Pleistocene Shimosa Group Research Articles

Reexamination on stratigraphy of the Middle to Upper Pleistocene Shimosa Group beneath the Tsukuba Upland, Ibaraki Prefecture, central Japan

Reexamination on stratigraphy of the Middle to Upper Pleistocene Shimosa Group beneath the Tsukuba Upland, Ibaraki Prefecture, central Japan

八潮GS-YS-2コアに見られる更新統下総層群の堆積サイクルとテフロクロノロジー

Sedimentary facies and tephrochronology of the Pleistocene Shimosa Group in the GS-YS-2 core recovered from Yashio, Saitama Prefecture, central Japan are examined. Our detailed examination reveals that the depth range of 37.02-94.40 m which corresponds to the Shimosa Group is divided into 10 lithofacies units, A to J. Of them, Units G and J are composed mainly of humic mud interpreted as marsh facies. Units A, D, E, and H are characterized by bioturbated sandy mud and/or muddy sands indicating bay facies, and Units B, F, and I consist of well-sorted sands interpreted as shoreface to beach facies. We recognize four depositional cycles corresponding to the formations in the standard division of the Shimosa Group. Each of them comprises the marsh, bay, and shoreface to beach facies in ascending order. Consequently, the examined interval in the core section is divided into four formations; they are Formation I (Units A and B), Formation II (Units C, D, E, and F), Formation III (Units G, H, and I), and Formation IV (Unit J). The core section intercalates some tephra layers. The lowermost tephra layer in the core section is A1Pm (TE-5a) of the Omachi APm series, known as a marker tephra indicating MIS 11. It is intercalated in Formation II. A tephra layer which is similar to BT-72 considered to have falled at MIS 10, is recognized in the lowermost part of Formation III. Therefore, Formations I to IV are correlative with the Kasamori Formation of the Kazusa Group, the Jizodo Formation, the Yabu Formation, and the Kamiizumi Formation of the Shimosa Group, respectively. The correlation between the GS-YS-2 and the other cores in the central Kanto Plain makes it clear that each formation of the Shimosa Group becomes thicker and represents deeper distribution northeastward from the GS-YS-2 drill site. These characteristics are distinctive particularly in the lower formations. These indicate that the area northeast of the GS-YS-2 drill site was continuously subsiding during the deposition of the Shimosa Group.

Tidal deposits with climbing ripples in the Pleistocene Shimosa Group

Tidal deposits with climbing ripples in the Pleistocene Shimosa Group

Wave condition of Paleo-Tokyo Bay recorded in foreshore deposits in Pleistocene Shimosa Group of Ibaraki, Japan

Wave condition of Paleo-Tokyo Bay recorded in foreshore deposits in Pleistocene Shimosa Group of Ibaraki, Japan

Sedimentary facies and physical properties of the latest Pleistocene to Holocene sediment core (GS-KZK-1) in the Shibakawa Lowland, Kawaguchi City, central Japan

埼玉県川口市在家町地区でボーリングコアGS-KZK-1を掘削し，堆積相·物性·元素分析結果に基づいて沖積層の堆積環境変遷を明らかにした．また，既存ボーリングコアによる断面図を作成し，層相の空間分布を検討した．GS-KZK-1で観察された堆積物は下位より更新統下総層群と沖積層に区分され，沖積層は下位より河川流路堆積物，氾濫原堆積物，潮汐流路堆積物を含む内湾堆積物，塩性湿地∼後背湿地堆積物から構成される．既存ボーリングコアおよび断面図からは，沖積層の基底礫層が基盤の更新統下総層群に由来すること，埋没谷を構成する堆積物は概ね泥質で，基底部を除き砂層は側方の連続性が悪いことがわかった．

