Litho- and Biofacies of Mudstone in the Pliocene-Pleistocene Title Kazusa Group, Boso Peninsula, Central Japan
Author(s) Sato, Mika
Memoirs of the Faculty of Science, Kyoto University. Series of Citation geology and mineralogy (1992), 57(1): 1-31
Issue Date 1992-08-31
URL http://hdl.handle.net/2433/186673
Right
Type Departmental Bulletin Paper
Textversion publisher
Kyoto University MEMolRs oF THE FAcuLTy oF SclENcE, KyoTO UNIvERslTy,SERIEs oF GEoL. & MINERAL, , VoL LVII, No. 1, pp. 1-31, August 31, l992
Litho- and Biofacies of Mudstone in the Pliocene-Pleistocene Kazusa Group, Boso Peninsu la, Central Japan
By
Mika SATo"
(Manuscript received March 30, 1992)
Contents
Abstract ."...",".,,".,..H,,",,.,....",,,",H.H..,,".,,,...H.,.-H,,,",...,"..,,,,",,",.. 1 Introduction.,,.",,...,.".,,,.,.H,,.-,,,,-H.,.,,,,"...... ,.,,,,"...,.H.H.....,,,,,",....H 2 Geological setting ",,,"...... ,,,",,..H...,,.,...H...,-,,.,.H"...... ,.,,,,.,,...... , 3 Stratigraphy ...-,,...,,H",,-,",.H...,",,,..."...",,,,...H...-,,,,,-....H,.,,,m,,".... 3 Geological age of the Kazusa Group .,..,,,...... J¥¥¥-¥¥-¥¥¥¥--¥¥¥-¥¥¥¥J,,¥¥¥¥-¥¥¥¥¥¥J¥ 10 Sedimentological features ofmudstone and benthic faunas ,,..,...... ,..,,.,, 10 Sedjmentary environment ofmudstones in the Kazusa Group ...,.,,,...... 25 A. Stratigraphic change ofmudstone facies ..,...... ,,..,,,...... ,, 25 B. Reconstruction ofsedimentary environemtns ofthe laminated fine-grained mudstone (Facies E) ,...... ,,...... ,...,,...... 27 Conclusion ,,"..H.,,,.,,,,H..H..-,,,..,..H,,.,,",....H.,",,",..-"....,,..,,,,H...... ,H,,. 29 Acknowledgments .H...H,,",".,,...... ,,.,...... H.,,"...... "-,,,",,","-...H,,",,. 29 References .,".,,"..".,,,".,,.,,-...",,.","..",,,.",,....".,.",,,,.H.H....",,,,.,,....H 30 PIates l-3
Abstract
The Pliocene-Pleistocene Kazusa Group, deep- to shallow-marine clastic deposits attaining 3,000m thick, exposed in the Boso Peninsula, central Japan, is predominated by apparently monotonous mudstone, Based on the sedimentary structure, nature and intensity of bioturbation, and containirig mega-fossils, the mudstone can be divided into following five different facies; (l) Greenish gray massive fine-grained mudstone facies (facies A). Moderately bioturbated, fine-grained mudstone, in which small, horizontal tube burrows are quite common. Benthic assemblages indicate the bathyal environments, (2) Bluish gray mottled coarse-grained mudstone facies * Present address: Technical Center, Tokyo Kyuei Ce,, 6906-10, Shiba Tsurugamaru, Kawaguchi, 333 Japan 2 SATo M, 7 fossils are quite rare. rl"he distribution of facies types changes stratigraphically, the facjes A is seen in the Jower part of the group, and is replaced upward by the facies B, and C, successively. In general, grain size and degree of bioturbation increase upward. The succession probably refiect the shallowing upward environmental change of the group. The facies D, the muddy turbidite, is found locally within the distribution of the facies A. Thin parallel laminated mudstone of the facies E suggests an oxygen-deficient environment. The in sita bed of the facies E is distributed in a restricted western part of the Yoro area, while they occur commonly as allochthonous blocks of the slump deposits in the facies A of the eastern area. The facies E might originally be deposited in the shallower western area, and most part of the facies might be slumped eastward, and preserved as a]lochthonous blocks. Introduction The Kazusa Group, Upper Pliocene to Lower Pleistocene offshore deposits, is widely distributed in the middle part ofthe Boso Peninsula, centralJapan. It consists largely of massive bluish- to greenish-gray mudstone intercalated with light-colored sandstone beds and many traceable tuff beds, and yields numerous well-preserved mega- and microfossils. Many studies from various points of view have been made for the group since the last century. Detailed litho- and chronostratigraphy of the group have been established by MiTsuNAsHi <1954), MiTsuNAsHi et al. (1959, 1961, 1976), IsHiwADA et al. (1971), NiiTsuMA (1976), and ODA (1977). Many paleontological data ofthe group have been accumulated (e.g., OyAMA, 1959; IsHiwADA, 1964; BABA, 1990). Bathymetric study using benthic foraminifers indicates that the group was deposited in the depths from lower continental slope to shelf upward (AoKi, 1968). In addition, there are sedimentoiogical studies focused on the sandstone beds in the group. Some sandy turbidite beds in the group were traced and observed over 38 km (HiRAyAMA and SuzuKi, 1968). KATsuRA (1984) recog- nized twenty-five sandstone facies jn the group, and reconstructed their depositional processes. Cyclic change in thickness of the turbiditic sandstone layer was regarded to reflect the "Milankovitch cycle" by MAsuDA et al. (1989). In contrast, sedimentary features of mudstone have remained largely to be explored, although mudstone is dominant throughout the Kazusa Group. Mudstone may yield many important information on offshore litho- and bio-event, and may reflect sedimentary environments at that time. I made detailed observation on the mudstone of the Kazusa Group, focusing on lithology, sedimentary structures, fossil associations, and mode of their occurrences in mudstone. Some different types ofmudstone facies have been recognized. In this paper, I will describe sedimentary and biological features of the mudstone facies, and try to reconstruct depositional environments in the Kazusa Group. Offshore mudstone facies are predominant in the Kazusa Group except the uppermost part (Mandano and Kasamori Formations). Therefore, the discussion will be concen- trated mainly on sedimentary and biological features of the Iower 10 formations of the group, namely, the Kurotaki, Katsuura, Namihana, Ohara, Kiwada, Otadai, Umegase, Kokumoto, Kakinokidai, and Chonan Formations, all of which are composed mainly of muddy deposits, and are well-exposed in the Yoro area, the study area in the central part ' i ' ti ÅëL ' t ' > ' --.. ' )l ii'/ ' ! t e /x }> SE) ./ 13 ' ' 130 "...t ' -.¥ t' 129 ' ' " t- 128 126 t 277 ' tt-)Xci 125 t .---274 g" ' / 2' 7{[tig:s?..1 T Oh .--- -v"- ---" '-"' -- o ------.-t ' 124 .t o '-'-'-'---'-'t ' 1.i?g.. -V6t "s pt. - ' . -i F. ' "g v. Chonan .... - 123A 06i!ofs.>A CdS' .r;: l2 -:---:; rt '1 114 :: -- i-- --: o t KO :-- :: - - ¥;; Mobara ' -:--: -/ Kakinokidai E 120 1 11 112 :. 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Index- , B nd C in the index map show he location f the oro, Chonan, nd atsuura-Ohara reas espectively. Litho- and Biofacies of Mudstone 3 ofthe Peninsula, and the Katsuura-Ohara area in the eastern part ofthe Peninsula (Fig. 1; Table 1). As the distinction of cJaystone and si)tstone is diflicult without precise grain size analysis, I divide the mudstone into fine-grained and coarse-grained based mainly on the observation with binocular microscope. Geological setting The Boso Peninsula is situated in the southern part ofnortheastJapan near the triple junction of the Philippine sea, Pacific, and North American Plates. The Neogene and Quaternary sediments deposited in the forearc slope and basin, ranging from deep- to shallow-marine environments, are widely distributed in the Peninsula. The sediments have been divided into the Lower Miocene to Lower Pliocene Miura Group, the Upper Pliocene to Lower Pleistocene Kazusa Group, and the Middle to Upper Pleistocene Shimosa Group. The Kazusa Group is exposed in the middle and northern parts of the Peninsula, showing northwestward-dipping homoclinal structure with dips of 20e in the Iower part, and 50 in the upper part in general. There are many normat fauits of N-S strike in the eastern part of the distribution area. Main part of the group is well exposed in the Yoro, Chonan, and Katsuura-Ohara areas (Figs. 1-3). . Stratigraphy of the Pliocene-Pleistocene Kazusa Group The Kazusa Group is characterized by thick, clastic sequence attaining 3,OOO m in maximum thickness. It overlies unconformably the Miura Group, with the Kurotaki Unconformity, and unconformably overlain by the Shimosa Group. )N" 4,,Å~.KasaoMOriK,os Kt"aSaM04or;E,to, choi'llilan2Mgolal)xch x 409 Chonan F. /403 40 40C 411 403 L :o2 TsurumaioX 0 O 2km 401 Kakinokidai F. Fig, 2. Locality map of the Chonan area (B in Fig. 1). 4 SATo, M. 313 Otadai F. 312 k314 ' 311 SU pt 1 . 310 oj b &h Kiwada F. Ohara F. -- 309 308 NamihanaF. 307 N. N 306 Katsuura F. 305 304 Katsuura City 303 o " o 5km 301 302 Fig. 3. LocaLity map of the Katsuura-Ohara area (C in Fiq, ,1). Stratigraphic subdivision of the Kazusa Group differs in places because the group shows remarkable lithologic change laterally from west to east. In the western area, sandy deposits predominate, and are partly interrupted by small-scale unconformities, while muddy deposits are widely distributed in the central to eastern parts of the Peninsula. In the latter parts, the Kazusa Group is about 3,OOO m thick and is subdi- vided into 9 formations, namely the Kurotak i, Kiwada, Otadai, Umegase, Kokumoto, Kakinokidai, Chonan, Mandano, and Kasamori Formations in ascending order (MiTsu- NAsHi et al., 1959, 1961, 1979). They are conformable with each other. Among them, the lower 6 formations, from Kurotaki to Kakinokid ai Formations, are well-exposed in the Lithe- and Biofacies ofMudstone 5 Yoro area, where is the chief area studied (A in Fig. 1). The Chonan Formation is well-exposed in the Chonan area, located at 9 km northeast of the Yoro area (Fig. 2; B in Fig. I). In the Katsuura-Ohara area of the eastern coast, located 25 km east of the Yoro area (Fig. . 