Depositional Environments of the Sagara and Kakegawa Groups (Middle Miocene-Early Pleistocene), and the Evolution of the Sedimentary Basin, Central Japan

A thick marine clastic sequence, the Sagara and Kakegawa Groups of middle Miocene to early Pleistocene age, is successively developed in the Kakegawa district, central Honshu, Japan. It was deposited in a fore-arc basin lying in an arc-trench gap between the subducting Philippine Sea Plate and the overriding Honshu Arc. Fourteen depositional facies are identified within the two groups, and depositional environments are reconstructed on the basis of these facies in order to delineate their depositional environments in time and space. As a result, it becomes clear that the arrangement of depositional facies in the Sagara and Kakegawa Groups was mainly controlled by the global sea-level changes and a series of local tectonic movements induced by the initiation of subduction of the Philippine Sea Plate beneath the Honshu Arc along a plate boundary to the east of the sedimentary basin at 3.6 Ma, and it is inferred that the plate has changed its moving direction from northwards to west-northwestwards at that time

Composition and Provenance of Clay Minerals in the Northern Part of Lake Tonle Sap, Cambodia

金沢大学環日本海域環境研究センターエコテクノロジー研究部

北陸地方に分布する更新統大桑層ならびに同層相当層の層位学的研究

一昨年度および昨年度に実施した石川県金沢市東部〜北東縁部地域ならびに富山県小矢部市南西部地域に分布する下部更新統大桑層の地質調査をさらに発展拡大させ,本年度には石川県北部ならびに南部,および富山県福光市地域に分布する同層ならびに下位層となる高窪層以下の諸層について,5,000〜1万分の1地形図にもとづく地質調査を終了した.これで大桑層の主要分布地域の地質調査はほぼ終了したといえる.これまでの成果に本年度の調査結果をあわせ,金沢市地域から富山県西部地域に分布する大桑層には,青灰色〜暗灰色砂質泥岩・泥質砂岩が卓越し凝灰岩の多数挟在する下部,青灰色細粒砂岩からなり貝化石層ならびに凝灰岩が挟在する中部,そして塊状黄褐色細粒砂岩から構成される上部という岩相区分がほぼ全域にわたって適用されることが明らかとなった.また,同層中への挟在がこれまでに確認された15枚の凝灰岩層に加え,鍵層として有効性はいまだ未確認ではあるが,さらに6枚の凝灰岩の挟在を確認するとともに,下位層である高窪層の地質調査結果とあわせ,これまでに調査を実施したすべての地域で大桑層と高窪層以下の諸層とは緩やかな傾斜不整合関係にあることが明らかとなった.そして,凝灰岩鍵層ならびに不整合面の野外での追跡結果から,金沢市地域に分布する大桑層は,水平方向への岩相変化にきわめて乏しいこと,そして同市北東部では北東―南西方向あるいは北西―南東方向の軸をもつ緩やかな背斜・向斜構造を呈するものの,さらに東には北北東―南南西の軸をもつ向斜構造ならびに同方向で東南東傾斜の衝上断層が存在し,この衝上断層が富山県福光市北西部をへて同県小矢部市南西部にまで達することが判明した.なお,今後は調査区域をさらに南北へと拡大し同様の調査を継続することで,大桑層堆積当時の堆積盆の構造発達史・古環境変遷史を解明する予定である.The research results on the lithostratigraphical study on the Pleistocene Omma Formation and the related strata distributed in the Hokuriku Province, central Japan are as followings :1. The Omma Formation is distributed mainly in the eastern to northern parts of the Kanazawa City, Ishikawa Prefecture, and the western part of the Oyabe City and Fukumitsu town, Toyama Prefecture in the investigated areas.2. The Omma Formation is divided lithologically into the lower member composed mainly of bluish-grey mudstone, muddy sandstones and sandy mudstones with intercalations of pyroclastic layer, the middle member consisting of bluish-grey fine-grained sandstones with intercalations of volcanic ash layer and molluscan shell bed, and the upper member composed of yellowish brown fine-grained sandstones through most part of the investigated areas.3. More than 15 traceable volcanic ash layers are recognized in the lower and middle members of the Omma Formation. Since a certain number of them are traceable wholly in the investigated areas, they are effective to correlate within the formation among the distributed areas.4. The Omma Formation unconformably covers such Mio-Pliocene strata as the Takakubo and Saikawa Formations in the investigated areas. On the other hand, it is conformably overlain by the Utatsuyama Formation, but interfingering relationships between the Omma and Utatsuyama Formations are recognized in part.5. The Omma Formation distributed in the investigated area are folded in broad anticlines and synclines trending NNE-SSW in general. Existence of a major thrust fault trending N-S to NE - SW is inferred from the eastern marginal part of the Kanazawa area to the southwestern part of the Oyabe area through the northern marginal part of the Fukumitsu area, and the Miocene strata distributed on the southeast of the fault are thrust over the younger strata such as the Omma and Takakubo Formations on the northwest.研究課題/領域番号:09640555, 研究期間(年度):1997-1999出典：「北陸地方に分布する更新統大桑層ならびに同層相当層の層位学的研究」研究成果報告書　課題番号09640555 (KAKEN：科学研究費助成事業データベース（国立情報学研究所）)　　　本文データは著者版報告書より作