沖積層ボーリングコアＧＳ‐ＳＳＳ‐１（埼玉県さいたま市）の堆積相と堆積物物性

The stratigraphy and transitions of sedimentary environments in the latest Pleistocene to Holocene incised valley fills in the Arakawa Lowland are revealed based on sedimentary facies, physical properties and chemical elements contents in the GS-SSS-1 core, Saitama City, Saitama Prefecture.The sedimentary successions are composed of Pleistocene Shimosa Group (Unit 1), gravelly fluvial deposits (Unit 2), sandy fluvial deposits (Unit 3), salt marsh deposits (Unit 4), inner bay to delta front deposits (Unit 5) and salt marsh to back marsh deposits (Unit 6) in ascending order. The depositional units excepting Unit 1 are the latest Pleistocene to Holocene incised valley fills, indicating a transgression (Unit 2 to the middle part of Unit 5) followed by a regression (the upper part of Unit 5 to Unit 6). In a transverse section, borehole logs correlated with the GS-SSS-1 core show that the gravelly fluvial deposits, sandy fluvial deposits and salt marsh deposits fill the narrow axis of the incised valley under the Arakawa Lowland, the inner bay deposits extend over buried terraces, and the delta front deposits and the salt marsh - back marsh deposits cover the entire lowland.

沖積層ボーリングコアＧＳ‐ＴＫＴ‐１（埼玉県戸田市）の堆積相と堆積物物性

The stratigraphy and transitions of sedimentary environments in the latest Pleistocene to Holocene incised valley fills in the Arakawa Lowland are revealed based on sedimentary facies, fossil shell assemblages, physical properties, chemical element contents and radiocarbon dating in the GS-TKT-1 core, Toda City, Saitama Prefecture.The sedimentary successions are composed of Pleistocene Shimosa Group (Unit 1), gravelly fluvial deposits (Unit 2), sandy fluvial to salt marsh deposits (Unit 3), inner bay to delta front deposits (Unit 4) and sandy fluvial deposits (Unit 5) in ascending order. Unit 2 and upper units are the latest Pleistocene to Holocene incised valley fills, indicating a transgression (Unit 2-3) followed by a regression (Unit 4-5). Depositional ages are 10000 cal yBP or before for Unit 2, 10000-9000calyBP for Unit 3, 9000-5000calyBP for Unit 4 and 5000calyBP to present for Unit 5.In a transverse section, the gravelly fluvial deposits and the sandy fluvial deposits fill the narrow axis of the incised valley under the Arakawa Lowland, the salt marsh deposits and lower part of the bay deposits extend over a buried terrace south of the valley axis, and upper part of the bay deposits, the delta front deposits and the fluvial deposits cover the entire lowland.

Depositional cycles and tephrochronology of Pleistocene Shimosa Group in central Kanto Plain, central Japan

堆積サイクルとテフロクロノロジーに基づき，関東平野中央部地下のC～F層と仮区分されていた更新統が，それぞれ下総層群の清川層，上泉層，藪層，地蔵堂層に対比されることを明らかにした．それぞれの累層は，基本的に陸成層と海成層からなる1堆積サイクルとして認識され，累層境界はシーケンス境界に相当する．これら累層から房総半島の模式地の下総層群に挟在する複数のテフラを見いだした．本研究の対比に基づくと，下総層群の基底に相当する地蔵堂層の基底は，調査地域において標高－100～－140 mに位置すると推定され，従来解釈されていた基底深度よりも極めて浅いことが明らかとなった．

Taphonomy of large-scale oyster shell beds : examples from the Pleistocene Shimosa Group around the Lake-Kasumigaura area, Ibaraki Prefecture, Japan

Taphonomy of large-scale oyster shell beds : examples from the Pleistocene Shimosa Group around the Lake-Kasumigaura area, Ibaraki Prefecture, Japan

茨城県行方台地の更新統下総層群にみられる外浜堆積物

茨城県行方台地の更新統下総層群にみられる外浜堆積物

Temporal change of life-history traits in fossil bivalves: an example of Phacosoma japonicum from the Pleistocene of Japan

