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

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 fluctuation 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 trans-

gression 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 siltstone. 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 environment 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, with 0 reflecting a non-bioturbated sediment and 3 a strongly bioturbated sediment. 1995年3月 Sedimentary Facies and Paleoenvironment of the Sogwipo Formation, Korea 25 species are over-sized Mizuhopecten tokyoensis hoku- sandstone with volcanic granules arranged in rikuensis, Mercenaria stimpsoni, Mytilus coruscus and linear lamination (Fig. 5). The lithofacies Sm is Crassostrea gigas. The boundary between the mol- massive, semiconsolidated, light grey, very fine to luscan shell bed and the massive sandstone bed is fine sandstone with infaunal bivalvian shells and generally sharp and sometimes eroded. shell fragments. The lithofacies M commonly Facies association B: alternated bed of thin occurs as a thin massive or laminating bed. The bedded sand and mud thickness of the bed is 5～12cm. The biofacies The facies association B consists of alternative MSb is mainly homogeneous and/or occasionally beds of lithofacies Sg, Sp, and MS or M (Fig. 4). parallel stratified, light grey muddy sandstone. General rock facies is normal graded, stratified, This biofacies is characterized by densely and per- very thinly laminated, yellowish grey, consolidated, pendicular burrow holes, Skolithos ichsp. The bio- fine to medium grained sandstones with inter- facies FBs contains sporadic shells and hatched shell layered muddy sandstones and mudstones. The fragments. Molluscan species, usually seen in life beds comprising this facies association B are lo- positions, includes Lucinoma annulata. Trace fossils, cated at the lower part of the Sogwipo Formation. mostly Skolithos ichsp., are seen as perpendicular

They overlie the beds consisting of facies associa- burrow holes; 0.5～3cm in diameter and 8～15cm tion A with sharp and eroded surface (Fig. 2). in length. This intense Skolithos layer (Fig. 5) The total thickness of the beds is 1.85m. The sand which is a good tracer of this facies association can grains of Sg and Sp are mostly composed of angu- be distinguished from the non-bioturbated facies lar, well-sorted basaltic fragments. They are linear- association B. The Thalassinoides burrows also ly concentrated in the basal part of each layer. occur on the bedding surface, and the bivalves Each layer composed of lithofacies Sg, Sp, Ms and with life position yield in lithofacies Sg. M is in normal graduation. The lithofacies MS Facies association D: bioturbated sand with and M consisting of thinly laminated fine grained burrow holes layers (less than 5cm) is massive, consolidated, The facies association D is mainly composed of weathered, whitish or dark grey mudstones and lithofacies Sm and biofacies Sb with FBs. General muddy sandstones. In general, these lithofacies are rock facies is massive, structureless, whitish grey, situated on the normal graded one unit layer com- high bioturbated, very fine to fine sandstones, posed of lithofacies Sg and Sp (Fig. 4). The num- though it is characterized by moderately well- ber of layers of this facies association total 28. The sorted very fine sandstone. Primary sedimentary thickness of one layer is 7cm on average. The up- structures were destroyed by intensive bioturba- per and lower boundaries of each thin layer are tion. The beds of this facies association D con- fairly sharp and planar. Paleo-current ripple scars formably overlie the facies association C, and is were occasionally found on the bedding surface, about 1m in thickness. These beds and the beds and the strikes of current ripples were estimated at consisting of the following facies association E NW 36°～46°(NW 38° on average). occupy the middle part of the Sogwipo Formation Facies association C: high bioturbated mud- (Fig. 2). The lithofacies Sm is massive, light grey, dy and fine sandstone semiconsolidated, moderately well sorted very fine The facies association C consists of lithofacies to fine sandstones. The biofacies Sb is similar to Sg, Sm and M with biofacies MSb and FBs. The the lithofacies Sm lithologically. Two bioturbated beds composed of the facies association C overlie layers can be distinguished with two types of spe- those of the facies association B with sharp and cialized burrow holes characteristically. These are

non-erosive boundary (Fig. 2). Its total thickness considered to be the trails of Polychaeta (2～3mm

is 1.3m. The lithofacies Sg is fairly normal in diameter)and the Skolitkos burrows (1～3cm in graded, thick bedded, light grey, fine to medium diameter) with full-relief. Molluscan shells (e. g., 26 Soon-Seok Kang Mar. 1995

Fig. 4 Detailed section and photograph of facies association B

Sm, M, etc. show sedimentary lithofacies. Note the sharp and eroded facies association boundary at the lower part of photograph.

