Characteristics of sediments and their dispersal systems along the shelf and slope of an active forearc margin, eastern Hokkaido, northern Japan Atsushi Noda a, Taqumi TuZino a a Geological Survey of Japan, National Institute of Advanced Industrial Science and Technology, Tsukuba, Higashi --, Ibaraki 5-8567, Japan Abstract This study revealed sediment distributions and their dispersal systems in a shelf slope setting along an active forearc margin off eastern Hokkaido, northern Japan, where the Pacific Plate is being subducted beneath the Okhotsk (North American) Plate at the 7 deep Kuril Trench. The findings were based on analyses of the textures and structures of surface sediments, components of the sampled sand fractions, high-resolution acoustic reflection records, and sediment budgets. The studied margin has a wide shelf and steep (5 ) slope marked by faults and folds associated with transpressional tectonics. The shelf sediments are classified into gravelly coarse to fine sand from the shelf edge to the uppermost slope (, m water depth), gravel and coarse sand on the shelf off Hanasaki, fine sand on the middle outer shelf off Kiritappu, and very fine sand on the shelf off. The coarse sediments on the eastern shelf off Hanasaki and the shelf margin are interpreted to represent lag deposits from coastal erosion associated with a post-glacial transgression across topographic highs. The two depocenters of shelf sediments subsequent to the Last Glacial Maximum are situated in the middle outer shelf off Kiritappu and in the inner middle shelf off ; these are interpreted as transgressive highstand sequences whose locations were controlled by ridges on the shelf margin and topographic highs on the shelf. The shelf sediment thickness reaches m, and the total volume is estimated at ca. 6. The slope sediments are grouped into medium fine sand on the uppermost and upper slopes (5, m water depth) and mud-dominated sediments on the upper to middle slope (,, m water depth). The sandy sediments on the slope at water depths greater than, m are regarded as being derived from the shelf break by gravitational redistribution. The occurrence of diamictite-like sediments around gullies that incise into the slope indicates that mass movement and sediment gravity flows play an important role in the redistribution of sediments upon the slope. On the upper middle slope, hemipelagic fallout and advection of turbid water from the shelf edge provide a supply of suspended material. The main contributor of sediment supply to the area is coastal erosion (.8 Mt y ), along with a lesser contribution from fluvial input (. Mt y ). During the latest Quaternary, the rates of sediment accumulation upon the shelf are estimated to have been.47 Mt y.the remainder of the material transported into the sea is probably redistributed to deeper parts of the slope via gravitational sediment flows or transported outside of the study area by alongshore and tidal currents. The studied shelf that kept less than a quarter of the mass from land is comparable with other active margin shelves of New Zealand and California, although they are fed by high sediment-laden rivers. Key words: sediments, shelf, slope, grain size, sand, composition, budget, Japan NOTICE: this is the author s version of a work that was accepted for publication in Sedimentary Geology. Changes resulting from peer review are reflected, but editing, formatting, and pagination from the publishing processes are not included in this document. A definitive version will be published in DOI:.6/j.sedgeo.7.7.. Corresponding author. Fax: +8 9 86 65. Email address: a.noda@aist.go.jp (Atsushi Noda). Preprint submitted to Sedimentary Geology 7 July 7
. Introduction Shelves and slopes at forearc margins are some of the most active regions in the world in terms of the erosion, transport, and deposition of sediment (e.g. Milliman and Syvitski, 99). Sediment dynamics in such settings interplay with the characteristic occurrence of faulting and folding associated with regional tectonics and the creation of accommodation space related to these structures, in addition to hydrographic conditions, sea-level change, and climate. Several studies have investigated sediment dispersal systems on tectonically active marginal shelves and slopes such as the California margin, USA (Nittrouer, 999; Edwards, ; Spinelli and Field, ; Sommerfield and Lee, 4), Papua New Guinea (Kineke et al., ; Walsh and Nittrouer, ; Keen et al., 6), and the northern Hikurangi margin of New Zealand (Lewis, 97; Foster and Carter, 997; Orpin, 4; Orpin et al., 6). Such studies have emphasized the importance of structural controls on the distribution of shelf slope sediments (Spinelli and Field, ; Orpin et al., 6): thick sediment packages accumulate in structural lows, whereas little or no sediment is deposited upon structural highs. In other words, the preservation of lowstand and transgressive deposits is controlled by structures or incised valley systems (e.g. Zaitlin et al., 994; Ercilla and Alonso, 996). Although comprehensive and quantitative studies are necessary in order to gain a better understanding of shelf slope sediment dynamics along active forearc margins, only limited areas have been well studied. The area of eastern Hokkaido, northern Japan, is an active forearc margin where the Pacific Plate is being subducted along the Kuril Trench (Fig. ). The purpose of this study is to reveal sediment characteristics and dispersal systems upon the shelf and slope quantitatively. This paper presents new information on the texture, volume, and accumulation rate of sediments upon the shelf and slope of the eastern Hokkaido forearc margin, based on granulometric and mineralogic analyses of surface sediments and high-resolution seismic records. The sediment budget and factors controlling the observed sediment-dispersal systems are then discussed. The observations reported in this study will serve as a useful tool in understanding the interactions among the supply, redistribution, and accumulation of sediments and sedimentary rocks upon shelves and slopes along tectonically active forearc margins.. Physical setting.. Geology The study area is situated on the eastern Hokkaido margin, which descends to the 7, m deep Kuril Trench (Fig. ). In this region, the Pacific Plate is being subducted beneath