Shell microgrowth and oxygen isotope patterns were examined in fossil specimens of a venerid bivalve Phacosoma japonicum from the Kami-iwahashi shell bed (c. 2.5 m thickness) in the middle Pleistocene Shimosa Group (c. 0.2 Ma) in central Japan. Winter and spawning breaks in this species can be distinguished by shell microgrowth analysis, and oxygen isotope values are high near winter breaks and low around spawning breaks. These breaks, therefore, can be used to estimate age of sexual maturity and shell growth rate in fossil specimens. Shell microgrowth analysis in the three fossil samples from the lower, middle and upper horizons in the Kami-iwahashi shell bed reveals that the maximum shell height and age of sexual maturity decrease gradually toward the top of this shell bed. Extant populations around the Japanese coast show a progressive change of life-history traits along a north–south gradient. Northern populations generally exhibit more delayed sexual maturity and larger shell size at a given age than southern ones. Life-history traits of the fossil sample from the lower horizon of this shell bed resemble those of the extant population from Ishikari Bay, Hokkaido, northern Japan, and those from the upper horizon of this shell bed are similar to those of the extant population in Tokyo Bay, central Japan. Sequence stratigraphic analysis has regarded this shell bed as a part of the transgressive systems tract, and the observations of the warm- and cold-water bivalve species suggest that mean annual water temperature in Paleo-Tokyo Bay increased during deposition of this shell bed. The geographic variation of life-history patterns among the living populations is related to the difference of mean water temperature during the growing season for each population. Judging from these facts, the temporal change of life-history traits observed in the fossil samples reflects the increase of mean annual water temperature during the deposition of this shell bed.

古東京湾に発達した２つのタイプの堆積シークェンス 下総層群上泉層，清川層および横田層

古東京湾に発達した２つのタイプの堆積シークェンス 下総層群上泉層，清川層および横田層

中‐上部更新統下総層群に産する“アナジャコ巣穴化石”Ｐｓｉｌｏｎｉｃｈｎｕｓ

中‐上部更新統下総層群に産する“アナジャコ巣穴化石”Ｐｓｉｌｏｎｉｃｈｎｕｓ

Diachronous evolution of systems tracts in a depositional sequence from the middle Pleistocene palaeo-Tokyo Bay, Japan

Palaeo-Tokyo Bay is a relic of the Plio-Pleistocene Kazusa forearc basin in the Boso Peninsula of Japan. The sedimentary infill of palaeo-Tokyo Bay is characterized by shallow marine to paralic sediments of the middle to upper Pleistocene Shimosa Group. Sequence stratigraphical analysis has been used to describe spatial and temporal variations in the depositional systems of the lowest units of the Shimosa Group, deposited during the early stage of development of palaeo-Tokyo Bay. Three different type of depositional systems were recognized: sand ridge to shelf (SRS), shelf to delta (SDL) and shelf to non-deltaic nearshore (SNS) systems. They overlie early transgressive estuarine deposits infilling lowstand valleys incised in the south-eastern margin of palaeo-Tokyo Bay. These systems were developed during late transgressive through highstand stages of a relative sea level cycle, which may have been controlled by a glacio-eustatic sea level change at about 0·4 Ma. Spatial variation in depositional systems is largely identical to that in modern Tokyo Bay; environmental conditions similar to those prevailing at the present day probably characterized the early history of palaeo-Tokyo Bay. The timing of highstand systems tracts within a high frequency depositional sequence was analysed in terms of the effect of sedimentation rate, based on the mapping of a chronostratigraphical surface marked by the Hy4 volcanic ash layer. From spatial variations in sedimentation rate, it was possible to identify the diachronous evolution of highstand systems tracts from the SDL system, through the SNS system, to the SRS system. Time lag is indicated by major bounding surfaces, such as maximum flooding or downlap surfaces associated with a condensed section, which developed immediately above or below the Hy4 volcanic ash layer. The lag may be of the order of a few thousands to tens of thousands of years within a depositional sequence with a total of duration of about 100 000 years.

