Geology and Stratigraphy of Forearc Basin Sediments in Hokkaido, Japan: Cretaceous Environmental Events on the North-West Paciﬁc Margin

Cretaceous Research 25 (2004) 365e390 www.elsevier.com/locate/CretRes

Geology and stratigraphy of forearc basin sediments in Hokkaido, Japan: Cretaceous environmental events on the north-west Paciﬁc margin

a,) b a c Reishi Takashima , Fumihisa Kawabe , Hiroshi Nishi , Kazuyoshi Moriya , Ryoji Wanid, Hisao Andoe

aDepartment of Earth and Planetary Science, Graduate School of Science, Hokkaido University, N10W8, Kita-ku, Sapporo 060-0810, Japan bInstitute of Natural History, 3-14-24 Takada, Toshima-ku, Tokyo 171-0033, Japan cDepartment of Earth Science, Graduate School of Social and Cultural Studies, Kyushu University, 4-2-1, Ropponmatsu, Chuo-ku, Fukuoka 810-8560, Japan dDepartment of Geology, National Science Museum, 3-23-1, Hyakunincho, Shinjuku-ku, Tokyo 169-0073, Japan eDepartment of Environmental Sciences, Faculty of Science, Ibaraki University, 2-1-1, Bunkyo, Mito 310-8512, Japan

Received 13 August 2003; accepted in revised form 3 February 2004

Abstract

Litho-, bio-, and chemostratigraphy of the Cretaceous forearc basin sediments exposed in Hokkaido, northern Japan allow a synthesis of the faunal, sedimentological, and environmental history of the north-west Pacific margin. Although the succession, named the Yezo Group, has yielded an abundant record of mid- to late Cretaceous invertebrates, monotonous lithologies of sandstone and mudstone, showing occasional lateral facies changes, have caused confusion regarding the lithostratigraphic nomenclature. Based on our wide areal mapping of the sequence, and analysis of litho- and biofacies, a new lithostratigraphic scheme for the Yezo Group is proposed. In ascending order, the scheme is as follows: the Soashibetsugawa Formation (Lower Aptian mudstone unit); the Shuparogawa Formation (Lower Aptianelower Upper Albian sandstone-dominant turbidite unit); the Maruyama Formation (lower Upper Albian tuffaceous sandstone unit); the Hikagenosawa Formation (Upper AlbianeMiddle Cenomanian mudstone-dominant unit); the Saku Formation (Middle CenomanianeUpper Turonian sandstone-common turbidite unit); the Kashima Formation (Upper TuronianeLower Campanian mudstone-dominant unit); and the Hakobuchi Formation (Lower CampanianePaleocene shallow-marine sandstone-conglomerate unit). In addition, we designate two further lithostratigraphic units, the Mikasa Formation (Upper AlbianeTuronian shallow-marine sandstone-dominated unit) and the Haborogawa Formation (Middle TuronianeCampanian shelf mudstone/sandstone unit), which correspond in age to the shallower facies of the Saku and Kashima formations, respectively. Despite a lack of so-called ‘‘black shales’’, because of siliciclastic dilution, our stratigraphic integration has revealed the horizons of oceanic anoxic events (OAEs) in the Yezo Group. The OAE1a horizon in the Soashibetsugawa Formation is characterized by 13 a lack of foraminifers, macrofossils and bioturbation, and a prominent positive excursion of d Corg. A significant hiatus during the late Aptian and early Albian removed the OAE1b horizon. The OAE1c horizon in the Maruyama Formation shows a distinct 13 negative excursion of d Corg with a concomitant high productivity of radiolarians. The OAE1d horizon in the middle part of the Hikagenosawa Formation consists of weakly laminated, pyrite-rich mudstone. Planktonic and calcareous benthic foraminifers are absent, whereas radiolarians are abundant above the OAE1d horizon. The mid-Cenomanian event (MCE) horizon is identified at the top of the Hikagenosawa Formation. Stepwise extinction of calcareous benthic foraminifers and a decrease in radiolarian diversity become apparent above the MCE horizon. In the study area, the OAE2 horizon has been well documented, and is placed in the middle part of the Saku Formation. Ó 2004 Elsevier Ltd. All rights reserved.

Keywords: Cretaceous; Stratigraphy; Yezo Group; OAEs; Paciﬁc; Forearc basin

) Corresponding author. E-mail address: [email protected] (R. Takashima).

1. Introduction several names have been used synonymously for the same lithostratigraphic unit. We, therefore, propose Knowledge of Cretaceous ocean-climate systems here a new, synthesized stratigraphic framework, in- provides important information to help understanding cluding macro- and microfossil biostratigraphy, based a future warm greenhouse world. In particular, the mid- on extensive mapping of the entire area in the central Cretaceous warming around 120e90 Ma, enhanced by region of Hokkaido. Moreover, we organize the strati- a huge production of oceanic crust and plateaus in the graphic and geographic distribution of OAE horizons Pacific and Indian oceans, brought higher levels of recorded in the Yezo Group using the new standard greenhouse gases and high poleward heat transportation stratigraphy. (e.g., Larson, 1991a,b). This global warming induced a unique deep-ocean circulation and expansion of anoxic conditions, known as oceanic anoxic events (OAEs) that resulted in large turnovers of marine biota (Leckie 2. Geological setting et al., 2002). Many palaeontological and geochemical studies of OAEs have been conducted in land sections The Jurassic to Palaeogene sequences exposed in the across the Tethyan region (e.g., Erbacher et al., 1996), western and central parts of Hokkaido, northern Japan, on deep-sea cores from the Atlantic (e.g., Bralower are basically divided into three NeS trending tectono- et al., 1994; Wilson and Norris, 2001), the Antarctic stratigraphic divisions called the Oshima (Cretaceous (e.g., Bralower et al., 1993), and from equatorial Pacific volcanic arc on a Jurassic accretionary complex), the regions (e.g., Sliter, 1989), whereas there is little infor- SorachieYezo (CretaceousePaleocene forearc basin mation about Cretaceous environmental events in the and accretionary complex), and the Hidaka (lower North Pacific. Palaeogene accretionary complex) belts from west to The Cretaceous Yezo Group, exposed in central east (Kiminami et al., 1986; Ueda et al., 2000)(Fig. 1). Hokkaido, northern Japan, was probably deposited at These were formed along a westward-dipping sub- about 35e45(N(Hoshi and Takashima, 1999; Kodama duction of the IzanagieKula plates under the eastern et al., 2002) along a westward subduction margin in the margin of the Asian continent (e.g., Okada, 1974). north-eastern Asian continent during the Cretaceous. The SorachieYezo Belt consists of a coherent This group consists of a 10,000-m-thick forearc sedi- succession, from the Horokanai Ophiolite through the mentary sequence of sandstones and mudstones with Sorachi Group to the Yezo Group (Fig. 2), and the subordinate conglomerates. This forearc basin is called accretionary complexes of the Idonnapu and Kamuiko- the Yezo Basin (Okada, 1983), and it extends from tan zones (Ueda et al., 2000; Fig. 1). The Horokanai offshore of north Honshu, through Hokkaido, to Ophiolite and the lower part of the Sorachi Group Sakhalin Island, Russia (Fig. 1). As the rocks contain represent a piece of basaltic oceanic crust (Ishizuka, abundant, well-preserved macro- and microfossils, many 1981; Takashima et al., 2002a), while the upper part of biostratigraphic schemes have been established (Matsu- the Sorachi Group is represented by subaqueous calc- moto, 1942, 1977; Tanaka, 1963; Obata and Futakami, alkaline and alkaline volcano-sedimentary sequences, 1975; Futakami, 1982; Taketani, 1982; Maeda, 1986; suggesting an oceanic island arc setting (Girard et al., Toshimitsu and Maiya, 1986; Motoyama et al., 1991; 1991; Niida, 1992; Takashima et al., 2002b). The Yezo Kawabe et al., 1996, 2003; Takashima et al., 1997; Group conformably overlies the Sorachi Group and Toshimitsu et al., 1998; Kawabe, 2000; Wani and comprises very thick sandstone and mudstone sequen- Hirano, 2000; Moriya and Hirano, 2001; Ando et al., ces. The sand clastics of this sequence were derived from 2001; Nishi et al., 2003). In the last decade, carbon Cretaceous granitic rocks and Jurassic accretionary isotope excursions of organic materials have also been complexes of the Oshima Belt, which represents a reported across the Cenomanian/Turonian boundary contemporaneous continental arc setting (Kito et al., (OAE2) and from the upper Lower Aptian (Hasegawa 1986). Although a part of the Hakobuchi Formation and Saito, 1993; Hasegawa, 1995, 1997; Ando et al., includes the upper Paleocene in the Oyubari and 2002, 2003). Moreover, the thermal structure of the Nakatonbetsu areas (Ando et al., 2001; Ando, 2003), north-western Pacific of the Asian continental margin the geological age of this formation ranges mostly from has recently been investigated, based on oxygen isotopic early Aptian to early Maastrichtian. Many hiatuses exist analyses (Moriya et al., 2003). These studies provide between the Campanian and Maastrichtian. The sedi- important information of Cretaceous environmental mentary environment of this group shows an eastward- change and faunal turnover related to global events in deepening facies trend, from fluvial to continental slope the mid-latitude north-west Pacific. (Fig. 2). The Yezo Group is unconformably capped by However, the nomenclature of the lithostratigraphic late Eocene, non-marine and shallow-marine sediments divisions in the Yezo Group are complicated and differ of the Ishikari and Poronai groups, or by younger between the areas investigated within Hokkaido, and Neogene deposits. R. Takashima et al. / Cretaceous Research 25 (2004) 365e390 367