the Okhotsk (North American) Plate at about 8. cm y toward ca. N6 W (DeMets et al., 99, 994; Seno et al., 996), with eastern Hokkaido moving to the WSW at cmy (Ito et al., ) under the present tectonic regime. Oblique subduction of the Pacific Plate has caused dextral strike-slip movement within the Kuril forearc sliver (Fitch, 97). Although geodetic data reveal steady subsidence at average rates of 5 mm y over the past years (Shimazaki, 974; Kasahara, 975; Kasahara and Kato, 98), during the late Quaternary the area of eastern Hokkaido has risen slightly more than it has fallen. The uplift rate and accrued elevation over the past 5, years (Interglacial Stage 5e) are estimated to be.6.4 mm y and m, respectively, based on the elevations of coastal terraces (Okumura, 996). The rate of tectonic uplift continued to equal or exceed tectonic subsidence during the Holocene (Maeda et al., 99; Sawai, ; Atwater et al., 4). The uplift rate in this area is one or two orders of magnitude less than the general rate of increase in sea level over the period from the Last Glacial Maximum (LGM: ca. 8 ka) to the sea level maximum of about 6 ka (ca. m/ ky = mm y ). The coastal land area shown in Fig. is underlain by the Nemuro Group, which comprises a sequence of Cretaceous Eocene marine clastic and hyaloclastic rocks that total approximately, m in thickness. The marine clastic sequence is composed mainly of hemipelagic mudstone, turbidites, and submarine slump deposits (Kiminami, 98; Naruse, ). The hyaloclastic deposits in the lower part of the Nemuro Group are overwhelmingly intermediate mafic in composition, and the sedimentary rocks contain a large amount of detritus derived from these igneous rocks. Sandstones of the lower Nemuro Group are therefore rich in feldspar (5 8%), especially plagioclase, and rock fragments, while being quartz-poor (< %) (Kumon and Kiminami, 994). In contrast, sandstones of the upper Nemuro Group are relatively rich in quartz ( 6%) and K-feldspar (< %), and are characterized by additional sources of granitic rocks (Okada, 974; Kiminami, 979). These sequences form homoclinal structures that strike parallel to the coastline (east west) and
4 E 4 E 4 E 44 E 45 E 46 E N 45 44 N Japan Sea Okhotsk Plate Eurasia Plate Hokkaido (b) 4 Hokkaido 4 N (A) Tokyo Pacific Plate Philippine A Sea Plate 5 4 45 5 Kushiro River Kushiro Nemuro Hiroo KSC Survey area (Fig. ) 4 N 4 N (B) 5 7 4 5 7 5 6 Kuril Trench ca. 8 cm/y Fig.. (A) Index map of the survey area and location in Japan. (B) Bathymetry of the eastern Hokkaido forearc along the Kuril Trench. KSC, Kushiro submarine canyon. dip gently to the south. The Nemuro Group is overlain by the Kushiro Group and Kutcharo pyroclastic flow deposits. The former is composed of Pleistocene conglomerate and sandstone, while the latter consists of Pleistocene pumiceous rhyolitic pyroclastic deposits (Okumura, 99). These Pleistocene strata are partly covered by Holocene Mashu volcanic ashes (Katsui, 96)... Geography Pleistocene marine terraces are common along the Pacific coast in the present study area, forming cliffs as high as 8 m above the current sea level. The terrace heights indicate that the elevation of the paleo-shoreline gradually decreases eastward from to Hanasaki (Okumura, 996) (Fig. ). No rivers enter the shelf between Kiritappu and Hanasaki, except for very short ephemeral streams; however, short rivers ( in length) and estuarine mouths are found on the coast between and Kiritappu (Fig. ). The shelf has a steep and rocky coastline with a complex outline due to the presence of several capes, such as Cape Ochiishi, and numerous islets. The bathymetry is characterized by a moderate ( wide) shelf and steep (up to ) slope (Figs. and A). The shelf break off Hanasaki occurs in 5 m of water depth and deepens westward, reaching 7 8 m depth off (Fig. ). The shelf off is relatively
4 'N 4 'N 4 'N 4 5'N 4 4'N 44 5'E 45 'E 45 'E 45 'E Bay x 79 87 Hokkaido 8 y Fig. B 7 8 Lake 74 8 75 68 76 69 77 6 7 6 7 64 7 b 57 65 Kiritappu 58 66 Fig. A 5 59 67 5 6 5 6 45 46 54 Cape Ochiishi 55 9 47 56 4 48 4 45 'E 45 4'E 45 5'E 46 'E 46 'E 4 49 Nemuro Peninsula 4 5 4 5 6 6 44 Hanasaki 7 9 8 7 9 Fig. 8 4 4 5 4 x 5 6 6 5 7 7 y 8 8 b a 9 Fig.. Sampling localities of seafloor surface sediments and survey lines of the seismic and Parasound sub-bottom profiling records. Solid circles and open squares represent sampling points visited during Cruises GH (May June, ) and GH4 (July August, 4), respectively. Solid and dashed lines represent acoustic survey lines surveyed during Cruises GH and GH4, respectively. The transects labeled x x and y y show the locations of topographic cross-sections oriented parallel to the coast, while the transects a a and b b indicate cross-sections oriented perpendicular to the coast (Fig. ). Bold solid lines represent the Parasound sub-bottom and seismic profiles shown in Figs. and. gentle and has low relief, whereas the inner shelf off Hanasaki and Cape Ochiishi is more irregular due to the presence of topographic highs (line a a in Fig. B and C). The slope is subdivided into three parts in this study: the uppermost (shelf break to, m water depth), upper (,, m water depth), and middle (,, m depth) parts. The uppermost slope has the highest dip (average 5 6 ), reaching in places (Fig. A), whereas the middle slope is less steeply dipping ( ). No deep canyons cut into the outer shelf and slope; instead, a series of gullies runs down the uppermost and upper slopes, oriented perpendicular to the main trend of the shelf. The gullies are concentrated in the area off Cape Ochiishi, originating in the uppermost part ( 5 m) of the slope (Figs. and D)... Climate and oceanography The oceanography of the northwestern Pacific is dominated by the Oyashio Current, which is part of the western boundary current of the subarctic gyre (Ohtani, 99; Yasuda et al., 996). The present-day axis of the current lies within 4 4 5 N, outside of the study area. The Oyashio Current moves to the southwest at ca. 4 cm s (e.g. Isoguchi et al., 997; Ohshima et al., 5). The Off-Tokachi nearshore current (Sugiura, 956) also flows along the coastline toward the southwest or west-southwest (Fig. 4). The average and maximum velocities for this current are ca. 5 5 cm s and ca. cm s, respectively (Maritime Safety Agency, 987, 998, 999). The northeast southwest components in Fig. 4 represent local tidal currents. 4
