Texture and Petrology of Modern River, Beach and Shelf Sands in A

Texture and petrology of modern river, beach and shelf sands in a volcanic ? back-arc setting, northeastern Japan ∗ Atsushi Noda a, aGeological Survey of Japan, National Institute of Advanced Industrial Science and Technology, Tsukuba, Higashi 1-1-1, Ibaraki 305-8567, Japan

ABSTRACT

The focus in the present study is on characterizing spatial patterns of textural and petrological variabilities, and on evaluating mechanisms influencing the textural and petrological components of modern river, beach and shelf sands in a volcanically active back-arc tectonic setting. Abashiri Bay and the surrounding area in eastern Hokkaido, Japan, has volcanic source land within a back-arc region associated with subduction of the Pacific Plate beneath the Okhotsk (North American) Plate. A total of 41 river, beach and shelf sands were obtained for grain-size and modal composition analyses. Multivariate analytical techniques of hierarchical cluster and principal component analyses were performed on the textural and petrological data for investigating relations among quantitative variables. On the basis of grain-size data, four sedimentary zones were identified: zone I, palimpsest zone; zone II, relict zone; zone III, modern (proteric) zone; zone IV, coastal sedimentary zone. All sands are feldspatholithic and quartz-deficient. The framework (quartz, feldspar and rock fragment) modal compositions were also classified into four clusters, A–D. The characteristic components of each cluster are as follows: cluster A, felsic volcanic rock fragments; cluster B, andesitic– basaltic volcanic rock fragments; cluster C, mixed or plagioclase; cluster D, sedimentary rock fragments. Almost all sands in western and central Abashiri Bay belong to cluster A, where the original compositions are influenced by Kutcharo pyroclastic flow deposits. Andesitic–basaltic lava and Neogene volcaniclastic and sedimentary rocks have a major influence on the compositions of shelf sands in eastern Abashiri Bay. The modal compositions are basically controlled by the source lithology. Compositional maturity (percentage of quartz to feldspar and rock fragments; Q/FR%) slightly increased, in order, from river (1.2), zone IV (coastal, 1.7), zone II (relict, 2.2), zone I (palimpsest, 3.6), to zone III (modern proteric, 7.0). Greater maturity in the recycled sediments is indicative of weathering under the sea or abrasion by transportation induced by sea-level fluctuations, waves, or sea currents. Several controlling factors – (i) source lithological; (ii) mineralogical; (iii) climatic; and (iv) geomorphological controls – might still cause low maturity through all sedimentary zones other than the continental margin sands previously reported.

Key words: modern sand; beach; shelf; grain size; modal composition; multivariate analysis; provenance analysis

hydraulic, climatic, and geographic conditions. The petrology of clastic materials also has been used to elucidate orogenic INTRODUCTION process, unroofing, and plate tectonic evolution (e.g. Dickinson The texture and petrology of sedimentary rocks provide ba- & Suczek, 1979; Dickinson, 1985; Suczek & Ingersoll, 1985; sic clues to paleogeographic and paleogeologic reconstructions Dorsey, 1988; Lee & Lee, 2000). Petrological properties of of a basin and its hinterland. It is well known that grain-size sediments are, however, affected by various factors, including distributions are affected by selective transportation and depo- chemical weathering, physical breakdown, abrasion, and hydro- sition (Komar, 1977; Dacey & Krumbein, 1979; McLaren & dynamic sorting (Davies & Ethridge, 1975; Mack, 1978; Dacey Bowles, 1985). The texture of clastic sediments has been uti- & Krumbein, 1979; McLaren & Bowles, 1985; Basu, 1985; lized as a key to their sedimentary environments, such as the Grantham & Velbel, 1988; Johnsson, 1989, 1993). Mixing of detritus from multiple sources may further modify the initial ? sediment characteristics, especially when dispersal pathways NOTICE: this is the author’s version of a work that was accepted for are complex and involve recycling of previously deposited sed- publication in The Island Arc vol. 14(4), p. 687–707. Changes resulting from peer review are reflected, but editing, formatting, and pagination from the iments (e.g. Critelli et al., 1997; Arribas et al., 2000; Garzanti publishing processes are not included in this document. A definitive version et al., 2002; Critelli et al., 2003). An environmental signature were published in DOI: 10.1111/j.1440-1738.2005.00477.x. ∗ (e.g. bioclasts or glauconite) may be added to the sediment Corresponding author. Fax: +81 29 861 3653. causing a compositional change that is essentially unrelated to Email address: [email protected] (Atsushi Noda).

Article published in The Island Arc 14 (2005) 687–707 the initial detrital spectrum. pumices and ashes at ca. 7 ka, and thus, generated a caldera, Spatial patterns of textural and petrological variabilities of Lake Mashu (Katsui, 1955; Katsui & Satoh, 1963). sediments may provide constraints on the geography, climate, The Kutcharo caldera and surrounding mountains (Fig. 2) tectonics, and lithologies of sediment source areas. An under- are the main sources of rivers flowing into Abashiri Bay. The standing of the textural and petrological characteristics of mod- Kutcharo caldera was formed in middle–late Pleistocene age ern sediments is desirable to develop more refined provenance (Katsui & Satoh, 1963). Neogene sedimentary and volcanic interpretation schemes, to provide a basis for evaluating past rocks are extensively blanketed by the Kutcharo pyroclastic environmental conditions, and to evaluate the role of erosion flow (Kpfl) deposits produced by eight large-scale dacitic pyro- and sedimentation within the tectonic and hydrologic cycles. clastic flows during the last 300,000 years (Katsui, 1958; Kat- Although almost all the researchers referred to above have tar- sui & Satoh, 1963; Hirose & Nakagawa, 1995). The largest is geted fluvial or beach sands, few have studied shallow marine the Kpfl-IV of ca. 120 ka, and the youngest is the Kpfl-I of sands for the purpose of unraveling various influences on the ca. 32 ka (Kigoshi, 1967). Fluvial sediments composed of py- sediments. roclastic sands and gravels were described as Bihoro (Sassa & The main objectives of this study are (1) characterizing spa- Inoue, 1939), Yambetsu (Matsushita, 1960), and Sattsuru (Sug- tial patterns of textural and petrological variabilities, and (2) imoto & Hasegawa, 1959) Formations that were intercalated evaluating mechanisms influencing the textural and petrologi- into the Kpfl deposits of late Pleistocene age. Most of the Kpfl cal components of modern river, beach, and shelf sands. This deposits are composed of augite-bearing hypersthene dacitic study differs from previous works in that it focuses on beach pumice and ash. The deposits are characterized by absence of to marine sands in an actively volcanic back-arc setting. The bedding, lack of sorting, and abundance of glass shards (Katsui results suggest that textural and petrological data for modern & Satoh, 1963). sediments aid in interpretation of depositional environments in ancient sedimentary rocks of more ambiguous setting. For these purposes, modern sediments were collected from Abashiri Bay and the surrounding area in eastern Hokkaido, Japan (Fig. 1). Abashiri Bay is situated in the back-arc region GEOGRAPHY of the Inner Kuril volcanic arc where the Pacific Plate actively Marine terraces have developed at Cape Notoro and the converges under the Okhotsk (North American) Plate (Fig. 1). Shiretoko Peninsula. The lowermost terraces are 45 m above sea level at Cape Notoro and 80 m above sea level at the Shire- toko Peninsula (Fig. 3). These correspond to the interglacial stage 5e (ca. 125 ka; Okumura, 1991), and therefore, the estimated uplift rates are 0.36 m/ky at Cape Notoro and 0.64 m/ky PHYSICAL SETTINGS at the Shiretoko Peninsula. In contrast, marine terraces do not occur near Shari. The subsidence rate of the Shari Plain is estimated to be more than 0.50 m/ky (Koike & Machida, 2001). These rates are about an order of magnitude less than the gen- GEOLOGY eral rate of rise in sea level from the Last Glacial Maximum The Inner Kuril volcanic arc is made up of Tertiary volcani- (LGM: ca. 20 ka) to the sea level maximum of about 6 ka clastic and sedimentary rocks and Pliocene–Pleistocene calc- (ca. 120 m/14 ky = 8.6 m/ky). alkaline pyroxene andesites (Sato & Mitsunashi, 1970; Sato, Abashiri Bay encompasses an area of about 20 km (NS) 1970; Tsushima, 1974) (Fig. 2). Neogene strata in the Abashiri by 60 km (EW) (Fig. 4). Four submarine terraces (at 40, 75, (western part of the study area) and the Shiretoko (eastern) ar- 115, and 130 m water depths) have been noted off Cape No- eas are divided into five formations on the basis of lithology. toro (Fig. 5A). The terrace at 130 m depth (Notoro Spur) is the In the Abashiri area, these are defined from lower to upper as largest, and might be a wave erosional surface formed during the Kurumatonai (siltstone), Abashiri (andesitic volcanic clastic the LGM (Maritime Safety Agency, 1990). A flat area has de- sandstone and conglomerate), Notoro (siltstone), Yobito (silt- veloped off the Shari Plain between the water depths of 40 and stone), and Misaki (sandstone and conglomerate) Formations. 60 m (Fig. 5B). A long sandy beach (Shari Beach) fronts the In the Shiretoko area, they are named the Churui (andesitic– Shari Plain. Several lagoons, the Lake Notoro, Lake Abashiri, rhyolitic volcaniclastic mudstone and sandstone), Okushibetsu and Lake Tofutsu, have formed due to development of sandy (andesitic volcaniclastics and lavas), Koshikawa (mudstone and dunes during Holocene transgressions and regressions (Ko- sandstone), Ikushina (siltstone), and Chippudomari (mudstone daira, 1996). The largest river in the study area is the Abashiri and sandstone) Formations (Kato et al., 1990). River (drainage area, 1380 km2; length 115 km). The Lake The Akan–Shiretoko volcanic chain in the Inner Kuril Vol- Abashiri is a lagoon near the river mouth, which traps almost canic arc consists of subaerial andesitic stratovolcanoes and all the detritus derived from the river source. The second is calderas, such as Shari, Mashu, and Kutcharo (Goto et al., the Shari River (drainage area, 566 km2; length, 55 km) which 2000). Shari volcano is a cone-shaped stratovolcano consisting rises on Mt. Shari. The Shiretoko Peninsula has a steep and of basaltic to dacitic lavas and volcaniclastic rock (Sugimoto rocky coastline fronted by a relatively narrow continental shelf & Hasegawa, 1959). Mashu volcano explosively erupted felsic (less than 10 km in width) (Fig. 5C). The peninsula consists

