Depositional systems of the Lower Ordovician Mungok Formation in Yeongwol, Korea: implications for the carbonate ramp facies development

摘要

Detailed study on the Lower Ordovician Mungok Formation in the mideastern part of the Korean peninsula has led to recognition of seven lithofacies which can be grouped into seven facies associations (FAs): FA1 (lagoon), FA2 (shoal), FA3 (shoreface), FA4 (inner to mid-ramp), FA5 (inner to outer ramp), FA6 (mid- to outer ramp), and FA7 (outer ramp). Spatio-temporal organization of the FAs represents homoclinal ramp environments with fringing ooidpeloid shoals. Correlation and lateral thickness variation of the FAs suggest that deep part of the basin might have been located toward the northern part of the study area. The relative sea-level curve inferred from the facies analysis suggests that the Mungok Formation evolved through three depositional stages in accordance with 3rd-order sealevel change. The organization of the facies succession indicates that the inner ramp facies associations (FA1 to 3) of the Mungok Formation are characterized by non-cyclic facies succession whereas the midand outer ramp facies associations (FA 4 to 7) have distinct cyclicity except non-cyclic FA 7. The difference in the facies successions between the inner ramp and the mid- to outer ramp area may result from the various depositional regime across a ramp. A non-cyclic facies successions of inner ramp facies (e.g., packstone to grainstone and lime mudstone) are suggestive of mosaic-type distribution that is controlled mostly by wave, storms, tides, currents, and local geomorphology. In contrast, cyclic facies successions (e.g., pebbly limestone conglomerate, limestone-shale alternation, and calcareous shale) represent a simple belt-type distribution of mid- to outer ramp facies that seems to reflect shifts of facies belt caused by sea-level changes. The partitioning of inner and mid- to outer ramp facies is appropriate for understanding the facies development of the Mungok Formation and similar ramp successions with distinct inner and mid- to outer ramp facies.