Sedimentary Facies and Paleoenvironment of the Lower Pleistocene Sogwipo Formation, Cheju Island, Korea

Mar.1995 第四紀研究(The Quaternary Research) 34 (1)p. 19-38 Sedimentary Facies and Paleoenvironment of the Lower Pleistocene Sogwipo Formation, Cheju Island, Korea Soon-Seok Kang* The Sogwipo Formation is distributed along the southern coast of Cheju Island, Korea, and is covered by Middle Pleistocene volcanic lava flows. These sediments are composed of mainly light grey fine to medium sandstones, muddy sandstones, mudstones, volcanic ashes and volcanic clasts with characteristic shell beds and trace fossils. This formation contains the molluscan fossils of the Omma-Manganzian fauna. The deposits can be divided into eleven sedimentary facies associations which are made in a grouping of nine sedimentary lithofacies and six sedimentary biofacies. On the basis of facies analysis, it is determined that all sedimentary facies associations were deposited at the inner shelf to foreshore and bay environments and that they were formed in a open sea to bay fluc- tuation system, controlled by glacio-eustatic sea level change. The depositional process of the Sogwipo Formation is summarized on transgression, gradual regression and transgression stages with a ravinement surface ascendant upwards. The transgression stage mainly consists of massive shoreface sands within a channelized compact shell bed. The gradual regression stage showed change from a open shelf to a beach system grad- ually, while the transgression stage with transgressive conglomerate is indicated in shoreface to bay system produced by geographic changes of the sedimentary basin as the shoreline re- treated during the transgression. It consists of lower shoreface shell beds and tuffaceous silt- stone. The uppermost bay sediment suggests that the sedimentary basin was buried in rapid sedimentation by strong volcanic activity. It is interpreted that the Sogwipo Formation has been developed by shallow marine environ- ment on the paleo-Cheju volcano and produced two sedimentary stratigraphic cycles reflecting to the fifth order cyclic eustasy. Key Words: Sogwipo Formation, Early Pleistocene, sedimentary facies, progradation, ravinement surface, glacio-eustatic sea level change no other exposures on the Korean Peninsula. The I. Introduction Sogwipo Formation is characterized by compact The Sogwipo Formation is distributed along the fossil beds, composed of nannoplankton, foramini- southern coast of Cheju Island, Korea (Fig. 1), fer, coral, ostracod, mollusk, brachiopod, echinoid, and represents Plio-Pleistocene marine deposits. It shark teeth and trace fossils, and consists of con- underlies deposits of volcanic sequence in the Che- siderable shallow open sea deposits (Kim, 1972, ju Volcanic Series. This formation is very impor- 1984; Yoon, 1981, 1988; Paik and Lee, 1986, tant for the Plio-Pleistocene stratigraphical and 1988; You et al., 1986, 1987). These densely fossil- paleontological studies in Korea because there are iferous units have served as the basal beds for Received July 28, 1994. Accepted January 21, 1995. Partly presented at the 101st Annual Meeting of the Geological Society of Japan (1994, Sapporo). * Graduate School of Science and Technology , Niigata University. Ni-no-cho 8050, Ikarashi, Niigata, 950-21. (Present Address: Department of Oceanography, Cheju National University. Ara-dong 1, Cheju City, Korea, 690-756). 20 Soon-Seok Kang Mar. 1995 Fig. 1 Geological map of around area of Sogwipo City showing outcrop location of the Sogwipo Formation volcano-lithologic subdivisions owing to their sub- cessive central eruptions during the Plio-Pleisto- lateral traceability in Cheju Island. These marine cene. This volcano is mainly composed of volca- strata have figured prominently in the early Qua- nic rocks belonging to the alkaline rock series ternary history between Korea and Japan. Pre- (e.g., basalt, trachyte, andesite). While mainly vious studies have focused mainly on the valuable formed by active eruptions during the middle stratigraphical and paleontological informations, Pleistocene, it has nonetheless continued into his- especially with respect to the Neogene marine toric time. The geology of the island (Table 1) is shell-rich strata (Yokoyama, 1923; Haraguchi, characterized by thick voluminous basaltic lava 1931). flows, minor pyroclastic and sedimentary rocks Problems of the geological age, depositional en- (Won, 1976; Lee, 1982; Yun et al., 1987; Lee et vironment and tectonic history of the Sogwipo al., 1988; Kim et al., 1989; Nakamura et al., Formation, however, have remained unsolved. 