Half graben filling processes in the early phase of continental rifting: The Miocene Namurungule Formation of the Kenya Rift

Sedimentary Geology 186 (2006) 111 – 131 www.elsevier.com/locate/sedgeo

Half graben filling processes in the early phase of continental rifting: The Miocene Namurungule Formation of the Kenya Rift Mototaka Saneyoshi a,*, Katsuhiro Nakayama b, Tetsuya Sakai b, Yoshihiro Sawada b, Hidemi Ishida c a

c

Graduate School of Science and Technology, Niigata University, Niigata 950-2181, Japan b Department of Geoscience, Shimane University, Matsue, Shimane 690-8504, Japan School of Human Nursing, The University of Shiga Prefecture, Hikone, Shiga 522-8533, Japan

Received 7 August 2004; received in revised form 22 October 2005; accepted 7 November 2005

This paper is dedicated to one of the authors, Katsuhiro Nakayama, who tragically passed away in a traffic accident in Kenya in 2001.

Abstract The Miocene Namurungule Formation crops out on the eastern flank of the Kenya Rift, representing basin fill that developed in association with Miocene rifting. The formation is characterized by large volumes of volcaniclastics supplied passively by pyroclastic fall. Facies analysis reveals that this formation consists mostly of lacustrine delta deposits with minor alluvial fan deposits at its base. The conspicuous occurrence of a flood plain facies with infrequent channel fill deposits in the lower part of this formation suggest that the drainage area was limited in the early stage of deposition. Pyroclastic fall would therefore have been an important source of sediment during the early stages of rift development. The delta deposits are divided into two distinct successions based on lithological characteristics, separated by a thick pyroclastic layer in the middle part of the formation. The stacking pattern of the lower succession is retrogradational, whereas the upper succession is characterized by a pile of prograding bodies. Both delta successions are interpreted to have accumulated in an underfilled basin. The change in depositional mode from the lower to upper is considered to be due to a change in the balance between the sedimentation rate and the rate of lake-level rise. Assuming constant sediment supply, the apparent difference in flooding scale between the lower and upper successions is attributed to the topographical widening of the basin flat. Similar successions also occur in another basin in the East African Rift (Ngorora Formation, central Kenya), in which pyroclastic sediments are dominant and upward decrease in sedimentation rate is recognized. The differing in the stacking patterns between the Namurungule Formation and the Ngorora formation is probably induced by basin width. The narrow Namurungule basin seems to have been sensitive to increased sediment supply owing to the expansion of the catchment area. D 2005 Elsevier B.V. All rights reserved. Keywords: Flood plain deposits; Lacustrine delta; Half graben; Kenya Rift; Late Miocene

1. Introduction

* Corresponding author. Tel.: +81 25 262 6161. E-mail address: [email protected] (M. Saneyoshi). 0037-0738/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.sedgeo.2005.11.012

The basin fill of half grabens has been studied by many researchers (e.g., Flores, 1990; Schlische and Olsen, 1990; Browne and Plint, 1994; Cavinato et al., 2002), and a number of theoretical models have been

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proposed. Carroll and Bohacs (1999) classified half graben basins as overfilled, balanced-filled or underfilled based on the balance between both sediment supply + water supply rates and the potential accommodation rate, which is proportional to basin subsidence. Overfilled basins are defined as those in which the rate of water influx plus sediment fill generally exceeds the rate of potential accommodation creation, whereas in balanced-filled basins, the potential accommodation rate is approximately equivalent to the water influx plus sediment fill rate over the depositional time span of a unit. Withjack et al. (2002) suggested that the relationships between the incremental accommodation space, sediment supply and water supply determine which depositional system predominates in a given rift basin. In this model, the accommodation space is shaped by basin subsidence. Thus, if the rate of basin subsidence exceeds that of sediment supply, the accommodation space of the basin will increase. The rates of sediment and water supply and the change in accommodation related to the rate of basin subsidence are important factors in the fill processes of half grabens. Overfilled, balanced-filled and underfilled basins are practically categorized based on the facies distribution. If the entire extent of the basin deposits is fluvial, the basin can be recognized as an overfilled basin (e.g., Limarino et al., 2001; Cavinato et al., 2002). However, the control factors on basin filling must be difficult to understand in actual rift basins because of the post depositional deformation: basin geometry when the strata were formed is hardly reconstructed, particularly in the older rift basins. In the central part of the rift, small basins may have coalesced as the basin subsidence and sediment fill progressed, resulting in increasingly complicated sedimentation processes in the basin (Cowie et al., 2000; Gawthorpe and Leeder, 2000). In contrast, half grabens that develop in the early phase of rifting appear to be hydrologically closed, and doming may also prevent sediment supply from outside the rift zone (Frostick and Steel, 1993; Gawthorpe and Leeder, 2000). In such cases the basins are expected to be underfilled. However, sediment supply to such closed basins may also occur by gravity transport from volcanoes, such as pyroclastic flow and ash fall (cf. Ashley and Renaut, 2002). The Kenya Rift lies in the eastern branch of the East African Rift (Frostick, 1997; Fig. 1A), and was strongly affected by basaltic or trachytic volcanism (Einsele, 1992). The Kenya Rift has been active for the past 30 million years. By 12–15 million years ago, it began to experience broad regional uplift, local doming, increased heat flow, and the development of

magma centers in the upper crust (Barker, 1986; Morley et al., 1999). Barker (1986) indicates that a half graben formed in the Late Miocene by faulting succeeded by further eruption in the rift floor. By earliest Pliocene time a graben had formed with faulting of the flexed eastern margin of the depression. Succeeding periods of faulting and volcanism migrated inwards, creating step-fault platform and a narrow inner graben (Barker, 1986; Einsele, 1992). The sediment successions formed in the early stage of rifting are now exposed on the marginal part of the rift basin (Williams and Chapman, 1986; Ashley and Hay, 2002; Fig. 1B). To develop a better understanding of basin fill processes in half graben terranes, we undertook a detailed study of the fill of a small half graben fill in the marginal part settings of the Kenya Rift. Thick sedimentary successions filling small half grabens are well exposed on the east side of the Suguta Valley in the north of the Kenya Rift (Sawada et al., 1998). The sparse vegetation in this area allows for uninterrupted observation of the entire succession of small basin fill sediments, enabling a good detailed understanding of the basin fill processes. The present study focuses on the Miocene Namurungule Formation. 2. Geological setting The Samburu Hills are located on the eastern flank of the Kenya Rift, approximately 50 km south of Lake Turkana (Fig. 1B). Neogene volcanic and sedimentary rocks related to rift activity in the Miocene period are widely distributed in the hills (Fig. 2A). In this area, the rift margin is a steep flexure affecting the distribution of Miocene volcanics and sediments (Williams and Chapman, 1986; Sawada et al., 1998; Fig. 2A). The strata of this area make up part of the Samburu monocline and tend to incline toward the rift valley axis on a broad scale. The strata strike N–S and dip toward the west by up to 408 (average 10–158) (Fig. 2). The strata are subdivided into small blocks separated by faults extending NNE–SSW, subparallel to the rift axis. The strike and dip of strata in the blocks are strongly controlled by this NNE–SSW fault system. A more detailed description of the geologic structure of this area can be found in Makinouchi et al. (1984) and Sawada et al. (1987). The Miocene succession unconformably overlies gneisses of the latest Precambrian Mozambique Belt in many sites, and is bounded by local faults in some locations. The Miocene succession consists, from base to top, of the Nachola, Aka Aiteputh, Namurungule and Kongia formations (Sawada et al., 1998, 2001; Fig. 3).

