Abstract
The Plio-Pleistocene Gotsu Group is distributed in the western part of Shimane Prefecture, southwest Japan. The sedimentary environment of the Shimanohoshi Formation in the middle of the Gotsu Group has been elucidated as downriver sediment. However, detailed sedimentary facies of the Shimanohoshi Formation have not yet been understood because of the difficulty involved in the lateral correlation of lithofacies of this formation. In this study, I clarified the sedimentary environment of the Shimanohoshi Formation in the type locality around Mt. Murogami-yama, which is located in the east of the Gonokawa River, on the basis of the sedimentary facies, distributions, and paleocurrent directions of the formation.
Sedimentary facies of the Shimanohoshi Formation are classified as follows: (i) facies Gmm (debris-flow deposits), composed of unsorted massive matrix-supported or clast-supported conglomerates; (ii) facies Gb (mass-movement deposits), composed of unsorted clast-supported angular conglomerates directly covering basement rocks; (iii) facies St (braided-river deposits), composed of very coarse to coarse-grained sandstone with trough cross-bedding, showing paleocurrents in the west-northwest direction; (iv) facies Sm (subaqueous gravity-flow deposits), composed of massive or cross-bedded, very coarse to fine-grained pebbly sandstone with a muddy matrix; and (v) facies Fsm (lake deposits), composed of mudstone with fine laminae.
The succession around Mt. Murogami-yama comprises alternating debris-flow deposits (Gmm) and braided-river deposits (St) along with some lake deposits (Fsm). On the basis of the distribution of the dominant debris-flow deposits (Gmm) and braided-river deposits (St), the sedimentary environment of the Shimanohoshi Formation is inferred to be a debris-flow-dominated alluvial fan with a west-northwestward braided river.
