Interaction between alluvial fan sedimentation, thrusting, and sea-level changes: an example from the komeno formation (early pleistocene), Southwest Japan

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SEDIMENTARY GEOLOGY

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ELSEVIER

Sedimentary Geology 92 (1994) 97-115

Interaction between alluvial fan sedimentation, thrusting, and sea-level changes: an example from the Komeno Formation (Early Pleistocene), southwest Japan Fumio Yoshida Osaka Office, Geological Survey of Japan. Government Bldg., No.2, Bekkan, 4-1-67, Otemae, Chuo-ku, Osaka 540, Japan Received March 2, 1993; revised version accepted February 2, 1994

Abstract

The gravelly Komeno Formation, 300 m thick or less, accumulated during 1.0-0.7 Ma along the faulted, N-S-trending Suzuka Mountain front in southwest Japan. The deposits originated as an alluvial fan on the mountain front. Three facies, deposited by debris flow, debris flow and braided flow, and braided flow, can be recognized in the fan body. These facies built two downcurrent facies successions that enable reconstruction of a three-dimensional facies architecture of the body. This fan (Komeno Fan) can be divided into three smaller fans, each stratigraphically composed of two to four different facies and bounded on the west by a segment of the Ichishi Thrust. The bases of these fan bodies become tephrostratigraphically younger northward. The Ichishi thrusting, triggered by severe horizontal crustal compression in SW Japan, began at about 1 Ma. The Komeno Fan, therefore, can be interpreted to be a northerly-formed, coalesced, composite alluvial fan generated by the northerly-shifted Ichishi thrusting. The Komeno Fan sedimentation ended at 0.7 Ma, although the Ichishi thrusting in the study area persisted to near the end of the Middle Pleistocene. The cessation of deposition was probably due to extra-lowering of the base-level of erosion caused by a combination of basin uplift by compression and sea-level fall during deposition.

1. Introduction

Alluvial fan sedimentation at the base of mountains has been so far considered largely dependent on tectonics or climate or both (e.g. Heward, 1978b; Nilsen, 1982; Rust and Koster, 1984; Fraser and Sutter, 1986). Throughout these and other earlier studies, interactions between fault-bounded alluvial fan sedimentations and sea-level changes have been never fully discussed. Therefore detailed field studies on Quaternary

faulted alluvial fan deposits contribute much to our understanding of interactions of faulting and sea-level changes on alluvial fan sedimentation, because synsedimentary faults, mountains (drainage basins), and river systems producing alluvial fan sedimentation show more or less their original frameworks. In addition, the climate and the eustatic sea-level changes throughout the Quaternary have been well studied (e.g. Shackleton and Opdyke, 1976; Crowley a n d North, 1991). Herein I present a study of a well-preserved

0037-0738/94/$07.00 © 1994 Elsevier Science B.V. All rights reserved SSDI 0 0 3 7 - 0 7 3 8 ( 9 4 ) 0 0 0 3 4 - R

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F. Yoshida / Sedimentary Geology 92 (1994) 97-115

Early Pleistocene example of combined effects of thrusting and sea-level changes on alluvial fan sedimentation, that is, the Komeno Fan and the Ichishi Fault in SW Japan (Fig. 1). The Ichishi faulting (thrusting) caused the rapid uplift of the Suzuka Mountains and the Komeno Fan deposition at about 1 Ma. In this setting, not only fan deposits (Komeno Formation) but also synsedimentary fault (Ichishi Fault), drainage basin (Suzuka Mountains), and river systems in and around the fan are still preserved. In addition, the fan sedimentation was affected by the late Quaternary sea-level changes. Therefore this setting can provide deeper understanding of the effects of thrusting and sea-level changes on fault-bounded alluvial fan sedimentation.