M. Saneyoshi et al. / Sedimentary Geology 186 (2006) 111–131 Fig. 1. (A) Map showing the main lakes and structures in the East African Rift (modified from Frostick, 1997). (B) Structural and locality map of the Kenya Rift between the equator and southern end of Lake Turkana (modified form Williams and Chapman, 1986). The Samburu Hills is located on the northern part of the Kenya Rift. Inset shows the location of the geological maps in Fig. 2.

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114 M. Saneyoshi et al. / Sedimentary Geology 186 (2006) 111–131 Fig. 2. (A) Geological map of the Samburu Hills and (B) cross-section. The Samburu Hills are located on the eastern flank of the Kenya Rift. (Inset) (C-1) is a detailed geologic map of the Namurungule Formation and (C-2) shows the location of columnar sections in Fig. 4.

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Fig. 3. Generalized lithofacies, paleoenvironments, tectonics, whole-rock K–Ar ages, magnetostratigraphy (from N1 to N3) and intervals yielding hominoid fossils for the Neogene formations on the eastern flank and floor of the Rift Valley in the vicinity of the Samburu Hills (after Sawada et al., 1998, 2001; Sawada et al., in press). Whole-rock K–Ar ages and magnetic polarity are shown in Ma. Magnetostratigraphy of upper part of the Aka Aiteputu Formation and the Namurungule Formation was from Sawada et al. (in press).

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The Lower Pliocene Tirr Tirr Formation unconformably overlies the Kongia Formation in the north (Fig. 2B). The Nachola Formation is up to 200 m thick and consists, from base to top, of rounded gravel beds, arkosic sandstone beds, basalt lava, and trachytic pyroclastic rocks with welded tuff, secondary precipitated chert, quartz arenite and phonolitic trachytes. Wholerock K–Ar age measurements give a time range of 19– 15 Ma for deposition of this formation (Itaya and Sawada, 1987; Tatsumi and Kimura, 1991). The Aka Aiteputh Formation conformably overlies the Nachola Formation, is 370 m thick and consists of lower, middle and upper sub-units. The lower unit is comprised of trachytic pyroclastic flow deposits with welded tuff and fluvial sediments. Abundant fossils of Nacholapithecus kerioi fossils (Nakatsukasa et al., 1998; Ishida et al., 1999) have been collected from fluvial sediments and tuff beds from this unit (Sawada et al., 2001). The middle unit consists of basalt lavas with intercalated trachyte lavas and pyroclastic rocks, and the upper unit is characterized by basalt, sedimentary rocks, and red soil. Whole-rock K–Ar ages indicate that this formation was deposited between 15.0 and 9.9 Ma (Itaya and Sawada, 1987; Tatsumi and Kimura, 1991; Sawada et al., 1998). The Namurungule Formation overlies the Aka Aiteputh Formation. Sawada et al. (in press) recognized that the base of the Namurungule Formation was a conformity (Fig. 3). Some parts of the boundary are marked by local faults (Sawada et al., 2001), and whereas the two underlying formations appear to be continuously distributed on the geologic map (Fig. 2B), the Namurungule Formation is segmented by faults (Fig. 3). Each segment is somewhat elongated in the N–S direction, almost parallel to the fault system of this area. This formation is ca. 200 m thick, and consists predominantly of tuffaceous sandstone and siltstone beds (Tateishi, 1987). Poorly sorted conglomerate beds containing bouldersized clasts are interbedded in the basal and middle parts of the formation. The basal conglomerate is 3 m thick and is characterized by lapilli tuff beds with pumice and wood fragments, whereas the conglomerate bed in the middle of the formation is up to 30 m thick. Tateishi (1987) recognized that the Namurungule Formation consists of fluvio-lacustrine and mud flow deposits. The Namurungule Formation has yielded many bone fossils of mammals and reptiles (Nakaya et al., 1987; Kawamura and Nakaya, 1987; Nakaya, 1994), including Samburupithecus kiptalami (Ishida and Pickford, 1997), as well as footprints of mammals and birds (Nakano et al., 2001). Sanidine K–Ar ages of the pyroclastic flow deposit and pumice in the lower part of the formation are

9.47 F 0.22 and 9.57 F 0.22 Ma, respectively (Sawada et al., 1998). Magnetostratigraphic analysis has revealed three magnetozones; N2, R2 and N3 (Sawada et al., in press). The N2 zone is closely correspondent to the lower part of the Namurungule Formation (0–94.2 m), and the R2 zone correlates with the upper part of the formation (94.2–159 m). From these K–Ar ages, the N2/R2 polarity boundary can be correlated with the chron boundaries of C4Ar.2r/C4Ar.2n or C4Ar.3r/C5n (Cande and Kent, 1995). The latter is most plausible considering the K–Ar ages of the Namurungule and Aka Aiteputh formations. The N3 zone was only recognized in location NK1. Recent research has shown that the sediment characteristics of each segment (e.g., thickness, grain size, facies) vary slightly between segments, although the broad-scale stratigraphy remains similar (Sawada et al., in press). This implies that the segments were separated during sedimentation rather than by fault activity some time after deposition, probably in the last depositional stage of the Aka Aiteputh Formation when downward warping of the rift may have begun (Sawada et al., 1998). The Kongia Formation unconformably overlies the Namurungule Formation. Some parts of the Aka Aiteputh and Namurungule formations are deeply truncated by the gravel beds of the Kongia Formation, which is more than 400 m thick and consists predominantly of basalt lava. Whole-rock K–Ar ages indicate that this formation was deposited in the period 7.3–5.3 Ma (Itaya and Sawada, 1987; Tatsumi and Kimura, 1991; Sawada et al., 1998). The Tirr Tirr Formation, unconformably overlying the Kongia Formation, consists of basalt lava and has a maximum thickness of about 180 m (Kabeto et al., 2001). Whole-rock K–Ar ages indicate that this formation was deposited between 4.1 and 3.6 Ma (Baker, 1963; Itaya and Sawada, 1987; Tatsumi and Kimura, 1991). In the Suguta Valley, the recent rift center of this area, the predominant near-surface sediments are diatomites and basaltic lava of the Pliocene to Quaternary periods. The ages of basaltic rocks determined by whole-rock K–Ar age measurements range from 2.0 to 0.1 Ma (Itaya and Sawada, 1987; Tatsumi and Kimura, 1991). The estimated size of the basin in this study area is 6 km from north to south, and 2 km from east to west. The four main outcrops shown in (Fig. 4) are described in detail below. 3. Facies association The Namurungule Formation succession was characterized by facies analysis (Walker, 1984; Walker and James, 1992), which revealed alluvial fan and delta

