Stratigraphy, petrochemistry and Quaternary depositional record of the Songaksan tuff ring, Jeju Island, Korea

Journal of Volcanology and Geothermal Research 119 (2002) 1^20 www.elsevier.com/locate/jvolgeores

Stratigraphy, petrochemistry and Quaternary depositional record of the Songaksan tu¡ ring, Jeju Island, Korea Y.K. Sohn a; , J.B. Park b , B.K. Khim c , K.H. Park d , G.W. Koh e a

Department of Earth and Environmental Sciences, Gyeongsang National University, Chinju 660-701, South Korea b U.S. Army Corps of Engineers, Far East District, Seoul 100-195, South Korea c Department of Marine Science, Pusan National University, Pusan 609-735, South Korea d Korea Institute of Geoscience and Mineral Resources, P.O. Box 111, Taejon 305-350, South Korea e Water Resources Development O⁄ce, Jeju Provincial Government, Jeju 690-170, South Korea Received 5 October 2001; accepted 19 February 2002

Abstract The Songaksan tuff ring (STR) is one of several recent hydrovolcanic centers on Jeju Island, Korea, which provides an excellent example of proximal-to-distal facies changes in wet pyroclastic surge deposits. A multidisciplinary study has been carried out on the STR and adjacent lithostratigraphic units to constrain absolute age, geochemical characteristics, and Quaternary depositional history. A number of rock units were identified inside the crater of the STR, including Scoria deposit I, trachybasalt lava, Scoria deposit II, and a late-stage basaltic tuff, indicative of a rather complex sequence of magmatic and phreatomagmatic eruptions after the construction of the tuff ring. Petrochemical analysis shows that the STR was generated from different magma batches that fractionated from a homogenous magma chamber, and the early erupted magma was more evolved and volatile-rich. Reworking of the STR commenced shortly after the hydromagmatic eruption in a high-energy nearshore environment, resulting in deposition of the Hamori Formation. The formation is composed of planar-stratified and low- to high-angle crossstratified tuffaceous (pebbly) sandstones and occurs up to an altitude of about 4 m above present sea level. 14 C dating of molluscan shells beneath the formation indicates that it began to be deposited after about 4000 yr BP. Detailed sedimentary logging reveals that the formation consists of several stratal packages bounded by laterally persistent and distinct lithologic boundaries, probably formed by millennial-scale sea-level fluctuations. Occurrence of another hydrovolcanic sequence (the Sinyangri Formation) on the opposite side of Jeju Island, having similar sedimentary characteristics and ages, suggests that the sea-level fluctuations as seen in the Hamori Formation have affected a wide area of Jeju Island, probably related to the high-frequency sea-level oscillations during the post-6 ka BP regression period in the East Asian region. It can be concluded that the formation of the STR was possible because of the Holocene transgression, which made the present coastal areas water-saturated and adequate for hydrovolcanic eruptions. The STR in turn contributed to record high-frequency sea-level fluctuations during the Holocene via acting as a local and short-lived but affluent source of loose sediment. ; 2002 Elsevier Science B.V. All rights reserved. Keywords: Holocene sea-level change; hydrovolcanic sequence; Jeju Island; tuff ring * Corresponding author. Tel.: 82-55-751-6005; Fax: 82-55-757-2015. E-mail addresses: [email protected] (Y.K. Sohn), [email protected] (J.B. Park), [email protected] (B.K. Khim), [email protected] (K.H. Park), [email protected] (G.W. Koh).

0377-0273 / 02 / $ ^ see front matter ; 2002 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 7 - 0 2 7 3 ( 0 2 ) 0 0 3 0 2 - 5

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1. Introduction Hydrovolcanic centers, including tu¡ rings, tu¡ cones, and maars, are small, monogenetic volcanic craters that are second only to scoria cones in abundance on Earth (Cas and Wright, 1987; Thouret, 1999). They are produced by explosive interaction of magma and surface water, ground water, or wet sediment, and their evolution is principally controlled by the hydrogeologic factors around the vent regions such as the nature of the substrate and the abundance and type of external water (Dobran and Papale, 1993; Houghton et al., 1999; Kokelaar, 1986; Lorenz, 1986; Sheridan and Wohletz, 1981; Sohn, 1996; White, 1996; Wohletz, 1986). The products of hydrovolcanic eruptions can thus be good indicators of substrate lithology and the presence of underground aquifers, or of standing bodies of water, and can aid in paleoenvironmental interpretation of ancient volcanic successions (Sohn, 1996). Furthermore, hydrovolcanic eruptions result in a sporadic increase of sediment supply to nearby terrestrial or marine depositional systems

(White, 1991, 2001). Hydrovolcanic eruptions can thus create stratigraphic records in an otherwise sediment-starved volcanic ¢eld, and especially in a lava £ow-dominated area. Jeju Island is a major Quaternary volcanic ¢eld in Korea. The island, 73 km long and 31 km wide, is mainly composed of plateau- and shield-forming lavas and is covered with numerous (V360) monogenetic volcanic cones (Park et al., 2000c) (Fig. 1). Scoria cones are largely concentrated in inland or upland areas, whereas hydrovolcanic centers are distributed mainly along the coastal regions, suggestive of a causal link between the type of volcanic centers and the hydrogeological setting (Sohn, 1996). Major sedimentary formations on the island, e.g. the Sinyangri Formation (Han et al., 1987) and the Hamori Formation (Park et al., 2000a), are also composed of mainly reworked hydrovolcanic materials and found near the hydrovolcanic centers. This implies that the hydrovolcanic centers were the major sources of clastic sediments and played an important role in generating the Quaternary depositional records on the island.

Fig. 1. Geologic map of the southwestern part of Jeju Island (after Park et al., 2000a). Inset: Location of Jeju Island.