Fig. 5 Detailed section and photograph of facies association C 1995年3月 Sedimentary Facies and Paleoenvironment of the Sogwipo Formation, Korea 27

Mizuhopecten tokyoensishokurikuensis) occur in bio- can be subdivided to the two layers due to facies FBs sporadically and sometimes with life bioturbation grade. The lower layer, up to 1.5m position. in thickness, is mainly tuffaceous mudstones with Facies association E: unconsolidated and rare burrow holes. The upper layer, about 60cm massive very fine to fine sandstone in thickness, is marked by intense burrow holes fill- The facies association E is mainly composed of ed by whitish grey sands. Planolites burrows are lithofacies Sm with biofacies FBl and FBs. Main filled by sand sediments differing from the sur- rock facies of the bed consisting of the facies asso- rounding matrix in terms of composition, color, ciation E is massive, very well-sorted, unconsoli- texture, and fabric. In the upper bed consisting of dated, structureless and/or dime plane laminated, this facies association (Fig. 9), Thalassinoidesbur- whitish grey, very fine to fine sandstones. The bed rows yield remarkably. This trace fossil, repre- conformably overlies the underlying bed, and the sented by cylindrical burrows forming 3-D hori- thickness is up to 13m (Fig. 2). The lithofacies zontal branching networks, connected to the sur- Sm is massive, very well-sorted, very fine to fine face by vertical shafts, appears similar to feeding sandstones. The sands are dominantly composed of and dwelling burrows. Burrows are commonly fill- quartz grains up to 60%; basaltic fragments are ed with carbonates, and brachiopodian shells are rare. The biofacies FBl is a lenticular shell bed sometimes found inserted. and is linearly distributed in the middle part of the Facies association G: large-scaled cross-bed- beds. The occurrence of molluscan shells is exotic ded gravel sandstone and convex up in position. On the other hand, the The facies association G is composed of litho- molluscan shells of biofacies FBs are sporadic and facies SG and GS with M, and biofacies FBc and indicate in situ fauna. Vertical burrows similar to FBl. The beds consisting of the facies association an Ophiomorpha type are also found in biofacies G is located in the upper part of the Sogwipo FBs. They are 2～3cm in diameter, 20cm up to Formation, and overlies on the underlying bed

50cm in length. with a sharp and erosive boundary. The total Facies association F: high bioturbated tu- thickness of the bed is 2m. The bed is subdivided ffaceous muddy sandstone into the lower part of condensed shell layer and The facies association F is mainly composed of the upper part of cross-bedded conglomerate layer MS and MSb. Main rock facies of this facies asso- (Fig. 6). Lithofacies SG and GS are characteriz- ciation is massive, structureless, yellowish grey ed by largc-scale, low anglc (8° ～15°),planar or and/or light pink, tuffaceous muddy sandstone and trough cross-beds. It is poorly-sorted and contains volcanic ash. The bed consisting of the facies asso- well-rounded volcanic granulcs and pebblcs (0.3～ ciation F is distributed in two horizons of the Sog- 1.5cm in diameter, 0.8 cm on average). Individual wipo Formation (Fig. 2), namely in the upper bed sets of cross-stratified units range typically from 5 and lower bed. The base of the lower bed is not to 30cm in thickness, locally forming a herring- seen and the visible thickness is about 2m. The bone pattern. Clasts are mostly composed of lower boundary of the upper bed conformably vesicular basaltic fragments that are well rounded overlie the underlying bed of facies association J. and thinly coated with reddish iron oxides. Bio- The thickness is 30cm (Fig. 9). The lithofacies turbation is restricted. Shells are aligned convex- MS is massive, yellowish grey, weathered muddy up. Thin mud layers (<1cm) are commonly sandstones. Primary sedimentary structures are intercalated between sets of cross-strata. The destroyed by intense bioturbations. The biofacies estimated paleo-currents in cross-beds lie NW MSb is characterized as intensely bioturbated san- 41°, 12° E on average. Layers consisting of the con- dy mudstones and muddy sandstones. In the mid- densed Mizuhopecten tokyoensis hokurikuensis are situ- dle part of the Sogwipo Formation, the lower bed ated at the base of this facies association, and the 28 Soon-Seok Kang Mar. 1995

Fig. 6 Detailed section and photograph of facies association G

Fig. 7 Detailed section and photograph of facies association H volcanic gravels.

1995年3月 Sedimentary Facies and Paleoenvironment of the Sogwipo Formation, Korea 29

transported shells of Glycymeris rotunda are inter- are commonly wavy laminae. In the uppermost calated in cross laminae. part of these beds, the lithofacies M yields Thalassi- Facies association H : graded and wave-rip- noides burrows and crowded boring shells (e. g., ple lamination-dominated sandstone Barnea manilensis). The facies association H is mainly consists of Facies association I: poorly-sorted gravel lithofacies Sw, Sg and Sp with intercalated lithofa- sandstone cies MS and M. The general character of this Facies association I consists of lithofacies SG, facies association is graded and stratified, wave- GS and biofacies FBc. The general rock facies is ripple laminated fine to medium sandstones con- massive, poorly sorted, light grey, consolidated, taining characteristic sedimentary structures; gravelly muddy to coarse sandstones with abun- namely wavy laminae, flaser beddings, gravel dant molluscan shells and volcanic clasts, all of dunes and planar laminae (Fig. 7). The beds con- which are extensively bioturbated and commonly sisting of this facies association are located in two amalgamated (Fig. 8). The bed composed of the horizons of the upper part of the Sogwipo Forma- facies association I overlies the underlying bed tion, upper beds and lower beds (Fig. 2). The with erosive basal contact (Fig. 2). The total lower beds are 2.85m in thickness, and overlie an thickness is about 1.3m . The pebble graded clasts underlying bed with erosive basal contact resem- of lithofacies SG are randomly dispersed in muddy bling a small-scaled channel. The lower beds are sand matrix. The clasts comprise dominantly composed of lithofacies Sw, Sg, Sp, MS and M, basalt fragments and are relatively well-rounded from 3 to 15cm in thickness. The sands of this (0.5cm in diameter). Biofacies FBc is composed of lithofacies comprise abundant basaltic fragments shell beds distributed in the middle part and forms that are poorly-sorted and angular. Coarse grains a so-called basal condensed shell bed. The lower