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Fig. 4. Columnar cross-sections of the study sites. Magnetostratigraphy was from Sawada et al. (in press). The Namurungule Formation mainly consists of two magnetozones (N2 and R2). Rose diagrams show the paleocurrents (up = north). Delta front deposits are observed only in KI2. Channel fill deposits are well preserved in lacustrine delta Type 2 than lacustrine delta Type 1 (see in text). Brackets denote the intervals of columnar sections in Figs. 7 and 9.

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Table 1 Lithofaceis and sedimentary characteristics of the Namurungule Formation Facies Lithology code

1 2

3

5

6

7 8

9

10

Volcanic ash, lapilli, welded pumice Massive, crudely bedded, muddy matrix, partly coarse tail inverse grading Poorly sorted sandy mudstone Massive, deformed lamination with scattered gravel Sorted gravely sandstone, Trough or planar cross bedding very coarse to coarse sand with pebble Alternations of Sandstone A; very coarse to sandstone and mudstone coarse sand with rare pebbles and granule, trough cross-stratification, tabular cross-stratification, current ripple lamination Sandstone B; medium to fine sand with volcanic ash, hummocky cross-stratification, parallel lamination wave-ripple lamination Massive mudstone; small burrows, rootlets, desiccation cracks, bioturbated Sandstone Planar cross-stratification, coarse sediments concentrated base of the cross-stratification Mudstone Millimeter-scale lamination, green colored Gravely sandstone, Trough cross stratification, partly granule to medium current ripple cross lamination sand with pebbles Alternations of sandstone Sandstone; very coarse to fine and mudstone sand with granules, parallel stratification, current-ripple lamination Mudstone; massive, rootlets, desiccation cracks Mudstone Massive, rootlets, desiccation cracks, sandpiper, bioturbation Very poorly sorted Massive, volcanic ash, backset tuff beds structures, imbrication of gravel

Basal contact

Thickness (m)

Interpretation

Facies association Alluvial fan

Plane Plane

2.0–3.0 0.5–2.0

Pyroclastic flow Debris flow

+ +

Plane

0.1 N

Mud flow

+

Erosive

0.3–1.5

Fluvial channel fill

+

Erosive

0.1–1.0

Fluvial channel fill (delta plain)

Plane

0.1–0.5

Flood plain inundation

Plane

2.0 N Flood plain (average: 0.2–0.5) (delta plain) 0.6 N Delta front

Erosive

Plane

3.0 N (average: 1.0) Erosive (partly plane) 0.2–1.0

Erosive

0.1 N

Plane

0.1 N

Plane

0.3–3.0

Plane

20.0 N

This formation revealed alluvial fan and delta facies associations along with lahar deposits.

Lacustrine Lacustrine delta type 1 delta type 2

+

Lacustrine (prodelta)

+

+

+

+

Fluvial channel fill (delta plain)

+

Flood flow or natural levee (delta plain)

+

Flood plain (delta plain) Lahar deposits

+ Middle part of the Namurungule Formation

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4

Poorly sorted tuff beds Very poorly sorted conglomerate

Sedimentary structure

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facies associations along with lahar deposits (Table 1). Although the lahar deposits are not strictly representative of the depositional environment, they are of sufficient thickness to have a significant effect on sediment accumulation in this basin and are thus included in the present facies descriptions. 3.1. Alluvial fan association This facies association consists of (1) massive, poorly sorted tuff beds, (2) very poorly sorted conglomerate beds, (3) massive, poorly sorted sandy mudstone beds with scattered gravel and (4) well-sorted, trough or planar cross-stratified gravelly sandstone beds intercalated with the conglomerate beds. (1) Massive, poorly sorted tuff beds Description: The massive, poorly sorted tuff beds are composed mainly of tuff and lapilli tuff, and is welded in some locations (Fig. 5A). This facies is 2.0–3.0 m thick. Interpretation: Welded pumice particles in this facies clearly indicate that the sediment transport was related to a volcanic eruption. The massive, poorly sorted character of this facies implies rapid deposition from pyroclastic flow (Fisher, 1971). The absence of gas segregation pipes formed by interaction between hot pyroclastic flow and water implies that this facies was deposited in a subaerial setting. (2) Very poorly sorted conglomerate beds Description: The very poorly sorted angular conglomerate beds mainly consist of gravel-sized clasts of trachytic volcaniclastics, basalt lava, mudstones and sandstones (Fig. 5B). This facies is 0.5–2.0 m thick, and each constituent gravel bed is up to 1.0 m thick with sharp and flat bases and a sheet-like geometry with good lateral continuity. The maximum grain size is 50 cm in diameter. The gravel beds are massive. The matrix is composed of muddy sediments. In the lowest unit of the facies, the conglomerate beds are rich in basalt, and contain a red sandy-mud matrix. Clasts in the beds exhibit local coarse inverse grading. Local imbrication of gravel grains has also been observed. Paleocurrent analysis indicates flow directions towards the west to southwest (Fig. 4). Interpretation: Very poorly sorted conglomerate beds with a fine matrix and local inverse grading are typical of debris flow deposits (Miall, 1992, 1996). The debris flow deposits are only recognized in the coarse sediment interval of the lower part of the Namurungule Formation. Reworked pyroclastic flow deposits may be retransported as debris flow.

