Review On Paleoenvironments Of The Kunsan Basin (Northern South Yellow Sea Basin) Based On Palynofloral Assemblage

The paleoenvironments with geological ages of the northern South Yellow Sea Basin (Kunsan Basin) are reviewed on the basis of the tectonic interpretation and the previous palynological studies of the five wells (Kachi-1, Haema-1, IIH-1Xa, IIC-1X and Inga-1). Early Cretaceous strata identified only in lowermost interval of Kachi- and IIC-1X wells yield palynomorphs, freshwater algae, ostracods, and charophyta, although they are rare in occurrence. The palynomorph assemblages indicate a fluvial environment surrounding a mountain slope with gymnosperm forest under dry and warm temperate conditions. Late Cretaceous strata are found only in Kachi-1 and IIH-1Xa wells. During the Late Cretaceous, the sedimentary basin became a large lake due to post pull-apart rifting. Abundant and diverse flora of gymnosperm and angiosperm pollen, aquatic mega- and micro-spores, and freshwater algae reflect that the lacustrine environment was under wet and subtropic conditions. Paleocene strata are detected in Haema-1, IIH-1Xa, and IIC-1X wells. Sedimentary succession during this period was formed in a wedge-shaped syn-rift basin after half-graben normal rifting. Moreover, the strata yield abundant Paleocene pollen grains indicating wet, warm temperate conditions and fluvial environments. Eocene strata are identified only in Haema-1, IIH-1Xa, IIC-1X, and Inga-1 wells. Early to Middle Eocene successions with syn-rift sedimentary features are the thickest in the Kunsan Basin (northern South Yellow Sea Basin). Late Eocene strata are distributed in several small-scale wedge-shaped basins. Abundant deciduous broad-leaved tree pollens associated with rare conifer indicate dry and temperate climate conditions during the Early Eocene. During the Middle Eocene, however, pollen grains derived from the low-wetland vegetation and surrounding slope are predominant reflecting somewhat wet and cool temperate conditions. During the Late Eocene cold temperate climate developed and cold-tolerant, deciduous broad-leaved trees flourished on the slope surrounding the fluvial area. Oligocene was the main period of severe tectonic activity of the basin, leading to uplift, erosion and denudation of the southwest sag (Kachi-1 and Haema-1) and central sag (IIH-1Xa and IIC-1X). Absence of Oligocene palynomorphs also indicates a hiatus. Early-Middle Miocene strata are identified only in Haema-1, IIH-1Xa, IIC-1X, and Inga-1 wells. During the Early Miocene, the basin has maintained thermal subsidence to get thick sedimentary succession. Subsequently the basin was uplifted and denuded again during the Middle Miocene. Diverse deciduous broad-leaved trees prevailed in fluvial and slope areas, and aquatic plants also lived in wetlands or ponds. The pollen assemblages indicate that the Early Miocene was a wet, warm temperate period. During the Middle Miocene, subtropical vegetation replaced the former forest due to increasing temperature. Pliocene strata are recorded in Haema-1, IIH-1Xa, IIC-1X and Inga-1 wells. Pollen grains derived from grass and conifer, together with hardwood trees, reflect that the Pliocene environment was fluvial surrounded by slope under dry, cold temperate conditions.