Origin and modification of early dolomites in cyclic shallow platform carbonates, Yeongheung Formation (middle Ordovician), Korea

Abstract

The middle Ordovician Yeongheung Formation, Korea, consists of numerous metre-scale, shallowing-upward cycles, which were deposited on a shallow-marine carbonate platform. Depositional sequences developed on the inner platform were completely dolomitized regardless of constituting lithology, whereas those developed on the outer platform were partially dolomitized. The amount of replacement dolomite within a metre-scale, shallowing-upward peritidal cycle decreases downward from the peritidal cycle cap in the partially dolomitized depositional sequences. The cross-platform distribution of dolomites and their stratigraphic position in the sequences suggest that the Yeongheung dolomite formed syndepositionally. The common existence of pseudomorphs of evaporite minerals and nodules in the Yeongheung Formation indicates that dolomitization of the Yeongheung Formation occurred in a sabkha-type tidal-flat environment. It also occurred during each cycle of deposition influenced by high-frequency sea-level oscillations. The present-day Yeongheung replacement dolomites are characterized by near-stoichiometric to stoichiometric composition (49.9 to 52.1 CaCO 3 mole%), and low strontium concentration (52 to 133 ppm). The δ 18O value of the replacement dolomites ranges from −7.7 to −5.2‰ (PDB); this is much depleted relative to that of contemporary sabkha-type dolomites. These geochemical characteristics of Yeongheung dolomite indicate significant post-depositional modification. The common existence of oomolds, leached skeletal grains and dissolved evaporites in the Yeongheung Formation illustrates invasion of meteoric water into the sediments. Recrystallization of Yeongheung dolomites in the infiltrated meteoric water at slightly elevated temperatures during shallow burial resulted in lighter δ 18O values and lower strontium concentration.