2. Geologic and neotectonic setting Two major N-S-trending active faults, the Ichishi and Yoro faults, form the boundaries of a westward-tilting block that is now topographically divided into the Yoro Mountains in the east and a basin (hereafter called the Yoro Basin) in the west (Fig. 1A). To the east, the Yoro Mountains adjoin with another westward-tilting block, where the Nobi Plain (Basin) covers the subsided, western part of this block. The Yoro Basin is bounded on the west by the N-S-trending Suzuka Mountains. The Yoro Mountains are 600 to 800 m high, and the Suzuka Mountains 1000 to 1200 m high. The bedrocks of these mountains are made up of the Permo-Jurassic sandstone, mudstone,

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Fig. 1. Geologic outline of study and adjacent areas: (A) topographic and structural outlines; (B) geological map of the Tokai Group in the Yoro Basin (simplified from Yoshida, 1988) and the Permo-Jurassic rocks in the Suzuka Mountains (simplified from Miyamura et al., 1976) and in the Yoro Mountains (simplified from Takada et al., 1979).

F. Yoshida/ Sedimentary Geology 92 (1994) 97-115

chert, limestone, and basaltic rocks intruded sparsely by the Cretaceous Inugami Granite Porphyry (Fig. 1B). The Yoro and Nobi basins were both buried by a thick sequence of Late Pliocene to Early Pleistocene fluviolacustrine sediments (Fig. 2B), both becoming thicker than 1000 m southwestward. This major basin-fill, called the Tokai Group, is of a larger nonmarine, nonvolcanic basin named the Tokai Sedimentary Basin (Yoshida et al., 1991) which has existed in and around Ise Bay since the earliest Pliocene. Each group in the Yoro and Nobi basins is a part of the upper Tokai Group and consists of muds, sands, and gravels with some intercalated thin, but well traceable tufts (Fig. 3). The Komeno Formation here described was deposited at 1.0-0.7 Ma in the late Early Pleistocene and occupies the up-

99

permost unit of the succession in the Yoro Basin (Yoshida, 1988). The underlying Oizumi Formation is conformably overlain by the Komeno Formation in the Yoro Basin but unconformably overlain by the Yatomi Formation in the Nobi Basin (Fig. 2B). The Yatomi Formation, which is correlative with the Komeno Formation (Yoshida et al., 1990), contains brackish and marine diatom floras (Mori, 1986). Therefore the Tokai Basin suffered a first marine transgression from the Pacific Ocean at the Komeno time (Figs. 2C and 4B). Since then, the subsided area of a N - S trend, including Ise Bay and the Nobi Plain, has undergone periodic marine transgression and regression, during which a cyclic marine and nonmarine sequence of nearly 300 m thick accumulated above the Oizumi Formation (Figs. 2B and 2C). By

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100

F. Yoshida / Sedimentary Geology 92 (1994) 97-115

contrast, the Yoro Basin lacks a thick sequence of late Quaternary sediments above the Komeno Formation. The Ichishi Fault consists of many separate faults arranged in an en-echelon or parallel pattern along the Suzuka Mountain front, and was a synsedimentary fault during deposition of the Tokai Group. According to Yoshida et al. (1991) and Yoshida (1992), the fault movement began as normal faulting at the southern end in about the earliest Pliocene. Moving northward and uplifting the west side, it reached the Yoro Basin at 3 Ma. Since then, the Yoro and northern Ichishi faultings have induced the two block tectonic movements of the Yoro and Suzuka Mountains and the fluviolacustrine sedimentations in the two basins (Fig. 4A). This tectonosedimentary inter-

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action has continued in the Nobi Basin, but ended at about 0.7 Ma with the Komeno deposition in the Yoro Basin (Fig. 2B). The eastern part of SW Japan was widely considered to have been undergoing severe horizontal compressional stress in a WNW-ESE direction throughout the Quaternary (e.g. Huzita, 1976; Okada and Ando, 1979). Recently, Tsunakawa (1986) estimated that this stress field began at about 1 Ma. Also, Yoshida (1992) considered that this compression and the contemporaneous Ichishi thrusting took place at about 1 Ma, on the basis of Komeno deposition and some related geologic evidence around Ise Bay. In summary, the Tokai Basin underwent the following two major geologic events at about 1 Ma (Fig. 4B): (1) the beginning of compressional

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Fig. 3. Facies and unit-fan divisions of the Komeno Formation in cross-section, with the stratigraphy and sedimentary environment of the Tokai Group. The stratigraphy, sedimentary environment, and fission-track ages are after Yoshida (1988) and Yoshida et al. (1991). Locations a - f are shown in Fig. 5. Stages 1-3 are the division of the Komeno Fan development (see Fig. 12).