(A) a b b a (B) VE : C N 5 cm/s B N 5 cm/s 5 cm/s A N W E Water depth (m) (C) (D) 5 x y a b 4 6 8 4 6 8 5 5 VE : 5º º VE : º.5º.º.º VE : VE : º º º 4 6 8 Distance () Fig.. Bathymetric profiles across the study area. (A) Profiles across the shelf slope along lines a a and b b. The dip of the uppermost slope exceeds in parts, and continues to a depth of more than, m. (B) Details of across-shelf profiles along lines a a and b b. Topographic highs occur off Cape Ochiishi (line a a ). (C) Along-shelf profile along line x x. The seafloor off Cape Ochiishi is the shallowest area, and deepens both eastward and westward. (D) Along-slope profile along line y y. Sets of gullies cut the slope from off Cape Ochiishi to off. The locations of all the cross-sections are shown in Fig... Methods In May and June (Cruise GH) and July and August 4 (Cruise GH4), 8 stations were visited on the Pacific side of eastern Hokkaido as part of a cruise undertaken by R/V Hakurei-maru No. (Fig. and Table ). The targeted area was 4 5 N 4 N and 44 45 E 46 5 E, with positions being determined using differential GPS. The cruise involved sampling surface sediment using a grab sampler, taking photographs of the sea floor, collecting sediment cores using gravity and piston corers, and measuring oceanographic data (temperature, conductivity, ph, ORP, DO, and turbidity). Oceanographic data for the bottom water were measured m above the seafloor, and the turbidity of the water column was measured at every sampling locality (69 points). The specifications of the equipment precluded measurements of turbidity at localities with water depth in excess of x y W W D E S 5 cm/s 5 cm/s 5 cm/s 4 ' 4 ' 4 4' 45 ' 45 ' E D C B 45 4' 46 ' A Maximum current velocity Average current velocity 5 cm/s Fig. 4. Directions and velocities of nearshore currents in August of 986 (A and B), July of 997 (C), and July of 998 (D and E). The current data were obtained from m below the sea surface. Modified from Maritime Safety Agency (987, 998, 999)., m. Seismic reflection profiles were collected during Cruises GH and GH4 using a GI gun (5 in generator and 5 in injector airgun) with a 6 ch streamer cable. The survey speed was generally 8 knots (4.8 h ) and the shooting interval was 6 sec (ca. 5 m). The grid spacing was miles (.7 ) E W and 4.5 miles (8. ) N S (Fig. ). Parasound sub-bottom profiles (Atras, USA) were obtained at the same time as the seismic profiles; these proved to be useful in analyses of sediment distribution (e.g. Damuth, 98; Kuhn and Weber, 99). The grab sampler used in this study typically recovered sediments from the upper 5 cm of sand or cm of mud, and in most cases preserved the stratigraphy to a satisfactory degree. The uppermost 5 cm of the sediments was removed for analyses of grainsize and sand composition. Grain-size analyses of materials coarser and finer than 4φ (6.5 mm) were made using dry sieve and hydrometer methods, with intervals of.5 and about.5φ, respectively. These grain-size data were combined to produce complete grain-size distributions. Textural parameters such as mean grain size, sorting coefficient, and skewness were calculated based on Folk and Ward (957). To study the structures within the sediment, soft X- radiographs were taken from slab subsamples of the grab samples using a Sofron Type STA-5 operated at a voltage of 45 kv, current of ma, and irradiation time of 9 s. The compositions of medium-sand (.75.φ) frac- 5
Table Sampling localities, grain sizes, and sand compositions of surface sediments surface sediments. St. Water Mn Srt Sk Weight percent of each size fraction (wt%) Sand composition of fine sand fraction (%) Longitude Latitude depth (ø) G VCS CS MS FS VFS Slt Cly Lm Hm Lf Vol Ben Plank Glauc Plant 45.87 4. 6 -.6.9. 5. 5.9. 7.9.. 6.7 6.4 4..7 46.4. 4.5... 45.85 4. 6.5.7 -.4..7 7.4 9. 6. 7..5...9.5..... 45.766 4. 6.54.78...5.7. 66.6.7.. 45. 6..5..... 4 45.8 4.4 9.8.86 -.5..6 5.4 5. 9.4 4... 6. 4.9 8.9 9.6.5... 5 45.8 4.67 7 6 45.867 4. 889.6.67 -.9 4.7.8 6.. 7.9 8. 6.8. 7.4 8..9.7.9... 7 45.97 4.9 89 4.6.77.68..4. 4. 7.8.7 9. 4.9.9. 5. 65.9.6.4.. 8 45.967 4.8 7 6.59.9.5.... 4.5. 54.9 6.4. 9.4.8.7.5.5.. 9 46.7 4.7 679 7.8..4...6.9 6. 9. 46. 7. 5.4.4 8. 66. 4.8.4..5 46.66 4.6 89 7.8.8.4.... 6.4 6.8 49.5 6.4 4.6.5 4. 8..4 5... 45.667 4. 45 45.7 4. 7.6. -..5. 7.6.7 46.5 4.7 5.9. 48.5 5.8 5.5 9..... 45.7 4.67 5.99.9.8..8 8. 4. 4.7.6..6 5.. 4..7.5... 4 45.767 4. 44.8.55..9.8 5.9.7 49.7 8.7 4. 4. 4..9 4.5 9.5.... 5 45.8 4.94 4 4.6.89.7... 6. 8.9 5. 4. 5.7..9...4... 6 45.85 4.8 86 6..97......7 8.8 45.5.4 6.9.8 6. 7..9.9..5 7 45.899 4.75 449 6.8..5..8. 4.4 7. 9. 45..6 47.4 9.5 9.4..9.8.. 8 45.95 4.6 686 6.6.46 -.6..9.4 5.. 8.4 4..6 5..9 8. 8..9... 9 45.6 4.4 48 45.64 4.67 8.9.7.8.4.6 6.7 6.4 9.8 9.7.. 54.9.7 4.6.7 4.... 45.667 4. 45.7 4.9 596.7.88 -.8 6.4.7 7.9 9.8 7.6 6. 4.6 4..8 7.6 7..9.4... 45.7 4.867 677 5..89.58..6.. 8.7 6..9 7.9 6.8..8 5..6.7..5 4 45.78 4.767 9 6..9.9....5 6.7 5.9 5.4 4.5 4.8. 5.7 84...9.. 5 45.8 4.667 587 6.6..8......6 45. 8..8.9 6.5 6..9.9.. 6 45.5 4.67 5.8. -.9 8.4 7..8 4.4 6..9.8.4. 9...8 7.7... 7 45.567 4. 95..9..7. 7.4 6.5 4. 5.... 7.7 55.9 5.9.5... 8 45.599 4.9 77.47..... 7. 6. 9.6 4. 4.4 4.9.7. 69.7.5... 9 45.69 4.866 74. 4.6.5 5. 4. 5. 6.. 8.5 8.. 4.5 6.4 9.8 6.8 5.6.9.. 45.668 4.8 8 5..76.5.... 7..5 9.6 6.7.8.8 4.6 6. 8..8.. 45.77 4.7 8 6..9.....5.6 8.6 48.5.8.5.4. 7.. 8... 45.766 4.6 7 6.4.85.8....7 5.6 4.8 5.5 4.4.5 7.5. 4.4 6.6.9.. 45.44 4.67 4 -.58.46 -.5.7 4. 4.4 5.6 6.4.4 4.5. 9..7.8.4 6.... 4 45.467 4. 8..7 -.5 4.5 8. 9. 6.4. 4..9.4 56.8 7.7.8..4... 5 45.5 4.94.5.58 -.9....6 64.5 4.6 4... 4.5 4.9.9.5... 6 45.55 4.8 58 4.8.46.69...7 4. 6.5. 4.9. 9.5.4. 65.6 8.6.8.5.5 7 45.6 4.74 9 5.7.78.4... 4..8.8 44. 7.4 6. 5. 8. 66.7.7... 8 45.65 4.6 7 6..95.5...5.4 8. 6.8 5.. 6.9.8.6 56..9.9.. 9 45. 4.66 8.87.5.7....6 6.8.7 5.4 6. 4..8..9 8.7... 4 45.66 4. 64.87.85.4...7. 59.4 9. 4.5 4..6. 4.4 5.6 8..5..7 4 45.99 4.94.4.87.5.8.5.4 6. 6.5.4.5.5.5 5. 8.5..... 4 45.4 4.867 6.6.4 -...7 4.8. 4.8 8..8 4.6..4 4. 7.4.... 4 45.48 4.767 566 6.4.7.4..... 8.6 45.4.9.4 8.6 5.7.5.4.4.. 44 45.5 4.667 4 6.87.68...... 8.8 6.6 7. 5.8 8.7 7.9 6.8.6 9.7..6 45 45.66 4. 54.98.9.7....4 5. 6.6 6. 4.5 5.9 4..5 57..9...5 46 45.99 4.9 9.77.4.....8 67.9.8 4...7.8 4.9 88.4.... 47 45. 4.867 54.54.84 -.54 4.8. 5.9.7 4. 8.9 4.4. 