Fig. 5. Photographs of an outcrop showing the alluvial fan association. (A) Pyroclastic flow deposits at NK1 (Facies 1), displaying welded pumice (arrows). (B) Poorly sorted boulder conglomerate beds at NK1 (Facies 2). Outcrop is about 4 m high. (C) Granular to very coarse sandstone beds with trough cross-stratification at NM2 (Facies 4).

(3) Massive, poorly sorted sandy mudstone beds with scattered gravel Description: The fine sediment and gravel of this facies are characterized by volcaniclastics. The facies is up to 1.0 m thick, with sharp-based beds having a

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sheet-like geometry. The angular to subrounded gravel clasts consist mainly of basalt, trachyte, sandstone and mudstone, with sizes of up to 30 cm in diameter. Gravel is scattered throughout the muddy matrix in a disorganized fabric. The muddy component exhibits deformation and a faint folded lamination in some locations. Interpretation: The presence of scattered gravel in mudstone suggests deposition from a flow with sufficient matrix strength to support the coarse sediments (e.g., Middleton and Hampton, 1976). This feature, together with the faint deformed lamination, represents clear evidence of plastic deformation. Thus, this facies is thought to have been deposited from a highly concentrated sediment gravity flow (i.e., mud flow) in which sediment would have been supported by the matrix and transported as laminar flow. (4) Well-sorted, gravelly sandstone beds with trough or planar cross-stratification Description: These beds are composed mainly of coarse to very coarse sand with scattered pebble-sized clasts (Fig. 5C). This facies is 0.3–1.5 m thick. Gravels of this facies originate from basalt lavas. Each trough cross-stratification bed exhibits a concave-up erosional base. Distinct erosion surfaces at the base of this facies were observed at locations NM2 and NK1. Paleocurrent directions determined from the trough cross-stratification range from north to south but with dominantly westward paleoflow. Interpretation: These beds are interpreted as channel fill deposits, as the erosional bases and cross-stratification indicate unidirectional sediment transport as bedload. The predominance of trough cross-stratification throughout this facies is representative of active lateral channel shifting. The absence of upward-fining or epsilon cross-stratification, important indicators of meandering streams, implies that this facies is of braided stream origin. This coarse sedimentary facies association, with braided stream, debris flow and mudflow deposits and evidence of pyroclastic flow, is interpreted as a small alluvial fan system developed associated with volcanic activity. As such a coarse sediment interval is not found in other parts of the Namurungule Formation (in Fig. 2 C-1); this alluvial fan should be recognized as a transient event, probably forming as a result of the supply of coarse sediments following pyroclastic flow events. Many examples of alluvial fan growth with volcanic activity have been also reported in the literature (e.g. Ayder, 1998; Zanchetta et al., 2004). Although the upper reaches of the alluvial fan are not represented

due to subaerial erosion, paleocurrent analysis suggests sediment transport from the northeast to the southwest. 3.2. Lacustrine delta association The lacustrine delta deposits occupy the majority of the Namurungule Formation (Fig. 4). The characteristics of the delta deposits above and below the thick interbedded pyroclastic flow deposits differ appreciably. For this reason, the delta deposits are referred to here as type 1 (below the pyroclastic flow deposits) and type 2 (above the deposits). 3.2.1. Type 1 delta deposits The type-1 delta deposits include three facies; (5) alternating sandstone and mudstone beds, (6) planar cross-stratified sandstone beds, and (7) laminated mudstone beds. (5) Alternating sandstone and mudstone beds Description: The sandstone and mudstone beds contain volcaniclastics such as pumice and volcanic ash. The mudstone beds are white or green in color and include abundant scattered granule- to pebble-sized pumice fragments. Two types of sandstone were identified in the alternation. Sandstone A consists mainly of very coarse to coarse sand with rare pebbles and granules of pumice or mud clasts. These sandstone beds are 0.1–1.0 m thick, and exhibit trough cross-stratification, tabular cross-stratification, and current ripple lamination (Fig. 6A). The bases of these sandstone beds are erosional surfaces. The sandstone beds with current ripple lamination are comprised of coarse to fine sand and are 0.1–0.3 m thick. The ripplelaminated sandstone beds generally have flat bases, exhibit a sheet-like geometry, and can be traced for up to 500 m laterally in the vicinity of locations NK1 and NM2. Some beds exhibit wave ripple lamination at the top of the bed (Fig. 6B). This type of sandstone occurs less frequently than sandstone B, which overlies this subfacies. Paleocurrent analysis from cross stratification and current ripple lamination indicates wide ranges of flow directions, but a weak trend from north to south or east to west are recognizable in the rose diagram (Fig. 4). Sandstone B consists of medium to fine sand, most parts of which are characterized by fine materials of pyroclastic fall origin (very fine sand to coarse silt). This facies is 0.1–0.5 m thick, and displays hummocky cross-stratification (HCS), parallel stratification, and wave ripple lamination (Fig. 6C). Wavelengths in the HCS are up to 0.4 m and heights are less than 0.02 m. The HCS is covered by wave ripple lamination in some

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Fig. 6. Photographs of outcrop showing type-1 delta deposits. (A) Granular to very coarse sandstone beds with trough cross-stratification (sandstone A in Facies 5; indicated by an arrow). The hammer for scale is 30 cm long. (B) Sandstone A (black arrow) and overlying sandstone B at KI2 (Facies 5). Wave ripple laminations (white arrows) are observed at the top of sandstone A. (C) Fine to very fine sandstone bed with wave ripple laminations (arrows) in sandstone B at KI2 (Facies 5). (D) Sandstone beds with hummocky cross-stratification (sandstone B; indicated by an arrow) at NK1 (Facies 5). This bed is 20 cm thick, and is bounded above and below by mudstone flood plain deposits.

units. These sandstone beds also exhibit a sheet-like geometry with good lateral continuity; some beds are traceable for at least 1000 m in the vicinity of locations NK1 and KI2. The top of the sandstone is covered by mudstone with a sharp contact. The mudstone beds of this facies, typically light brown to pale yellow, are characterized by volcanic products (volcanic glass, anorthclase and pumice), and are up to 2.0 m thick (average: 0.2–0.5 m). The mudstone beds contains small tubate burrows from 0.5 to 5.0 cm in length, rootlets from 0.1 to 3.0 cm in length, and desiccation cracks. No first-order erosion surfaces occur within the HCS sand beds. Desiccation cracks with spacing from 0.1 to 2.0 cm are commonly found in the top of the mudstone beds. Some parts of the mudstone beds are intensely bioturbated. Interpretation: The presence of sedimentary structures formed by unidirectional currents, coarser sediments and erosional bases (i.e., sandstone A) indicates

that this facies is of fluvial channel fill origin. As each sandstone bed of this facies is thin and widely extensive, the channels are considered to have been broad and shallow. The wavy ripple-lamination and hummocky cross-stratification of sandstone B clearly show that the sandstone beds accumulated in a subaqueous environment. HCS formed by unidirectional flow, known as hummocky cross-stratification mimics, has been reported for the top of channel fill and natural levee deposits (Rust and Gibling, 1990; Masuda et al., 1993) and has been interpreted as being of antidunal origin (Rust and Gibling, 1990). Although there is no direct evidence indicating whether the hummocky cross-stratification was formed by wave action or unidirectional current flow, the beds can be inferred to have been deposited under oscillatory or combined flow conditions from the facts that the beds are laterally continuous for at least 1000 m, indicating submergence of the entire flood plain environment, and that some beds are topped by wavy ripple-

