PETROGRAPHIC STUDY OF SEDGWICK ROCK TUFA DEPOSIT, WAYNE COUNTY, INDIANA

This investigation is part of an undergraduate independent study project aimed at determining the geologic origins of Sedgwick's Rock, an unusual fresh water tufa deposit located on a NE facing ravine of Boston Creek in SE Wayne County Indiana. The tufa body has an outcrop area of about 11,400 m2 and is situated among horizontally bedded Upper Ordovician shallow marine limestones. The tufa is noteworthy because there are no others in east-central Indiana, it has a complex history that has not been previously studied and it may contain evidence of vanished hydraulic and climatic regimes.

The tufa is made of calcite as determined from XRD. Chaotically admixed textures belie differing modes of calcite precipitation. Textures include a fenestral, clotted, irregular material, smoothly laminated surface layer coatings, pendant cement incrustations and root and stem molds. The tufa ranges from from weakly consolidated to well cemented. Petrographic analysis reveals 4 distinct fabrics often found in a distinct sequence: 1.) an irregular, clotted spar 2.) finely crystalline microspar associated with organic discoloration, 3.) isopachous druzy cement, 3.) elongate palisade crystals often displaying radiaxial extinction.. Palisades crystals exhibit multistage growth bands and sometimes preserve what appear to be algal filaments supporting an algal origin. Modern algal growth in springflow over the tufa is characterized by calcite crystals growing in contact with filament tissue.

The tufa occurs below a 100 m long springflow discharge zone positioned 20 m above Boston Creek. Flow originates along a lense of sand and gravel enclosed within till. The discharge zone is characterized by mucky organic-rich hydric soils and abundant Equisetum growth. One puzzle is that the land surface area capable of contributing recharge to the spring line is puny comprising about 0.1 km2. This suggests a more complicated hydrogeologic picture, perhaps reflecting discharge of a regional flow system. Another possibility is that modern conditions represent the remnants of a much larger discharge system, perhaps driven by a plumbing system fueled by deglaciation.

Future research, including 14C dating and oxygen and carbon isotope analyses, may help to refine our understanding of Sedgwick's Rock.