4.6.8 8.9 6.7.... 48 45.67 4.8 749.86.4.4.5.6.9.9.7 8.6 7. 4.6.6 5.. 4.4.9... 49 45.47 4.7 759 5.7.88.5...4..8 9.7 45..5 6.4.4 8.8 6. 4.8.4.. 5 45.465 4.599 97 6.9.4.4....4 5..8 5.4 9..8. 5. 8..7 9... 5 45.67 4. 46.97.96....6. 49.4 5. 7. 4..4. 8..9.9.4.. 5 45. 4.94 8.8.98.5....9 47.6 9. 7.6 4.5..4. 95..8... 5 45. 4.867.77.4.9.4..6 7.4 58.5.4 5.8 4. 7.7. 9. 8..... 54 45.67 4.8 459.7.88 -. 6.9 4.8 5.7..8 9.6 6..6 9.8 7. 5.7 7.4.... 55 45. 4.7 5..98.49... 5.4 4.6. 8.6 7.6. 8...6..7.. 56 45.49 4.64 976 6..9.4....8 8.6. 49.6 9.5 6.7..7 6.. 6... 57 45.98 4.9 6.67.44 -. 5.7 4. 4.5 8..6 9. 7.9 6.8 9..9 8.4. 5.... 58 45. 4.866.4.4.5...8. 46. 6. 9. 5..9. 5.7 7.5.5.5.. 59 45.67 4.8 48.46.4.6.. 9.7 4.8 9.4 5..9.6.8.8 4.8.6.... 6 45. 4.7 86.9..9.7.8 5.8.9 8..5.8 7. 55.8.7.8 9.8...9. 6 45.4 4.667 488 4.9..6.5.9. 5.7..9 4. 5.5 4.. 4. 5.9..8.. 6 45. 4.9 56..9.55.... 44.4 4.5 9..8.5.8.8 6.4 9.5.5..5 6 45. 4.867 9.6..6....4 4. 8.7.9 4.6.6.6. 59..8.8..8 64 45.67 4.8.4.65 -.6 5. 5. 6.4 8.5 4.8.6.8.6 4.6 6.7 4. 5....4. 65 45. 4.74 7.94.6 -.. 5. 4.5 8.6.7.4 4.6.. 7.4 9.6.6.4.9.. 66 45. 4.667 7 5..5.6..8.8 6.8. 5. 4..7 4..4..4.4.4.. 67 45.67 4.6 64 6.5.9.7...4. 4.8.7 5.. 4.. 4.6 7..6.4.. 68 44.9 4.867 87.8.87.6.... 4. 45. 7. 4. 6.. 5.7 87.4.9... 69 44.968 4.8 5.85..4.6.5. 7. 5.5 6.7 4.7 5.5.9...9.5.5.. 7 45. 4.7 74.85.98 -.7.7.8. 6. 7. 6.4.7. 56. 4.6 4.8 4.4.... 7 45. 4.667 849.94.8...5. 9. 4.7 4. 7..9 6. 4.9 4.5 4.4.9... 7 45.67 4.6 56 4.7.8.45...6 6.. 9.8 6.9 4..4.. 94.8.9.9.. 7 44.87 4.867 8.6..7.... 4.8 46.4.5 5.8.7.4.9 8. 6... 5.6 74 44.867 4.799 9.9.9.47...4.8 44.9 9.5 7. 5..9.4 5.7 86.5..4..9 75 44.9 4.74 67.6.9...4. 7.7 49..6. 4.6 49.5 9.5...... 76 44.94 4.667 58.6.5 -.8.4. 5.4.5 8. 9. 5. 4. 8. 4.8 47. 8.....4 77 44.967 4.6 54 5.7.87.4...5 5. 7.5 8.5 4.9 6..6.. 9..4.8.. 79 44.767 4.8 9.64.66.8....5. 4.4.4 8.4 7.6.4 6.8 5.9.9.5..9 8 44.799 4.7 6.59.75....4. 57.6.6.8. 45....7.7... 8 44.8 4.666 4..9 -.4 6.9 5.8 6. 7.8 4. 6.9 4.6.. 8.. 8..5... 8 44.867 4.6 97 4.8.57.45..5. 6.6.9 6..9.6..5 9. 7...5.. 87 44.767 4.6 884 4.5..46..4. 5.9.7.4. 8..6 4.7 5.8 45.6..4.. Abbreviations, Mn: mean grain size, Srt: sorting, Sk: skewness, G: gravel, VCS: very coarse sand, C: coarse sand, MS: medium sand, FS: fine sand, VFS: very fine sand, Slt: silt, Cly: clay, Lm: light minerals, Hm: heavy minerals, Lf: lithic fragments, Vol: volcaniclastics, Ben: benthis remains, Plank: planktonic remains, Glauc: glauconite, Plant: marine plant fragments. 6
tions were determined from 4 points counted under a stereomicroscope. Sand grains were classified into eight categories: light minerals, heavy minerals, lithic fragments, volcaniclastics (pumice and volcanic glass), benthic remains, planktonic remains, glauconite, and marine plant fragments. The sediment budget was calculated to gain a semiquantitative understanding of the active sedimentdispersal systems. Coastal erosion was estimated on the basis of coastline retreat measured via a comparison of dated (published in 9) and recent (published in 996) topographic maps. To estimate the mass of eroded material, we assumed that the elevation and dry bulk density of rocks eroded from sea cliffs were m and.6 g cm (Suda et al., 99), respectively. Values of height and density adopted for beaches were mand.4gcm (using an average porosity for sandy sediments of 49%, as proposed by Pryor 97), respectively. The volumes of shelf sediments were calculated based on measurements of sediment thickness and distribution in sub-bottom profiling records and geological maps previously reported by Maritime Safety Agency (987, 998, 999). To estimate the mass, a dry bulk density of.4 g cm was assumed for the post-glacial shelf sediments. Sedimentation rates on the slope were estimated with reference to historical volcanic ash layers (dating from the 7th century) that are intercalated with the sediments. The value of dry bulk density was.4 g cm, as calculated from the water content (% wet weight) of muddy surface sediments in core samples obtained close to the study area (Noda et al., 5; Kuroyanagi et al., 6). 4. Results 4.. Textures Surface sediments were not recovered at station number 5,, 9, and because the seafloor at these sites was rocky (Table ); the nature of the seafloor was confirmed from photographs. The mean grain sizes of the inner shelf from off Hanasaki to off Cape Ochiishi and the shelf edge from off to off Kiritappu are all coarser than coarse sand ( φ) (Fig. 5A). Medium sand ( φ) was recorded on the outer shelf off Hanasaki, whilefine ( φ) tovery fine sand ( 4φ) occurs on the shelf off Kiritappu and. Fine sand also covers much of the uppermost slope (<, m), with very fine sand to medium silt (5 6φ) upon the upper slope (,, m). Fine silt (6 7φ) and very fine silt (7 8φ) are distributed upon the middle slope (>, m). The degree of sorting within the study area ranges from well sorted (.4) to extremely poorly sorted (4.) (Fig. 5B). Almost all of the shelf sediments are classified as moderately (.7.) or poorly sorted (..), with the exception being gravelly sediments on the inner shelf from off Hanasaki to off Cape Ochiishi. Well sorted (.5.5) to moderately well sorted (.5.7) sediments occur on the outer shelf off Kiritappu; silty sediments on the slope are generally very poorly sorted (>.). Skewness values for shelf sediments off Hanasaki are strongly negative (<.) to negative (..) (Fig. 5C), as are those for shelf-edge sediments off Kiritappu. In contrast, sediments on the shelf off show strongly positive skewness (>.). The obtained values reveal along-shelf variations, with a trend of gradually increasing skewness to the west (Fig. 5C). The upper slope sediments (,, m) are also strongly positively skewed, while those on the middle slope (deeper than,5, m) are positively skewed (..). High gravel contents (> wt%) are recorded for sediments on the inner shelf from off Hanasaki to off Cape Ochiishi and on the shelf edge off Kiritappu (Fig. 6). The sediments from the shelf edge to the uppermost slope contain in excess of wt% gravel. Mud content is generally low (< wt%) on the shelf off Hanasaki and the shelf edge, whereas the mud contents of shelf sediments off Kiritappu to and part of the area off Hanasaki (St. ) exceed wt%; on the middle shelf off, mud contents exceed 5 wt% (Fig. 6). On the slope, mud contents rapidly increase from the uppermost to upper slope. Sediments on the slope off Hanasaki in water depths greater than,8, m contain more than 6 wt% mud, as do sediments off at depths of,,5 m. 4 ' 4 ' 4 4' 45 ' 45 ' Hokkaido Bay % % % 5% Kiritappu Cape Ochiishi % 45 4' 46 ' Hanasaki % Fig. 6. Gravel and mud contents (wt%). % % 6% Rocky bottom Gravel (> wt%) Gravel ( wt%) Mud content 7