50°

45° SAKHALIN ISLAND 40°

35°

otan Zone 30°

Kamuik Idonnappu Zone Oshima Belt Sorachi-Yezo 25° Belt 0 500 1000 km Hidaka 20°N Belt 120°E 125° 130° 135° 140° 145° 150°

Teshionaka- Nakatonbetsu area gawa area OKHOTSK SEA Tomamae area (Fig. 5) 44 N Oyubari & Mikasa JAPAN SEA area (Fig. 4)

140 E

42 N 42 N

otan Zone otan PACIFIC OCEAN

144 E

142 E

Ido. Zone Ido. Kamuik

PACIFIC OCEAN Legend Cretaceous-Paleocene forearc basin sediments Palaeozoic to Lower Cretaceous shelf to continental sediments Upper Cretaceous to Eocene accretionary complexes Jurassic accretionary complexes

South Kitakami Upper Jurassic to Lower Cretaceous volcano-sedimentary successions Cretaceous volcano-sedimentary Belt TectonicHay Zone successions

achine Cretaceous granitic rocks

KesennumaHizume- High-pressure-type metamorphic rocks Fault Ultramafic rocks

Fig. 1. Simple geological map showing the distribution of the Mesozoic formations of north-east Japan and southern Sakhalin. Dashed-lined frames show the study areas (Figs. 4, 5). The central region (Fig. 4) consists of the south-eastern Oyubari area and the north-western Mikasa area. 368 R. Takashima et al. / Cretaceous Research 25 (2004) 365e390

Geological column Lithology Planktonic Age Lithostratigraphic units foraminiferal Environment Mikasa Tomamae Oyubari Mikasa Tomamae Oyubari zone

area area area beds Key area area area (Nishi et al.,2003)

Ishikari / Poronai Sst, coaly mdst and coal Fluvial Group Palaeogene

Fluvial-inner Hakobuchi Fm Bioturbated sandy slst-sst, congl, HCS-TCS sst Globotruncana arca shelf and coaly mdst with felsic tuff (<450m) Contusotruncana

Campan.- Maastricht. fornicata

Tsukimi Felsic volcaniclastic sst (prodelta / turbidite) and tuff Outer-inner Sst Mbr KY-6 TCS sst Marginotruncana shelf Strongly sinuosa bioturbated Strongly bioturbated mdst

Haborogawa (1670m) mdst-sandy mdst Fm

Kashima Fm Marginotruncana (1750-2250m) Felsic volcaniclastic sst (turbidite) and tuff

KY-5 pseudolinneiana Continental slope Alternating beds of sst (turbidite) & mdst

Sst>mdst Sstmdst W. archaeocretacea Sst Mbr Alternating beds of Saku Fm sst (turbidite) & mdst y-outer shelf (2300-1800m) Thick-bedded Shelf-coastal plain (400-750m)

Congl, HCS-TCS sst, Rotalipora Mikasa Fm congl & sst Sst

Incised valle

Cretaceous Group Yezo Rotalipora

Cenomanian Turonian Coniacian Santonian Weakly bioturbated Weakly bioturbated mdst globotruncanoides sandy mdst & massive mdst R. appenninica Kanajiri Alternating beds of sst (turbidite) & mdst Sst Mbr Sst>mdst Sst

Okusakainosawa Ticinella Continental slope Sst & Mdst Mbr Alternating beds of sst (turbidite) & mdst Sstmdst primula Kirigishiyama Olisto. Mbr Olistostrome occasionally containing lst olistoliths

KY-1 KY-2 KY-3 KY-4 Globigerinelloides Sstmdst Sstmdst Soashibetsugawa Leupoldina cabri No exposure Laminated mdst with frequent intercalations (<700m) Sorachi Fm of felsic tuff beds

Berrias.- Bar. Group Mafic-intermediate volcaniclastics and lava Oceanic Jurassic Horokanai Ophiolite Mafic-ultramafic igneous basement island arc

Fig. 2. Schematic diagram of the Yezo Group in the study areas. Note that the geological column shows eastward-deepening facies.

3. Stratigraphy of the Yezo Group units, with intercalations of six distinct stratigraphic key units (Fig. 2, Table 1). The revised lithostratigraphic Lithostratigraphic nomenclature for the Yezo Group definitions proposed in this paper are as follows, in as- is very confused. Various definitions have been proposed, cending order (Figs. 2, 3): the Soashibetsugawa Forma- depending on the area and the researchers, because this tion (mudstone unit); the Shuparogawa Formation group basically consists of a monotonous sequence of (sandstone-dominant turbidite unit); the Maruyama sandstone and mudstone and their alternating beds, Formation (felsic tuff and tuffaceous sandstone unit); which occasionally exhibit lateral facies changes. The the Hikagenosawa Formation (mudstone-dominant criteria of lithological boundaries have been defined by unit); the Saku Formation (sandstone-common turbidite sandstone intercalations and/or detailed changes of the unit); the Kashima Formation (mudstone unit) and sandstone/mudstone ratios in alternating beds. However, the Hakobuchi Formation (shallow-marine sandstone our study has revealed that the Yezo Group throughout unit). Hokkaido is basically characterized by six alternations We also recognize the sandstone-dominant, outer- of mudstone-dominant units and sandstone-common shelf to shoreface Mikasa Formation and the overlying, R. Takashima et al. / Cretaceous Research 25 (2004) 365e390 369

Table 1 List of the widely traceable key units of the Yezo Group Key units Thickness Lithofacies Horizons Age Picture KY-6 10e30 m Tuffaceous coarse Middle parts of the Kashima Latest Santonian Fig. 7N, O sandstone beds and Haborogawa formations (Tsukimi Sst Mbr) KY-5 10e20 m Greenish tuffaceous Lower parts of the Kashima Early Coniacian Fig. 7M coarse sandstone and Haborogawa formations beds with abundant I. uwajimensis KY-4 !300 m Greenish gray Middle part of the Early Turonian Fig. 7I tuffaceous muddy Saku Formation sandstone with abundant (Hakkin Muddy Sst Mbr) Planolites trace fossils and very thick felsic tuff beds KY-3 100e300 m Sandstone-conglomerate Lower middle part of the Latest Albian Fig. 7G beds Hikagenosawa Formation (Kanajiri Sst Mbr) KY-2 !82 m Felsic tuff and tuffaceous Maruyama Formation Early Late Albian Fig. 7E sandstone beds KY-1 !400 m Olistostrome or debris flow Middle part of the Late Aptiane Fig. 7C deposits occasionally containing Shuparogawa Formation Early Albian ‘‘Orbitolina’’ limestone blocks (Kirigishi. Olistostrome Mbr) sandy mudstone-dominant outer-shelf Haborogawa Lithology. The formation consists of dark grey Formation. The former is exposed in the Mikasa area, parallel-laminated mudstone with many intercalations and the latter in the Tomamae and Mikasa areas (Figs. 2, of felsic tuff beds (Fig. 7A). The laminae are composed 4, 5). The geological ages of these formations correspond of felsic tuff and very fine-grained sand layers. Deep-sea to the Saku and Kashima formations exposed in the trace-fossils, such as Lorenzinia and Cosmorhaphe, are Oyubari area, respectively (Fig. 2). Additionally, in the found occasionally in the mudstone. The tuff beds are study areas we name six stratigraphic key units as KY-1 white, hard and generally 10e30 cm thick, though some to KY-6 consisting of olistostrome (KY-1), tuffaceous beds attain a thickness of 1e7 m. They contain fine- sandstone (KY-2, KY-4 to 6), and sandstone-conglom- grained, bubble-wall glass shards with minor amounts of erate units (KY-3) (Fig. 2, Table 1). idiomorphic feldspar and biotite.

