**7. Summary**

(1)

the Q Bed and the P Bed (Figure 3). The alternating characteristics of the S, R, and Q beds indicate short-term cycles in the depositional environment. These cycles may represent periods of uplift of surrounding areas, with subsequent erosional readjustments, or periods of subsidence of the lake basin with subsequent infilling. A study of the lithology and chronology of the Kobiwako Group, the Plio-Pleistocene freshwater sediments distributed around Lake Biwa, reveals that the history of this basin is divisible into two main stages; the Older and the Actual [27]. The deposits of the Older Stage are mainly distributed southward of the present Lake Biwa. Accordingly, the sedimentary depocenter of the Older Stage was located south‐ ward of the modern Lake Biwa. Between the Older Stage and the Actual Stage, the depocenter evidently shifted northward to its present location, as indicated by the stratigraphy and chronology of the Kobiwako Group, creating the basin of modern Lake Biwa. Large amounts of gravel began to deposit at the northern part of the Older Stage Lake Biwa basin starting about 1.5 Ma. This occurrence is thought to record the beginning of the migration of the lake depocenter. The R bed corresponds to the sediments of the early stage of Actual Stage. The upper part of the S bed and the entire T bed represent the Actual Stage. The high and constant sedimentation rates within the drilling sequences in present Lake Biwa represent a time of rapid infilling. Rapid subsidence was necessary for the deposition of the continuous sequence in the present Lake Biwa with a sedimentation rate of 0.57 m/kyr [13] and inferred a subsidence rata of 0.74 m/kyr from the data of age of T bed and water depth at the 1400 m drilling site. Reconstruction of these tectonic implications cannot be done competently from data from one

224 Mechanism of Sedimentary Basin Formation - Multidisciplinary Approach on Active Plate Margins

core site, and we must use surrounding geological data and geophysical data.

*hg*

*g*

faulting during the Quaternary.

Figure 15 shows the first-order horizontal derivative of the Bouguer gravity anomalies (the horizontal gradient of gravity anomaly) more than 2 mGal/km and contour interval is 1 mGal/ km. The horizontal gradient of gravity anomaly (Δ*ghg*) is defined as the following equation.

> ( ) ( ) 2 2 , ,

*gxy gxy*

*x y* æ öæ ö ¶ ¶ D= + ç ÷ç ÷ ¶ ¶ è øè ø

The horizontal gradient of gravity anomaly emphasizes shorter wavelength signals of subsurface structures. Therefore, it is a good indicator of a conspicuous density change and/or a large fault. High gradient anomalies of greater than 2 mGal/km correspond well with large faults and tectonic lines such as the Rokko-Awaji fault zone, Arima-Takatsuki tectonic line, Ikoma fault, Kanbayashigawa fault, Hanaore fault, Biwako-seigan fault and others. They have NE-SW and E-W trends in their strike directions. However, distribution of high gradient anomalies around the Lake Biwa is extremely complex. It is difficult to find a specific trend in the strike directions of the horizontal gradient of gravity anomaly. As one reason, it can be inferred that they reflect subsurface structures caused by extreme crustal activities including

Here, g(*x, y*) are gravity anomaly data given on *xy* mesh with a constant interval.

The present Lake Biwa is a tectonic basin under the E-W compressional stress state. Distribu‐ tion of active faults is characterized by the western boundary of the lake (Biwako-seigan Fault zone; Hira, Katata Faults with N-S direction) and the northeast region of Lake (Yanagase Fault etc). Activity of faults (mainly Biwako-seigan Fault zone) of more than 1 m/kyr along the west side of Lake Biwa is important for the present lake basin formation.

Basement topography revealed by the seismic reflection survey shows alignment of the valley and range with a N-S direction. The "A" horizon (bottom of T bed since about 0.43 Ma) topography of the seismic reflection profile revealed its tilting topography from east to west. The sedimentary record from present Lake Biwa by deep drilling includes a shift of the lake depocenter from farther to the south to its current location. The sedimentary record shows constant sedimentation from at least 1.3 Ma. The sedimentation rate of the present Lake Biwa is about 0.57 m/kyr as revealed by age depth curve of 1400-m deep drilling core taken in 1982-1983. The subsidence rate is calculated as 0.74-m/kyr by data of the T bed (thickness 250 m, duration 430 kyr, water depth 68 m).

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Paleogeographic evidence since Pliocene in central Kinki district including Paleo-Lake Biwa area shows the characteristics by which the history of this basin has been divided into two main stages; the Older and the Actual. Deposits of the Older Stage are distributed mainly southward of the present Lake Biwa. The sedimentary depocenter of the Older Stage was located south of the modern Lake Biwa. The depocenter evidently shifted northward to its present location creating the modern Lake Biwa Basin. Those changes and basin migration since the Pliocene are regarded as tectonic basin formation under the influence of superposed two tectonic stress states represented by upheaving of the southern area and tilting of eastern area. Tectonic stress change from a N-S compressional stress state to the E-W one was conspicuous.

Lake Biwa area (Ohmi Plain) is a large negative gravity anomaly area in the 'Kinki-Triangle' area with -60 mGal indicating the inhomogeneity of lower crust, or lower crust thickness or existence of very low density materials because of faulting. Distribution of high gradient anomalies around Lake Biwa is extremely complex. It is difficult to find a specific trend in the strike directions of the horizontal gradient of the gravity anomaly, perhaps because they reflect subsurface structures because of extreme crustal activities including faulting during the Quaternary.