45 E 45 E 4 54 E 46 E 4 'N 4 'N 4 5'N 4 4'N 4 Kiritappu Ochiishi 7 Hanasaki 4 5 6 7 Mean (ø) 6 7 6 5 4 4 'N 4 'N 4 5'N 4 4'N Kiritappu Ochiishi Hanasaki Sorting 4..5..5..5..5 4 'N 4 'N 4 5'N Kiritappu.4.4.4 Ochiishi.4 Hanasaki.4.4.4 Skewness.8.6.4... 4 4'N.4.4.4.6 Fig. 5. Spatial distribution of (A) mean grain size, (B) degree of sorting, and (C) skewness of the surface sediments. 8
4 ' 4 ' 4 4' 45 ' 45 ' Hokkaido Benthic remains (> %) Planktonic remains (> 5%) Kiritappu Cape Ochiishi Heavy minerals (> %) Marine plant fragments (> %) 45 4' 46 ' Hanasaki Pumice & volcanic glass (> 5%) Fig. 7. Areas of concentrations of selected components within the medium-sand fraction (.75.φ) of the sampled sediments. 4.. Sand composition The main components of the sand-sized fraction are light and heavy minerals, lithic fragments, and volcaniclastic grains (Table ); in combination, these components make up more than 8% of the total grains in the majority of the analyzed samples. Quartz and feldspar are both classified as light minerals, as their similar shapes and densities mean that they exhibit similar behaviors under sea water. The concentrations of light minerals are relatively high (> %) in shelf sediments from off Hanasaki to off Cape Ochiishi, at the shelf edge off, and on the upper slope. The heavy minerals observed within the analyzed samples are dominantly orthopyroxene and clinopyroxene, with lesser amounts of amphibole, mica, and opaque minerals such as magnetite. High concentrations of heavy minerals (> %) are recorded from the shelf to the uppermost slope off Hanasaki, and from the shelf edge to the uppermost slope off the region from Kiritappu to (Fig. 7). Concentrations of heavy minerals in excess of 5% are recorded on the shelf off Hanasaki (St.,,, and ), the shelf edge (St. 64), and the uppermost slope (St., 9, and 54). Lithic fragments are also one of the main components within the sand fraction. Fragments of intermediate mafic igneous rocks and sedimentary rocks (chert, mudstone, and sandstone) are dominant in this category. The sediments on the shelf off Hanasaki and the shelf edge record the highest percentages of lithic fragments (more than %). Many sediment samples obtained from the shelf and slope contain high concentrations of pumice and volcanic glass. Pumice grains have a very low density, and are sometimes transported by suspension on the sea surface. The shelf sediments off and those on the upper and middle parts of the slope have high pumice contents (> 5%) (Fig. 7). Benthic remains observed within the analyzed samples include shells, foraminifera, sponges, bryozoa, ostracoda, and echinoid spines. Shell fragments are abundant in the shelf sediments, while foraminifera are dominant in the slope sediments. The inner shelf sediments from off Cape Ochiishi to off have high contents (more than %) of benthic bioclasts (Fig. 7). Planktonic remains include radiolarians, diatoms, and foraminifera. Diatoms and radiolarians are dominant in the study area, whereas planktonic foraminifera are rare. Areas of high concentrations of planktonic remains (> 5%) are zonally distributed along the middle slope (Fig. 7). Marine plant fragments consist mainly of algae and sea grass fragments. Sediments on the inner shelf off contain more than % plant fragments (Fig. 7). 4.. Turbidity Turbidity generally decreases with increasing water depth. Relatively high (>. ppm) bottom-water turbidity was observed on the inner shelf from off Kiritappu to off (St. 5, 6, 7) and on the shelf edge off Kiritappu (St. 4 and 47) (Fig. 8A). The shelf from off Hanasaki to off and shelf edge off Kiritappu recorded moderate turbidity values (.5. ppm), whereas very low values were recorded for the bottom water on the upper and middle slope. Exceptionally high turbidity was recorded at a single station on the upper slope (St. 7). Intermediate and bottom nepheloid layers from the shelf edge to areas off the shelf were identified in the vertical turbidity profiles (Fig. 8B). 4.4. Sediment structure Sediment structures within the surface sediments were examined using soft X-radiographs. Fine sands upon the middle outer shelf off Hanasaki (St. and ) lack gravel and contain weakly developed parallel laminae and burrows (Fig. 9). The medium to coarse sands and gravels from off Hanasaki to off Cape Ochiishi (Figs. 5 and 6) are devoid of cross and ripple laminations. Muddy sands on the shelf from off Kiritappu to off contain numerous burrows near the sediment water interface ( cm below the seafloor) (Fig. 9). The lower sediments at St. 7 contain lami- 9
(A) 4 ' 4 ' 4 4' (B) Water depth(m) X 4 6 45 ' 45 ' >. ppm.. ppm.5. ppm..5 ppm Kiritappu X Cape Ochiishi Y 45 4' 46 ' Hanasaki Turbidity (ppm) >....5...5 <. <. ppm 8 45 46 47 48 49 Station number Fig. 8. (A) Spatial distribution of turbidity of the bottom water ( m above the seafloor). (B) Vertical profiles of turbidity along the line X Y. nated sands and shell fragments (Fig. 9). The gradual upward reduction in the brightness of the radiographs indicates an upward reduction in grain size with ongoing accumulation. Surface sediments on the upper slope contain unconformities at cm below the seafloor (Fig. 9). These boundaries are easily identified in the radiographs as layers of sharp contrast. The unconformities generally consist of poorly sorted gravelly and muddy (diamictitelike) sediments that overlie semi-consolidated muddy deposits. Y I was further subdivided into two sub-facies based on the form of the surface reflection: irregular (Facies IA) and smooth (Facies IB) (Fig. ). Facies II was further subdivided into the following four types as Facies I: continuous (Facies IIA), discontinuous (Facies IIB), hyperbolic (Facies IIC), and weak (Facies IID), with each of these being subdivided in turn into the following five classes based on the observed internal reflection pattern: no internal or sub-bottom reflectors (a), sub-bottom but not internal reflectors (b), several continuous stratified reflectors (c), several