**2. Sedimentary sequences in present Lake Biwa Basin**

Lake sediments are important archives for understanding tectonic history at different scales. Several attempts to recover core sediments from Lake Biwa have been made, mainly in the 65-70 m deep depression situated in the southern part of the Northern Lake (Figure 1). Horie et al. first recovered a 6-m-long sediment core in 1965 and then an 11.5-m-long piston core in 1967 [2]. In 1971, with considerable effort, they drilled sediments in the same depression (Figure 2) and obtained core samples of about 200 m in all [3]. Finally, in 1982 and 1983, they recovered a 1400-m-long core covering the entire sediment sequence to the basement.

© 2013 Takemura et al.; licensee InTech. This is an open access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2013 Takemura et al.; licensee InTech. This is a paper distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

**Figure 1.** Geomorphology and active faults around Lake Biwa (illustrated by D. Ishimura) Surface traces of active faults are from [1]. This map is from 10m DEM of the Geospatial Information Authority of Japan. The bathymetric contour interval in Lake Biwa is 10 m.

This record confirmed that the basin is filled with lacustrine and fluvial sediments about 800 m thick with a ca 100 m thick pebbles and cobbles layer resembling debris flow deposits piled on Mesozoic- Paleozoic basement rocks [5,6]. Sediments were divided into five units based on differences in predominant grain-size distributions [9, 10]. These units have been named the P (ca 100-m-thick pebble and cobble layer) and fluvial and lacustrine sediments (Q, R, S, and T beds) from deepest to most shallow. The Q bed is a 72.3m thick unit (731.8-804.1 m below lake floor, mblf) composed of alternating layers of sand, gravel, and silt. The R bed is 149.9 m thick (581.9-731.8 mblf) and is considered to be continuous with the S Bed above it. Subunits of bluish gray nonlaminated clay and of layers of silt, sand, and sandy gravel alternate at approximately 10 m intervals throughout this unit. The S bed is 332.4 m thick (249.5-581.9 mblf) and is believed to be continuous with the overlying T Bed. It consists of thin alternations of sands and silts interspersed with sandy gravels. The T bed is 249.5 m thick (0-249.5 mblf) and is composed of bluish gray, nonlaminated clay. They contain 54 layers of volcanic ash inter‐ calated throughout them [5]. The uppermost unit (T bed) was estimated as having been deposited continuously during the last 430 ka [5,11] (Figure 3). A horizon in Figure 3 is correlated with the bottom of T bed. In 1986, additional samples of 141-m-thick sediment were recovered about 5 km northeast of older drilling sites (Figure 1; [7]). Although the neighboring (ca. 20 km) basin of Lake Suigetsu has varved sediments of the past 150 ka[12], Lake Biwa has

continuous sediments of a million years age. Therefore, the two lake basin records together will facilitate understanding of the Quaternary climate and tectonics at annual to orbital time

line 9-1 and 14 are the survey lines of seismic reflection shown in Figure 3 and Figure 8.

**Figure 2.** Map showing locations of principal coring sites in Lake Biwa [4]. BB (□:200 m drilling in 1971; [3]), B1400 (□: 1400 m drilling in 1982-1983; [5, 6]), BT (□: 141 m drilling in 1986; [7]), Site 1, 2, 3 (○:BIW95; Piston cores in 1995; [8]), BIW07-1 to BIW07-6 (●:Piston cores in 2007; [4]), BIW08-A and BIW08-B (⋆:Drilled cores in 2008; [4]). Blue

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Initially, the scientific value of Lake Biwa sediments was not properly acknowledged because the first attempt of fission track dating assigned an incorrect Pliocene age to the basal part. This suggested a markedly crooked sediment accumulation rate curve, casting doubt on the continuity of the Lake Biwa sediment record. In 1993, based on a stratigraphic correlation of the Biwa core with marine data, Meyers et al. [11] reported that the fission track dates were erroneous. Then in 2005, improvements on the fission track timescale identified the paleo‐

scales.

**Figure 2.** Map showing locations of principal coring sites in Lake Biwa [4]. BB (□:200 m drilling in 1971; [3]), B1400 (□: 1400 m drilling in 1982-1983; [5, 6]), BT (□: 141 m drilling in 1986; [7]), Site 1, 2, 3 (○:BIW95; Piston cores in 1995; [8]), BIW07-1 to BIW07-6 (●:Piston cores in 2007; [4]), BIW08-A and BIW08-B (⋆:Drilled cores in 2008; [4]). Blue line 9-1 and 14 are the survey lines of seismic reflection shown in Figure 3 and Figure 8.

