Trace-Fossil Distribution across a Marine-to-Nonmarine Gradient in the Pennsylvanian of Southwestern Indiana

Facies representing eight deltaic environments are recognized in the Mansfield Formation (Pennsylvanian: Morrowan) and Dugger Formation (Pennsylvanian: Desmoinesian) of Indiana. These facies are interpreted as A) lower interdistributary bay, B) middle interdistributary bay, C) upper interdistributary bay, D) tidal flat, E) distributary-mouth bar, F) crevasse splay, G) active channel fill, and H) floodplain and lacustrine. Despite some overlap, each facies is characterized by a specific assemblage and diversity of trace fossils. Although certain burrows (e.g., Chondrites type C and Palaeophycus sp.) occur in several facies, over 70% of the ichnogenera are restricted to a single facies. Trace-fossil taxonomic diversity decreases from a maximum in environments dominated by marine conditions (A, B) to a minimum in transitional environments of mixed marine and freshwater conditions (C-E) and increases again in freshwater environments (F-H). Under specialized preservational circumstances, a diverse assemblage of freshwater traces can be preserved. Thus it is apparent that high trace-fossil diversity in Pennsylvanian deposits cannot always be assumed to indicate marine conditions.