F. Yoshida/ Sedimentary Geology 92 (1994) 97-115

101

Middle Pleistocene, having a mean vertical slip rate of more than 0.3 m/1000 yr., but have been inactive since about 0.2 Ma. To sum up the above description, I consider that motion of the Ichishi Fault in the study area can be divided into three periods (Fig. 2A): (1) a period of gentle normal faulting (3 to 1 Ma), (2) a period of active reverse faulting (1 to 0.2 Ma), and (3) an inactive period (0.2 Ma to the present).

stress in a WNW-ESE direction, and (2) a first marine transgression from the Pacific Ocean. The first event triggered (1) the tectonic changes of the Ichishi and Yoro Faults from gentle normal faulting to severe thrust faulting, (2) the rapid uplift of the Suzuka and Yoro Mountains, and (3) the contemporaneous Komeno deposition. The second episode indicates that the base level of erosion for Komeno deposition was influenced by eustatic sea-level changes. Three segments of the Ichishi Thrust in the study area cut the Komeno Formation on its western boundary. All the fault planes observed in the field show the westward-dipping thrusts with overturned strata of the Komeno Formation (Yoshida, 1988). This means that the activities of these thrusts extended into post-Komeno deposition in the Middle Pleistocene. According to Ota and Sangawa (1984), some segments of the Ichishi Fault were active but some were inactive in the late Quaternary. Also they confirmed that the segments in this area were active in the Early and

3. Sedimentology

3.1. Depositional facies and sedimentary environments The Komeno Formation, less than 300 m thick, is dominantly composed of gravel with common mud and very rare sand, in which three depositional facies are recognizable as in the N-Strending cross-section (Fig. 3) and as in the plan view (Fig. 5).

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Fig. 5. Facies distribution and unit-fan division of the Komeno Formation in the plan view, with the palaeocurrents of the Komeno Formation and a network of present rivers•

F. Yoshida/Sedimentary Geology 92 (1994)97-115 3.1.1. Depositional Facies I (debris flow facies) Description. The first facies is comprised of dominant gravel with rare thin mud and lack of sand, and is subdivided into two variants: Facies Ia and Facies Ib. Facies Ia (Fig. 6) was deposited at the mountain front (Fig. 5), occupies the upper part of the formation (Fig. 3), and consists of gravel or gravel with thin mud. Facies Ib (Fig. 7) is present

103

in the lower part of the formation (Fig. 3), contains more mud and matrix-supported gravel than Facies Ia, and is thinner-bedded than Facies Ia. The clasts are smaller than those of Facies Ia. Gravel beds of Facies Ia, less than a few metres thick, consist of very poorly sorted, subangular to angular, pebble to boulder-sized clasts with a matrix of sandy silt to silty sand (Figs. 8A-8C). They are mostly clast-supported, but

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F. Yoshida / Sedimentary Geology 92 (1994) 97-115

104

occasionally matrix-supported gravel beds that lack internal stratification and show most commonly reverse grading in the lower parts. The base contacts were found to be fiat, while the top contacts often are convex upward (Figs. 8A and 9). Mud beds, several centimetres to a little less than 1 m thick, extend laterally for as much as 20 m or occasionally more in width, and consist mostly of brown or grayish black, massive, but sometimes thinly parallel-laminated mud. The base and top contacts were observed to be distinct and fiat.

rapid sedimentation in the form of clay-poor, noncohesive debris flows as described by Lowe (1982). In conclusion, Facies I is characterized by the dominance of debris flow deposits and therefore represents a sedimentary environment dominated by debris flow events. In a modern alluvial fan this facies occurs mainly in proximal reaches (e.g. Bull, 1963; Hooke, 1967). Facies Ia must occupy a more proximal reach than Facies Ib does, simply because Facies Ia contains more gravel beds and larger clasts than does Facies lb.

