Shallow-marine records of pyroclastic surges and fallouts over

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INTRODUCTION. Explosive volcanism provides a unique and locally abundant source of sediment to many of Earth's subaerial and subaqueous environ-.

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Shallow-marine records of pyroclastic surges and fallouts over water in Jeju Island, Korea, and their stratigraphic implications Article  in  Geology · July 2010 DOI: 10.1130/G30952.1

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Shallow-marine records of pyroclastic surges and fallouts over water in Jeju Island, Korea, and their stratigraphic implications Y.K. Sohn1 and S.-H. Yoon2 1

Department of Earth and Environmental Sciences, Research Institute of Natural Science, Gyeongsang National University, Jinju 660-701, Republic of Korea 2 Faculty of Ocean Science, Marine and Environmental Research Institute, Jeju National University, Jeju 690-756, Republic of Korea

SITE CHARACTERISTICS Jeju Island is a shield volcano produced by alkalic volcanism associated with decompression melting of the shallow asthenosphere (Choi et al., 2006). The volcanism occurred throughout the Quaternary and continued into historic times (Koh, 1997). The island was constructed upon the ~100-m-deep continental shelf in the southeastern Yellow Sea (Fig. 1A)

DEPOSIT CHARACTERISTICS The Seoguipo Formation is exposed at only one locality along the south-central coast of the island (Fig. 1B). The outcrop section comprises ten fossil-bearing and four fossil-free units with sharp boundaries (Fig. 2A). The former are composed of quartzose sand and mud as well as rounded basaltic gravel and sand. They contain abundant shallow-marine fossils, including molluscs, brachiopods, foraminifera, ostracodes, sponges, corals, barnacles, echinoids, and bryo-

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where tidal currents have been one of the most important agents of sediment deposition, erosion, and resuspension in association with global sea-level fluctuations during the Quaternary (Liu et al., 1998; Shinn et al., 2007). Extensive fields of tidal sand ridges have formed in places under the combined effects of tidal currents and fluctuating sea levels, including the Jeju area. Jeju Island is thus floored by continental shelf sediment composed of quartzose sand and mud with marine microfossils (Koh, 1997; Sohn and Park, 2004). Above the shelf sediment lies the Seoguipo Formation, ~100 m thick (Koh, 1997), which is composed of numerous superposed phreatomagmatic volcanoes (tuff rings and cones) and their remnants intercalated with marine or nonmarine, volcaniclastic or nonvolcaniclastic deposits (Sohn et al., 2008). The assemblage suggests that hydrovolcanic activity and attendant volcaniclastic sedimentation prevailed in the Jeju area under the influence of fluctuating Quaternary sea levels, before effusion of the shield-forming lavas.

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INTRODUCTION Explosive volcanism provides a unique and locally abundant source of sediment to many of Earth’s subaerial and subaqueous environments (Fisher and Smith, 1991), resulting in a wide range of volcaniclastic deposits (White, 2000). Moreover, rapid input of abundant volcanic detritus can result in sedimentary responses not encountered in nonvolcanic areas where the sedimentation rates either are relatively low or do not fluctuate so rapidly (e.g., Manville, 2002). In this paper, we introduce a unique volcaniclastic deposit from Jeju Island, Korea (Fig. 1), which we interpret as having resulted from syneruption reworking of volcanic detritus derived from pyroclastic clouds passing over water in a shallow sea. Here we show that the deposit accumulated in a very short period of time with extremely high sedimentation rate and could record the ordinary or fair-weather processes in the depositional site, which couldn’t be recorded by nonvolcaniclastic deposits. This finding illustrates that significant information on paleoenvironmental conditions can be retrieved from such volcaniclastic deposits in addition to the nature of past volcanic eruptions.

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ABSTRACT Explosive volcanism results in a wide range of volcaniclastic deposits in many of Earth’s subaerial and subaqueous environments. In this paper, we introduce a unique, shallow-marine volcaniclastic deposit from Jeju Island, Korea, for which the materials were transported to the water surface by pyroclastic clouds and then settled from the surface as they were entrained in the water. The deposition occurred under alternating currents and still waters, which is most plausibly attributed to tidal processes. Mud flasers or drapes intercalated in the deposit, which indicate periods of slack water during tidal cycles, suggest that the deposit accumulated in a very short period of a fortnight or a month, about a million times faster than the adjacent sedimentary strata. Because of the unusually high sedimentation rate, the volcaniclastic deposit could record the “usual” fair-weather processes in the depositional site at a resolution that is almost never provided by ordinary sedimentary deposits. This finding highlights the biases in Earth’s stratigraphic records and teaches us that volcanic deposits, commonly regarded as the products of catastrophic events, can in some cases record more faithfully the ordinary and usual processes that nonvolcanic deposits cannot.