This record confirmed that the basin is filled with lacustrine and fluvial sediments about 800 m thick with a ca 100 m thick pebbles and cobbles layer resembling debris flow deposits piled on Mesozoic- Paleozoic basement rocks [5,6]. Sediments were divided into five units based on differences in predominant grain-size distributions [9, 10]. These units have been named the P (ca 100-m-thick pebble and cobble layer) and fluvial and lacustrine sediments (Q, R, S, and T beds) from deepest to most shallow. The Q bed is a 72.3m thick unit (731.8-804.1 m below lake floor, mblf) composed of alternating layers of sand, gravel, and silt. The R bed is 149.9 m thick (581.9-731.8 mblf) and is considered to be continuous with the S Bed above it. Subunits of bluish gray nonlaminated clay and of layers of silt, sand, and sandy gravel alternate at approximately 10 m intervals throughout this unit. The S bed is 332.4 m thick (249.5-581.9 mblf) and is believed to be continuous with the overlying T Bed. It consists of thin alternations of sands and silts interspersed with sandy gravels. The T bed is 249.5 m thick (0-249.5 mblf) and is composed of bluish gray, nonlaminated clay. They contain 54 layers of volcanic ash inter‐ calated throughout them [5]. The uppermost unit (T bed) was estimated as having been deposited continuously during the last 430 ka [5,11] (Figure 3). A horizon in Figure 3 is correlated with the bottom of T bed. In 1986, additional samples of 141-m-thick sediment were recovered about 5 km northeast of older drilling sites (Figure 1; [7]). Although the neighboring (ca. 20 km) basin of Lake Suigetsu has varved sediments of the past 150 ka[12], Lake Biwa has

**Figure 1.** Geomorphology and active faults around Lake Biwa (illustrated by D. Ishimura) Surface traces of active faults are from [1]. This map is from 10m DEM of the Geospatial Information Authority of Japan. The bathymetric contour

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

interval in Lake Biwa is 10 m.

continuous sediments of a million years age. Therefore, the two lake basin records together will facilitate understanding of the Quaternary climate and tectonics at annual to orbital time scales.

Initially, the scientific value of Lake Biwa sediments was not properly acknowledged because the first attempt of fission track dating assigned an incorrect Pliocene age to the basal part. This suggested a markedly crooked sediment accumulation rate curve, casting doubt on the continuity of the Lake Biwa sediment record. In 1993, based on a stratigraphic correlation of the Biwa core with marine data, Meyers et al. [11] reported that the fission track dates were erroneous. Then in 2005, improvements on the fission track timescale identified the paleo‐

**Figure 3.** Multichannel seismic reflection profile along the Line 9-1. The core site is indicated by an arrow. The inset roughly correlates reflectors with the stratigraphic column [5].

magnetic reversal near the base as Jaramillo rather than Olduvai, estimating the time coverage of the Lake Biwa core as ca. 1.5 Ma ([13]; Figure 4). Figure 4 shows the nearly linear sediment accumulation from 0.57m/kyr to 0.60 m/kyr during the sedimentary record in present Lake Biwa, despite the lithologic units are different. The average sedimentation rate is calculated from the data of depth of 695.6 m with about 1211 ka.

sedimentation rates, and (3) to recover the longest possible undisturbed sediment sequence. Analyses of the core samples include paleomagnetism, environmental magnetism, physical properties, organic and inorganic chemistry, pollen analysis and 14C dating. We also demon‐ strated that magnetic susceptibility data are extremely useful to find microscopic tephra horizons, and establish correlation and age assignment of core sediments from different

**Figure 4.** Summary of chronology of core B1400 based on the re-investigation of fission track ages, tephra identifica‐ tion and magnetostratigraphy [13]. Tephra horizons: K-Ah, U-Oki, AT, Aso-4, K-Tz, Ata, Ata-Th, Aso-1, Tky-Ng1, Kkt, Imakuma II, Ss-Pink; Paleomagnetic information: B/M (Brunhes/Matuyama) boundary, top of Jaramillo event, bottom

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Drilling challenges are continuing for high-resolution studies of island arc tectonics. In 2007 and 2008, we obtained six new piston cores covering at least 50 ka, two longer cores covering 300 ka [4], and 300-km-long shallow seismic surveys. Various interdisciplinary analyses are expected to generate high-resolution records of the dynamics of the tectonic convergence. Sato et al. [15] described that two stages indicating shallow lacustrine delta formation were intercalated with sediment cores during 300 ka at the present depth of 50m. This intercalation is important evidence of tectonic deformation of the lake basin from

locations.

coastal area migration.

of Jaramillo event, Cobb mountain event.