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lamination. Because shoreline indicators have not been found, the water depth of deposition of HCS sandstone beds remains unknown. In another example, Greenwood and Sherman (1986) reported HCS sand beds from the surf zone of the Lake Huron (ca. 2 m depth). We believe that the Namurungule Formation HCS sandstone beds were also accumulated under a water depth almost the same depth as in the Lake Huron. In each HCS bed, the first-order erosion surface was not found. Because the bed consists of tuffaceous material which can be more readily suspended than quarts or feldspar grains, the sediments could be fully suspended during the highwater stage. Only the HCS beds which accumulated during the final phase of the high-water stage could be preserved in the stratigraphic record. The mudstone beds are interpreted as being deposited in a flood plain environment based on the presence of rootlets, desiccation cracks, and bioturbation. The sand beds would have been deposited during periods when the flood plain environment was submerged, whereas the mud beds accumulated during dry periods, leading to root growth, bioturbation and the formation of desiccation cracks. The sequence of mudstone and sandstone beds is therefore representative of a flood plain environment that was repeatedly inundated and dried up. The rare fluvial channel fill deposits in this facies are overlain by sandstone B and mudstone, indicating that these channels were also submerged and covered with sand and mud during periods of flood plain inundation (Fig. 7). (6) Planar cross-stratified sandstone beds Description: Sandstone beds with planar cross-stratification were observed only at KI2. This facies reaches 0.6 m in thickness and can be traced laterally for at least 50 m. Coarse sediments tend to be concentrated near the base of the cross-stratification. This facies overlies laminated mudstone beds of facies 7 and is itself overlain by sandstone A (up to 10 cm thick) which is, in turn, overlain by the next laminated mudstone beds of facies 7. Interpretation: The concentration of coarser grains near the base of the cross-stratification suggests that these planar cross-stratifications were formed by smallscale grain flows (Middleton and Hampton, 1976). As there is no evidence of sediment reworking by unidirectional flow in this interval (such as trough crossstratification), this facies can be distinguished from fluvial channel fill deposits. The character of deposits above and below this facies suggests this thin planar cross-stratified facies represents delta-front sedimentation. However, the thinness of the facies indicates that the delta was very small.

Fig. 7. Detailed stratigraphic columnar sections of type-1 delta deposits at NK1 (see Fig. 4). The alteration of sandstone and mudstone beds (Facies 5) consist of this succession. Massive mudstone beds are covered by HCS sandstone beds (sandstone B) in many horizons.

(7) Laminated mudstone beds Description: The laminated mudstone beds are up to 3.0 m thick (average: 1.0 m), are green in color, and contain millimeter-scale laminations. This facies is very well sorted. Rootlets and bioturbation are not evident in this facies. Interpretation: The laminated mud facies is interpreted as representing lacustrine deposits, supported by the lack of bioturbation (probably anoxic conditions), very well sorted fine-grained laminated mudstone, and similarity to other lacustrine facies (e.g., Martel and Gibling, 1991; Browne and Plint, 1994). Stacking pattern The lower part of the formation thins toward the north (i.e., landward). The succession is slightly thinner at NM2 than at NK1, KI1 and KI2 (Fig. 2; C-1 and Fig. 4). In the area of the Namurungule Formation under study, the alternating sandstone and mudstone bed facies is dominant. Planar cross-stratified sandstone beds (delta-front deposits) appear only at sites of thicker strata on the same horizon. Thicker laminated mudstone beds (lacustrine deposits) are observed at KI2, transi-

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tioning northward into flood plain deposits, which are recognized near the top of the lower part of the formation at the north, where the lower part is capped by flood plain mudstone. This lateral facies change suggests an overall retrogradational pattern in this interval, from the crosssection along basin-tilting trend except for the uppermost 5 m in which flood plain deposits and prograding delta deposits were observed (Fig. 4). In addition, the stratigraphic section showing retrogradational stacking pattern is almost parallel to paleoflow trends that trend toward the south or west (Fig. 4). The existence of thicker strata in the south probably reflects an in increase in the rate of subsidence, whereas the thinner succession at KI1 and the presence of thin lake deposits at NK1 suggest local uplift around NK1. The flood plain deposit at the top of this interval is indicative of a stabilization of lake level. 3.2.2. Type 2 delta deposits The type-2 delta deposits are composed of (8) gravelly sandstone beds with trough cross-stratification and current ripple lamination, (9) alternations of sandstone with current ripple lamination and mudstone beds, (10) massive mudstone beds, (6) planar cross-stratified sandstone beds, and (7) laminated mudstone beds. This facies association also contains volcaniclastics such as pumice and volcanic ash. The planar cross-stratified sandstone beds and laminated mudstone beds are essentially identical to those of the Type-1 delta deposits and are ascribed the same enumeration. (8) Gravelly sandstone beds with trough cross-stratification and current ripple lamination Description: These beds are mainly composed of granule and very coarse to medium sand, with pebbles scattered near the bottom of the beds. The gravel clasts are commonly rounded. Basalt grains are also found in this facies. This facies is 0.2–1.0 m thick (Fig. 8A), and beds have local concave-up erosional bases. The dominant paleocurrent direction based on cross-stratifications is toward the west to southwest (Figs. 4 and 9), although some areas exhibit northwestward flow (Fig. 4). This facies is overlain by facies 6 and 9. Interpretation: These gravelly sandstone beds with erosional bases are interpreted to have accumulated from unidirectional flow and are interpreted as fluvial channel fill deposits. The good lateral continuity of the facies but lack of distinct upward-fining or lateral accretion pattern (indicators of meandering streams) implies that this facies consisted of fluvial channels forming a braided stream network.