discontinuous reflectors (d), and several weak reflectors (e) (Fig. ). The facies maps were then compiled based on data obtained from the N S survey lines. Facies IA occurs in parts of the inner shelf from off Hanasaki to off, along the shelf margin from off Hanasaki to off Kiritappu Ochiishi, and on the outer shelf off (Fig. ). This facies corresponds to outcrop or gravelly sediment (Fig. ). Facies IB, which represents fine to coarse sand, is largely distributed on the shelf off Cape Ochiishi and the outer shelf off. Facies IIAa, IIAb, and IIAc are distributed on the shelf off Hanasaki, Kiritappu, and, as well as areas upon the middle slope (Fig. ). These facies are deposited on the shelf upon the erosional surface of the Last Glacial Maximum (Fig. ). The facies off Bay change to the west from IIAb to IIAa and IIAc (Fig. ). Facies IIAa and IIAb represent fine to medium sand, while Facies IIAc is silt to very fine sand. Facies IIDa and IIDe are characteristic of the uppermost (average gradient in excess of 4 6 ) and upper ( 4 ) slope, respectively. The lack of penetration of acoustic waves reflects the steepness of the slope. Facies IIAd and IIBd are recognized around gullies developed upon the middle slope. 4.6. Seismic profiles 4.5. Sub-bottom profiles The classification scheme adopted for the Parasound sub-bottom profiles (SBPs) obtained in the present study was based on acoustic classes related to the form and signature of the echo from the seafloor. Two broad facies were identified based on the intensity of the surface reflection: distinct (Facies I) and indistinct (Facies II). Facies I showed strong reflection with no penetration or internal reflection, while Facies II exhibited a certain width of reflection with internal reflectors or subbottom reflection beneath the surface reflection. Facies Aseismicprofile across the shelf to the slope reveals a clear two-part division of the strata based on an onlap unconformity (Fig. ). The lower unit (U) dips steeply seaward and is characterized by continuous but weak reflectors; accordingly, it cannot be traced to deeper levels. Unit U is evident in the SBP records around the shelf edge (Fig. A), which represents the Paleogene from the shelf to the upper slope (TuZino et al., 4, 5). The unit was uplifted and tilted subsequent to the late Paleogene, as the uppermost part of the Nemuro Group extends to the Eocene where exposed onland, and Unit L onlapped the underlying Unit U (TuZino
St. (6 m) Hanasaki Slope St. (6 m) St. 7 (8 m) St. 79 (9 m) St. 9 (74 m) St. 67 (64 m) Weak laminae Burrows Burrows Burrows? Shell Burrows Laminae 5 cm Fig. 9. Soft X-radiographs of the shelf sediments off Hanasaki (St. and St. ), off (St. 7 and St. 79), and slope sediments (St. 9 and St. 67). Fine sands on the outer shelf off Hanasaki contain weakly developed laminae and burrows. The muddy sands on the shelf off contain numerous bioturbation traces. Unconformities are recognized 4 6 cm below the surface of the slope sediments, where poorly sorted gravelly and muddy (diamictite-like) sediments are deposited upon semi-consolidated muddy sediments. et al., 4, 5). The upper unit can be subdivided into subunits Qa, Qb, P, M, and L, which are characterized by strong subhorizontal reflectors. Based on a comparison with the acoustic units, more than 4, m of drill core obtained southwest of Kushiro (Sasaki et al., 985), and onland exposures, the units Qa, Qb, P, M, and L are assigned to the Quaternary, the Pliocene, the upper Miocene, the middle Miocene, and the lower Miocene, respectively (TuZino et al., 4, 5). Anticlines trending ENE WSW are observed in the middle slope at water depths of,,5 m (Fig. ). Although the length of each fold axis is 5, several anticlines occur in the middle slope (Fig. ). The seismic record shown in Fig. reveals that a greater thickness of sediments was deposited on the landward side of the anticline. The thickness of sediment along the anticline axis is similar to that in the limbs for units M to P; in contrast, a greater thickness is recorded along the limbs for unit Qb and Qa. This indicates that the anticline developed mainly during deposition of unit Qb (Pliocene). 4.7. Sediment budget Comparisons of dated and recent topographic maps revealed that the combined erosion of sea cliffs (.44 ) and beaches (.84 ) over the past 75 years considerably exceeds the amount of coastal accretion over that time (.8 ) (Fig. ). The net rate of coastline erosion was calculated to be.66 y (4.94 /75 years), while the total erosion rate in the study area was estimated to be.67 Mt y for sea cliffs and.4 Mt y for beaches, representing a total of.8 Mt y (Fig. ). The volumes and rates of coastal erosion determined for three sections (Hanasaki, Kiritappu, and ) were.6 y and.46 Mt y,.467 y and.8mty,and. y and.7 Mt y, respectively. Although there is no data for the sediment supply of small streams entering the shelf, the transported volumes are known for the Kushiro River and its tribu-
Nemuro 4 'N 44 5'E 45 'E 45 'E 45 'E 45 'E 45 4'E 45 5'E 46 'E 46 Nemuro Peninsula Hanasaki 'E 4 'N Hokkaido Cape Ochiishi Kiritappu 4 'N Bay 4 5'N 4 4'N IA IB IIAa IIAb IIAc IIAd IIBd IICa IIDa IIDe Distinct, irregular, and no internal reflectors. Rough topography on the shelf and shelf edge. Distinct, smooth, and prolonged bottom reflectors. No internal reflectors. Inner outer shelf. Indistinct, smooth, prolonged bottom reflector. No subbottom and internal reflectors. Inner outer shelf. Indistinct, smooth, prolonged bottom reflector with subbottom. No internal reflectors. Inner to middle shelf. Indistinct, smooth, prolonged bottom reflector. Continuous and parallel internal reflectors. Inner and middle shelf, and middle slope. Indistinct, smooth, and prolonged bottom reflector. Discontinuous internal reflectors. Occurs in vicinity of facies IIAc. Indistinct and discontinuous bottom and internal reflectors, associated with rough topography, especially channelized slope. Hyperbolae bottom reflector. No internal and subbottom reflectors. Rough topography on slope. Weak bottom reflector without internal reflector. Very steep slope. Weak bottom reflector with low penetration. Stratified or discontinuous internal reflectors. Steep slope. Anticline Gully Fig.. Spatial distribution of the acoustic facies determined from the Parasound sub-bottom profiler records.