カンボジアのトンレサップ湖における過去2万年間の環境変遷史

本年度5月に実施した現地調査をもって試料の採集を終了し,前々年度ならびに前年度に採集した湖底堆積物試料の解析およびその結果のとりまとめをまず解析分野ごとに実施した.そして,本年度10月にカンボジア王国プノンペン市鉱工業省講堂にて開催したシンポジウムでそれぞれの成果を発表し,同年度11月に金沢市で開催した総合討論会および将来検討会においてそれぞれの解析分野を相互に参照しながらとりまとめた.その結果として明らかになった事実は以下のとおりである.1.トンレサップ湖は約7,600年前に誕生した.2.誕生当時から約5,500年前までのトンレサップ湖は現在のトンレサップ盆地に散在する小湖沼群であった.これを古トンレサップ湖と名付ける.3.古トンレサップ湖は周辺からもたらされる土砂によっての埋積が年平均1mm前後で進行していた.4.約5,500年前の海面高頂期にトンレサップ盆地へメコン河の水が流れ込むようになり,それによって巨大湖トンレサップが出現した.5.約5,500年前以降のトンレサップ湖は流入する土砂と流出する土砂の均衡がとれるようになり,そのため湖の埋積は事実上停止し安定した水域として存在している.6.湖底柱状堆積物に含まれる珪藻化石群集は,数百年周期での量的な増減はみられるものの過去7,600年間をとおして淡水生種のみから構成される.7.湖底表層堆積物に含まれる介形虫群集は常時水のある区域に生息する種類と浸水林域を含む区域に棲む種類とに大きく2分され,これはその生活様式に起因するものと推定される.8.底生・浮遊性有孔虫は,柱状試料・表層試料ともにまったく見いだせない.9.過去7,600年間をとおして花粉群衆組成に変化は見いだせない.しかし,約15,000年前の試料に含まれる花粉は当時の気候がより湿潤であったことを示しており,これはカンボジアでは約15,000年前から約7,600年前にかけて乾燥化が進行したことを意味する可能性がある.During the course of this programme, twelve short and six long cores were recovered from north Lake Tonle Sap to reconstruct environmental history of the lake and its environs. On the other hand, about 100 surface sediment samples were obtained to obtain compositional features of micro-organisms and surface sediments, and their spatial distribution in the present lake floor and adjacent fluvial systems such as the Tonle Sap and Mekong rivers. Although some studies are still under investigation, the environmental changes and geological development of Lake Tonle Sap and the lower Mekong River during the last 10,000 years can be summarized. Lake "Palaeo-Totue Sap" appeared at about 7.6 Ka, and it existed to 5.5 Ka. The lake was a considerably smaller freshwater lake or lakes, rather than the large lake of the present time, situated in the central axis area of the modern Lake Tonle Sap. At that time the palaeo-lake(s) was isolated from the major fluvial system in the Indochina Peninsula and muddy sediments derived mainly from alluvial plains were filling up the lake(s). Because of the connexion of the lake with the Mekong River, caused by the rapid rise of sea-level after the Last Glacial Maximum and its high-stand at the Holocene Climatic Optimum, the present environment of Lake Tonle Sap appeared at 5.5 Ka and the environment has existed continuously till the present day. The exact shape and topography of Lake Palaeo-Tonle Sap still remains in question. Thus, shallow water seismic surveys should be carried out in the whole area of the lake to reveal paleogeography of the lake during the period from 7.6 to 5.5 Ka. Further, since the basement strata in the long coring sites can be interpreted sedimentologically as sub-aqueous deposits, a long drilling programme in the central part of the lake should be made in order to develop the mult-history of the lake.研究課題/領域番号:12573014, 研究期間(年度):2000-2002出典：「カンボジアのトンレサップ湖における過去2万年間の環境変遷史」研究成果報告書　課題番号12573014 (KAKEN：科学研究費助成事業データベース（国立情報学研究所）)　　　本文データは著者版報告書より作

