ORIGIN AND MODIFICATION OF EARLY DOLOMITES IN CYCLIC SHALLOW PLATFORMCARBONATES, YEONGHEUNG FORMATION (MIDDLE ORDOVICIAN), KOREA

The middle Ordovician Yeongheung Formation, Korea, consists of numerou s metre-scale, shallowing-upward cycles, which were deposited on a sha llow-marine carbonate platform. Depositional sequences developed on th e inner platform were completely dolomitized regardless of constitutin g lithology, whereas those developed on the outer platform were partia lly dolomitized, The amount of replacement dolomite within a metre-sca le, shallowing-upward peritidal cycle decreases downward from the peri tidal cycle cap in the partially dolomitized depositional sequences. T he cross-platform distribution of dolomites and their stratigraphic po sition in the sequences suggest that the Yeongheung dolomite formed sy ndepositionally. The common existence of pseudomorphs of evaporite min erals and nodules in the Yeongheung Formation indicates that dolomitiz ation of the Yeongheung Formation occurred in a sabkha-type tidal-flat environment. It also occurred during each cycle of deposition influen ced by high-frequency sea-level oscillations. The present-day Yeongheu ng replacement dolomites are characterized by near-stoichiometric to s toichiometric composition (49.9 to 52.1 CaCO3 mole%), and low strontiu m concentration (52 to 133 ppm). The delta(18)O value of the replaceme nt dolomites ranges from -7.7 to -5.2 parts per thousand (PDB); this i s much depleted relative to that of contemporary sabkha-type dolomites , These geochemical characteristics of Yeongheung dolomite indicate si gnificant post-depositional modification. The common existence of oomo lds, leached skeletal grains and dissolved evaporites in the Yeongheun g 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 resulte d in lighter delta(18)O values and lower strontium concentration. (C) 1998 Elsevier Science B,V. All rights reserved.