(A) Line SSW NNE 5 m 5 IBa IA IIAb IIAa IIAb IA (B) Line WSW ENE IIAc IIAa IIAb IA Fig.. Parasound sub-bottom profiling records for (A) a transect across the shelf off Kiritappu and (B) a transect along the shelf off. The maximum penetration depth is about 5 m. The location of the survey line is shown in Fig.. Facies labels are shown in Fig.. taries (,5 ), flowing in the west of the study area (Fig. ). The annual rate of the transported materials is about 4,9 m y (ca.. Mt y if the density is.4 g cm ) (Ohtsuka, ). We applied the value of the Kushiro River for estimation of the input volumes and rates in the drainage areas of each sections (4 for Hanasaki, for Kiritappu, and 94 for ). The approximation resulted in.4,.7, and.79 Mt y ;thefirst two were two order of magnitude less than the values of coastal erosion. The last value in area was about one forth of the coastal erosion, and the total sediment input was therefore.5 Mt y (Fig. Fig. ). If possible, volumes of sediment deposited on the 7th-century tephras or the transgressive surface should be used for estimation of accumulation rates on the shelf, because the sediment supply related to the sea level might be stable during the Holocene in this area (Ohta et al., 99). Nevertheless, we here used the volumes since the Last Glacial Maximum (ca. 8, years), as the tephra layers or the transgression surface were hardly identified in the shelf sediments or the seismic data, like other margins around Japan (cf. Saito, 994). The volumes of sediment accumulated on the shelf since the LGM were calculated to be.7 off Hanasaki,.9 off Kiritappu, and.47 off (Fig. ). The average mass accumulations for each area were therefore.,.7, and.7mty (using an average dry bulk density of.4 g cm ), respectively. The sum of these values (.47 Mt y ) represents approximately 5% of the sediment supply from land. The sedimentation rate on the upper middle slope over the past 4 years was..88 cm y, as con-
CDP 5 5 U. VE: 8.8 TWT (s) NNW. Qa Anticline Qb P. SSE M L 4. 5. Fig.. Representative seismic profile for the area off Cape Ochiishi. Horizontal axis, CDP, is ca. 5 m; Vertical axis TWT is ca. 75 m. Labels for seismic units are explained in the text. 44 5'E 45 'E 45 'E 45 'E 45 'E 45 4'E 45 5'E 46 'E 4 'N Coastal erosion.7 Mt y- Fluvial input.79 Mt y -.5 m m y- ) 7 k Mt.. ( 5m m 5 m 4 'N Coastal erosion.46 Mt y- Fluvial input.4 Mt y - Coastal erosion.8 Mt y- Fluvial input.7 Mt y - Kiritappu 5 5 5 5 IA SBP facies.47 cm y- - (.8 g cm y ) (.9. cm y- - g cm y ) (. (. 5 4 5'N - ) Mt y (.7.47 Sedimentation rate Sediment gravity flow deposits m - )- ) k yt y.99 M.7 tm 7 (. 4 'N 46 'E Sediment thickness. cm y- - (. g cm y ) (. 4 4'N.88 cm y- - (.5 g cm y ) 5.4 cm y- - (. g cm y ) (.4 Fig.. Estimated rates of eroded mass along the coast and fluvial input (Mt y ). Also shown are sediment thicknesses (m), volumes ( ), and mass accumulation rates (Mt y ) for the modern shelf sediments and sedimentation rates (cm y ) for the slope sediments. Data on sediment thickness upon the shelf were derived from Maritime Safety Agency (987, 998, 999). 4
strained from the 7th-century tephras. Assuming a dry bulk density of.4 g cm for muddy sediments on the seafloor, the rate of sediment accumulation is estimated to be ca..4 g cm y (..5 g cm y ). 5. Discussion 5.. Sediment characteristics and dispersal systems 5... Shelf area off Hanasaki From off Hanasaki to Cape Ochiishi, high concentrations of heavy minerals in the sand fraction reflect the fact that the sediments are coarse-grained (Fig. 7). The occurrence of abundant intermediate mafic volcanic and sedimentary lithic fragments and pyroxenes suggests that the sands were originally derived from the Nemuro Group, as exposed upon the Nemuro Peninsula. Transgressive erosion of the cliffs along the coast is likely to be a major contributor to the gravelly and sandy detritus accumulating on the inner shelf, mainly at depths of less than m (Figs. and 4B). Under the present conditions, the small stream inputs result in low mud contents in sediments of the middle outer shelf (Fig. 6), so that relict sediments remain as a thin (.5 m) cover over the middle outer shelf. A small amount of mud, partly found in the gravelly bottom on the inner shelf (Fig. 6), is considered to be derived from suspended muddy particles produced mainly by coastal erosion (Fig. ). 5... Shelf area off Kiritappu The sediments are characterized by high percentages of benthic remains in the inner-shelf sediments and pumice grains in the outer shelf. The volcaniclastic grains are possibly derived from the Kutcharo and Mashu pyroclastic flow deposits (Katsui, 96) or the other Holocene airfall tephras that originated from western Hokkaido (Furukawa and Nanayama, 6). The Holocene tephras are widespread off eastern Hokkaido and are intercalated as 5 cm thick layers within surface marine sediments between Hiroo and Nemuro (Noda et al.,, 4). Because of their low density, pumice grains generally behave in the same manner as finer grains. Sediments in excess of 5 m thickness are deposited in the middle outer shelf (Fig. ). The location of the depocenter suggests that the distribution of postglacial shelf deposits was controlled by ridges on the shelf margin (SBP Facies IA). This depositional area might be generated by north-trending normal faults associated with strike-slip (transtensional) tectonics from oblique subduction of the Pacific Plate (Fitch, 97; Kimura and Tamaki, 986), as observed in the shelf along the eastern Nankai subduction zone (Yamaji et al., ), in the submarine Aleutian forearc (Geist et al., 988), and in the Tibet Plateau (McCaffrey and Nabelek, 998). Several buried channels might have acted as active conduits to the outer shelf and beyond during lowstand periods (Fig. 4A), which are buried by transgressive or highstand sediments and connected to the depocenters (Fig. ). These sediments are interpreted to represent a discrete sand sheet within a transgressive systems tract. Thenearshoreareaiscoveredby 5 m of sediments with minimal input from streams, has probably been directly derived from coastal erosion. Strong wintertime storm waves attack the cliffs and remove beach sand to offshore areas. Landslides that develop within the coastal cliffs, especially earthquake-induced slope failures, also frequently deliver sediment into the sea (Tajika et al., 994a,b; Amemiya and Tajika, 999). The middle outer shelf sediments have relatively low mud contents (< % in Fig. 6), and some (St. 4 and 46) are moderately well sorted to well sorted (Fig. 5). These features indicate that little mud is deposited in the area deeper than the storm wave base (ca. 4 m in Sunamura, 987), although a zone of high turbidity (St. 4 and 47) extends from the nearshore area to the outer shelf off Kiritappu (Fig. 8). The lack of mud can be explained by the fact that the suspended particles within the bottom water (Fig. 8B) are transported to deeper waters as both bottom and intermediate nepheloid layers, as observed in other slope settings (Mc- Cave, 97; Pak and Zaneveld, 977; Pak et al., 98; McGrail and Carnes, 98; Cacchione and Drake, 986; Durrieu de Madron et al., 99; Walsh and Nittrouer, 999, ). In addition, the resuspension of fine particles by physical processes and subsequent offshore transport from the slope may leave behind only sandy sediment. The Off-Tokachi nearshore current flows westward at speeds of up to cm s, making it capable of transporting suspended material and redistributing previously deposited sediments (Fig. 4). The simple calculation proposed by Miller et al. (977), in which ū =.6D.9 for D. cm, where ū is the average current velocity cm above the seafloor and D is grain diameter, suggests that the threshold flow velocities for φ and 4φ grains are 4.4 cm s and 4.4 cm s, respectively. Although the velocity just above the seafloor is unknown, the Off-Tokachi nearshore current is almost certainly able to transport fine fractions from within the shelf sediments (Fig. 4B). 5