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In this paper, the authors present the results of a multidisciplinary study on the Songaksan tu¡ ring (abbreviated as STR hereafter) and a reworked hydrovolcanic sequence (the Hamori Formation) in the southwestern part of Jeju Island (Fig. 1), including geological mapping, petrochemical analysis, sedimentary logging, and 14 C age determination. This study supplements the previous sedimentological study on the tu¡ ring (Chough and Sohn, 1990) and provides new insights into the recent eruptive history and the late Quaternary depositional processes of Jeju Island.

2. Regional geology Jeju Island is a shield volcano that lies on the ca. 100-m-deep continental shelf o¡ the Korean Peninsula. The island is mainly composed of basaltic lava £ows and subordinate amounts of volcaniclastic sedimentary rocks (Park et al., 2000a; Park et al., 1998, 2000b). A variety of age estimations on the volcanic and sedimentary rocks show that the island has formed mostly since the late Pliocene (Khim et al., 2001; Won et al., 1986; Yi et al., 1998). Geochemical analyses suggest that the volcanic rocks are made up of oceanic-island basalt produced by plume-related hotspot magmatism (Lee, 1982; Park, 1994; Park and Kwon, 1993a,b). About 360 monogenetic volcanic cones cover the surface of Jeju Island, most of which are scoria cones formed by Strombolian or Hawaiian eruptions, whereas about 20 are tu¡ rings and cones formed by explosive hydrovolcanic eruptions (Sohn, 1996). The STR and the Hamori Formation, which are the youngest units on Jeju Island, occur in the southwestern margin of the island above an extensive and low-altitude, plateau-forming basalt lava (Fig. 1). Another tu¡ rings/cones and a lava dome also occur along the coastal region. The Dangsanbong Tu¡, the oldest stratigraphic unit in this area, includes the Yongmeori and the Dansan tu¡ rings/cones. They are composed of thinly and wavy-strati¢ed, basaltic lapilli tu¡ and tu¡ produced by explosive hydrovolcanic eruptions (Sohn, 1995). The Dangsanbong Tu¡ is overlain by the Sanbangsan trachyte dome (the Hallasan

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Trachyte; Park et al., 2000a), which has been dated to be ca. 0.8 Ma (Lee et al., 1988; Won et al., 1986). A sequence of block-and-ash £ow deposits, formed during the extrusion of the lava dome, covers the Yongmeori tu¡ ring. The tu¡ rings/cones and the lava dome are overlain by the Kwanghaeak Basalt (KB), which is characterized by abundant olivine phenocrysts and acicular feldspar laths and is tholeiitic andesite in composition (Park et al., 2000a). The basalt forms a lowrelief plateau, commonly shows tumuli and ropy structures (Park et al., 2000a), indicative of emplacement by pahoehoe lava £ows (Crown and Baloga, 1999; Rowland and Walker, 1990). The basalt is overlain by the STR, which is in turn overlain by the Hamori Formation (Fig. 1). The formation is a several meter-thick, shallow marine sedimentary formation composed of mostly basaltic tu¡aceous sandstone.

3. The Songaksan hydrovolcanic center 3.1. Tu¡ ring The STR is located at the southwestern margin of Jeju Island (Fig. 1). Its rim beds are up to 80 m thick and extend northward and northwestward for more than 2 km (Fig. 2). The tu¡ ring mainly comprises thin-bedded and gently dipping tu¡ with abundant megaripple bedforms as well as bedding sags and U-shaped channels (Chough and Sohn, 1990). The deposits are composed mainly of dark gray or brownish sideromelane ash and minor amounts of lapilli and blocks. A signi¢cant portion of the lapilli and blocks are angular crystalline basalt and sedimentary rocks derived from the underlying lavas and sedimentary sequences. On the basis of these accessory components and subsurface stratigraphy, Sohn (1996) inferred that the hydrovolcanic explosions excavated at least 300 m of the volcanic and sedimentary rocks below the pre-eruption surface. The proximal^distal facies relationships in the STR are described in detail by Chough and Sohn (1990). Lapilli tu¡ and tu¡ beds in the proximal part are generally crudely strati¢ed and rarely massive. In the medial part, thinly strati¢ed and

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Fig. 2. Detailed geologic and topographic map of the STR and the Hamori Formation. Locations of columnar logging (L1^L9), photographs (P1 and P2), and sampling sites for geochemical analysis (B1^B7) and 14 C dating (C14) are shown. Contours in meters and 5-m intervals.

wavy- or megaripple-bedded tu¡ is dominant. Mantle-bedded tu¡s with accretionary lapilli layers are common in the distal part. Chough and Sohn (1990) interpreted these lateral facies changes as a result of waning pyroclastic surges whose particle concentration and suspended-load fallout rate decreased in a downcurrent direction. Later, Sohn (1996) suggested that the pyroclastic surges in the STR waned rapidly in £ow power and became wet based on the common occurrence of accretionary lapilli and plastered tu¡s in the distal part, general lack of internal truncation surfaces within pyroclastic surge bedsets, and rapid downcurrent thinning of surge deposits.

