Modal composition of pre-Paleogene sandstones scattered in the southern Ashikita region, southwestern Kyushu, Southwest Japan

Abstract

The southern Ashikita region in southwestern Kyushu, southwest Japan, is underlain by Jurassic to Lower Cretaceous accretionary units (Chichibu Belt) and the Cretaceous Saiki Subgroup (Shimanto Belt) in its northern and southern parts, respectively. These pre-Paleogene units are widely covered by Neogene and younger formations, and thus are exposed in isolated outcrops over a narrow area. The modal compositions of representative pre-Paleogene sandstones in this region were determined. A ternary Q–F–R plot of the sandstone compositions indicates the sandstones are mostly feldspathic. A Qm–F–Lt plot constructed using the Gazzi–Dickinson method indicates the sandstones can be divided into two groups based on the content of monocrystalline quartz (Qm). Seven samples from the northern part of the study area have high Qm/F values (≥0.69) and 10 samples from the southern part of the study area have low Qm/F values (≤0.50). The boundary between the locations of the high- and low-Qm/F samples coincides with the known position of the Butsuzo Tectonic Line in this region.