A B Sediments derived from coastal erosion Along shelf transport by oceanic current Suspended transport pathway Gravitational transport pathway Fig. 4. (A) A schematic diagram of paleoenvironment upon the shelf during the early transgression stage. (B) Present-day distribution of sediments and their dispersal systems. 5... Shelf area off The depocenter in this area is located along the coast in water depths of between 4 and 8 m, extending to the west (Figs. and ). This area is also surrounded by topographic highs similar to those off Kiritappu. The presence of a high-turbidity area (Fig. 8) suggests that suspended particles are being supplied to the area, probably from coastal erosion, river discharge, and the estuarine mouths of Lake and Bay via wave or tidal action (cf. Hubbard et al., 979; Dalrymple et al., 99). The material supplied to the ocean is probably deposited on the seafloor at depths below the storm wave base (ca. 4 m), following transportation by alongshore and tidal currents. Accordingly, these sediments are interpreted to represent modern sediments of present highstand systems tracts. 5..4. Shelf margin uppermost slope Poorly sorted medium to gravelly sands are distributed in water depths of between and, m (Fig. 5), and contain gravel contents of more than wt% (Fig. 6); in addition, outcrops were recognized at a number of localities off Hanasaki (St. 5 and ). The geometry in the area around the shelf break is irregular, bearing features such as ridges (Fig. ) and slump scours (Fig. ). The water depths at the shelf edge ( 8 m) are too deep to be affected by the types of modern storm waves that typically produce lag deposits. The coarse sediments around the shelf edge are regarded as relicts of the period from the last glacial age to the early transgression stage (Fig. 4A). At this time, river mouths might have connected to gullies that incised into the slope during lowstand periods, and thereby might directly transport terrigenous detritus to the deep sea (Fig. 4A). The gullies developed further with repeated submarine slope failures that occurred in response to the large amount of material produced during the following transgression with failure triggered by earthquakes related to the subduction of the Pacific Plate. The coarse sandy sediments extend to the uppermost slope (, m water depth) where hemipelagic muddy sediments are generally deposited (e.g. Doyle et al., 979; Stanley et al., 98). The slope is characterized by its steepness and the presence of incised gullies that represent repeated downcutting by gravity flows (e.g. Field et al., 999). The large amounts of gravel and 6
heavy minerals, in combination with small amounts of planktonic remains, suggest that the uppermost slope sediments were supplied from the shelf edge under the influence of gravity mainly during the Last Glacial Maximum. The steepness (5 ) of the slope might have prevented hemipelagic suspended particles from settling to the sediment surface. 5..5. Upper slope Poorly sorted and positively skewed sandy sediments are deposited on the upper slope (Fig. 5). The textural characteristics of these sediments can be explained by the mixing of coarse deposits and fine hemipelagic particles. The coarse fractions may have been gravitationally derived from the shelf edge or the uppermost slope. The advection of turbid water (Fig. 8) implies intermediate and bottom nepheloid-layer transport from the shelf edge. Nepheloid-layer transport involves the seaward movement of shelf-generated turbid water along isopycnal surfaces (Fig. 8). The turbid water that led to the development of intermediate and bottom nepheloid layers may originally have been produced mainly by coastal erosion, resuspension by storm waves and currents, and perhaps infrequently by river discharge, and estuarine plumes (Fig. 4B). The particles within the turbid water and hemipelagic fallout could have contributed to the slope sediment, producing poorly sorted and positively skewed fine-grained deposits. The location of the highest accumulation rate within the slope, situated at the extension of the high-turbidity area off Kiritappu, indicates that nepheloid-layer transport is an important component of the off-shelf sediment dispersal system. 5..6. Middle slope Muddy sediments are distributed on the upper to middle slope at water depths in excess of, m (Fig. 5). High proportions of volcaniclastic detritus and planktonic remains are interpreted to reflect primary deposition by the precipitation of surface-plume fallout or nepheloid transport (Fig. 7). Sediments on the steep slope and in areas around gullies occasionally contain unconformities, with the deposits immediately above the unconformities being very poorly sorted muddy sediments (Figs. 9 and ). These observations suggest that mass movements or sediment gravity flows might erode and redeposit the surface sediments upon the slope. Rapid subduction of the Pacific Plate beneath Hokkaido acts to compress the forearc margin (Tada and Kimura, 987), which may in turn act to steepen and deform the slope (Fig. ). In addition, large earthquakes occur frequently along the subduction zone (Kanamori, 97; Shimazaki, 974; Kikuchi and Fukao, 987; Hirata et al., ; Yamanaka and Kikuchi, ), and these events may reduce the strength of the slope sediments (e.g. Lee and Edward, 986; Normark and Piper, 99; Hampton et al., 996). Such seismicity could potentially trigger mass movements or sediment gravity flows. In the eastern Hokkaido forearc (off Kushiro), the recurrence interval for sediment gravity flows over the past 4 years is estimated to be ca. 6 7 years (Noda et al., 4); accordingly, sediment gravity flows are interpreted to be an important component of the sediment dispersal system upon the steep part of the slope. 5.. Sediment budget 5... Shelf area The mass accumulation rates off Hanasaki (. Mt y ) and Kiritappu (.7 Mt y ) represent approximately 6% and 6% of the sediment supply around Hanasaki (.46 Mt y ) and Kiritappu (.8 Mt y ), respectively. Coastal erosion can therefore explain the entire postglaciation sediment mass on the shelf. The remainder of the material derived from coastal erosion must have been transported to deeper areas as a nepheloid layer or moved out of the study area by nearshore and tidal currents. This implies that the majority of sediment input is dispersed seaward to the slope, as reported from other active margins (cf. Sommerfield and Nittrouer, 999; Orpin et al., 6). Small inputs of terrigenous material could have not covered much of the outer shelf, particularly in the east. The accumulation rate off (.7 Mt y )is nearly balanced by the sediment supply (.5 Mt y ) in the area. The higher ratio of accumulation to input rates than other areas could be attributed to transportation by the Off-Tokachi nearshore current from the east. Sedimentation with along-shelf transport would be more important, as the shelf widened with sea-level rise. 5... Slope An accumulation rate of about..88 cm y (mass accumulation rate of..5 g cm y ) was estimated for the middle slope (Fig. ). The low rate of mass accumulation reflects the low dry bulk density (.4 g cm ) of the surface sediments, which may in turn reflect high levels of diatom productivity within the Oyashio Current during the Holocene (Shiomoto et al., 994; Narita et al., ; Ikehara et al., 6). Assuming that the sedimentation area on the slope is,,, the sedimentation rate is calculated 7