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

## **2.1. Koura formation**

and sediment supply and so on. Pre-rift basement structures also affect the evolution of the

Case A Case B Case C

LRAZ

HRAZ

RR

Case D Case E Case F

LRAZ

HRAZ

Case H Case I

slip accommodation zone, RR: Relay ramp.

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 indicate the direction of normal fault displacement. LRAZ: Low relief accommodation

RR

Case G

SSAZ

(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

basin as well as fills of the early rift basins (for example, [13, 22-23]).

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

LRAZ

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-However, studies of the rift basin fills with active volcanism have been limited and their basin-fill processes are poorly understood 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].

The basement rock of the basin has not been confirmed yet, but granitic or metamorphic rocks are inferred to be the basement on the basis of the gravels contained in the formation [31].

conglomerate interbeds filling small sublacustrine channels developed on the fan delta slope —which were deposited in a blackish lake, as suggested by the presence of *Ostrea* and *Corbicula* fossils and burrows by *Teredo* sp. as well as geochemical data [29, 34, 36-37]. The upper Koura Formation contains hummocky cross-stratified (HCS) sandstone beds, implying that the lake size became wide enough for generating large waves. The sediment supply to the basin was mainly from the east during the deposition of the upper formation [30, 35]. Three thick lapilli tuff beds (T3–T5) are strong tools for correlation within the basin (Figure 3). The chronology of the Koura Formation has been insufficient. Ages of 16–24 Ma were obtained from the upper formation through fission-track dating [38], and the age of the base of the overlying Josoji Formation (i.e. the top of the Koura Formation) was estimated to be 20–18 Ma or younger [39].

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

Black Shale Slumped beds

Lithology and sedimentary structures

Alternation of sandstones & mudstones with local conglomerate Wave ripple lamination Hummocky cross-strat. Pebbly sand (debrite) Graded sand (turbidite)

Alternation of sandstones & mudstones

Andesitic clasts Wave ripples Parallel strat. Graded sand (trubidite)

Sheet or lenticular sandstone with lateral accretion Trough cross & parallel stratification

Trough & parallel stratification

5

6

1 km

Alluvial fan Conglomerate beds

**Figure 3.** Lithostratigraphy of the Koura and lowest Josoji Formations. T3–T5 indicate lapilli tuff beds in the upper

The boundaries of the lower-middle and middle-upper formations are marked by a surface that is then overlain by an up to 10 m sediment interval consisting of cross-stratified sandstone or conglomerate beds (Figure 5). It is interpreted that each cross-stratified interval was deposited from a basin-wide flood-flow incoming from another basin, and subsequent lakelevel rise occurred when this and the other basin were merged [34], on the basis of the following

Josoji Fm

T5

T4

T3

Koura Fm

Middle

Upper

Lower

Koura Formation.

7

Figure 3. Lithostratigraphy of the Koura and lowest Josoji Formations. T3–T5 indicate lapilli tuff beds in the upper Koura Formation.

Josoji Fm

3

Koura Fm

Conglomerate Sandstone Mudstone Andesitic pyroclastic sediments Lapilli Tuff (T3–T5)

Middle

W E

4

Fan delta

Shallow lake and flood plain

Meandering stream, braided stream, flood plain, marsh and shallow lake

Depositional environment

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87

Marine slope

Figure 4. Columnar cross-sections of the Koura Formation (modified from [34]). m: mud, s: sand, g: gravel.

100 m

Lower Legend

The boundaries of the lower-middle and middle-upper formations are marked by a surface that is then overlain by an up to 10 m sediment interval consisting of cross-stratified sandstone or conglomerate beds (Figure 5). It is interpreted that each cross-stratified interval was deposited from a basin-wide flood-flow incoming from another basin, and subsequent lake-level rise occurred when this and the other basin were merged [34], on the basis of the following reasons: (1) absence of a major erosion surface within both cross-stratified intervals and homogeneous lithology imply their deposition within a short period; (2) both the cross-stratified intervals cover terrestrial deposits with tree trunks, and change upward into the alternations of sandstone and mudstone bed and the andesitic volcaniclastic beds containing both pyroclastic fall, turbidite and beds with small-scale slump structures (lake deposit)

T4

?

T3

T5

2

Shale (Josoji Fm) Terrestrial facies (Facies 1 & 2) Sublacustrine channel

> m s g grain size

Trunk Root

Upper

?

1

**Figure 2.** Location and geologic map of eastern Shimane. 1–7 in Figure 4 show the sections.

It is difficult to discuss the basin morphology at the time of deposition because of the limited extent of exposure. However, the seismic cross-sections of this area show the presence of a series of half-grabens under the bottom of the Japan Sea [32], suggesting that the Koura Basin fills a half-graben. Although the border fault of this basin has not been confirmed either, one of the major faults of this region, the Shinji Fault (or Kashima Fault), running just south of the distribution area of the Koura Formation (Figure 2) and acting presently as a right-lateral strikeslip fault [33], is most probably the border fault of the basin.

The Koura Formation consists of three members (Sakai et al., 2013). For simplicity, these three members are referred to as "lower", "middle" and "upper" formations. The lower formation consists of conglomeratic sandstone beds (alluvial fan origin [30]) and the overlying alternation of the sandstone and mudstone beds (meandering and braided streams, floodplain, marsh and shallow lake origin [34]) (Figures 3 and 4). The middle formation consists of andesitic volca‐ niclastics deposited in a shallow (probably fresh) lake and floodplain (Figures. 3 and 4). The sediments in this interval are predominated by those from pyroclastic fall and small-scale gravity flows (Kano, 1991; Sakai et al., 2013). The upper formation is characterized by alter‐ nations of tuffaceous sandstone and mudstone beds (fan delta deposit; Figures 3 and 4) and conglomerate interbeds filling small sublacustrine channels developed on the fan delta slope —which were deposited in a blackish lake, as suggested by the presence of *Ostrea* and *Corbicula* fossils and burrows by *Teredo* sp. as well as geochemical data [29, 34, 36-37]. The upper Koura Formation contains hummocky cross-stratified (HCS) sandstone beds, implying that the lake size became wide enough for generating large waves. The sediment supply to the basin was mainly from the east during the deposition of the upper formation [30, 35]. Three thick lapilli tuff beds (T3–T5) are strong tools for correlation within the basin (Figure 3). The chronology of the Koura Formation has been insufficient. Ages of 16–24 Ma were obtained from the upper formation through fission-track dating [38], and the age of the base of the overlying Josoji Formation (i.e. the top of the Koura Formation) was estimated to be 20–18 Ma or younger [39].

The basement rock of the basin has not been confirmed yet, but granitic or metamorphic rocks are inferred to be the basement on the basis of the gravels contained in the formation [31].

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

Lake Nakaumi

It is difficult to discuss the basin morphology at the time of deposition because of the limited extent of exposure. However, the seismic cross-sections of this area show the presence of a series of half-grabens under the bottom of the Japan Sea [32], suggesting that the Koura Basin fills a half-graben. Although the border fault of this basin has not been confirmed either, one of the major faults of this region, the Shinji Fault (or Kashima Fault), running just south of the distribution area of the Koura Formation (Figure 2) and acting presently as a right-lateral strike-

The Koura Formation consists of three members (Sakai et al., 2013). For simplicity, these three members are referred to as "lower", "middle" and "upper" formations. The lower formation consists of conglomeratic sandstone beds (alluvial fan origin [30]) and the overlying alternation of the sandstone and mudstone beds (meandering and braided streams, floodplain, marsh and shallow lake origin [34]) (Figures 3 and 4). The middle formation consists of andesitic volca‐ niclastics deposited in a shallow (probably fresh) lake and floodplain (Figures. 3 and 4). The sediments in this interval are predominated by those from pyroclastic fall and small-scale gravity flows (Kano, 1991; Sakai et al., 2013). The upper formation is characterized by alter‐ nations of tuffaceous sandstone and mudstone beds (fan delta deposit; Figures 3 and 4) and

Sections in Fig. 3

<sup>3</sup> <sup>2</sup> <sup>4</sup> <sup>5</sup>

1

Shinji Fault

10 km

Sakaiminato

<sup>6</sup> <sup>7</sup>

Shimane Peninsula

Yumigahama Peninsula

N

Japan Sea

133°0

Matsue

Legend

Takashibiyama Formation Wakurayama Andesite Neogene intrusive rocks Quaternary

Matsue Formation Fault

slip fault [33], is most probably the border fault of the basin.

Syncline Anticline

**Figure 2.** Location and geologic map of eastern Shimane. 1–7 in Figure 4 show the sections.

Lake Shinji

35°30

Pre-Tertiary granitic rocks Hata Formation Koura Formation Kawai, Kuri & Josoji Fms. Omori & Ushigiri Fms. Fujina & Furue Fms.

Figure 3. Lithostratigraphy of the Koura and lowest Josoji Formations. T3–T5 indicate lapilli tuff beds in the upper Koura Formation. W E **Figure 3.** Lithostratigraphy of the Koura and lowest Josoji Formations. T3–T5 indicate lapilli tuff beds in the upper Koura Formation.

T4 Josoji Fm Upper T5 1 2 3 4 5 6 7 The boundaries of the lower-middle and middle-upper formations are marked by a surface that is then overlain by an up to 10 m sediment interval consisting of cross-stratified sandstone or conglomerate beds (Figure 5). It is interpreted that each cross-stratified interval was deposited from a basin-wide flood-flow incoming from another basin, and subsequent lakelevel rise occurred when this and the other basin were merged [34], on the basis of the following

1 km

Figure 4. Columnar cross-sections of the Koura Formation (modified from [34]). m: mud, s: sand, g: gravel.

100 m

Lower Legend

The boundaries of the lower-middle and middle-upper formations are marked by a surface that is then overlain by an up to 10 m sediment interval consisting of cross-stratified sandstone or conglomerate beds (Figure 5). It is interpreted that each cross-stratified interval was deposited from a basin-wide flood-flow incoming from another basin, and subsequent lake-level rise occurred when this and the other basin were merged [34], on the basis of the following reasons: (1) absence of a major erosion surface within both cross-stratified intervals and homogeneous lithology imply their deposition within a short period; (2) both the cross-stratified intervals cover terrestrial deposits with tree trunks, and change upward into the alternations of sandstone and mudstone bed and the andesitic volcaniclastic beds containing both pyroclastic fall, turbidite and beds with small-scale slump structures (lake deposit)

Koura Fm

Conglomerate Sandstone Mudstone Andesitic pyroclastic sediments Lapilli Tuff (T3–T5)

Middle

T3

Shale (Josoji Fm) Terrestrial facies (Facies 1 & 2) Sublacustrine channel

> m s g grain size

?

Trunk Root

?

Black Shale Slumped beds

Lithology and

Alternation of sandstones & mudstones with local conglomerate Wave ripple lamination Hummocky cross-strat. Pebbly sand (debrite) Graded sand (turbidite)

sedimentary structures

Alternation of sandstones

Sheet or lenticular sandstone with lateral accretion Trough cross & parallel stratification

& mudstones

Andesitic clasts Wave ripples Parallel strat. Graded sand (trubidite)

Josoji Fm

T5

T4

T3

Koura Fm

Middle

Upper

Lower

Figure 3. Lithostratigraphy of the Koura and lowest Josoji Formations. T3–T5 indicate lapilli tuff beds in the upper Koura Formation.

Fan delta

Shallow lake and flood plain

Meandering stream, braided stream, flood plain, marsh and shallow lake

Depositional environment

Marine slope

Figure 4. Columnar cross-sections of the Koura Formation (modified from [34]). m: mud, s: sand, g: gravel. **Figure 4.** Columnar cross-sections of the Koura Formation (modified from [34]). m: mud, s: sand, g: gravel.

reasons: (1) absence of a major erosion surface within both cross-stratified intervals and homogeneous lithology imply their deposition within a short period; (2) both the crossstratified intervals cover terrestrial deposits with tree trunks, and change upward into the alternations of sandstone and mudstone bed and the andesitic volcaniclastic beds containing both pyroclastic fall, turbidite and beds with small-scale slump structures (lake deposit) (Figure 5). The rapid lake-level rise suggests the merger with a basin having a base level higher than that of the Koura Basin. The second event may record the merger of this basin with a marine one to become a blackish lake basin. The boundaries of the lower-middle and middle-upper formations are marked by a surface that is then overlain by an up to 10 m sediment interval consisting of cross-stratified sandstone or conglomerate beds (Figure 5). It is interpreted that each cross-stratified interval was deposited from a basin-wide flood-flow incoming from another basin, and subsequent lake-level rise occurred when this and the other basin were merged [34], on the basis of the following reasons: (1) absence of a major erosion surface within both cross-stratified intervals and homogeneous lithology imply their deposition within a short period; (2) both the cross-stratified intervals cover terrestrial deposits with tree trunks, and change upward into the alternations of sandstone and mudstone bed and the andesitic volcaniclastic beds containing both pyroclastic fall, turbidite and beds with small-scale slump structures (lake deposit)

The top of the Koura Formation is marked by a major flooding surface below the black marine shale of the Josoji Formation. The Josoji Formation is interpreted to consist of sediments of the climax phase of the Japan Sea opening. In the context of sequence stratigraphy, the lower and middle formations are interpreted to be the lowstand systems tract (LST). The base of the upper formation is interpreted to be the first flooding surface and the upper formation is interpreted to be the transgressive systems tract (TST) togeth‐ er with a part of the overlying Josoji Formation.

The upper Koura Formation hosts sediment cycles (Figure 6). The thickness of each cycle ranges from 5 to 20 m. Some of the cycles are bounded by flooding surfaces (see [40]) (Figure 6A). Such cycles mainly consist of sediments with an upward-shallowing trend. The basal flooding surface is covered with a massive mudstone bed (outer shelf equivalent deposit of the fan delta) or an alternation of HCS sandstone and mudstone beds of inner shelf equivalent

deposits of the fan delta [34]. On the other hand, most of the cycle bases are represented by a surface covering a slumped deposit (Figures 6B–D). Each surface is undulating (i.e. erosional) (Figures 6B and 6D), and is then covered with facies beds shallower than those below the surface (Figure 6). Parts of the sediments just above the surface are also dragged into the slumped deposits in some places (Figure 6C), indicating that sediment accumulation above the surface occurred almost simultaneously with slumping. The sediment overlying the surface is then punctuated by the flooding surface (Figure 6), covered by a shallowing-upward succession. The cycle boundaries are, therefore, interpreted to have been formed by relative

interpreted to have been formed by relative uplift of this area at the time of deposition.

