MICROBIAL ORIGIN OF LATE SILURIAN CARBONATE MUD-MOUNDS, NORTH-CENTRAL INDIANA

The Upper Silurian Wabash Formation contains carbonate mud-mounds that are exposed along the Wabash River throughout north-central Indiana. These mounds contain a thick-bedded, stromatactis-bearing core facies composed of skeletal wackestone to mudstone, that are surrounded by steep flanks beds (dips up to 45°). The core facies is highly dolomitized, and direct evidence of microbes is absent. However, some obscure textures (clotted mudstone) and the abundance of carbonate mud suggest a microbial origin. The flank facies is well-bedded, and is composed of skeletal wackestone and packstone. Crinoids are the most common skeletal elements, but brachiopods, bryozoans, mollusks, trilobites, ostracods, and sponge spicules are also common. The paucity of these organisms in core facies compared to the flank facies suggests that they grew on the mound, but were not directly responsible for its construction.

The Wabash mud mounds investigated represent the earliest phases of an important yet enigmatic type of Paleozoic carbonate factory. Based on the estimated synoptic relief, lack of photozoan organisms, and lack of evidence for wave reworking or transportation of sediment, the Wabash mounds are interpreted to have formed under relatively deep water conditions. They were initiated directly upon bioturbated dolomitic shale identical to contemporaneous intermound facies. At first, stromatactis cavities in the core facies were small (<1 cm), and they gradually increased in size through time until they formed a reticulated network that composes about 50% of the mound core. Growth of the mound was interrupted periodically, and individual growth phases are separated by thin beds of dolomitic shale similar to the intermound facies. Younger phases of nearby reefs contain fauna and facies that suggest the mounds grew into shallow water before mound growth was terminated.

Stromatactis is interpreted to have originated as a cavity network in the cores during the early phases of the mound development, possibly through the decay of sponges, but likely also as vugs constructed via microbial growth processes. The cavities were modified by several generations of internal erosion and sedimentation, and eventually filled by marine cements under shallow burial conditions.