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And Biofacies of Mudstone in the Pliocene-Pleistocene Title Kazusa Group, Boso Peninsula, Central Japan

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And Biofacies of Mudstone in the Pliocene-Pleistocene Title Kazusa Group, Boso Peninsula, Central Japan 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 <facies B). Intensely bioturbated, coarse-grained mudstone containing large vertica] tube burrows, Lower-neritic environments are suggested from benthic assemblages, (3) Light gray massive fine-grained mudstone facies (facies C). Intensely bioturbated. fine-grained mudstone, in which large vertical tube burrows are abundant. Benthic assemblages indicate environments from outer she]f to uppermost bathyal. (4) Facies composed of alternating beds of greenish gray fine-grained mudstone and bluish gray fine-grained mudstone (facies D). This is the low-density turbidite facies; the bluish bed is a layer formed by muddy turbidity current and intercalated in the greenish gray massive fine-grained mudstone similar to the facies A. (5) Greenish gray laminated fine-grained mudstone facies (facies E). Parallel and narrowly spaced lamination is distinct in this fine-grained mudstone. Degree of bioturbation is low, and benthic mega- * 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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  • Molecular Evidence for the Existence of Five Cryptic Species Within The
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