**1. Introduction**

There are numerous stratigraphic studies regarding rift valley fill successions. The major understanding of the rift basins' filling process was obtained from the Basin and Range regions and Rio Grande Rift, USA (for example, [1-4]); East African Rift Valley (for example, [5-7]); Suez Rift, Egypt (for example, [8-10]); Corinth Basin, Greece (for example, [11-13]) and so on. The initial stage of the rift basin evolution is characterized by the development of a series of small half-grabens. Basins become larger through the linkage of border faults of individual half-grabens [14]. Even after basin mergers, topographic lows of footwall among basins (accommodation zones before basin mergers) play an important role for sediment supply. The relay ramp developed between two normal faults dipping in the same direction (Figure 1), and its evolution, is crucial because it acts as the major entry point of the water and sediments to the basins [14-15]. The manner of sediment entry to the basins and the subsidence pattern strongly affect the architecture of basin-fills (for example, [16-17]), resulting in the formation of different systems tracts in different places within a basin at a given time [18]. In case of continental rift basins with lakes, the strata formation is much more complicated than in marine basins (for example [19-20]). The differences in sedimentation process between lake and marine basins are summarized in [21], suggesting that the terrestrial basin fills are not miniature marine basins because there were different amplitude base-level variations, linkage of climate

© 2013 Sakai 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 Sakai 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.

and sediment supply and so on. Pre-rift basement structures also affect the evolution of the basin as well as fills of the early rift basins (for example, [13, 22-23]).

indicate the direction of normal fault displacement. LRAZ: Low relief accommodation zone, HRAZ: High relief accommodation zone, SSAZ: Strikeslip accommodation zone, RR: Relay ramp. However, studies of the rift basin fills with active volcanism have been limited and their basin-fill processes are poorly understood **Figure 1.** Various types of linked half-grabens and characteristics of accommodation zones modified from Figure 6a-c in Rosendhal (1987). Arrows indicate the direction of normal fault displacement. LRAZ: Low relief accommodation zone, HRAZ: High relief accommodation zone, SSAZ: Strike-slip accommodation zone, RR: Relay ramp.

Figure 1. Various types of linked half-grabens and characteristics of accommodation zones modified from Figure 6a-c in Rosendhal (1987). Arrows

However, studies of the rift basin fills with active volcanism have been limited and their basinfill processes are poorly understood (see [24]). Pyroclastic fall may supply sediments from the air nearly evenly within a basin if the basin size is small relative to the pyroclastic fall area. The reworked volcaniclastics (mainly ash) supplied via rivers can be more widely spread in the lake than is the case of the siliciclastic system. This is because of smaller grain density (for example [25]), resulting in faster sedimentation even in parts of a basin starved of sediments transported by streams. Such faster sedimentation may provide opportunities to decode the high-resolution tectonic and basin-fill history through the reconstruction of environmental changes. Some examples of the basin-fill successions affected strongly by sediment supply via pyroclastic fall are, therefore, shown to discuss the evolution of the early rift basin fills. Early rift basins are expected to experience a complicated history in association with merging small basins when border fault tips propagate laterally to the next basin [14] or when one basin is filled out and sediments and water spill over to the next basins beyond the accommodation zone [16]. Examples of studies discussing such events are also limited to a small number [4]. The basins filled rapidly with pyroclastic fall are suitable for detecting such basin-merging

Early Continental Rift Basin Stratigraphy, Depositional Facies and Tectonics in Volcaniclastic System…

http://dx.doi.org/10.5772/56804

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The Miocene successions exposed along the Japan Sea on the Japanese main island contain the early rift basin fills, which were formed when the Japan Sea was opened; they are now exposed on the land because of tectonic inversion (for example, [26]). One of the basin fills was targeted in this study—the Miocene Koura Formation, exposed in SW Japan. Other targets here were the Miocene half-graben fills in Kenya (Namurungule Formation in Samburu Hills and Nakali Formation in Nakali, northern and central Kenya, respectively). The basin fills adjacent to the volcanoes must be strongly affected by the supply of volcaniclastics and lava flow, as well as subsidence/uplift related to volcanism (for example [24, 27]). However, the local volcanorelated tectonics (such as caldera formation) are excluded in this study for simple discussions. Because the centre of the eruption or intrusion of magma during the deposition of the Na‐ murungle and Nakali Formations has not been discovered around the target basins, it is considered that the tectonic subsidence or uplift induced by local volcanisms (see [24, 28]) can be ignored for these cases. The Koura Formation example is unclear for the strong tectonic control from local volcanoes, but its effect can be ignored as well, because lava has not been found and only pyroclastic fall or flow deposits have been described from the formation.

