Textural and Mineralogical Properties of Turbidite Sandstone of Upper Miocene to Pliocene Tamugigawa Formation in the Niigata Basin

The Upper Miocene to Pliocene Tamugigawa Formation, distributed in the western part of the Niigata Sedimentary Basin, Northern Fossa Magna, is confined trough-filled turbidite (TAKANO, 1990). Textural properties and mineral composition of this turbidite sandstone were investigated in this study. The textural analysis shows that the turbidite sandstone is composed of sand-sized and silt- to clay-sized populations. Both populations are clearly distinguished, and are normally distributed respectively. Accordingly the sandstone is very poorly sorted and very positively skewed. Although the vertical variation of grain size distribution throughout a thick-bedded sandstone bed is generally obscure, the upward increase of quantity of fine population sometimes results in the positive grading. The modal analysis shows that the sandstones are mainly lithic greywacke, and have similar composition in Q-F-R diagram. However, the detail investigation of minerals and rock fragments revealed that they are classified into the following two types: one including clastics from granite together with older sedimentary rocks and volcanic rocks, and the other which doesn't include clastics derived from granite. These sandstones were supplied from the Mikuni Mountains to the northwest. Early to middle Miocene volcanic rocks covering basement rocks in the Mikuni Mountains were firstly eroded and supplied, and then granitic rocks constituting the basement were supplied as the uplift of mountains advanced.