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The Great Bank of Guizhou (GBG) is an isolated carbonate platform in the Nanpanjiang Basin. Contacts between limestone and dolomitized material cross-cut bedding and form an irregular lobe shaped body that extends from the interior and terminates toward the basin. Precursor carbonate facies include oolite grainstone and packstone (with some containing a microbial matrix), clotted microbialite boundstone, and massively dolomitized carbonate mudstone containing stromatolitic and fenestral fabrics indicating a range of high-energy subtidal shoal to intertidal tidal flat environments. The mudstones are present lower in the outcrop, followed by grainstones and packstones moving upwards. Samples range from partially to fully dolomitized, with partially dolomitized samples occurring near the dolomitization front. In partially dolomitized oolite, the ooids are selectively replaced by dolomite. The paragenetic sequence interpreted from petrography includes early marine intergranular cement, early fracturing associated with possible tepee formation, neomorphism of aragonitic mollusk shells and cortical laminae in ooids, anhedral replacement dolomite, dissolution forming dissolved ooids and vugs, late stage coarse euhedral dolomite cement within intergranular pores, vugs, and early fractures, twinned calcite fill of early fractures and vugs, late stage fractures filled with twinned calcite, and finally stylolites. Previous data from the western GBG using fluid inclusion and clumped isotope geothermometry indicates that dolomitization occurred with high temperatures at burial, within a spatially variable range of 90°C to 185°C. 87Sr/86Sr values of 0.707677 to 0.708601 are similar to the ratios found in Triassic seawater. δ18O (VPDB) values for dolomite range from -7.68‰ to -1.53‰, consistent with evaporative enrichment of seawater and high burial temperatures. The spatial distribution of the dolomite, strontium isotopes and oxygen isotopes are consistent with reflux dolomitization. In other areas, the platform interior contains evidence for hypersaline conditions and evaporite mineral precipitation also consistent with the reflux model. The geothermometry data indicates that early reflux dolomite was replaced at high temperatures during burial.
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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.
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- Award ID(s):
- 2041315
- PAR ID:
- 10510590
- Publisher / Repository:
- Geological Society of America
- Date Published:
- Journal Name:
- Geological Society of America Abstracts with Programs
- Volume:
- 56
- Issue:
- 3
- Subject(s) / Keyword(s):
- doi: 10.1130/abs/2024NC-398794
- Format(s):
- Medium: X
- Location:
- Springfield, MO
- Sponsoring Org:
- National Science Foundation
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