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Tsunami-induced conglomerates in Miocene upper bathyal deposits, Chita Peninsula, central Japan
SEDIMENTARY GEOLOGY ELSEVIER
Sedimentary
Geology
104 (1996) I75- 188
Tsunami-induced conglomerates in Miocene upper bathyal deposits, Chita Peninsula, central Japan Tsunemasa Shiki a, Teiji Yamazaki b aKitabatake, Kohata, Uji City 61 I, Japan b Institute of Earth Science, Osaka Kyoiku University, Asahigaoka, Kashiwara City 582, Japan Received
10
March 1993;revised version accepted 10 September 1994
Abstract The Tsubutegaura conglomeratic tsunamiites occur in the middle section of the Miocene storm-related sand-silt alternation system in the Chita Peninsula, central Japan. Deposition of the system in the upper bathyal environment in a bay and synsedimentary seismic activity has been elucidated by palaeontological, palaeogeographical and sedimentological studies. Two coupled units, each built up by a conglomerate layer with an overlying sandstone layer and the alignment of lenticular sedimentary bodies are exceptional in this sequence. Some typical lenticular bodies, confined within sedimentary troughs, consist of a boulder bearing conglomerate layer and calcareous sandstone layer of the lower couple, and another conglomerate layer of the upper couple. The tuffaceous sandstone layer of the upper couple is distributed more widely than the other lithologies. The framework gravels in the coupled conglomerates form a clast-supported fabric and are quite notably monomictic. The clasts are angular and imbricated partly, forming peculiar gravel clusters due to traction current transportation. Conspicuous laminations develop in the coupled sandstone layers. Antidunes with chute and pool structure in the upper couple sandstone layer indicate deposition from an upper flow regime current. On the other hand, the calcareous composition of the lower couple sandstone layer reveals shallow-water provenance. Thixotropic deformations and diastasis cracks in the siltstone bed immediately beneath the layers of the couples show that a severe earthquake tremor and a rapid change of water pressure occurred just before the deposition of the tsunamiites. Submarine debris flows due to collapse of a fault scarp in a shallow bank and the ensuing wash by two pulses of the tsunami-induced ebb current comprise the scenario for the formation of the Tsubutegaura conglomeratic tsunamiites.
1. Introduction Numerous studies have been made on the roles of various events in the transportation and deposition of marine elastic sediments. While storms have received much attention (Einsele and Seilacher, 1982; Aigner, 198.5; Einsele et al., 1991), tsunami events were also assumed by Kuenen (19501, as one of the important generative triggers of turbidity currents. Coleman (1968) mentioned the possibility of a rela0037-0738/96/$15.00 0 1996 Elsevier Science B.V. All rights reserved SSDI 0037.0738(95)00127-I
tion between tsunamis, high-density turbidity currents, and submarine canyons. In recent years, tsunami deposits have attracted considerable attention. For instance, thick homogeneous sediments (called homogenites) in the eastern Mediterranean Sea were interpreted as tsunami-induced sediments triggered by the collapse of the Santorini Caldera about 3500 years B.P. (Kastens and Cita, 1981; Cita et al., 1984; Hieke, 1984). Bourgeois et al. (1988) noticed that the Upper Creta-
T. Shiki, T. Yamazaki/Sedimentary
176
ceous to Paleocene deposits around the Gulf of Mexico and in the western Atlantic contain coarsegrained sandstone beds at or close to the Cretaceous-Tertiary boundary. They suggested that these coarse-grained beds were deposited by a tsunami induced by a bolide-water impact. Sedimentary bodies, including some boulder beds which formed by bolide-generated wave trains were also postulated by Smit et al. (1992), Alvarez et al. (19921, and Poag et al. (1992). As for the earthquake-induced tsunami sediments, the so-called tsunami-ishi (tsunami-transported stones> have been reported and studied to determine the highest run-up level of the Yaeyama Earthquake tsunami which occurred in 1771 near Okinawa (Miyoshi, 1968, 1987; Miyoshi and Makino, 1972; Kato and Kimura, 1983). Minoura and his colleagues reported some sand layers which were transported and deposited in inter-dune ponds by the run-up
Geology 104 (1996) 175-188
tsunamis triggered by the Japan Sea Earthquake (19831, by the Meiji Samiku tsunami (1896) of the northeast coast of Japan, and by several other historical tsunamis (Minoura et al., 1987, 1994; Minoura and Nakaya, 1991). No reports on tsunami-affected offshore sediments in the trench-forearc region around the Pacific, however, had been published before our preliminary report on the Miocene tsunami-reworked offshore cobble layers (Yamazaki et al., 1989) (Figs. 1 and 2). These are the Tsubutegaura Conglomerates, exposed on the Tsubutegaura coast of Chita Peninsula, near Nagoya, central Japan. In this paper, we provide a detailed description and discussion on the depositional process of these conglomerates and the overlying sandstones. We hope that documentation will be carried out of many examples of various types of tsunami-reworked sediments, tsunamiites, in both recent and ancient sediments in and around
135’E
l---nTl
UBUTEGAURA
Fig. 1. Palaeogeographic reconstruction area is shown by a solid square.)