3.2. Scoria and lava complex A complex of scoria deposits and a ponded lava £ow is contained inside the 800-m-wide crater of the STR (Figs. 2 and 3). Along a sea cli¡ exposure, the tu¡ ring is overlain by a 1-m-thick deposit of scoria (Scoria deposit I; Fig. 3a). The scoria deposit is massive or crudely strati¢ed and consists of dark gray or reddish scoria and spatter, generally lacking ash-size matrix (Fig. 3b). Large clasts are commonly £attened parallel to the bedding plane and have a ropy and breadcrusted surface texture. The scoria deposit demarcates the cessation of phreatomagmatic eruptions

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Fig. 3. (a) Sea cli¡ exposure at P2, showing contact relationship between tu¡-ring deposits, Scoria deposit I, trachybasalt lava, and Scoria deposit II. (b) Close-up of the contact between dark gray Scoria deposit I and trachybasalt lava at P2. (c) Trachybasalt lava is several tens of meters thick at P1, and is overlain by Scoria deposit II and late-stage tu¡. Several tiny scoria cones are shown in the background. (d) Distal deposits of primary tu¡-ring deposits, overlying the KB and overlain by the Hamori Formation (HF) at locality B1. Tu¡-ring deposits = TR; Scoria deposit I = SC I; trachybasalt lava = TBL; Scoria deposit II = SC II; late-stage tu¡ = LT. See Fig. 2 for locations.

and the onset of magmatic (¢re-fountaining) eruptions in the STR. The scoria deposit is overlain by trachybasalt lava (Fig. 3a,b). The lava has a variable thickness, ranging between a few meters to several tens of meters, although the upper surface is relatively level. This suggests that the lava £ow was ponded inside the crater of the tu¡ ring and its thickness was controlled by the pre-eruption topography of the intracrater area.

A large scoria cone with a diameter of about 500 m and a group of tiny scoria cones ( 6 100 m in diameter), which are collectively designated as Scoria deposit II, overlie the trachybasalt lava (Fig. 3a,c). The large scoria cone shows £attened and agglutinated clasts of dark brown spatter and scoria along its inner crater wall, about 100 m wide. The tiny scoria cones occur in the south of the large scoria cone. They are composed of dark gray or reddish brown scoria and spatter,

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locally showing steeply inclined strati¢cation. Some of the tiny scoria cones appear to be paired and aligned along an inferred ¢ssure, which radiates from the crater of the large scoria cone (Fig. 2). The contact between Scoria deposit II and the underlying trachybasalt lava is irregular due to loading of lava by the scoria and intrusion of the lava into the scoria (Fig. 3a). This contact relationship suggests that the lava was still £uid during construction of the scoria cones, and the emplacement of the lava and the formation of the scoria cones occurred almost contemporaneously. Several meter-thick deposits of tu¡ and lapilli

tu¡ occur along the southeastern part of the intracrater area, overlying the trachybasalt lava and the Scoria deposit II (Fig. 3c). Constituent facies of the deposit are dominated by scour-¢ll-bedded and inverse-to-normally graded deposits (Chough and Sohn, 1990). These deposits suggest deposition from highly concentrated and turbulent proximal pyroclastic surges, indicative of rejuvenation of a hydrovolcanic eruption after the magmatic eruption that produced the lava and scoria cone complex. Possible causes and the location of the source vent of the hydrovolcanic eruption remain problematic.

Table 1 Trace element abundances (ppm) in the volcanic rocks at Songaksan area Sample No. Stratigraphic unit Rock type Sr Ba Zr Rb Y Nb Ni Co Cu Li Sc V Cr Zn Hf Th U Pb La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu (La/Yb)na a

B1 KB lava 379 128 147 7.3 19.1 18.8 195 67 58 6.9 20.9 172 266 108 3.7 2.1 0.4 1.7 15.2 31.6 4.2 18.8 4.8 1.7 5.0 0.7 4.3 0.77 2.0 0.3 1.55 0.21 6.61

B2 TR lapilli 609 441 257 43.9 19.8 45.8 85 731 58 9.1 14.2 138 85 117 5.3 5.8 1.4 3.3 35.3 71.5 7.9 30.8 6.2 2.2 5.6 0.8 4.4 0.78 2.0 0.3 1.52 0.20 15.73

B4 TR lapilli 650 441 263 44.2 20.7 47.1 70 224 52 8.5 13.2 136 64 118 5.6 5.7 1.4 3.0 36.7 74.5 8.3 32.1 6.8 2.3 6.1 0.8 4.7 0.80 2.2 0.3 1.69 0.21 14.72

B8 SC I scoria 632 467 282 47.8 21.2 48.6 107 140 49 9.4 12.8 129 64 119 6.0 6.3 1.5 3.2 38.9 79.0 8.7 33.6 6.8 2.4 6.1 0.9 4.9 0.87 2.1 0.3 1.65 0.23 15.93

B5 TBL lava 574 399 242 41.3 20.4 43.1 123 70 57 8.1 16.0 154 146 113 5.5 5.5 1.4 3.1 34.0 69.2 7.7 30.4 6.4 2.3 6.1 0.9 4.6 0.81 2.1 0.3 1.59 0.21 14.43

Normalized to chondrite abundances (Taylor and McLennan, 1985).

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B7 TBL lava 585 416 245 39.7 20.1 43.8 121 66 54 8.8 15.3 151 138 113 5.6 5.8 1.3 3.1 34.2 69.1 7.7 30.4 6.3 2.2 5.9 0.8 4.7 0.84 2.1 0.3 1.75 0.23 13.24

B6 SC II scoria 549 365 229 38.2 20.1 43.9 160 154 57 6.4 17.2 166 184 113 5.3 5.0 1.2 2.7 31.5 64.1 7.2 28.6 6.3 2.1 5.9 0.8 4.6 0.85 2.0 0.3 1.55 0.22 13.73

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4. Petrochemistry of the STR 4.1. Samples and methods Trace elements and Sr^Nd isotopic ratios were analyzed in order to evaluate the genetic relationship between the stratigraphic units in the Songaksan area. Representative whole-rock samples of unweathered juvenile lapilli, scoria, and lava were carefully collected from the KB and the STR. Trace element data, along with rare earth elements (REEs), were determined by inductively coupled plasma-atomic emission spectrometry and mass spectrometry at the Korea Basic Science Institute. The accuracy for trace elements is within O 5%, whereas that for REEs is better than O 10% determined by multiple analyses of the BHVO-1 and BIR-1 standards. Sr and Nd isotopic ratios were measured using VG 54-30 thermal ionization mass spectrometer at the same institute. The average 87 Sr/86 Sr value for the NBS987 standard determined was 0.710237 O 0.000007 (n = 14, 2c), and the average 143 Nd/144 Nd ratio for the La Jolla standard was 0.511843 O 0.000005 (n = 19, 2c). Further details on preparation techniques and analytical procedures are given in Park et al. (1995).

