STRATIGRAPHIC ARCHITECTURE AND GROWTH HISTORY OF A SILURIAN CARBONATE MOUND AT WABASH, INDIANA

The origin of Silurian carbonate mounds of the Wabash Formation (north-central Indiana) remains unclear despite over 100 years of investigation. Previous studies have focused on the stratigraphic position, biodiversity, and paleoenvironmental interpretation of the mounds, and the origin of stromatactoid cavities that comprise the mounds. This study investigates the internal stratigraphic architecture that resulted from interplay between depositional and erosional processes in the classic mound exposed near Wabash, Indiana.

The Wabash mound is one of many roughly circular Silurian mounds in the upper Midwestern USA. It preserves a massive reef core composed primarily of carbonate mudstone with abundant stromatactis, surrounded by thick thick-bedded proximal flanks composed of skeletal packstone and thinner-bedded distal flanks composed of skeletal wackestone. Parts of at least five separate growth phases are exposed in a railroad cut passing directly through the mound. Subsequent growth phases are separated by periods of non-deposition in which the mound was covered by a thin layer of dolomitic mudstone that characterizes the background sedimentation of the intermound setting.

Mounds both aggrade and prograde laterally during each growth phase. Down-flank fining and the presence of slump deposits and intra-phase erosional surfaces attest to the lateral growth of the flank beds through shedding of material that is produced on the mound crest. Each growth phase is rather symmetric in cross-section suggesting that there is no preferred off-mound transport direction and that the mounds may have formed in relatively deep water. The nature of stromatactoid cavities varies within each growth phase, with well-developed and highly interconnected cavities occurring in the mound cores, and more poorly developed, isolated cavities occurring with distance from the core. In addition, the stromatactis appears to be larger and better developed toward the top of each growth phase. This distribution of stromatactis, along with the lack of frame-building organisms, supports an interpretation that stromatactoid cavities may have a microbial origin, and that the microbial organisms are responsible for the vertical growth of the mounds through stabilization of the core and possibly flank sediments.