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

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

A

Pumice Tuff

HCS Trough cross stratification

Fu

Fma

Pum

Exo

Legend

Accretionary lapilli

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

Desiccation crack

Mud & sand clast

Parallel stratification

Slumped deposit

B

Ma-tuff

s

C

HCS cgs

0.5 m

Figure 16.Example<\$%&?>of<\$%&?>the<\$%&?>columnar<\$%&?>cross-

Event bed

Lake slope deposit (upper formation)

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

>b:<\$%&?>large<\$%&?>slump<\$%&?>block.

half of the upper formation. b: large slump block.

m s g

Pyroclastic

flow dep.

 (mid. formation)

Fluvio-lacustrine deposit (upper formation)

10 m

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.

section<\$%&?>of<\$%&?>the<\$%&?>upper<\$%&?>formation,<\$%&?>showing<\$%&?>the<\$%&?>boundary<\$%&?>of<\$%&?>the<\$%&?>upper<\$%&

b

b

up<\$%&?>solid<\$%&?>line<\$%&?>indicates<\$%&?>an<\$%&?>erosion<\$%&?>surface.<\$%&?>A:<\$%&?>outcrop<\$%&?>photograph<\$%&?>of<\$% &?>the<\$%&?>boundary<\$%&?>of<\$%&?>the<\$%&?>lower<\$%&?>and<\$%&?>upper<\$%&?>halves<\$%&?>of<\$%&?>the<\$%&?>formation.<\$%&?

stratification,<\$%&?>cgs:<\$%&?>conglomerate<\$%&?>beds,<\$%&?>s:<\$%&?>slope<\$%&?>deposit.<\$%&?>C:<\$%&?>an<\$%&?>example<\$%&?>of< \$%&?>the<\$%&?>slumped<\$%&?>beds<\$%&?>in<\$%&?>the<\$%&?>upper<\$%&?>half<\$%&?>of<\$%&?>the<\$%&?>upper<\$%&?>formation.<\$%&?

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

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<\$%

?>and<\$%&?>lower<\$%&?>halves<\$%&?>of<\$%&?>the<\$%&?>upper<\$%&?>formation.<\$%&?>A<\$%&?>concave-

Ma

m s g

Slumped deposit

*Nakalipithecus*

>B:<\$%&?>close-up<\$%&?>photograph<\$%&?>of<\$%&?>the<\$%&?>boundary.<\$%&?>HCS:<\$%&?>hummocky<\$%&?>cross-

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

**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 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 subsidence within the basin [42].

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

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 case of the upper formation.

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 because of smaller fault displacement.

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.
