田辺層群朝来累層の古流系と堆積環境

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The Asso Formation of the Miocene Tanabe Group comprises thick conglomeratic sediments in the northern marginal part. This article deals with the sedimentological characteristics and sedimentary environments of these sediments. The sediments in Member Alt which occupies the lower part of the Asso Formation, is classified into 5 lithotypes, namely lithotypes A, B, C, D and E. Lithotype A consists of mudstone with coal seams and coal flakes. Lithotype B consists of alternation of conglomerate and mudstone. Lithotype C consists of thick-bedded conglomerate accompanied with thin mudstone intercalations. Lithotype D consists of massive conglomerate without mudstone intercalations. Lithotype E is characterized by stratified conglomerate with sandy matrices. Conglomerates in lithotypes B, C and D show no clear laminations and internal stratifications, and are composed of subangular gravels and muddy matrices, showing a distinctive contrast to conglomerates in lithotype E. In Member A_1, distinctive three fan-shaped sedimentary bodies, which are composed of lithotypes A, B, C and D, can be recognized. Each body shows the general tendency of fining stratigraphically upward and laterally away from the apex. That is, within each sedimentary body, lithotype D is developed at the basal part and the apex of the fan. Away from lithotype D, lithotypes C, B and A are distributed in this order vertically and laterally. The paleocurrent system, which are deduced from clast-fabrics of conglomerates, comprises three radially dispersing flow systems. It is worthy to note that lithotype D develops at the apex of each dispersing system. At the stage of deposition of Member A_1; the closed and stagnant sea area was existed in the study area. This closed sea was separated from the open sea by a presumable barrier complex, and in the back-barrier area mud and coaly materials were deposited quietly. From the land area situated to the north, large amount of gravels were transported into this closed sea, and dispersed radially. The intermittent supply of gravels resulted in the alternation of conglomerate and mudstone. The conglomeratic sediments of Member A_1 had been deposited as marginal gravels of Tanabe sedimentary basin, under the influence of upheaval movement in the hinterland and of the gradual transgression in the sedimentary basin.
地学団体研究会の論文
1987-07-25

田辺層群朝来累層の古流系と堆積環境

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