## 2 Geological Setting  
The study area mostly consisted of the Eocene Urahoro Group, which is unconformably covered by the Upper Miocene Atsunai Group (Figure 1, Table 1). The Urahoro Group consists of shallow marine to brackish water sediments intercalated with non-marine sediments and contains many coal layers, which were exploited in the past and are known as the Kushiro coal field [27]. The Eocene Urahoro Group consists of the Beppo, Harutori, Tenneru, Yubetsu, Shitakara and Shakubetsu formations in ascending order (Table 1, [28; 29]). The basement of the Urahoro Group is not exposed in the study area but unconformably underlain by the uppermost Cretaceous sandstone and mudstone in the area c. 30 km west (1:50,000 Geological Maps of Japan, Onbestu by [30] and Tokomuro by [31]). Further, since the sandstones of the Urahoro Group are arkosic and contain many plagioclase, K-feldspar, quartz and biotite detrital grains, the basement was inferred to have been composed of mostly granitic rocks pre-Eocene in age. The sedimentary age of the Urahoro Group, which was previously inferred to be from the Late Eocene from shell, plant [28] and foraminifera fossils [32; 33], has recently been determined to be from c. 39 Ma (Middle Eocene) based on the U-Pb age of detrital zircon grains from tuff beds [34]. The Middle Eocene Urahoro Group is overlain by the Onbetsu Group from the latest Late Eocene to the earliest Early Oligocene (Table 1, [32; 33; 35]) and consists of sandstone and mudstone deposited in a shallow marine environment with a very slight unconformity [30]. The Atsunai Group is also mostly shallow marine sediments, consisting of the Chokubetsu, Atsunai and Shiranuka formations in ascending order (Table 1, [30; 36]), including many tuff beds. In the study area, only the Atsunai Formation is distributed, consisting of lapilli tuff (or tufufaceous sandstone), including many pumices and tufaceous mudstones [37]. While the age of the Atsunai Group has previously been inferred to be the Late Miocene based on diatom fossils [38], the U-Pb age of detrital zircon grains from lapilli tuff of the Atsunai Formation has yielded c. 7 Ma for the sedimentation ages (personal communication with [PERSON]). In the studied and surrounding areas, the structure of the Urahoro Group is characterized by fold-and-thrust belts, where the orientations of the fold axes and thrust faults trend NNE-SSW [30; 31; 37]. The fold-and-thrust belts were possibly formed by the collision of the forearc sliver of the E-W trending Paleo-Kuril arc against the northeast Japanese arc trending N-S represented by the Hidaka belt in Hokkaido (Figure 1a, [39; 40]). The overlying Upper Miocene Atsunai Group is concordantly folded together with the Middle Eocene Urahoro Group based on the geological maps of the previous studies [30; 31; 37; 41]. However, the degree of folding of the Upper Miocene Atsunai Group compared to that of the Paleogene groups has not been precisely known. Whether the former group rests on the latter groups with parallel unconformity or clino-unconformity has been debated [36]. Therefore, if the deformation (folding) occurred at a single-phase or polyphase and their timing remains to be investigated. In the study area, only the uppermost unit of the Shitakara Formation and the Shakubetsu Formation of the Urahoro Group and the Atsunai Formation of the Atsunai Group are distributed (Table 1, Figure 1b), and the Onbetsu Group is not distributed. The uppermost unit of the Shitakara Formation solely consists of fine- to medium-grained arkosical sandstones. While the Shakubetsu Formation also consists of fine- to medium-grained arkosic sandstones, which are intercalated with mudstones and many thin coal layers up to a few tens of a centimeter thick. The structure of the study area is characterized by a fold belt called the Sashiusu-Shiranuka-gawa folded belt with a wavelength of 1-2 km [37], which trends in the NNE-SSW direction (Figure 1b). The strata mostly gently dip towards either an SSE or WNW direction up to 30 to 45 degrees.