(Figure 5). The rapid lake-level rise suggests the merger with a basin having a base level higher than that of the Koura Basin. The

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89

Figure 5. Columnar cross-sections of the event beds. Each event bed is interpreted as having been deposited from an outburst flood associated with a basin merger, followed by a lake-level rise (modified from [34]). A: event beds at the base of the middle Koura Formation. Arrows indicate the palaeoflow direction (up = north). B: event beds at the base of the upper Koura Formation. Both horizons grade upwards into the lake deposits containing turbdite (T) and slumped deposit. See Figure 3 for legend. C: outcrop photograph of a part of the event beds (cross-stratified beds) at the

The top of the Koura Formation is marked by a major flooding surface below the black marine shale of the Josoji Formation. The Josoji Formation is interpreted to consist of sediments of the climax phase of the Japan Sea opening. In the context of sequence stratigraphy, the lower and middle formations are interpreted to be the lowstand systems tract (LST). The base of the upper formation is interpreted to be the first flooding surface and the upper formation is interpreted to be the transgressive systems tract

The upper Koura Formation hosts sediment cycles (Figure 6). The thickness of each cycle ranges from 5 to 20 m. Some of the cycles are bounded by flooding surfaces (see [40]) (Figure 6A). Such cycles mainly consist of sediments with an upward-shallowing trend. The basal flooding surface is covered with a massive mudstone bed (outer shelf equivalent deposit of the fan delta) or an alternation of HCS sandstone and mudstone beds of inner shelf equivalent deposits of the fan delta [34]. On the other hand, most of the cycle bases are represented by a surface covering a slumped deposit (Figures 6B–D). Each surface is undulating (i.e. erosional) (Figures 6B and 6D), and is then covered with facies beds shallower than those below the surface (Figure 6). Parts of the sediments just above the surface are also dragged into the slumped deposits in some places (Figure 6C), indicating that sediment accumulation above the surface occurred almost simultaneously with slumping. The sediment overlying the surface is then punctuated by the flooding surface (Figure 6), covered by a shallowing-upward succession. The cycle boundaries are, therefore,

base of the middle formation. D: photograph showing the wide view of the event beds at the base of the upper Koura Formation.

second event may record the merger of this basin with a marine one to become a blackish lake basin.

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

C

T

D

0.5 m

1 m

**Figure 5.** Columnar cross-sections of the event beds. Each event bed is interpreted as having been deposited from an outburst flood associated with a basin merger, followed by a lake-level rise (modified from [34]). A: event beds at the base of the middle Koura Formation. Arrows indicate the palaeoflow direction (up = north). B: event beds at the base of the upper Koura Formation. Both horizons grade upwards into the lake deposits containing turbdite (T) and slump‐ ed deposit. See Figure 7 for legend. C: outcrop photograph of a part of the event beds (cross-stratified beds) at the base of the middle formation. D: photograph showing the wide view of the event beds at the base of the upper Koura

uplift of this area at the time of deposition.

(TST) together with a part of the overlying Josoji Formation.

m s g

1 m

1 m

Formation.

m s g

C

D

<sup>A</sup> <sup>B</sup>

T

(Figure 5). The rapid lake-level rise suggests the merger with a basin having a base level higher than that of the Koura Basin. The Early Continental Rift Basin Stratigraphy, Depositional Facies and Tectonics in Volcaniclastic System… http://dx.doi.org/10.5772/56804 89

second event may record the merger of this basin with a marine one to become a blackish lake basin.

Figure 5. Columnar cross-sections of the event beds. Each event bed is interpreted as having been deposited from an outburst flood associated with a basin merger, followed by a lake-level rise (modified from [34]). A: event beds at the base of the middle Koura Formation. Arrows indicate the palaeoflow direction (up = north). B: event beds at the base of the upper Koura Formation. Both horizons grade upwards into the lake deposits containing turbdite (T) and slumped deposit. See Figure 3 for legend. C: outcrop photograph of a part of the event beds (cross-stratified beds) at the base of the middle formation. D: photograph showing the wide view of the event beds at the base of the upper Koura Formation. The top of the Koura Formation is marked by a major flooding surface below the black marine shale of the Josoji Formation. The **Figure 5.** Columnar cross-sections of the event beds. Each event bed is interpreted as having been deposited from an outburst flood associated with a basin merger, followed by a lake-level rise (modified from [34]). A: event beds at the base of the middle Koura Formation. Arrows indicate the palaeoflow direction (up = north). B: event beds at the base of the upper Koura Formation. Both horizons grade upwards into the lake deposits containing turbdite (T) and slump‐ ed deposit. See Figure 7 for legend. C: outcrop photograph of a part of the event beds (cross-stratified beds) at the base of the middle formation. D: photograph showing the wide view of the event beds at the base of the upper Koura Formation.

Josoji Formation is interpreted to consist of sediments of the climax phase of the Japan Sea opening. In the context of sequence

punctuated by the flooding surface (Figure 6), covered by a shallowing-upward succession. The cycle boundaries are, therefore,

reasons: (1) absence of a major erosion surface within both cross-stratified intervals and homogeneous lithology imply their deposition within a short period; (2) both the crossstratified intervals cover terrestrial deposits with tree trunks, and change upward into the alternations of sandstone and mudstone bed and the andesitic volcaniclastic beds containing both pyroclastic fall, turbidite and beds with small-scale slump structures (lake deposit) (Figure 5). The rapid lake-level rise suggests the merger with a basin having a base level higher than that of the Koura Basin. The second event may record the merger of this basin with a

The boundaries of the lower-middle and middle-upper formations are marked by a surface that is then overlain by an up to 10 m sediment interval consisting of cross-stratified sandstone or conglomerate beds (Figure 5). It is interpreted that each cross-stratified interval was deposited from a basin-wide flood-flow incoming from another basin, and subsequent lake-level rise occurred when this and the other basin were merged [34], on the basis of the following reasons: (1) absence of a major erosion surface within both cross-stratified intervals and homogeneous lithology imply their deposition within a short period; (2) both the cross-stratified intervals cover terrestrial deposits with tree trunks, and change upward into the alternations of sandstone and mudstone bed and the andesitic volcaniclastic beds containing both pyroclastic fall, turbidite and beds with small-scale slump structures (lake deposit)

T4

?

T3

T5

2

Shale (Josoji Fm) Terrestrial facies (Facies 1 & 2) Sublacustrine channel

> m s g grain size

Trunk Root

Upper

?

1

Figure 4. Columnar cross-sections of the Koura Formation (modified from [34]). m: mud, s: sand, g: gravel.

**Figure 4.** Columnar cross-sections of the Koura Formation (modified from [34]). m: mud, s: sand, g: gravel.

100 m

Lower Legend

Figure 3. Lithostratigraphy of the Koura and lowest Josoji Formations. T3–T5 indicate lapilli tuff beds in the upper Koura Formation.

Josoji Fm

3

Koura Fm

Conglomerate Sandstone Mudstone Andesitic pyroclastic sediments Lapilli Tuff (T3–T5)

Middle

W E

4

Fan delta

Shallow lake and flood plain

Meandering stream, braided stream, flood plain, marsh and shallow lake

Depositional environment

Marine slope

The top of the Koura Formation is marked by a major flooding surface below the black marine shale of the Josoji Formation. The Josoji Formation is interpreted to consist of sediments of the climax phase of the Japan Sea opening. In the context of sequence stratigraphy, the lower and middle formations are interpreted to be the lowstand systems tract (LST). The base of the upper formation is interpreted to be the first flooding surface and the upper formation is interpreted to be the transgressive systems tract (TST) togeth‐

The upper Koura Formation hosts sediment cycles (Figure 6). The thickness of each cycle ranges from 5 to 20 m. Some of the cycles are bounded by flooding surfaces (see [40]) (Figure 6A). Such cycles mainly consist of sediments with an upward-shallowing trend. The basal flooding surface is covered with a massive mudstone bed (outer shelf equivalent deposit of the fan delta) or an alternation of HCS sandstone and mudstone beds of inner shelf equivalent

marine one to become a blackish lake basin.

Black Shale Slumped beds

Lithology and

Alternation of sandstones & mudstones with local conglomerate Wave ripple lamination Hummocky cross-strat. Pebbly sand (debrite) Graded sand (turbidite)

sedimentary structures

Alternation of sandstones

Sheet or lenticular sandstone with lateral accretion Trough cross & parallel stratification

Trough & parallel stratification

5

6

1 km

Alluvial fan Conglomerate beds

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

& mudstones

Andesitic clasts Wave ripples Parallel strat. Graded sand (trubidite)

Josoji Fm

T5

T4

T3

Koura Fm

Middle

Upper

Lower

7

er with a part of the overlying Josoji Formation.

deposits of the fan delta [34]. On the other hand, most of the cycle bases are represented by a surface covering a slumped deposit (Figures 6B–D). Each surface is undulating (i.e. erosional) (Figures 6B and 6D), and is then covered with facies beds shallower than those below the surface (Figure 6). Parts of the sediments just above the surface are also dragged into the slumped deposits in some places (Figure 6C), indicating that sediment accumulation above the surface occurred almost simultaneously with slumping. The sediment overlying the surface is then punctuated by the flooding surface (Figure 6), covered by a shallowing-upward succession. The cycle boundaries are, therefore, interpreted to have been formed by relative uplift of this area at the time of deposition. stratigraphy, the lower and middle formations are interpreted to be the lowstand systems tract (LST). The base of the upper formation is interpreted to be the first flooding surface and the upper formation is interpreted to be the transgressive systems tract (TST) together with a part of the overlying Josoji Formation. The upper Koura Formation hosts sediment cycles (Figure 6). The thickness of each cycle ranges from 5 to 20 m. Some of the cycles are bounded by flooding surfaces (see [40]) (Figure 6A). Such cycles mainly consist of sediments with an upward-shallowing trend. The basal flooding surface is covered with a massive mudstone bed (outer shelf equivalent deposit of the fan delta) or an alternation of HCS sandstone and mudstone beds of inner shelf equivalent deposits of the fan delta [34]. On the other hand, most of the cycle bases are represented by a surface covering a slumped deposit (Figures 6B–D). Each surface is undulating (i.e. erosional) (Figures 6B and 6D), and is then covered with facies beds shallower than those below the surface (Figure 6). Parts of the sediments just above the surface are also dragged into the slumped deposits in some places (Figure 6C), indicating that sediment accumulation above the surface occurred almost simultaneously with slumping. The sediment overlying the surface is then

interpreted to have been formed by relative uplift of this area at the time of deposition.

ES: erosion surface, FS: flooding surface, FBS: flooding-surface bounded cycle, H: hummocky cross-straification, W: wave ripple lamination, O: fan delta slope environment equivalent to outer shelf, I: fan delta slope environment equivalent to inner shelf, S: shoreface, F: fluvial.

Figure 6. Columnar cross-section of a part of the upper Koura Formation and changes in depositional environment. A: a sediment cycle showing

Such sediment cycles were not identified in the lower and middle formations. The detailed outcrop observations in the lower formation revealed that either the top or the base of the sandstone intervals (fluvial channel facies) is marked by a surface associated with minor sliding (Figure 7); the former case is the most common (Figure 7). The surface is then covered with a thin poorly sorted silty sandstone bed (up to 0.1 m thick)(Figure 7A). Some of the silty sandstone beds contain pebble-sized sandstone or mudstone clasts of the underlying beds (Figure 7). In some places, the very small syndepositional faults extending almost parallel to the bedding plane are recognized below the silty sandstone beds. The silty sandstone beds are then covered with a massive or a laminated mudstone bed of a small and shallow lake origin (Figure 7B), showing a lake-level rise immediately after the sliding event. The slide may have

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

been associated with subsidence of the basin.

Figure 7. Example<\$%&?>of<\$%&?>the<\$%&?>columnar<\$%&?>cross-

A

Tree trunk Mud clasts

Tuff Pumice

Legend

B

>long.<\$%&?>B:<\$%&?>close-

**y** 

3 m

section<\$%&?>of<\$%&?>the<\$%&?>lower<\$%&?>Koura<\$%&?>Formation<\$%&?>taken<\$%&?>along<\$%&?>section<\$%&?>6.<\$%&?>The<\$%&?>a

5 cm

s

m

m

Parallel strat. Trough cross-strat. Convolute str.

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91

HCS

scale<\$%&?>slide.<\$%&?>A:<\$%&?>outcrop<\$%&?>view<\$%&?>of<\$%&?>the<\$%&?>fluvial<\$%&?>deposit.<\$%&?>The<\$%&?>arrow<\$%&?>indi cates<\$%&?>the<\$%&?>horizon<\$%&?>of<\$%&?>Figure<\$%&?>7B.<\$%&?>The<\$%&?>scale<\$%&?>(hammer)<\$%&?>is<\$%&?>0.3<\$%&?>m<\$%&?

up<\$%&?>photograph<\$%&?>of<\$%&?>the<\$%&?>top<\$%&?>of<\$%&?>the<\$%&?>fluvial<\$%&?>channel<\$%&?>fill<\$%&?>sandstone<\$%&?>bed s.<\$%&?>The<\$%&?>dotted<\$%&?>white<\$%&?>line<\$%&?>indicates<\$%&?>the<\$%&?>surface<\$%&?>of<\$%&?>the<\$%&?>slide,<\$%&?>which< \$%&?>is<\$%&?>then<\$%&?>overlain<\$%&?>by<\$%&?>a<\$%&?>silty<\$%&?>sandstone<\$%&?>bed<\$%&?>with<\$%&?>abundant<\$%&?>sand<\$% &?>clasts<\$%&?>originating<\$%&?>from<\$%&?>the<\$%&?>underlying<\$%&?>sandstone<\$%&?>bed.<\$%&?>The<\$%&?>scale<\$%&?>(a<\$%&?>pa rt<\$%&?>of<\$%&?>the<\$%&?>hammer<\$%&?>head)<\$%&?>is<\$%&?>0.05<\$%&?>m<\$%&?>long.<\$%&?>m:<\$%&?>massive<\$%&?>sandstone<\$%

**2.2.<\$%&?>Examples<\$%&?>from<\$%&?>the<\$%&?>East<\$%&?>Africa<\$%&?>rift<\$%&?>valle**

fill<\$%&?>successions<\$%&?>are<\$%&?>exposed<\$%&?>(Figure<\$%&?>8).<\$%&?>The<\$%&?>activity<\$%&?>of<\$%&?>the<\$%&?

rrows<\$%&?>indicate<\$%&?>the<\$%&?>horizons<\$%&?>showing<\$%&?>evidence<\$%&?>of<\$%&?>a<\$%&?>small-

**Figure 7.** Example of the columnar cross-section of the lower Koura Formation taken along section 6. The arrows indi‐ cate the horizons showing evidence of a small-scale slide. A: outcrop view of the fluvial deposit. The arrow indicates the horizon of Figure 7B. The scale (hammer) is 0.3 m long. B: close-up photograph of the top of the fluvial channel fill sandstone beds. The dotted white line indicates the surface of the slide, which is then overlain by a silty sandstone bed with abundant sand clasts originating from the underlying sandstone bed. The scale (a part of the hammer head) is

&?>bed,<\$%&?>s:<\$%&?>small<\$%&?>lake<\$%&?>deposit,<\$%&?>HCS:<\$%&?>hummocky<\$%&?>cross-stratification.

