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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
Tropical reefs are of great importance to geologists and biologists alike. Not only are reefs areas of high biodiversity in the modern oceans, they are also important sources of new species over evolutionary time. Moreover, they form distinctive sedimentary deposits in the geological record that commonly contain high primary porosity and, therefore, can serve as important reservoirs for petroleum. The geological importance of reefs as reservoirs depends in large part on the specific groups of organisms that served as the primary framework-builders and on the associated environmental conditions that governed the degree of cementation and lithification in the reefs prior to burial. Mass extinction at the end of the Permian Period (252.6 Ma) eliminated nearly all of the important Permian reef-builders and reefs are absent from the margins of Early Triassic (252.6-247 Ma) carbonate platforms. The re-establishment of platform-margin reef complexes during the Middle Triassic (247-237 Ma) has been viewed as an important sign of biotic recovery. Although Middle Triassic reefs contain potentially reef-building organisms such as scleractinian corals, calcareous sponges, and calcareous algae, quantitative analyses of most Middle Triassic reefs indicate a limited role for macroscopic skeletal animals and algae in constructing the reefs. Instead, framework organisms of unknown biological affinities, particularly Tubiphytes, and large volumes of marine cements contribute most to the structure and volume of the reef. Thus, although the onset of platform margin reef development has been seen as an indicator of biotic recovery in the aftermath of the end-Permian extinction, perhaps enabled by environmental amelioration reflected in carbon cycle stabilization, the degree to which the development of Middle Triassic reefs in fact reflects the recovery of animals from the end-Permian extinction is not clear. Decoupling of platform-margin reef formation from the participation of framework-building metazoans suggests the existence of important controls on Triassic platform-margin reef development beyond the evolution of animals. Additional controls may include, for example, platform geometry or carbonate chemistry of seawater (including inhibitors of carbonate precipitation). The lag between reef development and increase in the abundance of framework-building metazoans suggests that rather than reflecting recovery and diversification, Middle Triassic reef development may have served as a driver of diversification by increasing differentiation of microenvironments within shallow platform environments, much like reefs today. However, the best-studied Middle Triassic reefs do not date to the earliest part of the Middle Triassic and pervasive dolomitization has further limited detailed analyses of these reefs in situ. Large carbonate platforms in the Nanpanjiang Basin of southern China contain extensive reefs that date to the earliest Middle Triassic and are preserved as limestone in situ. Consequently, they provide an unparalleled opportunity to examine temporal and spatial differences in reef diversity and reef geometry and to better identify relevant controls on these aspects of reef development.
During the summer of 2007, we mapped and sampled the Middle Triassic reefs of the Nanpanjiang Basin at three localities. We sampled the reefs on the northern and southern margins of the isolated Great Bank of Guizhou between the towns of Bianyang and Loudian and the reef on the margin of the attached Yangtze Platform near Guanling. From field observations we were able to determine that the diversity and abundance of framework-building corals, sponges, and algae increased from the oldest to the youngest parts of the reefs at all localities. The framework of the oldest parts of the reefs at all localities was dominated by Tubiphytes and early marine cements and lacked corals, demonstrating that the recovery of reefs occurred prior to the local advent of reef-building corals. The Yangtze Platform reef generally exhibited greater biodiversity and more limited syndepositional cementation than the GBG. Field observations and mapping will be supplemented through quantitative analysis of polished slabs and thin sections from samples collected during fieldwork. The relevant samples are currently in transit to the United States, and so more robust conclusions will await the detailed analysis of these samples.
Financial support from the Petroleum Research Fund has had a substantial positive impact on the research in my lab. It allowed me to initiate this major, field-based research project in southern China. Moreover, it has provided summer tuition and stipend support for a PhD student who participated in the summer fieldwork and plans to develop a dissertation project from this work. Although there is still much work to do within the context of the current grant, the large sample suite collected this summer 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.
