| Abstract |
Sandstones contained in radiolarian cherts and siliceous mudstones in the Mesozoic of Southwest Japan have often been regarded as conformable interlayers, hence not a few workers have referred the che...rt of this Erathem to as shallow-water deposits. In this connection, analogous sandstones in siliceous rocks of the southern Chichibu Terrane in Kyushu are described in this paper as clastic injections and their origin is discussed. The rocks studied belong to the Yoshio Formation along the midstream of the Kuma River, Kumamoto Prefecture, and to the Shakumasan Formation in the Youra Peninsula, Oita Prefecture. These two formations are composed of sandstone, mudstone, and siliceous rocks comprising thin-bedded chert and siliceous mudstone. The siliceous rocks usually occur as displaced blocks of various sizes and extensive sheets commonly with a fault relation with clastic rocks, and are accompanied with pebbly mudstone and conglomerate and exotic blocks of greenstone and limestone in neighboring areas. Sandstone injections are founded in many blocks and some sheets of siliceous rocks and characteristically occur in and near the junctions between the chert unit and the overlying siliceous mudstone unit. These injections show a great variety of form, and most common is of a tabular to lenticular sill-form. In places, they crosscut bedding surfaces, laminations, and even folded structure of the host rock. In addition to such a simple injection form, much complicated ones including ramifying, laterally discrete, and reticulate-vein forms occur. These sills and dikes range from a few millimeters to a few meters thick. Most of the sandstone sills and dikes are internally structureless, but some exhibit multiple sets of injection layers. Microscopic examination revealed that they show a crude orientation of sand-grains nearly parallel to their sides. They are medium-to coarsegrained feldspathic arenite, and occasionally contain chert breccias of a granule to pebble size that were probably detached from the host rock. They are similar in composition to the sandstones that occur adjacent to the siliceous rocks. The contact between the sandstone injections and the enclosing siliceous rocks is quite sharp. The stratigraphic position of the junctions between the bedded chert and the siliceous mudstone unit is different in individual bodies of the siliceous rocks and ranges in age from Early Jurassic to Middle Jurassic. The cause of concentrated occurrence of sandstone sills and dikes in and near the junctions stated above is not known, but formation of fractures or weakened structural surfaces into which sands intruded may have been caused by the differences of physical properties including that of competency between those two kinds of rocks. Sandstone sills and dikes apparently resulted from forceful injection of liquified sand from a source bed that lies adjacent to siliceous rocks. Liquifaction and fluidization of the source bed are likely to have been created when the unconsolidated source bed had been overlain by or mixed with allochthonous sheets or blocks of consolidated siliceous rocks of submarine gravity-sliding origin and had obtained overpressured conditions due to the overburden.show more
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