Sedimentary geology of the Great Bank of Guizhou: Birth, evolution and death of a Triassic isolated carbonate platform, Guizhou Province, south China.

摘要

A complete cross section of the Great Bank of Guizhou (GBG) exposed in a syncline provides a unique opportunity to study the overall history of an isolated carbonate platform.; The stratigraphic framework indicates that the GBG formed near the margin of the Yangtze platform during a transgression that expanded the Nanpanjiang basin and drowned the region surrounding the GBG in the latest Permian. Initially the GBG was a low-relief bank with mobile oolite sands at the margin, shallow-subtidal to peritidal deposits on the interior, and gentle slopes dominated by pelagics, turbidites, and debris-flow deposits at the basin margin. In the early Middle Triassic the GBG developed a progressively steepening profile rimmed with massive Tubiphytes reefs. The platform was flat topped with tidal-flat deposits across the interior. Basin-margin deposition was dominated by turbidites and debris-flow deposits as the slopes steepened and eventually shifted to avalanche and rock-fall deposits as slopes reached the angle of repose. In the late Middle Triassic an erosional escarpment with up to 1700 m of relief developed at the margin. Platform-margin strata are bedded packstones similar to interior strata, whereas breccias at the basin margin contain coral-boundstone clasts suggesting erosion of reefs from the escarpment. A restricted subtidal lagoon formed in the interior producing an atoll-like morphology. Later a flat-topped profile was restored as tidal flats spread across the interior. In the beginning of the Late Triassic deepening and siliciclastic influx contributed to termination of the GBG before shales were deposited over the platform. Siliciclastic turbidites filled the basin and onlap the escarpment.; Sequence-stratigraphic correlation with the Yangtze platform indicates that development of the GBG was strongly influenced by regional fluctuations in relative sea level.; The GBG contains muddy carbonates and a progressively steepening profile. In contrast, Middle Triassic platforms of the Dolomites, Italy contain little mud and have angle-of-repose geometries. The GBG's larger size increased mud production and protected its interior from winnowing of mud. Muddy deposits were stable on relatively gentle, basin-margin slopes. The lack of mud on the Dolomites platforms resulted in avalanche and talus deposition at the basin margin, which in turn produced their angle-of-repose geometries.