3.1. Soashibetsugawa Formation (redefined) Thickness and distribution. The formation is 450e 700 m thick in the eastern part of the Oyubari area, and Definition. This formation is characterized by a pre- not exposed in the Mikasa and Tomamae areas. dominance of dark grey siliceous mudstone, and corresponds to the Soashibetsugawa Mudstone Member of Fossils and age. No macrofossils have been found in the Shuparogawa Formation of Takashima et al. (2001) this formation. Radiolarians occur abundantly through- (Fig. 3). Takashima et al. (2001) defined the member as out the sequence of this formation, but there are no the basal unit of the Yezo Group because of the age-diagnostic species. Planktonic foraminifers appear lithofacies shift from in situ volcanic and volcaniclastic rarely in the upper part (Fig. 8). The early Aptian spe- rocks of the Sorachi Group to terrigenous, dark grey cies, Leupoldina cabri (Sigal), occurs from the upper- mudstone at the base of the member. As the member is most part of the formation (Saito and Ando, 2000; widely traceable and clearly distinguishable from other Takashima et al., 2001; Nishi et al., 2003). Agglutinated members (sandstone-dominant facies) of the Shuparo- benthic foraminifers, such as Bathysiphon, also occur gawa Formation of Takashima et al. (2001), we redefine occasionally. the member as the basal formation of the Yezo Group. Depositional environment. Microfossil assemblages Stratigraphic relationship. This formation conform- and deep-sea trace fossils suggest an abyssal environment. ably overlies the Shirikishimanaigawa Formation of the Sorachi Group. 3.2. Shuparogawa Formation (redefined)

Type section. The Soashibetsu River section, northern Deﬁnition. This formation, corresponding to the Oyubari area (Figs. 4, 6; section 11). Tomitoi and Shuparogawa formations of Motoyama 370 R. Takashima et al. / Cretaceous Research 25 (2004) 365e390

Mikasa area Tomamae area Oyubari area

Nishida et al. Wani & Hirano Matsumoto Motoyama et al. Ando (1990b) This paper This paper Takashima et al. This paper (1997) (2000)* (1942) (1991)** (2001)*

Hakobuchi Hakobuchi Hakobuchi Hakobuti Hakobuchi Hakobuchi Group Fm Fm Group Group Fm

Upper Haboro- Upper IIId Upper Tsukimi Sst Mbr gawa Fm Haborogawa Ammonite IIIc Kashima Kashima Yezo Group Haborogawa Middle Haboro- Fm Group IIIb Fm Fm Fm gawa Fm IIIa Twd IIs IIr Shirogane Fm Twc Gp Upper Yezo IIq Mikasa Twb Mikasa Fm Shirochi Fm Hakkin Muddy IIp Hakkin Muddy Fm Sst Mbr IIn Takinosawa Sst Mbr Twa My7-8 Saku Fm IIm Saku Fm Saku Fm Middle Fm Me Mbrs IIk-g Ammonite IIf

Main Group Yezo Hikagenosawa Group IIe Hikagenosawa Md Tenkaritoge Part Hikagenosawa IId Hikagenosawa Hikagenosawa Middle Yezo Group Middle Yezo My1-6 Fm Kanajiri Fm Kanajiri Fm Fm IIc Fm Fm Subgroup Mbrs Group Yezo Sst Mbr Group Yezo Group Yezo Sst Mbr Middle Yezo Main Part Yezo Supergroup Yezo IIb Middle Yezo Group Middle Yezo Maruyama Fm Takimibashi Fm Maruyama Fm IIa Maruyama Fm Maruyama Fm Maruyama Fm Okusakai. Sst If Okusakai. Sst Okusakai. Sst Shuparogawa Ie &mdst Mbr Id &mdst Mbr &mdst Mbr Fm Group Yezo Shuparogawa Kirigishiyama Lower Kirigishiyama Kirigishiyama Ic Fm Ly Mbr Olist. Mbr Ammonite Olist. Mbr Olist. Mbr Yezo Group Yezo Group Ib Refureppu Refureppu Refureppu Subgroup

Lower Yezo Lower Sst Mbr Sst Mbr Sst Mbr Ia Tomitoi Fm Shuparogawa Fm Shuparogawa Shuparogawa Fm Shuparogawa

Shuparogawa Fm Shuparogawa Soashibetsu- Sorachi Group Sorachi Group Onisashi Group Sorachi Group Soashibetsugawa (Uppermost part) (Uppermost part) (Uppermost part) gawa Mdst (Uppermost part) Mbr Fm

* Same definition is used in Nishi et al. (2003) * *Same definition is used in Hasegawa & Saito (1993), Kaiho et al. (1993), Kaiho & Hasegawa (1994), Hasegawa (1995) and Hasegawa (1997)

Fig. 3. Comparison of lithostratigraphic subdivisions proposed by several workers in the study areas. et al. (1991), is distinguished from the mudstone- ratio in the lower part is about 5/1. Sandstones in the dominant Soashibetsugawa Formation by the onset of upper part are less than 5 cm thick, with Tc-e divisions, common to frequent intercalations of sandstone beds. and the S/M ratio is 1/2 to 1/5. Mudstones in this member are uniformly less than 10 cm thick, dark grey Type section. Along the Shuparo River section, the and weakly bioturbated. In the northern Oyubari area, central Oyubari area (Figs. 4, 6; section 12). this member is dominated by sandstone throughout (Fig. 6; section 11). Stratigraphic relationships. The Shuparogawa For- The Kirigishiyama Olistostrome Member represents mation conformably overlies the Soashibetsugawa an olistostrome bed containing huge allochthonous Formation in the Oyubari area, while the basal part of blocks of massive sandstone (!40 m thick), alternating this formation is not exposed in the Tomamae and beds of sandstone and mudstone (!20 m thick) and Mikasa areas because of poor exposure. limestone (!60 m thick) in a muddy matrix (Fig. 7C). Limestone olistoliths consist of corals, large foraminifers Lithology. The Shuparogawa Formation is mainly (Orbitolina), rudists and ooids, contaminated with composed of alternating beds of turbiditic sandstone pebbles of chert, granite and sandstone. The width and and dark grey mudstone. This formation incorporates thickness of limestone olistoliths is greatest in the a thick olistostrome bed (Kirigishiyama Olistostrome northern Oyubari area where a 60-m-thick, slab-shaped Member) in the middle portion, and is subdivided block is continuously exposed for a distance of 3 km in into three members as follows: the Refureppu Sand- aNeS direction (Fig. 4, around Mt. Kirigishiyama). The stone, Kirigishiyama Olistostrome, and Okusakaino- limestone olistoliths thin out in the Tomamae and sawa Sandstone and Mudstone members, in ascending southern Oyubari areas. However, the olistostrome bed order (Figs. 2, 6). is widely traceable throughout the study area as a key The Refureppu Sandstone Member consists of a very unit (KY-1) (Fig. 6). thick- to medium-bedded, sandstone-dominant sequence The Okusakainosawa Sandstone and Mudstone in the lower part (Fig. 7B) and thin to very thin-bedded, Member basically comprises mudstone-dominant alter- mudstone-dominant alternating beds in the upper part. nating beds, and varying thicknesses of sandstone, from It contains slump beds locally. Lower sandstones are 5 to 50 cm depending on the area (Fig. 7D). Sandstones stratified, well sorted, usually ranging from 10 to 50 cm are turbiditic, exhibiting Tb-e sequences with sole marks thick, and occasionally more than 1 m thick. They are and numerous plant fragments, while thick sandstones turbidites showing the S3eTb-e divisions of Lowe (1982), show the S1-3 divisions of Lowe (1982). In the Oyubari with many sole marks. The sandstone/mudstone (S/M) area, intercalations of sandstone become common to R. Takashima et al. / Cretaceous Research 25 (2004) 365e390 371 abundant in the middle part of this member (Fig. 6; 3.3. Maruyama Formation section 11). Mudstones are bioturbated with moderate numbers of trace fossils. Definition. This formation is defined by an assemblage of hard felsic volcaniclastic sandstones, tuffs and Thickness. The formation is approximately 1500 m associated conglomerates (Motoyama et al., 1991) thick in the type section. However, it varies in thick- (Fig. 3). It also yields an excellent stratigraphic marker nesses, ranging from 800 to 2450 m. (KY-2), which extends throughout Hokkaido (Table 1).