688 Okhotsk Plate 130° 135° 140° 145° 140° 142° 144° 146°

45° A B B Okhotsk Sea 300 km 60 km Fig. 2 N N Abashiri Bay Shiretoko Peninsula Mombetsu Eurasia Plate 40° 44° Abashiri Japan Sea Shari Inner Kuril volcanic arc

Tokyo Hokkaido 35° Pacific Plate 42°

Philippine Sea Plate 8.3 cm/y

Fig. 1. Index map of study area: (A) Map showing tectonic setting around Japan. The Paciﬁc Plate is subducting beneath the Okhotsk (North American) Plate at the rate of ca. 8.3 cm per year (Riegel et al., 1993; DeMets et al., 1994). (B) Location of Abashiri Bay. Active volcanoes (indicated by the solid trapezoids in the map) are common along the Inner Kuril arc.

144° 00' 144° 20' 144° 40' 145° 00'

Lake Notoro Alluvium Cape Notoro Mashu volcanic ashes Abashiri Bay Holocene

44° 00' Abashiri Kutcharo pyroclastic

Lake Tofutsu – flow deposits (dacitic) Shari o volcanics a tok (pyroxene andesite ire sul Lake Abashiri Sh nin Pliocene Pleistocene and basalt) Pe

Sedimentary rocks Mt. Shari (partially volcaniclastic Mt. Mokoto sediments and lavas) Neogene

10 km

Lake Kutcharo (Kutcharo caldera)

Fig. 2. Generalized geological map around the study area (compiled from Sato, 1970, Sato & Mitsunashi, 1970, and Tsushima, 1974).

WEST EAST 0 50 km Shiretoko 100 Peninsula

50 CLIMATE AND OCEANOGRAPHY Lake Saroma Shari Plain The study area lies in a region where the climate is classified Elevation (m) 0 as subpolar. The mean annual temperature is 6.2 ◦C (monthly Mombetsu Tokoro Abashiri Pt. Shiretoko averages; 19.4 ◦C in August, and −6.6 ◦C in February). The area Fig. 3. Profiles of marine terraces corresponding with the interglacial stage 5e has the lowest annual precipitation in Japan, with an average along the coast line around the study area (solid lines). Dashed lines indicate value of ca. 800 mm/year (National Astronomical Observatory, inferred profiles of marine terraces (modified from Okumura 1996). 2003). Oceanic conditions in the Okhotsk Sea, sited along east- of a series of volcanoes (the Akan-Shiretoko volcanic chain), ern Hokkaido, are characterized by the Soya Warm Current including active volcanoes higher than 1000 m above sea level. in summer and drift ice in winter. The velocity of the Soya

689 144˚10' 144˚20' 144˚30' 144˚40' 144˚50'

–900 –1000 A –800 N –100 Notoro –600 –700 –80 Spur –500 44˚10' –60 –40 m1 –20 –400 10 km Cape m2 m4 –300 m9 Notoro –200 B m6 m8 m12 m16 C Lake m18 Notoro –100 c1 m3 m5 m10 m14 m19 –80 Abashiri m7 m13 m17 44˚00' Abashiri Bay Lake c2 –60 Abashiri m15 Abashiri c3 Lake Tofutsu m11 c4 c14 River c5 –40 c6 c11 c13 c7 c8 c10 –20 c12 c9 500 oko a sul Shari Beach Shari iret r8 Sh nin r7 r4 Pe 500 r5 r6 Shari Plain Shari 1000 River 43˚50' r1 r2 r3

Fig. 4. Geography around Abashiri Bay. Sampling localities are shown as solid circles. Black lines A–C indicate topographic cross-sections in Fig. 5.

0 5 10 15 20 25 30 35 km / 0 m Warm Current in summer is ca. 0.5–1.0 m s, moving from west (A) to east around Abashiri Bay (Fig. 6). After passing off Cape –40 Notoro, part of the current forms a back-flow to the west in –80 Abashiri Bay. When drift ice covers Abashiri Bay from January –120 Notoro Spur to March, the Soya Warm Current weakens and is considered to form an undercurrent beneath the drift ice (Nakamura et al., –160 1985). Autumn–winter storms are one of the essential factors –200 Off Cape Notoro (line A) influencing the sedimentary dynamics, generating high waves, 0 m which during rare extreme events could attain significant wave heights of Hs = 7.7 m, and wave periods of T = 12.8 s (Nagai, –40 (B) 2002). –80

–120

–160 METHODS –200 Off Shari (line B) 0 m

–40 (C)

–80 SAMPLING A total of 41 samples were collected during a ship survey –120 in July 2001 and a land survey in August 2001 (Fig. 4 and –160 Table 1). Nineteen shelf sands (m1–m19) were obtained from water depths of 35–265 m by a grab sampler, and 2–3 cm was –200 Off Shiretoko Peninsula (line C) stripped oﬀ the surface for analysis. Fourteen beach sands (c1– Fig. 5. Topographic cross-sections in Abashiri Bay. Locations of lines A–C c14) were obtained from a 2 cm thick layer from the surface on are shown in Fig. 4. Arrows indicate submarine terraces on line A and ﬂat the foreshore. A series of samples (c3–c12) was obtained from area on line B. Shari Beach, and other samples were collected from smaller

690 pocket beaches. River sands (r1–r8) were also collected from to maﬁc (Rpi–m) categories. Fresh vitric fragments without any medium-grained sands at point or longitudinal bars in the mid- phenocrysts were grouped as volcanic glasses, which were cat- dle reaches of rivers. egorized into four types: bubble-wall type (VGb), pumice-type (VGp), massive and colorless (VGmt), and massive and colored (VGmc).