This was evidence for the stable sedimentary environment of the basin and was evidence for the suitability of Lake Biwa as a tectonic archive. Moreover, progress in Japanese tephrochro‐ nology in recent decades has enabled the identification of several marker tephras [14] in and around the basin. Lake Biwa is therefore an ideal terrestrial site for exploration of the paleo‐ climate and tectonic history of eastern Asia during at least the past 1.5 Ma.

Although Lake Biwa sediments have been analyzed using various methods, high-resolution studies have not yet been conducted. In most studies, a single core was obtained at a single site. It was therefore difficult to evaluate the completeness of core recovery and the disturbance of core samples. For example, during deep drilling of 1982 and 1983, it is known that rotary coring caused a disturbance of the upper sediment samples. For a detailed study of the sedimentary record, in 1995, we recovered seven piston cores (10-15 m long) at three localities (sites 1, 2, 3) in the northern part of Lake Biwa (Figure 1). We designed the coring plan (1) to take at least two cores from each site; (2) to take cores at three locations having different

**Figure 4.** Summary of chronology of core B1400 based on the re-investigation of fission track ages, tephra identifica‐ tion and magnetostratigraphy [13]. Tephra horizons: K-Ah, U-Oki, AT, Aso-4, K-Tz, Ata, Ata-Th, Aso-1, Tky-Ng1, Kkt, Imakuma II, Ss-Pink; Paleomagnetic information: B/M (Brunhes/Matuyama) boundary, top of Jaramillo event, bottom of Jaramillo event, Cobb mountain event.

magnetic reversal near the base as Jaramillo rather than Olduvai, estimating the time coverage of the Lake Biwa core as ca. 1.5 Ma ([13]; Figure 4). Figure 4 shows the nearly linear sediment accumulation from 0.57m/kyr to 0.60 m/kyr during the sedimentary record in present Lake Biwa, despite the lithologic units are different. The average sedimentation rate is calculated

**Figure 3.** Multichannel seismic reflection profile along the Line 9-1. The core site is indicated by an arrow. The inset

This was evidence for the stable sedimentary environment of the basin and was evidence for the suitability of Lake Biwa as a tectonic archive. Moreover, progress in Japanese tephrochro‐ nology in recent decades has enabled the identification of several marker tephras [14] in and around the basin. Lake Biwa is therefore an ideal terrestrial site for exploration of the paleo‐

Although Lake Biwa sediments have been analyzed using various methods, high-resolution studies have not yet been conducted. In most studies, a single core was obtained at a single site. It was therefore difficult to evaluate the completeness of core recovery and the disturbance of core samples. For example, during deep drilling of 1982 and 1983, it is known that rotary coring caused a disturbance of the upper sediment samples. For a detailed study of the sedimentary record, in 1995, we recovered seven piston cores (10-15 m long) at three localities (sites 1, 2, 3) in the northern part of Lake Biwa (Figure 1). We designed the coring plan (1) to take at least two cores from each site; (2) to take cores at three locations having different

climate and tectonic history of eastern Asia during at least the past 1.5 Ma.

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

from the data of depth of 695.6 m with about 1211 ka.

roughly correlates reflectors with the stratigraphic column [5].

sedimentation rates, and (3) to recover the longest possible undisturbed sediment sequence. Analyses of the core samples include paleomagnetism, environmental magnetism, physical properties, organic and inorganic chemistry, pollen analysis and 14C dating. We also demon‐ strated that magnetic susceptibility data are extremely useful to find microscopic tephra horizons, and establish correlation and age assignment of core sediments from different locations.

Drilling challenges are continuing for high-resolution studies of island arc tectonics. In 2007 and 2008, we obtained six new piston cores covering at least 50 ka, two longer cores covering 300 ka [4], and 300-km-long shallow seismic surveys. Various interdisciplinary analyses are expected to generate high-resolution records of the dynamics of the tectonic convergence. Sato et al. [15] described that two stages indicating shallow lacustrine delta formation were intercalated with sediment cores during 300 ka at the present depth of 50m. This intercalation is important evidence of tectonic deformation of the lake basin from coastal area migration.