Fossils and age. Mudstones of the Refureppu Sand- Type section. Along the Shuparo River section, cen- stone Member yield a few foraminifers and common tral Oyubari area (Figs. 4, 6; section 12). radiolarians, but no macrofossils (Fig. 8). Planktonic foraminifers include the Upper Aptian species, Globiger- Stratigraphic relationship. This formation conform- inelloides barri (Bolli, Loeblich and Tappan) and G. ably overlies the Shuparogawa Formation. duboisi (Chevalier). Agglutinated and calcareous benthic foraminifers also occur occasionally. The Okusakaino- Lithology. This formation is composed of tuffaceous sawa Sandstone and Mudstone Member contains abun- sandstone beds (Fig. 7E), locally accompanied by dant radiolarians and planktonic foraminifers, with a conglomerates at the base. The tuffaceous sandstone subordinate proportion of common agglutinated and beds are 0.3e2 m thick, white to pale brown, siliceous calcareous benthic foraminifers. The Upper Albian and very hard. These beds have turbiditic features of ammonoid, Mortoniceras cf. geometricum Spath, occurs graded to parallel-laminated sequences (Tb-e) and basal in the upper part of this member (Kawabe, 2000). This rip-up mudstone clasts. Microscopically, the sandstones member also includes the transitional interval from the are composed of platy glass shards, subordinate idio- Ticinella primula planktonic foraminifera (pf) Zone to morphic plagioclase, biotite, quartz and hornblende. The the lower part of the Biticinella breggiensis pf Zone, tuffaceous unit varies from 4 to 84 m thick throughout assigned to the Lowerelower Upper Albian (Nishi et al. the study area. 2003)(Fig. 8). A significant hiatus is present from The conglomerates accompanying the base of this the Planomalina cheniourensis to Hedbergella planispira formation are usually less than 2 m thick. However, in zones of Hardenbol et al. (1998), approximately 115.5e the central part of the Tomamae area (Fig. 5), the total 108.21 Ma (7 myr). It might have been eroded during thickness of the conglomeratic beds exceptionally attains deposition of the Kirigishiyama Olistostrome Member. as much as 900 m (Fig. 6; section 10). The conglomerates The limestone olistoliths yield larger foraminifers of are thick-bedded, poorly sorted, and clast-supported, orbitolinids, e.g., Orbitolina lenticularis (Blumenbach), structureless or exhibiting R3eS1 divisions of Lowe and its age is determined as Late AptianeEarly Albian (1982). The conglomerates consist mainly of subrounded (Matsumaru, 1971). pebbleseboulders of rhyolite, with subordinate, well- rounded granules to pebbles of mudstone, chert and Depositional environment. Lithology and benthic siliceous mudstone in the Tomamae area, whereas in foraminiferal data suggest that this formation was the Oyubari area they are composed of granules to deposited on the continental slope (e.g., Motoyama pebbles of chert, sandstone, siliceous mudstone and et al., 1991). Benthic foraminifers of the Refureppu limestone. Sandstone Member and the Okusakainosawa Sandstone and Mudstone Member, consisting of species of Bathy- Thickness. This formation attains thicknesses of siphon, Gaudryina, Gyroidinoides, Gavelinella and Nodo- 180e900 m in the Tomamae area and 4e84 m in the saria, indicate the upper bathyal zone or deeper (e.g., Oyubari area. The horizon and thickness of this for- Sliter and Baker, 1972). However, the limestones of mation are disputable in the Mikasa area because of the Kirigishiyama Olistostrome Member are considered poor exposure. to have been formed on a carbonate platform, such as a rimmed shelf along the Asian continental margin (Sano, Fossils and age. Although no planktonic foraminifers 1995). The olistostrome indicates the collapse of the and macrofossils have been obtained from this forma- shallow carbonate platform and mixing with the bathyal tion, radiolarians (spumellarians) occur abundantly mudstone around the Aptian/Albian boundary. (Fig. 8).

Fig. 4. Geological map and structural proﬁle section of the Oyubari and Mikasa areas, central Hokkaido Island region. The south-eastern Oyubari and north-western Mikasa areas are bounded by the Katsurazawa Thrust Fault. Although most areas in this map are based on our original data, the southern Mikasa area (the Manji and Hatonosu domes) and northeastern Oyubari area (around the Mt. Yubaeyama) are modiﬁed from Tanaka (1970), Obata and Futakami (1975), Futakami (1982) and Hashimoto (1953), respectively. 372 R. Takashima et al. / Cretaceous Research 25 (2004) 365e390 R. Takashima et al. / Cretaceous Research 25 (2004) 365e390 373 374 R. Takashima et al. / Cretaceous Research 25 (2004) 365e390

Fig. 5. Geological map and structural proﬁle section of the Tomamae area; see Fig. 4 for legend. R. Takashima et al. / Cretaceous Research 25 (2004) 365e390 375

Depositional environment. Abundant volcaniclastic igneous rocks. The horizon and thickness of this materials in this formation suggest that huge, felsic member are disputable in the Mikasa area because of volcanic eruptions episodically occurred along the poor exposure and scarce fossil data. western circum-Paciﬁc/Asian continental margin. Main transporting channels for the volcanic sediments are Thickness and distribution. 1900e2600 m thick in the inferred to have been located in the Tomamae area. Oyubari area, and 1900e2000 m thick in the Tomamae area. The thickness in the Mikasa area is uncertain 3.4. Hikagenosawa Formation because of poor exposure.

Definition. This formation is defined by the pre- Fossils and age. The occurrence of macrofossils dominance of dark grey mudstone (Motoyama et al., (ammonoids and inoceramids) becomes common above 1991; Fig. 3), although there are intercalations of a thin the lower part of this formation (Fig. 8). The late sandstone-dominant unit (Kanajiri Sandstone Member) Albianemiddle Cenomanian ammonoids Mortoniceras in the lower middle part. rostratum (Sowerby), Mariella bergeri (Brongniart), Mantelliceras saxbii (Sharpe) and Cunningtoniceras Type section. Along the Hikageno-sawa Valley sec- cunningtoni (Sharpe) occur in the formation. Micro- tion, the central Oyubari area (Fig. 4). fossils are abundant throughout the sequence. Excep- tionally, planktonic foraminifers and calcareous benthic Stratigraphic relationship. The formation overlies, foraminifers are lacking around the Kanajiri Sandstone conformably, the Maruyama Formation. Member, whereas radiolarians become very abundant and agglutinated benthic foraminifers are common in Lithology. The dominant lithology of the Hikageno- this interval (Fig. 8). Planktonic foraminifers indicative sawa Formation is dark grey mudstone (Figs. 6, 7F). A of the Biticinella breggiensis to Rotalipora cushmani pf thin unit of alternating beds of sandstone/conglomerate zones, for example, Biticinella breggiensis (Gandolfi), and mudstone (Kanajiri Sandstone Member) is in- Ticinella subticinensis (Gandolfi), Rotalipora appenninica tercalated in the lower middle part of this formation. (Renz), R. globotruncanoides Sigal, and R. cushmani This unit is used as a stratigraphic marker, the KY-3 (Morrow), occur, indicating a Late AlbianeLate Cen- (Fig. 2, Table 1). omanian age (Nishi et al., 2003; Fig. 8). The lower part of this formation is characterized by weakly-laminated mudstone. The intercalations of thin- Depositional environment. Benthic foraminiferal as- bedded felsic tuffs and distal turbiditic sandstones semblages suggest that this formation was deposited in showing Tc-e divisions are common in the Tomamae the lower part of the upper bathyal zone under relatively and Mikasa areas. In the northern Tomamae area, oxygenated conditions (Motoyama et al., 1991; Kaiho thick- to very thick-bedded sandstones with S3eTb-e are, et al., 1993). exceptionally, also intercalated (Fig. 6; section 9). On the other hand, rare sandstones are intercalated in the 3.5. Saku Formation Oyubari area. Mudstones in the upper part of this formation are moderately bioturbated, and occasionally Definition. This formation is defined by the frequent weakly laminated. intercalations of turbiditic sandstone beds (Matsumoto, The Kanajiri Sandstone Member is composed of 1942). thick- to very thick-bedded sandstone with very thick- bedded conglomerates in the Tomamae area, whereas in Type section. The Abeshinai River section, Teshio- the Oyubari area it comprises thin- to medium-bedded, nakagawa area, northern Hokkaido (Fig. 1). The mudstone-dominant alternating beds of sandstone and lithology and distribution of this formation were well mudstone (Fig. 7G). Sandstones are proximal (S3eTb-e) documented by Matsumoto (1942) and Matsumoto and in the former area and distal (Td-e) in the latter. The Okada (1973). conglomerates are well-rounded pebbles to granules, exhibiting R3eS1 divisions. Their compositions are Stratigraphic relationship. The formation overlies con- mostly chert with minor sandstone, mudstone and formably the Hikagenosawa Formation.

Fig. 6. Correlation of selected sections from the study areas. 1e5, Mikasa area. 1, Shikoro-zawa Valley; 2, Ponbetsu River; 3, Yonno-sawa Valley; 4, Ganseki-zawa ValleyeNanashi-zawa Valley; 5, Tsukimi-sawa Valley. 6e10, Tomamae area. 6, Sankebetsu River; 7, Nakafutamata River; 8, Shumarinai River; 9, Sounnai RivereKotanbetsu River; 10, Gosen RivereKanajiri-zawa ValleyeAkanosawa Valley. 11e17, Oyubari area. 11, Soashibetsu River; 12, Shuparo River; 13, Tengu-sawa Valley; 14, Hakkin-zawa Valley; 15, Oyubari dam; 16, Horokakuruki River; 17, Mayachi- zawa Valley. Each section is cited in Figs. 4 and 5. Note that the precise thicknesses of the KY2 (sections 11e13) and KY-5,6 could not be depicted because of scale problems; see Table 1 for these.