of southern
m
gravelly bottom
/J
shore line ditto of low reliability
fresh water area
,,t,’
m
muddy bottom
...I...’ 200m depth line
m
ditto assumed
nn
a
sandy - silty bottom
steep mountain
Central Japan during the Middle Miocene. (After Shibata and Itoigawa,
1980. Study
T. Shiki, T. Yamazaki/ Sedimenrary Geology 104 (1996) 175-188
island arcs, because great earthquakes and tsunamis have occurred frequently along converging plate boundaries. We use the term tsunamiites not only for sediments transported by the tsunami wave itself, but also for tsunami-induced current deposits. This usage is much the same as that of the term tempestites, which is used for storm-induced sediments. 2. Geologic setting The Tsubutegaura Conglomerates which are noted for their conspicuously large boulders, occur in the
middle part of the Miocene Morozaki Group. The Morozaki Group, more than 1000 m thick, is mainly composed of alternations of siltstone and sandstone. The alternations form fining-upward sequences overlying the coarse-grained lowest formation of shallow tempestite facies which exhibit hummocky crossstratification, ripples, load casts, and so on. Many tuff beds were intercalated in the Morozaki Group and provide good key beds for correlation of the strata (Doi, 1983; Yamaoka, 1984; Kondo and Kimura, 1987; Yamaoka, 1993). Shibata and Itoigawa (1980) pointed out, in their comprehensive palaeogeographic study, that the mid-
CHITA PENINSULA
m
dark
m
&2j
sandstone dominated alternation (storm -induced) wavy or cross laminated or contorted sandstone mostly massive or faintly laminated sandstone conglomerate
m
tuff
IF
fault
B a
grey
siltstone
layer
Fig. 2. Lithological map showing major distribution of the Tsubutcgaura 1-6 = measured columnar sections shown in Fig. 4.
tsunami conglomerate:
I-V = numbers of conglomeratic
bodies;
178
T. Shiki, T. Yamazaki/Sedimenrary
--
_
-
- _ _-
Fig. 3. Schematic profile of the tsunami sediment bodies along the well exposed part of the Tsubutegaura coast: A, C = tempestitic sand-silt alternation; B = tsunami-induced sediments (tsunamiite); B-l = conglomerate lens; B-2 = widely extended sandstone.
dle part of the Morozaki Group was deposited in the center of a southwardly opening palaeobay (PalaeoIse Bay) at a water depth of more than 200 m (Fig.
r
w
Geology
104 (1996)
175-188
The Morozaki Group yields many fossils of mollusts, fishes, crustaceans, asteroids, whale bones, and others (The Tokai Fossils Editing Committee, 1993). Benthonic molluscs are common in every horizon of the Morozaki Group. The benthonic mollusts from mudstone of the middle Morozaki Group consist of an Acilanu assemblage characterized by the predominance of Acilana tokunagai. Many species in this assemblage have a close affinity with living molluscs that inhabit the sea bottom at depths from 200 to 400 m deep or more. The other benthonic fauna (crustaceans, asteroids and echinoids) also indicate similar uppermost bathyal environments (Yamaoka, 1985, 1987; Takeda et al., 1986; Hachiya et al., 1993; The Tokai Fossils Editing Committee, 1993). Besides these, the nektonic fossil communities from the mudstone consist mainly of a deep-sea fauna. In particular, the occurrence of small fishes having luminous organs is noteworthy.
U-I
Fig. 4. Detailed columnar sections of the Tsubutegaura tsunamiik Upper coupled unit: U-2 = widely extended tuffaceous upper sandstone layer; U-Z = upper conglomerate layer. Lower coupled unit: L-2 = calcareous lower sandstone layer; L-l = lower conglomerate layer.
T, Shiki, T. Yamazaki/ Sedimentary Geology 104 (1996) 175-188
179
sedimentation and synsedimentary tectonics of the Morozaki Group took place coeval with intermittent movements of the Median Tectonic Fault Line, which is the most important major strike slip fault in the geotectonic development of the Japanese Islands, and its conjugate fault systems. The left-lateral slip phase of the Atsumi Fault, one of the conjugate faults, occurred concurrently with the deposition of the middle Morozaki Group.
Fig. 5. The largest and exceptionally rounded boulder in the first conglomeratic body sticking out into the overlying tuffaceous sandstone
layer.
The fossil molluscs from the intercalated sandstone layers in the same formation, on the other hand, are composed of both the Lucinoma-Cyclocur& and Aciluna assemblages. These assemblages indicate shallow sea bottom environments, 100 to 200 m deep. The palaeontological data thus show that the intercalated sandstone layers were transported from a shallow sea bottom to the uppermost bathyal environment (Yamaoka, 1985, 1993; Yamazaki et al., 1988). Inorganic sedimentary structure in the Morozaki Group include sandstone dikes, diapiric foldings, convolute laminations, and ball-and-pillow structures (Hayashi, 1985). Hayashi (1987) clarified that the
3. Sedimentary tsunamiites
features of the conglomeratic
3.1. Occurrence The conglomeratic tsunamiites (i.e., the Tsubutegaura Conglomerates and associated sandstones) occur along the Tsubutegaura beach over a distance of about 150 m. They form some horizons in a stratigraphic interval of several metres, intercalated in the tempestitic sandstone-siltstone alternation (Figs. 2 and 3). They are characterised by aligned lenticular conglomeratic bodies filling erosional troughs. Only one of the horizons of the aligned conglomeratic bodies is well exposed since most of the others are usually under water or buried by beach sands. The occurrence of conglomeratic bodies in this horizon was described preliminarily by Yamazaki et al. (1989). The bodies are 10 to 40 m across and a few SE
NW Land
Fig. 6. Sketch of the fourth conglomeratic body viewed from the sea-side: A = underlying pate bluish gray siltstone; B = conglomerate layer of the lower unit; C = calcareous sandstone layer of the lower unit; D = conglomerate layer of the upper unit; E = tuffaceous laminated sandstone layer of the upper unit; F = overlying sand-silt alternation. The stratigraphic relation between the layers D and E is not obvious from this view.
