Sequence stratigraphy of the cyclic successions in the Dumugol Formation (Lower Ordovician), mideast Korea

Abstract

This study focuses on the cyclic successions of the Dumugol Formation (Early Ordovician), Taebaeksan Basin, mideast Korea. The successions consist of meter-scale cycles, represented by alternation of shale- and limestone-dominated beds, and can be grouped into three cycle types: shale-lime-mudstone, shale-wacke/packstone, and shale-grainstone cycles. The meter-scale cycles most likely formed in shoreface and offshore transition zone where there was abrupt change between the depositional environments with siliciclastic-dominated sediment supply and carbonate-dominated sediment supply. The change most likely resulted from variations in clastic input and carbonate productivity during fourth- or fifth-order sea-level fluctuations accompanied with climatic changes. The cycles can be stacked into four third-order transgressive-regressive sequences, showing a systematic change in cycle thickness, cycle type, and intracycle facies proportion within a sequence. Each sequence is characterized by an upward-thickening to-thinning trend in cycle thickness, an upward-fining to— coarsening trend in cycle type, and an upward increase to decrease in proportion of shale-dominated facies. Sequence boundaries are picked out by a distinct surface (or zone) that shows a turnaround from gradual upward decrease to abrupt increase in cycle thickness and an abrupt shift in cycle type from shale-grainstone cycle to shale-wacke/packstone cycle. The sequence can be divided into transgressive and highstand systems tracts. The transgressive cycles are characterized by upward-thickening in thickness, upward-fining in cycle type, and higher and upward-increasing proportion of shale-dominated facies, whereas the regressive cycles are by upward-thinning, upward-coarsening, and lower and upward-decreasing proportion of shale-dominated facies. The systematic change in cycle thickness, cycle type, and facies proportion helps deduce the stacking patterns of the cycle units in the Dumugol Formation. The stacking pattern analysis suggests that the cycles are retrograded during rapid increase in accommodation and prograded during slow increase in accommodation. During relative fall of sea level, the platform sediments were affected by intense submarine erosion and redistribution, forming submarine hiatus.