Ironstones and green marine clays in the Dongjeom Formation (Early Ordovician) of Korea

Abstract

Abstract In the Sino–Korean (North China) block the occurrence of ironstone is reported for the first time from the Lower Ordovician Dongjeom Formation, Korea. The Dongjeom Formation comprises much quartzose sediment deposited on the extensive carbonate platform, and is composed of three coarsening-upward sequences. Ironstone beds are distributed on the tops of the lower two sequences, suggesting that they mark (approximately) maximum flooding events. During the relative lowstand of sea-level when the Dongjeom Formation was deposited, extensive weathering in the source areas, coupled with higher-order sea-level rises, resulted in the formation of the Dongjeom ironstones. The Dongjeom ironstones include much green marine clays. Ironstones of the lowest sequence are characterized by glaucony facies and those of the middle sequence by chamosite facies. Chemical compositions of Dongjeom glauconies reveal that they were originally nascent to slightly evolved (3 to 5 K 2 O wt.%), and that they were affected by illitization after burial. Their immature compositions suggest that glauconies were formed in about 10 3 to 10 4 years or less on the seafloor. The substrates for glauconitization are interpreted as detrital mineral grains. Regressions after glauconitization introduced iron-oxides to the green grains, eventually forming the iron-oxide-rich band under the most oxidizing conditions. The distribution of glauconies in limited horizons suggests that the Dongjeom glauconies are autochthonous in origin. Chamosites have granular shapes and are mixed with quartzose sand grains, suggestive of reworking during storms. The Dongjeom chamosites were transformed from the precursor berthierines. The occurrence of glauconies in correlative horizons in Korea and North China may indicate that glauconitization in the host sediments, which were deposited in far-northern Gondwana, occurred under the influence of a coeval transgressive event.