3; C in Fig. 1), the beds synchronous with the Kurotaki Formation become very thick, and are further subdivided into 3 formations, namely Katsuura, Namihana, and Ohara Formations in upward sequence. Summary of stratigraphic subdivisions and correlation is shown in Table 1. Table 1, Stratigraphic summary ofthe Kazusa Group in the Boso Peninsula. (Modified from MiTsuNAsm, 1979), Westernpart Easternpart SHIMOSAGROUPSUAF. KASAMORIF. Ks5 - a=oece CHONANF. h3 Kal ICHtJIKU F.KAK!NOKIDAIF.Ka2.3 Ku2 KOKUNOTOF.Ku6 WA$AKAF. HfGASHtGASA UMEGASEF.U6 07 TAKAMIZO OTADAIF. 027 F. ltS9, T . TAKEOKAF. KtWADAF. ,K,d gg es OHARAF. s NoANAMIHANAF. stfKATSUURFA. 7.". I MIURAGROUP The uppermost parts of the group, the Mandano and Kasamori Formations, are composed of sandy deposits. Therefore they are out of the scope of the this study. Thicknesses of the formations shown below are mainly measured along the Yoro River route. The columnar sections in the Yoro and Katsuura-Ohara areas are shown in Figs. 4-6. Kurotaki Formation [UEDA, 1930] StratotlPe: Kurotaki, east of the Kameyama Lake, Kimitsu City, about 6 km west of the Yoro area. Thickness: 280 m along the Yoro River. The formation is composed of basal conglomerate of 5 m thick in the western area (Locs. 181, 200), being succeeded by brownish gray, bedded tuffaceous sandstone of 6 SATo, M. W Yoro River E ul . Kd2 :""Nt. Tsuchisawa ShiroujiSaWa shichiri ::::r:: q Yuuki ..g'.s Kurotaki r:¥':t, a ::::t::: :::::::::::::lt n:.:.:C.:':':" '::it;:: :;::f:::::-': ( :::[:1.::::::-::: :I:::I.:1.I:1:l.1, llllltEi¥;l ;llilil g llllilll: --' .Kd5 I{'/i'i'ii l{:.}ll{llll{¥ :::::s.:¥L¥:¥i; llll¥il¥¥l¥i 1':;¥f,i 9 :iif}'ii' Hirasawa ttt .t tt tt tlttt v' v ifvtVvttvvtVVgV vivvvvvvvtvvvvvvvt Kd8"tvvvtJv"vvvv"vvv L.L: VV.VvV vvtJifttvttVVgVV tttvtfVtifV t::t:"." :.:-:.p:c.: ,tlll: ::s::::::r::: t:,x" .":';'::::S""x' :':i:iÅ}:'t':i::'::' /E :1.:::::::::::. U' .:.:c.:" 1¥::::::::::::¥, '{'{{':}i{'i} ::::;::. y ¥::::::::::::k .:::;.:::. = ¥:¥:t::':a¥':::: . vt'vVtvvUVV .:::::::5::::: '::s::::, : KKd,ilg t:t:t:t:t):t:t :.:,:.:.) tV'tvt ( ' :c.":.):.:,: /!'('r¥L-:,';, llliillS .:::s::s Kdlcr' vtv"ivtg"vvVJV ' rllll'lt',ll'- :'):':x - tt ):t:t:t ' v t:t:t:t:t:::t) "VttWt e}E o l..}//,l/r¥t//i e¥i:i! .Kd19 ec L tttit/tt 'il':l:l L t/ ¥ri:i:iE 7¥ ¥:IG:::I Ltt ¥iE:i:i:¥:i:i: ; l . E:;¥:Il¥ - ,.tt¥//:fl:t¥' . tlj1/1!¥i'//s-. = tt.-. t- .:::::::::s::: :ltf#" Ltt. Ill;Xe.t¥ Y Iilil:,l liil/Ulll tLL ::I:i:1:1'ia:l t-if :t)tt:t:t L:.::'", v- ,' :r.¥i, vvvvvvvvvvvvJvvvvvv Kd20L,vvvtvvvvt.vtvvv vv 'l'{;e:il¥l,¥" I{{¥iS,. lit t"t t vv 'vV .,NeXfS,x¥ b- Kd23v. .N:, .;, '"-ISt/ t, ' leL {g¥i¥; VF i:\\'l'<¥. 1¥. 1. .::::p" ILt"}Y{li. -LLLL I [x; ., y iS'ts' l"lx','¥l'L ¥' .1/ ;1ljly: tkL.,i,;.tt>.t¥.¥ LLLLL uss li:1:1¥ ,kl-ec,f. t":t:t: LEGEND K6i, i¥li¥//¥Utk 1IIi{. 'lli;11/El//f,i,)¥ II#ilif/ii{i¥;¥li¥ Greenish gray ma$sive tine mudstone LtLL o 2km ).- S' 'JiL J Alternating beds ot greenish gray tine mudstone l',t(x;/i¥/il/ - and biuish gray tjne mudstone ez tS¥L¥ , ,d N Greenish gray lamtnated fane mudstene )t t.;/1[rtTyl. /:¥ ..b ;c.g),i. ii!- Alternating beds of sandsrone and fine mudstone fl¥)¥"g;,lgt// ;i,'l-1'/ll' Tuffaceous$andstone ,Jtlftt,x-4L. 1[Å},C't,L)//U,c"i•¥L.t 'k.t..'.tr/¥. ny. Slumpedbeas .t.-x1.L as Part intercalated wdth"mudstone breccLa" xL.11¥NV)ltlt t- x llf'(S¥t'".- v....vv Tufflayer 300m - Kd 19 --- Markertuff Layer ColumnarFig. 4. section ofthe Kurotaki and Kiwada Formations in the Yoro area. Deposits thicken in the eastern part, in which mass-gravity flow deposits originated from slump and turbidity current are frequently intercalated in the mudstone. various grain size, 270 m thick, which becomes finer upward, and muddy tuffof 10 m thick at the top. Katsuura Formation [SAwADA, 1939] Stratot"Pe: Katsuura Coast, Katsuura City, about 20 km east of the Yoro area. Thickness: 250 m in the Katsuura City. The formation is composed of bedded mudstone, alternating beds of fine- to coarse- grained turbiditic sandstone of 30-200 cm thick and bedded mudstone of 10-30 cm thick. There are frequent intercalations of slumped beds of 1-20 m thick. Namihana Formation [KoiKE, J9491 StratotyPe: Misone Coast, Ohara, about 25 km east of the Yoro area. Thickness : 220 m in Ohara. The formation consists of bedded fine-grained mudstone, 30-100cm thick, and intercalated fine-grained sandstone beds, 5 to 30 cm thick, with frequent intercalations of slumped beds of 50-100 cm thick. Litho- and Biofacies of Mudstone 7 t"'t'i'1 KAKINOKIDAI e Kal z F. q tseUl tt E'''t'''il .x Ka ,6 Z ut o ¥¥ ¥¥ ----E e Kul r t--¥-sw --- o e Ch3 --- L,n,: -- i-- U2 9oE=\o tttt .' :'¥':::',';.='L'"1¥ ttt ¥:t'IitYl¥.:.:r'," ";:,.;;:.Sli/:¥:¥ e Ku2 eU4 :::ll:!II ;--+i---- '!'-'L''' iii' eU6 Y e Ku5,:.t:.'.(,r/' r.ttttt/.ti,tt.t. e Ku6t. e U8 e Ull LEGEND ------ny--- -"------}h- -il{lllill!i{iliiii Greenish gray massive fine mudstone ------ 11/ll'i' l,ll:,l'I'l:lii Biuish gray mottled coarse mudstone e o7 ;,l"'I¥'ti-;-;'?¥i Light gray massive fine mudstone '¥r".t -4t .,., .' Alternating beds of greenish gray fine mudstone 300m and bluish gray fine mudstone e Oi8 [EI GreenishgrayEammatedfinemudstone fo22 --- :- .'- Alternating beds of sandstone and mudstone ÅÄ '. l.. :::"ii Beddedsandstone --- . 026 eo27 rMX¥}li ,-.. Slumped beds @ Part intercaiatedwith "mudstonebreccia" O e 027------Marker tuff layer Fig. 5. Columnar sections of the Otadai to Chonan Formations along the Yoro River, 8 SATo. M, Yoro River Katsuura-Ohara i}i¥ii{E¥ llllll¥ll ":' tr'¥' Illli'llll - Hirasawa 't-'"t.. 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Columnar sections of the Kurotaki to Kiwada Formations in the Yoro and Katsuura- Ohara area, Symbols are same with Fig. 4. Litho- and Biofacies ofMudstone 9 Ohara Formation [MiTsucHi, 1933] Stratotlpe: Ohara Coast, Ohara, about 28 km east of the Yoro area. Thickness: 200m in the Ohara town. The formation is characterized by frequent intercalation of sandstone beds. It consists of alternating beds of bedded fine-grained mudstone, 30-200m thick, and sandstone of 10-100 cm thick with slumped beds of5-20 m thick. Kiwada Formation [UEDA, 1933] Stratot?pe: Kiwada, Kimitsu City, about 3.5 km west of the Yoro area. Thickness: 600 m along the Yoro River. The formation is characterized by dominance of bedded fine-grained mudstone, 50- 100 cm thick. The mudstone is intercalated with sandstone of 5-30 cm thick at the east ofthe Yuuki River, and slumped beds of5-35 m thick in the middle part of the Yoro area. Otadai Formation [MiTsucHi, 1933] StratotyPe: Otadai, the Yoro area. Thickness: 540 m along the Yoro River. This is characterized by frequent intercalation of sandstone beds in mudstone. It consists of bedded fine-grained mudstone, 30-80m thick, and alternating beds of fine- grajned mudstone, 1-50 cm thick, and sandstone of 30-300 cm thick, and slump beds of 30-300 cm thick intercalated at several stratigraphic levels. Umegase Forrnation [MiTsucHi, 1933] StratotJPe: Umegase, west ofAsobara, Ichihara City, about 2.2 km west of the Yoro area. Thickness: 530 m along the Yoro River. The formation is characterized by more frequent intercalation ofsandstone beds than the Otadai Formation. The formation consists of alternating beds of fine-graned mud- stone, 1-50cm thick, and turbiditic sandstone of 50-200cm thick, and bedded fine- grained mudstone, 30-60 cm thick, with intercalated several slump beds of 2-3 m thick. Mudstone becomes coarser in the upper part of the formation. Kokumoto Formation [MiTsucHi, 1933] Stratotmpe: Kokumoto, Ichihara City, Yoro area. Thickness: 300 m along the Yoro River. The formation consists of bedded coarser-grained mudstone, 50--200 cm thick, and alternating beds of sandstone, 30-300 cm thick, and coarser-grained mudstone, 5-50 cm thick. Kakinokidai Formation [MiTsucHi, 1933] StratotlPe: Kakinokidai, Ichihara City, about 2 km east of the Yoro area. Thickness: 65 m along the Yoro River. The formation is composed of coarse-grained mudstone, in the lower 35 m, and sandy 10 SATo, M, mudstone, 30 m thick, in the upper part. Chonan Formation [MiTsucHi, 1933] Stratotlpe: Chonan Town, about 9km northeast ofthe Yoro area. The lower boundary of the Chonan Formation is demarcated here according to ToKuHAsHi and ENDo (1984). Thickness: 85 m along the Yoro River. The formation is characterized by frequent intercalation of sandstone beds. It consists of alternating beds of fine-grained mudstone, 5-50 cm thick, and fine- to coarse- grained turbiditic sandstone of 30-300 cm thick. Geological age of the Kazusa Group The age of the Kazusa Group ranges from Late PJiocene to Early Pleistocene. According to the magnetostratigraphic correlation (NiiTsuMA, 1976), the boundary be- tween Pliocene and Pleistocene lies at the lower part of the Kiwada Formation, near the Kd38 tuff ODA (1977) indicated that the boundary between the Matsuyama Reversed Epoch and the Brunhes Normal Epoch, based on the planktonic foraminiferal assem- blages, lies in the midd)e part of the Kokumoto Formation. He also reported that the Jaramillo Event lies at the uppermost part of the Otadai Formation. The LAD of Discoaster brouweri, 1.91 Ma, is known at the middle part of the Ohara Formation, and the Kasamori Formation is older than the FAD Helicosphaera inversa, O.48 Ma, both of cal- careous nannofossils (SATo et al., 1988). Sedimentological features of mudstone and benthic faunas Apparently monotonous muddy deposits predominate throughout the Kazusa Group ofthe study area. However, detailed observation ofstratification, grain size, color, degree