In<\$%&?>the<\$%&?>Kenya<\$%&?>Rift,<\$%&?>the<\$%&?>Miocene<\$%&?>rift<\$%&?>basin-

0.05 m long. m: massive sandstone bed, s: shallow lake deposit, HCS: hummocky cross-stratification.

an upward-shallowing trend. The cycle base is represented by a flooding surface. B: an erosion surface truncating the inner shelf equivalent deposit, and is then covered with HCS sandstone beds of the shoreface origin. s.b.: slump block. Note the hammer (circled) for the scale. C: a close-up photograph of the sediment just below the erosion surface (cycle boundary). There are several slip surfaces of the slump in the sediments. Coarsesediment grains, which can be found only above the surface, are also incorporated (probably dragged) into the slumped horizon. The scale is 0.2 m long. D: a basal erosion surface of the cycle, truncating the inner-shelf-equivalent deposit and being covered with fluvial channel deposit. The white arrow indicates the surface. See Figure 6 for the legend of the columnar section. Such sediment cycles were not identified in the lower and middle formations. The detailed outcrop observations in the lower formation revealed that either the top or the base of the sandstone intervals (fluvial channel facies) is marked by a surface associated with minor sliding (Figure 7); the former case is the most common (Figure 7). The surface is then covered with a thin **Figure 6.** Columnar cross-section of a part of the upper Koura Formation and changes in depositional environment. A: a sediment cycle showing an upward-shallowing trend. The cycle base is represented by a flooding surface. B: an ero‐ sion surface truncating the inner shelf equivalent deposit, and is then covered with HCS sandstone beds of the shore‐ face origin. s.b.: slump block. Note the hammer for the scale. C: a close-up photograph of the sediment just below the erosion surface (cycle boundary). There are several slip surfaces of the slump in the sediments. Coarse-sediment grains, which can be found only above the surface, are also incorporated (probably dragged) into the slumped horizon. The scale is 0.2 m long. D: a basal erosion surface of the cycle, truncating the inner-shelf-equivalent deposit and being cov‐ ered with fluvial channel deposit. The white arrow indicates the surface. See Figure 7 for the legend of the columnar section.

Such sediment cycles were not identified in the lower and middle formations. The detailed outcrop observations in the lower formation revealed that either the top or the base of the sandstone intervals (fluvial channel facies) is marked by a surface associated with minor sliding (Figure 7); the former case is the most common (Figure 7). The surface is then covered with a thin poorly sorted silty sandstone bed (up to 0.1 m thick)(Figure 7A). Some of the silty sandstone beds contain pebble-sized sandstone or mudstone clasts of the underlying beds (Figure 7). In some places, the very small syndepositional faults extending almost parallel to the bedding plane are recognized below the silty sandstone beds. The silty sandstone beds are then covered with a massive or a laminated mudstone bed of a small and shallow lake origin (Figure 7B), showing a lake-level rise immediately after the sliding event. The slide may have been associated with subsidence of the basin.

section<\$%&?>of<\$%&?>the<\$%&?>lower<\$%&?>Koura<\$%&?>Formation<\$%&?>taken<\$%&?>along<\$%&?>section<\$%&?>6.<\$%&?>The<\$%&?>a rrows<\$%&?>indicate<\$%&?>the<\$%&?>horizons<\$%&?>showing<\$%&?>evidence<\$%&?>of<\$%&?>a<\$%&?>smallscale<\$%&?>slide.<\$%&?>A:<\$%&?>outcrop<\$%&?>view<\$%&?>of<\$%&?>the<\$%&?>fluvial<\$%&?>deposit.<\$%&?>The<\$%&?>arrow<\$%&?>indi cates<\$%&?>the<\$%&?>horizon<\$%&?>of<\$%&?>Figure<\$%&?>7B.<\$%&?>The<\$%&?>scale<\$%&?>(hammer)<\$%&?>is<\$%&?>0.3<\$%&?>m<\$%&? >long.<\$%&?>B:<\$%&?>closeup<\$%&?>photograph<\$%&?>of<\$%&?>the<\$%&?>top<\$%&?>of<\$%&?>the<\$%&?>fluvial<\$%&?>channel<\$%&?>fill<\$%&?>sandstone<\$%&?>bed s.<\$%&?>The<\$%&?>dotted<\$%&?>white<\$%&?>line<\$%&?>indicates<\$%&?>the<\$%&?>surface<\$%&?>of<\$%&?>the<\$%&?>slide,<\$%&?>which< \$%&?>is<\$%&?>then<\$%&?>overlain<\$%&?>by<\$%&?>a<\$%&?>silty<\$%&?>sandstone<\$%&?>bed<\$%&?>with<\$%&?>abundant<\$%&?>sand<\$% &?>clasts<\$%&?>originating<\$%&?>from<\$%&?>the<\$%&?>underlying<\$%&?>sandstone<\$%&?>bed.<\$%&?>The<\$%&?>scale<\$%&?>(a<\$%&?>pa rt<\$%&?>of<\$%&?>the<\$%&?>hammer<\$%&?>head)<\$%&?>is<\$%&?>0.05<\$%&?>m<\$%&?>long.<\$%&?>m:<\$%&?>massive<\$%&?>sandstone<\$% **Figure 7.** Example of the columnar cross-section of the lower Koura Formation taken along section 6. The arrows indi‐ cate the horizons showing evidence of a small-scale slide. A: outcrop view of the fluvial deposit. The arrow indicates the horizon of Figure 7B. The scale (hammer) is 0.3 m long. B: close-up photograph of the top of the fluvial channel fill sandstone beds. The dotted white line indicates the surface of the slide, which is then overlain by a silty sandstone bed with abundant sand clasts originating from the underlying sandstone bed. The scale (a part of the hammer head) is 0.05 m long. m: massive sandstone bed, s: shallow lake deposit, HCS: hummocky cross-stratification.

&?>bed,<\$%&?>s:<\$%&?>small<\$%&?>lake<\$%&?>deposit,<\$%&?>HCS:<\$%&?>hummocky<\$%&?>cross-stratification.

In<\$%&?>the<\$%&?>Kenya<\$%&?>Rift,<\$%&?>the<\$%&?>Miocene<\$%&?>rift<\$%&?>basin-

**2.2.<\$%&?>Examples<\$%&?>from<\$%&?>the<\$%&?>East<\$%&?>Africa<\$%&?>rift<\$%&?>valle**

fill<\$%&?>successions<\$%&?>are<\$%&?>exposed<\$%&?>(Figure<\$%&?>8).<\$%&?>The<\$%&?>activity<\$%&?>of<\$%&?>the<\$%&?

Figure 7. Example<\$%&?>of<\$%&?>the<\$%&?>columnar<\$%&?>cross-

**y** 

Figure 6. Columnar cross-section of a part of the upper Koura Formation and changes in depositional environment. A: a sediment cycle showing an upward-shallowing trend. The cycle base is represented by a flooding surface. B: an erosion surface truncating the inner shelf equivalent deposit, and is then covered with HCS sandstone beds of the shoreface origin. s.b.: slump block. Note the hammer (circled) for the scale. C: a close-up photograph of the sediment just below the erosion surface (cycle boundary). There are several slip surfaces of the slump in the sediments. Coarsesediment grains, which can be found only above the surface, are also incorporated (probably dragged) into the slumped horizon. The scale is 0.2 m long. D: a basal erosion surface of the cycle, truncating the inner-shelf-equivalent deposit and being covered with fluvial channel deposit. The white

Such sediment cycles were not identified in the lower and middle formations. The detailed outcrop observations in the lower formation revealed that either the top or the base of the sandstone intervals (fluvial channel facies) is marked by a surface associated with minor sliding (Figure 7); the former case is the most common (Figure 7). The surface is then covered with a thin

arrow indicates the surface. See Figure 6 for the legend of the columnar section.

H: hummocky cross-straification, W: wave ripple lamination, O: fan delta slope environment equivalent to outer shelf,

H

H

H H

FBS

W

Abbreviations

m s g

?

10 m

section.

FS FS

ES

ES

FS

ES

ES

FS

FS

FS

ES

H

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

O ISF Environment A

B

slumped

H

C

s.b.

D

ES: erosion surface, FS: flooding surface, FBS: flooding-surface bounded cycle,

I: fan delta slope environment equivalent to inner shelf, S: shoreface, F: fluvial.

**Figure 6.** Columnar cross-section of a part of the upper Koura Formation and changes in depositional environment. A: a sediment cycle showing an upward-shallowing trend. The cycle base is represented by a flooding surface. B: an ero‐ sion surface truncating the inner shelf equivalent deposit, and is then covered with HCS sandstone beds of the shore‐ face origin. s.b.: slump block. Note the hammer for the scale. C: a close-up photograph of the sediment just below the erosion surface (cycle boundary). There are several slip surfaces of the slump in the sediments. Coarse-sediment grains, which can be found only above the surface, are also incorporated (probably dragged) into the slumped horizon. The scale is 0.2 m long. D: a basal erosion surface of the cycle, truncating the inner-shelf-equivalent deposit and being cov‐ ered with fluvial channel deposit. The white arrow indicates the surface. See Figure 7 for the legend of the columnar

H

1 m

FS

W

#### **2.2. Examples from the East Africa rift valley**

?>in<\$%&?>the<\$%&?>present<\$%&?>study.

In the Kenya Rift, the Miocene rift basin-fill successions are exposed (Figure 8). The activity of the rift system started in the Oligocene and attained its maximum in the middle to late Miocene [41]. We targeted the half-graben fills exposed in the Samburu Hills, northern Kenya [42-44], and Nakali, central Kenya [45]. The target sediment successions of both areas (Namurungule and Nakali Formations) have not been classified into members based on the international stratigraphic nomenclature, although each formation can be divided into three units. Therefore the terms, the lower, middle and upper formations, are used for three units of each formation in the present study. >rift<\$%&?>system<\$%&?>started<\$%&?>in<\$%&?>the<\$%&?>Oligocene<\$%&?>and<\$%&?>attained<\$%&?>its<\$%&?>maximum <\$%&?>in<\$%&?>the<\$%&?>middle<\$%&?>to<\$%&?>late<\$%&?>Miocene<\$%&?>[41].<\$%&?>We<\$%&?>targeted<\$%&?>the<\$% &?>halfgraben<\$%&?>fills<\$%&?>exposed<\$%&?>in<\$%&?>the<\$%&?>Samburu<\$%&?>Hills,<\$%&?>northern<\$%&?>Kenya<\$%&?>[42- 44],<\$%&?>and<\$%&?>Nakali,<\$%&?>central<\$%&?>Kenya<\$%&?>[45].<\$%&?>The<\$%&?>target<\$%&?>sediment<\$%&?>success ions<\$%&?>of<\$%&?>both<\$%&?>areas<\$%&?>(Namurungule<\$%&?>and<\$%&?>Nakali<\$%&?>Formations)<\$%&?>have<\$%&?> not<\$%&?>been<\$%&?>classified<\$%&?>into<\$%&?>members<\$%&?>based<\$%&?>on<\$%&?>the<\$%&?>international<\$%&?>stra tigraphic<\$%&?>nomenclature,<\$%&?>although<\$%&?>each<\$%&?>formation<\$%&?>can<\$%&?>be<\$%&?>divided<\$%&?>into<\$ %&?>three<\$%&?>units.<\$%&?>Therefore<\$%&?>the<\$%&?>terms,<\$%&?>the<\$%&?>lower,<\$%&?>middle<\$%&?>and<\$%&?>up per<\$%&?>formations,<\$%&?>are<\$%&?>used<\$%&?>for<\$%&?>three<\$%&?>units<\$%&?>of<\$%&?>each<\$%&?>formation<\$%&

**a.** Samburu Hills

mall<\$%&?>half-

igure<\$%&?>9).

body are punctuated by faults (Figure 9).

Basalt Lava

Tirr Tirr Fm

Alluvium and terrace deposits

Legend

Diatomite and gravel

Suguta Valley (rift floor)

are locations of columnar cross-sections in Figure 10.

Fault

Precambrian basement complex

Nachola Fm Aka Aiteputh Fm Namurungule Fm Kongia Fm

36 25' E °

1 45' N

Miocene

KI2 KI1 NM2

Quaternary

Pliocene

Samburu Hills are located in the eastern shoulder of the eastern branch of the East African Rift Valley system, northern Kenya (Figure 8). The Nachola, Aka Aiteputh, Namurungule and Kongia Formations (ca. 20–5.3 Ma [43]) make up the Miocene succession, which covers the Precambrian Mozambique Belt rocks (gneiss and granitic rocks)(Figure. 9). The upper Aka Aiteputh to the Namurungule Formations' phase (ca. 10–9.3 Ma) was one of the major rifting periods in this area, as suggested by the development of a series of small half-grabens, which is indicated in the geologic map as the scattered distribution of the Namurungule Formation [42] (Figure 9). Each formation body has a lenticular plan view and one or both sides of the

9.3<\$%&?>Ma)<\$%&?>was<\$%&?>one<\$%&?>of<\$%&?>the<\$%&?>major<\$%&?>rifting<\$%&?>periods<\$%&?>in<\$%&?>this<\$%& ?>area,<\$%&?>as<\$%&?>suggested<\$%&?>by<\$%&?>the<\$%&?>development<\$%&?>of<\$%&?>a<\$%&?>series<\$%&?>of<\$%&?>s

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

grabens,<\$%&?>which<\$%&?>is<\$%&?>indicated<\$%&?>in<\$%&?>the<\$%&?>geologic<\$%&?>map<\$%&?>as<\$%&?>the<\$%&?>sc attered<\$%&?>distribution<\$%&?>of<\$%&?>the<\$%&?>Namurungule<\$%&?>Formation<\$%&?>[42]<\$%&?>(Figure<\$%&?>9).<\$% &?>Each<\$%&?>formation<\$%&?>body<\$%&?>has<\$%&?>a<\$%&?>lenticular<\$%&?>plan<\$%&?>view<\$%&?>and<\$%&?>one<\$% &?>or<\$%&?>both<\$%&?>sides<\$%&?>of<\$%&?>the<\$%&?>body<\$%&?>are<\$%&?>punctuated<\$%&?>by<\$%&?>faults<\$%&?>(F

Figure 9. Geologic<\$%&?>map<\$%&?>of<\$%&?>the<\$%&?>Miocene<\$%&?>in<\$%&?>Samburu<\$%&?>Hills.<\$%&?>The<\$%&?>enclosed<\$%&?> part<\$%&?>is<\$%&?>the<\$%&?>studied<\$%&?>area.<\$%&?>KI1,<\$%&?>KI2,<\$%&?>NM2<\$%&?>and<\$%&?>NK5<\$%&?>are<\$%&?>locations<\$%

Baragoi River

NK5

Samburu Hills

N

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

93

10 km

S<\$%&?>(Figure<\$%&?>9).<\$%&?>Although<\$%&?>the<\$%&?>western<\$%&?>margin<\$%&?>of<\$%&?>the<\$%&?>basin<\$%&?>is