Fig. 8. Outcrop photographs of the Type 2 delta deposits. (A) Sandstone beds with trough-cross stratification at Loc. KI2 (Facies 8). Scale is 10 cm long. Arrows indicates basalt gravels. (B) Massive mudstone beds with rootlets (Facies 10). (C) Type 2 delta succession at Loc. NK1 (Facies 1 8 anf 10). The outcrop is 2.5 m high.

(9) Alternations of sandstone with current ripple lamination and mudstone beds Description: The sandstone alternations consist of very coarse- to fine-grained sand with granule-sized clasts. The muddy part is massive. This facies is 0.1– 0.2 m thick, and each sandstone and mudstone bed is a

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Fig. 9. Detailed columnar sections of Type 2 delta deposits at KI2 (see Fig. 4). The left columnar section shows the interval marked with a bracket in KI2 in Fig. 4. A complete delta sequence is characterized by PD: prodelta deposits (Facies 7), DF: delta-front deposits (Facies 6) and DP: deltaplane deposits (8, 9 and 10) in this successions.

few centimeters thick. Each bed has a sheet-like geometry. The sandstone beds exhibit current ripple and climbing ripple lamination. Rootlets, in length from

0.1 to 3.0 cm, and desiccation cracks, in spacing from 0.5 to 5.0 cm, are apparent in the mudstone beds. This facies is overlain by facies 10.

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Interpretation: The alternation of thin sandstone and mudstone indicates the cyclic weak flow and calm conditions. The presence of rootlets, desiccation cracks, and frequently interbedded thin sandstone beds above fluvial channel fill deposits implies that this facies represents a natural levee or flood plain environment adjacent to channels. The sandstone beds with current ripple lamination would have been deposited during flood flow, and the overlying mudstone beds may have been deposited as suspension fallout during stagnation. (10) Massive mudstone beds Description: The massive mudstone beds, typically brown to pale yellow, are characterized by volcanic products (volcanic glass, anorthclase and pumice), and are very well sorted. Gravelly sandstone beds generally overlie these mudstone beds. This mudstone facies is 0.3–3.0 m thick, and contains burrows 0.1 cm in diameter and 10 cm at most in length, as well as rootlets, from 0.5 to 5.0 cm in length, and desiccation cracks with spacing from 0.1 to 0.5 cm. The mudstone beds are intensely bioturbated in some locations. This facies is overlain by facies 6 and 8. Interpretation: The presence of rootlets and desiccation cracks is a good indicator of a flood plain environment. The presence of fluvial channel deposits above and below this facies also supports this interpretation. Stacking pattern This upper part of the Namurungule Formation is comprised of several prograding delta deposits (Figs. 8C and 9). A complete delta sequence is characterized by laminated mudstone beds (prodelta), tabular crossstratified sandstone beds (delta front) and repeated appearance of gravelly sandstone beds, alternations of thin sandstone and mudstone beds, and massive mudstone beds (fluvial channel and flood plain deposits). Complete successions are only found at KI2, and each succession is about 2 m thick with a lateral extent of at least 300 m to the southwest (the progradation direction; Fig. 9). This delta progradation direction is almost parallel to paleoflow trends which are toward the south to west (Figs. 4 and 9). At other locations, part of the delta succession, typically the delta front, is missing due to truncation by fluvial channels. As all of the delta successions exhibit similar characteristics in the upper part of the formation, the overall stacking pattern of this interval can be recognized as aggradational, although the distribution of the lacustrine facies is shifted landward com-

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pared to the lower part of the formation. That is, the delta front progradation of the upper part occurred further basinward than in the lower part. The broadscale upward facies change indicates a progradational stacking pattern. 3.3. Lahar deposits Description: A thick gravel bed in the middle part of the formation (up to 20 m thick) was found in all segments of this formation (Fig. 4). This facies is represented by a single gravel bed that is poorly sorted, and matrix supported. The angular to subangular gravel bed consists of a muddy matrix with basalt, trachyte, sandstone and mudstone clasts of up to 5.0 m in diameter. Matrix consists mainly of volcanic glass fragments with alkali feldspar. Local imbrications of gravel grains and backset structures have also been observed (Nemec, 1990) (Fig. 10). Paleocurrent data obtained from coarse pebble or cobble-clast imbrication demonstrate sediment transport from the northeast to southwest. Interpretation: This matrix-supported coarse gravel bed is interpreted to represent sedimentary gravity flow, most likely debris flow, pyroclastic flow or lahar. The exact flow type could not be determined due to the lack of welded structures, but it is believed to be lahar considering that the unit is found in almost all of the small separated basins of the Namurungule Formation. Furthermore, the unit was deposited in the early phase of rifting when it is unlikely that a topographic high sufficient to trigger such a voluminous debris flow would have existed in the marginal part of the rift basin. Thus, it is considered that only a large-scale

Fig. 10. Pyroclastic flow deposits in the middle part of the Namurungule Formation at NM2, showing imbrication of gravels (long arrows) and backset structure (dotted lines). The outcrop is 15 m high.

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lahar could have resulting in the widespread deposition of this unit. Imbrication directions indicate that the sediments were supplied from the northwest. 4. Discussion 4.1. Basin type and stacking pattern The Namurungule Formation is considered to have accumulated in an under-filled type basin (Carroll and Bohacs, 1999) for two key reasons: (1) sediments of the delta deposits do not contain material from outside of the basin such as gneisses of the Mozambique Belt, and (2) sediment accumulation was continuous throughout the section. The dominance of flood plain deposits in the upper part (Fig. 9) also demonstrates continuous sediment accumulation associated with long-term lakelevel rise, which indicates that the potential accommodation space in the basin was maintained or increased throughout this depositional timeframe. The stacking pattern of delta deposits in the lower part of the formation differs from that in the upper part. The lower part is characterized by thick delta plain deposits and lateral equivalent lacustrine deposits in the south (type 1) with an overall retrogradational stacking pattern (Fig. 4), while the upper part consists of a pile of delta successions (type 2) throughout the study area (Fig. 9). Delta-front deposits and probably part of the prodelta (lacustrine) deposits are missing in the north. The retrogradational pattern in the lower part is considered to have been caused by lake level rising faster than the sedimentation rate. In contrast, the pile of prograding delta successions in the upper part would have formed as a result of alternating long term lakewater flooding and sediment progradation. There are several possibilities for the causes of these flooding events including local tectonic subsidence and lake level rise associated with a transient increase in water influx. Fig. 8 shows the lateral change in thickness of three deltas in the upper part of the successions. The middle prodelta deposits taper out toward the NE in a 200 m-wide outcrop, even though it covers the flood plain deposits suggestive of nearly flat topography. As the flood plain around the lake can be recognized as being almost flat and the top of the flood plain deposits is not deeply truncated, the thickness change is attributable to tectonic subsidence in the southwest. The lowest prodelta deposits maintain a near-constant thickness and the uppermost delta-front deposits, the prodelta deposits of which are indistinct, are preserved throughout, indicating that there was no differential