are concentrated in the basal part of each layer part of this bed,30～50cm in thickness, is char- showing coarse-tail grading. Each layer has both acterized by large-scaled, low-angled, cross-bedded

upper and lower sharp boundaries. The lower beds (NW 24°,7°E) volcanic sandy gravel layer (Fig. are subdivided into three units due to lithologic 8). Furthermore, the uppermost part occurs a sig- characters. The lower unit (109cm in thickness) is nificant trace fossil layer, including Cruziana, Chon- characterized in graded and stratified sandstones drises and Helminthopsisat the bedding surface. with wavy laminae and gravel dunes. Occasional- Facies association J: poorly-sorted muddy ly, slump scarring also occurs in this layer. The sandstone middle unit (80cm in thickness) is characterized This facies association J consists of biofacies by graded and wavy sandstones, composed of nine MSb and FBb. The general rock facies is character- wavy layers. Each layers are 3～15cm in thick- ized by high bioturbated, whitish grey, poorly- ness. The upper unit (96cm in thickness) is char- sorted muddy sandstones. The upper and lower acterized in wavy and plane laminated sandstones, molluscan shell beds consisting of this facies asso- and locally yields burrow holes. Wavy sandstones ciation are located in the upper part of the Sogwi- locally consist of gravel dunes and flaser beddings. po Formation. Both molluscan shell beds overlie A plane laminated layer has dominated abundant each underlying bed with erosive basal contact (Fig. 9). Thickness of the lower shell bed is 73cm The upper beds are 0.8m in thickness, and and the upper shell bed is 30cm. The biofacies overlie on the facies association I with erosive ba- MSb is whitish grey, semiconsolidated, carbon- sal contact (Fig. 2). The upper beds are composed ated, poorly-sorted muddy sandstones with shell of lithofacies Sw, Sg and M. The upper beds are fragments and volcanic clasts. The biofaciesFBb is characterized in graded and stratified, wave-ripple usually characterized in basally concentrated shell laminated sandstones, and sedimentary structures beds. The main species is represented Chlamys 30 Soon-Seok Kang Mar. 1995