GRAIN-SIZE ANALYSIS The grain size of all 41 sands was determined as a textural parameter of the sediments. Bulk dry samples of 100–150 MULTIVARIATE ANALYSES g were split using the quartering method. Each sample was Many researchers have applied multivariate techniques, such vibrated for 10 minutes by the Retsch type RV sieve shaker, as factor analysis on grain size to unravel paleosedimentary using 200 mm diameter sieves at 0.25 φ intervals. Grain-size environments or provenances (Klovan, 1966; Solohub, 1970; parameters (mean, median, and sorting) were calculated by a Allen et al., 1971, 1972; Dal Cin, 1976), principal component graphical method (Folk & Ward, 1957). analysis on grain size (Davis, 1970; Chambers & Upchurch, 1979; Lirer & Vinci, 1991; Zhou et al., 1991), factor analysis on sandstone composition (Clemens & Komar, 1988; Wong, 2002), and cluster analysis on sandstone composition (Smosna MODAL COMPOSITION ANALYSIS et al., 1999; Hounslow & Morton, 2004). Cluster and principal The modal compositions were analyzed only for medium- component analyses on both grain size and modal compositions grained sand fraction (1.75–2.00 φ) from the sieved samples, in were applied here to investigate relations among the quantitative order to reduce the effect of grain-size variation (Ingersoll et al., variables. 1984; Decker & Helmold, 1985; Heins, 1993; Nesbitt et al., Grain-size measurements are a type of compositional data 1996; Whitmore et al., 2004). The sands were impregnated with as well as modal compositions. Each category is represented epoxy resin, and thin sections were made to allow point count- by a percentage, which must be positive or zero, and each ob- ing to be carried out using a polarization-microscope. Each thin servation must sum to a constant. This means that composi- section was etched with hydrofluoric acid and stained by im- tional variables are not free to vary independently. Because of mersion in sodium cobaltinitrite solution to identify potassium the constant-sum constraint (closure effect; Chayes 1960), at feldspar (Bailey & Stevens, 1960). Modal counts of 300–350 least one negative correlation must exist between the variables. points were performed following the ‘traditional’ method (Sut- Therefore, they are not suitable for conventional statistical anal- tner et al., 1981; Suttner & Basu, 1985). The Gazzi–Dickinson yses. method was not used in this study because it does not yield in- In order to remove the effects of the constant-sum and non- formation about rock fragment survivability (Suttner & Basu, negativity constraints on covariance and correlation matrices, 1985). the raw percentage data are expressed as logarithms of ratios The probable errors in percent of individual components at (Aitchison, 1981, 2003). Because ratios of constituents in com- the 50 and 95.4 confidence levels can be calculated by E50 = positional data do not change during the closure operation, 0.5 0.5 0.6745 [P(100 − P)/N] and E95.4 = 2 [P(100 − P)/N] , the use of logratios in statistical analysis removes the effect where E is probable error in percent, N is total number of of closure for a given sample A = (x1, x2, ··· , xN ), where xi points counted, and P is percentage of N of an individual are the percentages of N components, the sample transformed component (van der Plas & Tobi, 1965; Galehouse, 1971). For into a centered logratio transform can be expressed as A0 = 1/N example, if the total number of sand grains counted is 300 and (ln[x1/g], ln[x2/g], ··· , ln[xN /g]), where g = (x1 x2 ··· xN ) is 90 (=30%) of these are feldspar, then the probable errors E50 the geometric mean of sample A. This transformation makes and E95.4 are 1.17% and 3.46%, respectively. the variables in each samples sum to zero. To apply logratio The main constituents were quartz, feldspar, rock fragments, transformations to the data sets, all zero values must be re- volcanic glass, heavy minerals, and biogenic clasts. The vol- placed by small positive values (zero-replacement). Zero values canic rock fragments were classified as Rvf (felsic), Rvv (vit- are present in both grain-size and point-count data sets. Values ric), Rvm (microlitic), and Rvl (lathwork). The felsic volcanic of 0.01% for grain size and 0.1% for modal composition were rock fragments (Rvf) were anhedral, microcrystalline mosaic, adopted here as input values for zero-replacement. either granular or seriate, and composed mainly of quartz and feldspar. The vitric volcanic rock fragments (Rvv) were defined by partially to wholly altered glass and vitrophyric grains. The microlitic volcanic rock fragments (Rvm) were defined Grain-size data as fragments containing subhedral to euhedral feldspar plates Multivariate analysis was applied to grain-size data of beach and prisms in pilotaxitic, felted, trachytic, or hyalopilitic pat- and shelf sands in order to classify them into several sedimen- terns of microlites. The lathwork volcanic rock fragments (Rvl) tary zones, and in order to reveal their interactions and rela- contained plagioclase laths in intergranular and intersertal tex- tionships. Grain sizes of river sands generally depend on chan- tures. Plutonic rock fragments (holocrystalline igneous rock nel morphology, stream power, or bed rock lithology, and vary fragments) were subdivided into felsic (Rpf) and intermediate within a sampling site. In addition, river sands have not been

691 Table 1 Localities and grain-size parameters of the sand samples. Orig. Depth Median Mean Sorting GR VCS CS MS FS VFS No. longitude latitude num. (m) (ø) (ø) (ø) (%) (%) (%) (%) (%) (%) m1 263 144º 19.92' 44º 08.00' 80 –0.40 –0.51 1.44 31.2 26.9 26.8 9.7 2.0 3.5 m2 196 144º 22.00' 44º 06.02' 72 2.19 2.18 1.05 2.0 1.3 3.0 24.1 54.6 15.1 m3 197 144º 22.27' 44º 02.03' 42 2.94 2.92 0.30 0.0 0.0 0.1 0.9 37.6 61.3 m4 265 144º 26.16' 44º 05.95' 265 1.71 1.59 1.19 7.9 2.9 6.0 42.5 34.4 6.1 m5 266 144º 26.07' 44º 02.04' 64 2.77 2.78 0.37 0.0 0.0 0.2 1.1 62.1 36.6 m6 209 144º 28.04' 44º 03.92' 94 2.14 1.92 0.85 1.5 2.5 9.8 18.8 57.7 9.7 m7 210 144º 27.85' 44º 00.04' 47 –0.4 –0.59 1.85 41.0 13.7 20.2 17.5 4.5 3.1 m8 268 144º 32.08' 44º 04.03' 116 2.12 1.91 0.75 0.4 1.8 8.9 29.6 53.4 5.9 m9 221 144º 34.00' 44º 05.98' 254 2.87 3.04 1.59 0.0 0.2 0.2 2.6 47.8 49.2 m10 222 144º 34.06' 44º 01.96' 77 2.36 2.13 0.84 0.8 1.4 8.0 18.3 58.0 13.5 m11 223 144º 33.87' 43º 57.99' 46 –1.53 –1.29 1.86 58.7 14.3 12.8 10.6 2.2 1.4 m12 235 144º 40.04' 44º 04.06' 144 1.50 1.50 0.97 2.6 3.4 19.7 36.1 33.4 4.8 m13 236 144º 39.90' 43º 59.97' 63 1.46 1.52 1.16 3.3 6.3 19.0 33.0 28.2 10.3 m14 248 144º 46.09' 44º 02.00' 87 1.87 1.76 0.80 0.1 1.9 13.5 32.2 45.6 6.6 m15 249 144º 45.94' 43º 57.98' 35 1.73 1.76 0.51 0.0 0.1 3.3 55.5 39.0 2.1 m16 272 144º 48.01' 44º 04.01' 145 1.98 1.84 0.96 2.0 2.8 11.8 29.6 43.9 9.9 m17 273 144º 14.98' 43º 59.98' 57 0.91 0.78 1.30 9.3 14.8 24.8 29.7 17.4 4.0 m18 261 144º 52.08' 44º 04.02' 116 1.91 1.81 0.77 1.1 1.8 10.7 31.0 50.4 5.1 m19 274 144º 52.04' 44º 01.99' 69 1.80 1.71 0.62 0.3 1.6 10.0 40.4 45.6 2.1 c1 1 144º 16.36' 44º 01.46' 2.16 2.14 0.31 0.0 0.1 0.3 17.3 81.8 0.5 c2 56 144º 18.40' 43º 58.65' 1.70 1.73 0.59 0.0 0.2 6.3 50.2 42.9 0.2 c3 57 144º 19.80' 43º 58.00' 2.30 2.28 0.28 0.0 0.0 0.0 8.3 90.7 1.0 c4 59 144º 21.70' 43º 57.33' 2.10 2.08 0.30 0.0 0.0 0.0 23.7 76.0 0.3 c5 61 144º 25.10' 43º 56.60' 1.88 1.88 0.43 0.0 0.1 1.8 42.6 55.3 0.2 c6 63 144º 27.26' 43º 56.00' 0.91 0.80 0.56 0.2 6.8 41.2 51.3 0.4 0.0 c7 65 144º 29.70' 43º 55.60' 1.35 1.33 0.55 0.0 0.3 19.4 61.2 19.0 0.0 c8 66 144º 31.60' 43º 55.30' 1.48 1.48 0.49 0.0 0.2 9.5 66.9 23.1 0.4 c9 51 144º 34.70' 43º 55.00' 1.78 1.82 0.40 0.0 0.0 1.7 54.4 43.5 0.4 c10 54 144º 37.60' 43º 55.00' 1.41 1.43 0.27 0.0 0.0 1.9 90.9 7.2 0.0 c11 49 144º 39.73' 43º 54.86' 2.09 2.04 0.33 0.0 0.0 1.9 90.9 7.2 0.0 c12 37 144º 44.60' 43º 55.10' 1.98 1.99 0.31 0.0 0.2 1.1 28.5 70.0 0.1 c13 44 144º 47.87' 43º 55.70' 1.37 1.49 0.45 0.0 0.0 4.5 72.5 22.8 0.2 c14 43 144º 50.10' 43º 56.81' 1.63 1.63 0.32 0.0 0.1 0.2 31.1 68.5 0.1 r1 1a 144º 04.93' 43º 49.70' 2.41 2.26 0.90 5.0 1.3 2.6 23.6 40.6 27.0 r2 2a 144º 06.59' 43º 49.57' 2.78 2.88 0.66 0.2 0.6 0.4 6.7 49.6 42.5 r3 3a 144º 13.22' 43º 49.05' 1.60 1.67 0.64 0.0 0.5 9.7 57.0 25.0 7.7 r4 4a 144º 18.07' 43º 51.51' 0.14 0.59 1.76 16.9 26.4 19.5 12.2 17.0 8.0 r5 5a 144º 22.65' 43º 50.74' –1.09 –0.63 2.36 49.6 10.1 9.5 18.0 9.9 2.7 r6 6a 144º 24.70' 43º 51.88' 2.43 2.34 0.78 0.8 1.0 3.0 24.0 45.7 25.5 r7 7a 144º 39.24' 43º 52.09' 3.00 0.00 0.78 0.6 0.2 0.7 6.1 35.3 57.1 r8 8a 144º 39.87' 43º 53.03' 2.92 2.97 0.74 0.1 0.2 0.5 5.8 43.7 49.6 Abbreviations: GR, gravel; VCS, very coarse-grained sand; CS, coarse-grained sand; MS, medium-grained sand; FS, fine- grained sand; VFS, very fine-grained sand.

