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45329-G8
Controls on Reef Recovery in the Aftermath of End-Permian Extinction: Comparison of Reefs on Isolated and Attached Carbonate Platforms in South China

Jonathan L. Payne, Stanford University

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Tropical reefs leave distinctive sedimentary deposits in the geological record that commonly contain high primary porosity and, therefore, can serve as important reservoirs for petroleum.  Consequently, controls on reef development are of interest not only to biologists and paleontologists but also to carbonate sedimentologists and industry geologists.  The re-establishment of reefs following the end-Permian mass extinction (252 Mya) provides an ideal opportunity to study the pattern and process of reef formation from a sedimentary and biological perspective. 

When reefs reappeared in the Middle Triassic, they contained potentially reef-building organisms such as scleractinian corals, calcareous sponges, and calcareous algae. However, quantitative analyses suggest these organisms were not volumetrically important within the reef framework.  Instead, organisms of unknown biological affinities, particularly Tubiphytes, and large volumes of marine cements contribute most to the structure and volume of Middle Triassic reefs.  Thus, the recovery reef-building metazoans and algae may be more a reflection of the return of reefs than a cause. 

During August and September of 2007, we conducted detailed sample of the reef margins on two large carbonate platforms in the Nanpanjiang Basin of southern China where earliest Middle Triassic reefs are preserved as limestone in situ.  We have since determined the stratigraphic and environmental distributions of reef-building organisms via petrographic analysis of polished slabs and thin sections from three study localities: Bianyang and Xiliang on the Great Bank of Guizhou and Guanling on the Yangtze Platform.  We found that the diversity of co-occurring reef builders increased from the oldest to the youngest parts of the reefs at all localities, and that local diversity was greatest closest to the platform margin.  We also found that the order of appearance and occurrence frequency of reef-builders was similar at all study localities.  Tubiphytes is the only framework-building organism in the oldest samples and occurs in all samples.  Calcareous algae, calcareous sponges, and taxonomically problematic organisms such as Plexoramea and Bacinella reappeared somewhat later and became increasingly common through time.  However, they remained much less common than Tubiphytes and were relatively minor contributors to the reef framework through the Middle Triassic.  Scleractinian corals are rare and occur exclusively in younger parts of the reefs, likely post-dating reef initiation by as much as 5 My.  At all study sites, the physical structure of the reef as a wave-resistant, cemented feature on the margin of the platform was well established prior to the first local occurrences of calcareous algae, calcareous sponges, and scleractinian corals.  Thus, it appears from our study that Tubiphytes was not only responsible for the initiation and development of platform-margin reefs in the early Middle Triassic, but it may have also been the only organism capable of sustaining reef growth.

Based upon our observations of the stratigraphic and environmental distribution of fossils in the reefs in south China, it is clear that understanding the controls on the occurrence of Tubiphytes is central to deciphering the controls on Triassic reef recovery more generally.  To that end, we have selected a number of the best-preserved samples containing Tubiphytes for additional thin section analysis in an effort to better understand the biological affinities of this fossil and the diagenetic controls on its early lithification by marine cements.   Our initial observations suggest that Tubiphytes reflects the microbially-induced precipitation of micrite around otherwise soft-bodied, siphonous algae.  Difference in the morphology of Tubiphytes specimens suggests that there may be several morphologically distinct groups of algae that have been identified as Tubiphytes due to this common mode of preservation.

Financial support from the Petroleum Research Fund has had a substantial positive impact on my research.  It has allowed a PhD student (Brian Kelley) who participated in fieldwork during 2007 to continue his research by making large format thin sections of our samples.  Funding from PRF also supported a second research trip to China during August and September, 2008, with two PhD students (Ellen Schaal and Katja Meyer).  Samples from the second research trip are currently in transit to the United States and are expected to arrive in November.  These will be used to study the environmental change associated with reef initiation through the analysis of molecular biomarkers and stable carbon isotopes.  The large sample suite collected during these two research trips and the field localities identified present the opportunity to continue research on these samples and at these localities far beyond the scope of the currently funded research project.

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