Interpretation. Gravel beds can be interpreted to

3.1.2. Depositional Facies H (debris flow and braided flow facies)

be the products of debris flows on the basis of their lithologic characteristics. But they consist mostly of clast-supported gravel with a sandy matrix. Therefore they were, perhaps, formed by

Description. The second facies is made up of gravel and mud with very rare sand. Gravel beds

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F. Yoshida / Sedimentary Geology 92 (1994) 97-115

are divided into two types. One is of debris flow origin and another is characterized as follows. The gravel beds, typically up to 5 m thick, are

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comprised of subrounded to subangular, clastsupported gravel with a matrix of sand, and contain occasionally fossil plant debris, logs, and

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Fig. 8. Depositional features of the Komeno Fan deposits. (A) Debris flow deposits with thin mud beds in Facies II. The rule is 1 m long (a part of Fig. 9). (B) Matrix-supported debris flow deposit showing inverse to normal grading in Facies II. The rule is 65 cm long (a part of Fig. 9). (C) Clast-supported debris flow deposit in Facies Ia, Tanihata in Kami-ishizu. (D) Small and deep channel-fill deposit in Facies II. The rule is 1 m long (a part of Fig. 9). (E) Broad and shallow channel-fill deposits in Facies III, Komeno in Fujiwara (type locality of the Komeno Formation). (F) Sieve deposit in Facies III, 1.5 km east of Nobesaka in Kami-ishizu. The rule is 1 m long.

106

F. Yoshida / Sedimentary Geology 92 (1994) 97-115

intraformational gravel, and thin lensing layers of sand (rarely planar cross-bedded) or mud. They show mostly crude parallel stratification (rarely planar cross-bedded). Clasts present are frequently up to boulder-size. Basal surfaces of these beds are concave-upward, sharp, and erosional, with 1 m or more of relief (Figs. 8D and 8E). Normal grading is common. Although mud beds display much the same lithofacies as those of Facies I, basal contacts with their underlying gravel beds are gradual.

Interpretation. The above-mentioned gravel type is in agreement with Miall's (1977) lithofacies code Gm. Therefore they are interpreted to be the results of deposition by longitudinal gravel bars in a proximal, gravel-dominant braided stream as described by Miall (1977, 1978) and Rust (1978). They interpreted these deposits as formed during high discharge events in shallow braided channels. Consequently, this facies can be interpreted to be mixed deposits of debris and braided stream origins, analogous to Miall's (1978) Trollheim type. Therefore Facies II is considered to have been deposited in shallow braided environments with frequent debris flow events, occupying the portion between Facies I (debris flow facies) and Facies III (braided flow facies as described later). Actually this facies is present more in the downstream portion than Facies Ia in the upper part of the Komeno Formation (Fig. 5).

Interpretation. Facies III is made up of gravel beds of a braided stream origin. Moreover this facies can be interpreted to be the upper reach products of a gravel-dominant braided stream, largely because clasts contained in them are commonly up to boulder-size. The absence of debris flow deposits in this facies indicates that Facies III must occupy a more basinward position than Facies II does. 3.2. Paleocurrent and clast composition My earlier study (Yoshida, 1988) provided paleocurrent data and gravel compositions of the Komeno Formation. They revealed that the detritus must have come from the Suzuka Mountains, not from the Yoro Mountains, for the following reasons. The paleocurrents all flowed basinward from the Suzuka Mountains (Fig. 5). Gravel compositions include abundant sandstone and chert, rare mudstone, basaltic rocks, and granite porphyry. The bedrock in the Suzuka Mountains is characterized by the dominance of sandstone and chert and by the presence of granite porphyry (Fig. 1); therefore these clasts are thought to have been derived from the Permo-Jurassic rocks and the Inugami Granite Porphyry of the Suzuka Mountains.

