牟婁層群の「サラシ首層」 : オリストストロームの一型式

概要

論文の詳細を見る
Peculiar angular fragments-bearing mudstones, called "Sarashikubi(gibbeted heads) beds" as a field name by KISHU SHIMANTO RESEARCH GROUP (1969), are distributed in the southern coastal area of the Kii Peninsula. The "Sarashikubi beds" belong to the upper formation of the Oligocene to Lower Miocene Muro Group, representing the later stage of the Shimanto geosyncline. Sedimentological study of the beds is important to make clear not only the latest development of the Shimanto geosyncline, but also sedimentation of a certain type of olistostrome bed. The "Sarashikubi beds" (310 m in thickness) are lithologically subdivided into three units, that is, Units A, B and C in ascending order. Unit A is mostly composed of massive angular fragments-bearing mudstones, and intercalates in the middle part a sequence of normal beds, consisting of pebble conglomerate, sandstone and calcareous siltstone. Unit B consists of coarse-grained sandstone, conglomerate and angular fragments-bearing mudstone. Unit C is composed of angular fragments-bearing mudstone bedded in 50 to 300 cm thick. In the present paper, sedimentological properties such as stratification, form, roundness, gravel-size, mineral composition and fabric of gravels and occurrence of matrix were described in detail. Matriceous mudstone is poorly sorted, massive and black siltstone in which no tectonic shear or fissility is observed. "Angular" gravels consisting mainly of sandstone, are classified into three types of occurrence, namely, sharpstone type, segregated type and flowed type. Based on mineralogical composition, these sandstone gavels are derived from the Muro Group itself. Difference of their occurrence reflects the degree of consolidation of source rocks from which their gravels were formed. It is probably thought that the geanticlinal upheaval, grew up at the later stage of the Muro sedimentary history, was collapsed and supplied various types of "angular" gravels. According to the gravel fabric, the upheaval existed to the south of the basin. It is estimated that angular fragments-bearing mudstones of Unit A transported by submarine debris flow under the condition of abundant muddy matrices and low viscosity of the flow, differed from the typical submarine debris flow (MIDDLETON and HAMPTON, 1976). It might be that the ones of Unit C were deposited by immature turbidity current. On the contrary, the rounded pebbly conglomerate, intercalated in the middle part of Unit A, is quite different from angular fragments-bearing mudstones and other conglomerates in the "Sarashikubi beds", in the respects such as composition, roundness, sorting and fabric of gravels. Based on the fabric of gravels, these gravels were supposed to be derived from the northern geanticlinal land, where sandstone glavels of the Muro Group were rounded by fluvial and/or wave action, and then transported by grain flow and/or turbidity current into the sedimentary basin. Muddy flysch facies and abundant slump beds underlying the "Sarashikubi beds" show that the basin was not so shallow. The basin was filled up by deposition of the Unit A, and become shallower. The abundance of shallow-facies trace fossils, trough-type cross lamination and coaly materials, observed in Unit B, supports this interpretation. It is probable that the basin became a little deeper in the stage of the Unit C sedimentation. In the later stage of the Muro Group, geanticlinal upheavals arose here and there in the basin, and the basin became differentiated. The Muro basin became shallower as a whole, though differentiated small basin repeated to become a little deeper or shallower. And then, it was disappeared and uplifted. It is worth to be noted that the "Sarashikubi olistostrome beds" was not formed on trench floor or deep sea terrace, but on the shallower sedimentary basin.
1980-03-25

牟婁層群の「サラシ首層」 : オリストストロームの一型式

スポンサーリンク

概要

著者

関連論文

スポンサーリンク