岩国市柱野付近で見いだされた礫質泥岩の起源

Hashirano, the western part of Iwakuni city, occupies a small area in the northern-most part of the zone of schistose hornfelses (NUREKI, 1960). All of the rocks found in this zone have been weakly metamorphosed, but original structures of the rocks are often well preserved. There has been found neither typical turbidite nor any type of slump-structures in the zone.
So-called pebbly mudstone can be found in several exposures near Hashirano. The pebbly mudstones occur in a limited stratigraphic horizone as one bed, which is inserted in an alternated member of banded chert with pelite. The pebbly mudstones are mainly composed of breccias or pebbles of chert and sandstone with subordinate amount of pebbles of quartz and diabasic rock. The breccias of chert occur most frequently among them, and in general are not rounded but angularly shaped. There is, however, a conspicuous tendency that the longest diameter of individual breccia of chert are preferably oriented parallel to the trend of lineation on the neighboring rocks.
The writer studied various features of pebbly mudstones, especially of the breccias of chert, from the petrographic and structural petrologic view points, and discussed their origin. It is assumed that the pebbly mudstones would have been produced during an unstable phase in sedimentation. The unstable phase seems to have been originated in abrupt increase of velocity of stream, and in a little desiccation of the watery sediments. Such changes of environmental condition during sedimentation may have not occurred probably on a large scale but only on a small scale.