4. Komeno coalesced composite alluvial fan

3.1.3. Depositional Facies III (braided flow facies) Description. Facies III is composed of dominant gravel with rare mud and sand. Although almost all the gravel beds consist of lithofacies code Gm, gravel beds of another type are observable. They are less than 2 m thick and consist of very poorly sorted, subrounded to subangular, pebble- to boulder-sized clasts with poor matrix (Fig. 8F), and extend laterally for as much as 20 m in width. This lithofacies is assignable to open-framework gravel known as sieve deposits, following descriptions of some modern alluvial fan deposits (e.g. Hook, 1967; Bull, 1972).

The Komeno Formation can be regarded as alluvial fan deposits (hereafter called the Komeno Fan) derived from the Suzuka Mountains based on all the above-mentioned sedimentological and topographic evidence. The long and narrow fan surface allows the whole Komeno Fan to be divided into several smaller fans. This division is also supported by the distribution patterns of the Ichishi Thrust and the present river systems. For instance, the Ichishi Thrust in the study area consists of three segmented faults here tentatively named Thrusts A to C. Also the river systems in and around the fan are loosely grouped into three smaller river systems, that is, the Inn-

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F. Yoshida / Sedimentary Geology 92 (1994) 97-115

be, Makida, and Kajiya rivers, with their tributaries (Fig. 5). These present features must be the remains of their original frameworks, largely because they were formed in the early Quaternary. Therefore the whole Komeno Fan was most probably produced by the following three combinations: Thrust A and the Inabe River, Thrust B and the Makida River, and Thrust C and the Kajiya River. Furthermore, the facies distribution and paleocurrents in the plan view (Fig. 5), and the thickness changes of the facies and the facies accumulation in the cross-section (Fig. 3) suggest that the Komeno Fan can be roughly divided into three unit-fans, here tentatively named Fans 1 to 3. In addition, all the unit-fans are stratigraphically composed of two to four different facies (Fig. 3). Therefore, it can be concluded that the Komeno Fan is a coalesced, composite alluvial fan. Each unit-fan is in contact with a thrust segment of the Ichishi Fault at its western limit. The base horizons of these fan bodies become tephrostratigraphically younger in a northward direction (Fig. 3). Thus the Komeno Fan was also influenced by the northward movement of the Ichishi thrusting. This northerly shift may have been caused by a slight directional gap between the WNW-ESE-trending O'Hmax of the compression and the N-S-trending Ichishi Fault (Fig. 4B).

The succession of Stage 3 can be easily reconstructed based on the fact that the upper Komeno Fan body shows a downward succession of Facies Ia to Facies II (Fig. 5), which corresponds to Succession 1. On the other hand, the complete successions of Stages 1 and 2 are now unrecognizable, because later sediments buried the proximal- and mid-fans of these stages. However, these successions can be rebuilt by applying Succession 1 or2. Facies Ib and III are now exposed in the distal portions of Stages 1 and 2 (Figs. 3 and 5). In the case of Facies Ib, the presence of Facies Ia in the proximal reach can be inferred by applying Succession 1. Succession 2 is applicable to the case of Facies III, in which case Facies Ia and II can be estimated to be present in the proximal and mid parts of the fan (Figs. 10 and 11).

6. Ichishi thrusting and Komeno Fan sedimentation

I will now discuss the tectonosedimentary interaction between the Ichishi thrusting and the Komeno Fan sedimentation in three depositional stages (Fig. 3), also mentioning briefly the formative process of the present river systems.

6.1. Pre-Stage 1 (prior to 1 Ma: Fig. 12A) 5. Three-dimensional facies architecture

In the cross-fan profile (Fig. 3), the thickness change of the facies and the depositional feature of the facies accumulation allow the Komeno Fan development to be roughly divided into three depositional stages. This fan development can be discussed in detail by reconstructing the downslope facies succession in each stage. The lateral relationships between Facies I, II, and III mentioned earlier permit a proposal of two downcurrent facies successions for the Komeno Fan sedimentation. The first succession (Succession 1) is from Facies Ia to Facies Ib, and the second succession (Succession 2) is from Facies Ia through Facies II into Facies III (Fig. 10).

Until the Komeno deposition, a gentle westward-tilting and uplift of the Yoro and Suzuka

Fig. 10. Two downstream facies successions for the Komeno Fan sedimentation.