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Figure 1. Physiographic setting and outcrop location. A: Bathymetry and the distribution of tidal sand ridges (elongate black bars) in the southeastern Yellow Sea (after Shinn et al., 2007). The tidal sand ridges are now moribund but were generated when sea level was lower than at present. B: Outcrop location of the Seoguipo Formation exposed beneath a 0.4 Ma hawaiite lava. Bathymetric and topographic contours are in meters.

zoans as well as prolific trace fossils. Sedimentary structures in these units suggest deposition in storm-dominated shoreface to outer shelf environments (Yoon and Chough, 2006). In contrast, the fossil-free units (Units III, IX, XI, and XIII) are composed of fresh basaltic glass grains and pyroxene and olivine crystals, the former preserving delicate irregular margins in contrast to the generally rounded volcaniclasts in the fossil-bearing units (see Fig. DR1 in

© 2010 Geological Society of America. For permission to copy, contact Copyright Permissions, GSA, or [email protected] GEOLOGY, August 2010 Geology, August 2010; v. 38; no. 8; p. 763–766; doi: 10.1130/G30952.1; 3 figures; Data Repository item 2010206.

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B (m) Thin- and planar-bedded basaltic tuff 3.0 Bioturbated gravelly muddy sandstone

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Figure 2. Logs of the Seoguipo Formation and the volcaniclastic Unit IX. A: The Seoguipo Formation contains four volcaniclastic units (Units III, IX, XI, and XIII) that are devoid of fossils and bioturbation. B: Unit IX consists mainly of normally graded and ripple cross-laminated deposits, which we divided into 24 subunits. They have uniform thicknesses throughout the entire outcrop extent except for subunit i.

the GSA Data Repository1). These units thus have components and grain texture that are indistinguishable from those of the primary volcaniclastic deposits that form the phreatomagmatic volcanoes of Jeju Island (e.g., Sohn and Chough, 1989; Chough and Sohn, 1990). They

are, however, characterized by an abundance of ripple cross-laminations and mud laminae (Fig. 2B), apparently indicating deposition in a shallow-marine environment. In addition, the sedimentary structures in these units are distinctly different from those of the under- and

overlying fossil-bearing units, in spite of their deposition in similar, if not identical, settings. Bioturbation is also absent. The paleocurrent direction is generally southeastward in contrast to the northeastward paleocurrent direction of the fossil-bearing units (Yoon and Chough, 2006). The depositional processes of the fossilfree units thus appear to be fundamentally different from those of the fossil-bearing units. In order to unravel the depositional processes, we examine the characteristics of the thickest (3 m thick) fossil-free Unit IX in more detail below. Unit IX is sharply under- and overlain by cross-stratified and partly bioturbated gravelly sandstone units containing abundant palm-size shells (Fig. 2A) that are interpreted to have been deposited in a shallow-marine skeletal bank (a gravel/sand bar containing abundant bioclastic debris) with local development of tidal channels or inlets (Yoon and Chough, 2006). We divided the unit into 24 subunits that are several centimeters to a few decimeters thick (Fig. 2B). They are commonly normally graded from very coarse sand or granule grade at the base to silt or mud at the top. The coarsegrained lower part of individual subunits is either normally graded (subunits c, h, j, k, q, and r), parallel-stratified (subunits b, s, t, and w), or low- to high-angle cross-stratified (subunits d, e, f, g, l, m, n, o, p, u, v, and x), whereas the fine-grained upper part is commonly ripple cross-laminated. Some subunits contain or are bounded by millimeter- to centimeter-thick mud flasers or drapes that have pinch-andswell and undulatory geometries. Mud laminae are scarce in the fossil-bearing units, but were found along 24 horizons within Unit IX. Normally graded divisions are massive or very crudely stratified and pass upward into ripple cross-laminated, very fine sandy to silty divisions (Figs. 3A–3C), mimicking the sequence of divisions in turbidites (Bouma, 1962). The normally graded division is, however, distinguished from that of typical turbidites in that the lower surface is mostly nonerosional and occasionally transitional (Figs. 3A and 3B). Evidence of erosion is also absent at the base of the parallel-stratified divisions, which have thin, continuous, parallel but not necessarily planar stratification, in some cases mantling the undulatory or rippled upper surface of the underlying subunit (Figs. 3D and 3E). The stratified divisions also contain a thin and continuous layer of accretionary lapilli with diffuse boundaries (Fig. 3F). Erosion surfaces are also rare in cross-stratified or ripple crosslaminated divisions, found only in some divisions composed of trough- or wedge-shaped sets (Fig. 3D). Ripples are generally climbing with deposition on the stoss side (Figs. 3A and 3C).