378 R. Takashima et al. / Cretaceous Research 25 (2004) 365e390

Lithology. This formation comprises alternating beds The sandstones are very thick-/thick-bedded, showing of turbiditic sandstone and mudstone, and intercalates S1eTb-e divisions. The second and fourth cycles show the with a unit characterized by a predominance of greenish facies changes from very thick-/thick-bedded, sandstone- grey muddy sandstones (Hakkin Muddy Sandstone dominant alternating beds to thin-bedded mudstone- Member; KY-4) in the middle part (Fig. 2, Table 1). dominant ones. The sandstones are proximal turbidites, Sandstones in the lower part (below the KY-4) are exhibiting S1eTb-e divisions. less than 40 cm thick and exhibit typical turbiditic sequences, having Tb-e divisions. Mudstones are dark Thickness and distribution. 2300 m in the Oyubari grey-coloured and moderately bioturbated. The S/M area, 1800e2000 m in the Tomamae area. The upper ratios change from 1/10 to 1/1 (Fig. 7H). part of this formation is truncated by the NeS trending A very characteristic lithofacies, greenish grey muddy thrust fault (Kashima Thrust Fault) in the northern sandstone with abundant trace fossils of Planolites Oyubari area (Fig. 4). (Fig. 7I), is observable in the middle part of this formation. This greenish unit extends from the Oyubari Fossils and age. Except for the Hakkin Muddy area to the central Tomamae area, about 200 km, and Sandstone Member the formation contains abundant is defined as the KY-4 (Hakkin Muddy Sandstone macrofossils (Fig. 8). The Cenomanian ammonoids, Member). The member incorporates medium-/thick- Calycoceras spp., occur in the lower part of the for- bedded sandstones, with S3eTb-e divisions, and frequent mation. The blackish-grey mudstone intercalated in the felsic tuff beds. The tuff beds vary in thickness from 0.02 basal part of the Hakkin Muddy Sandstone Member is to 2 m, and are altered to bentonites. Finely laminated, correlated with the Cenomanian/Turonian boundary, pyrite-rich, dark greyegreyish black mudstones are based on the results of carbon isotope, mega- and intercalated just below or in the lower part of this microfossil stratigraphy (e.g., Hasegawa and Saito, member in the Oyubari and Tomamae areas. 1993; Hirano, 1995). Although macrofossils are few in The upper part of the formation begins with dark the Hakkin Muddy Sandstone Member, Lower Turo- grey mudstone and changes to alternating beds of nian indicators, such as Pseudaspidoceras flexuosus sandstone and mudstone. Sandstones are thin- to (Powell) and vascoceratids, occur. The upper sequence medium-bedded turbidites with Tb-e sequences, and the is marked by an abundant occurrence of heteromorphic S/M ratios are not high, 1/10 to 1/5. Slump beds and ammonoids, including Nipponites, Eubostrichoceras and sandstone dykes are common in the southern Oyubari scaphitids, with minor Middle Turonian indicators, such area. Sandstones are occasionally intensively biotur- as Romaniceras spp. (ammonoid) and Inoceramus bated and bedding planes of some beds are destroyed. hobetsensis Nagao and Matsumoto (bivalve). Micro- Mudstones and sandy mudstones are dark grey to grey fossils commonly occur throughout the sequence; radio- and intensively bioturbated. larians are especially abundant in the Hakkin Muddy On the other hand, the Saku Formation, exposed Sandstone Member, and occupy 80e90% of the total. in the northern Tomamae area, is marked by four This formation ranges from the Rotalipora cushmani to thinning-upward sequences (Fig. 6; section 8). A channel- Helvetoglobotruncana helvetica pf zones, assigned to the filling conglomerate/sandstone unit constitutes the Upper CenomanianeMiddle Turonian (Nishi et al., first and third sequences. The conglomerates are clast- 2003; Fig. 8). supported, and 0.5e4 m thick, with disorganized erosional bases (Fig. 7J). Graded bedding, from well-rounded cob- Depositional environment. The benthic foraminif- ble to coarse sandstone (R2-3eS1 divisions), is common. eral assemblages indicate the lower part of upper

Fig. 7. Photographs of the representative lithofacies of the Yezo Group. A, laminated mudstone with frequent intercalations of felsic tuff beds; Soashibetsugawa Formation, Soashibetsu River, northern Oyubari area. B, sandstone-dominant alternating beds of sandstone and mudstone; Shuparogawa Formation, Refureppu Sandstone Member, Soashibetsu River, northern Oyubari area. C, olistostrome containing large limestone blocks; Shuparogawa Formation, Kirigishiyama Olistostrome Member (KY-1), Okusakaino-sawa Valley, central Oyubari area. D, alternating beds of sandstone and mudstone; Okusakainosawa Sandstone and Mudstone Member, Shuparo River, central Oyubari area. E, felsic tuffaceous sandstones; Maruyama Formation (KY-2), Uenbetsu River, southern Tomamae area. F, weakly laminated mudstone; Hikagenosawa Formation, Tengu-sawa Valley, central Oyubari area. G, mudstone-dominant alternating beds of sandstone and mudstone; Hikagenosawa Formation, Kanajiri Sandstone Member (KY-3), Shuparo River, central Oyubari area. H, alternating beds of sandstone and mudstone; Saku Formation, Shumarinai River, northern Tomamae area. I, greenish grey muddy sandstone with abundant Planolites (KY-4), Saku Formation, KY-4, Hakkin-zawa Valley, southern Oyubari area. J, conglomerate bed with erosional base; Saku Formation, Shumarinai River, northern Tomamae area. K, sandstone with hummocky cross-stratification; Mikasa Formation, western Katsura-zawa Lake, southern Mikasa area. L, massive mudstone; Kashima Formation, Horokakuruki River, southern Oyubari area. M, Inoceramus uwajimensis shell bed (KY-5), Kashima Formation, Okusamata-zawa Valley, central Oyubari area. N, felsic volcaniclastic sandstone; Haborogawa Formation, Tsukimi Sandstone Member (KY-6), Ashibetsu River, eastern Mikasa area. O, coarsening upward sequences, from sandy mudstone to medium-grained sandstone; Haborogawa Formation, Nakanofutamata River, northern Tomamae area. P, sandstone with hummocky cross-stratification; Hakobuchi Formation, Hachigatsu-zawa Valley, eastern Mikasa area. R. Takashima et al. / Cretaceous Research 25 (2004) 365e390 379 Fig. 7 (continued ) R. Takashima et al. / Cretaceous Research 25 (2004) 365e390 381

Fig. 8. Composite column of the Yezo Group, summarized lithology, macro-, microfossil and carbon isotope stratigraphy and microfossil biotic events. Note that ﬁve OAE horizons are identiﬁed within the Yezo Group. bathyal depth-zone with relatively low-oxygen condi- Type section. North-west of Katsurazawa Lake, in tions (Kaiho et al., 1993). the eastern Mikasa area (Fig. 4).

Lithology. The Mikasa Formation is characterized by 3.6. Mikasa Formation a predominance of sandstones with hummocky and trough cross-stratification (HCS and TCS) and gravel/ Definition. This formation is the contemporaneous, shell lags, and intensively bioturbated sandy mudstone shallower facies of the Saku Formation, and is defined (Fig. 7K). It is divided into three coarsening-upward by the predominance of sandstones exhibiting hum- successions, assigned as third-order depositional sequen- mocky and trough cross-stratification (Matsumoto, ces (DS1,DS2 and DS3), each of which further includes 1951; Ando, 1990a). three fourth-order sequences (4th DS) (Ando, 1997), 382 R. Takashima et al. / Cretaceous Research 25 (2004) 365e390