of bioturbation, and other features of mudstone revealed that the mudstone vary stratig- raphically and geographically. Following five mudstone facies can be distinguished in the main part of the Kazusa Group: (1) greenish gray massive fine-grained mudstone facies (facies A), (2) bluish gray mottled coarse-grained mudstone facies (facies B), (3) light gray massive fine-grained mudstone facies (facies C), (4) facies of alternating beds of greenish gray fine-grained mudstone and bluish gray fine-grained mudstone (facies D), and (5) greenish gray laminated fine-grained mudstone facies (facies E). The facies A is widely distributed in the lower part of the group, the facies B is mainly distributed in the upper part of the group, and the facies C is restricted to the uppermost part, the Chonan Formation. On the other hand, the facies D and E is found intercalating in the facies A. Many well-preserved fossil mollusks and echinoids were obtained from the mudstone. The faunal composition and their modes of occurrence are closely related to the facies types of the mudstone, reflecting the sedimentary environments. Litho- and Biofacies of Mudstone 11 (1) Greenish gray massivejine-grained mudstonefacies (facies A) Distribution: The fine-grained mudstone of the facies A is distributed extensively in the lower part of the group, from the lowermost part of the Kiwada Formation to the uppermost part of the Umegase Formation in the Yoro area. This mudstone is also distributed in the Ohara to Otadai Formations in the Katsuura-Ohara area, at Locs. 309- 313. TJPe Localit": Loc. 16, along the Yoro River, the middle part of the Kiwada Formation.¥ Characteristics: Fine-grained mudstone ofmassive, moderately bioturbated, and greenish gray (5 GY 611: wet) in color. Descni tion: The mudstone is sparsely bedded with 30-200 cm interval intercalated with sandstone beds and slumped beds (described later). The mudstone is greenish gray (5 GY 6!1: wet), fine-grained, containing dark-colored Planolites type burrows (unlined horizontal tube of1cm in diameter) at places (Plate 2, fig. 1). It is almost massive, but in some cases, thin, lamina-like sandstone layers of 1 mm thick are preserved. Horizontal burrows predominate in the facies. Among them, Planolites type burrows, Chondrites of O.5 mm in diameter, echinoid burrows of 2-4 cm in diameter, and Ancenichnus horizontatis' are quite common. Anconichnus horicontalis burrow is a dark-colored, narrow, discontinuous, twisting, muddy fecal string, O.5-1 mm in diameter, within a light-colored, poorly defined burrow fi11, 2-5 mm in diameter, depleted in mud and inertinite (Figs. 7, 8; Plate 2, fig. 5; GoLDRiNG et al., 1991). " kif.gfflj.{i, i ., ''l, pt t,t .ht .esptegSb ,Las, .Y' S'-, II 1ii ;i"': Sptr .t ;il 's.kL ''l lt2/i,.ffr) th,?ajhi7, :" k.¥eX '' :"tr"): tiri.1I.pt}xX.i...tt[.l'tl"'>/s.-ti.itx¥ift¥,:¥ifitts'!t¥l' h tt- ti "g,ls.S-.pt `"' -lli¥/ ".... L-'Xi?.sii"''" 'i¥@':,. S"'`:[L fx..,.5.i.'i Z¥9)rs g/ .['Lttat'"L"' 'i..-t,--,...... s"'t"'';'''2i,l¥i` " ' t'v t'- i.,,"ig-'tltW1{.`:[`Y'--¥ig-i,&.,,II.i-:;w"-.".x.'" -- ,s/.'t)lt,.¥in' ,/ u'.J? tkt Lstt 'ttTttt"- -" ("t '---:-E-LLst LLs.xNxxVXL"-'fY ttttt S 1 cm """'Tx. . ,.y'''' "'-..-..':;X,. '., ;i'ktt¥iY"/"' "''¥¥-.1> ':1 .Yx" s '¥v' Fig. 7. Schematic diagram showing Anconichnus hori(ontatis burrows, ' The ichnofossil is widespread in muddy deposits of various ages and parts in Japan other than the Kazusa Group. They are known from the Lower Cretaceous Monobegawa Group, Upper Cretaceous Yezo and Himenoura Groups, as well as from the recent sediments recovered from 1,800---4,OOO m off Muroto (personal communication of Mr, M. MuRAyAMA, Ocean Res. Inst,, Univ. Tokyo). 12 SATo, M. ". ,sl/L/tstksgkss./ee"oseceeeeew,me (S: ...i.-....:., n; es lcm fu x N Accumulationl l .:;;:1 - NC. 'v"IESS.".{3...4"L')ll' =--7-s Accumulationll he ' di AL-s';:,'.-r"l>."SH=' A-,Åí>`J".p' tt I¥, t-t ¥11¥1¥1¥,'¥ii,¥,,i¥¥',11,:-.,l¥1¥il:¥I:.`i.i,Ii-I'i.i.1,i'ili:1¥i'¥,1,¥1' .tt 't't Finesand t' estsofechinoid 'f ft'H--. av Tf'X-VL.-N( "t ' ' tt tt ¥' ¥¥' L¥ :¥`:i.t¥-i;;s,l/.i,{;l¥lllli.,t.tj,/i/{/ig,//r¥li,li//1////{i//L¥.s/`-f//i¥itl'///"ti'ii"l'11jt¥l t.t t' ;'(`(Z'"../,.l,.,,.///,// tttt "-'"-"-'-- t- - ; ScoriaN -- T' ia-ceYossil"'--"1/ "/I'"'t':' ? 21'-!ltle.rl/ngng.s.tll,,!g!ctu.r.e?t' : t:itt/tt' -- tt /t 1¥-.. Polynemamussium ¥:S] `:lh.(J;1[)'' J,' .J.r eium ttt t/ i tt t,;- ', 1 ..X,l' ,/-¥ l;tlt/thL.NA-:./i.Jt-,t. 5-15cm .--t A ., ."'-aj Fig. 9. Schematic diagram showing rnodes of occurrence of ichno- and benthic assemblages in the greenish t ray massive fine-grained mudstone (facies A). Litho- and Biofacies of Mudstone 13 Fine- to coarse-grained scoria beds of2-5 mm thick are also intercalated. The upper part of scoria beds is, however, usually bioturbated, and scoria grains are mixed with surrounding mudstone. Several types of trace fossils penetrate such scoria beds (Fig. 9). Such traces are sometimes fi11ed with fine scoria grains, and exhibit tiering structure. Chondrites (1-2mm in diameter), echinoid burrows (2-3cm in diameter) are usually recognized in the upward order. Under the microscope, diatoms, sponge spicules, planktonic and benthic foraminifers, and volcanic glass fragments are quite common. Turbiditic sandstone showing Bouma sequence of5-200 cm thick (Fig. 10), and two types ofsandy slump deposits of30-100 cm thick ("mudstone breccia I and II" in Fig. 10) which contain abundant mudstone biocks, are intercalated in the facies A. Mudstone breccia I is composed of three parts, coarse-grained sandstone of 10-30 cm thick showing graded bedding at the base, fine- to coarse-grained poorly-sorted sandstone in the middle part, 10-50 cm thick and containing few to numerous angular mudstone blocks of 1-100 cm in diameter, and parallel laminated, fine-grained sandstone of l-5 cm thick at the top (Fig. 10). Parallellamination oO o"0yp Mudstone blocks gllli)g.g?iltl-g---- abundant lrregularmudstone .Y blocksabunaant "BoumasequenCe " -Jt" ' t,ttt tt - ttt .tt .¥ ,L, ¥' / normal ' ' ,, ttt-t'1 t'/tr tt tt 1 t tt.- ' '' ' t/ ,. .<¥;;.,,, .. graded t .. /.I..t,, g E t tt -tttt tt E ' t, .TEL' ¥.1¥1,¥'¥¥¥',i1¥i'k,. g¥ g¥ ;' i'.c .. ql: '1 /1' ''"''' li "'" g¥ C:nL'';¥:.teL>¥-¥L Nir.¥ ,J- :)¥ ix¥ t- ,¥1> '¥ra'' I -,, 1 1 Turbiditic sandstone Mudstone breccia S Mudstone breccia " Fig. 10, Sketches ofsandy beds intercalated in the mudstone. These are quite cornmon in the eastern part ofthe area surveyed, Mudstone breccia II is poorly-sorted, medium-grained sandstone including numerous contorted blocks of mudstone (Fig. 10). These three types of gravity-flow deposits are quite common particularly in the eastern part of the area studied (Fig. 4). In the upper part of the Umegase Formation, fine-grained mudstone of the facies A grades upward into coarser-grained mudsone (facies B; described later). Associated fauna: Molluscan fossils are monotonous. A molluscan association of low diversity, the Portlandia-Neilonella-Ancistrolepis Association, is recognized in the mudstone of the facies A (Table2). It is mainly composed of three thin-shelled deposit-feeding bivalves, Portlandia lischkei (SMiTH), Neilonella j'aPonica OKuTANi, and Nuculana robai (KuRo- 14 SATo, M. Table 2. Faunal list ofthe main part of the Kazusa Group. Numbers show individual MudstoneFacies A 30313162041814926311 SeciesLocalitMollusca -"-CYac-ha"e-ropLaxedHe-lic-a!u.s-(.D-al!)---ff---ff7Bathybembixargenteonitenshiraset 99------N------F---"hn---fftT---r-- -""-.--..------.(T-a-k't.&-O-tu-ka)----.- ---Na-ti-co:d"spR.---.m------. ---'-- 5--- 3--- 1' 2------1---- 4---'------Jmap.eLio-n-pertc-oc-hLio-n-(S.c-hr-en-c.k)---.------tJ------H------}.-tfl,!cLst-ro-lepi[srtrQc-ho-id-e-us-CDglL)r--L-N-- -¥------"---- 2- 2-"-- 2--M i---3--i"------tYep-tu-ne-a-in-tqrs-c-utpt-aKS-owe-rb-y)------II[IIIII[:IIIIIII[I[I[IIII[III.------2--.------.--r------r-----r---.------ -B.- -' -- -- "s- p7-T------!-.------'-"-'"'--.------.E"ulgo-ra-ri[apce-vo-stta-na-tso-sh-ib-eos-is-O.tu-ta---.F.sp."--hfor7id7o"m-ed-eh-Ba-rt-sc-h-"---'-'----- =:II===i==I==I==:=11------H------vt------.------p----+------t------m-----'------1---1-5--3---"1'------'"-"-----ih----1-""H--"-'---'---1--'2------1---1'---'--- "- -- -- 4TJ---'--sp-,------il----'iZ----1-----e- g-h-d- rt:ttt5.-m-5-d"i-ic-a- .rin-iti-(y--o-k'o-9.a" .m-tilt!ta"lis-cheb-d-ot.um-gPJ/ls-bry)m------m.--- -1- ---SLij/Ltio"n-od.e-nt-al!u-m-is-a-ot.a.k"itLH-ab.e..-----"Episinophonmakiyamai(Kuroda&Kikuchi)'--Ghttlis-a-girn-ie'n'sis-K-u-ro-d58-H'ab-e-'----' -r ------v------" ------11----4'----'-'------1'------'-- Acharaxjohnsoni(Dal1)-"--'---.-'T-.-----'------'--' ---4c-ila--m.s,gn-is-(-G.ou.ldr)----.-----.---- I--:--:=I--J--5--IIII=II:=I=I=21---M'"---3-'---8--"3-'----'tTo------s"'--3---"--2+'----'--2------2---2"-1'2--2"------'---'------2-'-5---i3-"'7--'-----1---'----1"2-fi---"------N----'-r-----"H'"-----J:--i---"L---2------J---TJ-T---Ti'--J-213(2)1i-----.E-TJ4:--6--ili-o-)-'-g---1------6Kb'----i------".E.neu-cu-la.t"en-uts-(M"o-n!agu-)------..--tYeil.on-eLla-Lap.ooiLca-O-k-ut-an-i---.--"--. .--ty.-"--s-p.------M----'- ---N-u-cu!a-na-pe-rn-uLa-ppr-nu-lo-iae-s-(D-unk-er-)---- -."tY¥--.-Mo-koLyarnatK"u!o-da-."--.-T"--N.robai(Kuroda)---Z"en-u7'le-d-a-ik-eb-e7'(-Su-iuk-i-&-K5n"e'ha-ri)'--" ---- po- rt7a-nd-ia- iap-o-n,'ci(Ad"a-m-sg-Re-e-ve-)---" --' P,---'ttsc-h-ke-i(-S-m-ith- )------L7m-o-ps-is- u-w-ad- o-ko- i-Oy-a-m-a"--'----"------m----..-----.-5m---- =:l.:==taii.la--e:SIoWe--rb-orI=III=II:--"L.-uT-qbtiquaEI.-A-da-mLus-.-L-----L--.paQd-io"lu"s-sp-.------"----"---.