692 144° 20' 144° 40' 145° 00'

–90 0 –1000 –80 0 –100 –600 –70 –80 0 –60 –50 –40 400 0 –20 – –300

–200

–100 –80 00'

° –60 Abashiri 44

N –40 500 –20

Shari 10 km 1000

0.20 m/s 0.40 m/s 0.80 m/s

Fig. 6. Distribution of current velocities around Abashiri Bay during summer (Japan Oceanographic Data Center, 2003). The westerly Soya Warm Current dominates in the surface water.

ff –2 1 a ected by oceanic waves, currents, or sea level changes that Md (ø) have influenced beach and shelf sands. It is meaningless to try –1.5 Sorting (ø) 0.8 to incorporate river sands in the same classification as that of –1

–0.5 ) ø beach and shelf sands, so they were excluded from the analysis. ) 0.6 The usefulness of the median grain size (Mdφ) as a gen- ø 0 0.5 eral discriminator is limited because it does not reflect the Md ( 0.4 overall grain-size distribution of a sediment. The interpreta- 1 Sorting ( 1.5 tion of Mdφ and standard deviation (sorting value or σφ) as- 0.2 2 sumes that the grain-size distribution of a sample is approxi- 2.5 0 mately log-normal. Cluster and principal component analyses c2 c4 c6 c8 c10 c12 c14 were here applied to whole grain-size data grouped into six c1 c3 c5 c7 c9 c11 c13 classes: gravel, very coarse-grained sand, coarse-grained sand, φ σφ medium-grained sand, fine-grained sand, and very fine-grained Fig. 7. Median grain size (Md ) and sorting values ( ) of sands in order from west (left) to east (right). sand (Table 1). Due to low percentages of materials > 4.0 φ, this fraction was included in the very fine-grained sand class. sent both the samples (Q-mode) and the variables (R-mode) of Percentage data of the six grain-size classes were transformed compositional data. into centered logratios before multivariate analysis was applied. Hierarchical cluster analysis was used for determining the membership of groupings of samples with similar sets. The analysis was performed employing the Euclidean distances and Modal composition data Ward’s method (Swan & Sandilands, 1995). Behaves of each grain depends on its size, shape, and spe- Principal component analysis was used to determine intercor- cific gravity during transportation. It is necessary to filter out relations between variables and similarities between samples the compositional variation induced by selective transport in or- (Swan & Sandilands, 1995). Biplots were used here to graph- der to evaluate sediment dispersal patterns within the sedimen- ically depict patterns of relative variations of multivariate data tary system. For this purpose, it is important that the sediment sets by projection onto planes fixed by principal components composition should be insensitive to effects of selective trans- (e.g. Aitchison, 1990; von Eynatten et al., 2003). The origin and port due to variations of size, shape, and density. This has been axes of the biplot correspond to the center (geometric mean) the standard approach in petrographic modal analysis of sands, of the entire data and principal components of the log-centered in which the definition of framework composition is based on data, respectively. An advantage of biplots is that they repre- proportions of quartz, feldspar, and rock fragments, provided

693 that the grains are of approximately the same size. In general, Normalized distance most of these framework grains show high sphericity and simi- 0 0.2 0.4 0.6 0.8 1.0 Cluster lar density. It indicates that their proportions are not likely to be m17 affected by selective transport. This allows distinction of com- m4 m12 positional variation induced by selective transport from other m13 mechanisms, such as weathering or mixing of multiple source m2 m10 I compositions (Weltje, 1995; Weltje & Prins, 2003). m6 m16 Multivariate analyses were applied for the framework (QFR) m14 m19 compositions classified into 10 subgroups: total quartz (Q), m8 potassium feldspar (K), total plagioclase (P), felsic volcanic m18 c6 rock fragments (Rvf), vitric volcanic rock fragments (Rvv), mi- m1 II m7 crolitic volcanic rock fragments (Rvm), lathwork volcanic rock m11 fragments (Rvl), felsic plutonic rock fragment, intermediate– m9 m3 III mafic plutonic rock fragment (Rpi–m), and sedimentary rock m5 c1 fragments (Rs). The sum of 10 subgroups was recalculated to c14 c3 100%, and each group was transformed into centered logratios c4 in the same way as the grain-size data. m15 c9 c13 c10 c11 IV c7 c5 c12 RESULTS c2 c8

Fig. 8. Dendrogram given by Q-mode cluster analysis using logratio grain-size GRAIN SIZE distribution, showing clusters I–IV.

Coastal sands Fourteen of the beach sands are medium- to fine-grained sands in Cluster IV are concentrated between 1.33–2.30 φ and (1.35–2.16 Mdφ) and only one (c6) that is coarse-grained (0.91 0.27–0.59 φ, respectively (Fig. 9). The Mdφ of sands in cluster Mdφ) (Table 1 and Fig. 7). The samples are very well to mod- IV are similar to those of Cluster I, but the former shows better erately well sorted (0.27–0.59 φ). Median grain size (Mdφ) and sorting than the latter. sorting values (σφ) delineate a change of grain-size character- The principal component analysis of the grain-size data istics along the shoreline (Fig. 7). The median grain size of shows that the first four components contain 99% of the total sample c6 is the coarsest and its sorting is the poorest among a variance, and thus are sufficient to represent the variation of series of Shari Beach sands (c3–c12). Median grain sizes fine, the data (Table 2). Principal Components 1 (PC1) and 2 (PC2) and sortings improve both eastward and westward from c6. contain 52.8% and 35.2% of the total variance, respectively. The biplot diagram shows that adjacent size classes are lo- cated relatively close to each other, indicating their close correlations (Fig. 10). Arrows in Fig. 10 show the variable load- Multivariate analysis ings on the first two principal components. The first eigenvec- An easily readable dendrogram displays the four clusters I to tor shows a positive and negative loadings on fine- (> 1φ) and IV, which are separated using an approximate distance of 0.7 coarse-grained (< 1 φ) classes, respectively. Clusters I and II de- (Fig. 8). Clusters are also plotted in the graph of median grain pend on gravel and very coarse-grained sand contents, whereas size and sorting value for checking the relationship between Clusters III and IV are determined by medium-, fine-, and very the clusters and grain-size parameters (Fig. 9). Median grain fine-grained sand contents. The average proportions of coarse- sizes of sands in Cluster I are 1.0–2.5 φ and the sorting values grained sand classes (< 1 φ) of Clusters I and II are 18.2 wt% were 0.5–1.5 φ (Fig. 9). The shelf sands in Cluster II show and 73.4 wt%, respectively. In contrast, those of Clusters III the coarsest Mdφ and the poorest σφ among all sands. Median (0.25 wt%) and IV (3.8 wt%) are low. Cluster I, the Mdφ of grain sizes of sands in Cluster III are 2.77–2.94 Mdφ (fine- which is similar to that of Cluster IV, contains more gravel grained sand). Two of the three sand samples (m3 and m5) and very coarse-grained sand, which causes poorer sorting and are very well to well sorted (0.30–0.37 φ), while sample m9 is therefore is separated from Cluster IV. The second eigenvector poorly sorted (1.59 φ). Sample m9 obtained from shelf slope shows positive weighting for the very fine-grained sand class shows higher fine fractions than shelf sands of m3 and m5 and negative weights for the coarse- and medium-grained sand (Table 1). It includes materials from suspension, which make classes. The average content of the very fine-grained sand class the sorting poorer. Median grain sizes and sorting values of in Cluster III is very high (49.1%).

694 2

Q Q 0 )

ø shelf beach river 20 1

Sorting ( 40

0 80 shelf beach –2 –1 0 1 2 3 river Md (ø) 100 100 80 60 40 20 0 Cluster I Cluster II Cluster III Cluster IV F R

Fig. 9. Median grain size (Mdφ) and sorting values (σφ) of beach and shelf EMA sands. PMA

Table 2 Results of principal component analysis for grain-size composition. F R Variable Comp.1 Comp.2 Comp.3 Comp.4 GR –0.866 0.287 –0.329 0.241 Fig. 11. Ternary diagrams for quartz (Q), feldspar (F), and rock fragments VCS –0.910 0.025 0.043 –0.411 (R). Fields for discriminating provenance types of PMA (primitive volcanic CS –0.406 –0.810 0.391 0.153 arc) and EMA (evolved and mature magmatic arc) were proposed by Kumon et al. (1992); Kumon & Kiminami (1994). Shaded areas of shelf, beach, and MS 0.547 –0.802 –0.209 0.006 river in the small diagram represent 90% confidence regions of the mean FS 0.976 0.025 –0.093 –0.146 (Weltje, 2002) for each sampling province. VFS 0.417 0.855 0.286 0.087 Eigen values 3.169 2.114 0.397 0.279 Proportion 52.817 35.229 6.622 4.647 General Cumulative proportion 52.817 88.045 94.667 99.315 All sands are deficient in quartz and rich in feldspar or rock fragments (Fig. 11). These sands show typical characteristics of sediments related to volcanic arcs, and fall within the primitive –6 –4 –2 0246 volcanic arc field of the QFR discriminant diagram (Kumon et al., 1992; Kumon & Kiminami, 1994). Shelf sands contain PC1 = 52.8% PC2 = 35.2% slightly more quartz and fewer rock fragments than beach and VFS river sands (Fig. 11). Quartz contents in beach sands are similar

0.2 to those in river sands. One river sand sample (r8) shows a high

GR proportion of rock fragments (90.5%). The distribution of mineral and rock fragments percentages FS