180
T. Shiki, T. Yamuzaki/Sedimentary
metres high. The distance between individual conglomeratic bodies is several metres (Figs. 2 and 3). They are succeeded by a widely extended white tuffaceous sandstone layer in which conspicuous lamination develops, and underlain by a pale bluish or purplish grey siltstone bed with eroded upper surface. Where the conglomeratic body or even pebble lags are absent, the tuffaceous sandstone layer succeeds directly the pale bluish or purplish grey siltstone bed (Figs. 3 and 4). 3.2. Lithologic composition of the conglomerates and the coupled sandstones One of the most conspicuous features of the Tsubutegaura conglomeratic tsunamiites is their more or less monomictic composition (Yamazaki et al., 1989). Almost all the clasts are gneiss or gneissose igneous rocks from the Ryoke metamorphic belt which is one of the most remarkable geological belts
Geology 104 (1996) 175-188
of the Japanese Islands. Rip-up clasts from the subjacent siltstones are minor constituents. Coaly wood shreds and fragments of gastropods are accessories. The associated sandstones are either tuffaceous or calcareous. Sand grains in the tuffaceous sandstone are mainly composed of intermediate volcanic rock fragments, volcanic glass shards, feldspars, and quartz. Gneissose rock fragments, metamorphosed pelitic and siliceous rock fragments, pyroxenes, biotite and siliceous fossil fragments are the minor constituents. Sand grains in the calcareous sandstone are similar to those in the tuffaceous sandstone, except for the predominance of calcareous fossil fragments. The predominance of a carbonate matrix, and the calcification of the effusive rock fragments and feldspars characterise this sandstone. 3.3. Internal structure of the conglomeratic bodies The conglomerates of the Tsubutegaura tsunamiites are mainly composed of pebbles and boulders
Fig. 7. lmbricated cobbles in the conglomerate of the upper unit in the fourth body. Palaeocurrents the photo). Note the very angular shapes and well sorting of the cobbles.
are from northeast (from right to left in
T. Shiki, T. Yamazaki/Sedimentaty
but carry also some giant boulders as large as the thickness of its bed. The largest boulders in the conglomerates, reaching 3 m in diameter, appear in the extreme northwestern body which is rather small in size (Fig. 5). The fourth body from the northwestern end of the well exposed horizon shows the most conspicuous structure (Fig. 6) and was mentioned in our previous paper (Yamazaki et al., 1989). The conglomeratic body is composed of three layers: (1) a lower thin conglomerate layer, (2) an intercalated calcareous laminated sandstone layer, and (3) an upper conglomerate layer. The lower two layers form a coupled sedimentary unit, and the upper conglomerate layer forms a subunit coupled with the above-mentioned overlying tuffaceous sandstone layer. The lower coupled unit is found only in the fourth and fifth conglomerate bodies filling small channels in the underlying siltstone, but does not occur in the other bodies. Two-coupled units in the other bodies were amalgamated by some sedimentary process which took place in the narrow channels. The lower conglomerate layer, if present, consists of subangular pebbles of gneissose rocks and is only 50 cm thick at maximum. Reverse grading is obvious in some basal parts, and a crude concentration of larger gravels is seen in the upper half. The fabric of the conglomerate is clast-supported, and parallel orientation and imbrication of gravels are observed in some parts. The voids between gravels are filled with the detritus of the overlying calcareous sandstone. The lower conglomerate layer is covered and coupled with the calcareous sandstone layers. This ranges in thickness from several to 50 cm in the fourth body and is thinner in the fifth body. Parallel lamination and dish structures develop in some parts of the layer. The parallel lamination is contorted, though rarely, by the loading of cobbles in the overlying upper conglomerate layer as reported by Hayashi (1985, 1987). As stressed by Yamazaki et al. (19891, the most significant part of the conglomeratic deposits is the upper conglomerate layer which contains the giant boulders. The giant boulders tend to lie in the northwestern part of the upper conglomerate layer in each body, where it is also thicker. The conglomerate layer gradually thins out southeastward. The conglomeratic bodies therefore have the form of a bunch
Geology 104 (1996) 175-188
181
of grapes, with giant boulder gravels at their northwestern ends. The fabric of the upper conglomerate layer has been described in detail by Yamazaki et al. (1989). Gravels in this layer are clast-supported, and most of the clasts are angular to subangular and bladed in form. Voids among the framework gravel are filled with tuffaceous sand of the overlying layer. Shapes of gravels are usually asymmetrical around their longest axis as seen in fluvial gravels. As a matter of interest, northeastward imbrication develops in many parts, where the gravels are relatively well sorted (Fig. 7). The longest axis of the imbricated gravels is transverse to the palaeocurrents. The boulders and the smaller clasts accumulate like gravel clusters as seen in alluvial channels.
_:
/: ::’
,..’
. ..
,_....I
Fig. 8. High-energy current structures developed in the widely distributed tuffaceous sandstone layer (scale 5 cm): A = supercritical parallel lamination at the lowest part of the layer; B = antidune cross lamination with chute and pool structure; C = large convolute lamination, faintly visible.
T. Shiki, T. Yamazaki/ Sedimentary Geology 104 (1996) 175-188
.._ ,. ,,_,-, ._._._,__.._
.._,._.. - ..._
,, ...__......_.,...... ..-. ,__.... _ n. _..
:
,,;..
.. _. .- _....
i
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,.....
..___
. “....I
._
..._ _ _..._..... .. . ._. ..__.......-. . . ..
.._._
,/
‘ 1 I I/ M
B
1
t// Fig. 9. Diastasis cracks and horizontally expanded fractures in the siltstone. Wall-derived angular siltstone clasts (shaded) and sandstone fill the diastasis cracks (arrow I). Note the sand-filled horizontal fractures (arrow 2) due to expansion. Scale is 5 cm. A = fine laminated sandstone layer (part of tsunamiite) with water-escape pillar structures; B = siltstone layer.