Fig. 4. Variation of transitional metal elements (Ni, V, Cr, Sc) and incompatible elements (Sr, Nb, Ba, Y, Rb, La) against Zr.

4.2. Results The abundance of trace and REEs and the Sr and Nd isotopic ratios are given in Tables 1 and 2. Trace elements vs. Zr variation diagrams show that the abundance of transitional metal elements such as Ni, Cr, V and Sc decreases, whereas that of LILE (large ionic lithophile elements) such as Sr, Ba, Rb, Nb, Y and LREE (light REE) increases continuously as Zr content increases Table 2 Sr and Nd isotopic ratios of the volcanic rocks at Songaksan area Sample No. 87 Sr/86 Sr

143

B1 B4 B5 B6 B8

0.512697 O 0.000012 0.512792 O 0.000014 0.512795 O 0.000016 0.512778 O 0.000017 0.512799 O 0.000013

0.705386 O 0.000011 0.704124 O 0.000011 0.704242 O 0.000011 0.704249 O 0.000010 0.704189 O 0.000012

Nd/144 Nd

(Fig. 4). The variation patterns of most trace elements generally show a simple fractional crystallization trend except for the KB, which higher Ni, Cr and lower LILE contents. The variations of trace element abundances illustrate that the rocks of the STR were a¡ected by crystal fractionation, especially of olivine (Ni and Cr) and clinopyroxene (Cr, V and Sc) (Fig. 4). In general, primary magmas in equilibrium with typical upper mantle mineralogies (olivine+orthopyroxene+clinopyroxene O garnet O spinel) have high Ni ( s 400^500 ppm) and high Cr ( s 1000 ppm) contents (Wilson, 1989). With regard to the Ni and Cr contents, the rocks of the STR and the KB are evolved, not primitive in composition. Chondrite-normalized (Taylor and McLennan, 1985) REEs patterns are generally parallel within a narrow range of REEs abundances (Fig. 5). As a generalization, total REEs abundances and the

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Fig. 5. Chondrite-normalized REEs patterns.

ratio of light to heavy REEs decrease in an ascending order of stratigraphy except for the KB, which has lower LREE contents than the units of the STR. The 87 Sr/86 Sr and 143 Nd/144 Nd ratios for the KB are 0.705386 and 0.512697, while those ratios for the STR are 0.704124^0.704249 and 0.512778^ 0.512799, respectively (Fig. 6). The Sr and Nd isotopic data for the STR and the KB fall within the range reported previously for other volcanic rocks in Jeju Island (Park et al., 1996).

1989). The variations in trace elements concentrations and ratios and the isotopic element ratios can be used for evaluating petrogenetic models. If magmas are a¡ected by crustal contamination and/or originate from heterogeneous magma sources, scattering and variation of compositional data should appear. The Sr^Nd isotopic element ratios (Fig. 6) and the Ba/La and Zr/Nb ratios (Fig. 7) for the rocks of the STR fall in a narrow range within the analytical uncertainty, suggesting that the rocks have originated from a homogeneous parental magma and the crustal contamination was not signi¢cant. For a small volume of parental basaltic magma to ascend from a mantle source to the crustal surface without structural and geodynamic e¡ects, the magma should be fractionated in the magma chamber. As discussed above, the rocks of the STR are evolved in composition and were a¡ected by crystal fractionation. Deep-level crystallization of clinopyroxene was postulated for the alkaline rocks in the northern part of Jeju Island (Park and Kwon, 1993a,b). Spinel^lherzolite xenoliths were reported from the lava of the STR (Yim et al., 2000). These studies suggest that the parental

4.3. Discussion A magma generally undergoes a variety of fractionation, mixing, and contamination processes en route to the surface, after a primary magma became segregated from its source region (Wilson,

Fig. 6. Plot of

87

Sr/86 Sr and

143

Nd/144 Nd.

Fig. 7. Plots of (top) (La/Yb)n and Zr/Nb and (bottom) Ba/ La and Zr/Nb.

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Fig. 8. Photographs of the Hamori Formation that overlies the distal deposits of the STR. (a) Low-angle inclined-strati¢ed sandstone of Unit I underlain by planar- and parallel-laminated primary tu¡ (T) and overlain by coarser-grained and high-angle cross-strati¢ed sandstone of Unit II in Log 4. The strati¢cation in Unit I is indicative of repetitive shallow scours and deposition in a swash zone. (b) Alternations of ripple cross-laminated siltstone and planar-strati¢ed sandstone in Log 7. (c) Symmetrical ripples with linear, parallel, and evenly spaced crestlines observed on the upper bedding plane of planar strati¢ed sandstone in Log 7. (d) Thinly strati¢ed pebbly sandstone of Unit II, erosionally overlying thin-laminated primary tu¡ (T) between logs 1 and 2. Unit II is overlain by decimeter-thick clayey layer (Unit IV), which is in turn overlain by massive coarse sand (Unit V). The sand of Unit V contains some pebble-size fragments of underlying strata near the base. Scale arrow is 10 cm long. (e) Crossstrati¢ed pebbly sandstone of Unit II at Log 4, overlain by massive pedogenic layer of Unit IV. Note the di¡use contact between the units. (f) Decimeter-thick set of cross-strati¢ed sandstone at Log 8 with countercurrent ripples at base of the set. The pencil for scale is 15 cm long.

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VOLGEO 2502 30-10-02 Cyaan Magenta Geel Zwart Fig. 9. Graphic logs of the Hamori Formation. The vertical position of each column is shown relative to the present sea level. See Fig. 2 for locations of the logs. The facies terminology is after Clifton et al. (1971) (see Fig. 10).