0246 provides a general indication of the depositional provinces in

0.0 VCS the study area (Table 3 and Fig. 12). Plagioclase is one of the PC2

2 main constituents in all sands (Fig. 12A). Average abundances – of plagioclase in shelf, beach, and river sands are 30.2, 34.3, and MS 0.2

– 29.6%, respectively. Two types of plagioclase grains have been CS 4 – recognized. One type is fresh, with or without normal zoning, some of which are surrounded by volcanic glasses around the 6 – rim. This fresh type is in the majority in all samples. The other 0.4 – type is altered, some of which partly replaced by secondary –0.4 –0.2 0.0 0.2 sericite, calcite, or epidote. PC1 Contents of felsic volcanic rock fragments in beach and shelf sands are relatively high (20–40%) in the western side of Fig. 10. Biplots for grain size compositions. The vertical and horizontal Abashiri Bay compared to less than 15% in the eastern side axes correspond to the ﬁrst (PC1) and second (PC2) principal components, (Fig. 12B). respectively. Percentages indicate proportions of total variability explained by Vitric volcanic rock fragments are minor components in al- PC1 and PC2. Symbols as in Fig. 9. most samples (Fig. 12C). The proportions in shelf and beach sands average 5–6%, while those in river sands are more variable (4.3–34.4%), indicating the local eﬀects of source lithologies. In particular, river sand sample of r8 shows the highest MODAL COMPOSITION proportion of vitric volcanic rock fragments.

695 144º 20' 144º 40' 44º 10' (A) Plagioclase (B) Felsic volcanic

44º 00' Abashiri Abashiri

Shari Shari

43º 50' 10 km 10 km

Abashiri Abashiri

Shari Shari

10 km 10 km

(E) Volcanic glass (F) Heavy mineral

Abashiri Abashiri

Shari Shari

10 km 10 km

80% 60% 40% 20%

Fig. 12. Plots of minerals and rock fragments as a percentage of point count. Each symbol is centered on the sample location.

Microlitic and lathwork volcanic rock fragments are cor- related to intermediate–mafic volcanic rocks (andesite–basalt) (Dickinson, 1970). Sands from the eastern side of Abashiri Bay Multivariate analysis for framework composition (c10, c12, c13, c14, m16, m18, and m19) contain a higher per- Numerical classification of the framework compositions centage (more than 20%) of these rock fragments (Fig. 12D). (quartz–feldspar–rock fragment) of total 41 samples was ac- Sedimentary rock fragments are also very minor components complished using hierarchical Q-mode cluster analysis. The in all samples except sample r8. Sample r8 shows higher per- resulting dendrogram groups them into four clusters (A, B, C, centage (15.6%) of sedimentary rock fragments dominated by and D) using a normalized distance of 0.5 (Fig. 13). siltstone. Almost all of the siltstone fragments in sample r8 are Applying the principal component analysis to the framework angular, and contain diatom fragments in silty matrices. compositions yields 10 principal components. Of these, the first The distribution of volcanic glasses is fairly restricted four contain 72.4% of the total variance (Table 4). Loadings of (Fig. 12E). River sands contain relatively high proportions variables Rvm, Rvl, Rpi–m, Rvv, and Rs are positive in PC1, of volcanic glasses (10.1–45.3%). Samples c2, c13, m3, m5, whereas those of Q, K, Rpf, Rvf, and P are negative (Fig. 14). and m9 contain more than 10% volcanic glass. Massive-type The variable Rs has a strong influence on the second principal volcanic glasses are dominant in the beach sands, whereas component. PC1 is controlled by contents of detritus derived pumice-type volcanic glasses are dominant in the shelf and from felsic volcanic rocks and minerals. Cluster A shows a river sands (Table 3). good correlation to variables Q, K, Rpf, and Rvf. Cluster B is Heavy minerals are abundant in the shelf sands obtained characterized by variables Rvv, Rpi–m, Rvl, and Rvm. Samples from central and eastern Abashiri Bay (Fig. 12E). In particular, φ in Cluster C are concentrated near the origin of the biplot. sands coarser than 2 Md have higher percentages of heavy Cluster D is influenced by Rs. The distance Q–K–Rpf is very minerals. Clinopyroxene, orthopyroxene, and opaque minerals small in the biplot, and shows close correlation among the are the dominant minerals. Some beach (c7 and c8) and river > variables (Fig. 14). There is also little distance between Rvv, sands (r5) also have high heavy mineral contents ( 10%). Rpi–m, and Rvl.

