LATE SILURIAN KOKOMO LIMESTONE - MUDCRACKS OR SYNERESIS CRACKS?

The Kokomo Limestone Member of the Wabash Formation is of late Silurian age. About 30 meters of thinly laminated carbonates are exposed in the Logansport area of north-central Indiana. Larger scale (0.1 – 2 m) beds containing laminated carbonates alternate in bands of greenish gray and tan colors. Almost complete lack of fossils throughout the member indicates a hypersaline environment. Broken laminations in many parts of the member were interpreted in previous studies either as subaerial mudcracks or subaquaeous syneresis cracks. Consequently, the inferred depositional environment of Kokomo Limestone member has been interpreted either as an intertidal or restricted lagoon. In this paper we show strong evidence of mudcracks indicating an intertidal environment during deposition of the Kokomo. Carbonate mud doesn't crack subaquaeously unless it contains significant clay component. Insoluble residue of the Kokomo amounts to less than 1% of total rock and it mostly includes detrital or authigenic quartz and pyrite. Fossil absence indicates a relatively stable, hypersaline environment, which precludes changes in water chemistry that might have caused expulsion of water from clays. Couplets of dark and light laminae were interpreted as annual cycles but we interpret them as a daily tidal cycles. Light laminae represent accumulation of carbonate crystals during an incoming tide while dark laminae are settling of algal mats during an outgoing tide. We found incipient mudcracks offsetting 11-16 couplets and interpret them as a brief, daily exposure during the spring tides. Other microscopic evidence for an intertidal environment includes intraformational micro breccia/conglomerate, abundance of dolomite and anhydrite/gypsum pseudomorphs. In the outcrop, we observe at several intervals, from the base to the top of the Kokomo, intraformational conglomerates, anhydrite/gypsum chicken wire structures, birdseye structures and vugs lined with calcite, dolomite and tar that support the interpretation of an intertidal environment.