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magma of the STR has fractionated in the deep level of a magma chamber after segregation from the source, and the evolved magma ascended to the surface very rapidly (probably hours to days). The more evolved magma in the chamber with high magma volatility (mostly erupted as scoria cones) indicates a positive volatile pressure of the magma during its ascent (McDonough et al., 1985). The variations of trace element contents and chondrite-normalized REE patterns show that the lower stratigraphic units of the STR were more evolved, implying that the eruptive activities in the STR started from a compositionally more evolved magma and ended with a less evolved one. It is highly unlikely that a magma chamber beneath a volcano remains as a closed system. Instead, fresh batches of relatively primitive magma may be periodically injected into the chamber from the underlying mantle source, and this may trigger a contemporaneous volcanic eruption (Wilson, 1989). In summary, the overall petrochemical characteristics indicate that the magma of the STR is unrelated to that of the KB and the geochemical evolution of the STR from the tu¡ ring at the bottom to the Scoria deposit II at the top can be explained by a series of eruptions involving di¡erent magma batches from a homogenous magma chamber. The early erupted magma of the STR was more evolved and volatile-rich, probably increasing explosivity the during the early phreatomagmatic stage of the STR eruption.

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subrounded, compared with the generally angular grains in the primary tu¡-ring deposits. The sandstones are also depleted in ¢ne ash and enriched in quartz sand and fragments of molluscan shells. The deposits of the Hamori Formation show planar strati¢cation, low- to high-angle cross-strati¢cation, climbing ripple and trough (festoon) crosslamination, and countercurrent ripple cross-lamination (Fig. 8). Detailed logging of the Hamori Formation reveals that the formation can be divided into ¢ve units of strata, which are bounded by laterally persistent erosional surfaces and have distinct sets of lithofacies (Fig. 9). 5.1.1. Unit I This unit unconformably overlies the thinly laminated tu¡ of the STR. It is composed of planar to low-angle inclined-strati¢ed sandstones (Fig. 8a) locally with small scours ¢lled by subrounded basalt gravel between Log 3 and Log 4 (Fig. 9). Large pebble- to cobble-size clasts of laminated tu¡ are observed directly above the erosional lower contact of the unit. Between Log 5 and Log 6, angle-of-repose cross-strati¢ed pebbly sand deposits are common. Between Log 7 and Log 8, an assemblage of lithofacies, including climbing ripple cross-laminated siltstones (Fig. 8b), planar-strati¢ed sandstones, and low- to high-angle inclined-strati¢ed sandstones, are observed. The planar-strati¢ed sandstones are commonly capped by symmetrical ripples that have linear, parallel, and evenly spaced crestlines (Fig. 8c).

5. The Hamori Formation 5.1. Sedimentary characteristics The thinly strati¢ed distal facies of the STR are unconformably overlain by reworked volcaniclastic deposits of the Hamori Formation (Fig. 3d). These deposits are generally less than a few meters thick and occur along the coast both to the east and west of the STR (Park et al., 2000a) (Fig. 1). The reworked deposits are composed of basaltic pebbly sandstone, basaltic tu¡aceous sandstone and siltstone. Individual grains of basaltic sand and gravel in this formation are subangular to

5.1.2. Unit II This unit occurs more landward than Unit I and up to about 4 m above present sea level (Fig. 9). The unit unconformably overlies the thinly laminated primary tu¡ of the STR between Log 1 and Log 2 (Figs. 8d and 9). Here, the unit is dominantly composed of planar- to low-angle inclined-strati¢ed pebbly sandstones. Between Log 3 and Log 4, the unit is composed of decimeter-thick sets of angle-of-repose cross-strati¢ed pebbly sandstones (Fig. 8e), locally containing large pebble- to cobble-size, subrounded basalt clasts. Between Log 5 and Log 7, the unit is again

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dominated by planar- to low-angle inclined-strati¢ed sandstones. The sandstones here are, however, ¢ner-grained than those of Log 1 and Log 2, and are mostly devoid of pebble clasts. To the west of Log 8, large-scale cross-strati¢ed sandstones are prominent, some of which have countercurrent ripples at the base of the cross-strati¢ed sets (Fig. 8f). The cross beds mostly dip eastward. The east^westward facies change in Unit II is very similar to that of Unit I. Unit II is, however, distinguished from Unit I by a laterally persistent erosional contact, its coarser grain size (more abundant pebble gravel between Log 1 and Log 5) and eastward or landward shift of overall facies (Fig. 9). 5.1.3. Unit III Unit III is thickest in Log 6 and pinches out on both sides, having an overall concave-up lenticular geometry with deep erosion into the underlying unit (Fig. 9). It is mainly composed of largescale trough-cross-strati¢ed pebbly sandstones. 5.1.4. Unit IV Unit IV consists of dark gray or brown, clayey materials containing altered basaltic lapilli, whose size and content decrease upward and westward (Figs. 8d,e and 9). The lower contact of the unit is locally sharp (Fig. 8d), but is generally di¡use and gradational (Fig. 8e) as the upper parts of units II or III were fragmented prior to deposition of Unit IV.

high-energy nearshore environment during and after the eruption of the STR. A variety of cross-strati¢cation and the scarcity of mudstone interlayers and muddy matrix in the sandstones indicate continuous action of waves and currents above a fair-weather wave base. Lateral facies changes in units I and II are interpreted to record zoned wave activities in a high-energy nearshore environment. The planarto low-angle inclined-strati¢ed (pebbly) sandstones (Unit I in logs 3 and 4 and Unit II in logs 1^3; Fig. 9) are most likely indicative of swash on a beach (foreshore) environment (Clifton, 1969), corresponding to the ‘inner planar facies’ of Clifton et al. (1971) (Fig. 10). The smallto medium-scale (several cm- to decimeter-thick), angle-of-repose cross-strati¢ed deposits adjacent to the inner planar facies (Unit I in logs 5 and 6 and Unit II in logs 3 and 4) are interpreted to have formed by a series of ripples and dunes. Overall seaward-dipping cross-beds indicate that these bedforms migrated towards the sea. The cross-strati¢ed deposits are interpreted to correspond to the ‘inner rough facies’ produced between the surf zone (upper shoreface) and the swash zone (foreshore) (Clifton et al., 1971) (Fig. 10). The mainly planar-strati¢ed deposits to the west of the inner rough facies (Unit I in logs 7 and 8 and Unit II in logs 5^7; Fig. 9) probably represent ‘outer planar facies’ of Clifton et al.