696 Table 3 Detrital modes of the sand samples. Q F R VG HM BC Clusters No. Orig. Total num. Qs Ks Ps Pme Rvf Rvv Rvm Rvl Rpf Rpi–m Rs VGb VGp VGmt VGmc Cpx Opx Op Hbl Bt Bioclast Count size mode m1 263 2.7 1.2 25.0 1.5 20.5 4.5 7.1 6.3 2.1 0.0 0.0 0.0 0.0 1.5 0.3 2.1 7.4 0.3 0.0 0.0 17.6 336 II A m2 196 2.9 1.3 28.1 1.0 34.0 8.2 8.5 9.8 0.7 0.0 0.0 0.0 0.0 2.0 0.0 1.0 2.3 0.3 0.0 0.0 0.0 306 I A m3 197 5.0 1.3 37.7 2.2 29.9 1.6 2.8 0.6 0.0 0.0 0.6 0.0 8.5 4.1 0.6 1.6 1.3 0.6 0.0 0.0 1.6 318 III A m4 265 5.5 0.0 21.8 0.3 33.4 5.2 6.7 9.8 1.2 0.0 0.0 0.3 0.9 0.6 0.0 1.8 7.4 3.7 0.0 0.0 1.2 326 I C m5 266 3.9 1.0 32.4 0.6 15.2 4.9 7.1 5.8 0.0 0.0 0.0 1.0 16.1 3.5 1.9 3.2 2.6 0.6 0.3 0.0 0.0 309 III A m6 209 1.8 0.6 19.0 0.3 17.2 4.3 7.1 4.6 0.6 0.0 0.0 0.0 0.3 2.5 0.0 5.5 24.5 11.3 0.0 0.0 0.3 326 I A m7 210 0.6 0.6 23.3 0.6 4.3 0.6 0.9 1.2 0.3 0.0 0.0 0.0 0.6 0.6 0.3 6.2 29.8 29.2 0.0 0.0 0.6 322 II A m8 268 2.3 0.0 33.3 0.3 6.3 3.4 3.4 6.0 0.6 0.3 0.0 0.0 1.1 1.1 0.9 5.7 11.8 23.0 0.0 0.0 0.3 348 I C m9 221 6.7 1.8 19.0 1.8 29.1 10.7 7.0 5.2 2.4 0.0 0.0 0.0 9.5 4.9 0.0 0.9 0.6 0.0 0.0 0.3 0.0 327 III A m10 222 2.1 0.3 39.3 0.3 14.4 5.8 5.5 1.5 2.8 0.0 0.0 0.0 1.2 3.1 0.6 6.1 12.0 4.9 0.0 0.0 0.0 326 I A m11 223 1.0 0.0 40.3 0.7 6.7 2.7 3.3 2.7 1.3 0.3 0.0 0.0 0.7 1.7 0.3 10.3 23.7 4.3 0.0 0.0 0.0 300 II C m12 235 3.1 0.8 48.4 0.0 7.1 1.7 10.8 3.1 0.8 0.0 0.0 0.0 0.3 1.4 0.6 4.5 14.4 3.1 0.0 0.0 0.0 353 I A m13 236 1.8 0.3 40.4 0.0 15.4 2.1 6.9 0.9 0.0 0.0 0.0 0.0 0.3 2.4 0.9 2.4 22.7 3.3 0.0 0.0 0.0 332 I A m14 248 0.3 0.0 37.1 0.9 14.5 5.3 4.4 5.0 0.0 0.0 0.0 0.0 0.3 2.2 1.3 2.8 10.4 15.4 0.0 0.0 0.0 318 I B m15 249 1.2 0.3 45.4 0.3 11.0 5.2 4.6 4.6 2.4 0.0 0.0 0.0 0.0 0.3 0.0 8.0 16.5 0.0 0.0 0.0 0.0 328 IV A m16 272 2.8 0.0 17.6 0.8 11.2 4.2 14.8 10.6 2.2 1.1 0.0 0.0 0.8 1.4 0.6 4.5 18.8 8.4 0.0 0.0 0.0 357 I C m17 273 0.9 0.0 5.6 0.0 3.7 1.6 2.2 1.9 0.0 0.0 0.3 0.0 0.0 0.0 0.0 1.9 27.3 54.7 0.0 0.0 0.0 322 I D m18 261 0.3 0.3 35.8 0.3 14.1 8.9 12.8 15.9 0.6 0.0 0.0 0.0 0.6 2.4 1.2 2.8 2.8 1.2 0.0 0.0 0.0 327 I B m19 274 3.0 0.6 25.1 0.3 15.1 13.6 13.0 9.5 1.2 0.6 0.9 0.0 1.2 3.2 1.5 3.2 7.7 0.6 0.0 0.0 0.0 338 I D c1 1 2.6 0.3 13.5 1.9 38.5 12.5 3.8 16.7 1.6 0.0 1.9 0.0 0.3 5.1 0.3 0.0 0.0 0.0 0.0 0.0 1.0 312 IV D c2 56 0.6 0.6 15.4 0.9 24.1 7.4 13.0 19.4 0.6 0.3 0.0 0.0 0.0 9.9 5.2 0.6 0.6 0.0 0.0 0.0 1.2 324 IV B 697 c3 57 3.3 0.0 27.8 1.6 30.4 10.8 7.8 12.1 1.3 0.3 1.0 0.0 0.0 2.3 0.3 0.3 0.0 0.0 0.0 0.0 0.3 306 IV D c4 59 1.7 0.0 25.1 0.3 40.6 6.3 10.7 8.9 3.5 0.0 0.0 0.0 0.0 2.3 0.6 0.0 0.0 0.0 0.0 0.0 0.0 347 IV C c5 61 2.1 0.0 38.7 0.3 37.2 4.3 3.7 9.5 0.6 0.0 0.0 0.0 0.0 1.5 0.3 0.3 1.5 0.0 0.0 0.0 0.0 328 IV C c6 63 1.2 0.0 64.0 0.3 11.4 3.3 7.5 6.3 0.9 0.0 0.0 0.0 0.0 1.2 0.6 0.0 3.3 0.0 0.0 0.0 0.0 333 II C c7 65 0.3 0.0 40.7 0.7 16.9 0.3 3.6 5.3 0.3 0.3 0.0 0.0 0.0 0.0 0.3 7.0 23.8 0.3 0.0 0.0 0.0 302 IV C c8 66 0.7 0.7 36.5 0.7 22.0 1.3 3.3 9.9 2.0 0.0 0.0 0.0 0.0 1.0 0.3 3.9 17.4 0.3 0.0 0.0 0.3 304 IV A c9 51 1.2 0.0 53.4 0.9 20.2 4.9 6.1 8.0 0.9 0.3 0.0 0.0 0.0 0.6 0.3 0.9 2.1 0.0 0.0 0.0 0.0 326 IV C c10 54 0.6 0.0 50.2 1.3 13.8 6.8 13.5 10.0 0.0 0.0 0.0 0.0 0.0 2.3 1.3 0.0 0.0 0.0 0.0 0.0 0.0 311 IV B c11 49 1.9 0.0 40.7 2.5 12.4 5.3 2.8 14.3 1.2 0.0 0.0 0.0 0.0 0.3 0.6 4.0 13.4 0.6 0.0 0.0 0.0 322 IV C c12 37 1.6 0.6 44.0 2.8 11.7 6.6 9.8 15.2 0.6 0.9 0.0 0.0 0.0 2.2 1.9 1.0 0.3 0.3 0.0 0.0 0.0 316 IV B c13 44 0.5 0.0 17.3 1.1 23.9 11.8 14.6 14.6 0.3 2.7 0.3 0.0 0.5 8.0 3.3 0.8 0.0 0.0 0.0 0.0 0.3 364 IV B c14 43 1.5 0.9 12.8 0.9 8.6 5.8 6.7 16.5 2.1 9.8 0.0 0.0 0.0 0.0 0.0 13.8 19.0 1.2 0.0 0.0 0.3 327 IV B r1 1a 1.9 0.0 33.7 0.3 25.6 15.7 3.5 6.7 2.6 0.6 0.0 0.0 0.6 3.2 1.0 3.8 0.6 0.0 0.0 0.0 0.0 312 C r2 2a 1.6 0.0 31.9 0.6 23.0 13.4 5.1 5.1 0.0 0.6 0.0 0.0 9.9 3.2 1.9 1.0 1.9 0.6 0.0 0.0 0.0 313 B r3 3a 1.3 0.0 51.3 0.3 9.7 8.1 5.2 5.2 0.0 0.6 0.0 0.0 4.2 3.6 2.3 7.8 0.3 0.0 0.0 0.0 0.0 308 B r4 4a 1.0 0.0 37.7 0.3 6.6 15.2 6.6 5.6 0.0 0.3 0.0 0.0 6.6 3.3 2.0 7.0 4.6 3.0 0.0 0.0 0.0 302 B r5 5a 0.3 0.0 32.4 0.7 7.0 4.3 2.3 1.7 0.0 1.0 0.0 0.0 1.0 0.7 0.7 25.3 1.7 21.0 0.0 0.0 0.0 302 B r6 6a 0.3 0.0 26.3 0.3 4.2 12.7 10.4 5.8 0.0 1.0 0.0 0.3 19.2 3.9 6.2 7.5 0.6 1.3 0.0 0.0 0.0 308 B r7 7a 0.3 0.0 16.8 0.0 3.6 13.6 9.1 8.1 0.0 1.0 0.0 1.0 35.9 2.3 6.1 1.3 0.0 1.0 0.0 0.0 0.0 309 B r8 8a 0.3 0.0 6.6 0.0 4.0 34.4 1.7 8.6 0.0 1.7 15.6 0.3 23.8 2.0 1.0 0.0 0.0 0.0 0.0 0.0 0.0 302 D Abbreviations: Q, quartz; F, feldspar; R, rock fragments; VG, volcanic glass; HM, heavy mineral; BC, bioclast; Qs, single quartz; Ks, single potassium feldspar; Ps, single plagioclase; Pme, altered plagioclase; Rvf, felsic volcanic rock fragments; Rvv, vitric volcanic rock fragments; Rvm, microlitic volcanic rock fragments; Rvl, lathwork volcanic rock fragments; Rpf, holocrystalline felsic igneous rock fragments; Rpi–m, holocrystalline intermediate–mafic igneous rock fragments; Rs, sedimentary rock fragments; VGb, bubble-wall type volcanic glass; VGp, pumice type volcanic glass; VGmt, massive-colorless type volcanic glass; VGmc, massive-colored volcanic glass; Cpx, clinopyroxene; Opx, orthopyroxene; Op, opaque mineral; Hbl, hornblende; Bt, biotite. Normalized distance –5 0 5 10

0 0.2 0.4 0.6 0.8 1.0 Cluster 10 0.6 PC1 = 31.0% m3 PC2 = 16.7% m5 m13 m2 m6 m1 0.4 m9 A

m10 5 m15 Rs c8 m7

m12 0.2

r5 PC2 r4 r2 Q r3 Rvv c13 K Rpf 0 r6 0.0 Rpi–m r7 B Rvf c14 m14 Rvl c10 c12

m18 0.2 c2 – P c4

c11 5 m4 Rvm – c5 c7 C m16 –0.2 0.0 0.2 0.4 0.6 r1 c6 c9 PC1 m8 m11 r8 m17 Cluster A Cluster B Cluster C Cluster D c1 D m19 c3 Fig. 14. Biplots for the framework compositions. Percentages indicate proportions of total variability explained by PC1 and PC2. Fig. 13. Hierarchical Q-mode cluster analysis grouping the 41 sand samples into clusters A–D, based on the framework (quartz–feldspar–rock fragments) graphic conditions are the most important factors that modified compositions. the texture and petrology of the sands. Wave erosion surfaces Table 4 during LGM can be estimated at 130 m below the present level, Results of principal component analysis for QFR compositions. based on the undersea topography such as Notoro Spur (Mar- Variable Comp.1 Comp.2 Comp.3 Comp.4 itime Safety Agency, 1990) (Fig. 16A). Terrestrial sediments Q –0.708 0.196 –0.268 –0.007 might be deposited in the present-day shelf area (previous land), K –0.635 0.106 –0.009 –0.469 P –0.296 –0.617 –0.408 –0.030 such as the Bihoro, Yambetsu, and Sattsuru Formations. Dur- Rvf –0.565 –0.142 –0.203 0.650 ing the following rapid transgression (20–11 ka), the terrestrial Rvv 0.730 0.102 –0.271 0.098 sediments were submerged under the sea. Rvm 0.171 –0.780 –0.217 –0.055 Rvl 0.565 –0.290 0.458 0.436 Zone I (Cluster I) is a recycled sedimentary environment, ly- Rpf –0.484 0.059 0.793 0.068 ing on the middle and outer shelf between water depths of 50 Rpi–m 0.748 –0.063 0.081 –0.402 and 200 m. Transgression progressively eroded the paleoshore- Rs 0.338 0.712 –0.390 0.238 line and reworked the sediments previously deposited. Such Eigen values 3.102 1.668 1.397 1.070 recycled sediments are referred as palimpsest sediments (Swift et al., 1971); that is, a relict deposit composed of particles sup- Proportion 31.015 16.679 13.967 10.700 plied to this zone before the present. This zone is dominated Cumulative proportion 31.015 47.695 61.662 72.361 by moderately to poorly-sorted and medium- to coarse-grained sands with gravels or very coarse-grained sands. Zone II (Cluster II) is a high-energy relict sedimentary envi- DISCUSSION ronment. The sediments in this zone are characterized by poor sorting and contents of coarse-grained sands to granules. This zone is geographically divided into two areas. One is in the flat area in the central Abashiri Bay between the depths of 40 and SEDIMENTARY ZONES AND THEIR EVOLUTION 60 m (Zone II-1), and the other lies off Cape Notoro at a water The spatial distribution of sediment grain size indicates that depth of 80 m (Zone II-2). The latter corresponds to the second the study area can be divided into four sedimentary zones submarine terrace (Fig. 5A). Transgressional or regressional (Zones I–IV) corresponding to four grain-size clusters (Fig. 15). reworking might selectively remove fine-grained particles and The four sedimentary zones and their evolution from late Pleis- leave coarse-grained detritus. During the Younger Dryas (11– tocene to present time are discussed below (Fig. 16). 10 ka; Keigwin & Gorbarenko, 1992; Gorbarenko et al., 2002), The rate of sea level rise after the Last Glacial Maximum the sea level may have been 40–60 m below the present sur- (LGM: ca. 20 ka) is an order of magnitude greater than tectonic face level (Fig. 16B). Falling of sea level during this cooling uplift/subsidence in the study area (e.g. Okumura, 1996). This event increased reworking of previous transgressive sands, and suggests that transgression and associated changes in oceano- remained gravels and coarse-grained sands the eroded surface,