<\$%&?>which<\$%&?>is<\$%&?>interpreted<\$%&?>as<\$%&?>having<\$%&?>been<\$%&?>deposited<\$%&?>during<\$%&?>the<\$%&

the<\$%&?>border<\$%&?>fault<\$%&?>of<\$%&?>the<\$%&?>basin<\$%&?>runs<\$%&?>in<\$%&?>the<\$%&?>western<\$%&?>margin, <\$%&?>as<\$%&?>suggested<\$%&?>by<\$%&?>the<\$%&?>Namurungule<\$%&?>sediments<\$%&?>thickening<\$%&?>to<\$%&?>the <\$%&?>west<\$%&?>[42].<\$%&?>There<\$%&?>is<\$%&?>a<\$%&?>gap<\$%&?>in<\$%&?>fault<\$%&?>location<\$%&?>in<\$%&?>the< \$%&?>northern<\$%&?>and<\$%&?>southern<\$%&?>halves<\$%&?>of<\$%&?>this<\$%&?>basin.<\$%&?>In<\$%&?>the<\$%&?>earliest <\$%&?>phase<\$%&?>of<\$%&?>basin<\$%&?>evolution,<\$%&?>there<\$%&?>may<\$%&?>have<\$%&?>been<\$%&?>an<\$%&?>acco mmodation<\$%&?>zone<\$%&?>in<\$%&?>the<\$%&?>boundary<\$%&?>between<\$%&?>the<\$%&?>northern<\$%&?>and<\$%&?>so

The<\$%&?>northern<\$%&?>half<\$%&?>of<\$%&?>the<\$%&?>basin,<\$%&?>where<\$%&?>spectacularly<\$%&?>well<\$%&?>exposu res<\$%&?>allow<\$%&?>sediment<\$%&?>correlation<\$%&?>among<\$%&?>outcrops,<\$%&?>was<\$%&?>targeted<\$%&?>in<\$%&?

graben<\$%&?>fill<\$%&?>consists<\$%&?>of<\$%&?>the<\$%&?>upper<\$%&?>Aka<\$%&?>Aiteputh<\$%&?>Formation,<\$%&?>whic h<\$%&?>is<\$%&?>characterized<\$%&?>by<\$%&?>red<\$%&?>soil<\$%&?>beds<\$%&?>with<\$%&?>abundant<\$%&?>calcrete<\$%& ?>layers<\$%&?>and<\$%&?>basalt<\$%&?>lavas<\$%&?>with<\$%&?>basalt<\$%&?>conglomerate<\$%&?>layers<\$%&?>[44].<\$%&?>T he<\$%&?>overlying<\$%&?>Namurungule<\$%&?>Formation<\$%&?>consists<\$%&?>of<\$%&?>four<\$%&?>parts:<\$%&?>the<\$%&? >basal<\$%&?>conglomerate<\$%&?>beds<\$%&?>of<\$%&?>alluvial<\$%&?>fan<\$%&?>origin,<\$%&?>the<\$%&?>alternations<\$%&?>

?>rejuvenated<\$%&?>phase<\$%&?>of<\$%&?>the<\$%&?>rift<\$%&?>after<\$%&?>7<\$%&?>Ma<\$%&?>(see<\$%&?>[43])—

The<\$%&?>target<\$%&?>basin<\$%&?>has<\$%&?>a<\$%&?>lenticular<\$%&?>shape<\$%&?>extending<\$%&?>N–

The target basin has a lenticular shape extending N–S (Figure 9). Although the western margin of the basin is truncated by the overlying Kongia Formation— which is interpreted as having been deposited during the rejuvenated phase of the rift after 7 Ma (see [43])—the border fault of the basin runs in the western margin, as suggested by the Namurungule sediments thickening to the west [42]. There is a gap in fault location in the northern and southern halves

**Figure 9.** Geologic map of the Miocene in Samburu Hills. The enclosed part is the studied area. KI1, KI2, NM2 and NK5

<\$%&?>truncated<\$%&?>by<\$%&?>the<\$%&?>overlying<\$%&?>Kongia<\$%&?>Formation—

&?>of<\$%&?>columnar<\$%&?>cross-sections<\$%&?>in<\$%&?>Figure<\$%&?>9.

uthern<\$%&?>halves<\$%&?>of<\$%&?>this<\$%&?>basin.

>the<\$%&?>present<\$%&?>study.<\$%&?>A<\$%&?>half-

Samburu<\$%&?>Hills<\$%&?>are<\$%&?>located<\$%&?>in<\$%&?>the<\$%&?>eastern<\$%&?>shoulder<\$%&?>of<\$%&?>the<\$%&?> eastern<\$%&?>branch<\$%&?>of<\$%&?>the<\$%&?>East<\$%&?>African<\$%&?>Rift<\$%&?>Valley<\$%&?>system,<\$%&?>northern< \$%&?>Kenya<\$%&?>(Figure<\$%&?>8).<\$%&?>The<\$%&?>Nachola,<\$%&?>Aka<\$%&?>Aiteputh,<\$%&?>Namurungule<\$%&?>an

5.3<\$%&?>Ma<\$%&?>[43])<\$%&?>make<\$%&?>up<\$%&?>the<\$%&?>Miocene<\$%&?>succession,<\$%&?>which<\$%&?>covers<\$% &?>the<\$%&?>Precambrian<\$%&?>Mozambique<\$%&?>Belt<\$%&?>rocks<\$%&?>(gneiss<\$%&?>and<\$%&?>granitic<\$%&?>rocks) (Figure.<\$%&?>9).<\$%&?>The<\$%&?>upper<\$%&?>Aka<\$%&?>Aiteputh<\$%&?>to<\$%&?>the<\$%&?>Namurungule<\$%&?>Form

Figure 8. Location<\$%&?>map<\$%&?>of<\$%&?>Samburu<\$%&?>Hills<\$%&?>and<\$%&?>Nakali<\$%&?>in<\$%&?>central<\$%&?>and<\$%&?>nort hern<\$%&?>Kenya. **Figure 8.** Location map of Samburu Hills and Nakali in central and northern Kenya.

d<\$%&?>Kongia<\$%&?>Formations<\$%&?>(ca.<\$%&?>20–

ations'<\$%&?>phase<\$%&?>(ca.<\$%&?>10–

a. Samburu<\$%&?>Hills

#### **a.** Samburu Hills

igure<\$%&?>9).

**2.2. Examples from the East Africa rift valley**

?>in<\$%&?>the<\$%&?>present<\$%&?>study.

in the present study.

&?>half-

hern<\$%&?>Kenya.

TimboroaI

0°

Uasin Gishu

Cherangani

1 N°

Sekerr F.

N

Anticline

50 km

a. Samburu<\$%&?>Hills

Londiani

Elgeyo F.

Lomut F.

Steep monocline Basement

Dips away from rift center

Legend B

Dips toward rift center

Kula F.

Kapkut

**Figure 8.** Location map of Samburu Hills and Nakali in central and northern Kenya.

Kamasia

Bogoria

F.

L. Bogoria

L. Baringo

Korosi

RIibkwo

Titai

Paka

Silali

EMURUAN-GOGOLAK Emuruan-Ggolak

Orus

Western

branch

d<\$%&?>Kongia<\$%&?>Formations<\$%&?>(ca.<\$%&?>20–

ations'<\$%&?>phase<\$%&?>(ca.<\$%&?>10–

In the Kenya Rift, the Miocene rift basin-fill successions are exposed (Figure 8). The activity of the rift system started in the Oligocene and attained its maximum in the middle to late Miocene [41]. We targeted the half-graben fills exposed in the Samburu Hills, northern Kenya [42-44], and Nakali, central Kenya [45]. The target sediment successions of both areas (Namurungule and Nakali Formations) have not been classified into members based on the international stratigraphic nomenclature, although each formation can be divided into three units. Therefore the terms, the lower, middle and upper formations, are used for three units of each formation

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

Loriu

>rift<\$%&?>system<\$%&?>started<\$%&?>in<\$%&?>the<\$%&?>Oligocene<\$%&?>and<\$%&?>attained<\$%&?>its<\$%&?>maximum <\$%&?>in<\$%&?>the<\$%&?>middle<\$%&?>to<\$%&?>late<\$%&?>Miocene<\$%&?>[41].<\$%&?>We<\$%&?>targeted<\$%&?>the<\$%

graben<\$%&?>fills<\$%&?>exposed<\$%&?>in<\$%&?>the<\$%&?>Samburu<\$%&?>Hills,<\$%&?>northern<\$%&?>Kenya<\$%&?>[42- 44],<\$%&?>and<\$%&?>Nakali,<\$%&?>central<\$%&?>Kenya<\$%&?>[45].<\$%&?>The<\$%&?>target<\$%&?>sediment<\$%&?>success ions<\$%&?>of<\$%&?>both<\$%&?>areas<\$%&?>(Namurungule<\$%&?>and<\$%&?>Nakali<\$%&?>Formations)<\$%&?>have<\$%&?> not<\$%&?>been<\$%&?>classified<\$%&?>into<\$%&?>members<\$%&?>based<\$%&?>on<\$%&?>the<\$%&?>international<\$%&?>stra tigraphic<\$%&?>nomenclature,<\$%&?>although<\$%&?>each<\$%&?>formation<\$%&?>can<\$%&?>be<\$%&?>divided<\$%&?>into<\$ %&?>three<\$%&?>units.<\$%&?>Therefore<\$%&?>the<\$%&?>terms,<\$%&?>the<\$%&?>lower,<\$%&?>middle<\$%&?>and<\$%&?>up per<\$%&?>formations,<\$%&?>are<\$%&?>used<\$%&?>for<\$%&?>three<\$%&?>units<\$%&?>of<\$%&?>each<\$%&?>formation<\$%&

Barrier

36 E° 37 E° L. Turukana

Tirr Tirr

SAMBURU HILLS

2 N°

5 S

0

IndianOcean

5 N

N

A

Suguta Valley

Namarunu

Figure 8. Location<\$%&?>map<\$%&?>of<\$%&?>Samburu<\$%&?>Hills<\$%&?>and<\$%&?>Nakali<\$%&?>in<\$%&?>central<\$%&?>and<\$%&?>nort

30 E 45 E

L. Turkana

Fig. 7B

Kenya Rift

40 E Ethiopian Rift

Eastern

L. Victoria

L. Tanga nyika

branch

35 E

Nakali

Samburu<\$%&?>Hills<\$%&?>are<\$%&?>located<\$%&?>in<\$%&?>the<\$%&?>eastern<\$%&?>shoulder<\$%&?>of<\$%&?>the<\$%&?> eastern<\$%&?>branch<\$%&?>of<\$%&?>the<\$%&?>East<\$%&?>African<\$%&?>Rift<\$%&?>Valley<\$%&?>system,<\$%&?>northern< \$%&?>Kenya<\$%&?>(Figure<\$%&?>8).<\$%&?>The<\$%&?>Nachola,<\$%&?>Aka<\$%&?>Aiteputh,<\$%&?>Namurungule<\$%&?>an

5.3<\$%&?>Ma<\$%&?>[43])<\$%&?>make<\$%&?>up<\$%&?>the<\$%&?>Miocene<\$%&?>succession,<\$%&?>which<\$%&?>covers<\$% &?>the<\$%&?>Precambrian<\$%&?>Mozambique<\$%&?>Belt<\$%&?>rocks<\$%&?>(gneiss<\$%&?>and<\$%&?>granitic<\$%&?>rocks) (Figure.<\$%&?>9).<\$%&?>The<\$%&?>upper<\$%&?>Aka<\$%&?>Aiteputh<\$%&?>to<\$%&?>the<\$%&?>Namurungule<\$%&?>Form

Samburu Hills are located in the eastern shoulder of the eastern branch of the East African Rift Valley system, northern Kenya (Figure 8). The Nachola, Aka Aiteputh, Namurungule and Kongia Formations (ca. 20–5.3 Ma [43]) make up the Miocene succession, which covers the Precambrian Mozambique Belt rocks (gneiss and granitic rocks)(Figure. 9). The upper Aka Aiteputh to the Namurungule Formations' phase (ca. 10–9.3 Ma) was one of the major rifting periods in this area, as suggested by the development of a series of small half-grabens, which is indicated in the geologic map as the scattered distribution of the Namurungule Formation [42] (Figure 9). Each formation body has a lenticular plan view and one or both sides of the body are punctuated by faults (Figure 9). 9.3<\$%&?>Ma)<\$%&?>was<\$%&?>one<\$%&?>of<\$%&?>the<\$%&?>major<\$%&?>rifting<\$%&?>periods<\$%&?>in<\$%&?>this<\$%& ?>area,<\$%&?>as<\$%&?>suggested<\$%&?>by<\$%&?>the<\$%&?>development<\$%&?>of<\$%&?>a<\$%&?>series<\$%&?>of<\$%&?>s mall<\$%&?>halfgrabens,<\$%&?>which<\$%&?>is<\$%&?>indicated<\$%&?>in<\$%&?>the<\$%&?>geologic<\$%&?>map<\$%&?>as<\$%&?>the<\$%&?>sc attered<\$%&?>distribution<\$%&?>of<\$%&?>the<\$%&?>Namurungule<\$%&?>Formation<\$%&?>[42]<\$%&?>(Figure<\$%&?>9).<\$% &?>Each<\$%&?>formation<\$%&?>body<\$%&?>has<\$%&?>a<\$%&?>lenticular<\$%&?>plan<\$%&?>view<\$%&?>and<\$%&?>one<\$% &?>or<\$%&?>both<\$%&?>sides<\$%&?>of<\$%&?>the<\$%&?>body<\$%&?>are<\$%&?>punctuated<\$%&?>by<\$%&?>faults<\$%&?>(F

Figure 9. Geologic<\$%&?>map<\$%&?>of<\$%&?>the<\$%&?>Miocene<\$%&?>in<\$%&?>Samburu<\$%&?>Hills.<\$%&?>The<\$%&?>enclosed<\$%&?> part<\$%&?>is<\$%&?>the<\$%&?>studied<\$%&?>area.<\$%&?>KI1,<\$%&?>KI2,<\$%&?>NM2<\$%&?>and<\$%&?>NK5<\$%&?>are<\$%&?>locations<\$% &?>of<\$%&?>columnar<\$%&?>cross-sections<\$%&?>in<\$%&?>Figure<\$%&?>9. **Figure 9.** Geologic map of the Miocene in Samburu Hills. The enclosed part is the studied area. KI1, KI2, NM2 and NK5 are locations of columnar cross-sections in Figure 10.

The<\$%&?>target<\$%&?>basin<\$%&?>has<\$%&?>a<\$%&?>lenticular<\$%&?>shape<\$%&?>extending<\$%&?>N– S<\$%&?>(Figure<\$%&?>9).<\$%&?>Although<\$%&?>the<\$%&?>western<\$%&?>margin<\$%&?>of<\$%&?>the<\$%&?>basin<\$%&?>is <\$%&?>truncated<\$%&?>by<\$%&?>the<\$%&?>overlying<\$%&?>Kongia<\$%&?>Formation— <\$%&?>which<\$%&?>is<\$%&?>interpreted<\$%&?>as<\$%&?>having<\$%&?>been<\$%&?>deposited<\$%&?>during<\$%&?>the<\$%& ?>rejuvenated<\$%&?>phase<\$%&?>of<\$%&?>the<\$%&?>rift<\$%&?>after<\$%&?>7<\$%&?>Ma<\$%&?>(see<\$%&?>[43]) the<\$%&?>border<\$%&?>fault<\$%&?>of<\$%&?>the<\$%&?>basin<\$%&?>runs<\$%&?>in<\$%&?>the<\$%&?>western<\$%&?>margin, <\$%&?>as<\$%&?>suggested<\$%&?>by<\$%&?>the<\$%&?>Namurungule<\$%&?>sediments<\$%&?>thickening<\$%&?>to<\$%&?>the The target basin has a lenticular shape extending N–S (Figure 9). Although the western margin of the basin is truncated by the overlying Kongia Formation— which is interpreted as having been deposited during the rejuvenated phase of the rift after 7 Ma (see [43])—the border fault of the basin runs in the western margin, as suggested by the Namurungule sediments thickening to the west [42]. There is a gap in fault location in the northern and southern halves

uthern<\$%&?>halves<\$%&?>of<\$%&?>this<\$%&?>basin.

