Quantitative Analysis and Paleoecology of Earliest Mississippian Microbial Reefs, Gudman Formation, Queensland, Australia: Not Just Post-Disaster Phenomena

摘要

Small (> 30 m diameter, ~ 9 m thick) reefs in the Gudman Formation of Queensland, Australia are the oldest known Mississippian reefs, occurring very near the Devonian-Carboniferous boundary following Late Devonian extinction events. They occur in high-energy, shallow, oolitic grainstones and consist of > 70% microbialite and bound detritus on the basis of point-count analysis. Skeletal organisms, including potential frame-building rugose and tabulate corals, ramose bryozoans, crinoids, and algae, occur in growth position but account for only 4.4% of sampled framework. The microbial framework was syndepositionally rigid on the basis of: (1) vertical and overhanging relief in a high-energy setting, (2) export of framework intraclasts, (3) hard-substrate-encrusting organisms, (4) abundant hard-rock borings, and (5) neptunian dikes. Unusually for Mississippian reefs, stromatolites make up a large part of the framework (~ 32%), but more typical thrombolites are equally abundant. Interfingering of skeletal organisms and microbialites suggests that they grew in welloxygenated, normal marine waters and that microbial biofilms competed effectively with skeletal organisms for available substrate. Abundant and diverse co-occurring skeletal flora and fauna are inconsistent with interpretations of Gudman stromatolites as post-extinction "disaster taxa." Hence, Gudman reefs are stromatolite-rich examples of a larger class of microbialite-dominated Phanerozoic reefs that occurred with abundant skeletal metazoans in normal marine settings.