698 144˚10' 144˚20' 144˚30' 144˚40' 144˚50'

44˚10' – –60 500 km –400 –40 0 5 10 –300

20 – –200

–100 –80 –60 Abashiri 44˚00'

–20 –40 50 0 Shari

43˚50'

Fig. 15. Distribution of each cluster of the grain-size data in the study area. Symbols as in Fig. 9. acting as marine basal lags. In addition, the gravelly sediments Zone III-2 (m9 of Cluster III) is situated in the shelf slope in this zone are not covered by modern sediments, and keep where fine-grained and poorly-sorted sediments are distributed their character as relict deposits. After the Younger Dryas age, (Fig. 15). Additional data sets imply that the shelf slope sedi- sea level rose 2.5–3.5 m above the present level between 6–7.6 ments around the sampling site of m9 are texturally and petro- ka (Umitsu, 1983; Hirai, 1987; Kodaira, 1996). In this period, logically similar to sample m9 (Noda et al., in press). It is con- river valleys were drowned and estuaries developed in a trans- sidered that sample m9 represents shelf slope sediments in this gressive barrier island system which trapped sediments (Ko- zone. Muddy particles from suspension presently blanket the daira, 1996). Subsequently, sea level fell to its present level at 4 shelf edge to the shelf slope, and make the sorting poorer. The ka, and remained high-stand with only small-scale fluctuations bottom current energy in this zone may be low enough to al- through to the present time (Fig. 16C). low fine particles to settle due to the deep water. These are also Another interpretation is possible for the sediments in Zone considered to be as proteric deposits whose suspended parti- II-2. Oceanic current is one of the main processes for sediment cles may be derived from pre-existing sediments on the shelf transport under the sea. This zone is overlain by the path of due to resuspension. the Soya Warm Current. The current speed is sometimes over Zone IV (Cluster IV) consists mainly of sands in Shari Beach 100 cm/s at the sea surface during the summer (Japan Oceano- (Fig. 15), which is considered to have originally formed as a graphic Data Center, 2003). It is possible that the bottom sed- barrier island system until ca. 4 ka (Kodaira, 1996). Sands are iments are winnowed by the current and coarse lags remain. medium- to fine-grained, and well to very well sorted. Grain Zone III-1 (m3 and m5 of Cluster III) is characterized by sizes become finer and sortings improves both eastward and modern terrigenous sedimentary materials deposited in the in- westward from sample c6 among a series of the Shari Beach ner shelf of the western Abashiri Bay (Fig. 15). These sed- sands. A possible reason of this tendency is that wave energy is iments are well-sorted and fine-grained. The westerly Soya strongest at locality c6 and weakens to both the east and west. Warm Current winnows detritus from offshore of Cape Notoro, and in the east to the cape it expands and decelerates as the shelf widens. The current loses successively coarser fractions of the transported load, and in the process, progressively sorts EFFECTS OF SOURCE LITHOLOGY ON COMPOSITION it. The deposits are composed of particles that were supplied to Distributions of the clusters based on the framework com- the depository in the past, and the process may be in progress. positions were mapped to aid interpretation of the relation- This type of sediment is called a proteric sediment (McManus, ship between source lithology and the sedimentary environment 1975). Parasound subbottom profiler records show a transpar- (Fig. 17). By restricting the concept of framework composition ent seismic facies with a subbottom reflector overlying Zones I to a subpopulation of hydraulically equivalent grains, the ef- and II-1 in this area (Noda et al., in press). It supports modern fect of selective transport is eliminated. Accordingly, the results sedimentation on the previously deposited shelf sediments. from the framework compositions can suggest two features: (1)

699 effects of source lithology on the composition, and (2) effects of mixing, recycling, and transportation on the composition. Cluster analysis does not consider the spatial relationships 144° 20' 144° 40' among samples and simply classifies them on the basis of their numerical distances. In the actual space, compositions of the A coast line sediments gradationally change among the samples. In practice, Clusters A, B, and C partly overlap in the biplot diagram of the framework composition (Fig. 14). The cumulative proportion of total variability of PC2 was only 47.7%, which suggests a 00' complexity of the composition. ° Abashiri 44 Almost all samples in Cluster A are distributed in Abashiri N Bay (Zones I, II, and III) (Fig. 17). Just before the LGM, the Kutcharo pyroclastic flow (Kpfl-I) deposits of ca. 32 ka may Shari have spread extensively over the Shari Plain and the present- 10 km day Abashiri Bay area. Rapid sea level rise after the LGM submerged the huge amounts of dacitic pyroclastic detritus, and caused enrichment of felsic volcanic rock fragments in the B shelf sediments. Modern sediments in Zone III-1 are classified into Cluster A. In this zone, distributors of modern sedimentary particles, mainly the Soya Warm Current, transport and redistribute sed- 00' iments deposited in the past. Therefore, the framework com- ° Abashiri 44 positions are influenced not by modern coastal sands, but by N pre-existing shelf sands. Clusters B and D in eastern Abashiri Bay (Fig. 17) are in- Shari fluenced by andesitic–basaltic volcanic and sedimentary rocks 10 km exposed in the Shiretoko Peninsula. This area may have been outside the depositional area of the Kutcharo pyroclastic flows, ff –600 and the sand compositions are less a ected by felsic volcanic C –500 detritus. A steep shelf slope and a rocky coast in front of the –400 –300 Shiretoko Peninsula with high mountains could generate and –200 transport many volcanic and sedimentary rock fragments to the –100 –80 shelf. –60 00' Most river sands were classified into Cluster B (Fig. 17). ° Abashiri –40

Early Pleistocene andesite and basalt are exposed in the head- 44 N –20 waters (Katsui, 1962) of several rivers (samples r3–r7) and may influence the sediment composition. Sample r8 shows a high Shari proportion of sedimentary rock fragments, and is in Cluster D. 10 km At the source area of the locality r8, Tertiary sedimentary rocks are widely outcropped (Fig. 2). Almost all of the sedimentary rock fragments are angular in sample r8, and infer short trans- Zone I Zone III-1 portation with less abrasion. Beach sands of Cluster C (Fig. 17) fell near the origin of the Zone II-1 Zone III-2 biplot (Fig. 14). This suggests that the beach sands have been homogenized by longshore drift or mixing by fluctuations of Zone II-2 Zone IV transgression and regression during the Holocene. Sands in the central beach have higher plagioclase contents than other beach Fig. 16. Evolution of the sedimentary zones. (A) 20 ka: Paleoshoreline is sands (Fig.12A). A large part of these plagioclase grains may ca. 100–130 m during the Last Glacial Maximum. Fluvial systems were developed in the present-day shelf area, and transported detritus. Terrigenous be derived from rivers due to high plagioclase contents of river sediments were distributed in the present-day Abashiri Bay area. (B) 10 ka: samples r3–r6. Beach sands in Cluster B are also affected by Rapid transgression had submerged the sediments and recycled them (Zone I). andesitic–basaltic volcanic rock fragments derived from moun- In the Younger Dryas age, a submarine terrace formed off Cape Notoro and in tains or coastal cliffs in the Shiretoko Peninsula. the flat area in the central Abashiri Bay, on which lag deposits were deposited (Zone II). (C) present: High-stand of the sea level allows sedimentation in western Abashiri Bay (Zone III-1), which may be caused by the Soya Warm Current. Lag deposits still remain as relict sediments in Zone II.