------J"------r------"-.L------tuzLm------"---L--tT--L-J-r. ---S-oLacpgn-d-iap-ha-n-a-(D-aLl).----.-.---"------'- 5(3)------'------S,---sp.------.---"------"----''-----"-- i(-1)------'--- -.-eoYy-netna-m-us-sLu-m-in.tu-sgo-st-at-um-SY-otsoya-m-a} ----' 1-'-L 5--- 39- (6- )--2------4-'-5---- -L"D-eLegtoLppc-te-n-p-egk-ha-m.iGABTB----Tfi------"- 6d)-'------'------""-Lim-atgla-k-u-ro-dqi-Oya.-m-a------..-.---7--"LqcLnQtna-sp,---L-."------.-Conchoceiebisecta(Conrad)------¥------m-m--L-rT----r.t--LL--mnu------2--'---- 5--'------'------pH------f------Thyqs!rqsp.-----T.--LLT.----" --J- -- -- L-z- "- m------' L- -- r'-t n- -- u------"- 't- u' -- -- -' L------tYeac-om-a-c-a-tc-ar-ea-SG-m-eLln)------.t 21"--'2-"----3---'-']----"-1"'-- -..P-er-ib"to-m.apLane.O-z-akj---m-.-.------"Gy-a!h-od.oot.a.gcaou-.io"sa-(-Ad-a-m-s.e.t-R-eev-e)-"-Cardiomyasp. ----e"------.------3"--2N"T3H-2--""----'2'---- [Echinoids] --"B-ris-a-st-er-la..ti-f.ro-n-s(-Aga-s.siz->--m------.m- --Hetts-sopsis-l-uz-ooic-a-(Gr-ay)-----r"---.Linopneustesmurrayi(Agassiz) -"------"-----P------T-- 3---7 4-M' 2--L i---2--Jr"-2ntrrrTJ Litho- and Biofacies of Mudstone 15 number, those of disarticulated bivalves is shown in parentheses. 37160163 79812412429498251255257403 --..---.-"------"-3-.-.------.. ------'" 1--- i--- 6-- -H"'-6---2--"--"---1-1--'---4'"'-5"' ------T-"------f ------T--r -- "- --L ------n ------1-" ------JJL'-2--";m-J 131---.tt----T-----rrr----mT-"--TTm- -- ny------.------;--:J--JJ:JJ------.'-L----J--J--JJ.-L.----r ------r----- 410'------'-'--"-'------M-'------r----- 2'-- --- ".---.n b"- ----'--"----'--'-- 1-'---"-'------. -"-P.----r- ---"- 1-----'--nt---- 3'--"---"-- -- -m-"---t-"- ---' 2H-----'-----'2------2------m------M----'"-"---'---'----- T- -- -.-.------P :=III::I]I===:--:=:2-:II=111r-----tP---m----r------T-t------"------r------P------T------ih-----' --r- -+------J-1------1-.-T---ZT-T---'3.--T.--T-T---'------" '-" 2-"- 2--- 3n -t- 322'4-'-i---3-----'3------'------'- ----"------I= -' 2--- i7r-"- -T-- 3(25-3-(3-)-'------4--'-'-2-"--- -'Å}- 4(i)---- TMI----r---tt---rT-t'--r-J------T "--'- 3---" .------T------.------'-- 2------.------e------!-----P.------"e------N- "!----"P"--T---N---"------""- '---'---' 1(-1>- '8I45-s-(2)------'--4-(4-)--5('5)- "- i--1(15------'--'T-- 1(IJ-'----ff------P------m- ---- 4(35'6-(4")--'-'T--"6-(4-)-'i(i)--'---5-(5-)- '----J--L l(1)- -J."J--.3.-.TJ------J-'JJ------'-----"-r-LJ--.-"-T---.15(22)4(5)26(15)46(20)'--"-"----"--3(T)-"----J'-"-'------'------3-(T)---J'------'-5(-5)- ---r--ff--- t------"-"- ----"------r i-J-- .6- (-4- 6(3)---r"--'---hl(IJ`-"r---'-----nt ---'- 4-'+'-- 5I5-)-----5(-4)-'----'--" i2- (8- )'rl--'------3--2(3)-----'----' -ff---P---P--"------m------..----r--TJ-r--tr------2(2)-3---6(15'---'------2'-'--M'------1"---'-'----"---'-'- 1-8(- 5r------1"----'--"--'----'-'22" (5- )--4----- Mlr--Mr---T--rU--"' ft---rtTL----- =I--:I=II=:=III2.-I:::1:=I-- 1142-1--"'"'----"--"T----'"-n-H 3'---"- 34272'-r---'-"-'-5-H-"-" 1----t ------"e- 1---"------u--tt--1" 11 mJ--T---rr"--r"--t.---r---T-J--T 16 SATo, M. DA). In addition, Acila insignis (GouLD), PolLpanemamussium intuscostatum (YoKoyAMA), Peri- Ploma Plane OzAKi, Cardiomla sp., and AncistrolePis trochoideus (DALL) are also common. Bivalves mostly occur with conjoined valves or slightly dislocated two valves. Pollnema- mussium occur at places. The species usually forms clump consisting of 5-10 articulated individuals distributed within about 10cm long of rnudstone (Fig.9). Five to seven individuals of Neilonella also occur with conjoined valves, oriented parailel to bedding plane, although they are few in general. On the other hand, allochtonous shell accumulations are contained in the mudstone as small pathces (accumulation I in Fig. 9; Plate 3, figs. 1, 2). Unbroken shells of above mentioned association including articulated bivalve individuals form the accumulation I, in which planktonic and benthic shells and corals are arranged parallel to the bedding plane, and accumulated as small patches ofO.5-2 cm thick, 1-30 cm wide in cross-section. Numerous Maki ama sp, are sometimes accumulated in 2 to 10 mm thick bed in the fine-grained mudstone. This is the accumulation II in Fig.9. It is more continuous laterally than the accumulation I. The matrix is same as the surrounding mudstone. Besides these, LinoPneustes murra2 (AGAssiz), a spatangoid echinoid, is quite common in the mudstone (Table 2). The urchin usually occurs as fragments or as entire individuals. The small fragments are concentrated into thin lenses or layers, mostly arranged parallel to the bedding plane (Plate3, fig.2), and are frequently associated with spines, and sometime with other molluscan fossils. On the other hand, the entire shells are found sporadically. The tests are usually coliapsed by compaction, but still keeps their living orientation. (2) Bluish gray mottled coarse-grained mudstonefacies (facies B) Distribution: This facies is distributed in the Kokumoto and the Kakinokidai Formations at Locs. 78-118. T)pe Localitl: Loc. 98, along the Yoro River, the middle part of the Kokumoto Forma- tion. Characteristics: Facies characterized by massive, mottled or intensely bioturbated bluish gray (5B 511: wet), coarse-grained mudstone. It sometimes intercalates muddy sand- stone beds. Description: The coarse-grained, and poorly sorted mudstone of bluish gray in color (5 B 511: wet), and is sparsely bedded with 30-200 cm interval. The mudstone is mottled or intensely bioturbated, and several kinds of burrow are observable (stated later). Fine sand-size quartz, feldspars and mica grains, plant debris, diatoms, sponge spicules, planktonic and benthic foraminifers are contained in the mudstone, and volcanic glass grains are quite common. Fine- to coarse-grained turbiditic sandstone beds of 3-300 cm thick are interacalated at the several stratigraphic levels. The coarse-grained mudstone is frequently interca- lated, 5-20 cm interval, with sandy mudstone beds of 5-10 cm thick with diffused upper and lower boundaries (Fig. Il; Plate 1, fig. 1). Such sandy mudstone layers are some- times preserved as patches ofO.5-2 cm thick, 1-15 cm wide in cross-section or mottled in Litho- and Biofacies of Mudstone 17 tx. s ¥/ (s} NJ s 1' .7, ,: l' ¥i, ¥ii ll', .. '1,"¥il' Til,,li''''i,,i.III¥ illilll¥i,'1';' :-i¥l./i r 'ti Fine to coarse .J¥¥ , o 1¥ Muddy tube burrows t-/-) I LI;.' '/¥, l .1.1. t, ¥¥ 1.1'.¥,¥1.1.;..:.:,: ¥'t¥ :: '?i' "' abundant .¥ ,7 sand ,e '' .. .¥ . tt y" 1.tl t .J.t- t ' 't''" t ' .L. "'¥/ c). rd-zaKu.,ptit ' l' . T':':,:' :n .t.t I} ..- Vertical cylindrical Nv., j Scoria ' ., {rk""e '.t- .x: :. .",, a,- g¥;:',:..ffl/'F isx¥.tr4.e;::.¥9s}'¥¥' burrows tt lt t -¥ :r' x 14¥.l-¥ . ,,e. Si-?l'1 -' -= -;JlJ/ i ;'n ,:-.¥¥',1 i t :.v--(r`' --/ l ¥.... ' ¥ ¥ ¥ t ) 'v L,- 'Anconichnus burrowsJ ' ' pt N/7 ':' < -'5 , Lr ¥0¥¥ 1, . }l .t -. .. -. N,.' - - -r > . abundant -i Sandy mud rl {' v=./",i'¥¥'¥4;'--" ' i.Y'¥' ' t- -. "t':'' - M.. -v x. ). -.1/, tt. t.t N --- KN' )¥'' : ¥g¥;'ii-1¥¥¥,1i-'¥,. o /t/ t ' N ),/i ¥¥;' gcf.. ' Lucinoma sp. IJ il>x. J ,/ tt ' .1, tt {;;lg., t ."i ,i ' ;../.'"i"' .L ,, -L ' V¥r" - .--. -- Accumulation l .1 -"v M;G}k g-t.,,'itF.v"..tt..s(uij' ts)¥ ttl -t L- 'L y .:., .. :, ¥1, :1¥ 1¥ t -'""/'//' lt' '''/t ''' ' t.'r ,, .,,;'rt.)¥rg'i¥''.'' ¥ ttt tt tt tt ' ':iif.tiS2,,.. ;.if'IF51Illl-.' tt ' .t. ttt t ''' ttt' .1 ' ' tt"i' 1tt ' ¥'2 IJ- ii 'i 5-15 cm Sandy mud t./ tt t t. ''t"' ttt '" / it4;ltL"l't,"'¥.st't/i6..r,l,';J'"''''' . ttt ttt .t t L tt ttt risaster Ia tifrons t . Fig. I1. Schematic diagram showing modes ofoccurrence ofichno- and benthic assemblages in the bluish gray mottled coarse-grained mudstone (facies B). surrounding mudstone (Plate 2, fig. 2). This facies can be distinguished from greenish gray fine-grained mustone of the facies A in coarser grain-size, bluish color, and more intensive bioturbation. Laminations are sometimes preserved in part in the facies A. In contrast, laminations are never preserved, and even thin scoriaceous sandstone beds intercalated are destroyed by bioturbation in the facies B (Fig. 11; Plate 2, fig. 2). Many trace fossils are found throughout coarse-grained mudstone becls. Three characteristic traces are observed: (1) elliptical pellets, (2) vertical cylindrical burrows, and (3) muddy tube burrows. Both of (1) and (2) are commonly found in the coarse- grained mudstone, while (3) are usually seen in sandstone beds. Elliptical pellets of O.5 mm in diameter and 1mm in length are common throughout this facies. Vertical cylindrical burrows (Fig. 1 1), 2-3 cm in diameter and 5-7 cm Iong, consist of fine-grained muddy sediments, and usually form concentrated layers of 1-3m thick. The muddly 18 SATo, M. tube burrows of O.5-2 cm in diameter and 5-30 cm in length consist of muddy deposits. They densely occur in sandstone beds, orienting vertical to the bedding and penetrating the sandstone beds (Fig. 9). In addition, numerous Anconichnus hori4ontalis burrows in 1 mm in diameter and echinoid burrows of 2-4 cm in diameter are frequently found forming beds of5-20 cm thick (Fig. 11). Ichnofossil assemblages differ between the facies A and B. The assemblages in the facies A consist mainly of small horizontal tube burrows, and large tube-burrows, vertical ones in particular, are not found. The mudstone shows coarsening upward succession within the facies' B, and coarse- grained sandy mudstone predominates in the upper part of the Kakinokidai Formatjon along the Yoro River. Associatedfauna: The following three molluscan associations can be recognized in the facies B (Table2); the Limepsis-Tenuileda-Bath)bembix