F. Yoshida/Sedimentary Geology 92 (1994) 97-115

Mountains was underway. The Komeno Fan deposits conformably overlie the Oizumi Formation (high-sinuosity river deposits) in the south and the Tara Formation (low-sinuosity stream deposits) in the north (Fig. 3). The paleocurrents derived from these underlying formations flowed dominantly south-southeastward (Yoshida, 1988). Thus, immediately before the Komeno deposition, a basin-axis trunk stream was flowing south-southeastward, producing a transition from low-sinuosity to high-sinuosity river deposits.

6.2. Stage 1 (Fig. 12B) The severe compressional stress began in and around the Tokai Basin and thus the Ichishi thrusting ensued. Initially the thrusting took place at Thrust A, followed by Thrust B. These thrustings formed Fans 1 and 2. At the end of this stage, Fan 1 became a larger, mature-shaped alluvial fan with a downslope facies succession of debris facies (Facies I) through debris and braided facies (Facies II) to braided facies (Facies III),

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Fig. 11. Three-dimensional facies architecture of the Komeno Fan body. Locations of sections G-I are shown in Fig. 5.

F. Yoshida / Sedimentary Geology 92 (1994) 97-115

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Fig. 12. T e c t o n o s e d i m e n t a r y i n t e r a c t i o n b e t w e e n the K o m e n o F a n s e d i m e n t a t i o n and the Ichishi thrusting. I - / V d e p o s i t i o n a l facies: I = F a c i e s Ia, / / = F a c i e s Ib; l I I = Facies II; I V = F a c i e s III; a-e s e d i m e n t a r y e n v i r o n m e n t : a = alluvial cone, b = lowsinuosity river, c = high-sinuosity river, d = marsh, e = alluvial fan to low-sinuosity river; 7 = reverse faulting, 2 = n o r m a l faulting, 3 = d i r e c t i o n of tilting. L o c a t i o n s of sections G - I are shown in Fig. 5.

F. Yoshida/ Sedimentary Geology 92 (1994) 97-115

while Fan 2 was still a smaller, immature-shaped fan with only debris facies. As Fan 1 became larger, it displaced the trunk stream toward the Yoro Mountain front. Eventually Fan 1 partially dammed the trunk stream, causing the formation of a marsh. This allowed fine sediment to accumulate in the marsh, that now corresponds to the uppermost part of the Tara Formation (Fig. 3).

6.3. Stage 2 (Fig. 12C) Fans 1 and 2 became larger than in Stage 1. As their fan fringes expanded over the Yoro Mountain front, the topographic high entirely dammed the trunk stream and, as a result, formed the northward-flowing Makida and Kajiya rivers and the southward-flowing Inabe. Thrusting C became active and constructed immature-shaped Fan 3 with debris flow facies.

6.4. Stage 3 (Fig. 12D) The continued thrustings enlarged the three fans. Debris facies prograded typically basinward and occupied most parts of the fan surfaces. By the end of this stage, the Yoro Basin was almost totally covered by the Komeno Fan.

6.5. Post-Stage 3 (0. 7 Ma to the present: Fig. 5) The fan sedimentation ceased at 0.7 Ma. Since then, the fan surface has been denuded by the present rivers.

7. Why did debris flow deposits prograde in Stage 3? The reconstructed facies architecture of the Komeno Fan body is typified by the frequency of debris flow events, especially in Stage 3 (Figs. 3 and 11). Many researchers have confirmed that frequent debris flows occur in alluvial fanheads (e.g. Reineck and Singh, 1980; Fraser, 1989). This has been reported not only from modern fans (Hooke, 1967; Bull, 1972; Wasson, 1977) but also from ancient fans (Bluck, 1967; Nilsen, 1969; Steel, 1974; Heward, 1978a; Brady III, 1984; De