Fig. 8 Detailed section and photograph of facies association I

Fig. 9 Detailed section and photograph of facies association J with intercalated facies association F 1995年3月 Sedimentary Facies and Paleoenvironment of the Sogwipo Formation, Korea 31 cosibensis,though infaunal molluscan species includ- sediment that characterized in sharp based thin ing Panopea japonica, Ezocallista brevisiphonata and bedded parallel lamination, non-bioturbation, and Phacosoma japonicum are seen in life position. alternations of sand and mud (Fig. 4). The Sg, Sp Brachiopodian, echinoidian and coralian shells are and laminated fine sediments of this facies associa- also yielded. Boring burrows sometimes with shells tion are also interpreted as shelf sediments result- are drilled into the underlying bed. This layer is ing from turbidity currents. It is suggested that disturbed and irregular as to the result of a crowd this facies association is produced by BC beds burrowing of boring shells (e.g., Barnea manilensis), above the storm wave base (Walker and James, and rip-up mud clasts are common (Fig. 9). 1992). Occasionally, current ripples as a result of Facies association K: tuffaceous siltstone storms occur in the uppermost bedding plane, and This facies association K mainly consists of it indicates south-westward in the direction of the lithofacies ST and is characterized as tuffaceous paleo-current. siltstone occurring as volcanic ash. The bed com- Facies association C: inner shelf posed of the facies association K is situated at the The inner shelf commonly consists of alternating uppermost part of the Sogwipo Formation. It con- sediments of sand and mud (Saito, 1989). High formably overlies to the underlying bed. The total densely burrowed sandstones and muddy sands are thickness is up to 6m. This bed is unconformably indicative of a changing stage from offshore to low- covered by thick volcanic conglomerates and lavas er shoreface. Moreover, moderately poorly-sorted, (Fig. 2). Lithofacies ST is homogeneous, non- very fine sandstones concentrated in the burrow bioturbated, bluish grey, tuffaceous siltstone. It hole layer are considered shallower than the lower also contains a kind of volcanic ash flow marks facies association. This facies contains two types of and a few in situ bivalves (e. g., Macoma sp.). trace fossils : in the lower part is Thalassinoides, while in the upper part is Skolithos.In general, Sko- IV. Depositional Environment lithos ichsp. is shallower than Thalassinoidesichsp., 1. Depositional environments of each facies in accordance with environmental conditions (Fig. association 5). Facies association A: lower shoreface Facies association D: lower shoreface Massive sandstone (lithofacies Sm) is inter- This facies association is indicative of the transi- preted as shoreface sediments on the back-barrier tion sand from shelf to shoreface, and highly system. Relatively well-rounded volcanic clasts and bioturbated sand beds are also inferred to the low- sparse bioturbation suggest a shoreface erosion on er shoreface (Fig. 11). Massive sand (facies Sm) a transgression surface (Fig. 2). The condensed is characterized of marine sediments, composed a and lenticular shell bed with gravels indicates in two types of trace fossils (e. g., burrows by Poly- existence of the erosive-based tidal inlet on barrier chaeta and Skolithosichsp.), and randomly dispersed migration during the shoreface retreat. It was shells (e. g., Mizuhopectentokyoensis hokurikuensis). probably deposited as sheet-like channel on the All of these species are found in coastal regime. It shoreface floor, and the molluscan species, repre- is suggested that this facies association has been a senting mixed species from intertidal zone to eu- well conditioned biological field showed relatively neritic zone in bathymetric distribution, are be- slow sedimentation. lieved to be coastal species, in accordance with the Facies association E: lower shoreface environmental conditions. In general, channel-floor This facies association is indicative of typical deposits contain abundant transported shells and sheet-like sandbodies in the lower shoreface (Fig. gravels (Fig. 11). 2). This SCS shoreface sandbodies characteristical- Facies association B: inner shelf (turbidite) ly occur in the upper parts of sandier-upward This facies association indicates the inner shelf facies successions (Fig. 11). These successions typ- 32 Soon-Seok Kang Mar. 1995 ically begin with bioturbated mudstones and inter- Characteristic bedding features such as wavy bedded fine to very fine sandstone beds a few bedding, flaser bedding, gravel dune are indicative centimeters or tens of centimeters in thickness of middle to upper shoreface as a storm deposit. In (Rosenthal and Walker, 1987). The massive sand the upper part, plane lamination is developed of body is very well sorted, mainly marine quartz the foreshore deposits as a typically gravel beach- particles. This facies association also consists of in face system (Fig. 7). This foreshore facies is local- situ bivalves and Ophiomorphaburrows, a tracer of ly disturbed by the characteristical coastal burrow the lower shoreface. holes. This densely compact layer is interpreted as Facies association F: lower shoreface upper shoreface to beach deposits in beach system, (event) resulting as small-scaled sea-level fluctuation on The tuff beds of muddy sand are described as a the regression stage (Figs. 10 and ii). event layer. Intensive burrow holes of the Planolites Facies association I: lower shoreface (trans- and Thalassinoides, deposit-feeding and dwelling gressive lag) animals, are seen. This facies association is situ- Lithofacies SG and GS in the lower part are ated on the sand body, facies association E, or in- characterized in massive or high angled cross bed- tercalated into the compact molluscan fossil beds, ded layers (Fig. 8). This conglomerate is a redep- facies association J (Figs. 2 and 9). These tu- ositional sediment, transported as erosive coarse ffaceous muddy sand beds are inferred to the lower sediments by strong wave action on the transgres- shoreface sediments on the open-bay system. Bur- sive surface. This facies association is mainly com- row holes are concentrated in the upper portion of posed of the poorly-sorted shell sands within con- this facies, burrowed from the upper bed, but this densed shell-beds. It also indicates transgressive facies may indicates a rapid deposition as to the lower shoreface sediments on the shoreface erosion event bed. surface (S. E. S.) by shoreface retreat during the Facies association G: upper shoreface (tidal transgression (Heward, 1981). Consequently, this bar and trough) facies association is interpreted as lower shoreface This facies association of the large-scaled cross sediments from ravinement surface to open bay on bedded gravel sandstone is interpreted as bar and the transgression stage (Fig. 10). trough deposits of the upper shoreface (Fig. 11). Facies association J: lower shoreface The facies SG occasionally has a herringbone cross The basal contact of this facies association is an stratification, indicative of tidal current dominated erosion surface with a long period. It consists of environments where speed of current is sufficiently the crowded boring shells and rip-up mud clasts strong (Fig. 6). In general, tide-dominated, high- (Fig. 9). Shells are usually made of basally con- stand systems tracts are characterized by prog- centrated shell bed or locally lenticulated shell bed radation, either of tide-dominated deltas and/or in the central part linearly, and also have a sparse- open-coast tidal flats. Both of these should extend ly in situ fauna. This facies association is deposited seaward into a regressive shelf (Walker and James, along the lower shoreface of the large scaled open 1992). In addition, this facies represents a tidal bay with the environment of intensive bioturba- channel to erode of lower bed which consists of tion. bioturbated and tuffaceous sandy mud. The com- Facies association K: embayment mon intercalation of thin mud layers reflects The massive tuffaceous siltstone (facies ST) is periodic slack water conditions during which fine characterized in thick volcanic ash fall and flow suspended sediments settle to the bottom (Reineck sediments. This facies association consists of a few and Singh, 1973). in situ molluscan shells, whose are indicated of the Facies association H: upper shoreface to embayment fauna. From these features, it can be foreshore inferred that this facies association is deposited in 1995年3月 Sedimentary Facies and Paleoenvironment of the Sogwipo Formation, Korea 33

Fig. 10 Interpreted stratigraphic change in water depth and sedimentary environmental system of the Sogwipo Formation The relative sea level curve (solid line) is made of sedimentary facies analysis. Three arrows indicate transgression to regression stages of the 5th-order stratigraphic cycle.