EFFECTS OF MIXING, RECYCLING, AND TRANSPORT ON THE COMPOSITION

700 Shelf sediments in Zones I and II may have been recycled tances from the source to the sedimentary area, which re- within the sedimentary system. A predicted effect of polycyclic- duce the times that sediment is exposed to weathering or is ity on sediment composition is an enrichment in chemically and abraded during transport (Ruxton, 1970; Mack, 1984; Basu, physically stable phases and textures at the expense of more 1985; Johnsson, 1989). Additionally, it has been suggested that labile ones. There are surprisingly few published studies doc- fluvial transport is not a major influence on compositional ma- umenting the survivability of minerals and rock fragments in turity (Basu, 1976; Breyer & Bart, 1978; Nesbitt & Young, modern shelf sands. Those studies suggested that maturity of 1996). The lengths of all the rivers in the study area, except marine or beach sand samples was higher in fluvial samples the Abashiri River, are less than 100 km, and the drainage ar- (e.g. Mack, 1978; McBride et al., 1996). For example, Mack eas are also small. Especially, there is no river longer than 10 (1978) described how there was significantly less total feldspar km in the Shiretoko Peninsula. Short transport distances tend and rock fragments in shallow marine sands than in fluvial sam- to allow retention of the original sand compositions from the ples. There was a greater than 50% decrease in feldspar or rock source to the sampling sites. fragments between braided channels and marine sands. The re- The poor representation of sedimentary rock fragments sug- sults of this study, however, do not show such major composi- gests depletion of these grains from source to sand when com- tional changes among river, beach, and shelf sands. The matu- pared with the percentages of areal extent in the drainage ar- rity index Q/FR shows a small increase in the order: river (1.2), eas. This is due to the poor capability of sedimentary rocks to Zone IV (beach, 1.7), Zone II (relict, 2.2), Zone I (palimpsest, generate lithic sand or rapid destruction of sedimentary lithic 3.6), and Zone III (modern proteric, 7.0) (Table 5). Higher ma- grains during transport (Cameron & Blatt, 1971; Grantham & turity in the recycled (palimpsest and modern proteric) sedi- Velbel, 1988; Cavazza et al., 1993; McBride & Picard, 1987; ments suggests that mechanical, chemical, and possibly biolog- Ingersoll et al., 1993). Siltstones, one of the main constituents ical reduction of labile grains could occur under the sea. of the sedimentary rocks in this area, appear to be selectively The compositional maturities in all sedimentary zones destroyed by wave action in high-energy coastal environments are still as low as other arc-derived detritus (Ruxton, 1970; or fluvial transport. As the coastline from Abashiri to Cape No- Marsaglia & Ingersoll, 1992; Marsaglia et al., 1992). Sev- toro is rocky, the potential supply of sedimentary rock detritus eral controlling factors of (1) to (4) are considered for the from the rocky coast is considerable. However sedimentary rock compositional maturity in this study. fragments are absent, although they are exposed around Cape (1) Source lithological control: scarcity of quartz grains in Notoro (Fig. 2). These sedimentary rocks (mainly Miocene silt- the original source rocks. An active volcanic island arc com- stones) are very labile owing to weak consolidation, and can monly contains much dacitic to basaltic volcanic rock, which be easily broken by wave activity. generally lack quartz. Actually, volcanic rocks in the eastern Hokkaido are deficient in quartz (Katsui, 1962). In addition, loose pyroclastic flow deposits that are widely distributed in land are easily eroded, and thus, may lead to an increase in volcanic rock fragment contents and a dilution of quartz contents CONCLUSIONS in the sediments. Detailed textural and petrological study of modern river, (2) Mineralogical controls: the durability of fresh plagioclase beach, and shelf sands in and around Abashiri Bay yields the grains and felsic volcanic rock fragments. Nesbitt & Young following conclusions. (1996) also reported that there was no significant change in the Four sedimentary zones are identified in the study area: Zone feldspar to quartz ratio of sands over the entire length of the I, palimpsest; Zone II, relict; Zone III, modern (proteric); Zone Genoba River fluvial system, and concluded that abrasion dur- IV, coastal sedimentary environments. Zone I is distributed on ing bedload transport was insufficient to comminute feldspar. the middle–outer shelf in Abashiri Bay. The sediments are in- The high durability of felsic volcanic rock fragments to fluvial terpreted as palimpsest sediments that are medium- to coarse- transport or beach abrasion is also known (Cameron & Blatt, grained sands with gravels or very coarse-grained sands. Grav- 1971; McBride & Picard, 1987; McBride et al., 1996; Garzanti elly sediments in Zone II are considered as relict deposits et al., 2002). Fresh plagioclase grains are dominant in the total formed during the Younger Dryas (11–10 ka). Zone III is re- feldspar category in this study. They are first-cycle detritus from garded as a modern (proteric) sedimentary zone, which is sub- volcanic rocks or tephras, and have few cracks in the crystals divided into III-1 and III-2 due to geographical setting and sort- and sometimes are surrounded by volcanic glass shards. They ing characteristics. Zone III-1 is a sedimentary environment in may resist mechanical abrasion or chemical weathering. which the westerly Soya Warm Current transports and accu- (3) Climate controls: Low precipitation and cool climate de- mulates well-sorted and fine-grained sands. Sediments in Zone crease the intensity of chemical weathering of feldspar and III-2 are also fine-grained but poorly sorted, and are added to rock fragments in sediments (Grantham & Velbel, 1988; Heins, suspended fine particles. Zone IV is coastal, modern beach to 1993; Nesbitt et al., 1996; Nesbitt & Young, 1996). The study littoral environment. Sands are fine-grained and well to very area has the lowest annual precipitation in Japan (ca. 800 mm), well sorted. and lies in subpolar climate. This climatic circumstance pro- Framework compositions of river, beach, and shelf sands ba- hibits the sediments from chemical weathering. sically reflect their source lithology. Western and central shelf (4) Geomorphological controls: steep slopes or short dis- sands (Cluster A) are controlled by dacitic Kutcharo pyroclastic

701 Table 5 Averages of framework compositional data. σ means standard deviation of Q/FR ratio. Abbreviations as in Table 3. Q K P Rvf Rvv Rvm Rvl Rpf Rpi–m Rs Q F R Q/FR σ Cluster I 3.4 0.5 43.4 22.1 7.7 11.7 9.4 1.2 0.2 0.3 3.4 43.8 52.7 3.6 1.9 Cluster II 2.2 0.9 62.0 16.4 4.1 6.6 5.9 1.8 0.1 0.0 2.2 62.9 35.0 2.2 1.2 Cluster III 6.6 1.7 40.1 30.9 7.2 7.3 5.1 1.0 0.0 0.3 6.6 41.8 51.7 7.0 1.5 Cluster IV 1.6 0.3 39.9 25.3 7.3 8.6 13.9 1.5 1.5 0.2 1.6 40.2 58.2 1.7 0.9 River 1.2 0.0 43.0 13.8 20.9 8.3 8.5 0.4 1.3 2.7 1.2 43.0 55.9 1.2 0.7

Cluster A 3.7 1.2 48.7 24.8 5.7 8.2 6.1 1.6 0.0 0.1 3.7 49.9 46.5 3.9 2.2 Cluster B 1.0 0.2 43.0 16.2 12.8 11.4 12.8 0.4 2.1 0.0 1.0 43.3 55.7 1.1 0.7 Cluster C 2.6 0.0 47.8 23.7 6.1 7.7 10.0 1.7 0.4 0.0 2.6 47.8 49.6 2.7 1.9 Cluster D 3.2 0.2 24.3 23.9 19.6 8.7 13.0 0.9 0.7 5.5 3.2 24.5 72.3 3.3 2.0

Soya Warm Current 144˚10' 144˚20' 144˚30' 144˚40' 144˚50' 44˚10' – –60 500 km – 400 –40 0 5 10 –300

20 – –200

proteric –100 –80 –60 palimpsest 44˚00' Abashiri relict –20 –40 coastal 500 Lithic detritus Shari (volcanic and Feldspar sedimentary lithic Volcanic lithic fragmetns fragments)

43˚50'

Size zones Sand clusters Direction of current Zone I Zone III-1 Cluster A Direction of sediment dispersal Zone II-1 Zone III-2 Cluster B

Zone II-2 Zone IV Cluster C

Cluster D

Fig. 17. Schematic model of the sediment dispersal systems.

flow deposits. Shelf sands in the eastern Abashiri Bay (Clus- compositions slightly. The active volcanic arc generates large ters B and D) are controlled by Neogene andesitic–basaltic vol- volumes of volcaniclastic detritus with less quartz grains. The canic and sedimentary rock fragments. Beach sands (Cluster C) source lithology, mineralogy, climate and geomorphology con- show mixed or plagioclase-rich characteristics. Compositions trol the maturity of the sediments. Sedimentary rock fragments of beach sands in pocket beaches are influenced by their local are weakly consolidated, and are poorly preserved in the sed- sources. iments due to rapid destruction during transport or nearshore There is a small-scale increase of compositional maturity processes. Processes operating during erosion, transport, and in the order river, Zone IV (beach), Zone II (relict), Zone I deposition are intimately interlinked, creating a complex mech- (palimpsest), and Zone III (modern proteric). This indicates anism within the sedimentary system controlling the texture submarine weathering and abrasion have modified sediment and petrology of clastic sediments.

702 (Senio River, north-central Italy): composition of source rock, soil profiles, and fluvial deposits. Geological Society ACKNOWLEDGMENTS of America Special Paper 284, 247–261. I am much indebted to the officers, crew, and research staff of Chambers, R. L. & Upchurch, S. B. 1979. Multivariate analy- the Research Vessel Hakurei-maru No. 2 for their kind support sis of sedimentary environments using grain-size frequency during the survey. I am grateful to Y. Okamura, F. Nanayama, distribution. Mathematical Geology 11, 27–43. M. Kametaka, and T. Tsujino for their useful comments on Chayes, F. 1960. On correlation between variables of constant an early draft of this paper. Thanks go also to A. Owada and sum. Journal of Geophysical Research 65, 4185–4193. to K. Fukuda for providing the excellent thin sections. This Clemens, K. E. & Komar, P. D. 1988. Oregon beach-sand com- manuscript has benefited from constructive reviews by B. 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