Harutaka Sakai

A fossil freshwater Thalassiosira, T. inlandica sp. nov. (Bacillariophyta), with semicontinuous cribra and elongated marginal fultoportulae

T. Hayashi, Y. Tanimura and H. Sakai. 2007. A fossil freshwater Thalassiosira, T. inlandica sp. nov. (Bacillariophyta), with semicontinuous cribra and elongated marginal fultoportulae. Phycologia 46: 353–362. DOI: 10.2216/06-60.1 A fossil freshwater diatom species, Thalassiosira inlandica sp. nov., is described from Pleistocene lacustrine sediments of the Kathmandu Basin on the southern slope of the Nepal Himalaya. Thalassiosira inlandica is characterized by (1) a nonplicated (flat) valve face, (2) networks of ribs and costae, (3) semicontinuous cribra composed of the aggregate of many pore rows, (4) absence of valve face fultoportula, (5) elongated marginal fultoportulae, and (6) multiple rimoportulae in the mantle area. This new species is compared with allied taxa based on several morphological characters.

Puncticulata versiformis sp. nov. and Cyclotella kathmanduensis sp. nov. (Bacillariophyta), new fossil species from Middle Pleistocene lacustrine sediments, Kathmandu, Nepal Himalaya

New fossil species, Puncticulata versiformis sp. nov. and Cyclotella kathmanduensis sp. nov., are described from lacustrine sediments in the Kathmandu Basin on the southern slope of the Nepal Himalaya. They were dominant in the Middle Pleistocene. Both LM and SEM observations reveal their unique morphological features. Puncticulata versiformis is characterized by (1) various valve outlines (circular, elliptical, or oval), (2) a tangentially undulate central area, (3) a complex alveolar structure composed of three kinds of costae (thick costae, thin costae, and thin and short costae), (4) well‐developed spines (Y‐shaped and tapering) located on one side of the valve face/mantle area junction, and (5) a valvocopula with an extremely undulate margin. The species‐specific feature of C. kathmanduensis is the presence of two kinds of alveolate zones in a single valve: type‐1 zone composed only of normal costae, and type‐2 zone composed of both normal costae and recessed costae bearing fultoportulae. Changes in valve ornamentation occur in these two species from initial valves to vegetative valves. In P. versiformis, the arrangement of areolae with internal domed cribra and fultoportulae in the central area changes from radial rows in the initial valve to groups in the vegetative valve. In the initial valve of C. kathmanduensis, the type‐1 alveolate zone is generally absent.

Songpan-Garze Fold Belt, Eastern Margin of the Tibetan Plateau

松潘－甘孜(Songpan-Garze)褶曲帯はチベット高原の東縁に位置する, 揚子地塊とチャンタンーラサ地塊および古いユーラシアの地塊に挟まれた逆三角形の造山帯である. その東縁は竜門山スラストーナップ帯(幅30-50km, 総延長500km)を挟んで, 西四川前縁盆地に衝上している. またその西縁には左横ずれセンスの活断層である鮮水河(Xianshui He)断層が走っており, それに沿って中新世の花崗岩が貫入している. この褶曲帯は3つのユニットから構成されている. 基盤をなす原生代中期の花崗岩類と震旦系の酸性～中性火山岩類, カンブリア紀からペルム紀の堆積岩類と少量の炭酸塩岩・玄武岩類および厚さ6kmを越える三畳紀のタービダイトである. 古生層と三畳紀のタービダイトは, 揚子地塊上の受動的大陸縁辺堆積物とみなされる. この地層群は三畳紀後期(220～190Ma)のインドシニアン造山運動により, 変成作用と花崗岩の貫入を被り, 丹巴地域の古生層はガーネットー十字石－珪線石を含む変成岩になっている. 松潘－甘孜帯の衝上運動に伴い, その東縁のデボン紀から三畳紀後期の地層は南東方向に押し被さり, 竜門山帯のナップ・クリッペ群を生じている. その後ジュラ紀から白亜紀にかけて燕山造山運動を受け, 180～110Maの花崗岩が貫入した. さらに後期中新世のヒマラヤ期の運動によりスラスト群は再活動している.この報告は中国, 成都で開催された, 第15回ヒマラヤーカラコルムーチベットワークショップのプレ巡検(2000年4月15～20日)に基づいている. ワークショップの内容については, 日本地質学会News, 第3巻, 6号を参照されたい.