the central part of a large scaled embayment sys- As to the result of sedimentary facies analysis, tem with a rising sea level by the active volcanic the depositional process of the Sogwipo Formation ash fall and flow process (Fig. 10). can be divided into three stages due to the eustatic 2. Stratigraphical change of depositional sea-level fluctuations (Fig. 10), namely a trans- environment gression stage, a gradual regression stage, and a 34 Soon-Seok Kang Mar. 1995 transgression stage from the base upwards. Each This transgressive sequence includes a signifi- stage reflects successive changes in the geographic cant erosion surface produced by shoreface retreat. environment (Fig. 11) that the sedimentary stra- These thin transgressive facies commence above a tigraphy of the Sogwipo Formation has previously planar erosion surface and fine upwards as wave been explained. energy decreased, and can be explained by trans- 1) Transgression stage gressive systems tract (TST). On the other hand, This stage is situated in the lowermost part of Ryer (1977) proposed that thick transgressive se- the Sogwipo Formation. It is indicated as quences actually accumulated during periods of sedimentary facies association A and suggests low- shoreline still-stand or progradation which inter- er shoreface sand sediments with shell beds on the rupted the transgression. The facies association H back barrier system. This facies reflects a shoreface and K of this stage can be interpreted in a terms retreat, represents back-barrier sand body under of progradation system during a continuous trans- condition of moderately rapid sea-level rise. Fur- gression (Fig. 10). The conglomerate of facies thermore, this facies is overlain by thin bedded association I was formed by shoreface-erosion on turbidite of the inner shelf with eroded surface. In the transgression surface. Facies association I, con- this facies, large molluscan shells compose a mixed taining abundant shells with poorly-sorted sand- assemblage from intertidal to euneritic zones in stone, also indicates shoreface sediments. The up- bathymetric distribution, indicating a relatively per shoreface sediments of this stage, facies asso- high energy condition, as seen when a rip current ciation H, is intercalated into the facies association channel exists on the shoreface of the open sea. I and J which are compact shell beds, indicating 2) Gradual regression stage small scaled sea level fluctuation on the transgres- The depositional environments of this stage are sion stage. The poorly sorted muddy sandstone interpreted by inner shelf to shoreface models (Fig. and intact shells in facies association J were i11). This regression stage is characterized by high- perhaps deposited in lower shoreface along the stand systems tract (HST) which indicates gener- progressive shoreline. The boring shells and rip-up ally coarsening-upwards sequence produced by mud clasts at the basal contact of this facies asso- progradation. Facies association B represents a de- ciation indicate a hiatus to have a long period of posit of inner shelf in shallow system with turbid- erosion and the upper shoreface erosion by ity currents above storm wave base. Facies associa- shoreface retreat. This facies association interca- tion C, containing abundant burrow holes, corres- lates with facies association F which consists of ponds to the inner shelf sediments. The sandy de- weathered muddy sand with the intensive burrow posits of facies association D, E, F indicate a holes of a Thalassinoidesichsp. Tuffaceous silt in shoreface sand body deposited on the lower facies association K is indicative of a change to the shoreface in the regressive shoreface system. Facies embayment system marked by rapid sedimentation association D, E, F are also characterized by a due to the strong volcanic activity. densely bioturbated sand, massive sand body V. Discussion and tuffaceous muddy sand, respectively. The herringbone and large-scaled cross stratifications in 1. Paleoenvironment of the Sogwipo facies association G were formed by bimodal tidal Formation currents in the tidal bar and trough of the upper Previous paleoenvironmentalreports of the Sog- shoreface. Facies association H is characterized by wipo Formation are based on the analyses of fora- wave ripple lamination, gravel dune, parallel minifera (Kim, 1972), ostracodal fauna (Paik and lamination indicative of upper shoreface to fore- Lee, 1984, 1986, 1988; Park et al., 1986), nanno- shore deposits. plankton (You et al., 1986, 1987), molluscan fauna 3) Shoreface to bay transgression stage (Yoon, 1988) and sedimentary facies (Yoon and 1995年3月 Sedimentary Facies and Paleoenvironment of the Sogwipo Formation, Korea 35

Fig. 11 Block diagram showing the positions of sedimentary litho- and biofacies and sedimentary facies associations with water depth

Sw, FBb, etc. are sedimentary lithofacies and sedimentary biofacies. A-K are sedimentary facies association.

