湖成デルタの堆積物と形成過程 : 琵琶湖愛知川デルタを例として  [in Japanese] Deposits and Sedimentary Processes of Lacustrine Delta : An Example from Echi-River Delta, Lake Biwa  [in Japanese]

愛知川デルタは幅約2kmの非対称なカスブ状デルタで, 波浪の影響が比較的大きいため, 沿岸は砂質堆積物に富み, 沿岸州列が形成されている。デルタ堆積体は, 厚さ約14mで, 堆積相および珪藻化石群集の解析から, 上中下に3分される。下部は厚さ6m以下の内湾的な砂質堆積物からなる;主部は厚さ10mほどの上方粗粒化シーケンスで, 下位からプロデルタ, 泥質および砂質デルタフロント, 河口州, ビーチ相からなり, 砂質堆積物には波浪や洪水の影響が見られる;上部は河川や氾濫原堆積物などからなり, 上流側ほど厚い。^<14>C年代, テフラおよび花粉化石帯から時間面を求めると, 層相区分と斜交し, 河道方向の成長速度は平均3m/年で, 形成開始から水面は相対的に約6m上昇したこと, 成長とともにデルタフロントの傾斜が増大したことなどがわかる。また, 河口州とデルタフロントの砂質堆積物は, 粒度分布, 孔隙率, 浸透率からみて, 良好な貯留層特性を示している。

Geological investigations of modern freshwater deltas are needed in order to develop and verify the formation process model of petroleum reservoir sandstones. The Echi-River delta, selected as a model field, is an assymetrical cuspate and wave-dominated delta with a width of about 2 km. Near shore and upper slope deposits are dominated by sand and pebbles, and a fan-shaped plume of sand and silt is in the lake. Three sedimentary sub-sequences are recognized in the shallower parts of boreholes: the lower sub-sequence is less than 6 m thick, the main subsequence is about 14 m thick, the upper subsequence is up to 12 m thick. Sedimentary facies analysis and paleoecological study of molluskan and diatom assemblages have shown the sedimentary architecture of the delta. Basal prodelta mud with authigenic vivianite in the lower sub-sequence gradually coarsens upward to sandy sediments rarely affected by wave agitation. The main part of the deltaic sedimentary body is the middle, coarsening upward sub-sequence which consists of muddy delta front, sandy delta front, and coarser mouth bar and beach facies in ascending order: the coarse sediments are affected by frequent wave actions. Coarse channel and swamp sediments of the upper sub-sequence show fluviatile, delta-plain facies and thicken upstream. Age data from ^<14>C, marker tephras, and palinology show that time-lines cross the sedimentary-facies divisions. Progradation rates have been estimated at 3 m/yr. along the river, along with a 6 m water-level rise during the last 7,000 years of the delta formation. Acoustic structure of abandoned the Pleistocene delta and bathymetric change of the Echi-River delta during the last 30 years show that the inclination of the delta slope increased concordantly with delta growth into the deeper basin. Grain-size, porosity, and permeability studies show that mouth-bar and sandy delta front deposits have the best initial reservoir characteristics. These results are essential not only for developing petroleum exploration systems but are also useful for sedimentological research of lakes.