Association, the Limopsis-Buccinam Association, and the Conchocele-Th2asira-Acharax Association. The former is the most common in this facies. It consists of LimoPsis taj'imae Soweby or L. uwadokoi Oyama, Tenuileda ikebei (SuzuKi & KANEHARA), Lucinoma sp., PeriPloma Plane OzAKi, and Bath2bembix hirasei (TAKi & OTuKA). Two LimoPsis species are not coexisting and replace with each other. Twenty to thirty individuals of Limopsis, articulated or disarticulated, are some- times aggregated within 1mXlm area of a bedding plane (Plate3, fig.4). Other bivalves occur sporadically. More than a half of the bivalve individuals occur as conjoined. Most bivalves and gastropods never show preferred orientation, except for Lucinoma and Pen'Ploma. Both species occur in living position: the former is in an upright position (Fig. 1 1) and the latter species lay horizontally. Besides these autochthonous mollusks, disarticulated bivalves and gastropods are accumulated as small patches, O,5-2cm thick, 1-15cm wide in cross-section, together with plant debris and fine-grained sand. In the sandy mudstone of this facies at the upper part of the Kakinokidai Formation, Limopsis-Buccinum Association is common. It is predominated with Limopsis tay'imae SowERBy and Buccinum isaotakii KiRA. Tenuileda ikebei (SuzuKi & KANEHARA), and Modiolus sp. are also common. Buccinum, 5-10cm in shell-length, is abundant in this association, e.g. 5 individualslm2 at Loc. 257. Small disarticulated bivalve shells and plant debris show "sheltered preservtion" (MAEDA, 1991) in horizontally laying Buccinum shells (accumulation I in Fig, 11). The Conchocele-Th"asira-Acharax Association occurs from a restricted horizon of 30-200 cm thick (Loc. 246, 251) in which impure calcareous nodules of 3-10cm in diameter are common. It is predominated by Con'chocele sp., Th2asira sp., and AcharaxJ'ohnsoni (DALL). The shells are usually found as conjoined valves in upright living position. Two spatangoid species, Brisaster latz:flrons (AGAssiz) and Bn'ssoPsis luzonica (GRAy), are common in the coarse-grained mudstone. Brisaster occurs quite commonly, and Brissopsis occurs sporadically. Both species are found in good state of preservation; being not fragmented nor collapsed, occasionally associated with spines, and usualiy keeping their ventral side down (Fig.11). On the other hand, many individuals of Brisaster and Litho- and Biofacies of Mudstone 19 Brissopsis are associated with pumice grains, plant debris, and some other disarticulated bivalves, and concentrated in the upper 3-7 cm thick part of turbiditic sandstone bed of 15-150 cm thick (= accumulation III; Locs. 103, 241; Plate 3, fig. 5). In contrast to the accumulation I in mudstone, the accumulation III is restrictedly found in the top part of turbiditic sandstone beds, in which the test and shells are usually broken. (3) Light gray massive jine-grained mudstonefacies (facies C) Distribution: The fine-grained mudstone ofthis facies is in the Chonan Formation at Locs. 130, 131, 402-405. T2Pe Locality: Loc. 405, Chonan area, the middle part of the Chonan Formation. Characteristics: The facies characterized by massive, intensely bioturbated, light gray (N 6: wet) to medium light gray (N 5: wet), fine-grained mudstone. Descn-ption: Fine-grained mudstone of 5-100 cm thick is usually alternated with fine- to coarse-grained turbiditic sandstone of 5-200 cm thick. The mudstone is light gray (N 6: 0 risaster .; Ia tifrons @ @ ' (g9gyP @ Z v"tvrt ttt tt. t-iiliili-:-?S(:.{,d.-;¥t;X.-...'.'b',,i.i-i-,ivi'`..:.!.):¥.,,.tt.t....-.t.- I¥1¥¥t¥ Accumulation 111 :J.' ' ./.1./..vl 2{: ." ..-. .t .- ,..,. .. i -- .. t t. '- o11¥. t. 'J t ..¥ x-.' .lt .t ' ' - '.L-' tt' ' Finetocoarse '' ' '' ' sand tt ' '' `-=-tt '' '¥: '. J ' ' ' ,. '' lt- 'i¥¥ '-ttt 'tt'-tttt '': tt-- tt tt 'tt' -' tttt tt- p 5-15 cm k --- vt " @ -;." e ttt 1¥'- .L)Y,, T.3 t - " "' '-7 t-i F ------, e ¥tlk'"'":¥t,s-gL,,eqgi.t.tttt. ttttt ' '' ' tlll ' Muddy tube burrowg l ': Finetocoarse ' Mud ' ' L):1' il sand ttt'tt abundant l ] i.I ',.1:. 1t"'' '-- - -th.. - ' ; ' --- o @ -l 0 -5 v-" - Fig. 12. Schematic sketch of the light gray massive fine-grained mudstone (facies C), 20 SATo M, ' wet) to medium light gray (N 5: wet), well-sorted, and fine-grained. It is almost massive, and it usually contains Planolites type, small, horizontal tube burrows (Plate 2, fig. 3). Under the microscope, diatoms, sponge spicules, and volcanic glass grains are quite common, but in contrast, planktonic and benthic foraminifers are rather rare. The mudstone resembJes the facies A in grain size, but laminations are indiscernibJe in the facies C. Unlined burrows, e.g. Anconichnus horizontalis and Planolites type burrows ofO.5-1 cm in diameter, oriented parallel or oblique to the bedding plane are quite common in the fine-grained mudstone. Occasionally, numerous muddy tube burrows, and cylindrical burrows of O.5-3 cm in diameter and 10 cm long, penetrate vertically the intercalating sandstone beds of5-200 cm thick (Fig. 12). The feature is similar to those ofthe facies B. The bottom surface of the fine-grained mudstone bed is sometimes quite irregular by intense bioturbation. The mudstone becomes coarser upward in the uppermost part of the Chonan Formation in the Chonan area. Associatedfauna: The Macoma Association, composed ofMacoma calcarea (GMELiN), Limopsis sp., and Periploma Plane OzAKi, is characteristic in the Facies C (Table 2). Bivalves mostly occur as conjoined valves, and sporadically, usually lying parallel to the bedding plane. Disarticulated bivalve shells, such as some Macoma individuals are accumulated in several sandy layers or patches of 5-10cm thick. These shells show horizontally to random orientation (accumulation III in Fig. 12). A spatangoid species, Brisaster lattfrons (AGAssiz) is also common in the fine-grained mudstone. Individuals of the species are found sporadically keeping their ventral side down. (4) Facies of alternating beds ofgreenish gray fine-grained mudstone and bluish gray jine-grained mudstone (facies D) Distribution: It is mainly distributed in the lower part of the Kazusa Group, and intercalated with the mudstone of the facies A. The facies D appears in the Kiwada Formation at Locs. I4-17, 25, 26, 149 and the middle part of the Kokumoto Formation at Locs. 93-95 in the Yoro area, and in the Katsuura to Otadai Formations at the Katsuura-Ohara area. Partly, thin blue mudstone units of 1--5 cm thick are also found in other parts of the group from the Kiwada to Chonan Formations. T)pe Localitl: Loc. 16, along the Yoro River, the middle part ofthe Kiwada Formation. Characteristics: Mudstone consisting ofbluish gray (5B 612: wet) fine-grained mudstone (blue unit), and greenish gray (5GY 6/I: wet) moderately bioturbated fine-grained mudstone (green unit), are rhythmically interbedded with each other. The blue unit is most probably turbiditic in origin as described below. Description: Mudstone is rhythmically bedded with 10-50 cm interval (Fig. 13; Plate 1, fig. 2). Turbiditic sandstone of 5-30 cm thick, and the mudstone breccia I and II (Fig. IO) of 10-50 cm thick, are in some places intercalated in the alternating fine-grained mudstone units. Litho- and Biofacies ofMudstone 21 ' ' 4 N '- '. tt-., (s tY .lc.=;, s .:; .; . tt - "- "t -t- ` t " c' ..Ot." i1ts- '-' rl -z glll}l- ' t' ¥, ,¥I .1 .l E' : S-[P)',- 'S - Muddy turbidite - vt ll ,i .1 ' N 1-: "n dr t"/''t';' 5-15 cm o " ¥¥ 'illlii;¥i.iiiiitli,1,1iiil¥tli'l.i¥i¥iilii}iiiiiiiilil,iiliil.S,lliii{ii.ii,iiiili.iili¥iliii¥lilli,?,iiii.....'',,,,...ii.I,iiP.,iii Muddyturbidit '1,. ¥r;:' Fig. 13, Schematic sketch of alternating beds ofthe greenish gray fine-grained mudstone (white parts) and bluish gray fine-grained mudstone (dotted parts) (facies D). A cycle ofmudstone beds consists, in upward sequence, of thin sandstone ofO.1-5 cm thick, blue fine-grained mudstone of 1-10 cm thick, and green fine-grained mustone of 3- 50cm thick (Plate2, fig.4). The sandstone is cross- or parallel laminated, very fine- grained, and the base slightly cuts into the lower bed (Fig. 13). The succession of the blue unit, including basal sandstone, is comparable with the uppermost part (D and E) of "Bouma sequence," and may be regarded as muddy turbidite (GRiGGs et al., 1969; RupKE, 1975). The main part ofthe blue unit is almost homogeneous or very weakly laminated, and the top grades upward into the green unit. The green unit is almost massive, and quite similar to the fine-grained mudstone of the facies A described above. Under the microscope, the mudstone contains diatoms, sponge spicules, and volcanic glass fragments. The green units usually contain abundant planktonic and benthic foraminifers. On the other hand, density and diversity of foraminiferal fauna is low in the blue units (Table 3). The blue unit is usually poor in bioclasts. 22 SATo, M. Table 3. List of fossil formanifers from alternating beds of greenish gray fine-grained mudstone and bluish gray fine-grained mudstone (facies D) at Loc. 95, blue 1: lower part ofthe bluish gray fine-grained mudstone (l-3.5 cm thick), blue 2: upper part of the bluish gray fine-grained mudstone (3,5-7cm thick), green l: greenish gray massive bioturbated fine-grained mudstone (7-10,5 cm thick), 260: individual numberl100 g mudstone. pa h e8.b Fdi-)Q Sample di-)Q Species BENTHONICFORAMINIFERS BuliminaspissaCushman 2 PseudononionJ'aponicmAsano 1 Nonionsp. 