200 Paleontology VII - Consequences of Extinction and Radiation

Tuesday, 2 November 2010
1:30 PM - 5:30 PM
Room 605

THE ENVIRONMENTAL CONTEXT OF REEF RECOVERY FOLLOWING END-PERMIAN EXTINCTION ON AN ISOLATED CARBONATE PLATFORM IN THE NANPANJIANG BASIN, SOUTH CHINA

Paper number: 200-11
Program: Technical Sessions
Time: 4:00 PM – 4:15 PM

Authors

Abstract

Following end-Permian extinction, metazoan and algal reefs were absent from the oceans for several million years. The return of reefs and the first appearance of scleractinian corals during the Middle Triassic marked important milestones in both the recovery of marine life from mass extinction and the development of modern reef ecosystems. However, the pattern and timing of Triassic reef recovery remain poorly constrained, in part due to the shortage of well-preserved and well-documented early Anisian (earliest Middle Triassic) reefs. The oldest and best-preserved outer-platform reef of this age is exposed in cross-section on the north margin of the Great Bank of Guizhou (GBG), a Late Permian to Late Triassic isolated carbonate platform in the Nanpanjiang Basin of southern China. Establishing a paleoenvironmental and stratigraphic context for reef initiation and growth on the GBG, however, has been challenging because the reef exhibits massive lithology with little observable bedding.

In this study, we developed new constraints on the stratigraphic architecture of the Anisian reef and the overall platform-to-basin transition on the GBG from field-based measurements of bedding attitudes, tracings of bedding on a satellite image, petrographic analyses of hand samples and thin sections, and correlations of δ13Ccarb excursions across measured stratigraphic sections. A long-standing model of the architecture of the GBG suggests that the reef was part of a shallow-water, aggrading and prograding margin facies. In contrast, our results indicate that the reef was part of a prograding slope facies. The architecture of the GBG margin and slope implies that framework-building organisms, primarily Tubiphytes and calcareous sponges, grew on slope clinoforms over a significant range of water depth, approaching or possibly exceeding the lower limit of the photic zone. These findings suggest that the Anisian reef on the GBG was not restricted to the shallow-water platform-margin conditions that are generally considered most favorable for the growth of later Mesozoic and modern reefs. Instead, the oldest-known outer-platform reef of the Mesozoic Era grew over a wider range of physico-chemical conditions on a steep platform slope.