更新統下総層群の砂層やれき層にみられる暗褐色斑点物質：バーネス鉱等のマンガン鉱物

更新統下総層群の砂層やれき層にみられる暗褐色斑点物質：バーネス鉱等のマンガン鉱物

下総層群の堆積サイクルと堆積環境に関する一考察

下総層群の堆積サイクルと堆積環境に関する一考察

Arcuate and bird’s foot deltas in the late Pleistocene Palaeo-Tokyo Bay

Summary Two types of prograding deltaic sequences are recognized in the upper part of the late Pleistocene Shimosa Group, which crops out in southern Ibaraki Prefecture and northern Chiba Prefecture, eastern Kanto, Japan. The older sequence is interpreted as a wave- and tide-dominated arcuate delta and the younger sequence as a fluvial-dominated bird’s foot delta. The older sequence is characterized by thick molluscan shell beds ( c. 10 m) in the middle part. In contrast, the younger sequence is characterized by the rhythmic alternation of fine-grained sand and mud with ripples in the lower part and coarse-grained sandy deposits with trough cross-stratification in the upper part. Both sequences show distinct coarsening-upward sequences. The difference between these two types of delta was caused by the water depth in the delta front during the lowering of the sea level from the interglacial to the glacial period during the last stage of Palaeo-Tokyo Bay.

Wave Ripples in the Pleistocene Paleo-Tokyo Bay

Abundant wave ripples are found in the Pleistocene Shimosa Group distributed in Chiba and Ibaraki Prefectures, central Japan. Wave ripples in the Shimosa Group, which were deposited in the Paleo-Tokyo Bay, are intercalated in the following facies : the upper part of the point bar deposit of fluvial channel facies, distributary bay or lagoonal facies, tidal flat facies, tidal inlet facies, beach facies, shoreface to offshore facies, and deltaic facies.In this paper, it is introduced wave ripples in the distributary bay facies, tidal inlet facies, beach facies, and shoreface facies, which especially contain many typical wave ripples; and their paleo-environmental settings.Most of wave ripples were constructed under several meters deep in the Pleistocene Paleo-Tokyo Bay. Wave ripples in the distributary bay facies were constructed under the 'post-storm condition '. These in the shoreface facies were under both of the fair weather ' and post-storm ' conditions. These in the tidal inlet facies were mainly under the fair weather condition ' and in part under the post-storm condition '. Wave ripples are generally preserved by deposition of the overlying clay. Clay was supplied by floodwater of rivers running into the Paleo-Tokyo Bay.

古東京湾堆積物中のウエーブリップル

Abundant wave ripples are found in the Pleistocene Shimosa Group distributed in Chiba and Ibaraki Prefectures, central Japan. Wave ripples in the Shimosa Group, which were deposited in the Paleo-Tokyo Bay, are intercalated in the following facies : the upper part of the point bar deposit of fluvial channel facies, distributary bay or lagoonal facies, tidal flat facies, tidal inlet facies, beach facies, shoreface to offshore facies, and deltaic facies.In this paper, it is introduced wave ripples in the distributary bay facies, tidal inlet facies, beach facies, and shoreface facies, which especially contain many typical wave ripples; and their paleo-environmental settings.Most of wave ripples were constructed under several meters deep in the Pleistocene Paleo-Tokyo Bay. Wave ripples in the distributary bay facies were constructed under the 'post-storm condition '. These in the shoreface facies were under both of the fair weather ' and post-storm ' conditions. These in the tidal inlet facies were mainly under the fair weather condition ' and in part under the post-storm condition '. Wave ripples are generally preserved by deposition of the overlying clay. Clay was supplied by floodwater of rivers running into the Paleo-Tokyo Bay.

Compilation of the stratigraphy and molluscan fossil assemblages of the Pleistocene Shimosa Group, eastern part of Kwanto Plain, central Japan

Compilation of the stratigraphy and molluscan fossil assemblages of the Pleistocene Shimosa Group, eastern part of Kwanto Plain, central Japan