新第三紀生物源珪質頁岩の堆積作用と古海洋環境の関係について(予報)  [in Japanese] Preliminary report on the relationship between the sedimentation of Neogene biosiliceous shale and paleoceanographic environments  [in Japanese]

北海道北部の上部中新統稚内層の生物源珪質頁岩は堆積様式の違いから, 5つの岩石型(タイプA〜タイプE)に分けられた(福沢, 1988)。これらの5つの岩石型の堆積場から, 沿岸から沖合にかけての堆積パターンおよび堆積過程が復元された。沖合性の生物源珪質泥は乱泥流によって海底斜面やトラフに堆積した。海底斜面周辺の陸棚縁やSwellの上の生物源珪質物質は, 局地的な湧昇によって引き起こされた植物プランクトンの高生産に直接的に由来した。この解釈は, 陸棚の縁やSwellの上における海縁石の存在(福沢, 1988)や珪藻の種数(多様度)(荒戸, 1982)の小さいことによって支持される。深いトラフの中の生物源珪質堆積物は, 陸棚上のそれよりも大きな有機炭素の含有量を示すため, トラフの堆積物が酸素の少ない環境下で堆積したことを推定させる。比較的小さな浅い海盆中に堆積した後期中新世の生物源珪質頁岩はおそらく酸素の少ない氷塊の局地的な湧昇で形成され, また局地的な湧昇は季節風によって引き起こされたものかもしれない。

The northern foreland basin of northern Hokkaido was flooded by a marine transgression during the Late Miocene. The south eastern arm of this transgression in northern Hokkaido formed the Late Miocene Wakkanai Formation. Based on mode of sedimentation, the Wakkanai biosiliceous shales are divided into five lithologic units (type A-type E) (Fukusawa, 1988). Depositional patterns and processes operating along a shore to offshore transect are reconstructed on the basis of sedimentary sites of these five lithologic units. Offshore biosiliceous muds were deposited on submarine slopes and troughs by turbidity currents. Biosiliceous materials on the edge of shelves around submarine slopes and swells were directly derived from high production of phytoplanktons caused by regional upwelling. This interpretation is supported by the existence of glauconite pellets (Fukusawa, 1988) and poor populations of diatom species (low diversity) (Arato, 1982) on the edge of shelves and swells. As biosiliceous sediments in the deeper trough include higher contents of organic carbon than that on shelves, it is considered that these sediments accumulated under poorly oxygenated waters. Late Miocene biosiliceous shales deposited in relatively small basins were probably formed during regional upwelling of poorly oxygenated waters. The regional upwelling may have been caused by seasonal winds.