subsidence. These two flooding events are therefore considered to be related to a transient increase in water influx. Similar lateral facies changes are observable on other horizons. The southward-thickening trend in the study area and the absence of delta front-deposits in the north is therefore attributed to faster subsidence in the south (or relative uplift in the north). From the chronological data, the stacking pattern changes are interpreted to be associated with a decrease in the rate of lake level rise. Two magnetozones, N2 and R2 (9.64–9.58 and 9.58–9.31 Ma, respectively), were mainly detected in the Namurungule Formation (Sawada et al., in press). The sedimentation rates estimated roughly from these ages are 151 cm/kyr in the period 9.64–9.58 Ma and 24 cm/kyr in the period 9.58– 9.31 Ma (Fig. 11). The lower part is associated with the N2 zone, while the majority of the upper part corresponds to the R2 zone. This indicates that the change from the type-1 to type-2 delta sequence was probably induced by a decrease in the rate of lake level rise during the period in which the upper part of the Namurungule Formation was deposited. Schlische and Olsen (1990) showed that a systematic decrease in sedimentation rate occurs as the depositional surface grows. This process may be related to the slowing of lakelevel rise. If the flat of the basin (the lake and delta plain) widened, a greater volume of water is required to cause a lake-level rise, leading to decrease in the rate of lake-level rise under constant water supply from the drainage area. In this case, both the sedimentation rate and rate of lake-level rise would decrease in proportion to the increase in the area of the lake and depositional surface. The prevailing sedimentation rate became higher than the rate of lake-level rise, resulting in progradation and the formation of the type-2 delta succession. 4.2. Sediment supply during deposition of the lower Namurungule Formation The majority of the type-1 delta plain deposits consist of flood plain deposits, characterized by alternating sandstone and mudstone beds, and some fluvial channel fill deposits. The fluvial channel fill deposits are overlain by sandstone beds with hummocky cross-stratification, wave ripple lamination and parallel stratification (type-B sandstone beds), and then by mudstone beds of flood plain origin. This upward facies change indicates that the channels were short-lived and completely buried by mud when the flood plain was submerged. The presence of thick flood plain deposits is inconsistent with the rarity of fluvial channel fill deposits. One

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Fig. 11. Estimated sedimentation rate for the Namurungule Formation at NK1. The age of magnetostratigraphy is based on Cande and Kent (1995) and Sawada et al. (in press). The sedimentation rates are estimated from these ages, and show decreasing toward the upper part of this formation.

possible reason for the rarity of fluvial channel sand bodies is smaller rate of subsidence (Robinson and McCabe, 1998). In the model shown in Robinson and McCabe (1998, Fig. 12), frequency of fluvial channel sand bodies in flood plain mudstones tend to increase to the depocenter of continental tilting basin. In the case of the Namurungule Formation the fluvial channel deposits are also rare in the thick succession (Fig. 4). Thus, a sediment supply system other than fluvial channels should be considered for the period of deposition of the lower part of the Namurungule Formation. Both the sandstone and mudstone beds of the lower part of the formation are volcanics, consisting mainly of ash (volcanic glass and anorthclase) and subrounded or rounded pumice grains of pyroclastic fall origin. A significant mode of sediment supply to this basin can therefore be interpreted to be pyroclastic fall. This interpretation is also suggested by the lack of sediments derived from basement rock or other underlying formations. This period is characterized by intense volcanic activity to the south (Key, 1987), and it appears that the basin fill is strongly associated with this volcanic activity. The rarity of fluvial channel fill deposits can also be explained by the immaturity of the drainage system and sediment reworking during inundation events. In the early phase of basin development, the drainage system

in the basin may have been poorly developed. It appears that the fluvial system was not connected to other half grabens at that time, and large fluvial channels may not have developed in the small basins. Small fluvial channels may have developed in the initial phase of flooding. As very fine sand to coarse silt material predominates in the sediment of the Namurungule Formation, the fluvial channel fill deposits would also have consisted of such fine materials. Thus, even though substantial fluvial channel fill sediments may have been deposited, subsequent wave or current action would have resuspended the fine sediments during periods in which the delta plain was submerged. The channel fill deposits that were preserved were then buried by sand, and mud beds accumulated during the inundation phase. In contrast, the fluvial channel fill deposits are distinct in the type-2 delta deposits. The good lateral continuity of prograding delta successions indicates that the fluvial channels were long-lived due to stabilization of the lake level. During this period, the fluvial channels would have avoided deep submergence that may have occurred due to short-period lake-level fluctuations, the lake level may have remain stable for a sufficient amount of time to allow the delta to prograde over a wide area (i.e., fluvial channel deposits distributed over wider area), and the basin

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margin may have become increasingly dissected, leading to the creation of larger channels. The presence of rounded basalt grains, probably derived from the Aka Aiteputh Formation, in the channel fill deposits also supports an intensification of dissection at the margins of the basin. 4.3. Difference in flooding style between the lower and upper parts of the Namurungule Formation The delta deposits, particularly the delta plain deposits, occurring above and below the thick pyroclastic flow deposits in the middle part of the Namurungule Formation differ appreciably. The flood plain deposits of the lower part (type 1) contain beds with wave-generated sedimentary structures and parallelstratified beds, and exhibit good lateral continuity. This indicates submergence of the delta plain environment. In contrast, the flood plain deposits of the upper part are characterized by massive mudstone beds and alternations of thin sandstone and mudstone beds interpreted as being deposited near fluvial channels. The presence of thin sandstone beds records flooding of the delta plain, while the absence of wave-generated sedimentary structures indicates that large-scale delta plain inundation did not occur in this phase. The difference between these deposits can be explained by a drying of the climate, or the suppression of short-term, large-amplitude lake-level fluctuations due to widening of the basin flat associated with basin filling. As previous studies concerning climate change in this area have not detected any distinct climatic change at around 9.5 Ma (Cerling, 1992; Kingston et al., 1994; Kingston, 1999), the latter is the most plausible explanation for the reduced frequency of inundation of the flood plain during deposition of the upper part of the formation. 4.4. Comparison with the Ngorora Formation in the Tugen Hills, central Kenya From the stratigraphic analysis, the following characteristics in the early phase of rifting are detected from the Namurungule Formation: (1) the early phase of the basin filling was strongly controlled by supply of volcanic detritus (2) sedimentation rate tended to decrease during the filling of the basin; and (3) the stacking pattern of the delta succession changed from retrogradational to progradational. What controls the early phase of the basin fill should be elucidated through the comparison with other basins. However, the de-

