The use of ancient fields with undulating, rippled surface patterns (indicating, in parallel-arranged ditches, a furrow-like structure) was investigated at the Nagahara NG02-8 site. The site, dating from the early Nara period (8th-century), was studied in comparison with two neighboring sites of similar structure. The fossil assemblage of pollen and seeds from the cultivated layer at Nagahara NG02-8 was mainly composed of wetland and aquatic plants, describing a wetland environment for the field during the period of its use. Pollen grains of Fagopyrum and Oryza, possibly implying cultivated plants, were also detected. Higher percentage occurrence of Oryza pollen with carbonized rice in the wetland indicates paddy rice cultivation in the field. Plant fossils at the two other sites, along with their geographical conditions and differing furrow structures, indicate that dry-land rice was likely cultivated at the Nagahara NG03-5, and paddy rice at the Uriwari UR00-8. Our results suggest that, without detailed study, a furrow-like structure does not always mean that a site was farmed, much less that it was used as a rice paddy.

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The core sediments, in ascending order, consisted of fluvial gravelly sand, tide-influenced estuarine sand and mud with molluscan shell and wood fragments, prodelta mud and upward-coarsening delta-front muddy sand, and modern fluvial sand and mud. The maximum flooding surface (MxFS) was identified between the estuarine and deltaic sediments, and was dated at about 7,000 calibrated radiocarbon years ago (ca. 7 cal kyr BP). At that time the shoreline was located near the site of core KM02, about 4 km inland from the present shoreline. Before the MxFS, the present delta area was a fluvial environment until 9 cal kyr BP, and then changed to an estuarine environment during 9-7 cal kyr BP. After the MxFS, the Kumozu River delta formed and prograded eastward in the incised valley between Pleistocene terraces during 7-6 cal kyr BP. After deltaic sediments filled the incised valley, the delta advanced into Ise Bay and became lobate. Delta-front migration shows that the delta prograded northeastward, mainly during 6-4 and 3-0 cal kyr BP, and southeastward during 4-3 cal kyr BP.

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The Ohmi basin is located at the northern part of the Kinki Triangle and is bordered by the Ibuki and Suzuka Mountains to the east and by the Nosaka, Hira, and Hiei Mountains to the west. Along the western margin of the Ohmi basin runs the Biwako-seigan fault zone, a 60-km-long and N10°E-trending reverse fault zone. Around the northern part of the Biwako-seigan fault zone (e.g., Aibano, and Kamidera faults), fluvial terrace surfaces are well developed along the Ado and Ishida Rivers, and are important indices in understanding the tectonic history of the area. However, these well-preserved geomorphic features have rarely been used as tectonic markers due to lack of age constraints on the middle and higher terraces.
The purpose of this study is to provide age constraints on the middle and higher terraces and thus to evaluate the activity of the Biwakoseigan fault zone. We first divided fluvial terraces, from the highest to lowest, into Aibano 1, Taisanjino 1∼4, Okuyama, and Nakano surfaces, based on aerial photograph interpretation and field surveys. We carried out a drilling survey on the terraces to sample loamy soil. We then identified widespread volcanic ashes within loamy soil by precise cryptotephra analysis. The main results obtained in this study are as follows : (1) The KikaiAkahoya tephra (K-Ah : 7.3 ka), Aira-Tanzawa tephra (AT : 26-29 ka), and Kikai-Tozurahara tephra (K-Tz : 95 ka) are identified within loamy soil ; (2) Ages of the fluvial terraces are estimated at 160-200 ka (Aibano 1), 105-140 ka (Taisanjino 1), 95-130 ka (Taisanjino 2), 90-100 ka (Taisanjino 3), 60-80 ka (Taisanjino 4), and 25-30 ka (Okuyama) ; (3) Average uplift rates estimated from the ages and vertical displacement of the terraces are : 0.6-1.2 mm/yr (Taisanjino 1), 0.6-1.1 mm/yr (Taisanjino 2), 0.6-0.8 mm/yr (Taisanjino 3), 0.6-0.9 mm/yr (Taisanjino 4), and 0.8-1.2 mm/yr (Okuyama). Thus, since Taizanjino 1 surface was formed, the terraces on the west side of the Kamidera fault seem to have been uplifted at an almost constant rate (0.6-1.2 mm/yr) by the activity of the Kamidera fault.

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