1 GSA Data Repository item 2010206, Figure DR1, photomicrographs of the volcaniclastic deposit of the Seoguipo Formation, is available online at www.geosociety .org/pubs/ft2010.htm, or on request from [email protected] or Documents Secretary, GSA, P.O. Box 9140, Boulder, CO 80301, USA.

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Figure 3. Deposit features of Unit IX. A: Normally graded and cross-laminated subunit k. Ripples are generally climbing with deposition on the stoss side and have only local and minor internal truncation surfaces. B: Transitional lower contact and faint internal laminations (arrows) near the base of subunit k indicate continuation of deposition across the subunit boundary without a break. Subunit j consists of three normally graded beds with a centimeter-thick mud drape in the middle part. It overlies the rippled and mud-draped upper surface of subunit i without erosion. C: A 55-cm-thick succession of normally graded and climbingripple cross-laminated deposits from subunits k to q (subunit q and the upper part of subunit p are not shown) lacks either erosion surfaces or mud laminae, suggesting pulsatory but continuous input of volcanic debris during a single phase of the tide. Mathematical and experimental analyses suggest that climbing-ripple beds can aggrade at a decimeter-perhour rate (Allen, 1971; Ashley et al., 1982), resulting in a several-decimeter-thick succession of climbing ripples during a single phase of the tide. Note that only an ~35-cm-thick portion of the succession is ripple cross-laminated, whereas the rest is massive. D: Parallel stratification in coarse-grained deposits is crude but laterally continuous with meager thickness variations. The cross-laminated upper division of subunit b is composed of wedge-shaped sets with intercalations of several mud flaser horizons, probably indicating reworking of volcanic debris for days during a pause of volcanic eruption. E: The parallel-stratified lower division of subunit s shows mantling geometry upon the rippled upper surface of subunit r without erosion. Some lapilli indent the fine-grained top of subunit r (arrows), suggesting fallout from suspension rather than emplacement from a bedload. F: Close-up of a thin but continuous accretionary lapilli layer within the parallel-stratified deposit of subunit t. The coin in A and B is 2.3 cm in diameter; the pencil in C and D is 14.5 cm long; the photo scale in E and F is graduated in centimeters.

TRANSPORT AND DEPOSITIONAL PROCESSES The deposit features suggest that the materials of the normally graded and parallel-stratified

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divisions were deposited from suspension with minimal tractional movement on the bed. The water column is therefore envisaged to have been still or sluggishly moving, and hence

unable to induce tractional movement of volcanic debris settling on the bed. The mud laminae also suggest suspension settling of fine grains when the water was still. On the other hand, the cross-stratified and ripple cross-laminated divisions are interpreted to have formed when there was sufficient movement of the water column to induce tractional movement of settling volcanic debris and/or entrain and move relatively fine-grained volcanic debris deposited on the bed. The overall rarity of erosion surfaces and the common occurrence of climbing bedforms suggest, however, that the traction sedimentation occurred in the presence of rapid suspension fallout (Ashley et al., 1982) and that the bed aggraded rapidly without either significant erosion or prolonged reworking of settled volcanic debris. Overall, Unit IX is interpreted to have formed by combined suspension and traction sedimentation under alternating currents and still waters, which is most plausibly attributed to a tidal regime. A number of features suggest that Unit IX was not deposited via reworking of a previously deposited volcaniclastic deposit, except for subunit i (Fig. 2B), which contains large intraclasts, indicating resedimentation by a mass flow. Unit IX as a whole is interpreted to represent a distal, shallow-marine equivalent of a tuff ring produced by a hydrovolcanic eruption in a nearby coastal area. An abundance of sideromelane ash compared with lapillus-size fragments suggests that the eruption generated mostly ash-laden and buoyancy-dominated eruption clouds and surges, which could travel over the sea (cf. Carey et al., 1996). The materials of Unit IX were probably transported to the water surface by these processes and then settled from the surface as they were entrained in the water. Delivery of the materials by these pyroclastic processes that can disperse volcanic detritus over wide areas is probably responsible for the lateral continuity and uniform thicknesses of individual subunits along the entire outcrop extent, except for subunit i, which thins rapidly westward. The majority of Unit IX can thus be termed a “primary volcaniclastic deposit” or “tuff” (sensu White and Houghton, 2006) in spite of the intervention of marine processes during its final deposition, because the deposit apparently did not involve interim storage of particles prior to arrival at the depositional site. DISCUSSION Hydrovolcanic eruptions forming tuff rings, tuff cones, and maars are known to be very short-lived, the periods of intense eruptive activity commonly being only days to months (Simkin and Siebert, 2000). Ancient hydrovolcanic deposits have therefore been assumed to have formed in a very short period of time. However, no attempts have been made to verify