5.1.5. Unit V Unit V is composed of massive or very crudely strati¢ed sand, up to several meters thick. The sand is moderately to well sorted, coarse- to very coarse-grained, and basaltic in composition. Individual grains are subrounded to rounded. The lower contact is sharp and laterally persistent, but generally non-erosional (Fig. 8d). Some pebblesize clasts of underlying units occur near the lower part of the unit. 5.2. Depositional environment The overall sedimentary characteristics suggest that the Hamori Formation was deposited in a

Fig. 10. Zonation of wave activities, substrate bedforms, and sedimentary structures near a high-energy nearshore environment (after Clifton et al., 1971).

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(1971), lying between the outer portion of the surf zone (upper shoreface) and the inner portion of the wave build-up zone (middle shoreface). The outer planar facies is distinguished from the inner planar facies by its ¢ner grain size and common intercalation of ripple cross-laminated layers. Clifton et al. (1971) report that sand transport on the outer planar facies is dominantly by sheet £ow over a planar surface, and small ripples may form over the planar surface when landward surges wane. The large-scale (more than several decimeterthick), cross-strati¢ed deposits to the west of the outer planar facies (Unit II in logs 8 and 9; Fig. 9) were probably formed by large-scale bedforms (dunes or bars) in the outer rough facies zone, which develops seaward from the outer planar facies and beneath the outer portion of the wave build-up zone (middle to lower shoreface). The bedforms in the area migrate landward, resulting in large-scale cross-strati¢cation with foresets dipping landward (Clifton et al., 1971). The crossstrati¢cation in Log 8 and Log 9 with landwarddipping foresets and seaward-migrating countercurrent ripples (Fig. 8f) formed probably by landward migration of longshore bars or dunes in the middle to lower shoreface. The lenticular unit composed of large-scale trough cross-sets (Unit III) formed probably in a runnel developed across a shoreface environment, where a variety of bedforms, bars, and runnels form by longshore and onshore currents (Walker and Plint, 1992). Unit IV is interpreted to be a pedogenic layer (paleosol) formed after the underlying units were subaerially exposed. Fragmented nature and reddish or brownish color of the topmost parts of units II and III suggest subaerial exposure, oxidation, and subsequent fragmentation and pedogenesis of the units. The well-sorted and rounded coarse sand of Unit V is similar to beach sand in grain size characteristics, but the lack of planar strati¢cation as well as any obvious sedimentary structures negates the possibility of deposition in a foreshore to shoreface environment. The unit represents more likely a backshore or supratidal beach ridge deposits that have formed above mean high-tide level by storm waves.

13

5.3. Relative sea level Occurrence of marine-reworked deposits up to about 4 m above the present sea level indicates that there was a period of relative sea level higher than the present after the eruption of the STR. Furthermore, the facies organization in the Hamori Formation suggests that there were at least two cycles of relative sea-level rises and falls. The distal facies of the STR directly beneath the Hamori Formation is composed of thinly laminated tu¡ with low-amplitude waveforms and interlayers of accretionary lapilli (Figs. 3d and 8). These deposits are interpreted as distal pyroclastic surge and co-surge airfall deposits that accumulated on a subaerial setting (Chough and Sohn, 1990). Superposition of Unit I above the subaerially emplaced tu¡ therefore indicates a rise of relative sea level after the eruption of the STR. The landward (eastward) facies dislocation in Unit II relative to Unit I indicates that the relative sea level rose further and the shoreline transgressed farther landward during deposition of Unit II. Marked grain size contrast and lateral persistency of the erosional contact between units I and II suggest that there may have been an erosional break between deposition of units I and II, possibly caused by a brief period of relative sealevel fall and erosion. Superposition of the paleosol (Unit IV) above the oxidized and fragmented topmost parts of units II and III clearly indicates that the nearshore area during deposition of units II and III was subaerially exposed and subject to pedogenic processes. Unit V, formed probably on a backshore or supratidal area, suggests that the relative sea level rose once again, but not much higher than the present sea level. 5.4. Absolute age Abundant molluscan shells, fragmented or intact, are present along the contact between the KB and the Hamori Formation. The shells are interpreted to have inhabited upon the basalt in a shallow marine setting and then buried by the volcaniclastic materials produced by the eruption of the STR. Radiocarbon dating of the shell speci-

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mens (a bivalve and an avalone) collected at locality C14 (Fig. 2) has been carried out to constrain the age of the Hamori Formation and the STR. The outer surface of the shell specimens were etched with weak (0.5 N) hydrochloric acid, rinsed with deionized water, and dried to remove any secondary carbonate. The shells were sampled with a dental drill along the growth axis to avoid e¡ects of seasonal variation. Carbonate powders were placed in 10-ml vacutainers and evacuated through the rubber stoppers using a hypodermic needle. The evolved carbon dioxide was reduced to graphite using hydrogen with a cobalt catalyst. 14 C/13 C ratios were measured by accelerator mass spectrometry at Seoul National University, and 14 C ages were determined following the conventions of Stuiver and Polach (1977). The radiocarbon ages of the shell specimens are about 4000 yr BP (Table 3). These radiocarbon dates are interpreted to demarcate the onset of deposition of the Hamori Formation and provide an upper age limit of the STR eruption. The time gap between the eruption of the STR and the onset of deposition of the Hamori Formation must have not been large because the duration of monogenetic hydrovolcanic eruptions is almost instantaneous on a geological time scale and the reworking processes commence almost contemporaneously with the eruptions (e.g. Kienle et al., 1980; Kokelaar and Durant, 1983; Moore, 1967; Thorarinsson et al., 1964; Waters and Fisher, 1971).