Chough, 1990). According to Kim (1972), forami- 2). It represented that biotope A is deposited from nifera of the Sogwipo Formation suggests a shallow bay or shallow sea regime to deepening shallow warm open sea environment in Middle to Late sea, but ostracod species are not yielded from the Pliocene. You et al. (1986, 1987) also insist the nan- facies association B. Biotope B of his work corre- noplankton is indicative of a shallow and warm lates to the facies association F and G. They have open sea environment in the Plio-Pleistocene. proposed in warm water regime for offshore. Moreover, Yoon (1988) argue as the result of mol- However, it suggests based on the sedimentary luscan faunal analysis that the Sogwipo Fauna facies analysis that the facies association F is lower flourished in sublittoral shallow water under the shoreface and the facies association G is upper intermingling influence of warm and cold waters. shoreface developed tidal bar and trough. Biotope These are suggestive of the whole environment of C corresponds to the facies association F, they the Sogwipo Formation. have proposed in bay or shallow regime, but this Paik and Lee (1984, 1986, 1988) and Park et al. facies is suggestive to produce lower shoreface en- (1986) reported on cluster analysis of ostracod vironment by means of a eventual tuff bed during assemblage of the Sogwipo Formation, recognizing the regression. Moreover, Biotope D corresponds that there are four biotopes in the Sogwipo out- to the facies association I and J, in the upper part crops ; namely Biotope A, B, C and D. Biotope A of the Sogwipo Formation, they have proposed in has become to the lower and middle parts (facies cold water regime of offshore. This is coincident association A～E) of the Sogwipo Formation (Fig. with the result of sedimentary facies analysis. 36 Soon-Seok Kang Mar. 1995

Thus, it suggests that the facies association I and J fossil, Turritella saishuensissaishuensis, is a key spe- are lower shoreface environment with shoreface cies of the Sogwipo fauna. The Turritella Zone has erosion surface due to shoreface retreat during the been considered a tracer of the Neogene boundar- transgression stage. ing (Kotaka and Ogasawara, 1977; Kotaka, Yoon and Chough (1990) performed a sed- 1978). In Ishikawa Prefecture, the Turritella imentary facies analysis, classified into detailed en- saishuensis saishuensis Zone is placed in the Early vironment. Tidal flat sediments has become to the Pleistocene (Hasegawa, 1979; Ogasawara, 1981). sedimentary facies association B and H of this According to the nannoplankton analysis, the Sog- paper. The sedimentary facies association B is wipo Formation yields abundant Gepryocapsaoceani- characterized by thin bedded, normal graded and ca (You et al., 1987). It is interpreted that this spe- parallel stratified alternated beds of sand and mud cies is a key of Pleistocene in geological age. As to that are not found any sedimentary structures of the above results, the Sogwipo Formation suggests tidal flat. Moreover, the lower facies indicates a that the geological age is perhaps the Early Pleis- transgressive shoreface sand sediment with sharp tocene. based erosion surface. Therefore, this facies is sug- Moreover, the Sogwipo Formation is obtained to gestive of inner shelf sediments which represent a two cycles of periodic sedimentary stratigraphy BC bed produced by turbidity current on the that are identified in this outcrop sections (Fig. transgression (Walker and James, 1992). The 10). This periodicity is within fifth order strati- sedimentary facies association H is also suggestive graphic cycles (0.03～0.08 Ma) and is interpreted of upper shoreface to foreshore sediments com- to be of glacioeustatic origin (Vail et al., 1991). posed of characteristical sedimentary structures The oxygen isotope sea-level index indicates an such as wavy beddings, gravel dunes, parallel approximately 0.05 to 0.04m.y. periodicity with laminations (Fig. 7). Beach sediments have be- an amplitude about 1% during Late Pliocene to come to the facies association F of the upper part Early Pleistocene (older than ca. 0.85Ma) , and

of the Sogwipo outcrops. This facies consists of 0.05～0.04m.y. to 0.1m.y. periodicity during ab- muddy sandstones, yield Thalassinoidesburrows re- out 0.85 through 0.6Ma (Williams et al., 1988; markably. It is indicative of lower shoreface sedi- Joyce et al., 1990). Consequently, it is interpreted ments as to the event bed intercalating into shell that this formation is produced for about 80,000 to beds (Fig. 9). In uppermost part of the Sogwipo 200,000 years in Early Pleistocene. outcrop, the sedimentary facies association K consisting of tuffaceous siltstones is suggestive of vol- VI. Conclusion canic ash. It is interpretive of bay sediments with 1. The Sogwipo Formation can be classified into molluscan shells, Macomasp. Thus, the Sogwipo nine sedimentary lithofacies and six biofacies. sedimentary basin suggests that there were alterna- Eleven sedimentary facies associations were dis- tive fluctuations of shallow marine and bay en- tinguished, owing to the compound of their lithofa- vironment with cyclic changes of sea water temper- cies and biofacies. ature, and the cause was interpreted in terms of 2. Each sedimentary facies associations are glacial and interglacial cycles during Early Pleisto- mainly suggestive of shallow marine to embayment cene. environments (Fig. 10). 2. Geological age of the Sogwipo Formation 3. As to the result of sedimentary facies analy- The geological age of the Sogwipo Formation sis, the three stages of transgression, gradual re- can be inferred to the analysis of existing paleonto- gression and transgression with ravinement sur- logical data such as mollusca, foraminifers, face, were recognized. Each stage consists of a brachiopods and nannoplankton (Yoon, 1988; small scaled sea level fluctuation that can be ex- Kim, 1972, 1984; You et al., 1987). The molluscan plained by successive geographic changes. 1995年3月 Sedimentary Facies and Paleoenvironment of the Sogwipo Formation, Korea 37

