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Middle Mississippian (Meramecian) predominantly carbonate units of south-central Indiana, which include the Harrodsburg, Salem, St. Louis, and Ste. Genevieve Limestones, included the following environments: (1) sabkha (supratidal flats); (2) lagoons; (3) oolitic-and calcareous-sand bars and shoals; and (4) deeper shelf impure carbonates. Probably as functions of salinity and rate of deposition, these rocks range from being intensely bioturbated to a lack of recognizable bioturbation. Although several of the trace fossils cannot be currently assigned to recognized ichnogenera, the following forms have been recorded: Chondrites, Cylindrichnus(?), Planolites, Teichichnus, and a Rhizocorallium-type trace.
Degree of weathering exerts a considerable control on field recognition of the various forms. Intense weathering obliterates the slight tonal differences that aid in identification of traces in the fine-grained carbonates. However, in some calcarenites, certain taxa are only recognizable in weathered material and are nearly impossible to discern in polished slabs.
Factors related to episodic influx of fine-grained terrigenous detritus exert a considerable control on preservation of the various forms. Field exposures of carbonates that contain shale partings commonly exhibit abundant traces that are only apparent in polished slabs of more massive carbonates. The similarity of trace-fossil assemblages from these shallow-water carbonates with traces of deeper shelf Cretaceous chalks of the U.S. Western Interior further supports the contention of a high degree of preservational control.
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