EXTRA MUDDY GREENHOUSE CARBONATE PLATFORM DOMINATED BY SUBTIDAL METER-SCALE CYCLE

Meter scale cycles in geologic record have been interpreted to be related to the combined effects of eustatic oscillations, tectonism, and autocyclicity. Subtidal cycles are distinct from peritidal cycles, in that they do not shoal upward to sea-level, and hence difficult to establish in micritic limestone without presence of easily recognizable lithofacies such as shale, grainy facies.

We report an unusually muddy Early to early Middle Ordovician carbonate platform from the Taebaeksan Basin, Korea, in the eastern margin of the Sino-Korean Block, located in equatorial region near Siberia and Tarim in the Early Ordovician. We investigated a 220 m-thick and a supplementary 30 m-thick sections employing microfacies analysis with vertical sampling interval of less than 20 cm to differentiate subtidal cycles within micritic limestone succession. The studied interval is extremely micritic, with cumulative thickness of lime mudstone and wackestone comprising over 87% of the section, while packstone to grainstone, laminite and marl account for 4%, 5% and 4%, respectively.

Four types of meter-scale cycle are recognized: outer platform cycle composed of shale to lime mudstone, middle platform cycle of homogeneous lime mudstone to bioturbated wackestone, inner platform cycle of bioturbated lime mudstone to bioturbated wackestone, and peritidal cycle of lime mudstone to laminite. Peculiarly, the middle and inner platform cycles are only composed of micritic facies, which comprise 53 (66%) of 80 meter-scale cycles documented. These subtidal cycles show systematic change in thickness and facies proportions, which define prograding and retrograding cycle sets. Limited correlation between two sections 10 km apart comprised of middle platform to peritidal cycles demonstrate that type and composition of cycles are mostly well correlated, indicating overall dominance of allocyclic control with local record of autocycles.

This study highlights that low-amplitude, high-frequency sea-level fluctuations characteristic of the greenhouse period can be successfully deciphered from Early Paleozoic micritic dominated carbonate succession by careful analysis of subtidal meter-scale cycles aided by microfacies analysis, which will enable us to understand the nature and origin of these subtidal meter-scale cycles.