1989). These problems are important keys to understand- The Sogwipo Formation forms the lowermost ing Plio-Pleistocene paleogeography and pale- unit of the stratigraphic succession of Cheju Island oceanography in the north-eastern area of East Chi- (Table 1). The outcropping is limited to a sea cliff na Sea. The present study undertook detailed along the southwestern coast of Sogwipo City, sedimentological and paleontological observations Cheju Island (Fig. 1). According to drilling data, of outcrop sections, unraveling the change of de- however, the marine strata about 100m in thick- positional environments of the shallow marine de- ness are found under thick basaltic lava piles posit in the Sogwipo Formation by sedimentary throughout the island, except in the eastern part facies analysis. (KAPC, 1971-1991; Koh, 1991). On the basis of the characters of sediments and molluscan fossils, II. Outline of Geology this marine strata corresponds possibly to the Sog- Cheju Island is a shield volcano formed by suc- wipo Formation. 1995年3月 Sedimentary Facies and Paleoenvironment of the Sogwipo Formation, Korea 21 Table 1 Stratigraphic correlation between volcanic lava flows and sedimentary strata in the Cheju Island These successions are based on data in Haraguchi (1931), Won (1976), Lee (1982), Lee et al. (1988), Tamanyu (1990). Fission track analysis was carried out in the tuff basalts (Yun et al., 1987). The Turritella saishuensis bed (Fig. 2) of the Sogwipo Formation. The isotop- (s. s.) Zone is Early Pleistocene fauna based on ic ages of Zircon were determined to 2～3 Ma., the analysis of foraminiferal zone (Hasegawa, 80.5 Ma. and 161 Ma., values indicative of geolo- 1979; Ogasawara, 1981). gical age of the Cenozoic volcanic rocks of Paleo- The outcrop section of the Sogwipo Formation Cheju Island and the Cretaceous and Jurassic gran- (Fig. 2) is measured at 36 meters in thickness. ites distributed in the southern parts of Korean The strata have strikes at N12°Eand dips 7°W. Peninsula. It is overlain by a volcanic conglomerate and The geological age of the Sogwipo Formation is the Sogwipo Hawaiite. Unfortunately, the lower inferred to 1.2～0.5 Ma. based on the K-Ar agc boundary of the formation remains hidden beneath datings, namely 0.41～0.55 Ma. in the Sogwipo the sea. The Sogwipo Formation mainly consists of Hawaiite (Yun et al., 1987; Tamanyu, 1990) and light grey, fine to coarse sandstones, siltstones, 1.2～0.94 Ma. in the gravel of basal olivine augite mudstones, volcanic ashes and volcanic clasts. It is 22 Soon-Seok Kang Mar. 1995 1995年3月 Sedimentary Facies and Paleoenvironment of the Sogwipo Formation, Korea 23 Table 2 Sedimentary lithofacies and sedimentary biofacies of the Sogwipo Formation also characterized by molluscan fossil beds, trace Facies association A: massive sandstone fossils and various types of sedimentary structures with molluscan fossil bed such as wave ripples, parallel lamination, gravel The facies association A mainly consists of litho- dunes, cross-beds and herringbone structures. facies Sm, GS and biofacies FBc, FBl. It is located in the lowest part of the outcrop sections of the III. Sedimentary Facies and Sogwipo Formation along the southern coast of Facies Association Sogwipo City. The total thickness of the beds con- 1. Sedimentary lithofacies and biofacies sisting of this facies is about 3m, but the lower As the result of detailed field observations on the boundary cannot be seen. Lithofacies Sm compris- scale of 1:10 or 1:5 in the outcrop sections of es fine to medium grained, massive, moderately the Sogwipo Formation, nine sedimentary lithofa- poorly-sorted, light or yellowish grey, unconsoli- cies were defined using grain size, sedimentary dated and/or semiconsolidated sandstones. Litho- structure, composition and texture of the beds, and facies GS is dark grey, semiconsolidated, medium- six sedimentary biofacies were also defined using grained sandstones which contain well rounded bioturbation grade and occurrence of shell beds volcanic pebbles. The molluscan shell bed interca- (Table 2). lated in Sm (Fig. 2) is composed of FBc and FBl 2. Sedimentary facies association with GS, and is 30～50cm in thickness. Con- On the basis of characters of the sedimentary densed and lenticular shell bed (FBc, FBl) with the lithofacies and biofacies, eleven sedimentary facies mostly well-rounded volcanic fine pebbles (2～7 association were recognized (Fig. 3). These sedi- cm in diameter) are occupied by moderately mentary facies associations are accumulated as bioturbation in the middle part of the beds. Mol- shown in Figure 2, suggestive of shallow marine luscan shells are disarticulated, tightly packed, deposits as described and interpreted in the follow- convex up and concave down, parallel to bedding ing sections. plane, and usually fragmented or abraded. Main 24 Soon-Seok Kang Mar. 1995 Fig. 3 Schematic overview of the 11 sedimentary facies associations of the Sogwipo Formation In the bioturbation column, the degree of bioturbation is indicated,

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