates rapid community succession. A decrease in faunal diversity from the centers to the tops of the limestone beds indicates community degradation. The writers propose that a subtidal, terrigenous mud substrate was colonized by thin, flat, brachiopods (Onniella). The accumulation of these valves provided a pavement upon which erect bryozoans could grow. Further stabilization of the substrate allowed crinoids to become established. A wave-current baffle was produced in the denser parts of the community by the abundant growth of bryozoans and crinoids creating low energy conditions favorable for the simultaneous accumulation of unsorted allochems and ooze. The community spread laterally as skeletal debris swept from atop the growing centers stabilized the peripheral muds. Carbonate accumulation terminated when mobilized, muddy, bottom sediment, thrown into suspension under storm conditions, settled and smothered the community. The model proposed for the accumulation of the carbonate sediments is independent of bottom topography and conditions of accumulation of terrigenous mud.