4. The sedimentary process of the Sogwipo wipo Formation of Jeju Island, Korea. Jour. Nat. Acad. Formation is summarized by the open sea to bay Sci. Korea Natu. Sci. Series, 23: 167-194 Kim, D. H., Hwang, J. H., Hwang, S. K, and Choi, S. J. fluctuation. The geographic change suggests that (1989) Volcanic activity in Cheju island (II): Volca- the sedimentary basin is changed to the growth of nic stratigraphy and eruptive history on the western barrier system, development of open sea, develop- side of the island. Report of the Korea Institute of Energy ment of open bay and rapid sedimentation on the and Resources,KR-88-(B)-4* bay. Koh, G. W. (1991) Subsurface distributions of Seoguipo 5. The development of the Sogwipo Sedimentary Formation in Cheju Island, Korea. Bull. Mar. Res. Inst. ChejuNat. Univ., 15: 8192* Basin is controlled on the eustatic sea-level fluctua- Korean Agricultural Promotion Corporation (1971～ tions, and can be identified about two cycles in 1991) Ann. Rept.* outcrop section. This sedimentary cycle is consi- Kotaka, T. (1978) World-wide biostratigraphic correla- dered to the fifth order stratigraphic cycles, and in- tion based on turritellid phylogeny. The Veliger, 21: terpreted to be of glacioeustatic origin in Early 189-196 Kotaka, T. and Ogasawara, K. (1977) Turritellid zones Pleistocene. along the Japan Sea borderland, Honshu, Japan. Prof. Acknowledgements K. Huzioka Mem. Vol., 345-351, 1 p1.* Lee, D. Y., Yun, S. K., Kim, J. Y. and Kim, Y. J. The author wishes to thank J. S. Youn at Cheju (1988) Quaternary geology of the Jeju Island. Report of National University and S. Yoon at Pusan Nation- the Korea Institute of Energy and Resources,KR-87-29* al University for generous assistance in successful Lee, M. W. (1982) Petrology and geochemistry of Jeju volcanic Island, Korea. Tohoku Univ. Sci. Rept., 3rd field work and collecting geological information in Ser., 15 (2): 177-256 Korea. He is also grateful to I. Kobayashi, M. Nakamura, E., Campbell, I. H., McCulloch, M. T. and Tateishi and T. Ueda at Niigata University for Sun, S. S. (1989) Chemical geodynamics in a back arc their thoughtful consideration during the prepara- region around the Sea of Japan: implications for the tion of this text. Dr. H. Ohira at Kyushu Universi- genesis of alkaline basalts in Japan, Korea and China. ty provided assistance with fission track analysis. J. Geophys.Res., 94: 4934-4954 Ogasawara, K. (1981) Paleogeographic significance of My colleagues, Dr. A. Urabe, H. Kojima, H. the Omma-Manganzian fauna of the Japan Sea Nagamori, H. Yabe and Dr. J. A. Wren at Niigata Borderland. Saito Ho-on Kai Mus. Res. Bull., 49: 1-16 University, are acknowledged for their beneficial Paik, K. H. and Lee, E. H. (1984) A Plio-Pleistocene discussion and support. ostracod assemblage from the Seogwipo Formation, Cheju Island, South Sea of Korea. Y. A. Park et al. References (eds.) Marine geologyand physicalprocesses of the Yellow Sea: 223-234, Proceedingsof US-KOREASymposium and Haraguchi, K. (1931) Geology of Cheju Island. Bull. Workshopon the YellowSea Geol. Surv. Korea, 10: 1-34* Paik, K. H. and Lee, E. H. (1986) Ostracode fauna from Hasegawa, S. (1979) Foraminifera of the Himi Group, the Sogwipo Formation, Cheju Island. The Memoirs for Hokuriku province, central Japan. Sci. Rep. Tohoku Prof. Bong-kyunKim's Retirement: 375-389* Univ., 2nd Ser. (Geol.), 49: 89-163 Paik, K. H. and Lee, E. H. (1988) Plio-Pleistocene Heward, A. P. (1981) A review of wave-dominated elas- ostracods from the Sogwipo Formation, Cheju Island, tic shoreline deposits. Earth-Sci. Rev., 17: 223-276 Korea. T. Hanai, N. Ikeya and K. Ishizaki (eds.) Evo- Joyce, J. E., Tjalsma, L. R. C. and Prutzman, J. M. lutionarybiology of ostracoda: its fundamentalsand applica- (1990) High-resolution planktonic stable isotope re- tions: 541-556, Kodansha,Japan cord and spectral analysis for the last 5.35m.y.: Ocean Park, K. B., Lee, E. H. and Paik, K. H. (1986) Faunal drilling program site 625 Northeast Gulf of Mexico. analysis and paleoenvironmentof the Plio-Pleistocene Paleoceanography,5 (4): 507-529 ostracod assemblages from the Sogwipo Formation, Kim, B. K. (1972) A stratigraphic and paleontologic Cheju Island. KoreaUniv. Sci. Rep., 27 : 133-147* study of the Seogwipo Formation. The Memoirs for Prof. Reineck, H. E. and Singh, I. B. (1973) Depositional Chi Moo Son's Sixtieth Birthday: 169-187* sedimentaryenvironments. 579p, Springer-Verlag Kim, B. K. (1984) Pliocene brachiopods from the Seog- Rosenthal, L. R. P. and Walker, R. G. (1987) Lateral 38 Soon-Seok Kang Mar. 1995