>the<\$%&?>present<\$%&?>study.<\$%&?>A<\$%&?>half-

<\$%&?>west<\$%&?>[42].<\$%&?>There<\$%&?>is<\$%&?>a<\$%&?>gap<\$%&?>in<\$%&?>fault<\$%&?>location<\$%&?>in<\$%&?>the< \$%&?>northern<\$%&?>and<\$%&?>southern<\$%&?>halves<\$%&?>of<\$%&?>this<\$%&?>basin.<\$%&?>In<\$%&?>the<\$%&?>earliest <\$%&?>phase<\$%&?>of<\$%&?>basin<\$%&?>evolution,<\$%&?>there<\$%&?>may<\$%&?>have<\$%&?>been<\$%&?>an<\$%&?>acco mmodation<\$%&?>zone<\$%&?>in<\$%&?>the<\$%&?>boundary<\$%&?>between<\$%&?>the<\$%&?>northern<\$%&?>and<\$%&?>so

The<\$%&?>northern<\$%&?>half<\$%&?>of<\$%&?>the<\$%&?>basin,<\$%&?>where<\$%&?>spectacularly<\$%&?>well<\$%&?>exposu res<\$%&?>allow<\$%&?>sediment<\$%&?>correlation<\$%&?>among<\$%&?>outcrops,<\$%&?>was<\$%&?>targeted<\$%&?>in<\$%&?

graben<\$%&?>fill<\$%&?>consists<\$%&?>of<\$%&?>the<\$%&?>upper<\$%&?>Aka<\$%&?>Aiteputh<\$%&?>Formation,<\$%&?>whic h<\$%&?>is<\$%&?>characterized<\$%&?>by<\$%&?>red<\$%&?>soil<\$%&?>beds<\$%&?>with<\$%&?>abundant<\$%&?>calcrete<\$%& ?>layers<\$%&?>and<\$%&?>basalt<\$%&?>lavas<\$%&?>with<\$%&?>basalt<\$%&?>conglomerate<\$%&?>layers<\$%&?>[44].<\$%&?>T he<\$%&?>overlying<\$%&?>Namurungule<\$%&?>Formation<\$%&?>consists<\$%&?>of<\$%&?>four<\$%&?>parts:<\$%&?>the<\$%&? >basal<\$%&?>conglomerate<\$%&?>beds<\$%&?>of<\$%&?>alluvial<\$%&?>fan<\$%&?>origin,<\$%&?>the<\$%&?>alternations<\$%&?> of this basin. In the earliest phase of basin evolution, there may have been an accommodation zone in the boundary between the northern and southern halves of this basin.

The northern half of the basin, where spectacularly well exposures allow sediment correlation among outcrops, was targeted in the present study. A half-graben fill consists of the upper Aka Aiteputh Formation, which is characterized by red soil beds with abundant calcrete layers and basalt lavas with basalt conglomerate layers [44]. The overlying Namurungule Formation consists of four parts: the basal conglomerate beds of alluvial fan origin, the alternations of tuffaceous mudstone and sandstone beds (mudstone-dominated) of the lower part (both parts form the lower formation) and an about 20-m-thick lahar deposit of the middle formation (Figure 10). The upper formation is represented by a pile of sediment cycles, each of which consists of a sandstone-dominated and an overlying mudstone-dominated interval, as mentioned below. The age of the Namurungule Formation ranges from 9.6 to 9.3 Ma [43], and the rapid sedimentation rate was estimated to be 1.52 m/ky for the lower formation and 0.24 m/ky for the upper formation [42].

From the viewpoint of sequence stratigraphy, the red soil beds and basalt lava interval of the upper Aka Aiteputh Formation and alluvial fan interval of the basal Namurungule Formation are interpreted as the LST; most of the lower Namurungule Formation, except for its basal and uppermost part, is the TST showing retrogradational succession. The remaining part is the highstand systems tract (HST) (Figure 10: see also [44]).

The TST is characterized by a rapid lateral facies change from the thick lake facies in the southern part to the terrestrial facies represented by the alternations of root-bearing mudstone and sandstone beds in the northern part. The up to 20-m-thick terrestrial sediments in the TST contain a few stream deposits represented by an up to 0.5 m of sheet sandstone beds with parallel and trough cross-stratification. Other sandstone beds in the terrestrial deposits are associated with temporary lake expansions, as suggested by the wave-generated sedimentary structures (wave ripple lamination and small hummocky cross-stratification) in sandstone beds [42] (Figure 11). There are local slide deposits, represented by pebble- and cobble-sized mudstone breccia in the succession (Figure 11).

On the other hand, the HST is represented by a pile of sediment cycles [42]. Each cycle consists, from the base to the top, of the conglomeratic sandstone beds of fluvial channel fill origin, rootbearing mudstone beds of floodplain origin, laminated mudstone beds of lake origin and tabular cross-stratified sandstone beds of delta front origin (Figure 12). The cycle boundaries are sharp and undulating, and truncate the underlying sediments (delta front and lake deposit) (Figure 12).

Individual cycles tend to thicken to the west, which are interpreted to be owing to tectonic subsidence in the western part of the basin. The basal surface truncates the underlying sediments in each cycle, indicating a lake-level fall probably because of the migration of lake water to the basin centre when the basin subsidence occurred (for example, [2]).

ment of the drainage system. On the other hand, the sediment grains in the upper formation supplied from the basement (Mozambique Belt), such as quartz and microcline, suggest that the drainage basin became wider through time [46]. This trend, later appearance of the grains originating from basement rocks, has been reported from other rift basins (for example, [46]).

**Figure 10.** Columnar cross-sections of the Namurungule Formation (modified from [44]). Graphs indicate the petro‐ graphical analysis results. Grey bars indicate the horizons of lake deposits. C: fluvial channel fill, F: delta front deposit.

quartz,<\$%&?>M:<\$%&?>microcline,<\$%&?>F:<\$%&?>alkali-feldspar,<\$%&?>R:<\$%&?>rock<\$%&?>fragment.

sections<\$%&?>of<\$%&?>the<\$%&?>Namurungule<\$%&?>Formation<\$%&?>(modified<\$%&?>from<\$%&?>[44]).<\$%&?>Graphs<\$%&?>indicate<\$ %&?>the<\$%&?>petrographical<\$%&?>analysis<\$%&?>results.<\$%&?>Grey<\$%&?>bars<\$%&?>indicate<\$%&?>the<\$%&?>horizons<\$%&?>of<\$% &?>lake<\$%&?>deposits.<\$%&?>C:<\$%&?>fluvial<\$%&?>channel<\$%&?>fill,<\$%&?>F:<\$%&?>delta<\$%&?>front<\$%&?>deposit.<\$%&?>Q:<\$%&?>

Paleoflows (cross-stratification)

Rose diagrams (up = north) Paleoflows

n = 21

(imbrication)

TS

of<\$%&?>tuffaceous<\$%&?>mudstone<\$%&?>and<\$%&?>sandstone<\$%&?>beds<\$%&?>(mudstone-

<\$%&?>each<\$%&?>of<\$%&?>which<\$%&?>consists<\$%&?>of<\$%&?>a<\$%&?>sandstone-

Q M F R

n = 14

formation)<\$%&?>and<\$%&?>an<\$%&?>about<\$%&?>20-m-

HST

C

F

C C C

F

F C C

KI1

n = 19

Legend

n = 31

m s g

F

C C

C

C

C C C C KI2

TST

LST

Poorly-sorted gravel bed

Trough cross-stratification

Tabular cross-stratification

mation<\$%&?>[42].

Namurungule Formation

50 m

Figure 10.Columnar<\$%&?>cross-

Q: quartz, M: microcline, F: alkali-feldspar, R: rock fragment.

HCS rootlets

alluvial fan

Aka Aiteputh Formation

lacustrine delta

deposits lahar

lacustrine delta

Interpretation

dominated<\$%&?>and<\$%&?>an<\$%&?>overlying<\$%&?>mudstone-

dominated)<\$%&?>of<\$%&?>the<\$%&?>lower<\$%&?>part<\$%&?>(both<\$%&?>parts<\$%&?>form<\$%&?>the<\$%&?>lower<\$%&?>

thick<\$%&?>lahar<\$%&?>deposit<\$%&?>of<\$%&?>the<\$%&?>middle<\$%&?>formation<\$%&?>(Figure<\$%&?>10).<\$%&?>The<\$% &?>upper<\$%&?>formation<\$%&?>is<\$%&?>represented<\$%&?>by<\$%&?>a<\$%&?>pile<\$%&?>of<\$%&?>sediment<\$%&?>cycles,

dominated<\$%&?>interval,<\$%&?>as<\$%&?>mentioned<\$%&?>below.<\$%&?>The<\$%&?>age<\$%&?>of<\$%&?>the<\$%&?>Namur ungule<\$%&?>Formation<\$%&?>ranges<\$%&?>from<\$%&?>9.6<\$%&?>to<\$%&?>9.3<\$%&?>Ma<\$%&?>[43],<\$%&?>and<\$%&?>th e<\$%&?>rapid<\$%&?>sedimentation<\$%&?>rate<\$%&?>was<\$%&?>estimated<\$%&?>to<\$%&?>be<\$%&?>1.52<\$%&?>m/ky<\$%&? >for<\$%&?>the<\$%&?>lower<\$%&?>formation<\$%&?>and<\$%&?>0.24<\$%&?>m/ky<\$%&?>for<\$%&?>the<\$%&?>upper<\$%&?>for

> C C

> C C

> C

NK5

C

S 2.1km N

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

C

C

C

C

C

Fig. 11

Pyroclastic flow deposits Laminated mudstone bed

Basalt lava layer

m s g

Alluvial fan deposit Lacustrine deposit

C

n = 33 n = 31

Fig. 12

<sup>C</sup> <sup>C</sup>

n = 17 n = 13 n = 13 n = 19

m s g

C C C C C C

n = 19

NM2

n = 13

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

95

n = 15

n = 19

C

C

n = 13

C

C

C

m s g

Petrographical analysis indicates that the sediments (feldspar and rock fragments, mainly basalt grains) were supplied only from the adjacent area during the deposition of the upper Aka Aiteputh and lower Namurungule Formations (Figure 10), showing the poor develop‐

ungule<\$%&?>Formation<\$%&?>ranges<\$%&?>from<\$%&?>9.6<\$%&?>to<\$%&?>9.3<\$%&?>Ma<\$%&?>[43],<\$%&?>and<\$%&?>th e<\$%&?>rapid<\$%&?>sedimentation<\$%&?>rate<\$%&?>was<\$%&?>estimated<\$%&?>to<\$%&?>be<\$%&?>1.52<\$%&?>m/ky<\$%&? Early Continental Rift Basin Stratigraphy, Depositional Facies and Tectonics in Volcaniclastic System… http://dx.doi.org/10.5772/56804 95

dominated)<\$%&?>of<\$%&?>the<\$%&?>lower<\$%&?>part<\$%&?>(both<\$%&?>parts<\$%&?>form<\$%&?>the<\$%&?>lower<\$%&?>

thick<\$%&?>lahar<\$%&?>deposit<\$%&?>of<\$%&?>the<\$%&?>middle<\$%&?>formation<\$%&?>(Figure<\$%&?>10).<\$%&?>The<\$% &?>upper<\$%&?>formation<\$%&?>is<\$%&?>represented<\$%&?>by<\$%&?>a<\$%&?>pile<\$%&?>of<\$%&?>sediment<\$%&?>cycles,

dominated<\$%&?>interval,<\$%&?>as<\$%&?>mentioned<\$%&?>below.<\$%&?>The<\$%&?>age<\$%&?>of<\$%&?>the<\$%&?>Namur

>for<\$%&?>the<\$%&?>lower<\$%&?>formation<\$%&?>and<\$%&?>0.24<\$%&?>m/ky<\$%&?>for<\$%&?>the<\$%&?>upper<\$%&?>for

of<\$%&?>tuffaceous<\$%&?>mudstone<\$%&?>and<\$%&?>sandstone<\$%&?>beds<\$%&?>(mudstone-

<\$%&?>each<\$%&?>of<\$%&?>which<\$%&?>consists<\$%&?>of<\$%&?>a<\$%&?>sandstone-

formation)<\$%&?>and<\$%&?>an<\$%&?>about<\$%&?>20-m-

mation<\$%&?>[42].

dominated<\$%&?>and<\$%&?>an<\$%&?>overlying<\$%&?>mudstone-

of this basin. In the earliest phase of basin evolution, there may have been an accommodation

The northern half of the basin, where spectacularly well exposures allow sediment correlation among outcrops, was targeted in the present study. A half-graben fill consists of the upper Aka Aiteputh Formation, which is characterized by red soil beds with abundant calcrete layers and basalt lavas with basalt conglomerate layers [44]. The overlying Namurungule Formation consists of four parts: the basal conglomerate beds of alluvial fan origin, the alternations of tuffaceous mudstone and sandstone beds (mudstone-dominated) of the lower part (both parts form the lower formation) and an about 20-m-thick lahar deposit of the middle formation (Figure 10). The upper formation is represented by a pile of sediment cycles, each of which consists of a sandstone-dominated and an overlying mudstone-dominated interval, as mentioned below. The age of the Namurungule Formation ranges from 9.6 to 9.3 Ma [43], and the rapid sedimentation rate was estimated to be 1.52 m/ky for the lower formation and 0.24

From the viewpoint of sequence stratigraphy, the red soil beds and basalt lava interval of the upper Aka Aiteputh Formation and alluvial fan interval of the basal Namurungule Formation are interpreted as the LST; most of the lower Namurungule Formation, except for its basal and uppermost part, is the TST showing retrogradational succession. The remaining part is the

The TST is characterized by a rapid lateral facies change from the thick lake facies in the southern part to the terrestrial facies represented by the alternations of root-bearing mudstone and sandstone beds in the northern part. The up to 20-m-thick terrestrial sediments in the TST contain a few stream deposits represented by an up to 0.5 m of sheet sandstone beds with parallel and trough cross-stratification. Other sandstone beds in the terrestrial deposits are associated with temporary lake expansions, as suggested by the wave-generated sedimentary structures (wave ripple lamination and small hummocky cross-stratification) in sandstone beds [42] (Figure 11). There are local slide deposits, represented by pebble- and cobble-sized

On the other hand, the HST is represented by a pile of sediment cycles [42]. Each cycle consists, from the base to the top, of the conglomeratic sandstone beds of fluvial channel fill origin, rootbearing mudstone beds of floodplain origin, laminated mudstone beds of lake origin and tabular cross-stratified sandstone beds of delta front origin (Figure 12). The cycle boundaries are sharp and undulating, and truncate the underlying sediments (delta front and lake deposit)

Individual cycles tend to thicken to the west, which are interpreted to be owing to tectonic subsidence in the western part of the basin. The basal surface truncates the underlying sediments in each cycle, indicating a lake-level fall probably because of the migration of lake

Petrographical analysis indicates that the sediments (feldspar and rock fragments, mainly basalt grains) were supplied only from the adjacent area during the deposition of the upper Aka Aiteputh and lower Namurungule Formations (Figure 10), showing the poor develop‐

water to the basin centre when the basin subsidence occurred (for example, [2]).

zone in the boundary between the northern and southern halves of this basin.