1 Rotaliainfiata(Seguenza) 1 Cibicidescf.refutgens(Montfort) 3 Cibicidesaknerianusd'Orbigny 2 CassidulinasubglobosadepressaAsano 1 1 20 CassidutinakasiwazakiensisHusezima&Maruhashi 1 CassidutinaasanoiUchio 1 1 Fissurimasubmarginata(Boomgart) 1 PLANKTONICFORAMINIFERS 15 13 260 Such alternating fine-grained mudstone is more or less bioturbated. While the green units are usually intensely bioturbated and appear to be massive, the blue units are relatively weak in bioturbation. Two types of bioturbation are recognized in the blue units. One is the association ofdifferent traces, which exhibit tiering structure, consisting of Chondrites (1-2 mm in diameter), Anconichnus horizontalis (O.5-1 mm in diameter), echi- noid burrows (2-3 cm in diameter) and Planolites ofO.5-1 cm in diameter in upward order (Fig.13; B in Fig.14; Plate2, fig.4). These burrows are fi11ed with greenish gray fine-grained mudstone of the overJying green unit. These features indicate that the deposition of the blue unit were rather rapid, and the benthic organisms burrowed into the blue unit from the top surface after the deposition. The other type of bioturbation was made by numerous Anconichnus horicontalis burrows (Fig. 11; B in Fig. 14; Plate 2, fig. 5). The burrows are densely crowded in the blue unit. The green unit is also bioturbated, but trace fossils are indiscernible. It is probably due to the low color contrast between traces and matrix of the green unit. Associatedfauna: Two types of molluscan association are recognized in the green unit, Litho- and Biofacies of Mudstone 23 'tt...i.,geiptttt.tv..-.t...vtt)¥ .w/.1,,tt)-Wli((I>),ti] lntensely biotutoated lntensely biotutoated EoNI' "massive" "rnasslve -iT S..y,S,z.¥-'.C) ',lili,i'1`'ix?9,/'s-"(-"'IL')d(''.-t2'YXs-,L¥-IS;if.I-lv;LLA-)"-.tl/tiC¥?'t,ji{1rJ.-x,-"-.tK-",.-'.k.-t'.-<,,i'.kvt,:.'r,,Nlr/-,-),' r¥,lttptt,t;/t// - --" -- -T' ------¥" k"ge"ii'Ill.i,l >,7i.{g.-,,--<':2"E/i:Nxxr-,.n(ft)j)'i:fo<,e Trace fossil -. Anconichnus burrows "tiering structure" abundant Fig, 14. Two types of bioturbation in alternating beds of greenish gray fine-grained mudstone and bluish gray fine-grained mudstone (facies D), A: showing "tiering structure" ofthe trace fossils, B: showing numerous Anconichnus hori4ontalis burrows. Portlandia-?Veilonella-AncistrolePis Association and A(aculana Association. The former asso- ciation is known in the facies A. It is also common in the facies D of the Yoro area, and the Ohara to lower part of the Otadai Formations in the Katsuura-Ohara area. Their composition and mode of occurrence are similar to those of the facies A. In addition, LimoPsis uwadokoi OyAMA is common in the Katsuura-Ohara area. The ?Vuculana Associa- tion is restrictedJy distributed in the Katsuura and Namihana Formations in the Kat- suura-Ohara area, about 25km east of the Yoro area. It consists only of IVuculana sagamiensis OKuTANI. The individuals occur as conjoined valves, and are scattered in the mudstone. Some sheils arranged parallel to the bedding plane (accumulation I). Echinoid fossils LinoPneustes murray (AGAssiz) sporadically occur in the green unit, and their mode ofoccurrence is similar to that in the facies A. Both ofmollusks and echinoids are rare in the blue unit. Maki ama sp. are sometimes accumulated immediate or2 to 10 mm below the thin sandstone, the basal part of the blue unit, in between the underlying green fine-grained mudstone beds (accumulation II in Fig.13). This type of fossil occurrence is sporadically found in the green mudstone. Numerous trace fossils, e.g. muddy horizontal tube of 5 mm in diameter and muddy pellets of 5-10 mm in diameter, also found below the sandstone. (5) Creenish graN laminatedjine-grained mudstonefacies (facies E) Distribution: The facies E is mainly distributed in the lower part of the group from the Kiwada to Umegase Formations, intercalating in the facies A in the western part of the Yoro area at Locs. 17, 19, 20, 38, 39, 40, 214, 218, 219, 184. 24 SATo, M. ' ')zi::'vt" Z ,.r"/' : - ...¥ 4;..r -,.".., ---- " -- -}tt. ¥,. tt.;,t,. t.,, -.. pt 4..,t.t.. t,t/t.t.tt, ¥-+¥ /f7if ------x 1 ..-¥V" -...¥ / ifs- su. - a- 'J ncb-n-ichnb-s5UFTr6ws ';:l:e ctEEp(p -.a=Swh v- de "zA t-ttt..t.# t - ip -- - -- l{(t¥¥..t¥. ------i ,t v .¥ r.- - ':lt 5-15 cm ,¥-¥ ¥. .K¥¥ili'illii''' ¥.liilli¥., ....i.t,i,,.IIIiii¥,i'1¥1i¥ 1¥1¥1¥it¥,i¥¥ -- ';)}: `" .7 ' l¥LL2::zil-L,' '¥¥ '- - :------.. 'e - = ..#------; .. ' l,¥: Neilonella . ,7. r,.:T. rr:. t.,. ,. 7- . ,: ,. ,- r ,.-. ,.. .,. tt-t tt "t tlttttt -.:L.))t:g.1¥'Lii//-. Z¥ .¥K Iaponlca Scoria --L -' h--r K --.-L--.--- - , -y T:=T:I;.;I,.s:.:,:.,:X,..."., .t...b-=h-tlt.-:t-:-.- .-.i 't--it'r.L' .-:A',..t...,.'" tttK'' ttfaiji t ¥/¥.g J¥,:T,:Tr. .v, Tri¥ - lv' ., .- .r¥ .,{¥, ap tF+gEaa- (r. =BistaTtff . Z. .---t,.t. ...-- ..rN=.::.:T-... =..-..rt...: Fig. 15, Schematic sketch ofthe greenish grayth laminated fine-grained mudstone (facies E). Tmpe Locality: Loc. 184, along the Tsuchisawa River, the lower part of the Kiwada Formation. Characteristics: The fine-grained mudstone characterized by frequent parallel lamination. Descn'ption: Although the mudstone ofthe facies E is similar to facies A in grain-size, and in greenish gray (5 GY 611: wet) color, it can easily be distinguished from other facies in its thin and parallel lamination (Plate 2, fig. 6). The mudstone is sparsely bedded. The greenish gray fine-grained mudstone is intercalated with intervals of 3-10mm by fine- grained sand layers of O.5-2mm thick. The sand layers are composed of quartz, feldspars, and foraminiferal tests. Laminated sandstone beds of5-30 cm thick, and fine- to coarse-grained scoria Jayers of 1-5 mm thick are sometimes intercalated in the mud- stone, but are not disturbed by burrows (Fig. 15). The mudstone contains diatoms, planktonic and benthic foraminifers, sponge spicules, and volcanic glass. Under the microscope, most of calcareous benthic foraminifers are concentrated in thin lamination composed of very fine sand grains. Such foraminifers may not be indigenous but probably transported with sand grains from other places. Few trace fossils are observed in the fine-grained mudstone. Small Anconichnus horizontalis burrows ofO.5 mm in diameter are only contained in the limited layers ofO.5-1 cm thick (Fig.15; Plate2, fig.6). This facies suggest a sedimentary environment in which activities of large benthic organisms are scarce. Two types ofoccurrence ofthe facies E fine-grained mudstone, "autochthonous beds" and "allochthonous blocks," are recognized. The autochthonous bed of facies E has very restricted distribution in the lower part of the Kiwada Formation exposed along the Tsuchisawa River (Loc. 184), in the western part of the Yoro area, where the laminated fine-grained mustone bed of2 m thick is intercalated in the mudstone of facies A. In contrast, the autochthonous facies E is not found in the eastern area. The laminated fine-grained mudstone is preserved as allochthonous blocks in the slumped beds and as mudstone rubbles in mudstone breccia I and II. Slumped beds of 5-30 m thick contain numerous blocks of greenish gray laminated fine-grained mudstone of 30-300 cm Litho- and Biofacies ofMudstone 25 thick (facies E), and greenish gray massive fine-grained mudstone of 30-300 cm (facies A). The allochthonous occurences of facies E are commonly found in the Kiwada to Umegase Formations in the eastern part of the Yoro area (Locs. 17, 19, 20, 38, 39, 40, 214, 218, 219). Associated fauna: Fossil mollusks are rarely contained in the laminated fine-grained mudstone. IVeitoneltaj'aPonica OKuTANi, rarely occurs with conjoined valves. In addition, Maki ama sp. is accumulated in some horizons. All of these fossiis are arranged parallel to the bedding piane. Sedimentary environment of mudstones in the Kazusa Group A. Stratigrophicchangeofmudstonefacies The mudstone facies of the Kazusa Group change stratigraphically, through the Kiwada to Chonan Formations in the Yoro area. The succession of the mudstone facies can be summarized as follows; facies A (the Kiwada, Otadai, and Umegase Formations), facies B (the Kokumoto and the Kakinokidai Formations), facies C (the Chonan Forma- tion), in upward sequence (Fig. 16). The facies D, alternating beds of greenish gray fine-grained mudstone and muddy turbidite beds, is intercalatjng in part of the facies A. Laminated mudstone of the facies E has a limited distribution, intercalating also in the facies A. Roughly speaking, mudstone tend to become coarse upward. The mudstone consists mainly of clay-size material in the facies A, becomes silty in the facies B, and again consists of clay-size material in the facies C in upper part of the Kazusa Group. The mudstone ofthe Kazusa Group is usually bioturbated, and degree ofbioturbation increases upwards. Thin, lamina-like sandstone layers are sometimes preserved in the facies A (particuiarly in the Kiwada Formation; Plate 2, fig. 1). Small horizontal tube burrows, such as Planolites and Anconichnus horicontalis, are predominant in the facies A (Fig. 9; Plate2, fig.1), but large vertical tube burrows are almost absent. In contrast, mudstone of the facies B, distributed in the upper part of the group, is much more intensely bioturbated than the underlying facies A. Thin sandy mudstone layers are frequently intercalating in the facies B, however, most of them are severely destroyed by benthic activities. Such layers are only preserved as patches or mottles in surrounding mudstone (Fig. 1l; Plate 2, fig. 2). Laminations are not preserved in the mudstone of the facies B. Various types of trace fossils are observed in the facies B. Among them, large vertical tube-burrows are quite common (Fig. 11). Autochthonous benthic fossils are quite common throughout the Kazusa Group (Table 2). The molluscan assemblages change stratigraphically parallel with the change in lithofacies. The Portlandia-2Veilonella-AncistrotePis Association, composed of small thin- shelled mollusks, is quite common in the facies A, while they are few in the facies B and C. The association indicate bathyal environments (BABA, 1990). The Limopsis-IVuculana-BathJbembix Association is dominant in the facies B. Deep- burrowing Lucinoma sp. also occurs in this association (Fig.11). In addition, large, thick-she}led mollusks, such as Buccinum appear in the facies B in the upper part of the 26 SATo, M. xbdNeS ,e;eApe6S '(I.eNNel)Npt"S v.N)e`oN09A ""6sNo"e Macorr;a CHONANF. .Ch'3 AKINOK)DAl,ltl,h¥I¥l',:i.:¥;r,t't:.1¥L,1.:.:,:zt: ¥ ¥ ¥¥¥ "¥ Limopsis-Buccinum tt-.