T. Shiki, T. Yamazaki/Sedimenrary
Obstacle and wake fabrics, common among recent stream gravel clusters (Dal Cin, 1968; Brayshaw, 1984), are also present in the conglomerate layer. On the northeast side of the giant boulders, relatively well sorted smaller gravels are concentrated, with no arrangement such as seen on the stoss side of a boss formed by unidirectional current deposits. The random fabric parts are covered and armoured by coarser-grained cobbles. These features show that the gravels have been selectively transported and deposited by southwestward-flowing traction currents. Palaeocurrents from the northeast are indicated also by loaded flute casts found at the northwestern end of the fourth body. The southwestward palaeocurrent direction deviate significantly even at right angle from that of the surrounding storm-induced sediments which reveal westward to northwestward palaeocurrents in the middle part of the Morozaki Group. The upper conglomerate layer does not scour the underlying sediments except in the fifth lenticular body. Here, the underlying calcareous sandstone layer was contorted and eroded out leaving thin lenses and fragments of the well laminated sandstone. Contrary to the lower sandstone layer which is calcareous, the laminated sandstone layer of the upper couple is tuffaceous, similar to those of the general storm-induced turbidite or tempestite sandstone in the Morozaki Group. The layer is graded from medium sand to silt and attains 1.5 m in total thickness. Cross-lamination of the antidune type and chute and pool structures with convolutions in the division of the graded sandstone indicate deposition from high-energy currents in the upper flow regime. Giant convolute structures are also seen in the bed just above the antiduned sandstone (Fig. 8). Many current shadows behind gravels, sticking out from the underlying conglomerate layer, are seen in the lower part of the laminated sandstone layer. The palaeocurrent directions shown by both the current shadows and the laminations in the sandstone layer are from northeast to southwest and coincide with that of the upper conglomerate layer. 3.4. The underlying bed of the tsunamiite The underlying bed is pale bluish or purplish gray siltstone. Diastasis cracks (Cowan and James, 1992)
Geology 104 (1996) 175-188
183
Fig. 10. Thixotropic deformations in the siltstone underlying the conglomeratic tsunamiite.
are observed just beneath the erosion surface of the bed. The cracks are filled with elastics of the overlying conglomerate and/or sandstone (Fig. 9). Thixotropic deformation of typical shock wave-induced type without any gravel loading is also observed in the siltstone bed. Where the conglomeratic bodies are absent, the tuffaceous sandstone layer directly covers the siltstone bed (Fig. IO> with local small erosional signs as mentioned above. Conglomerate lags are found in many places between the tuffaceous sandstone layer and the underlying siltstone bed.
4. Discussion 4.1. Transportation mechanism On the basis of the sedimentary records given above, we can assume that earthquakes, collapses of fault scarp, and tsunamis played important roles in the sedimentary processes of the Tsubutegaura Conglomerates and associated sandstones. As shown in the previous section, the Tsubutegaura Conglomerates occur in the storm-related systems deposited in 200 to 400 m or deeper submarine environments. Their sedimentary fabrics, however, are very exceptional to the system. In monomictic lithology, the very angular shape of many gravels and the lack of mud matrix, the upper conglomerate layer of the Tsubutegaura conglomeratic bodies resemble washed talus deposits on a bench at the foot of a coastal cliff. The sedimentary facies and fauna
T. Shiki, T. Yamazaki/Sedimentary
184
Geology 104 (1996) 17.5188
Such an assumption, however, is supported neither by the sedimentary features of the conglomerates, nor by palaeocurrent directions. The head or uppermost part of some debris flow deposits may have clast-supported, bedded and/or imbricated fabrics (Hirano and Iwamoto, 1981; Suwa and Okuda, 1983; Suwa et al., 1984), but such fabrics usually develop only poorly and change into matrix-supported massive textures. The Tsubutegaura Conglomerates, however, do not have any matrix-supported massive parts. As described in the previous chapter, the most significant part of the Tsubutegaura Conglomerates is the conglomerate layer in the upper couple. In this layer, the traction-current features, such as imbrica-
of the Middle Morozaki Group, however, argue against the coastal topography for the formation of the conglomerate layer. Soft siltstone clasts, picked up from the underlying strata, also testify against a simple origin as a talus deposit. The conglomeratic bodies seem to resemble the tongues of some debris flow. In fact, deposition of the conglomerates by submarine debris flows or submarine slides was suggested by a few workers (Hayashi, 1987; Kondo and Kimura, 1987). If the debris flow origin is adequate for the Tsubutegaura Conglomerates, concentrations of giant boulders and the thickening tendency of the lenticular bodies in the northwestern part of each body, might suggest that the debris flows were driven from the southeast.
1
Storm-induced current
gneissose carbonate
gravels muds
an
-N
tsunami
ebris
flow
collabse
3 back
tSunami:worked
gravel
flow
of tsunami
wave
layer
Fig. I I. Depositional scenario of the Tsubutegaura tsunamiites near the foot of a bank in the Palaeo-Ise Bay (see text). (1) Storm-induced current deposited on the bay floor (upper bathyal environment). Calcareous sands and giant boulders formed on top of a neighboring shallow bank, which is bordered by a fault scarp. (2) Severe earthquakes and resultant tsunamis attacked the palaeobay. The fault scarp partly collapsed and calcareous sand and boulders were trausported to the deeper basin floor. (3) Torrential tsunami ebb currents carrying debris swept the sea floor and deposited conglomeratic and successive sandy sediments.
