ABSTRACT

The South Wabash carbonate mud mound displays three main zones: (1) a lower fossiliferous and stromatactis dolosiltite zone representing a core that probably formed below wave base, (2) a middle zone, consisting of algal (spongiostromid)-constructed and brecciated dolomite that indicates growth into a more agitated environment, and (3) an upper fossiliferous and stromatactis dolosiltite zone representing flank bed accumulation and continued mound development above wave base.

This exposure represents a possible transitional growth stage (Stage 4) between carbonate mud mounds and true wave-resistant stromatoporoid-and coral-controlled reefs, as postulated by Textoris and Carozzi (1964) for the ideal Indiana Niagaran reef development sequence. However, instead of the hypothesized encrusting stromatoporoid cap, a spongiostromid algal cap has been discovered. These spongiostromids represent the first definitely known algae in the Niagaran reefs and mounds of Illinois and Indiana.