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

m/ky for the upper formation [42].

highstand systems tract (HST) (Figure 10: see also [44]).

mudstone breccia in the succession (Figure 11).

(Figure 12).

Figure 10.Columnar<\$%&?>crosssections<\$%&?>of<\$%&?>the<\$%&?>Namurungule<\$%&?>Formation<\$%&?>(modified<\$%&?>from<\$%&?>[44]).<\$%&?>Graphs<\$%&?>indicate<\$ %&?>the<\$%&?>petrographical<\$%&?>analysis<\$%&?>results.<\$%&?>Grey<\$%&?>bars<\$%&?>indicate<\$%&?>the<\$%&?>horizons<\$%&?>of<\$% &?>lake<\$%&?>deposits.<\$%&?>C:<\$%&?>fluvial<\$%&?>channel<\$%&?>fill,<\$%&?>F:<\$%&?>delta<\$%&?>front<\$%&?>deposit.<\$%&?>Q:<\$%&?> **Figure 10.** Columnar cross-sections of the Namurungule Formation (modified from [44]). Graphs indicate the petro‐ graphical analysis results. Grey bars indicate the horizons of lake deposits. C: fluvial channel fill, F: delta front deposit. Q: quartz, M: microcline, F: alkali-feldspar, R: rock fragment.

quartz,<\$%&?>M:<\$%&?>microcline,<\$%&?>F:<\$%&?>alkali-feldspar,<\$%&?>R:<\$%&?>rock<\$%&?>fragment.

ment of the drainage system. On the other hand, the sediment grains in the upper formation supplied from the basement (Mozambique Belt), such as quartz and microcline, suggest that the drainage basin became wider through time [46]. This trend, later appearance of the grains originating from basement rocks, has been reported from other rift basins (for example, [46]).

**Figure 11.** Example of the columnar cross-sections from the lower Namurungule Formation (modified from [42]). The zigzag line indicates an erosion surface that can be seen only in the northern part of the measured area.

#### **b.** Miocene Nakali Formation

Nakali is located about 50 km south of the Samburu Hills (Figure 8). The Miocene Nakali and Nasorut Formations are distributed in this area [45] (Figure 13). The lower part of the lower Nakali Formation is characterized by the alternations of tuffaceous sandstone and mudstone beds, which are interpreted to be turbidite and slumped deposits (delta front deposit), and the overlying thick lapilli tuff beds that bury the lake (Figure 14). The fluvial channel fill, floodplain and shallow lake deposits (conglomerate, tuffaceous sandstone and mudstone beds) charac‐ terize the upper part of the lower formation. The middle formation is represented by thick pyroclastic flow deposits (ca. 40 m). The lower part of the upper formation shows sediment characteristics similar to the upper part of the lower formation. The upper part of the upper formation is represented by tuffaceous mudstone beds and conglomerate and sandstone interbeds with slump structures. The slumped deposits in this interval indicate that this is of the lake slope origin (Figure 14). One of the important hominoid fossils, *Nakalipithecus* [45], was discovered near the top of this interval. In terms of sequence stratigraphy, most of the formation forms the LST—except for the upper part of the upper formation, where deeper lake mudstone facies predominates. This part is interpreted as the TST. The Nakali Formation is then overlain by trachitic or basaltic lava and volcaniclastics of the Nasorut Formation (Figure 14). The magnetostratigraphic reversed polarity zone in the middle portion of the Nakali

Formation was correlated to the Chron C5n.1n (9.88–9.74 Ma [46]) on the basis of the Ar–Ar

**Figure 12.** Example of the columnar cross-sections from the upper Namurungule Formation (modified from [42]). The arrowed intervals show individual cycles. The rose diagrams indicate palaeoflow directions shown by cross-stratifica‐

Legend

m s g

Tabular cross-strat. Lacustrine deposit

Burrow Channel-fill deposit

sections<\$%&?>from<\$%&?>the<\$%&?>upper<\$%&?>Namurungule<\$%&?>Formation<\$%&?>(modified<\$%&?>from<\$%&?>[42]).<\$%&?>The<\$%

Delta front deposit Flood plain deposit

Rootlet

100 m

m s g

0.5 m

Figure 12.Example<\$%&?>of<\$%&?>the<\$%&?>columnar<\$%&?>cross-

Parallel stratification Ripple lamination

Desiccation crack

Trough cross-strat.

On<\$%&?>the<\$%&?>other<\$%&?>hand,<\$%&?>the<\$%&?>HST<\$%&?>is<\$%&?>represented<\$%&?>by<\$%&?>a<\$%&?>pile<\$% &?>of<\$%&?>sediment<\$%&?>cycles<\$%&?>[42].<\$%&?>Each<\$%&?>cycle<\$%&?>consists,<\$%&?>from<\$%&?>the<\$%&?>base<\$ %&?>to<\$%&?>the<\$%&?>top,<\$%&?>of<\$%&?>the<\$%&?>conglomeratic<\$%&?>sandstone<\$%&?>beds<\$%&?>of<\$%&?>fluvial<

bearing<\$%&?>mudstone<\$%&?>beds<\$%&?>of<\$%&?>floodplain<\$%&?>origin,<\$%&?>laminated<\$%&?>mudstone<\$%&?>beds

stratified<\$%&?>sandstone<\$%&?>beds<\$%&?>of<\$%&?>delta<\$%&?>front<\$%&?>origin<\$%&?>(Figure<\$%&?>12).<\$%&?>The<\$ %&?>cycle<\$%&?>boundaries<\$%&?>are<\$%&?>sharp<\$%&?>and<\$%&?>undulating,<\$%&?>and<\$%&?>truncate<\$%&?>the<\$% &?>underlying<\$%&?>sediments<\$%&?>(delta<\$%&?>front<\$%&?>and<\$%&?>lake<\$%&?>deposit)<\$%&?>(Figure<\$%&?>12).

n = 7

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97

n = 5

n = 7

\$%&?>channel<\$%&?>fill<\$%&?>origin,<\$%&?>root-

n = 6

n = 11

m s g

<\$%&?>of<\$%&?>lake<\$%&?>origin<\$%&?>and<\$%&?>tabular<\$%&?>cross-

SW NE

n = 8

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

ages [45].

tion (up = north).

<\$%&?>of<\$%&?>lake<\$%&?>origin<\$%&?>and<\$%&?>tabular<\$%&?>crossstratified<\$%&?>sandstone<\$%&?>beds<\$%&?>of<\$%&?>delta<\$%&?>front<\$%&?>origin<\$%&?>(Figure<\$%&?>12).<\$%&?>The<\$ %&?>cycle<\$%&?>boundaries<\$%&?>are<\$%&?>sharp<\$%&?>and<\$%&?>undulating,<\$%&?>and<\$%&?>truncate<\$%&?>the<\$% Early Continental Rift Basin Stratigraphy, Depositional Facies and Tectonics in Volcaniclastic System… http://dx.doi.org/10.5772/56804 97

\$%&?>channel<\$%&?>fill<\$%&?>origin,<\$%&?>root-

On<\$%&?>the<\$%&?>other<\$%&?>hand,<\$%&?>the<\$%&?>HST<\$%&?>is<\$%&?>represented<\$%&?>by<\$%&?>a<\$%&?>pile<\$% &?>of<\$%&?>sediment<\$%&?>cycles<\$%&?>[42].<\$%&?>Each<\$%&?>cycle<\$%&?>consists,<\$%&?>from<\$%&?>the<\$%&?>base<\$ %&?>to<\$%&?>the<\$%&?>top,<\$%&?>of<\$%&?>the<\$%&?>conglomeratic<\$%&?>sandstone<\$%&?>beds<\$%&?>of<\$%&?>fluvial<

bearing<\$%&?>mudstone<\$%&?>beds<\$%&?>of<\$%&?>floodplain<\$%&?>origin,<\$%&?>laminated<\$%&?>mudstone<\$%&?>beds

&?>underlying<\$%&?>sediments<\$%&?>(delta<\$%&?>front<\$%&?>and<\$%&?>lake<\$%&?>deposit)<\$%&?>(Figure<\$%&?>12).

**b.** Miocene Nakali Formation

m s g

m s g

Nakali is located about 50 km south of the Samburu Hills (Figure 8). The Miocene Nakali and Nasorut Formations are distributed in this area [45] (Figure 13). The lower part of the lower Nakali Formation is characterized by the alternations of tuffaceous sandstone and mudstone beds, which are interpreted to be turbidite and slumped deposits (delta front deposit), and the overlying thick lapilli tuff beds that bury the lake (Figure 14). The fluvial channel fill, floodplain and shallow lake deposits (conglomerate, tuffaceous sandstone and mudstone beds) charac‐ terize the upper part of the lower formation. The middle formation is represented by thick pyroclastic flow deposits (ca. 40 m). The lower part of the upper formation shows sediment characteristics similar to the upper part of the lower formation. The upper part of the upper formation is represented by tuffaceous mudstone beds and conglomerate and sandstone interbeds with slump structures. The slumped deposits in this interval indicate that this is of the lake slope origin (Figure 14). One of the important hominoid fossils, *Nakalipithecus* [45], was discovered near the top of this interval. In terms of sequence stratigraphy, most of the formation forms the LST—except for the upper part of the upper formation, where deeper lake mudstone facies predominates. This part is interpreted as the TST. The Nakali Formation is then overlain by trachitic or basaltic lava and volcaniclastics of the Nasorut Formation (Figure 14). The magnetostratigraphic reversed polarity zone in the middle portion of the Nakali

**Figure 11.** Example of the columnar cross-sections from the lower Namurungule Formation (modified from [42]). The

zigzag line indicates an erosion surface that can be seen only in the northern part of the measured area.

m s g

SW NE

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

Legend

m s g

Tuff

0.5 m

m s g

100 m

Mud & tuff clasts Burrow Rootlet

Convolute lamination

Parallel stratification

Pumice

HCS

Figure 12.Example<\$%&?>of<\$%&?>the<\$%&?>columnar<\$%&?>crosssections<\$%&?>from<\$%&?>the<\$%&?>upper<\$%&?>Namurungule<\$%&?>Formation<\$%&?>(modified<\$%&?>from<\$%&?>[42]).<\$%&?>The<\$% **Figure 12.** Example of the columnar cross-sections from the upper Namurungule Formation (modified from [42]). The arrowed intervals show individual cycles. The rose diagrams indicate palaeoflow directions shown by cross-stratifica‐ tion (up = north).

Formation was correlated to the Chron C5n.1n (9.88–9.74 Ma [46]) on the basis of the Ar–Ar ages [45].

Two normal faults extending N–S separate the rocks of this formation into three blocks (eastern, central and western) (Figure 13). The displacement of the eastern fault is larger than those of the others and is estimated to be about 200 m on the basis of the altitude gap of the

>the<\$%&?>altitude<\$%&?>gap<\$%&?>of<\$%&?>the<\$%&?>middle<\$%&?>formation<\$%&?>between<\$%&?>the<\$%&?>eastern<

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

The<\$%&?>good<\$%&?>exposures<\$%&?>of<\$%&?>the<\$%&?>lower<\$%&?>part<\$%&?>of<\$%&?>the<\$%&?>upper<\$%&?>form ation<\$%&?>and<\$%&?>the<\$%&?>frequently<\$%&?>interbedded<\$%&?>tuff<\$%&?>beds<\$%&?>allow<\$%&?>observation<\$%& ?>of<\$%&?>lateral<\$%&?>facies<\$%&?>changes<\$%&?>in<\$%&?>the<\$%&?>field<\$%&?>within<\$%&?>and<\$%&?>among<\$%&? >blocks<\$%&?>(Figure<\$%&?>15).<\$%&?>Six<\$%&?>tuff<\$%&?>beds<\$%&?>were<\$%&?>identified<\$%&?>in<\$%&?>this<\$%&?> horizon,<\$%&?>and<\$%&?>are<\$%&?>named<\$%&?>Twin<\$%&?>(two<\$%&?>white<\$%&?>tuff<\$%&?>beds),<\$%&?>Exo<\$%&? >(white<\$%&?>tuff<\$%&?>bed<\$%&?>containing<\$%&?>abundant<\$%&?>trachyte<\$%&?>fragments),<\$%&?>Pum<\$%&?>(pumic e<\$%&?>tuff),<\$%&?>Fu<\$%&?>(poorly<\$%&?>sorted<\$%&?>pumice<\$%&?>tuff<\$%&?>bed),<\$%&?>Mfu<\$%&?>(poorly<\$%&?> sorted<\$%&?>pumice<\$%&?>and<\$%&?>accretionary<\$%&?>lapilli<\$%&?>tuff<\$%&?>bed)<\$%&?>and<\$%&?>Ma<\$%&?>(white< \$%&?>tuff<\$%&?>bed<\$%&?>containing<\$%&?>accretionary<\$%&?>lapillis)<\$%&?>(Figure<\$%&?>15).<\$%&?>The<\$%&?>thick<\$ %&?>cemented<\$%&?>beds<\$%&?>with<\$%&?>weakly<\$%&?>weathered<\$%&?>soil<\$%&?>beds<\$%&?>(termed<\$%&?>'terrace <\$%&?>forming<\$%&?>bed'<\$%&?>in<\$%&?>Figure<\$%&?>15,<\$%&?>because<\$%&?>this<\$%&?>bed<\$%&?>forms<\$%&?>a<\$% &?>wide<\$%&?>terrace<\$%&?>in<\$%&?>this<\$%&?>place);<\$%&?>the<\$%&?>White<\$%&?>beds,<\$%&?>represented<\$%&?>by<\$ %&?>the<\$%&?>sandstone<\$%&?>and<\$%&?>conglomerate<\$%&?>beds<\$%&?>rich<\$%&?>in<\$%&?>small<\$%&?>breccia<\$%&? >of<\$%&?>white<\$%&?>tuff,<\$%&?>and<\$%&?>the<\$%&?>Red<\$%&?>beds,<\$%&?>characterized<\$%&?>by<\$%&?>red<\$%&?>co nglomerate<\$%&?>and<\$%&?>sandstone<\$%&?>beds<\$%&?>of<\$%&?>fluvial<\$%&?>channel<\$%&?>fill<\$%&?>and<\$%&?>flood plain<\$%&?>origin<\$%&?>(Red<\$%&?>beds),<\$%&?>can<\$%&?>be<\$%&?>used<\$%&?>for<\$%&?>the<\$%&?>correlation<\$%&?>a s<\$%&?>well.<\$%&?>The<\$%&?>bases<\$%&?>of<\$%&?>the<\$%&?>lake<\$%&?>deposits,<\$%&?>flooding<\$%&?>surfaces,<\$%&?>

were<\$%&?>also<\$%&?>used<\$%&?>as<\$%&?>one<\$%&?>of<\$%&?>key<\$%&?>horizons<\$%&?>(Figure<\$%&?>15).