Within each DS, conglomerate and coarse-grained sand- been deposited in lower shoreface to outer shelf stone tend to thicken in the west, and interbedded sandy environments. mudstone becomes dominant and thick in the east. In the northwestern Mikasa area, the DS1 begins with 3.7. Kashima Formation alternating beds of turbiditic sandstone and mudstone, and grades into HCS sandstone with bioturbated, very Definition. The formation is defined by the pre- fine-grained sandstone/sandy mudstone. A basal thick, dominance of dark grey, massive mudstone overlying cross-stratified conglomerate of the DS2 covers HCS the Saku Formation (Motoyama et al., 1991; Fig. 3). sandstones of DS1 with a sharp erosional base (unconformity) (Fig. 6; section 2). The DS2 comprises Type section. Kashima village in the southern a wide variety of lithofacies, such as cross-stratified Oyubari area (Fig. 4). conglomerate, carbonaceous mudstone, sandy mudstone with oyster-shell beds, medium- to coarse-grained Stratigraphic relationship. This formation covers, con- sandstone with TCS, HCS fine sandstone and biotur- formably, the Saku Formation. bated sandy mudstone. The DS3 deposits are distributed in the southern Mikasa area, mainly comprising bio- Lithology. This formation consists mainly of dark turbated, sandy mudstone and HCS sandstones. The grey, bioturbated, massive mudstone (Fig. 7L), grading intercalations of HCS sandstone become frequent in into muddy sandstones in the uppermost part. The the upper part of each fourth DS. These DSs grade into mudstones are intercalated with two felsic volcaniclastic the Saku Formation northward and eastward. units, in the lower and middle parts, respectively. The lower volcaniclastic unit is 10e20 m thick, consisting of Thickness and distribution. This formation is exposed thick-bedded, coarse- to fine-grained, volcaniclastic only in the Mikasa area, forming the SorachieIkush- sandstones with interbedded dark grey mudstones. The umbetsu Anticline and the Manji and Hatonosu domes. sandstone beds are turbiditic with Tb-e divisions, and less Its thickness ranges from 400 to 750 m, and it tends to than 2 m thick, rarely attaining 4 m thick in the northern thicken eastward. Oyubari area. The thickness of the mudstones is variable, depending on the area, but all are less than Fossils and age. Shallow-marine bivalves (including 2 m thick. A diagnostic feature of this unit is abundant Apiotrigonia, Glycymeris, Meekia, Pinna, Pseudoptera, occurrences of Inoceramus uwajimensis Yehara from the Pterotorigonia, Thetis, Yaadia), gastropods (e.g., Mar- interbedded mudstone (Fig. 7M). This unit is an ex- garites, Semisolarium) occur as storm-lag deposits within cellent stratigraphic marker (KY-5) throughout the HCS sandstones. Bioturbated, muddy sandstones to study areas (Table 1). sandy mudstones contain rare to common ammonoids Compared with the lower unit, the upper volcani- and inoceramids, but no microfossils. The Cenomanian clastic unit differs in lacking mudstone intercalations ammonoids, Mantelliceras sp., Calycoceras sp. and and macrofossils. Tuffaceous sandstones or tuffs are Desmoceras spp. have been reported from the DS1 thick-bedded (!1 m thick), white, coarse- to very fine- deposits (Kawabe, 2003). The Turonian species, Inocer- grained. Some are altered to bentonite. amus hobetsensis and Inoceramus teshioensis Nagao and Matsumoto, are abundant in bioturbated sandy Thickness. About 1670 m in the southern Oyubari mudstones of DS2 and DS3, respectively (Ando, area. The thickness of this formation in the northern 1990a,b). Oyubari area is difficult to estimate because of the monotonous lithology and complex geological structure, Depositional environment. Three DSs show conspicu- with isoclinal folding and thrust faulting. ous lateral and vertical facies changes, representing repetitive delta progradations in the western margin of Fossils and age. This formation contains abundant the Yezo Basin. Judging from the stacking patterns of calcareous concretions, including macrofossils such as their facies, third- and fourth-order DSs, the delta ammonoids and inoceramids. I. uwajimensis is particu- system is presumed to have shifted southward within the larly abundant in the KY-5 unit (Fig. 8). distribution area (Ando, 1997, 2003). The depositional Microfossils (foraminifers and radiolarians) are environment of the DS1 deposits is interpreted as basin- abundant, although Tethyan marker-species are very plain on continental slope to lower shoreface through rare. The following four mid-latitude zones of plank- shelf. The DS2 represents non-marine to shallow-marine tonic foraminifers have, therefore, been proposed from environments, such as fluvial channel, back marsh, the Late Turonian to Campanian in the study area, flood plain and tidal flat in the lower part, followed by namely: the Marginotruncana pseudolinneana, M. sinu- deeper marine (shoreface to inner shelf, though partly osa, Contusotruncana fornicata, and Globotruncana arca outer shelf) in the upper. The DS3 is thought to have pf zones (Fig. 8). These can be correlated directly with R. Takashima et al. / Cretaceous Research 25 (2004) 365e390 383 the Tethyan planktonic foraminiferal zones (Nishi et al., Thickness. Approximately 1950 m in the Mikasa area, 2003). 1750e2250 m in the Tomamae area.

Depositional environment. The benthic foraminiferal Fossils and age. This formation yields abundant, well- assemblages indicate the upper part of the upper bathyal preserved macrofossils from calcareous concretions in environment with medium- to relatively high-oxygen both the Tomamae and Mikasa areas (e.g., Futakami, levels (Kaiho et al., 1993). 1986; Maeda, 1986; Wani, 2001; Moriya et al., 2003). Several regional biozones, the I. uwajimensis-I. mihoensis, 3.8. Haborogawa Formation (new) I. amakusensis,andI. japonicus inoceramid zones, have been proposed in the Tomamae area by Toshimitsu and Deﬁnition. The base of this formation is delineated by Maiya (1986). Microfossils are abundant throughout the bioturbated mudstone without sandstone intercalations, study areas. Although Tethyan planktonic foraminifers overlying the Saku Formation. While the lower part of are sporadic, the four mid-latitude zones, from the this formation is mainly composed of bioturbated Marginotruncana pseudolinneiana to the Globotruncana mudstone, the upper part is characterized by coarsening arca pf zones, can be identiﬁed in this formation. Its upward successions, from mudstone to muddy sand- geological age is considered to be ConiacianeCampanian stone and/or sandstone. (Moriya et al., 2001; Nishi et al., 2003; Fig. 8).

