SEDIMENTOLOGICAL, DIAGENETIC, AND STABLE ISOTOPE INVESTIGATION OF THE ORIGIN OF STROMATACTIS-BEARING CARBONATE MUD MOUNDS (SILURIAN) FROM NORTH-CENTRAL INDIANA

The origin of Silurian carbonate mud mounds from the Wabash Formation of north-central Indiana, and the associated feature stromatactis, remain enigmatic despite more than 100 years of investigation. In particular, it is still uncertain as to whether stromatactis represents a framework structure of biotic origin, or calcite cement in an abiotically-formed cavity system. These carbonate mud mounds are distributed throughout the northern Midwestern United States, and developed in a broad, shallow epeiric sea surrounding the deeper waters of the central Michigan Basin.

The classic mud mound exposed at Wabash, Indiana is one of many roughly circular mounds that preserves well-bedded distal flank beds (dipping about 40°) composed of skeletal packstone. These grade into thick-bedded proximal flank beds (also skeletal packstone), and finally into a structureless reef core composed primarily of carbonate mudstone. Stromatactis occurs in all facies, but is most common in the reef core. Four statistically different morphologies of stromatactis have been recognized, each with a unique distribution across the mound. The mound core facies contains large, well-developed stromatactis cavities with relatively high sinuosity cavity tops. It is the only facies in which multiple cavities are interconnected. Stromatactis size and cavity sinuosity progressively decrease toward the distal flanks, and stromatactis cavities have not been recognized in the carbonate mudstone intermound facies. These relationships suggest a systemic variation in either biological (for example, the ecological distribution of organisms) or diagenetic (for example, facies-controlled dissolution) processes in order to account for the distribution of the stromatactis.

Stable oxygen and carbon isotopes obtained from the dolomitized host material, partially-dolomitized stromatactis cavity cement, and later pore-filling calcite cement are all consistent with dolomitization of primary limestone and/or high temperature precipitation of pore-filling cement under deep burial conditions. They do not provide any conclusive evidence of biotically-mediated calcite precipitation to account for the stromatactis, and thus, the origin of the carbonate mud mound.