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this assumption as far as we know because no dating techniques available at present are accurate enough to resolve the duration of such eruptions. We believe that Unit IX provides the first physical evidence for the short duration of an ancient hydrovolcanic eruption on an approximately quantitative basis. As explained above, the laterally persistent mud flaser horizons represent periods of slack water during tidal cycles. The total number of the horizons within Unit IX is thus interpreted to reflect the duration of the hydrovolcanic eruption. There is some uncertainty in correlating the number of the mud flaser horizons with the eruption duration, because not every slack-water episode may be recorded as a mud lamina in the deposit, and multiple mud laminae can form during a single slack-water episode because of pulsatory input of volcanic debris. Nevertheless, we consider that the number of the mud flaser horizons within Unit IX can be used to estimate the approximate eruption duration, because the overall deposit features indicate rapid bed aggradation generally without erosion and completely without bioturbation. Twenty-four mud flaser horizons in Unit IX thus appear to indicate that the main phase of hydrovolcanic activity ended in just a fortnight or a month, depending on the tidal cyclicity (diurnal or semidiurnal) and the tidal regime (flood- or ebb-dominated). Sedimentary structures in Unit IX and other volcaniclastic units of the Seoguipo Formation are distinctly different from the adjacent nonvolcaniclastic units in spite of their deposition in a presumably identical setting. The volcaniclastic and nonvolcaniclastic units are thus interpreted to have recorded different processes in an identical setting owing to the extreme difference in sedimentation rate and the type of sediment delivered. The 30-m-thick section of the Seoguipo Formation exposed was deposited from the latest Pliocene to the end of the Early Pleistocene over a million years (Yi et al., 1998; Kim and Lee, 2000). The sedimentation rate of Unit IX (3 m per 0.1 yr) is therefore estimated to have been a million times higher than the average sedimentation rate of the formation (30 m per 1 m.y.). The extremely high sedimentation rate probably preserved in Unit IX the imprint of daily tidal processes, which are inferred to reflect the dominant fair-weather conditions in the depositional site throughout the Early Pleistocene. On the other hand, only catastrophic or energetic but less frequent events such as storms are inferred to have been recorded in the nonvolcaniclastic or fossil-bearing units, for which storm waves and currents were suggested as the

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main agent of sediment erosion, transport, and deposition (Yoon and Chough, 2006). One of the main concepts of stratigraphy is that the greater the magnitude of an event, the less frequent its occurrence and the greater its effect (Doyle et al., 2001). In other words, the stratigraphical records are inevitably biased because they tend to be dominated by the records of rare and bigger events that erode or redeposit sediment accumulated by daily processes. The Seoguipo Formation attests to this dilemma by showing that its “ordinary” sedimentary units failed to record ordinary (fair-weather) processes but instead recorded rare, bigger, and extraordinary events. On the other hand, the volcaniclastic deposits that accumulated very rapidly show that the ordinary (fair-weather) processes can be read from these deposits. It is paradoxical that a volcanic deposit, commonly regarded as the product of catastrophic events, can in some cases record more faithfully the ordinary and usual processes that nonvolcanic deposits cannot. ACKNOWLEDGMENTS This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (No. 2009-0079427). We acknowledge discussions with J.D.L. White and S. Cronin and careful reviews by two anonymous reviewers. REFERENCES CITED Allen, J.R.L., 1971, Instantaneous sediment deposition rates deduced from climbing-ripple crosslamination: The Geological Society of London Journal, v. 127, p. 553–561, doi: 10.1144/GSL. JGS.1971.127.06.02. Ashley, G.M., Southard, J.B., and Boothroyd, J.C., 1982, Deposition of climbing-ripple beds: A flume simulation: Sedimentology, v. 29, p. 67– 79, doi: 10.1111/j.1365-3091.1982.tb01709.x. Bouma, A.H., 1962, Sedimentology of some flysch deposits: A graphic approach to facies interpretation: Amsterdam, Elsevier, 168 p. Carey, S., Sigurdsson, H., Mandeville, C., and Bronto, S., 1996, Pyroclastic flows and surges over water: An example from the 1883 Krakatau eruption: Bulletin of Volcanology, v. 57, p. 493–511, doi: 10.1007/BF00304435. Choi, S.H., Mukasa, S.B., Kwon, S.T., and Andronikov, A.V., 2006, Sr, Nd, Pb and Hf isotopic compositions of late Cenozoic alkali basalts in South Korea: Evidence for mixing between the two dominant asthenospheric mantle domains beneath East Asia: Chemical Geology, v. 232, p. 134–151, doi: 10.1016/j.chemgeo .2006.02.014. Chough, S.K., and Sohn, Y.K., 1990, Depositional mechanics and sequences of base surges, Songaksan tuff ring, Cheju Island, Korea: Sedimentology, v. 37, p. 1115–1135, doi: 10.1111/j .1365-3091.1990.tb01849.x. Doyle, P., Bennett, M.R., and Baxter, A.N., 2001, The key to Earth history: An introduction to stratigraphy: Chichester, John Wiley and Sons, 293 p.