The Miocene successions associated with the Japan Sea opening widely spread along the coastal region of the Japan Sea. The Miocene Koura Formation is exposed in the Shimane Peninsula, on the western part of the main island of Japan (Honshu Island) (Figure 2). The Koura Formation distribution is elongated E–W—which is almost parallel to faults in and around the western Japan Sea (ENE–WSW)—and dips mainly to the north, allowing observa‐ tion of axial facies changes in the basin (Figure 2). The basin fill thickness exceeds 600 m [29-30].

events as well as another type of tectonic events such as subsidence.

**2. Case studies of the early rift basin fills**

**2.1. Koura formation**

(see [24]). Pyroclastic fall may supply sediments from the air nearly evenly within a basin if the basin size is small relative to the pyroclastic fall area. The reworked volcaniclastics (mainly ash) supplied via rivers can be more widely spread in the lake than is the case of the siliciclastic system. This is because of smaller grain density (for example [25]), resulting in faster sedimentation even in parts of a basin starved of sediments transported by streams. Such faster sedimentation may provide opportunities to decode the high-resolution tectonic and basin-fill history through the reconstruction of environmental changes. Some examples of the basin-fill successions affected strongly by sediment supply via pyroclastic fall are, therefore, shown to discuss the evolution of the early rift basin fills. Early rift basins are expected to experience a complicated history in association with merging small basins when border However, studies of the rift basin fills with active volcanism have been limited and their basinfill processes are poorly understood (see [24]). Pyroclastic fall may supply sediments from the air nearly evenly within a basin if the basin size is small relative to the pyroclastic fall area. The reworked volcaniclastics (mainly ash) supplied via rivers can be more widely spread in the lake than is the case of the siliciclastic system. This is because of smaller grain density (for example [25]), resulting in faster sedimentation even in parts of a basin starved of sediments transported by streams. Such faster sedimentation may provide opportunities to decode the high-resolution tectonic and basin-fill history through the reconstruction of environmental changes. Some examples of the basin-fill successions affected strongly by sediment supply via pyroclastic fall are, therefore, shown to discuss the evolution of the early rift basin fills. Early rift basins are expected to experience a complicated history in association with merging small basins when border fault tips propagate laterally to the next basin [14] or when one basin is filled out and sediments and water spill over to the next basins beyond the accommodation zone [16]. Examples of studies discussing such events are also limited to a small number [4]. The basins filled rapidly with pyroclastic fall are suitable for detecting such basin-merging events as well as another type of tectonic events such as subsidence.

The Miocene successions exposed along the Japan Sea on the Japanese main island contain the early rift basin fills, which were formed when the Japan Sea was opened; they are now exposed on the land because of tectonic inversion (for example, [26]). One of the basin fills was targeted in this study—the Miocene Koura Formation, exposed in SW Japan. Other targets here were the Miocene half-graben fills in Kenya (Namurungule Formation in Samburu Hills and Nakali Formation in Nakali, northern and central Kenya, respectively). The basin fills adjacent to the volcanoes must be strongly affected by the supply of volcaniclastics and lava flow, as well as subsidence/uplift related to volcanism (for example [24, 27]). However, the local volcanorelated tectonics (such as caldera formation) are excluded in this study for simple discussions. Because the centre of the eruption or intrusion of magma during the deposition of the Na‐ murungle and Nakali Formations has not been discovered around the target basins, it is considered that the tectonic subsidence or uplift induced by local volcanisms (see [24, 28]) can be ignored for these cases. The Koura Formation example is unclear for the strong tectonic control from local volcanoes, but its effect can be ignored as well, because lava has not been found and only pyroclastic fall or flow deposits have been described from the formation.
