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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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Environments of dolomitization and evolution of carbonate diagenesis in the Triassic Zhenfeng margin of the Yangtze Platform in the Nanpanjiang Basin, South China
The Yangtze Platform borders the Nanpanjiang Basin on its north and west. During the Early Triassic the platform evolved from low-relief ramp with oolitic margins to a steepening platform with a relatively flat-topped geometry with margin shoals and evaporitic interior facies. At the Zhenfeng margin the precursor depositional facies include: oolitic grainstone to packstone, skeletal peloidal packstone, clotted microbialites and fenestral laminites. The Anshun strata range from undolomitized to partially dolomitized oolite and microbial facies to partially and completely dolomitized facies such as fenestral laminites. Dolomitization changes upward through the section with fenestral laminate facies being more pervasively dolomitized than the oolitic and skeletal packstone facies. The diagenetic evolution (paragenetic sequence includes: neomorphic alteration of aragonite, marine cementation, replacement dolomite, euherdral dolomites, saddle dolomites, calcite veins, stylolites, and late-stage fractures with calcite and oxide fill. Previous data from the Yangtze Platform include dolomite showing δ¹⁸O values ranging from -7.7‰ to 0.75‰ (VPDB) and δ ¹³C values ranging from 0.77‰ to 4.0‰ (VPDB). Vein calcite values range from δ ¹⁸O -18.4‰ to -5.2‰ and δ ¹³C -6.1 to 3.4‰. ⁸⁷Sr/⁸⁶Sr values from dolomite ranges from 0.707677 to 0.708601 with the exception of elevated ⁸⁷Sr/⁸⁶Sr in three samples. Homogenization temperatures (Th) and freezing point depressions (Tmice) from primary fluid inclusion assemblages from dolomite crystals indicate entrapment of saline brines (9.5 to 16 wt. % NaCl) over temperatures of 80-185°C. The preferential dolomitization of mud-rich platform interior facies and preservation of oolitic limestone facies at the platform margin points to interior derived dolomitizing fluids consistent with evaporative reflux. The range in δ ¹⁸O is consistent with enrichment by evaporative concentration of seawater, but also includes negative values consistent with high temperature fluids. ⁸⁷Sr/⁸⁶Sr values in dolomite are consistent with modified seawater including a radiogenic contribution of hydrothermal fluids. The geothermometric data, oxygen isotope values and radiogenic Sr contribution indicates that early dolomites were recrystallized at high temperatures by burial fluids.
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- Award ID(s):
- 2041315
- PAR ID:
- 10510593
- Publisher / Repository:
- Geological Society of America
- Date Published:
- Journal Name:
- Geological Society of America Abstracts with Program
- Volume:
- 56
- Issue:
- 3
- Subject(s) / Keyword(s):
- doi: 10.1130/abs/2024NC-397122
- Format(s):
- Medium: X
- Location:
- Springfield, MO
- Sponsoring Org:
- National Science Foundation
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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.more » « less
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