カンボジアのトンレサップ湖における生物多様性維持機構の評価

平成15年11月のトンレサップ湖北部での高水位期調査,平成16年5月の同北部での低水位期調査,そして同年11月の同湖南部での高水位期調査それぞれの結果をふまえ,低水位期となる平成17年5月に同湖南部で現地総合調査を実施した.これで同湖北部および南部における高水位期および低水位期それぞれの調査を完了したことになる.これに加えて水位上昇期となる同年7〜8月に同湖北部および南部で補助的調査を行った.いずれの調査もアンコール遺跡整備機構,カンボジア鉱工業エネルギー省資源局,および同水資源気象省水文河川局との共同実施である.調査項目は(1)同湖南部ならびにトンレサップ川・メコン河の堆積物の採取と解析,(2)同湖北部・南部での水圧,流速,流向,濁度等の連続横断観測,(3)同湖および周辺水域における水質調査と水の採集・分析,(4)同湖北部シェムリアプ市の気象観測装置によるデータ収集と解析,(5)同湖北部・南部水文観測ステーションにおける水圧,水温等の継続観測,(6)浸水林域ならびに周辺陸域における植生調査ならびに植物試料の採集,(7)浸水林域ならびに周辺陸域における住民の森林利用聞き取り調査,(8)北部・南部における浮遊性ならびに底生無脊椎動物群の採集,(9)同湖北部・南部における一次生産の測定,(10)同湖北部および南部での魚類の採集と分類である.調査項目個々についてのデータのとりまとめ,ならびに全項目を総括しての生物多様性維持機構評価は現在進めている段階であるが,その予察的成果を平成17年12月1〜2日にカンボジアの首都プノンペンで国際シンポジウムを開催し公表した.Lake Tonle Sap, the largest lake in Southeast Asia, is famous as "the heart of Cambodia" referring to various and multifarious freshwater organisms. The lake is also well known as "the elastic water world" because its water area and depth change drastically between rainy and dry seasons. Although such settings hold out a promising prospect for investigations of unique freshwater ecosystem in the lake under the control of dynamic seasonal fluctuations in water level and area, no systematic research studies have been made on the lake. During the course on this programme, four overall field surveys consisting lacustrine sedimentology, hydrology and limnology, plant ecology, and invertebrate and vertebrate zoology, in November 2003 (high water season in north), May 2004 (low water season in north), November-December 2004 (high water season in south) and May-June 2005 (low water season in south) and six optional surveys in January 2004 (early falling water season in north), January 2005 (early falling water season in north), February-March 2005 (late falling water season in both south and north) and July-August 2005 (rising water season in both), December 2005 (high water season in both), and March 2006 (falling water season in north) were carried out to evaluate mechanisms sustaining the biodiversity in the lake. In spite of the compilation of individual scientific fields and the final conclusion on the programme have been still under the investigation, preliminary result on each subject was presented in an international symposium in Phnom Penh, Cambodia on the 1^ and 2^ of December 2005.研究課題/領域番号:15405004, 研究期間(年度):2003-2005出典：「カンボジアのトンレサップ湖における生物多様性維持機構の評価」研究成果報告書　課題番号15405004 (KAKEN：科学研究費助成事業データベース（国立情報学研究所）)　　　本文データは著者版報告書より作

Planktic foraminiferal assemblages and stable isotope records of the southern Japan Sea during the last 27,000 years

Abstrac

The characteristics of Lake Tonle Sap in Cambodia based on changes in conductivity

金沢大学環日本海域環境研究センターエコテクノロジー研究部門Lake Tonle Sap in Cambodia is the largest freshwater lake in Southeast Asia, covering an area of about 2,500 km2 in the dry season and 12,500 km2 in the monsoon season. Due to such a unique seasonal change in hydrological phenomena related to the Mekong River system, the lake\u27s water quality must change dramatically. As there is a lack of basic data on the water quality and hydrological phenomena of Lake Tonle Sap, we launched a limnological research survey. This paper deals with the characteristics of the lake based on conductivity changes. Between the monsoon dry seasons, conductivity and water depth in the lake shore changed dramatically. Both were low in the dry season when the water depth was 0.5 m and the conductivity was 40 μS cm-1, and they were both high in the monsoon season, at 8 m and 120 μS cm-1, respectively. However, the conductivity offshore was high at an approximately constant rate throughout the year at around 100 ∼ 120 μS cm-1. This is because the waters that flow back from the Mekong River in the monsoon season remain in the offshore area. Conductivity decreases in the lake shore area in the dry season, which causes a low level of conductivity water to flow in from surrounding regions of the lake. It is the influence of the water flowing back from the Mekong River that increases the conductivity in the lake shore region during the monsoon season