Type section. The Nakafutamata River section, in the Depositional environment. The mudstone in the northern Tomamae area (Figs. 5, 6; section 7). northern Tomamae area contains abundant benthic foraminifers, e.g., Gavelinella, Gyroidinoides, Hoeglundina, Stratigraphic relationships. The formation is exposed Oolina,andSilicosigmoilina. The palaeodepth of this in the Tomamae and Mikasa areas, and conformably assemblage is considered to be outer shelf (Sliter and overlies the Saku and Mikasa formations, respectively. Baker, 1972). The cross-stratified sandstone situated It is the synchronous shallower-water facies of the towards the top of each succession (i.e., KY-6) in the Kashima Formation. northern Tomamae area is inferred to have been deposited in inner shelf to lower shoreface environments Lithology. The lithology and sedimentary cycles of (Toshimitsu, 1985; Wani, 2003). this formation differ between the Tomamae and Mikasa areas. In the Mikasa area the formation forms a single, 3.9. Hakobuchi Formation (new) coarsening-upwards sequence, from bioturbated sandy mudstone to very fine-grained sandstone. Two felsic Definition. This formation is defined by the pre- volcaniclastic turbidite units of the KY-5 and KY-6 dominance of HCS and TCS sandstones and conglom- (Tsukimi Sandstone Member) are intercalated in the erates, and corresponds to the Hakobuchi Group of lower and middle parts, respectively. The KY-5 unit Matsumoto (1942). consists of thick beds of volcaniclastic sandstones with abundant Inoceramus uwajimensis. In the Tsukimi-sawa Type section. Downstream of the Shuparo River Valley section, the KY-5 becomes channel-fill conglom- (dam site of Lake Shuparo) in the Oyubari area (Figs. 4, erates, including well-rounded pebbles of chert, sand- 6; section 15). stone and mudstones and rhyolitic rocks, with abundant fragments of I. uwajimensis. They are high-density tur- Stratigraphic relationships. This formation conform- bidite beds, exhibiting the S1-2 divisions of Lowe (1982). ably overlies the Kashima Formation in the Oyubari The KY-6 unit (Tsukimi Sandstone Member) is 20 m area and the Haborogawa Formation in the north- thick and exposed in the eastern Mikasa area. This unit eastern Mikasa and Tomamae areas. However, the consists mostly of volcaniclastic sandstones with very unconformable relationship between the Haborogawa thin interbedding of dark grey mudstones. The sand- and Hakobuchi formations is recognized in the north- stone beds range from 0.2e2 m thick, and display the western limb of the Sorachi Anticline, north-western S1eTb-e divisions of Lowe (1982) (Fig. 7N). Mikasa area. The Hakobuchi Formation is overlain by In the Tomamae area, this formation consists of two a disconformity, or an angular unconformity, and is coarsening-upwards sequences. Both begin with strongly covered by deposits younger than the Paleocene, such as bioturbated mudstone, grade into bioturbated muddy the middleeupper Eocene Ishikari Group, containing sandstone, and end in medium- to coarse-grained, cross- coal measures, or the upper Eoceneelower Oligocene laminated sandstone (Fig. 7O). The lower coarsening- offshore-marine Poronai Group. upward sequence incorporates the volcaniclastic marker unit of the KY-5 in the lower part and the KY-6 at the Lithology. As the deposits of the Hakobuchi For- top, respectively. mation form complicated stacking patterns of the 384 R. Takashima et al. / Cretaceous Research 25 (2004) 365e390 third- and fourth-order depositional sequences (DSs), hemipelagic mudstone-dominant units (Fig. 2). The the lithology varies depending on areas and horizons, as AptianeAlbian sequences are laterally similar in all for the Mikasa Formation (Ando, 2003). The number of study areas, comprising the laminated mudstone Soa- sequences differs between sections, but the maximum shibetsugawa Formation, the turbiditic Shuparogawa reaches over 15, including third- and fourth-order DSs. and Maruyama formations, and the mudstone Hikage- They mainly consist of coarsening-upwards facies suc- nosawa Formation, in ascending order. Sedimentary cessions (CUS), a few tens to 100 m thick, of bioturbated structures, lithological associations and sedimentary se- sandy mudstone to HCS/TCS sandstone (Fig. 7P). quences show slope fan with channel-fill structures. Often, there is an associated thin, fining-upwards marine Scarce occurrences of macrofossils, and common occur- succession (FUS), not more than a few metres thick, rences of deep-sea trace fossils, suggest abyssal environ- below the CUS. Fluvial conglomerate, sandstone, ments in the Aptian. Benthic foraminifers indicate the mudstone and sometimes coaly beds are intercalated in upper bathyal zone, about 300e600 m in depth, in the the basal part of the DSs. Thick marine conglomerates Albian. and fluvial-channel conglomerates are subordinately de- The Cenomanianelower Campanian rocks display veloped at several horizons. Felsic tuffs are interbedded lateral variations in lithology, tending to north-westward at a few horizons, and thicken in the northern Mikasa coarsening and shallowing sequences (Fig. 2). The and northern Tomamae areas, where the beds attain 30 uppermost AlbianeTuronian Mikasa Formation is and 80 m in thickness, respectively. Compared with the typically composed of shallow-marine, including fluvial, Mikasa Formation, the Hakobuchi Formation is estuarine and outer-shelf, sediments, suggesting rapid characterized by a smaller amount of offshore mudstone uplift in the western margin of the Yezo Forearc Basin. and a larger amount of conglomerate and tuff. The overlying Haborogawa Formation represents deeper facies than the Mikasa Formation in terms of sedimen- Thickness and distribution. The thickness is variable, tary environment, as indicated by the occurrence of ranging from several tens to 450 m thick in the Mikasa shoreface to outer shelf, and partly upper bathyal basin- and Oyubari areas (Ando, 1997). The formation is 250 m plain deposits. On the other hand, the Cenomaniane thick in the northernmost part of the Tomamae area. Turonian Saku Formation and ConiacianeCampanian Kashima Formation exhibit slope fan on continental Fossils and age. Although a few macrofossils, such as slope and basin-plain successions, respectively. Based on Sphenoceramus schmidti (Michael), S. hetonaianus (Mat- benthic foraminiferal assemblages, these formations sumoto), and Inoceramus shikotanensis Nagao and were deposited in upper bathyal depths. Matsumoto, occur in this formation (Ando, 1997; Ando The neritic to non-marine Hakobuchi Formation, et al., 2001), there are no age-diagnostic species. The deposited during the CampanianeEarly Maastrichtian planktonic foraminifers Globotruncana rugosa (Marie) and Late Paleocene interval, completely covers the deep- and Subbotina triloculinoides (Plummer) occur in the sea sediments across the whole area from north to south. lower and upper parts of this formation, respectively This formation indicates the final stage of deposition (Yasuda, 1986). The former is assigned to the Campa- and uplift of the Yezo Forearc Basin. nian (Robaszynski et al., 1984), and the latter is of Paleocene age (Yasuda, 1986). The Cretaceous/Palae- ogene boundary sequence has not been detected in the 5. OAE horizons in the Yezo Group Hakobuchi Formation. Laminated, organic-rich marine sediments, indicating Depositional environment. Depending upon the strati- six oceanic anoxic events (OAEs 1ae1d, 2 and the mid- graphic position within the third- or fourth-order DSs, Cenomanian Event, MCE) have been reported from the depositional environments represented change regu- mid-Cretaceous deep-sea cores and land sections in the larly and repetitively. They include shallow-marine envi- European Basin, North and South Atlantic oceans, and ronments, such as outer to inner shelf and shoreface, equatorial mid-Pacific mountains and plateaus (e.g., and subordinately estuarine, incised valley, and riverine Erbacher et al., 1996; Leckie et al., 2002; Coccioni and gravelly/sandy river-channel, back-marsh and flood- Galeotti, 2003). These sediments are visually-distinctive, plain. so-called ‘‘black shales’’, found as black to dark grey intercalations in white, pelagic limestones or grey marlstones. These black shale intervals commonly consist 4. Lateral change of depositional environments mainly of finely alternating layers of calcareous fossil- in the Yezo Basin rich laminae and clay/organic matter (Arthur and Sageman, 1994). The Yezo Group is lithologically characterized by However, terrigenous, siliciclastic sequences exposed an alternating sequence of turbidite-dominant and in the North Pacific regions, such as the Yezo Group in R. Takashima et al. / Cretaceous Research 25 (2004) 365e390 385