Fisher, R.V., and Smith, G.A., 1991, Sedimentation in volcanic settings: Tulsa, Oklahoma, SEPM (Society for Sedimentary Geology) Special Publication 45, 257 p. Kim, I.-S., and Lee, D., 2000, Magnetostratigraphy and AMS of the Seoguipo Formation and Seoguipo Trachyte of Jeju Island: Journal of the Geological Society of Korea, v. 36, p. 163–180. Koh, G.W., 1997, Characteristics of the groundwater and hydrogeologic implications of the Seoguipo Formation in Cheju Island [Ph.D. thesis]: Pusan, Pusan National University, 326 p. Liu, Z.X., Xia, D.X., Berne, S., Wang, K.Y., Marsset, T., Tang, Y.X., and Bourillet, J.F., 1998, Tidal deposition systems of China’s continental shelf, with special reference to the eastern Bohai Sea: Marine Geology, v. 145, p. 225–253, doi: 10.1016/S0025-3227(97)00116-3. Manville, V., 2002, Sedimentary and geomorphic responses to ignimbrite emplacement: Readjustment of the Waikato River after the A.D. 181 Taupo eruption, New Zealand: Journal of Geology, v. 110, p. 519–541, doi: 10.1086/341596. Shinn, Y.J., Chough, S.K., Kim, J.W., and Woo, J., 2007, Development of depositional systems in the southeastern Yellow Sea during the postglacial transgression: Marine Geology, v. 239, p. 59–82, doi: 10.1016/j.margeo.2006.12.007. Simkin, T., and Siebert, L., 2000, Earth’s volcanoes and eruptions: An overview, in Sigurdsson, et al., eds., Encyclopedia of volcanoes: San Diego, California, Academic Press, p. 249–261. Sohn, Y.K., and Chough, S.K., 1989, Depositional processes of the Suwolbong tuff ring, Cheju Island (Korea): Sedimentology, v. 36, p. 837–855, doi: 10.1111/j.1365-3091.1989.tb01749.x. Sohn, Y.K., and Park, K.H., 2004, Early-stage volcanism and sedimentation of Jeju Island revealed by the Sagye borehole, SW Jeju Island, Korea: Geosciences Journal, v. 8, p. 73–84, doi: 10.1007/BF02910280. Sohn, Y.K., Park, K.H., and Yoon, S.-H., 2008, Primary versus secondary and subaerial versus submarine hydrovolcanic deposits in the subsurface of Jeju Island, Korea: Sedimentology, v. 55, p. 899–924, doi: 10.1111/j.1365-3091 .2007.00927.x. White, J.D.L., 2000, Subaqueous eruption-fed density currents and their deposits: Precambrian Research, v. 101, p. 87–109, doi: 10.1016/S0301 -9268(99)00096-0. White, J.D.L., and Houghton, B.F., 2006, Primary volcaniclastic rocks: Geology, v. 34, p. 677– 680, doi: 10.1130/G22346.1. Yi, S., Yun, H., and Yoon, S., 1998, Calcareous nannoplankton from the Seoguipo Formation of Cheju Island, Korea and its paleoceanographic implications: Paleontological Research, v. 2, p. 253–265. Yoon, S.-H., and Chough, S.K., 2006, Sedimentary facies and depositional environment of the Seoguipo Formation, Jeju Island: Journal of the Geological Society of Korea, v. 42, p. 1–17.

Manuscript received 10 December 2009 Revised manuscript received 24 March 2010 Manuscript accepted 26 March 2010 Printed in USA

GEOLOGY, August 2010

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