-, -Ka1eKe2.6 L9o5YY -Ke1.1 .:-.-os.tsvn ,s¥s xtriklX¥' gE$Åë li:- R"e ic'-' ,IL ,' :.:¥ 8"xFS ;r-¥"',--" Eg eKu2 di 'li'e"i;.'i's'ie ------i- ¥*coos g t-----i------¥------i-- 'i,.,,I.',,i ..' B , s o,, E -Ku5 li- tKu6 -:-:-:-:-:-:-:-:---!-- a :¥liiE¥:¥t:'S.lilll:iE:¥i¥l i,L,: -U1 ll,I;il¥i;l¥:,;i/Lgl,:iIil'l¥ ---t----- eU2 E,l,1 I------ny ------11jill,lli¥11illilliliiillli'i. ca------.--ny---.------ ----' mq l::il/r,l.II¥i:triS,,/Tl¥ --ttt-- N -g.hQ .u:l::::L}lge.ltdi' il{:¥:¥l¥IE:,rlY¥il 11Silllllllll/Giilillillilil .O7 'n¥¥¥¥¥ e < :.:.:.:gc.k .:.:.:- .O8 re:l:illSgl I¥:l g <5{¥:,:;;,l:i,i#,lii .- o22-- ltw,le{,L,i¥l '(mDN(x -- m --ii'll'll:'¥lis'i':i,i'ilt;' q )h'.o . ¥l:I:l l: S¥l:l:¥:i:l: Åë -----.--- ¥lll,l'A'i¥lll/i -+ -- ---. ------i -- l[III::.l/lill.illl.[lli.l g..sli:,:mai¥:::s.'s'sc::.: ti- illllilill¥lllil'llilll' Åë tt------t-ttttt----- i,L::{7¥Z'i""'X,#:i-- as -tr¥¥; .:.,s:.:.:.:.:.)..:.: '------= Q <-n¥:11X¥¥:lil:I:;¥:¥11,i:l(,$¥,i¥:i¥rt?,i¥/T,i .:::::::u..:::: s:¥:: Q:. l'I¥li¥lllli¥lli/i¥l¥ilii¥ll¥l 5RO #111ill'i':':;illilil asl l:i ,, {¥IE'lll i.2ir,.Il¥i:Iilill,li¥: lewi/':"lll//tt",s,.,,¥Kd't(s.;l:3"N ul9(poec=Y Conchocele-rhyasira o .Fig,16. Generalized columnar section of the Kazusa Group in the Yoro area, showing ' stratigraphic distributions ofmudstone facies and fossil assemblages, ' Litho- and Biofacies of Mudstone 27 Kakinokidai Formation. The assemblage including Buccinum isaotakii Kira is referable to the Buccinum-LimoPsis Association of BABA (1990), which may be indicative of outer shelf mud bottom. All of these features suggest the shallowing upward sequence of the Kazusa Group. From the view point of benthic foraminifers, AoKi (1965) presumed bathymetry of the depositional environments of the Kazusa Group. According to him, the lower part of the group, from the Katsuura Formation to the upper part of the Umegase Formation was deposited in the lower to upper continental slope environments, and the upper part of the group, from the uppermost part of the Umegase Formation to the Kasamori Formation was deposited in the outer to middle shelfenvironments. The sedimentary features of mudstone and molluscan associations may also support AoKi's interpretation. Massive fine-grained mudstone (facies C) is again predominant in the uppermost part of the Kazusa Group. The facies C is quite similar to the facies A in grain size, however, the former is much more intensely bioturbated than the latter. Ichno- and molluscan assemblages of the facies C are also quite different from the facies A. Many large vertical tube-burrows, which are absent in the facies A, are observed penetrating intercalated sandstone bed in the facies C (Fig..12). The Macoma calcarea (GMEuN) Association, which may be indicative of the outer shelf to ¥uppermost bathyal environments (BABA, 1990), is common in the facies C. The facies C is overlain by the sandstone facies of the shelf environment (OyAMA, 1952; 1959). Therefore, rather shallow and quiet environment, in which sand influx was prevented by the barrier may be considered for the deposition ofthe B. Reconstruction ofsedimentary enzn'ronments ofthe laminatedjine-grained mudstone (facies E) The greenish gray laminated fine-grained mudstone of the facies E is similar to the facies A in color, grain-size, and grain composition. However, it is different from the latter in the following features. (1) Well-preserved parallel laminations. (2) Rare bioturbation. Only small Anconichnus horizontalis burrows are partly aggregated in some narrow horizons. (3) Benthic fossils almost absent. (4) Calcareous benthic foraminifer ' concentrated in a laminae surface. They are not indigenous in the facies E but' may be transported form the other facies together with sandy materials. These characteristic features indicate low activity oflarge benthic animals and suggest dysaerobic condition of the environment in which the laminated fine-grained mudstone was deposited. The distribution of the laminated fine-grained mudstone facies is characteristic. (1) It is narrowly distributed in the middle part of the Kiwada Formation in the western part of the area studied (Loc. 184). The laminated mudstone is about 2m thick intercalating in the greenish gray massive fine-grained mudstone of the facies A. 28 SATo, M. (2) The mudstone is found as allochthonous blocks in the slump beds of 5-30 m thick, and in two types ofthe mudstone breccia, both ofwhich are intercalating in the facies A of the Kiwada to Umegase Formations in the eastern area. It is likely that the laminated fine-graied mudstone was deposited on the slope of the western area, and most part of the facies was transported downslope toward the east by slumping or debris flow, and preserved as allochthonous blocks. The original place of the deposition was thought to be relatively depleted with dissolved oxygen, possibly due to the local bottom topography. Mass movement from west to east is also supported by following observations. Deformation and thrust structures in the slump beds indicate that the slurnps were supplied from west to east or northwest to southeast. The trend is also shown as sole marks and cross-lamination in the turbiditic sandstone beds in the Kiwada to Umegase Formations. The mudstone breccia are also intercalated limitedly in the eastern area. The breccia is absent in the Kiwada Formation in the western area around Kurotaki, while more than 60 beds are observed along the Yoro River (Fig.4). Similar trend of distribution is recognized among muddy turbidite beds, particularly those in the middle part of the Kiwada Formation; few around Kurotaki and many along the Yoro River. The thickness of the Kiwada and Otadai Formations in the eastern area is as three times larger than that of the western area; e.g. the Kiwada Formation is 185 m around Kurotaki and 600m along the Yoro Kver (Fig.4). HiRAyAMA and SuzuKi (1968) also noted the eastward transportation of sandy turbidite along a datum plane defined with 07 marker tuff in the Otadai Formation. Fig. 17 shows a reconstruction of the paleoenvironment of mudstone in the Kazusa Group. The series of the facies A to C may reflect the shallowing sequence. The depositional environment of the facies C was probably a silled depression or a part shaded from wave and current action. The muddy turbidite facies appears within the facies A, . ,B,si, --- " wa"kdeeei=Siiljfi"/:'e,I./3i,.l-s}e,i#,/l¥g"i",l,tii/l,{X"toi"itye,L'l;'eq'S -gh -¥.,¥¥,IL-,¥,1,t,/.:i¥:.:¥i N s::'sSifi'es,,sreeijpt.N,S '.'.s '. -Low-oxygenated zone -i}iÅí nyEgapsXpsajtuKpex:g,css,,,xss,,,SttigtM¥P, ¥: s. - ee mu SHELF Muddy turbiditv current :nit.ag eei R.rfis' s. NitNesxxYeqgffwtpa.,ge,,eq.esssleee,twqu"\Ngrgg$fsi,,iiE$tugNs SLOPE Fig. 17. Schematic reconstruction ofsedimentary environments of the main part ofthe Kazusa Group. The sedimentary basin deepcns eastward. Greenish gray laminated rnudstone (facies E) might be deposited under dysaerobic condition, such as 02-minimum zone, and the most part is preserved as allochthonous blocks in slump deposits. Litho- and Biofacies of Mudstone 29 and tend to increase toward the eastern area. The facies E was accumulated within the area of the facies A deposition, but in the low oxygenated condition. Conclusion 1). Monotonous muddy deposits in the Kazusa Group, distributed in the central to the eastern part of the Boso Peninsula, can be divided into five major facies (A-E), namely greenish gray massive fine-grained mudstone facies (facies A), bluish gray mottled coarse-grained mudstone facies (facies B), light gray massive fine-grained mudstone facies (facies C), facies of alternating beds of greenish gray fine-grained mudstone and bluish gray fine-grained mudstone (facies D), and greenish gray laminated fine-grained mudstone facies (facies E). 2). In the Yoro area, the central part of the Boso Peninsula, the mudstone facies changes stratigraphically upward, from the facies A (from the Kiwada to Umegase Formations) to the facies C (the Chonan Formation) through the facies B (the Kokumoto and Kakinoki- dai Formations). In general, mudstone of the Kazusa Group shows coarsening upward, and the degree of bioturbation also increases upward. 