6. Discussion 6.1. Implications for recent eruptive history of Jeju Island Jeju Island is characterized by numerous volcanic cones, which cover the extensive, plateauand shield-forming basaltic lava £ows. Numerous

holes drilled to exploit ground water and hot springs reveal that these volcanic cones not only shape the surface landform of the island but also constitute an integral part of the subsurface volcanostratigraphic units (Koh, 1997; Koh et al., 1992, 1993). In spite of this importance, stratigraphic, geochronologic, and petrochemical studies on these volcanic cones have been generally rare and cursory, hampering proper understanding of the stratigraphy and eruptive history of Jeju Island. Tu¡ rings and cones, for example, have been grouped into a single formation (the Seongsan Formation) and were placed in a relatively old position in the stratigraphic classi¢cations (early Pleistocene), whereas scoria cones have been regarded as the product of the youngest (Holocene) eruption (Lee, 1982; Won, 1976) (Fig. 11). The volcaniclastic sedimentary formations, lying directly above the tu¡ rings/cones, were also attributed to middle to late Pleistocene (Lee, 1982; Won, 1976). It is only recently recognized that the eruption of the volcanic cones occurred almost continuously throughout the history of Jeju Island and the previous dichotomized stratigraphic classi¢cation of the volcanic cones is one of many misconceptions on the geology of Jeju Island (Yoon, 1997; Yoon et al., 1995). This study con¢rms that at least one of the hydrovolcanic cones on Jeju Island formed during the Holocene, and attendant volcaniclastic sedimentation occurred until very recent on Jeju Island. It is also highly probable that other hydrovolcanic edi¢ces that have fresh morphologies, such as the Suwolbong tu¡ ring (Sohn and Chough, 1989), the Ilchulbong tu¡ cone (Sohn and Chough, 1992), and the Udo tu¡ cone (Sohn and Chough, 1993), formed during the Holocene. Sedimentological study on the Ilchulbong tu¡ cone, for example, reveals that the sea level during the eruption of the tu¡ cone was almost

Table 3 AMS radiocarbon data for biogenic calcium carbonate samples from locality C14 Sample ID

Radiocarbon age (yr BP)

N13 C (x)

LAB code

Note

SAS-1 SAS-2

3900 O 100 4090 O 90

30.8 1.5

SNU00-216 SNU00-217

Bivalve Abalone

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15

Fig. 11. Stratigraphic classi¢cations of the lithologic units in Jeju Island. Stratigraphic units in the study area are shown on the right for comparison.

identical to that of the present (Sohn and Chough, 1992). 14 C dating of molluscan shells from the Sinyangri Formation, which is the reworked product of the Ilchulbong tu¡ cone, gives an age of 4780 O 60 yr BP (Sameshima et al., 1988) and 4400^1570 yr BP (Kim et al., 1999). It can be concluded therefore that explosive hydrovolcanic eruptions and attendant volcanogenic sedimentation were active until very recent on Jeju Island, necessitating modi¢cation of ideas on Jeju volcanism and stratigraphy. 6.2. Hydrovolcanic sequences as a sensitive record of depositional environment Sedimentary sequences composed of reworked tephra from hydrovolcanic cones are common because hydrovolcanism occurs mainly in aggrading lowland settings that are typically wet, whereas sequences produced by reworking of scoria cones are rare, in spite of their abundance, because they

form mostly in upland settings where the preservation potential is relatively low (White, 1991). This seems true for the monogenetic volcanoes on Jeju Island because major sedimentary formations on the island (e.g., the Hamori and the Sinyangri formations) as well as a subsurface sedimentary formation (the Seoguipo Formation; Khim et al., 2001; Yi et al., 1998) are hydrovolcanic sequences composed mainly of reworked hydrovolcanic materials. Hydrovolcanism is therefore interpreted to have played an important role in generating Quaternary stratigraphic records in Jeju Island. Distribution of several tu¡ rings and cones with fresh morphologies, presumably formed during the Holocene, along the present shoreline of Jeju Island suggests that the formation of these volcanic edi¢ces was possible because of the Holocene transgression, which made the present coastal areas water-saturated and adequate for hydrovolcanic eruptions. The hydrovolcanic edi¢-

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ces, in turn, contributed to record high-frequency sea-level £uctuations once they were built via acting as local and short-lived but a¥uent sources of loose sediment. Local oversupply of sediment could probably result in ‘forced aggradation’ in the nearshore area, where the degree of syn- and post-eruptive reworking is very high (cf., Ho¡meister et al., 1929; Thorarinsson et al., 1964). Coastal depositional systems with high sediment input are known to record sensitively the frequency and amplitude of relative sea-level (or lake-level) changes (e.g., Dominguez et al., 1987; Riggs et al., 2001). The Hamori Formation appears to provide an example of such depositional systems of hydrovolcanic origin, and which is an excellent record of both the eruptive activity of a

hydrovolcanic center and the changes in surface environment, especially, the recent sea-level £uctuations. 6.3. Causes of relative sea-level £uctuations in the Hamori Formation Jeju Island has been under an ENE^WSW compressional tectonic regime since the beginning of the eruption at about 2 Ma (Park et al., 2000c). The compressional deformation is interpreted to have caused regional uplift of the island and adjacent areas during the Quaternary. An extensional tectonic event with a direction of ENE^WSW has been suggested as the most recent tectonic activity on Jeju Island (Hwang et al., 1994). The

Fig. 12. Simpli¢ed geologic and topographic map of the eastern margin of Jeju Island, showing the morphology of the Ilchulbong tu¡ cone and the distribution of the Sinyangri Formation. Altitudes shown in meters and contours shown in 5-m intervals. The inset map shows the location of both the Ilchulbong tu¡ cone and the STR.