T. Shiki, T. Yumnzaki/Sedimentwy
tion and gravel cluster fabrics, are most conspicuous. They formed by southwestward flow, namely, almost at right angle to the suggested debris flow. Very rapid transportation with little grain collision and abrasion is confirmed by the presence of nonresident rip-up clasts and high angularity of the gneissose gravels in the upper conglomerate layer. The lower conglomerate rests on scours in the underlying siltstone. The fabrics of the conglomerates suggest that the lower conglomerates are principally of debris-flow origin. Sedimentary features of some parts of the conglomerate layer, however, such as clast-supported fabrics, crude bedding and imbrications of gravels, though not clear enough to determine the current direction, show superimposition of traction currents on the debris flow deposits. The supercritical bed forms of the overlying calcareous sandstone layer support the idea that rapid traction currents followed the debris flow. The traction current transported boulders in the uppermost bathyal sea floor, deeper than 200 m, cannot be a tidal current. Needless to say that seasonal storm-induced waves and the resultant currents at such depths are too weak to move giant boulders. A tsunami and its related currents are the only possible agent. If submarine avalanche deposits are reworked at the foot of a submarine cliff by a very powerful traction current and transported to the shallow bathyal sea floor, or if newly derived debris-flow deposits on the sea floor are reworked by such traction currents, the resultant deposits can have fabrics similar to those of the upper conglomerate layer. The foregoing discussions are mainly concerned with the deposition of the conglomerate layers. The overlying sandstone layers also reveal remarkable characters of the successive stage of the sedimentary processes. The supercritical bed forms of the sandstones are evidence that rapid traction currents followed the conglomerate deposition. Such high power current structures have never been observed in the storm-induced turbidites in the Morozaki Group. In turn, some severe earthquake tremors and quick water pressure changes just before the deposition of the conglomerates are evidenced by the diastasis cracks and thixotropic deformations in the underlying siltstone as described above. Needless to say that a tsunami wave is a more powerful agent than a
Geology 104 (1996) 175-188
is5
storm wave for formation of the diastasis cracks suggested by Cowan and James (1992). Since a tsunami induced by an earthquake has several pulses, the tsunami-induced ebb currents also act in several pulses. It must also be pointed out that tsunamis induced very close to the hypocenter of an earthquake generally have only a few pulses. The two coupled conglomerates and overlying sandstone layers were possibly produced by two big pulses, though there were some differences in the depositional processes between the two. Seemingly an alignment of conglomerate bodies may also have resulted from these pulses. 4.2. Possible tectonic and hydrodynamic origin of the processes
Large fault movements on both east and west sides of the Chita Peninsula are evident (Hayashi, 1987). With submarine fault movements, the fault scarp should collapse and produce an amount of debris at the foot of the scarp. A subaqueous slump leaving a vertical cliff can occur as a result of earthquakes and succeeding tsunamis, as illustrated by the example of the Flores Island tsunami of December 12, 1992 (Yeh et al., 1993). It is quite possible that most of the very angular gravels in the conglomerate layers represent a collapse breccia at a fault scarp. On the other hand, the rounded giant boulder, the largest in the Tsubutegaura conglomeratic deposits, must have been originally on a shallow bottom surface, such as a shallow bank, where seasonal storm waves could round the boulder. Although seasonal storm-induced currents could not remove the boulder, exceptionally high energy currents, such as tsunami-induced currents, could bring down the largest boulder from the surface of the bank. Big boulders occasionally can roll down further and greater distance than smaller gravels when the water is deep enough and/or the role of gravity is effective (Kimura, 1954, 1956). The shallow water derivation is supported by the intercalated calcareous sandstone layer. We suggest that some strong tsunami trains with a large amount of debris can transport cobbles on the upper-bathyal sea floor. In particular, the ebb current of tsunamis is powerful enough to remove the cob-
T. Shiki, T. Yamazaki/Sedimentary
186
bles deposited by previous processes and to produce sedimentary fabrics like the conglomerates of Tsubutegaura. Many oceanographers have reported on the propagation, wave length and wave height of tsunamis, and coastal disasters caused by the run-up of tsunamis (Miyazaki, 1971; Miyoshi, 1987; Abe and Ishii, 1987). Although the properties of an ebb current of a tsunami have not yet been observed, the energy transport of tsunami waves in deep water is widely accepted because of its long wave length. By the run-up of tsunamis, a large water mass is piled up on the coast and therefore generates a resultant rapid ebb current (return current). The offshore velocity of a rip-current produced even by seasonal storms can reach to about 1 m/s at its maximum (Nagata, 1971). It is quite possible that a tsunami-induced ebb current, with a large amount of debris in suspension may reach a velocity of several metres per second. In general, even a normal traction current of a velocity of several metres per second can transport giant boulders 2 or 3 m in diameter as shown by Baker (1984). Transportation of giant limestone boulders by the ebb current of the Yaeyama Earthquake tsunami has been suggested by Kato and Kimura (1983). Rather recently, Pickering et al. (1991) suggested that back-flow surges of a tsunami could be of sufficient magnitude to move all normal available grain-sizes. Together with the tsunami-induced current, the initial moment due to the collapse of a submarine scarp may have caused the movement of the giant boulders in the Tsubutegaura Conglomerates. All the sedimentary features of the Tsubutegaura Conglomerates and associated sandstones discussed above can be explained better by the combined effect of the collapse of submarine scarp and tsunami-induced currents. The hydrodynamic circumstances of the deep-sea tsunami-worked coarse-grained sediments will be discussed in more detail in a separate paper. 4.3. Depositional
scenario
of
the
Tsubutegaura
tsunamiite The sedimentary basin of the Morozaki Group of the Middle Miocene was a southerly facing bay with a shallow bank cut by fault scarps (see Fig. 2).