3). In accordance with the suceessional change of lithofacies, ichno- and molluscan assemblages change stratigraphically. These features of litho- and biofacies suggest shallowing upward sequence of the Kazusa Group, 4). The facies D consists ofalternating beds ofgreenish gray fine-grained mudstone and muddy turbidite layers. The greenish gray fine-grained mudstone is the same as that of the facies A, and the facies D is intercalated in the faces A,' particulariy in the middle and uppermost part of the Kiwada and lower part of the Kokumoto Formations in the Yoro area, and the Katsuura to the lower part ofthe Otadai Formations in the Katsuura-Ohara area. 5). The facies E, the laminated fine-grained mudstone, yields few benthic fossiJs, and is scarcely bioturbated. The feature suggests the deposition under dysaerobic environment. The autochthonous part of the facies E is distributed only in the western area, whiSe they occur as numerous allochthonous blocks contained in the slump deposits in the eastern area. The facies E might originally be deposited in the shallower, upper slope of the western area, and the most part of the facies were thought to be transported eastward by mass-gravity flow, and preserved as allochthonous blocks in the facies A. Acknowledgments I am especially indebted to Prof. Kiyotaka CHiNzEi, Drs. Terufumi OHNo and Haruyoshi MAEDA of Kyoto University for their helpfu1 discussion both in the field and laboratory. Dr. Seiichiro MATsui ofUtsunomiya University and Dr. Hiroshi HAyAKAwA of Waseda University gave me much appreciated help and advice in the field. Dr. Ken'ichi KANAzAwA of the University of Tokyo kindly identified the echinoids, and gave me usefu1 suggestions. Dr. Hisato YAsuDA of Kochi University kindly identified benthic 30 SATo, M. foraminifers. ProÅí Tsunemasa SHiKi and Dr. Akihisa KiTAMuRA of Kyoto University kindly helped me with discussion and advice. I arn much indebted to the oMce of the University Forest of the University of Tokyo and Mrs. Ikuyo KAsuyA for kind assistance during my field survey. I am also grateful to Mr. Hisao TsuTsuMi, Mr. Hiroyuki MAEDA, Mrs. Tamiko NAsu, Miss Hatsuko FuJiKAwA, Mrs. Machi MusAsHiNo, Mrs. Chie MiKATA, and ail people engaged in the Department, for their help thoughout my study. References AoKi, N. (1968): Benthonic foraminiferal zonation of the Kazusa Group, Boso Peninsula. Trans, Proc, Pataeont. Soc. Japan, N. S., 70, 238-266. AusicH, W. I. and BoTTJER, D. J. (1982): Tiering in suspension-feeding communities on soft substrata throughout the Phanerozoic. Science, 216, 173-174. BABA, K. (1990): MoUttscanfossil assembtages of the Kazusa GrouP, South Kanto, centrat.laPan. Publication of KeiQ Primary School (Keio Yochisha), 445 pp. [inJapanese with English abstract]. GoLDRiNG, R., PoLLARD,J. E., and TAyLoR, A, M. (1991): Ancenichnus horicontatis: a pervasive ichnofabric- forming trace fossil in post-Paleozoic offshore siliciclastic facies. Palaios, 6 (3), 250-263. GRiGGs, G.'B., CAREy, A. G. Jr., and KuLM, L. D. (1969): Deep-sea sedimentation and sediment-fauna interaction in Cascadia Channel and on Cascadia Abyssal Plain. Deep Sea Res., 16, 157-170. HiRAyAMA,J. and SuzvKi, Y. (1968): Analysis oflayers, an example in Flysch-type alternations, ChikLyu Kagaku, 22, 43-63. [inJapanese with Englsih abstract]. IsHiwADA, Y. (1964): Benthonic Foraminifera of the Pacific coast ofJapan referred to biostratigraphy of the Kazusa Group, ReP. Geol. Surv,JaPan, 205, 1-45, IsHiwADA, Y., Mitsunashi, T., Shinada, Y., and MAKiNo, T, (1971): Geologicat map ofoil and gasYield of laPan, "Adtobara." Geol. Surv. Japan. KATsuRA, Y. (1984): Depositional environments of the Plio-Pleistocene Kazusa Group, Beso Peninsula, Japan. Sci, Pep., Inst. Geosci., Univ, Tsukuba, Sec, B, 5, 69-104. KoiKE, K. (l949): Geology of the middle part of the Boso Peninsula (II), Bult, Phlsiogr. Sci. Res. Inst., Toklo Univ., 3, 22-26. [in Japanese with English abstract]. MAEDA, H. (1991): Sheltered preservation: a peculiar mode of ammonite occurrence in the Cretaceous Yezo Group, Hokkaido, northJapan. Lethaia, 24, 69-82, MAsuDA, F., KATsuRA, Y., WATANABE, K,, YosiNo, T., and ITo, M. (1989): Milankovitch cycles recorded in turbidite sequence: Plio-Pleistocene Kazusa Group in the Boso Peninsula, Japan, Jour. Sed. Soc. .laPan, 31, 43-48, [inJapanese with English abstract]. MiTsucHi, T, (1933): Plio-Pleistocene boundary in the Boso district. Jrour. Geol. Soc, .lapan, Abstract, 40, 400-401. [inJapanese]. MiTsuNAsm, T, (1954): Geology of the southern district of Kinadayama, Boso Peninsula: Notes on the extension of rock facies in time and space. Jour, Geol, Sec. Japan, 60, 461-472, [in Japanese with English abstract]. MiTsuNAs-, T. (1979): Geotogie maP of "Otaki," 1:200,OOe. Geol. Surv,Japan, MITsuNAsHI, T., KIKucHI, T., SuzuKI, Y., HIRAyAMA,J,, NAKAJIMA, T., OKA, S., KoDAMA, K., HoRIGucHI, M., KATsuRAJIMA, S., MIyAsHITA, M., YAzAKI, K., KAGEyAMA, K., NAsu, N,, KAGAMI, H,, HoNzA, E., KiMuRA, M., NiREi, H., HiGucHi, S., HARA, Y., FuRuNo, K., ENDo, T., KAwAsHiMA, S., and AoKi, S, (1976): Geelogicat maP of Toipo Bal and adj'acent area. Miscellaneous Map Series, 1 : 100,OOO, No, 20, Geol. Surv. Japan. MiTsuNAsHi, T., YAsuKuNi, N,, and SHiNADA, Y. (1959): Stratigraphical Section of the Kazusa Group along the Shores of the Rivers Yoro and Obitsu. Buii. Geol, Surv, JaPan, 10, 83-98. [in Japanese with English abstract]. MITsuNAsHI, T., YAzAKI, K., KAGEyAMA, K,, SHIMADA, T., ONo, E., YAsuKuNI, N., MAKINo, T., SHINADA, Y., FuJiwARA, K., and KAMATA, S. (l961): CeotogicalmaP ofthe oil andgasYietd ofJrapan, "Futtsu-Otaki." Geol. Surv. Japan. NnTsuMA, N. (1976): Magnetic stratigraphy in the Boso Peninsula. Jrour. Geet. Soc. .laPan, 82, 11-29. [in Litho- and Biofacies of Mudstone 31 Japanese with English abstract]. ODA, M. (1977): Planktonic foraminiferal biostratigraphy of the Late Cenozoic sedimentary seqeunce, Centeral Honshu,Japan. Sci. ReP., TDhoku Univ., 2ndser, (Geol.), 48, 1-72, OyAMA, K, (l952): On the vertical distribution ofmarine mollusca. Venus, 17, 25-35. [inJapanese]. OyAMA, K. (1959): Fossil molluscan succession of Lower Pleistocene in Chiba Prefecture. BuU, Geol. Surv. Jra)ban, 10, 9. 9-102. [in Japanese with English abstract]. RupKE, N. A. (1975): Deposition of fine-grained sediments in the abyssal environment of the Algro- Balearic Basin, Western Mediterranean Sea. Sedimentotogy, 22, 95-109. SATo, T., TAKAyAMA, T., KATo, M,, KuDo, T,, and Kameo, K, (1988): Calcareous microfossil biostra- tigraphy of the uppermost Cenozoic Formation distributed in the coast of the Japan Sea -Part4: Conclusion-. .lour. JraPan. Assoc. Petrol. Technot., 53, 475-491. [inJapanese with English abstract]. SAwADA, H. (1939): Geology ofKatsuura and Okitsu Towns in the Isumi Country. Jour. Geot. Soc.Japan, 46, 445-450 [inJapanese]'. ToKuHAsHi, S. and ENDo, H. (1984): Geotogy of the Anesaki District (1:50,OOO geological map and explanation text). Geol, Surv.Japan, 136 pp. [inJapanese with English abstract], UEDA, H. (1930): Geology ofthe northern part ofthe Boso Peninsula. Jlour. Geel. Sec,Jrapan, Abstract, 37, 250-253, [inJapanese], UEDA, H. (1933): Stratigraphy ofthe Cenozoic strata developed in the Boso and Miura Peninsulas. .lour. Geol. Soc, JraPan, Abstract, 40, 799--801 [inJapanese]. Explanation of Plate 1 Outcrop of the Kazusa Group, 1. Bluish gray mottled coarse-grained mudstone frequently intercalated by sandy mudstone layers (facies B). Loc. 128 (upper part ofthe Kakinokidai Formation). 2. Alternating beds of bluish gray fine-grained mudstone of5-6 cm thick and greenish gray fine-grained mudstone of 10-40 cm thick (facies D). Loc. 16 (Middle part of the Kiwada Formation). SAtO Litho- and Biofacies of Mudstone Plate 1 Explanation of Plate 2 Vertical profiles of the rock specimens (all figures are XO.6 and ofupright position) 1. Greenish gray fine-grained mudstone (facies A) containing Ptanotites and Anceichnus horicontalis burrows, JC9001, Loc, 25 (upper part of the Kiwada Formation). 2. Bluish gray mottled coarse-grained mudstone (facies B). A fine scoria layer (center) is disturbed by burrowing, JC9002, Loc. 98 (middle part ofthe Kokumo- to Formation). 3. Light gray massive fine-grained mudstone (facies C) containing many Ptanolites- type burrows. JC9003, Loc. 403 (lower part of the Chonan Formation). 4. Alternating beds ofthe facies D consistin{ three parts, Light gray laminated fine sandstone at the basal part, bluish gray bioturbated fine-grained mudstone in the middle, and greenish gray bioturbated fine-grained mudstone in the upper part, JC9004, Loc. 306 (lower part of the Namihana Formation), 5. Alterating beds of green unit and underlying blue unit (facies D). Numerous Ancoichnus hori(ontalis burrows are observable, JC9005, Loc, 37 (middle part ofthe Kiwada Forrnation). 6. Greenish gray laminated fine-grained mudstone (facies E). Thin laminations are frequently intercalated in the fine-grained mudstone. JC9006, Loc. I84 (middle part of the Kiwada Formation). SATO: Litho- and Bio facies of Mudstone Plate 2 Explanation of Plate 3 Mode of fossil occurrence (all figures are Å~0.7) 1. Horizontal profile of mudstone in which many small shells and solitary corals are aggregated (accumulation I>, JC9007, Loc. 204 (facies A; middle part of the Kiwada Formation), 2. Shells and an echinoid test accumulated on a bedding plane as a small patch, Horizontal profile of mudstone. JC9008, Loc. 204 (facies Ai middle part of the Kiwada Formation). 3, itfaki ama sp. accumulated on a bedding plane (accumulation II). Horizontal profile of mudstone, JC9009, Loc. I49 (facies D; upper part of the Kiwada Formation), 4. Limopsis showing autechthonous occurrence. Five articulated individuals are observable in the vertical profile ofmudstone. JC9010, Loc. 256 (facies B; lower part of the Kakinokidai Formation), 5. Vertical profile of sandy layers intercalated in coarse-grained mudstone, which shows accumulation of bivalve shells and echinoid tests (accumulation III). JC9011, Loc. 241 (facies B; middle part ofthe Kokumoto Formation). SATO: Litho- and Biofacies of Mudstone Plate 3 2