3

m s g

**Figure 15.** Columnar cross-sections of the lower part of the upper Nakali Formation. The black bar indicates the hori‐ zon of lake deposit. The dotted and solid lines indicate correlated tuff beds and flooding surface, respectively. A bold

NW (A) SE(A' )

Fu

Mfu Mfu

Exo

Pum

F

Wastern block Central block

Fault

4

Red beds

m s g

Twin

5

6

10 m

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

99

Ma

Major flooding Surface

m s g

Gravel Desiccation crack Erosion surface

Legend

m s g

section<\$%&?>of<\$%&?>the<\$%&?>lower<\$%&?>part<\$%&?>of<\$%&?>the<\$%&?>upper<\$%&?>Nakali<\$%&?>Formation.<\$%&?>The<\$%&?>blac k<\$%&?>bar<\$%&?>indicates<\$%&?>the<\$%&?>horizon<\$%&?>of<\$%&?>lake<\$%&?>deposit.<\$%&?>The<\$%&?>dotted<\$%&?>and<\$%&?>solid< \$%&?>lines<\$%&?>indicate<\$%&?>correlated<\$%&?>tuff<\$%&?>beds<\$%&?>and<\$%&?>flooding<\$%&?>surface,<\$%&?>respectively.<\$%&?>A<\$ %&?>bold<\$%&?>line<\$%&?>indicates<\$%&?>"the<\$%&?>terrace<\$%&?>forming<\$%&?>bed".<\$%&?>See<\$%&?>text<\$%&?>for<\$%&?>tuff<\$%&

Pumice Accretionary lapilli

Tuff

Middle formation

White beds

Terrace forming beds

The<\$%&?>correlation<\$%&?>results<\$%&?>(Figure<\$%&?>15)<\$%&?>show<\$%&?>thicker<\$%&?>sediments<\$%&?>in<\$%&?>th e<\$%&?>western<\$%&?>part<\$%&?>of<\$%&?>the<\$%&?>central<\$%&?>block<\$%&?>below<\$%&?>the<\$%&?>Twin<\$%&?>Tuff< \$%&?>bed.<\$%&?>The<\$%&?>thickness<\$%&?>of<\$%&?>sediments<\$%&?>between<\$%&?>the<\$%&?>base<\$%&?>of<\$%&?>the< \$%&?>upper<\$%&?>formation<\$%&?>and<\$%&?>Twin<\$%&?>Tuff<\$%&?>bed<\$%&?>consistently<\$%&?>increases<\$%&?>from< \$%&?>section<\$%&?>5<\$%&?>to<\$%&?>section<\$%&?>2,<\$%&?>even<\$%&?>though<\$%&?>section<\$%&?>2<\$%&?>is<\$%&?>loc ated<\$%&?>in<\$%&?>the<\$%&?>west<\$%&?>of<\$%&?>the<\$%&?>fault<\$%&?>separating<\$%&?>the<\$%&?>western<\$%&?>and< \$%&?>central<\$%&?>blocks.<\$%&?>This<\$%&?>may<\$%&?>show<\$%&?>that<\$%&?>the<\$%&?>fault<\$%&?>was<\$%&?>inactive <\$%&?>before<\$%&?>the<\$%&?>Twin<\$%&?>Tuff<\$%&?>deposition,<\$%&?>and<\$%&?>another<\$%&?>fault,<\$%&?>which<\$%&

The good exposures of the lower part of the upper formation and the frequently interbedded tuff beds allow observation of lateral facies changes in the field within and among blocks (Figure 15). Six tuff beds were identified in this horizon, and are named Twin (two white tuff beds), Exo (white tuff bed containing abundant trachyte fragments), Pum (pumice tuff), Fu (poorly sorted pumice tuff bed), Mfu (poorly sorted pumice and accretionary lapilli tuff bed) and Ma (white tuff bed containing accretionary lapillis) (Figure 15). The thick cemented beds with weakly weathered soil beds (termed 'terrace forming bed' in Figure 15, because this bed forms a wide terrace in this place); the White beds, represented by the sandstone and con‐ glomerate beds rich in small breccia of white tuff, and the Red beds, characterized by red conglomerate and sandstone beds of fluvial channel fill and floodplain origin (Red beds), can be used for the correlation as well. The bases of the lake deposits, flooding surfaces, were also

c. Subsidence<\$%&?>history<\$%&?>of<\$%&?>the<\$%&?>upper<\$%&?>Nakali<\$%&?>formation

middle formation between the eastern and central blocks.

**c.** Subsidence history of the upper Nakali formation

\$%&?>and<\$%&?>central<\$%&?>blocks.

used as one of key horizons (Figure 15).

Fu

Twin

Pum

Exo

2

1

Upper formation

m s g

Figure 15.Columnar<\$%&?>cross-

m s g

?>names.<\$%&?>F:<\$%&?>local<\$%&?>flooding<\$%&?>surface.

line indicates "the terrace forming bed". See text for tuff names. F: local flooding surface.

ca. 500 m

Figure 13.Geologic<\$%&?>map<\$%&?>of<\$%&?>the<\$%&?>Nakali<\$%&?>Formation<\$%&?>(modified<\$%&?>from<\$%&?>[45]).<\$%&?>A-A'<\$%&?>is<\$%&?>the<\$%&?>trend<\$%&?>of<\$%&?>the<\$%&?>columnar<\$%&?>crosssections<\$%&?>obtained<\$%&?>from<\$%&?>locations<\$%&?>1–6<\$%&?>in<\$%&?>Figure<\$%&?>15. **Figure 13.** Geologic map of the Nakali Formation (modified from [45]). A-A' is the trend of the columnar cross-sec‐ tions obtained from locations 1–6 in Figure 15. Figure 13.Geologic<\$%&?>map<\$%&?>of<\$%&?>the<\$%&?>Nakali<\$%&?>Formation<\$%&?>(modified<\$%&?>from<\$%&?>[45]).<\$%&?>A-A'<\$%&?>is<\$%&?>the<\$%&?>trend<\$%&?>of<\$%&?>the<\$%&?>columnar<\$%&?>crosssections<\$%&?>obtained<\$%&?>from<\$%&?>locations<\$%&?>1–6<\$%&?>in<\$%&?>Figure<\$%&?>15.

<\$%&?>and<\$%&?>chronostratigraphy<\$%&?>of<\$%&?>the<\$%&?>Nakali<\$%&?>and<\$%&?>Nasorut<\$%&?>Formations<\$%&?>(modified<\$%&? >from<\$%&?>[45])<\$%&?>and<\$%&?>facies<\$%&?>description<\$%&?>and<\$%&?>interpreted<\$%&?>depositional<\$%&?>environments. Figure 14.The<\$%&?>generalized<\$%&?>litho- <\$%&?>and<\$%&?>chronostratigraphy<\$%&?>of<\$%&?>the<\$%&?>Nakali<\$%&?>and<\$%&?>Nasorut<\$%&?>Formations<\$%&?>(modified<\$%&? >from<\$%&?>[45])<\$%&?>and<\$%&?>facies<\$%&?>description<\$%&?>and<\$%&?>interpreted<\$%&?>depositional<\$%&?>environments. **Figure 14.** The generalized litho- and chronostratigraphy of the Nakali and Nasorut Formations (modified from [45]) and facies description and interpreted depositional environments.

S<\$%&?>separate<\$%&?>the<\$%&?>rocks<\$%&?>of<\$%&?>this<\$%&?>formation<\$%&?>into<\$%&?>three<\$%&?>blocks<\$%&?>(e astern,<\$%&?>central<\$%&?>and<\$%&?>western)<\$%&?>(Figure<\$%&?>13).<\$%&?>The<\$%&?>displacement<\$%&?>of<\$%&?>th e<\$%&?>eastern<\$%&?>fault<\$%&?>is<\$%&?>larger<\$%&?>than<\$%&?>those<\$%&?>of<\$%&?>the<\$%&?>others<\$%&?>and<\$% &?>is<\$%&?>estimated<\$%&?>to<\$%&?>be<\$%&?>about<\$%&?>200<\$%&?>m<\$%&?>on<\$%&?>the<\$%&?>basis<\$%&?>of<\$%&?

S<\$%&?>separate<\$%&?>the<\$%&?>rocks<\$%&?>of<\$%&?>this<\$%&?>formation<\$%&?>into<\$%&?>three<\$%&?>blocks<\$%&?>(e astern,<\$%&?>central<\$%&?>and<\$%&?>western)<\$%&?>(Figure<\$%&?>13).<\$%&?>The<\$%&?>displacement<\$%&?>of<\$%&?>th e<\$%&?>eastern<\$%&?>fault<\$%&?>is<\$%&?>larger<\$%&?>than<\$%&?>those<\$%&?>of<\$%&?>the<\$%&?>others<\$%&?>and<\$% &?>is<\$%&?>estimated<\$%&?>to<\$%&?>be<\$%&?>about<\$%&?>200<\$%&?>m<\$%&?>on<\$%&?>the<\$%&?>basis<\$%&?>of<\$%&?

Figure 14.The<\$%&?>generalized<\$%&?>litho-

Two<\$%&?>normal<\$%&?>faults<\$%&?>extending<\$%&?>N–

Two<\$%&?>normal<\$%&?>faults<\$%&?>extending<\$%&?>N–

Two normal faults extending N–S separate the rocks of this formation into three blocks (eastern, central and western) (Figure 13). The displacement of the eastern fault is larger than those of the others and is estimated to be about 200 m on the basis of the altitude gap of the middle formation between the eastern and central blocks. >the<\$%&?>altitude<\$%&?>gap<\$%&?>of<\$%&?>the<\$%&?>middle<\$%&?>formation<\$%&?>between<\$%&?>the<\$%&?>eastern< \$%&?>and<\$%&?>central<\$%&?>blocks.

#### **c.** Subsidence history of the upper Nakali formation c. Subsidence<\$%&?>history<\$%&?>of<\$%&?>the<\$%&?>upper<\$%&?>Nakali<\$%&?>formation

Figure 13.Geologic<\$%&?>map<\$%&?>of<\$%&?>the<\$%&?>Nakali<\$%&?>Formation<\$%&?>(modified<\$%&?>from<\$%&?>[45]).<\$%&?>A-

Figure 13.Geologic<\$%&?>map<\$%&?>of<\$%&?>the<\$%&?>Nakali<\$%&?>Formation<\$%&?>(modified<\$%&?>from<\$%&?>[45]).<\$%&?>A-

Central Block

Central Block

7.74± 0.21Ma \*

7.74± 0.21Ma \*

N N N N

N N N N

N

R R

N

R R

<sup>N</sup> Eastern Block

Lapilli tuff

Tuff bed

Legend

Basalt and

trachyte lava

Lapilli tuff

Basalt and

trachyte lava

Slumped bed

Conglomerate

<sup>N</sup> Eastern Block

Slumped bed

Tuff bed

Conglomerate

Legend

9.82±0.09 Ma\*

m s g

m s g

9.82±0.09 Ma\*

9.90±0.09 Ma\*

9.90±0.09 Ma\*

E 36°22' N 1°11'

E 36°22' N 1°11'

**Figure 13.** Geologic map of the Nakali Formation (modified from [45]). A-A' is the trend of the columnar cross-sec‐

10.10±0.12 Ma\*

10.10±0.12 Ma\*

m s g

m s g

*10.10±0.12*

**NF-A**

*10.10±0.12*

**NF-A**

**NF-C**

3 4 5 6

**NF-C**

3 4 5 6

Central block

Central block

**Quat. basalt**

**Quat. basalt**

2

2

**NF-C**

1

**NF-C**

1

A

A

 *9.82±0.09 9.90±0.09*

 *9.82±0.09 9.90±0.09*

N

N

**NF-B**

**NF-B**

C4Ar. 2r C4Ar. 2n C4Ar. 3r

C4Ar. 2r C4Ar. 2n C4Ar. 3r

C5n. 1n C5n. 1r

C5n. 1n C5n. 1r

C5n. 2n

C5n. 2n

**NF-A**

**NF-A**

A'

A'

100m

100m

N N N N N

N N N N N

m s g

m s g

10.0

R

R

N

sl

N

sl

9.5

9.5

9.58

9.74 9.64

9.88

9.92

Age (Ma) Mag. Polar. Chron

9.58

9.74 9.64

9.88

9.92

10.0

Age (Ma) Mag. Polar. Chron

Eastern block

Eastern block

<\$%&?>and<\$%&?>chronostratigraphy<\$%&?>of<\$%&?>the<\$%&?>Nakali<\$%&?>and<\$%&?>Nasorut<\$%&?>Formations<\$%&?>(modified<\$%&? >from<\$%&?>[45])<\$%&?>and<\$%&?>facies<\$%&?>description<\$%&?>and<\$%&?>interpreted<\$%&?>depositional<\$%&?>environments.

<\$%&?>and<\$%&?>chronostratigraphy<\$%&?>of<\$%&?>the<\$%&?>Nakali<\$%&?>and<\$%&?>Nasorut<\$%&?>Formations<\$%&?>(modified<\$%&? >from<\$%&?>[45])<\$%&?>and<\$%&?>facies<\$%&?>description<\$%&?>and<\$%&?>interpreted<\$%&?>depositional<\$%&?>environments.

S<\$%&?>separate<\$%&?>the<\$%&?>rocks<\$%&?>of<\$%&?>this<\$%&?>formation<\$%&?>into<\$%&?>three<\$%&?>blocks<\$%&?>(e astern,<\$%&?>central<\$%&?>and<\$%&?>western)<\$%&?>(Figure<\$%&?>13).<\$%&?>The<\$%&?>displacement<\$%&?>of<\$%&?>th e<\$%&?>eastern<\$%&?>fault<\$%&?>is<\$%&?>larger<\$%&?>than<\$%&?>those<\$%&?>of<\$%&?>the<\$%&?>others<\$%&?>and<\$% &?>is<\$%&?>estimated<\$%&?>to<\$%&?>be<\$%&?>about<\$%&?>200<\$%&?>m<\$%&?>on<\$%&?>the<\$%&?>basis<\$%&?>of<\$%&?