Geology 104 (1996) 175-188
Intermittent storm-induced turbidity currents were the major sedimentary agencies of the middle formations of the Morozaki Group which were deposited in the upper bathyal environment of 200 to 400 m deep or deeper (Fig. 111. Severe earthquakes and tsunamis took place when one of the faults bordering the center of the basin and the bank moved. A submarine fault scarp in the Ryoke Gneiss collapsed. Newly collapsed debris and older talus deposits which has been at the foot of the scarp, formed debris flows and were transported down to the deeper part of the basin. In turn, the talus debris was caught by the tsunami ebb currents, and some kind of sediment-gravity flow with a large amount of gravels was initiated. The gravel-rich flow scoured the floor and was followed by its sandy tail, which flowed down from the shallow bank surface, carrying the calcareous materials. Soon after the deposition of the lower unit sediments, gravels of the upper conglomerate were brought down very rapidly by a succeeding severer tsunami pulse. Much of the collapsed and previously deposited debris may have been carried some tens of metres toward the shallower places by the up-rush of tsunamis. Gigantic ebb currents, however, could catch a considerable amount of newly collapsed gravels and carry them back to the deeper floor. The second current was more powerful than the preceding current. It carried away smaller debris, and imbricated larger gravels of the first tsunamiite. Following the deposition and rearrangement of the upper conglomerate layer, the tail of the powerful ebb current of the same pulse containing a large amount of sorted sand grains came from the shallow area in the bay and deposited the tuffaceous laminated sandstone layer on top of the conglomerates. It covered directly a wide area of the siltstone bed where no conglomerate bodies had been deposited. During these processes, previously deposited silty muds, namely, the bottom sediments, experienced seismic tremor and rapid pressure change due to the passage of the high-amplitude tsunami wave. Thixotropic deformations took place first with the seismic tremor. Succeeding compaction and depression due to the passage of the high-amplitude tsunami wave caused diastasis cracks and associated horizontal tear-off cracks. These cracks were filled with sand grains and small pebbles.
T. Shiki, T. Yamazuki/Sedimenrary
Acknowledgements We are grateful to Prof. H. Okada (Kyushu University) for his critical reading of the manuscript. We wish to thank Dr. T. Hayashi for drawing our attention to the interesting sediments, the Tsubutegaura Conglomerates. We are also indebted to Prof. J. Itoigawa (Nagoya University) and Mr. M. Yamaoka (Kita Junior High School) for providing palaeontological information, and to Mr. H. Komori for assistance in field work. Thanks are extended to Profs. S. Okunishi, H. Suwa and S. Nakamura (Disaster Prevention Institute, Kyoto University) and their colleagues for their valuable discussions. The text figures were kindly prepared by Mr. K. Irino (Kyoto University).
References Abe, K. and Ishii, H., 1987. Distribution of maximum water levels due to the Japan Sea Tsunami on May 26, 1983. J. Ocean. Sot. Jpn., 43: 169-182. Aigner, T., 1985. Storm Depositional Systems-Dynamic Stratigraphy in Modem and Ancient Shallow-Marine Sequences. Springer-Verlag, Berlin, 174 pp. Alvarez, W., Smit, J., Lowrie, W. Asaro, F. Margolis, S.V., Claeyes, P. Kastner, M. and Hildebrand, A.R., 1992. Proximal impact deposits at the Cretaceous-Tertiary boundary in the Gulf of Mexico: a restudy of DSDP Leg 77, Site 536 and 540. Geology, 20: 697-700. Baker, V.R., 1984. Flood sedimentation in bedrock fluvial systems. In: E. Koster and R.J. Steel (Editors), Sedimentology of Gravels and Conglomerates. Can. Sot. Pet. Geol., pp. 87-98. Bourgeois, J., Hansen, T.A., Wiberg, P.L. and Kauffman, E.G., 1988. A tsunami deposit at the Cretaceous-Tertiary boundary in Texas. Science, 241: 567-570. Brayshaw, A.C., 1984. Characteristics and origin of cluster bedforms in coarse-grained alluvial channels. In: E. Koster and R.J. Steel (Editors), Sedimentology of Gravels and Conglomerates. Can. Sot. Pet. Geol., pp. 76-86. Cita, M.B., Camerlenghi, A. Kasten, K.A. and McCoy, F.W., 1984. New findings of Bronze Age homogenites in the Ionian Sea: geodynamic implications for the Mediterranean. Mar. Geol., 55: 47-62. Coleman, P.J., 1968. Tsunamis as geologic agents. J. Geol. Sot. Aust., 15: 2577273. Cowan, C.A. and James, N.P., 1992. Diastasis cracks: mechantally generated synaeresis-like cracks in Upper Cambrian shallow water oolite and ribbon carbonates. Sedimentology, 39: 1101-1118. Dal Cin, R., 1968. Pebble clusters: their origin and utilization in the study of paleocurrents. Sediment. Geol., 2: 233-241.
