LIMESTONE-SHALE COUPLET REVISITED: UPPER CAMBRIAN (FURONGIAN) HWAJEOL FORMATION, TAEBAEKSAN BASIN, KOREA

So-called “ribbon rock” or limestone-shale couplet, which are represented by interlayered thin-bedded limestone and shale, commonly occurs throughout deep water carbonate succession of the early Paleozoic worldwide. However, their formation process remained under question for a long time. In this study, we discuss origin of the limestone-shale couplet from the upper Cambrian Hwajeol Formation, Korea, based on detailed microfacies analysis. Seven sedimentary facies are identified from ca. 2 m thick interval of the Hwajeol Formation where continuous samples were taken, including normally graded calcarenite to shale, parallel- to cross-laminated shale, homogeneous micritic limestone, micritic limestone with sponge spicule network, skeletal grainstone, micritic matrix-supported packstone to conglomerate, and flat-pebble conglomerate with grainstone matrix. Among these, shale, grainstone, and conglomerate facies most likely formed by storm-induced bottom current, whereas micritic limestone facies deposited in situ by suspension settling of micrite or growth of sponges on the seafloor. Formation of limestone-shale couplet would have been promoted by the characteristic environmental conditions of the early Paleozoic, e.g., calcite precipitation-promoting sea-water chemistry (calcite sea) and rarity of burrowers in the deeper part of the ocean. Similarities in between the Hwajeol limestone-shale couplet and those in the coeval successions worldwide suggest that the model proposed here can be applied to the other early Paleozoic limestone-shale couplets, thus providing a clue to understand sedimentary processes in the deeper part of carbonate platform during the early Paleozoic.