Hominoid fossils

Hominoid fossils

Magnetic polarity

Magnetic polarity

Normal

Reverse

Normal

Reverse

\* : 40Ar-39Ar ages

**Figure 14.** The generalized litho- and chronostratigraphy of the Nakali and Nasorut Formations (modified from [45])

\* : 40Ar-39Ar ages

R N

R N

S<\$%&?>separate<\$%&?>the<\$%&?>rocks<\$%&?>of<\$%&?>this<\$%&?>formation<\$%&?>into<\$%&?>three<\$%&?>blocks<\$%&?>(e astern,<\$%&?>central<\$%&?>and<\$%&?>western)<\$%&?>(Figure<\$%&?>13).<\$%&?>The<\$%&?>displacement<\$%&?>of<\$%&?>th e<\$%&?>eastern<\$%&?>fault<\$%&?>is<\$%&?>larger<\$%&?>than<\$%&?>those<\$%&?>of<\$%&?>the<\$%&?>others<\$%&?>and<\$% &?>is<\$%&?>estimated<\$%&?>to<\$%&?>be<\$%&?>about<\$%&?>200<\$%&?>m<\$%&?>on<\$%&?>the<\$%&?>basis<\$%&?>of<\$%&?

A'<\$%&?>is<\$%&?>the<\$%&?>trend<\$%&?>of<\$%&?>the<\$%&?>columnar<\$%&?>cross-

A'<\$%&?>is<\$%&?>the<\$%&?>trend<\$%&?>of<\$%&?>the<\$%&?>columnar<\$%&?>cross-

Basalt - trachyte lavas Conglomerate, pyroclastics

Unconformity

Massive or laminated tuffaceous mudstone beds Sand interbeds Slump structure

Unconformity

Massive or laminated tuffaceous mudstone beds Sand interbeds Slump structure

Basalt - trachyte lavas Conglomerate, pyroclastics

Welded lapilli tuff

Welded lapilli tuff

See upper part of the lower formation

See upper part of the lower formation

Cross-stratified sandstone and conglomerate beds

Bouma Ta-c turbidite beds Graded conglomerate beds Laminated tuffaceous mudstone beds

Bouma Ta-c turbidite beds Graded conglomerate beds Laminated tuffaceous mudstone beds

Slump strcture Wave ripples

Slump strcture Wave ripples

Root bearing tuffaceous mudstone beds Laminated tuffaceous mudstone beds Wave ripple lamination, HCS

Root bearing tuffaceous mudstone beds Laminated tuffaceous mudstone beds Wave ripple lamination, HCS

Cross-stratified sandstone and conglomerate beds

*10.10±0.12* Ar/ Ar age (Ma) 40 39 1 km

*10.10±0.12* Ar/ Ar age (Ma) 40 39 1 km

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

Figure 14.The<\$%&?>generalized<\$%&?>litho-

Figure 14.The<\$%&?>generalized<\$%&?>litho-

and facies description and interpreted depositional environments.

Legend Quaternary basalt ----- Unconformity -----

basalt and trachyte

basalt and trachyte

Legend Quaternary basalt ----- Unconformity -----

NF-C : Upper formation NF-B : Middle formation NF-A : Lower formation

NF-C : Upper formation NF-B : Middle formation NF-A : Lower formation

Dyke

Nasorut Formation

Dyke

Fault

Fault

tions obtained from locations 1–6 in Figure 15.

Upper

Upper

Middle

Lake slope

Fluvial channel Flood plain Shallow lake

Fluvial channel Flood plain Shallow lake

Pyroclastic flow deposit

Fluvial channel Flood plain Shallow lake

Lake slope

Pyroclastic flow deposit

Fluvial channel Flood plain Shallow lake

Nasorut

Fm.

Nasorut

Fm.

Middle

Nakali Fm.

Nakali Fm.

Lower

Lower

Delta front

Delta front

basalt lava conglomerate ----- Unconformity ----- Nakali Formation

trachyte lava

basalt lava conglomerate ----- Unconformity ----- Nakali Formation

trachyte lava

Nasorut Formation

Two<\$%&?>normal<\$%&?>faults<\$%&?>extending<\$%&?>N–

Two<\$%&?>normal<\$%&?>faults<\$%&?>extending<\$%&?>N–

sections<\$%&?>obtained<\$%&?>from<\$%&?>locations<\$%&?>1–6<\$%&?>in<\$%&?>Figure<\$%&?>15.

sections<\$%&?>obtained<\$%&?>from<\$%&?>locations<\$%&?>1–6<\$%&?>in<\$%&?>Figure<\$%&?>15.

Western

Block

Western

Block

*7.74±0.21* **Nasorut Fm.**

*7.74±0.21* **Nasorut Fm.**

Western block

Western block

The good exposures of the lower part of the upper formation and the frequently interbedded tuff beds allow observation of lateral facies changes in the field within and among blocks (Figure 15). Six tuff beds were identified in this horizon, and are named Twin (two white tuff beds), Exo (white tuff bed containing abundant trachyte fragments), Pum (pumice tuff), Fu (poorly sorted pumice tuff bed), Mfu (poorly sorted pumice and accretionary lapilli tuff bed) and Ma (white tuff bed containing accretionary lapillis) (Figure 15). The thick cemented beds with weakly weathered soil beds (termed 'terrace forming bed' in Figure 15, because this bed forms a wide terrace in this place); the White beds, represented by the sandstone and con‐ glomerate beds rich in small breccia of white tuff, and the Red beds, characterized by red conglomerate and sandstone beds of fluvial channel fill and floodplain origin (Red beds), can be used for the correlation as well. The bases of the lake deposits, flooding surfaces, were also used as one of key horizons (Figure 15). The<\$%&?>good<\$%&?>exposures<\$%&?>of<\$%&?>the<\$%&?>lower<\$%&?>part<\$%&?>of<\$%&?>the<\$%&?>upper<\$%&?>form ation<\$%&?>and<\$%&?>the<\$%&?>frequently<\$%&?>interbedded<\$%&?>tuff<\$%&?>beds<\$%&?>allow<\$%&?>observation<\$%& ?>of<\$%&?>lateral<\$%&?>facies<\$%&?>changes<\$%&?>in<\$%&?>the<\$%&?>field<\$%&?>within<\$%&?>and<\$%&?>among<\$%&? >blocks<\$%&?>(Figure<\$%&?>15).<\$%&?>Six<\$%&?>tuff<\$%&?>beds<\$%&?>were<\$%&?>identified<\$%&?>in<\$%&?>this<\$%&?> horizon,<\$%&?>and<\$%&?>are<\$%&?>named<\$%&?>Twin<\$%&?>(two<\$%&?>white<\$%&?>tuff<\$%&?>beds),<\$%&?>Exo<\$%&? >(white<\$%&?>tuff<\$%&?>bed<\$%&?>containing<\$%&?>abundant<\$%&?>trachyte<\$%&?>fragments),<\$%&?>Pum<\$%&?>(pumic e<\$%&?>tuff),<\$%&?>Fu<\$%&?>(poorly<\$%&?>sorted<\$%&?>pumice<\$%&?>tuff<\$%&?>bed),<\$%&?>Mfu<\$%&?>(poorly<\$%&?> sorted<\$%&?>pumice<\$%&?>and<\$%&?>accretionary<\$%&?>lapilli<\$%&?>tuff<\$%&?>bed)<\$%&?>and<\$%&?>Ma<\$%&?>(white< \$%&?>tuff<\$%&?>bed<\$%&?>containing<\$%&?>accretionary<\$%&?>lapillis)<\$%&?>(Figure<\$%&?>15).<\$%&?>The<\$%&?>thick<\$ %&?>cemented<\$%&?>beds<\$%&?>with<\$%&?>weakly<\$%&?>weathered<\$%&?>soil<\$%&?>beds<\$%&?>(termed<\$%&?>'terrace <\$%&?>forming<\$%&?>bed'<\$%&?>in<\$%&?>Figure<\$%&?>15,<\$%&?>because<\$%&?>this<\$%&?>bed<\$%&?>forms<\$%&?>a<\$% &?>wide<\$%&?>terrace<\$%&?>in<\$%&?>this<\$%&?>place);<\$%&?>the<\$%&?>White<\$%&?>beds,<\$%&?>represented<\$%&?>by<\$ %&?>the<\$%&?>sandstone<\$%&?>and<\$%&?>conglomerate<\$%&?>beds<\$%&?>rich<\$%&?>in<\$%&?>small<\$%&?>breccia<\$%&? >of<\$%&?>white<\$%&?>tuff,<\$%&?>and<\$%&?>the<\$%&?>Red<\$%&?>beds,<\$%&?>characterized<\$%&?>by<\$%&?>red<\$%&?>co nglomerate<\$%&?>and<\$%&?>sandstone<\$%&?>beds<\$%&?>of<\$%&?>fluvial<\$%&?>channel<\$%&?>fill<\$%&?>and<\$%&?>flood plain<\$%&?>origin<\$%&?>(Red<\$%&?>beds),<\$%&?>can<\$%&?>be<\$%&?>used<\$%&?>for<\$%&?>the<\$%&?>correlation<\$%&?>a s<\$%&?>well.<\$%&?>The<\$%&?>bases<\$%&?>of<\$%&?>the<\$%&?>lake<\$%&?>deposits,<\$%&?>flooding<\$%&?>surfaces,<\$%&?>

were<\$%&?>also<\$%&?>used<\$%&?>as<\$%&?>one<\$%&?>of<\$%&?>key<\$%&?>horizons<\$%&?>(Figure<\$%&?>15).

Figure 15.Columnar<\$%&?>crosssection<\$%&?>of<\$%&?>the<\$%&?>lower<\$%&?>part<\$%&?>of<\$%&?>the<\$%&?>upper<\$%&?>Nakali<\$%&?>Formation.<\$%&?>The<\$%&?>blac k<\$%&?>bar<\$%&?>indicates<\$%&?>the<\$%&?>horizon<\$%&?>of<\$%&?>lake<\$%&?>deposit.<\$%&?>The<\$%&?>dotted<\$%&?>and<\$%&?>solid< \$%&?>lines<\$%&?>indicate<\$%&?>correlated<\$%&?>tuff<\$%&?>beds<\$%&?>and<\$%&?>flooding<\$%&?>surface,<\$%&?>respectively.<\$%&?>A<\$ **Figure 15.** Columnar cross-sections of the lower part of the upper Nakali Formation. The black bar indicates the hori‐ zon of lake deposit. The dotted and solid lines indicate correlated tuff beds and flooding surface, respectively. A bold line indicates "the terrace forming bed". See text for tuff names. F: local flooding surface.

?>names.<\$%&?>F:<\$%&?>local<\$%&?>flooding<\$%&?>surface.

%&?>bold<\$%&?>line<\$%&?>indicates<\$%&?>"the<\$%&?>terrace<\$%&?>forming<\$%&?>bed".<\$%&?>See<\$%&?>text<\$%&?>for<\$%&?>tuff<\$%&

The<\$%&?>correlation<\$%&?>results<\$%&?>(Figure<\$%&?>15)<\$%&?>show<\$%&?>thicker<\$%&?>sediments<\$%&?>in<\$%&?>th e<\$%&?>western<\$%&?>part<\$%&?>of<\$%&?>the<\$%&?>central<\$%&?>block<\$%&?>below<\$%&?>the<\$%&?>Twin<\$%&?>Tuff< \$%&?>bed.<\$%&?>The<\$%&?>thickness<\$%&?>of<\$%&?>sediments<\$%&?>between<\$%&?>the<\$%&?>base<\$%&?>of<\$%&?>the< \$%&?>upper<\$%&?>formation<\$%&?>and<\$%&?>Twin<\$%&?>Tuff<\$%&?>bed<\$%&?>consistently<\$%&?>increases<\$%&?>from< \$%&?>section<\$%&?>5<\$%&?>to<\$%&?>section<\$%&?>2,<\$%&?>even<\$%&?>though<\$%&?>section<\$%&?>2<\$%&?>is<\$%&?>loc ated<\$%&?>in<\$%&?>the<\$%&?>west<\$%&?>of<\$%&?>the<\$%&?>fault<\$%&?>separating<\$%&?>the<\$%&?>western<\$%&?>and< \$%&?>central<\$%&?>blocks.<\$%&?>This<\$%&?>may<\$%&?>show<\$%&?>that<\$%&?>the<\$%&?>fault<\$%&?>was<\$%&?>inactive <\$%&?>before<\$%&?>the<\$%&?>Twin<\$%&?>Tuff<\$%&?>deposition,<\$%&?>and<\$%&?>another<\$%&?>fault,<\$%&?>which<\$%&

The correlation results (Figure 15) show thicker sediments in the western part of the central block below the Twin Tuff bed. The thickness of sediments between the base of the upper formation and Twin Tuff bed consistently increases from section 5 to section 2, even though section 2 is located in the west of the fault separating the western and central blocks. This may show that the fault was inactive before the Twin Tuff deposition, and another fault, which is not indicated on the geologic map and is running west of the study area, was active instead.

**3. Discussion**

graphic architecture.

basin evolution are discussed as follows.

**3.2. Record of basin mergers**

as a result of merging basins.

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

formed accommodation space was rapidly filled even near the basin centre.

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

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

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

101

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‐

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

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

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

Thickness of the sediments between the Twin and Pum Tuff beds is almost constant in the central block. However, the Red beds tend to be thicker to the east, and the sediments between the local flooding surface (F in Figure 15) and Exo Tuff bed tend to be thicker to the west, suggesting a temporary seesaw subsidence during the deposition between the Twin and Pum Tuff beds (Figure 15).

The sediments above the Pum Tuff bed (Figure 15) tend to be thicker in the eastern part of the central block. This thickness change and the appearance of the thicker lake depos‐ its in the eastern part clearly indicate that the depocentre was shifted in the eastern part of the basin. The seesaw subsidence seems to have ceased just before the Pum Tuff deposition. The thicker sediments to the east indicate that the fault separating the central and eastern blocks may have been formed in this phase. Note the thickness variation between the Pum and Ma Tuff beds—which tend to be thicker from section 3 to 5, but have thicker sediments in the same horizon in section 1, indicating larger subsidence around section 1 than section 3. This suggests that the fault separating the central and western blocks also became active after the Pum Tuff bed deposition.

Such a seesaw subsidence pattern suggests that the study area was located on the accom‐ modation zone [3, 5] during the deposition of the lower part of the upper Nakali Forma‐ tion. The Case C fault linkage and accommodation zone proposed in [5] (Figure 1) is inferred for this case.

Seesaw subsidence was reported from the Santo Domingo Basin in the Rio Grande Rift system [3], which has been long lived from the Oligocene to Pleistocene, because of changes in the shift of the active part of the faults forming the accommodation zone. This study showed a gradual facies shift because of such long-term seesaw subsidence (Figure. 8 in [3]). In case of the Nakali Formation, the movement's scale is much smaller and shorter than the case in [3]. This seesaw subsidence may have been related to the development of the block-bounding faults, which propagated either from the south or north. Such a temporary seesaw subsidence pattern may be the typical subsidence pattern of the Case C accommodation zone (Figure 1) when the zone is incorporated into a larger basin be‐ cause of the merger of smaller basins. This result additionally suggests that the constant thickness sediments within a half-graben fill could be the consequence of the seesaw subsidence happening in a short period.