tailed sediment records of the early phase of the basin development are quite rare in the East African Rift. Here we compare the characteristics of sediment successions with one of few examples, the Middle to Upper Miocene Ngorora Formation in Tugen Hills, central Kenya. The Ngorora Formation basins are located in the western margin of the central Kenya Rift, and crops out principally in the scarps and dip slopes of the Tugen Hills Tilt Block (Pickford, 1999). Estimated E– W basin width is about 20 km. According to Bishop and Chapman (1970) and Bishop and Pickford (1975), this formation sediments are divided into 5 units (Unit A–E). The Unit A consists of volcanics and the Unit B and D, and Unit C and E are represented by fluvial deposit and lake deposit respectively (Renaut et al., 1999). The Unit B–E also contains volcanics (Pickford, 1999). Sedimentation rate during deposition of the Unit A to the uppermost part of the Unit B (13.06–12.56 Ma, 123 m thick), the Unit C to Unit D (12.56–11.54 Ma, 131 m thick) and the lower half of the Unit E (11.54–10.51 Ma, 32 m thick) are estimated to be about 0.27, 0.13 and 0.03 m/ kyr, respectively, based on Ar–Ar age data in Pickford (1999). Although two dimensional facies distribution has not been published, the vertical change in depositional environment suggests that the Ngorora and Namurungule formation show the common characteristics of volcanic control of the early basin fill and decreased sedimentation during filling, as described above. However, the succession was finally changed into lacustrine setting, to form a retrogradational pattern even during the seasonal drier period (around 11 Ma: Kingston, 1999; Retallack et al., 2002). It, therefore, interpreted that decrease in sedimentation rate played an important roll for lake expansion in the Unit E period even though the lake level rise was inferred to have slowed down due to both base-level widening and drier climate. As is well known, in theory both sedimentation rate and lake level rise may slow through rise in base level. Whether the retrogradational or progradational stacking patterns are formed depends on which factor (sedimentation rate versus rate of lake-level rise) dominates sediment accumulation processes. In the case of Namurungule Formation, the maximum width of each basin is less than 5 km, one quarter than the Ngorora Formation. Catchment area widens during basin development (Gawthorpe and Leeder, 2000). In very small basins, subtle increase in sediment supply may be easily reflected in the sediment record. Increased sediment supply, associated with drainage area expansion, was

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probably sufficient to prevail over the rate of lake-level rise in the Namurugnule Basin. In the Ngorora Basin the reverse was the case. 5. Conclusions Facies analysis of the Namurungule Formation discriminated alluvial fan, delta successions and thick pyroclastic flow deposits. Two types of delta successions were identified in this formation. The type-1 delta succession occupies the lower part of the formation and consists predominantly of retrograding delta deposits and flood plain deposits with rare interbedded fluvial channel fill deposits. Delta front and prodelta deposits were only represented in the south of the study area. The type-2 delta deposits of the upper part of the formation consist of a pile of delta successions and form a progradational unit on a broad scale. Distinct differences in the environment of flood plain deposition were recognized between the type-1 and type-2 delta deposits. The type-1 delta succession, with alternations of sandstone exhibiting parallel stratification, hummocky cross-stratification and wave ripple laminations, and mudstone with rootlets, burrows and desiccation cracks, records cyclic inundation and drying periods and a distinct lack of well-developed fluvial channel fill deposits. On the other hand, the flood plain deposits of the type-2 delta succession consists of massive mudstone beds with roots and alternations of thin sandstone with current ripple lamination and mudstone beds with roots, indicating that inundation was infrequent in this period. The change in stacking pattern of delta successions from type 1 to type 2 was explained by a decrease in sediment supply and drop in the rate of lake level rise due to the widening of the area of the lake and depositional surface. Maintenance of the sedimentation rate would have resulted in the stacking of progradational deltas in the upper part of the formation. The rarity of fluvial channel fill deposits in the type-1 delta succession is interpreted as reflecting the post-depositional reworking of fine sediments during inundation of the delta plain. The lack of substantial dissection of the basin margin in the early phase of basin development may have played an important role in preventing the preservation of fluvial channel fill deposits. The differences between the flood plain deposits of the type-1 and type-2 successions are interpreted as being related to the flooding scale, which in turn is associated with the areal extent of the basin flat

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(delta plain and lake area). The limited area of the depositional surface during formation of the type-1 succession may have allowed the flood plain to have been deeply submerged briefly, leading to the formation of wave-generated bed forms. During deposition of the type-2 sequence, the wider areal extent would have prevented such deep submergence of the flood plain, thus preserving more of the channel fill deposits. Half grabens that develop near the flank of rift valleys are commonly small and have a poorly developed drainage system, particularly in their early phase. In the case of the Namurungule Formation, the input of a large amount of pyroclastic fall material may have significantly affected the basin fill process. We have recognized these features from the Namurungule Formation and Ngorora Formation of the Tugen Hills, central Kenya: (1) the early phase of the basin filling was strongly controlled by supply of volcanic ash and lahar deposits and (2) sedimentation rate tended to decrease as the basin filled. The difference of stacking pattern between the Namurungule Formation and the Ngorora formation is probably induced by width of basin. The narrow Namurungule basin seems to have been sensitive to increased sediment supply owing to expansion of the catchment area. In the case of intermontane basins, lake-level fluctuations may be affected by local topography. As basin fill progresses, the areal extent of the basin flat part will increase, causing lake-level rise to slow under constant net water influx. The results demonstrated that autogenic control factors play an important role in basin fill processes and should be considered carefully in the analysis of continental basins, and in particular, underfilled basins. Acknowledgements This research was greatly facilitated by the National Museum of Kenya. We thank Dr. M. Tateishi, Dr. H. Kurita, Dr. M. Hyodo, Dr. F. Masuda, Dr. S. Tanaka, Dr. M. Pickford and the staff and students of Niigata University and Shimane University for valuable suggestions and discussion. Our thanks are extended to Kyoto University research group and JSPS Nairobi Office for their support in fieldwork. The assistance of the Turkana people of Baragoi (northern Kenya) with fieldwork is also greatly appreciated. Thanks are also due to Dr. B.P. Roser for critical reading and helpful comments on the manuscript. Two reviewers and editor A.D. Miall are thanked for their constructive comments on the submitted

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