**3.1. Effects of supply mainly by pyroclastic fall on stratigraphic architecture**

Samburu Hills provide a good example of a basin that was strongly controlled by sediment supply from pyroclastic fall. The target basin did not seem to experience a complicated tectonic history during the Namurungule phase (interaction with another basin, such as a basin merger) like other examples, so it is a suitable place to discuss the contribution of fine volcaniclastics supplied by falls or streams on stratigraphic architectures. Because the border fault of this basin runs along the centre of the rift basin, sufficient sediment supply from the footwall slope would not have been expected, and the basin should have been starved in terms of sediment supply (particularly siliciclastic sediments). However, the supply by pyroclastic fall or by streams that transported reworked pyroclastic fall sediments to the lake contributed to the high rate of sedimentation. The total thickness of the lake deposit (TST) at the southern end of the study area becomes almost double that at the northern end of the basin. This suggests that the newly formed accommodation space was rapidly filled even near the basin centre.

The presence of different systems tracts within a half-graben in the same period was expected on the basis of computer simulations [18]. The study simulated marine basins, but its results are also applicable to continental basins, except for a different response of the lake- or sea-level changes compared with the tectonic subsidence (see [21]). As expected in [18], a high rate of sediment supply might have resulted in a progradational stacking pattern in the northern end of the target basin, where the subsidence rate was small. The absence of the progradational unit in this place can be explained by dispersion of the eroded sediments into the basin due to the larger mobility of fine volcaniclastics. However, we need more tests to evaluate the effect of the higher mobility of volcaniclastics compared with siliciclastic sediments on the strati‐ graphic architecture.

Another two basin sediments (Koura and Nakali Formations) were dominated by volcani‐ clastics, and show high sedimentation rates [44-45]. The high-resolution tectonics related to basin evolution are discussed as follows.

## **3.2. Record of basin mergers**

Both Koura and Nakali Formations record that terrestrial or shallow lake environments were finally changed to deep-water environments (Figures. 4 and 14) after several periods of rapid environmental change. As mentioned in Sakai et al. (2013), it is highly possible that the Koura Formation experienced at least two periods of outburst floods and subsequent lake-level rise as a result of merging basins.

The major flooding surface of the upper Nakali Formation is also interpreted as having been associated with a basin merger event. The hummocky cross-stratified beds and conglomeratic sandstone interbeds just below the flooding surface may be a record of strong waves and currents just before this basin was deeply submerged (Figure 16). Another basin merger event is expected to have occurred when the subsidence centre jumped from the western to eastern part of the central block around the deposition of the Pum Tuff bed. However, distinct evidence of basin merger cannot be found in the sediments. This was probably because of lower topographic relief in the accommodation zone (Figure 1), which was not high enough to cause the major shift in lake water when two basins were merged. Figure<\$%&?>1),<\$%&?>which<\$%&?>was<\$%&?>not<\$%&?>high<\$%&?>enough<\$%&?>to<\$%&?>cause<\$%&?>the<\$%&?>major

<\$%&?>shift<\$%&?>in<\$%&?>lake<\$%&?>water<\$%&?>when<\$%&?>two<\$%&?>basins<\$%&?>were<\$%&?>merged.

a rapid deepening event. Both basins finally submerged into the Japan Sea or deep lake in short periods, implying a high subsidence rate in these basins. Therefore, the tectonic merger of the basins (i.e. connection of border faults of adjacent basins) is strongly expected for these cases. Because the Japan Sea was opened rapidly during the middle Miocene, evidence of such basin

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

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

103

In the Namurungule and Koura Formations, sediment cycles appear in their upper parts[34, 44]. Similar types of cycles have been reported from other areas, and some of the cycle formation was explained simply by migration of the fluvial system ([47]). Strong pulse of pyroclastic sediment supply could form small cycles as well. The Namurungule case, thick‐ ening of individual cycles to the west, indicates that the cycle formation is controlled by

The Koura Formation example shown here is only a one-dimensional section, and is not enough to discuss the origin of the cycles. However, some of the erosion surface formation is clearly associated with tectonics. The shallower facies covering basal cycle surfaces without sedimentation gaps (Figures 6B and 6C) implies a lake-level fall induced by a relative uplift against the basin centre around the measured section. Although it is impossible to know the quantity of the relative uplift, the estimated uplift might be a few metres on the basis of the facies gap above and below the surface. The formation of the flooding surface and some of the

On the contrary, both formations do not contain such cycles in their lower and middle parts. The lower parts of both formations, however, show evidence of small-scale sliding in the sediments (Figures 7 and 11). There is a small gap in the environment above and below the slide interval of the lower Koura Formation, indicating that a small-scale subsidence occurred. However, the subsidence was not of sufficient amplitude to form a cycle boundary like the

This matches with the general understanding of the rift basin evolution, where the displace‐ ment of the border fault becomes larger through the basin enlargement (for example, [14, 16]). The absence of the poorly developed drainage system also contributed to the absence of sediment cycles in case of the Namurungule Formation, because streams do not have enough strength to form an erosion surface when the relative uplift occurred. Therefore, the earliest phase of the rifting is not favourable for generating small sediment cycles related to tectonics

The Nakali Formation does not contain such small sediment cycles, which indicates that the uplift or subsidence associated with fault displacement was not distinct in this place. Because the area we observed may have been situated near the accommodation zone when the upper formation was deposited, the fault displacement causing subsidence/uplift may have been

smaller than that near the basin centre and was not enough to form sediment cycles.

mergers is expected to be found from many basins along it.

cycle boundaries may be related to eustatic sea-level rise and fall.

subsidence within the basin [42].

case of the upper formation.

because of smaller fault displacement.