Japan, are generally composed of dark grey mudstones correlates with the OAE1a event (Fig. 8). The OAE1a and pale grey sandstones. Dark grey, monotonously- interval in the study area is very thick, about 110 m coloured deposits throughout the successions prevent (Fig. 6; section 11), compared with 1e5 m in the Italian visual recognition of OAE levels in background sedi- and Swiss sections (Menegatti et al., 1998). The extra- ments in certain outcrops. In the Yezo Group, the ordinarily thick interval in the Yezo Group resulted average total organic carbon (TOC) content is about from a huge terrestrial influx from the active Asian 0.5e1.2 wt% (Hasegawa and Saito, 1993), higher than continental margin. Mudstone in this interval is charac- those in the Tethyan and Atlantic carbonate rocks and terized by rare bioturbation and fine lamination, with marlstones (usually TOC is less than 0.5 wt%). The scarce or no benthic macro- and microfaunas. organic matter is mostly derived from terrestrial plants throughout the sequences, suggesting that organic frag- 5.2. OAE1b horizon ments (origin of the TOC) were transported from the westward Asian continental margin. Therefore, as the OAE1b was a long-lived event, extending from the TOC of the Yezo Group is not a useful indicator of Ticinella bejaouaensis to Hedbergella planispira pf zones anoxic environments, the biostratigraphy and chemo- (114.5e108.21 Ma, about 6.3 myr; Hardenbol et al., stratigraphy are both very important in identifying OAE 1998). This event is represented by several black shales: horizons in this sequence. Several chemostratigraphic (1) the uppermost Aptian Niveau Jacob in the Vocon- studies, using organic carbon isotopes from the Saku tian Basin and Livello ‘‘113’’ in central Italy, and (2) the 13 Formation, have reported a positive d Corg excursion lower Albian Niveau Kilian, Paquier and Leenhardt in record related to the global burial event of marine the Vocontian Basin, and Livellos Monte Nerone and organic matter around the Cenomanian/Turonian boun- Urbino in the Apennines of central Italy. dary (OAE2; Hasegawa and Saito, 1993; Hasegawa, In the Yezo Group, an interval spanning the T. 1995, 1997; Hasegawa and Hatsugai, 2000). Moreover, bejaouaensis to H. planispira pf zones was not detected, 13 another positive d Corg excursion (ca. 3.6&) was because the interval is placed within the olistostrome observed in the Soashibetsugawa Formation of the bed (Fig. 8). Consequently, OAE1b is missing owing to northern part of the Oyubari area (OAE1a; Ando et al., erosion or hiatus by a tectonic event in the study area. 2002, 2003). Here we summarize the published biostratigraphic and chemostratigraphic results of the study 5.3. OAE1c horizon areas using the new stratigraphic classification presented in this paper (Fig. 8), and identify and describe the OAE Black shales representing the OAE1c event are horizons in the sequence. characterized by abundant terrigenous organic matter in the lower Upper Albian Biticinella breggiensis pf Zone 5.1. OAE1a horizon in central Italy (Amadeus Level) (Erbacher et al., 1996). 13 The d Ccarbonate excursions within the B. breggiensis pf The late Early Aptian OAE1a, spanning about 1 myr Zone are not distinct in the Italian section (Erbacher (119.5e120.5 Ma; Larson and Erba, 1999), named as et al., 1996), whereas a prominent negative excursion of 13 the Niveau Goguel in the Vocontian Basin of France, d Corg (1.5&) has been identified in the Santa Rosa Livello Selli in Italy and Fischschiefer in Germany, is section, Mexico (Bralower et al., 1999). a global, organic-carbon burial-event. This OAE oc- In the Yezo Group, the B. breggiensis pf Zone spans curred within the basal part of the Leupoldina cabri pf the uppermost part of the Shuparogawa Formation to Zone (Premoli-Silva et al., 1999). There was a microfossil the basal part of the Hikagenosawa Formation (Fig. 8), 13 (planktonic foraminiferan and radiolarian) diversity and is 300 m thick. A negative d Corg excursion of about decrease, and nannoconid (calcareous nannoplankton) 1& occurs at the top of the Shuparogawa Formation species disappeared around the OAE1a event (Erba, (Hirano and Fukuju, 1997), and is followed by a radio- 1994; Premoli-Silva et al., 1999). The sharp negative larian high-productivity event in the Maruyama Forma- 13 d Ccarbonate excursion (0.5e3&), and the following tion (Fig. 8). This excursion interval could be correlatable abrupt, prolonged positive one (O2&), are recorded in with the OAE1c horizon of Leckie et al. (2002). this event (Menegatti et al., 1998). In the Yezo Group, a pair of negative and positive 5.4. OAE1d horizon 13 d Corg excursions have been found in the Soashibetsu- gawa Formation of the Sorachi Group in the Oyubari The uppermost Albian black shales (OAE1d, Breis- area (Ando et al., 2002, 2003). This formation has been troffer Level) occur in the upper part of the Rotalipora redefined as the Soashibetsugawa Formation of the appenninica pf Zone, as well as in the Stoliczkaia dispar Yezo Group, as described above. Because the paired ammonite Zone, in TethyaneAtlantic regions (Gale excursion spikes occur near the first appearance datum et al., 1996; Wilson and Norris, 2001). Broad positive 13 (FAD) of Leupoldina cabri, this positive excursion excursions of d Ccarbonate within this interval suggest 386 R. Takashima et al. / Cretaceous Research 25 (2004) 365e390 globally significant organic-carbon burial (Erbacher 1982; Arthur and Dean, 1986; Summerhayes, 1987). In et al., 1996; Gale et al., 1996; Nederbragt et al., 2001; central Italy, this event is associated with upper Wilson and Norris, 2001). The OAE1d horizon in the Cenomanian organic-rich layers up to the Cenoma- equatorial Atlantic area is marked by a collapse of nian/Turonian boundary. The MCE level shows a global upper water-column stratification due to intensified win- positive excursion in carbon isotope records, correlatable ter mixing and reduced summer stratification (Wilson within the middle part of the Dicarinella algeriana pf and Norris, 2001). Biological records indicate that this Subzone of the Rotalipora cushmani pf Zone. Above this OAE event damaged planktonic foraminiferal and event, radiolarian and benthic foraminifers decrease in radiolarian populations, as well as the carbonate plat- diversity in central Italy (Erbacher et al., 1996; Dameste´ forms (Erbacher et al., 1996; Nederbragt et al., 2001). and Ko¨ster, 1998; Coccioni and Galeotti, 2003). In the Yezo Group, the R. appeninnica pf Zone is Hasegawa (1997) proved a sudden positive excur- 13 correlatable with the lower middle part of the Hikage- sion of d Corg (about 1.2&) within KS19a (Z the nosawa Formation (Fig. 8). Although the base of the D. algeriana pf Subzone) in the Hakkin-zawa River R. appeninnica pf Zone is indefinite because of the pre- (Shirokin River in Hasegawa, 1997) section of the sence of a thick interval of rare to barren planktonic Oyubari area. This positive peak is probably correlatable foraminifers, mudstones in the lower part of this forma- with the MCE horizon in central Italy. Therefore, the tion contain the upper Albian ammonoids Mortoniceras horizon is placed at the top of the Hikagenosawa For- rostratum and Mariella bergeri (Kawabe et al., 2003; mation of our lithostratigraphic division. Benthic fora- Fig. 8). These ammonoids are index species of the upper miniferal assemblages show a stepwise extinction, marked Albian Stoliczkaia dispar ammonite Zone, as well as the by a 43% reduction of calcareous species during the 3 myr R. appeninnica pf Zone, in Tethyan biozonations. A after the MCE event in the southern Oyubari area (Kaiho 13 broad, positive excursion of the d Corg, which is similar and Hasegawa, 1994; Fig. 8). Radiolarian abundances 13 to the d Ccarbonate curve of OAE1d in the Tethyane decrease during the same interval (Taketani, 1982). Atlantic region, is observed in the lower part of the Hikagenosawa Formation, just below the Kanajiri 5.6. OAE2 horizon Sandstone Member (KY-3) of the Yezo Group (Hirano and Fukuju, 1997; Fig. 8). This horizon is located within OAE2 (the Bonarelli Event) was the largest environ- the upper part of the Stoliczkaia dispar ammonite mental and biotic disturbance during the mid-Creta- Chronozone (Kawabe et al., 2003). Hence, we concluded ceous interval. Major phenomena include maximum that this excursion level is correlatable with OAE1d. mixed-layer temperatures (about 33e34 (C; Wilson The rocks correlated to the OAE1d interval are et al., 2002), an abrupt drop in 87Sr/86Sr isotopic values weakly-laminated and pyrite-rich mudstones. Planktonic (Bralower et al., 1997), increased volcanic deposits and foraminifers are very rare to absent, whereas spumellar- intensive hydrothermal activity (Sinton and Duncan, ian radiolarians suddenly increased in abundance. 1997). This event is marked by a positive excursion of 13 Agglutinated benthic foraminifers are common, with d Ccarbonate (O2&) within the Whiteinella archaeocre- assemblages of Bathysiphon, Glomospira and Haplo- tacea pf Zone, close to the Cenomanian/Turonian (C/T) phragmoides, and rare Gyroidinoides and Lenticulina. boundary, and is considered to be related to increased The dominance of agglutinated forms might reflect productivity and global expansion of the oxygen relatively dysoxic conditions, as well as the Cenoma- minimum zone (e.g., Dameste´ and Ko¨ster, 1998; nian/Turonian boundary (Kaiho and Hasegawa, 1994). Premoli-Silva et al., 1999). A significant turnover of Although a dissolution effect on calcareous species can- radiolarians, calcareous nannofossils, and deep-dwelling not be ruled out, our results suggest that dysoxic con- planktonic foraminifers occurred, with a high extinction ditions developed and a radiolarian high-productivity rate (26% of all genera) of benthic foraminifers event took place during the OAE1d in the north-west (Erbacher et al., 1996; Erbacher and Thurow, 1997; Pacific. Premoli-Silva et al., 1999). The C/T boundary, based on macro- and microfossil 5.5. The mid-Cenomanian Event horizon biostratigraphy, is placed at the base of the Hakkin Muddy Sandstone Member of the Saku Formation in The mid-Cenomanian Event (MCE) is a major the Oyubari and Tomamae areas (Figs. 4, 6; sections 10, 13 turnover of foraminifers and radiolarians associated 13, and 14). A positive d Corg excursion of about 2.5& 13 with a 0.7& positive shift in d Ccarbonate and a positive is recorded at this boundary, indicating OAE2 (e.g., shift in the Sr/Ca ratio (Coccioni and Galeotti, 2003). Hasegawa and Saito, 1993). Investigation of benthic The organic-rich layers corresponding to this event have foraminiferal assemblages in the Oyubari area demon- been found in central Italy, the Vocontian Basin strates that the two lowest oxygen conditions devel- (Coccioni and Galeotti, 2003), and the North Atlantic oped during 0.15 myr before the C/T boundary and (Deep Sea Drilling Project Sites 367, 386 and 398; Cool, 1.5 myr after (Kaiho and Hasegawa, 1994). Radiolarians R. Takashima et al. / Cretaceous Research 25 (2004) 365e390 387 become abundant just above the C/T boundary (e.g., for support during our fieldwork. We thank T. Hatsugai, Hasegawa, 1997; Fig. 8). A. Ennyu, S. Egawa, F. Suzuki, A. Ando and T. Sakai for provision of unpublished data. This study was supported financially by the JSPS Research Fellows 6. Conclusions Scheme (No. 09898 to Takashima, No. 06365 to Wani, and No. 07622 to Moriya), Grants-in-Aid from JSPS Our study of the Yezo Group, a thick, forearc basin- (No. 14740302 to Kawabe, No. 10640446 to Ando, fill sequence, distributed across Hokkaido, Japan, serves and Nos. 13354006 and 15340176 to Nishi), and in part as an important record of tectonic, faunal and en- by a 21st Century Center of Excellence (COE) Program vironmental change on the north-west Pacific margin on ‘‘Neo-Science of Natural History’’ at Hokkaido during the Cretaceous Period. The litho- and bio- University. stratigraphic summaries of the sequence were previously little known, and various lithostratigraphic divisions had been proposed. Based on our wide areal mapping of the sequences, as well as analyses of litho- and biofacies, References we have here proposed a new lithostratigraphic scheme for the sequence. Ando, H., 1990a. Shallow-marine sedimentary facies distribution and The AptianeAlbian sequences (the Soashibetsugawa, progradational sequences of the Mikasa Formation, Middle Yezo Shuparogawa, Maruyama and lower Hikagenosawa Group (Upper Cretaceous). Journal of the Geological Society of formations) of the group were deposited on a continental Japan 96, 453e469 (in Japanese, English abstract). slope with a lowereupper bathyal palaeodepth in all Ando, H., 1990b. Stratigraphy and shallow marine sedimentary facies of the Mikasa Formation, Middle Yezo Group (Upper Creta- sections. The Cenomanian to lower Campanian sequen- ceous). Journal of the Geological Society of Japan 96, 279e295 ces exhibit lateral variations of sedimentary environ- (in Japanese, English abstract). ment in the study areas. Generally, the sequences Ando, H., 1997. 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