111

Feyter and Molenaar, 1984). But some fans are known to lack debris flow deposits (e.g. McGowen and Groat, 1971; Boothroyd and Ashley, 1975). Through these earlier studies, the frequency of debris flows in fan sedimentations has been considered as dependent on geologic, topographic, and climatic conditions of drainage basins. For instance, Nilsen (1982) stated that frequent debris flows are generated where sediment sources provided abundant muddy material, slopes were steep, vegetation was scarce, and rainfall was either seasonal or irregular. The first point regarding the Komeno deposition is that its drainage basin was presumably restricted to the eastern part of the Suzuka Mountains. This is because the clast composition of the fan deposits include abundant sandstone and rare basaltic rocks. The bedrock exposed in the northern Suzuka Mountains is now characterized by the dominance of sandstone in the east and basaltic rocks in the west (Miyamura et al., 1976 and Fig. 1). Thus this composition strongly suggests that the sediments have been mostly derived from the eastern part of the Suzuka Mountains. For this reason, the Komeno Fan deposits are inferred to have been the products of a network of short and steep rivers entrenched in the eastern part of the mountains. The next point is that lithofacies in the drainage basin would have been of little importance for the occurrence of debris flows. This can be inferred from the fact that debris flows occurred frequently during the deposition, despite a shortage of muddy material in the drainage basin. To sum up the above points, the frequency of debris flow events is related primarily to topographic and climatic conditions. The steep and short river networks could have rapidly eroded the exposed bedrocks and would have shed abundant sediment onto the fan surface by debris flows during rainfalls. In addition, the Tokai Basin suffered one marine transgressive-regressive episode during deposition (Figs. 2B and 2C). This must have lowered the base level of erosion in late Komeno sedimentation. Furthermore, the worldwide glacial cooling began at about 1 Ma (Crowley and North, 1991). This could also have

112

F. Yoshida / Sedimentary Geology 92 (1994) 97-115

caused a sparseness of vegetation in the mountains. The integrated effect by these two conditions must have accelerated downcutting in the Suzuka Mountains while the Komeno deposition was active, especially during the late depositional stage. Consequently, the debris flow deposits could have prograded in Stage 3.

8. Why did the Komeno Fan sedimentation end at 0.7 Ma?

The thrusting of the segmented faults in the study area extended until 0.2 Ma (Fig. 2A). Assuming that the Komeno Fan sedimentation had been completely controlled by this fault movement, it must have continued until 0.2 Ma, near the end of the Middle Pleistocene, and then abruptly ended at 0.7 Ma, without showing a fining-upward trend of the deposition (Fig. 3). Therefore, the Komeno sedimentary termination must not have been due to the end or decline of the fault movement. Why, then, did the Komeno Fan sedimentation end at 0.7 Ma? The Komeno deposition was influenced by eustatic sea-level changes, including one marine transgressive-regressive cycle during that time (Fig. 2C). This means that the base level of erosion for the fan surface most probably continued to fall during the late depositional stage. In addition, the compressional stress could have caused the tectonic uplift of the Yoro Basin as well as the Suzuka and Yoro Mountains, although its amount was probably much less in the basin than in the mountains (Fig. 13A). Thus the Yoro Basin must have gone through a continued lowering of the base level of erosion in late Komeno time, due to both the basin uplift by compression and the relative sea-level fall. An intersection point develops on a fan surface, where entrenchment occurs in the upstream portion, with deposition in the downstream portion (Hooke, 1967). When the base level of erosion continues to fall, entrenchment on the fan and its drainage basin (mountains) is promoted and, as a result, the intersection point moves

basinward. Also, rapid downcutting in the mountains yields a larger amount of detritus and thus supplies abundant sediments onto the fan surfaces. These integrated effects must have prompted the growth and progradation of the three unit-fans, because the sediments supplied were most probably deposited on the lower fan surfaces, as the intersection points shifted to the downslopes (Figs. 13A and 13B). A rapid and worldwide climatic cooling took place at about 1 Ma, near the end of the Early Pleistocene. This climatic event triggered shorter and larger sea-level fluctuations in the late Quaternary than those in the early Quaternary (e.g. Shackleton and Opdyke, 1976; Crowley and

i "~I~

! Komeno i

ICi'l~lhi ~t~ Thrust

(~

Alluvial plain

I I

~,

Intersection point ~/ Yoro Mount. , fronts

¢ Entrenchment

i Deposition '" ........................