**3.3. Appearance of cycles in the upper Koura and Namurungule formations**

Figure 16.Example<\$%&?>of<\$%&?>the<\$%&?>columnar<\$%&?>crosssection<\$%&?>of<\$%&?>the<\$%&?>upper<\$%&?>formation,<\$%&?>showing<\$%&?>the<\$%&?>boundary<\$%&?>of<\$%&?>the<\$%&?>upper<\$%& ?>and<\$%&?>lower<\$%&?>halves<\$%&?>of<\$%&?>the<\$%&?>upper<\$%&?>formation.<\$%&?>A<\$%&?>concaveup<\$%&?>solid<\$%&?>line<\$%&?>indicates<\$%&?>an<\$%&?>erosion<\$%&?>surface.<\$%&?>A:<\$%&?>outcrop<\$%&?>photograph<\$%&?>of<\$% &?>the<\$%&?>boundary<\$%&?>of<\$%&?>the<\$%&?>lower<\$%&?>and<\$%&?>upper<\$%&?>halves<\$%&?>of<\$%&?>the<\$%&?>formation.<\$%&? >B:<\$%&?>close-up<\$%&?>photograph<\$%&?>of<\$%&?>the<\$%&?>boundary.<\$%&?>HCS:<\$%&?>hummocky<\$%&?>cross-**Figure 16.** Example of the columnar cross-section of the upper formation, showing the boundary of the upper and lower halves of the upper formation. A concave-up solid line indicates an erosion surface. A: outcrop photograph of the boundary of the lower and upper halves of the formation. B: close-up photograph of the boundary. HCS: hum‐ mocky cross-stratification, cgs: conglomerate beds, s: slope deposit. C: an example of the slumped beds in the upper half of the upper formation. b: large slump block.

stratification,<\$%&?>cgs:<\$%&?>conglomerate<\$%&?>beds,<\$%&?>s:<\$%&?>slope<\$%&?>deposit.<\$%&?>C:<\$%&?>an<\$%&?>example<\$%&?>of<

filled<\$%&?>condition<\$%&?>(see<\$%&?>[19]).<\$%&?>In<\$%&?>the<\$%&?>present<\$%&?>examples<\$%&?>(Koura<\$%&?>and<\$ %&?>Nakali<\$%&?>cases),<\$%&?>each<\$%&?>event<\$%&?>seems<\$%&?>to<\$%&?>have<\$%&?>been<\$%&?>related<\$%&?>to<\$ %&?>the<\$%&?>outburst<\$%&?>flood<\$%&?>and<\$%&?>associated<\$%&?>with<\$%&?>a<\$%&?>rapid<\$%&?>deepening<\$%&?> event.<\$%&?>Both<\$%&?>basins<\$%&?>finally<\$%&?>submerged<\$%&?>into<\$%&?>the<\$%&?>Japan<\$%&?>Sea<\$%&?>or<\$%& ?>deep<\$%&?>lake<\$%&?>in<\$%&?>short<\$%&?>periods,<\$%&?>implying<\$%&?>a<\$%&?>high<\$%&?>subsidence<\$%&?>rate<\$ %&?>in<\$%&?>these<\$%&?>basins.<\$%&?>Therefore,<\$%&?>the<\$%&?>tectonic<\$%&?>merger<\$%&?>of<\$%&?>the<\$%&?>basin s<\$%&?>(i.e.<\$%&?>connection<\$%&?>of<\$%&?>border<\$%&?>faults<\$%&?>of<\$%&?>adjacent<\$%&?>basins)<\$%&?>is<\$%&?>st rongly<\$%&?>expected<\$%&?>for<\$%&?>these<\$%&?>cases.<\$%&?>Because<\$%&?>the<\$%&?>Japan<\$%&?>Sea<\$%&?>was<\$%

\$%&?>the<\$%&?>slumped<\$%&?>beds<\$%&?>in<\$%&?>the<\$%&?>upper<\$%&?>half<\$%&?>of<\$%&?>the<\$%&?>upper<\$%&?>formation.<\$%&? >b:<\$%&?>large<\$%&?>slump<\$%&?>block. The<\$%&?>process<\$%&?>of<\$%&?>the<\$%&?>basin<\$%&?>merger<\$%&?>and<\$%&?>related<\$%&?>basin<\$%&?>fill<\$%&?>ha s<\$%&?>been<\$%&?>modelled<\$%&?>in<\$%&?>some<\$%&?>previous<\$%&?>studies<\$%&?>[4,<\$%&?>16],<\$%&?>which<\$%&?> emphasized<\$%&?>the<\$%&?>hydraulic<\$%&?>connection<\$%&?>between<\$%&?>two<\$%&?>adjacent<\$%&?>basins<\$%&?>afte The process of the basin merger and related basin fill has been modelled in some previous studies [4, 16], which emphasized the hydraulic connection between two adjacent basins after one basin reached the over-filled condition (see [19]). In the present examples (Koura and Nakali cases), each event seems to have been related to the outburst flood and associated with

r<\$%&?>one<\$%&?>basin<\$%&?>reached<\$%&?>the<\$%&?>over-

a rapid deepening event. Both basins finally submerged into the Japan Sea or deep lake in short periods, implying a high subsidence rate in these basins. Therefore, the tectonic merger of the basins (i.e. connection of border faults of adjacent basins) is strongly expected for these cases. Because the Japan Sea was opened rapidly during the middle Miocene, evidence of such basin mergers is expected to be found from many basins along it.
