Sedimentary Features and Depositional Setting of the Cambrian Eau Claire Formation in Central Indiana
The Eau Claire Formation (upper Cambrian) of Indiana is a shallow marine succession, dominated by interbedded shale, siltstone, and sandstone. Cores from central Indiana were examined for sedimentary features and stratigraphic features. The lower part of the succession, overlying the Mt. Simon Sandstone, is dominated by glauconitic sandstones and siltstones with thin interbeds of dark black to greenish black glauconitic mudstones (~10% mudstone). Bioturbation and soft sediment deformation are pervasive. Multiple beds of reworked sandstone clasts indicate deep erosion and potentially mark sequence boundaries. This, together with abundant bioturbation and an overall low mud content suggests a shallow shelf setting, possibly above storm wave base.
The upper part of the succession is characterized by cm to dm thick interbeds of sandstone/siltstone and mudstone (~60% mudstone). Bioturbation is minor and primary sedimentary structures are well preserved. Sand/siltstones with sharp basal contacts and scour and fill structures indicate intermittent high energy events. Mudstones range in color from dark gray/black to maroon-red and are fossilifierous (low diversity fauna) with preserved trilobites and linguloid brachiopods. The red shales are unusual because such colors are typically associated with terrestrial deposits. Yet, their fossil content indicates a marine environment. Under the microscope red shales show particulate organic matter associated with framboidal pyrite, right next to detrital minerals coated with hematite. The low degree of bioturbation and the low diversity shelly fauna suggests a deeper (below storm wave base), dysaerobic setting with intermittent reworking by storm waves and bottom currents. In such a setting one would expect iron coatings on terrigenous minerals to be largely reduced to pyrite. The persistence of red color in these intervals may represent rapid influx of iron rich terrestrial sediments that overwhelmed the reduction capacity at their site of deposition. This in turn may also reflect lower surface productivity of Cambrian seas when compared with later time periods.
AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009