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Doi, K., 1983. On stratigmphy and age of the Miocene Utsumi Formation, Morozaki Group, southwest Japan. N.O.M., 10: 14-21 (in Japanese with English abstract). Einsele, G. and Seilacher, A. (Editors), 1982. Cyclic and Event Stratification. Springer-Verlag. Berlin, 536 pp. Einsele. G. Ricken, W. and Seilacher, A. (Editors), 1991. Cycles and Events in Saatigraphy. Springer-Verlag, Berlin, 955 pp. Hachiya, K., Yamanaka, M. and Mizuno, Y., 1993. Description of characteristic fossil beds. In: The Tokai Fossils Editing Committee (Editor), Fossils from the Miocene Morozaki Group, pp. 27-40 (in Japanese). Hayashi, T., 1985. Deformational sedimentary structures related with thixotropy. Based on the data obtained from the Miocene Morozaki Group. Mem. Symp. Formation of Slump Facies and its Relation to Tectonics, pp. 35-47 (in Japanese with English abstract). Hayashi, T., 1987. Synsedimentary tectogenesis in the Miocene Morozaki Group, Chita Peninsula, central Japan. J. Geogr., 96: 278-293 (in Japanese with English abstract). Hieke, W., 1984. A thick Holocene homogenite from the Ionian abyssal basin (eastern Mediterranean). Mar. Geol., 55: 63-78. Hirano, M. and Iwamoto, M., 1981. Experimental study on the grain sorting and the flow characteristics of a bore. Mem. Fat. Eng. Kyushu Univ., 41: 193-202. Kastens, K.A. and Cita, M.B., 1981. Tsunami-induced sediment transport in the abyssal Mediterranean Sea. Geol. Sot. Am. Bull., 92: 845-857. Kato, Y. and Kimura, M., 1983. Age and origin of so-called Tsunami-ishi, Ishigaki Island, Okinawa Prefecture. J. Geol. Sot. Jpn., 89: 471-474 (in Japanese). Kimura, H., 1956. Mechanism of sorting-a fundamental study of sedimentation (Part 7). J. Geol. Sot. Jpn., 62: 472-489 (in Japanese with English abstract). Kondo, T. and Kimura, I., 1987. Geology of the Morozaki district, with geologic map (1: 50,000). Geol. Survey of Japan, 93 pp. (in Japanese with English abstract). Kuenen, P.H.. 1950. Marine Geology. John Wiley and Sons, New York, 568 pp. Martel, A.T. and Gibling, M.R., 1993. Clastic dykes of the Dcvono-Carboniferous Harton Bluff Formation, Nova Scotia: Storm-related structures in shallow lakes. Sediment. Geol., 87: 103-119. Minoura, K. and Nakaya, S., 1991. Traces of tsunami preserved in inter-tidal lacustrine and marsh deposits: some example from northeast Japan. J. Geol., 99: 265-287. Minoura, K., Nakaya, S. and Sato, H., 1987. Traces of tsunamis recorded in lake deposits-an example from Jusan, Shiura. Aomori. J. Seismol. Sot. Jpn., Ser. 2.40: 183- 196 (in Japanese with English abstract). Minoura, K., Nakaya, S. and Uchida, M., 1994. Tsunami deposits in a lacustrine sequence of the Sanriku coast, northeast Japan. Sediment. Geol., 89: 25-3 I. Miyazaki, M., 1971. Tsunami and flood tide. In: J. Masuzawa @ditor), Physical Oceanology. Tokai Univ. Press, Tokyo, pp. 255-325 (in Japanese). Miyoshi, H., 1968. The tsunami of April 24, 1771. J. Seismol. Sot. Jpn., Ser. 2, 21: 314-316 (in Japanese).
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Miyoshi, H., 1987. True run-up height reached by the huge tsunami of 1986. J. Ocean. Sot. Jpn., 43: 159-168. Miyoshi. H. and Makino, K., 1972. The tsunami of April 24, 1771 (II). J. Seismol. Sot. Jpn., Ser. 2, 25: 33-43 (in Japanese with English abstract). Nagata, Y., 1971. Wave. In: J. Masuzawa (Editor), Physical Oceanology. Tokai Univ. Press. Tokyo, pp. l-107 (in Japanese). Ohe, F., 1993. Deep fish assemblage from the middle Miocene Morozaki Group, southern pat? of Chita Peninsula, Aichi Prefecture, central Japan. In: The Tokai Fossils Editing Committee (Editor), Fossils from the Miocene Morozaki Group, pp. 27-40 (in Japanese with English abstract). Pickering, K.T., Soh, W. and Taira, A., 1991. Scale of tsunamigenerated sedimentary structures in deep water. J. Geol. Sot. London, 148: 211-214. Poag, C.W., Powars, D.S., Poppe, L.J., Mixon, R.B., Edwards, L.E., Folger, D.W. and Bruce, S., 1992. Deep Sea Drilling Project Site 612 bolide event: new evidence of a late Eocene impact-wave deposit and a possible impact site, U.S. east coast. Geology, 20: 77 l-774. Shibata, H. and Itoigawa, J., 1980. Miocene paleogeography of the Setouchi province, Japan. Bull. Mizunami Foss. Museum, 7: l-49 (in Japanese with English abstract). Smit, J., Montanari, A. Swinbume, H.M., Alvarez, W., Hildebrand, A.R., Margolis, S.V., Ciaeyes, P., Lowrie, W. and Asaro, F., 1992. Tektite-bearing, deep-water elastic unit at the Cretaceous-Tertiary boundary in northeastern Mexico. Geology, 20: 99-103. Suwa, H. and Okuda, S., 1983. Deposition of debris flow on a fan surface, Mt. Yakedake, Japan. Z. Geomorphol. Suppl., 46: 79-101.
Suwa, H., Okuda, S. and Ogawa, K., 1984. Size segregation of solid particles in debris flows, Part 1. Accumulation of large boulders at the flow front and inverse grading by kinematic sieving effect. Ann. Rep. Disas. Priv. Inst. Kyoto Univ., 27: B- 1, 409-423 (in Japanese with English abstract). Takeda, M., Mizuno, Y. and Yamaoka, M., 1986. Some fossil crustaceans from the Miocene Morozaki Group in the Chita Peninsula, central Japan. Kaseki no Tomo, 28: 12-22 (in Japanese). The Tokai Fossils Editing Committee (Editor), 1993. Fossils from the Miocene Morozaki Group. The Tokai Fossil Society, 297 pp. (in Japanese with English abstract). Yamaoka, M., 1984. An outline of the Morozaki Group. Kaseki no Tomo, 26: 2-5 (in Japanese). Yamaoka, M., 1985. Molluscan fauna of the Morozaki Group. Kaseki no Tomo, 27: 13-23 (in Japanese). Yamaoka, M., 1987. Fossil asteroids from the Miocene Morozaki Group, Aichi Prefecture, central Japan. Kaseki no Tomo, 27: 13-23 (in Japanese). Yamaoka, M., 1993. Geology of the Morozaki Group. In: The Tokai Fossils Editing Committee (Editor), Fossils from the Miocene Morozaki Group, pp. 23-26 (in Japanese with English abstract). Yamazaki, T., Yamaoka, M. and Shiki, T., 1989. Miocene offshore tractive current-worked conglomerates-Tsubutegaura, Chita Peninsula, central Japan. In: A. Taira and M. Masuda (Editors), Sedimentary Facies and the Active Plate Margin. Terra Publ., Tokyo, pp. 483-494. Yeh, H. Imamura, F., Synolakis, C., Tsuji, Y., Liu, P. and Shi, S., 1993. The Flares Island tsunamis. Eos. 74: 369-372.
