Dolomitization of the periplatform carbonate slope deposit, the Machari Formation (Middle to Late Cambrian), Korea
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An ancient periplatform carbonate slope deposit, the Machari Formation (late Middle to early Late Cambrian), Korea shows five dolomite types of which dolomitization conditions, dolomitizing fluids and recrystallization are closely associated with depositional setting and burial history.
Mosaic dolomite I, uniformly red luminescent hypidiotopic to xenotopic medium- crystalline dolomite, forms massive dolomite rock about 40 m thick. The dolomite shows radiogenic87Sr/86Sr ratios (0.71160), low δ18O(−7.0‰ PDB) and δ13C (0.6‰, PDB) values, low Sr (47 ppm), high Fe (0.22 mol %) and Mn (0.13 mol %) contents. Petrography and geochemistry suggest that the dolomite has been initially formed from seawater in the shallow burial environment and then it was recrystallized by the basin-derived fluids from the underlying Sambangsan Formation in the deep burial setting during the Late Ordovician. Mosaic dolomite II, uniformly red luminescent hypidiotopic to xenotopic medium-crystalline dolomite forms discrete beds in limestone a few tens of cm to a few m in thickness with very radiogenic87Sr/86Sr ratios (0.71547), low δ18O(−8.3‰, PDB) and δ13C (0.8‰, PDB) values, 64 ppm Sr, 0.6 mol % Fe and 0.29 mol % Mn contents is interpreted to have formed by the fluids from crustal rocks in the deep burial setting during the Late Silurian. Patchy ferroan dolomite crystals of a few hundreds of (m to several cm consisting of fine- to medium-crystalline nonluminescent dolomite show an average87Sr/86Sr ratios of 0.71013, −6.3‰δ18O and 1.9‰δ13C values, and 110 ppm Sr, 4.38 mol % Fe and 0.51 mol % Mn contents; it is interpreted to have formed by the fluids from the siliciclastics of the Pyongan Supergroup in the deep burial setting during the Permian. The saddle dolomite, coarse crystalline xenotopic rhombs of severe undulose extinction is seemed to have formed during this orogeny. The disseminated dolomite which shows diverse texture and CL pattern is interpreted to have formed at various times based on cross-cutting relationship with other diagenetic features.
The Machari Formation reveals following aspects of ancient periplatform dolomitization: 1) Seawater was the main source of Mg for the initial dolomitization of ancient periplatform carbonate slope limestone. 2) The basin-derived fluids recrystallized the early dolomite in the deep burial setting. 3) After the initial formation of early dolomite, origin and modification of dolomites were affected by tectonic and burial histories. 4) Dolomitizing fluids were diverse; however, the fluids other than seawater dolomitized the limestone only at limited scale.
KeywordsDolomite Dolomitization Stylolite Deep Burial Burial History
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