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in£uence of these tectonic events on the Holocene sedimentation is dubious because the former, long-term uplift tectonics cannot account for the sea-level £uctuations with meter-scale amplitudes and millennial-scale frequencies as seen in the Hamori Formation. The latter event also appears too localized. Occurrence of another hydrovolcanic sequence (the Sinyangri Formation), showing several similarities to the Hamori Formation, in the easternmost margin of Jeju Island (Fig. 12) suggests that the £uctuations in relative sea level as recorded in the Hamori Formation were not local. The formation occurs up to about 3 m above present sea level and is composed of basaltic sand and gravel derived mainly from the Ilchulbong tu¡ cone. The formation is interpreted as having been deposited in a foreshore to upper shoreface environment under the in£uence of late Quaternary sea-level £uctuations (Han et al., 1987). Radiocarbon dates of molluscan shells from the formation also gives an age of 4780 O 60 yr BP (Sameshima et al., 1988) and between 4400 and 1570 yr BP (Kim et al., 1999). The formation is overlain by a thin, dark gray paleosol layer (the Dongnam Paleosol ; Yun et al., 1987) and modern beach and aeolian dune sands similar to Unit IV and Unit V of the Hamori Formation. The overall similarity in sedimentary characteristics, stratigraphy, age, and altitude of the marine limit of the Sinyangri and the Hamori formations suggests that the relative sea-level changes a¡ected a wide area of Jeju Island. A number of studies of recent sea-level changes in the East Asian region suggest that the sea level rose rapidly from 3130 m at about 18 ka BP to a few meters above present sea level in the middle of the Holocene (around 6 ka BP), followed by a regression trend with high-frequency oscillations superimposed on it (Feng and Wang, 1986; Fujji and Fuji, 1967; Jo, 1980; Pirazzoli, 1991, 1996; Zhao et al., 1982) (Fig. 13). The Hamori Formation, formed after about 4 ka BP and of which the marine limit is about 4 m above present sea level, may therefore have recorded the higher sea levels during the middle to late Holocene. Two cycles of relative sea-level £uctuations recorded within the formation may have resulted from the high-fre-

17

Fig. 13. Sea-level £uctuations during the Holocene in the East Asian region.

quency sea-level oscillations during the post-6 ka BP regression period.

7. Conclusions A multidisciplinary study on the STR and its reworked product (the Hamori Formation) has been carried out. Detailed mapping of the intracrater area reveals that predominantly magmatic eruptive activity continued after the cessation of the phreatomagmatic eruptions that formed the tu¡ ring. Petrochemical analysis indicates that different magma batches from a homogenous magma chamber were involved. The early erupted magma of the STR was more evolved and volatile-rich. Reworking of the STR commenced shortly after the eruption of the STR in a high-energy nearshore environment, resulting in deposition of the Hamori Formation. 14 C dating and sedimentary logging indicates that the formation began to be deposited after about 4000 yr BP and was subject to a few cycles of millennial-scale sealevel £uctuations. Occurrence of another formation (the Sinyangri Formation) with similar sedimentary characteristics and ages on the opposite side of Jeju Island suggests that the sea-level £uctuations recorded in the Hamori Formation are not local but can be related with the high-frequency sea-level oscillations during the post-6 ka BP regression period in the East Asian region. This result suggests that explosive hydrovolcanic eruptions and attendant volcanogenic sedimenta-

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tion were active until very recent on Jeju Island, and the formation of the STR was possible because of the Holocene transgression, which made the present coastal areas water-saturated and adequate for hydrovolcanic eruptions. It is highly probable that other hydrovolcanic centers with fresh morphologies [e.g., Suwolbong tu¡ ring (Sohn and Chough, 1989) and Ilchulbong tu¡ cone (Sohn and Chough, 1992)] formed also during the Holocene. These hydrovolcanic edi¢ces resulted in deposition of reworked hydrovolcanic sequences in generally sediment-starved, lava£ow-dominated areas via acting as local and short-lived but a¥uent sources of loose sediments. The hydrovolcanic sequences preserved high-resolution records of sea-level changes because of their high sedimentation rates. The STR and the Hamori Formation provide a good example of intimate feedback between Quaternary sea-level £uctuations and hydrovolcanism in coastal areas (cf., McGuire et al., 1997).

Acknowledgements This study was supported by Korea Research Foundation Grant (KRF-1999-015-DP0428). The authors would like to thank I. Skilling and S. Allen for their constructive review of the manuscript. The authors also thank logistical support from the Jeju Provincial Government during this study. All the help is gratefully acknowledged. References Cas, R.A.F., Wright, J.V., 1987. Volcanic Successions: Modern and Ancient. Allen and Unwin, London, 528 pp. Chough, S.K., Sohn, Y.K., 1990. Depositional mechanics and sequences of base surges, Songaksan tu¡ ring, Cheju Island, Korea. Sedimentology 37, 1115^1135. Clifton, H.E., 1969. Beach lamination ^ nature and origin. Mar. Geol. 7, 553^559. Clifton, H.E., Hunter, R.E., Phillips, R.L., 1971. Depositional structures and processes in the non-barred high-energy nearshore. J. Sediment. Petrol. 41, 651^670. Crown, D.A., Baloga, S.M., 1999. Pahoehoe toe dimensions, morphology, and branching relationships at Mauna Ulu, Kilauea Volcano, Hawaii. Bull. Volcanol. 61, 288^305. Dobran, F., Papale, P., 1993. Magma^water interaction in

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