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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.
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Massive Dolomitization of Interior and Slope to Basin-Margin Facies of the Triassic Yangtze Platform Through Superposed Earth-Surface and Burial Mechanisms, Nanpanjiang Basin, South China
Triassic strata of the Yangtze Platform at Guanling contain a dolomitized interior, undolomitized margin, and partially dolomitized slope to basin margin. Dolomitized microbial laminate caps of peritidal cycles and massive dolomite with associated evaporite nodules and solution collapse breccias are consistent with penecontemporaneous tidal flat and evaporative dolomitization in the platform interior. The preferential dolomitization of the slope and basin margin (up to 7 km basinward of the margin), dolomitization along fractures, and selective dolomitization of the matrix in slope breccia that diminishes toward the margin are interpreted to have resulted from the incursion of basin-derived fluids during burial. Integrated analysis of fluid-inclusion microthermometry, oxygen, carbon, and strontium isotopes, trace element geochemistry, U-Pb age dates of carbonate phases, and burial history support the recrystallization of interior dolomite and slope to basin-margin dolomitization by brines at high temperatures during burial. The Yangtze Platform at Guanling provides an excellent example of widespread stratiform dolomitization resulting from the superposition of multiple mechanisms, including penecontemporaneous dolomitization by evaporative seawater brines, high-temperature dolomitization of the slope and basin margin by basinal brines, and high-temperature recrystallization of dolomite by brines during burial. This study provides an example that suggests that widespread stratiform dolomite may result from superposed Earth surface and high-temperature burial dolomitization processes and provides a valuable analog for other carbonate platforms in which the margin remains undolomitized while the interior and basin margin are dolomitized. Similar mechanisms likely contributed to the widespread dolomitization of platforms across the Nanpanjiang and Sichuan basins.
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- PAR ID:
- 10611843
- Publisher / Repository:
- Minerals
- Date Published:
- Journal Name:
- Minerals
- Volume:
- 15
- Issue:
- 3
- ISSN:
- 2075-163X
- Page Range / eLocation ID:
- 324
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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