抄録
The geological history of Lake Biwa has been elucidated through the age and depositional environment of the Kobiwako Group, which is distributed around the lake. This group consists of freshwater sediments dating from 4.4 million years ago (Ma) to 0.35 Ma and is shown continuity with the sediment layers beneath Lake Biwa. It is postulated that the depositional basins of this group have shifted over time, resulting in the migration of the lake's location. The geological history of Lake Biwa from its ancient lake stages is reexamined through the review of previous research and descriptions of the Kobiwako Group.The earliest identified paleo- lake, dating from approximately 4 Ma to at least 3.5 Ma, existed as a small lake east of present-day Iga City in Mie Prefecture. This lake may have formed a larger body of water in conjunction with a paleo- lake in the eastern region, where the presence of such a lake has been inferred. The second paleo-lake, following the earliest one, extended from around Iga City in Mie Prefecture to Koka City in Shiga Prefecture until about 2.6 Ma. This lake represented the largest body of water during the Kobiwako Group period. Following the second lake, multiple smaller lakes and marshes formed in the Koto area of Shiga Prefecture, which served as the depositional basin from that period until approximately 1.8 Ma. Although it was previously believed that no stable lake existed during the Kusatsu Formation period until 1.3 Ma, evidence suggests the presence of a small lake in the southeastern part of present-day Lake Biwa. This lake is considered an early stage of a larger lake that formed around the southern area of present Lake Biwa around 0.9 Ma. This southern lake expanded northward in 0.43 Ma, ultimately leading to the formation of the northern part of Lake Biwa. However, the early northern lake was relatively small, comparable in size to the southern lake, and not as large as the modern Lake Biwa. The northern lake has continued to expand northward and eastward due to fault activity on the lake's western side. This study introduces two key findings: (1) a stable small lake existed during the Kusatsu Formation period, which may represent the precursor of the current southern lake; and (2) the initial northern lake that formed around 0.43 Ma, which was small and had expanded to the north and east by present.
https://ror.org/03esr8826 