© I

i

Channel profile

T~ie

~

~ relative fall

' Fan surface ~

¢

T~t~lu ~

Fig. 13. Komeno Fan developmentand its termination caused by the combined effects of basin uplift and sea-level fall. (A, B) The intersection point of the Komeno Fan moved basinward throughout Stages 1-3. (C) The point shifted onto alluvial plain at 0.7 Ma. Since then, dissection on the fan, including the alluvial plain, has continued.

F. Yoshida/Sedimentary Geology 92 (1994) 97-115

North, 1991). At the end of Komeno Stage 3, three fans expanded on the western flanks of the Yoro Mountains. This geomorphic constraint must have prohibited these fans to grow further. However, their intersection points must have moved from the distal portions through the fan fringes and finally onto the alluvial plains. This is because the base level still continued to fall, due to both the successive basin uplift and the intermittent sea-level falls (Fig. 2C). This must have been the reason for the end of sedimentation and it must have occurred at 0.7 Ma. Since then, the fan surfaces have been entrenched by the present river systems (Fig. 13C). Many authors so far have considered that fault-bounded alluvial fan depositions are basically controlled by tectonics a n d / o r climate (e.g. Heward, 1978b; Nilsen, 1982), and have inferred from their fining-upward sequence that they have dosed by the end or the decline of the fault movement (e.g. Rust and Koster, 1984; Fraser and Sutter, 1986). The Komeno Fan development was exclusively dependent on the Ichishi thrusting and the climate during deposition. Additionally, I emphasize that changes of the base level of erosion for fan surfaces also influence fault-bounded alluvial fan developments, as briefly mentioned by Heward (1978b). This is because the Komeno deposition must have ended by a rapid lowering of the base level of erosion, not by the end or decline of the Ichishi thrusting. Posamentier et al. (1988) already indicated in terms of sequence stratigraphy that dissection on the alluvial plain occurs during eustatic sea-level fall, in which case the alluvial fan surface at the base of the faulted mountain front must be also dissected. Sea (or lake)-level fluctuations trigger changes of the base level of erosion. Therefore sea (or lake)-level changes control fault-bounded alluvial fan sedimentation, as well as tectonics and climate.

9. Summary and conclusions The WNW-ESE-trending compressional stress at 1 Ma in SW Japan caused the N-S-trending

113

Suzuka Mountains, bounded by the Ichishi Thrust, to be rapidly uplifted, and this resulted in the deposition of the Komeno Fan along the mountain front from 1 to 0.7 Ma. This fan can be loosely divided into three smaller fans, in which Facies Ia and Ib (debris flow deposits), Facies II (debris flow and braided flow deposits), and Facies III (braided flow deposits) are recognizable. Two downcurrent facies successions (Facies Ia to Facies Ib, and Facies Ia through Facies II to Facies III) allow the reconstruction of a three-dimensional facies architecture for the three fan bodies. The fan development was characterized by the progradation of debris flow deposits in the last stage. This depositional feature most probably resulted from the total effects of the topographic and climatic conditions of the Suzuka Mountains and the relative sea-level fall during deposition. Each of the three fans are in contact with one segment of the Ichishi Thrust on the western border and are stratigraphically made up of several different facies. The bases of the fans become tephrostratigraphically younger to the north. Therefore, the Komeno Fan can be concluded to be a coalesced composite alluvial fan that migrated with time in response to the northerly Shift of the Ichishi thrusting. The Komeno Fan development is summarized as follows. The sedimentation was generated at 1 Ma by the Ichishi thrusting and was controlled by the combined effects of the thrusting and the late Quaternary climate. Finally it expanded to cover virtually the Yoro Basin and ended at 0.7 Ma by the extra-lowering of the base level of erosion for the fan surface, due to the combined effects of the basin uplift by compression and the relative sea-level fall during deposition.

Acknowledgements I thank Fuminori Takizawa (Geological Survey of Japan) and Manabu Miyamura (Kinki University) for encouragement and helpful advice. This manuscript benefited greatly from reviews of two anonymous reviewers.

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F. Yoshida / Sedimentary Geology 92 (1994) 97-115

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