STYLES OF DOLOMITIZATION OBSERVED IN THE GREAT BANK OF GUIZHOU, NANPANJIANG BASIN, SOUTH CHINA: EVIDENCE FOR MULTIPLE DOLOMITIZATION PROCESSES

The Great Bank of Guizhou is a 2.5 km thick isolated carbonate platform deposited during the Triassic period. The rocks preserve evidence for multiple episodes of dolomitization, spread across a range of geologic time. Different styles of dolomitization and geochemical evidence support this interpretation. Early dolomitization includes both peritidal cycle cap dolomites and large regions of massively-bedded dolomite in the platform interior, along with isolated dolomitized and partially dolomitized clasts in slope breccias derived from the platform interior. Forms of later stage dolomite include a widespread overprint and modification of massively bedded platform interior dolomites during burial; zones of pervasively dolomitized slope sediments (10s of m thick), some of which are discordant at various scales (0.1 m to 100s of m); partial dolomitization along fractures, bedding planes, and stylolites; alternating stratiform laminae of limestone and dolostone (mm to cm scale) in slope sediments; and matrix-selective dolomitization in some slope breccias.

Evidence for early dolomite includes isolated clasts of dolomite in Early Triassic slope breccias surrounded by lime mudstone, pervasive dolomite in platform interior sediments, Sr-isotopes and REE signatures consistent with Early Triassic seawater, and evidence for evaporites and solution collapse breccias in the platform interior. Textures and some geochemical indicators were modified during deep burial. Evidence for later stage dolomite (Late Triassic or later) includes zones of coarse massively dolomitized slope breccias surrounded by selectively dolomitized vertical and bedding plane fractures, stylolites, and alternating stratiform laminae of limestone and dolostone; fluid-inclusions containing brine (12-16 wt. %, NaCl equivalent) with homogenization temperatures of 100°C to 180°C, and some younger (post-burial) U-Pb age dates. Early evaporative-reflux dolomitization in the platform interior likely dominated the dolomite volumetrically before it was overprinted with burial signatures. Pervasively dolomitized slope breccias surrounded by selective dolomitized areas are interpreted to be the result of intrusion of late burial dolomitizing fluids into higher permeability units.