AAPG ANNUAL CONFERENCE AND EXHIBITION
Making the Next Giant Leap in Geosciences
April 10-13, 2011, Houston, Texas, USA
An Updated Model of the Stratigraphic Architecture of the Great Bank of Guizhou, an Isolated Carbonate Platform in the Nanpanjiang Basin of South China
(1) Department of Geological and Environmental Sciences, Stanford University, Stanford, CA.
(2) College of Resource and Environment Engineering, Guizhou University, Guiyang, China.
(3) Geosciences Department, Trinity University, San Antonio, TX.
Well-exposed isolated carbonate platforms provide valuable records of ancient ocean conditions and can also serve as models for investigating subsurface platforms and hydrocarbon reservoirs. The Great Bank of Guizhou (GBG) is an exceptionally exposed Late Permian to Late Triassic isolated carbonate platform in the Nanpanjiang Basin of south China. Because it spans the end-Permian extinction and Triassic biotic recovery, the GBG has been an important site for investigating end-Paleozoic and early Mesozoic ocean conditions. Establishing a complete model of platform margin architecture for the GBG, however, has been challenging because the Anisian (early Middle Triassic) margin is occupied by an extensive reef facies that exhibits massive lithology with little observable bedding.
In this study, we developed new constraints on the stratigraphic architecture of the 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. Our findings further indicate that (1) the GBG attained a stratigraphic height of approximately 900 meters during the Early Triassic, which more than doubles previous estimates. (2) The platform transitioned from a low-relief bank to a high-relief, steep-margined platform in the Early Triassic. (3) The platform developed steep slopes of approximately the angle of repose prior to the development of a reef-stabilized margin in the Anisian. (4) The sedimentation rate of the Anisian reef out-paced basin subsidence, causing slope angles to decrease as the reef prograded and filled accommodation space.
Our results suggest that sedimentation on the GBG shifted between two different depositional systems. During the Early Triassic, the dominant source of sediment on the GBG was the platform top, consistent with the high stand shedding model. During the Anisian, however, the primary source of sediment was the slope reef facies, which is consistent with a slope shedding depositional model. Consequently, the GBG provides a valuable example of both high stand shedding and slope shedding depositional systems.