Quantitative analysis and paleoecology of earliest Mississippian microbial reefs, Gudman Formation, Queensland, Australia: Not just post-disaster phenomena

摘要

Small (> 30 m diameter, similar to 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 (similar to 32%), but more typical thrombolites are equally abundant. Interfingering of skeletal organisms and microbialites suggests that they grew in well-oxygenated, 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.