RELATIONSHIP BETWEEN STROMATACTIS MORPHOLOGY AND FACIES IN THE SILURIAN WABASH REEF, INDIANA

Stromatactis (large, irregular cavities filled with calcite cement) is a conspicuous feature of a Silurian reef found in the Wabash Formation, exposed along a railroad cut near the historic Big Four Railroad Station, in Wabash, Indiana. This study examines and analyzes several morphological parameters of stromatactis cavities in order to investigate the relationship between reef facies and the shape and distribution of stromatactis.

The Wabash Reef is one of many roughly circular reefs that occur in the Wabash Formation. A railroad cut passes right through the reef center providing a complete NE-SW cross-sectional transect. The outcrop records a facies transition from well-bedded distal flank beds (dipping about 40°) composed of skeletal packstone, to thick bedded proximal flanks (also skeletal packstone), to a structureless reef core composed primarily of carbonate mud, back through fossiliferous proximal and distal flanks (dipping about 40°). Stromatactis occurs in all facies, but is most common in the reef core.

Stromatactis from core facies, proximal flank facies, and distal flank facies has been analyzed for size (height, length, and height/length ratio), shape (sinuosity of cavity tops and bases), and cavity interconnectedness. The core facies is characterized by large, well-developed stromatactis cavities with relatively high sinuosity on the tops of cavities. It is the only facies in which multiple cavities are interconnected. There is a progressive decrease in stromatactis size and cavity sinuosity toward the distal flanks. No stromatactis cavities are recognized in the carbonate mudstone facies that surround the reef.

The Wabash reef is considered to be a carbonate mud-mound based on the lack of reef-constructing organisms in the core of the mound. The development of stromatactis appears to be related to facies within the mound. The observed morphological trends may assist in interpreting the origin of stromatactis and its implications for Silurian reef development.