and vertical facies sequences in the Upper Cretaceous Yokoyama, M. (1923) On some fossil shells from the Is- Chungo Member, Wapiabi Formation, southern Alber- land of Saishu in the Strait of Tsushima. Tokyo Imp. ta. CanadianJournal of Earth Sciences,24: 771-783 Univ., Jour. Coll. Sci., 44, art. 7: 1-9, 1 p1. Ryer, T. A. (1977) Patterns of Cretaceous shallow- Yoon, S. (1981) The Seoguipo fauna (Mollusca) of the marine sedimentation, Coalville and Rockport areas, Jeju Island, Korea (abstract). Proc. 6th Internat. Work. Utah. Bull. Geol. Soc. Am., 88: 177-188 Group Meet., IGCP-114, 149p Saito, Y. (1989) Classification of shelf sediments and Yoon, S. (1988) The Seoguipo molluscan fauna of Jeju their sedimentary facies in the storm-dominated shelf : Island, Korea. Saito Ho-on Kai Spec.Pub. (Prof. T. Kota- A Review. Jour. of Geogr., 98 (3): 350-365* ka Commem.Vol.): 539-545, 5 pls. Tamanyu, S. (1990) The K-Ar ages and their strati- Yoon, S. H. and Chough, S. K. (1990) The Seoguipo graphic interpretation of the Cheju Island volcanics, Formation, Cheju Island, Korea. Field Excursion Guide, Korea. Bull. Geol. Surv.Japan, 41 (10): 527-537* The Hallim Publishers Vail, P. R., Audemard, F., Bowman, S. A., Eisner, P. N. You, H. S., Koh, Y. K. and Kim, J. Y. (1986) A study and Perez-Cruz, C. (1991) The stratigraphic signa- on the nanno fossils from the Seoguipo Formation, tures of tectonics, eustasy and sedimentology an Over- Cheju Island, South Sea of Korea. Thes. Coll. Chonnam view. Einsele et al. (eds.) Cycles and Events in Stratigra- Univ., Natu. Sci., 31: 127-136* phy: 955p, Springer-Verlag You, H. S., Koh, Y. K. and Kim, J. Y. (1987) A study Walker, R. G. and James, N. P. (1992) Facies model: on the micropaleontology and sedimentary petrology of response to sea level change. Geol. Asso. Canada, 409p the Seoguipo Formation, Cheju. Thes. Coll. Chonnam Williams, D. F., Thunell, R. C., Tappa, R., Rio, D. and Univ., Natu. Sci., 32: 23-36* Fafi, I. (1988) Chronology of the Pleistocene oxygen Yun, S. K., Han, D. S. and Lee, D. Y. (1987) Quater- isotope records : 0-1.88m. y. B. P. Palaeogeogr.,Palaeocli- nary Geology in the Southern Part of Jeju Island. Re- matol., Palaeoecol.,64: 221-240 port of the Korea Institute of Energy and Resourcev,KE- Won, C. K. (1976) Study of petro-chemistry of volcanic 86-2-(B)-2* rocks in Jeju Island. Jour. Geol. Soc. Korea, 12 (4): * in Korean or Japanese with English abstract. 207-226*

韓国済州島,下部更新統西帰浦層の堆積相と古環境

姜淳錫*

要旨

西帰浦層は,韓国済州島の西帰浦市の南部海岸にそっな海退期(堆積組相B～H),海進礫岩を伴う海進期(堆て分布し,中期更新世の溶岩流によっておおわれている. 積組相I～K)に分けられる.急速な海進期は,外浜のこの層は前期更新世に相当する海成層である.本層は, 砂質堆積物からなり,チャネル性貝化石帯を含む.漸進日本の大桑-万願寺動物群に対比される軟体動物化石を的な海退期は,陸棚から前浜システムによって支配され含み,浅海を指標する生痕化石と特徴的な堆積構造で構ていたと考えられる.これは,内側陸棚からはじまり下成されている.堆積物は主に明灰色の細粒-粗粒砂岩, 部外浜,上部外浜,前浜まで,典型的な前進する海岸線泥質砂岩,泥岩,火山灰,火山礫岩からなっている. システムで堆積された堆積物であると推定される.反面, 堆積相解析によって9個の堆積相と6個の生物相が海進礫層を伴う海進期は,主に内湾システムによって支認定され,さらにこれらの組み合せによって11個の堆配されていたと考えられる.これは,貝化石帯に代表さ積組相が識別できた.堆積組相が示す堆積環境の解析にれる外浜の堆積物とシルト質火山灰の内湾性堆積物からよると,西帰浦層は海進海退に伴う更新世の氷河性海水なる.最後は,活発な火山活動によって供給された火山準変動を反映する浅海から内湾の堆積環境を示している灰により,堆積盆地は内湾の中央部で急速に埋積されたと推定される. と推定される. 西帰浦層の堆積過程は海進期(堆積組相A),漸進的 *新潟大学自然科学研究科環境科学〒950-21新潟市五十嵐二の町8050.

(現:済州大学校海洋学科〒690-756韓国済州道済州市我羅洞1).