2008 Joint Meeting of The Geological Society of America, Soil Science Society of America, American Society of Agronomy, Crop Science Society of America, Gulf Coast Association of Geological Societies with the Gulf Coast Section of SEPM

Paper No. 3
Presentation Time: 8:00 AM-4:45 PM

High Resolution Lithostratigraphy and Facies Analysis of the Ediacaran (635-551 Ma) Doushantuo Formation, South China

MCFADDEN, K.A., Department of Geosciences, Virginia Polytechnic Institute and State University, Blacksburg, VA 24061, XIAO, Shuhai, Department of Geosciences, Virginia Polytechnic Institute and State University, 4044 Derring Hall, Blacksburg, VA 24061, JIANG, Ganqing, Department of Geoscience, University of Nevada Las Vegas, 4505 South Maryland Parkway, Box 454010, Las Vegas, NV 89154-4010 and ZHOU, Chuanming, Nanjing Institute of Geology and Paleontology, Chinese Academy of Sciences, Nanjing, 210008, China, pitkat@vt.edu

The Doushantuo Formation (635-551 Ma) in South China, characterized by organic-rich shale and carbonates, contains some of the earliest animal fossils and unusual geochemical anomalies. However, significant thickness and facies variability in the Doushantuo basin, lack of reliable marker beds, sequence boundaries, or biostratigraphic resolution has led to debate on the overall depositional setting and its potential controls on fossil preservation and geochemical signatures. Existing interpretations include a restricted intrashelf basin, an open-marine shelf, and an isolated alkaline lake. Here, we present a high-resolution lithostratigraphic profile using five sections across the Huangling Anticline in the Yangtze Gorges area, South China. Eight broad lithostratigraphic facies are identified, and their correlation suggests a restricted basin with perhaps episodic open marine influences. On the southern limb of the Huangling anticline, the Doushantuo is subdivided into four members: (1) a basal cap carbonate; (2) cherty shale and dolomicrosparite deposited below wave-base; (3) dolomitic mudstone-packstone passing into ribbon rock deposited in shallow subtidal environments; and (4) black shale lithologically comparable to the lower Doushantuo shale. In contrast, the northern limb consists of four dominantly shallow subtidal facies including (1) a basal cap carbonate, (2) quartose wacke and shale, (3) phosphorite, and (4) phosphatic packstone intercalated with cherty dolomite. All of the successions are overlain by peritidal dolomite of the Dengying Formation (551-542 Ma). The section farthest north (Zhangcunping) is considered as most proximal, based on a phosphorite deposit directly underlying an erosional unconformity. Episodic increase of quartz clasts updip suggests progradation of terrigenous sands during sea-level fall, while pure carbonates (dolomite and limestone) represent shut down of clastic supply associated with sea-level rise and/or open marine conditions. Condensed shales throughout the southern limb likely represent greater restriction in the deeper basin, which is supported by high organic matter content and enriched sulfur and carbon isotopes.