SEDIMENTATION AND GEOCHEMISTRY OF THE MANSFIELD FORMATION IN MARTIN COUNTY, INDIANA

Two Pennsylvanian-age rock outcrops from the Mansfield Formation at the Naval Surface Warfare Center, Crane Division, in Indiana are approximately one kilometer apart laterally and 40 meters apart stratigraphically. At the younger, more easterly outcrop laminated siltstones overlie a coal seam that in turn overlies a paleosol layer. Tetrapod tracks were discovered at this outcrop in 1992 approximately 20cm above the coal seam in the basal siltstone. At the older outcrop a black, organic-rich paleosol layer separates a younger siltstone bed and older, cross-bedded sandstone. Roots extend down into the upper part of the sandstone.

Bioturbation occurs at intervals in the siltstone layers of both outcrops. Ichnofossil analysis is beginning on samples collected from the outcrops with the intention of identifying a depositional environment.

Carbon/sulfur (C/S) analysis has been completed on samples collected from the younger outcrop and is in progress for samples from the older. Total organic carbon (TOC) in the siltstones is very low, generally in the 0.1-0.2% range, which is likely due to the impact of the bioturbation. C/S ratios fluctuate between 1.72:1 and 37.86:1.

Broadly fluctuating C/S ratios are often taken as an indication of freshwater deposition. Very low sulfur values and lack of pyrite in rock samples support a freshwater interpretation. However, aspects of the sedimentology suggest a marine (tidal) connection during deposition. One explanation is that the outcrops formed as freshwater tidal flats. The goal of the research is to evaluate this and other depositional models.