Sedimentary
Geology
104 (1996) I75- 188
Tsunami-induced conglomerates in Miocene upper bathyal deposits, Chita Peninsula, central Japan Tsunemasa Shiki a, Teiji Yamazaki b aKitabatake, Kohata, Uji City 61 I, Japan b Institute of Earth Science, Osaka Kyoiku University, Asahigaoka, Kashiwara City 582, Japan Received
10
March 1993;revised version accepted 10 September 1994
Abstract The Tsubutegaura conglomeratic tsunamiites occur in the middle section of the Miocene storm-related sand-silt alternation system in the Chita Peninsula, central Japan. Deposition of the system in the upper bathyal environment in a bay and synsedimentary seismic activity has been elucidated by palaeontological, palaeogeographical and sedimentological studies. Two coupled units, each built up by a conglomerate layer with an overlying sandstone layer and the alignment of lenticular sedimentary bodies are exceptional in this sequence. Some typical lenticular bodies, confined within sedimentary troughs, consist of a boulder bearing conglomerate layer and calcareous sandstone layer of the lower couple, and another conglomerate layer of the upper couple. The tuffaceous sandstone layer of the upper couple is distributed more widely than the other lithologies. The framework gravels in the coupled conglomerates form a clast-supported fabric and are quite notably monomictic. The clasts are angular and imbricated partly, forming peculiar gravel clusters due to traction current transportation. Conspicuous laminations develop in the coupled sandstone layers. Antidunes with chute and pool structure in the upper couple sandstone layer indicate deposition from an upper flow regime current. On the other hand, the calcareous composition of the lower couple sandstone layer reveals shallow-water provenance. Thixotropic deformations and diastasis cracks in the siltstone bed immediately beneath the layers of the couples show that a severe earthquake tremor and a rapid change of water pressure occurred just before the deposition of the tsunamiites. Submarine debris flows due to collapse of a fault scarp in a shallow bank and the ensuing wash by two pulses of the tsunami-induced ebb current comprise the scenario for the formation of the Tsubutegaura conglomeratic tsunamiites.
1. Introduction Numerous studies have been made on the roles of various events in the transportation and deposition of marine elastic sediments. While storms have received much attention (Einsele and Seilacher, 1982; Aigner, 198.5; Einsele et al., 1991), tsunami events were also assumed by Kuenen (19501, as one of the important generative triggers of turbidity currents. Coleman (1968) mentioned the possibility of a rela0037-0738/96/$15.00 0 1996 Elsevier Science B.V. All rights reserved SSDI 0037.0738(95)00127-I
tion between tsunamis, high-density turbidity currents, and submarine canyons. In recent years, tsunami deposits have attracted considerable attention. For instance, thick homogeneous sediments (called homogenites) in the eastern Mediterranean Sea were interpreted as tsunami-induced sediments triggered by the collapse of the Santorini Caldera about 3500 years B.P. (Kastens and Cita, 1981; Cita et al., 1984; Hieke, 1984). Bourgeois et al. (1988) noticed that the Upper Creta-
T. Shiki, T. Yamazaki/Sedimentary
176
ceous to Paleocene deposits around the Gulf of Mexico and in the western Atlantic contain coarsegrained sandstone beds at or close to the Cretaceous-Tertiary boundary. They suggested that these coarse-grained beds were deposited by a tsunami induced by a bolide-water impact. Sedimentary bodies, including some boulder beds which formed by bolide-generated wave trains were also postulated by Smit et al. (1992), Alvarez et al. (19921, and Poag et al. (1992). As for the earthquake-induced tsunami sediments, the so-called tsunami-ishi (tsunami-transported stones> have been reported and studied to determine the highest run-up level of the Yaeyama Earthquake tsunami which occurred in 1771 near Okinawa (Miyoshi, 1968, 1987; Miyoshi and Makino, 1972; Kato and Kimura, 1983). Minoura and his colleagues reported some sand layers which were transported and deposited in inter-dune ponds by the run-up
Geology 104 (1996) 175-188
tsunamis triggered by the Japan Sea Earthquake (19831, by the Meiji Samiku tsunami (1896) of the northeast coast of Japan, and by several other historical tsunamis (Minoura et al., 1987, 1994; Minoura and Nakaya, 1991). No reports on tsunami-affected offshore sediments in the trench-forearc region around the Pacific, however, had been published before our preliminary report on the Miocene tsunami-reworked offshore cobble layers (Yamazaki et al., 1989) (Figs. 1 and 2). These are the Tsubutegaura Conglomerates, exposed on the Tsubutegaura coast of Chita Peninsula, near Nagoya, central Japan. In this paper, we provide a detailed description and discussion on the depositional process of these conglomerates and the overlying sandstones. We hope that documentation will be carried out of many examples of various types of tsunami-reworked sediments, tsunamiites, in both recent and ancient sediments in and around
135’E
l---nTl
UBUTEGAURA
Fig. 1. Palaeogeographic reconstruction area is shown by a solid square.)
of southern
m
gravelly bottom
/J
shore line ditto of low reliability
fresh water area
,,t,’
m
muddy bottom
...I...’ 200m depth line
m
ditto assumed
nn
a
sandy - silty bottom
steep mountain
Central Japan during the Middle Miocene. (After Shibata and Itoigawa,
1980. Study
T. Shiki, T. Yamazaki/ Sedimenrary Geology 104 (1996) 175-188
island arcs, because great earthquakes and tsunamis have occurred frequently along converging plate boundaries. We use the term tsunamiites not only for sediments transported by the tsunami wave itself, but also for tsunami-induced current deposits. This usage is much the same as that of the term tempestites, which is used for storm-induced sediments. 2. Geologic setting The Tsubutegaura Conglomerates which are noted for their conspicuously large boulders, occur in the
middle part of the Miocene Morozaki Group. The Morozaki Group, more than 1000 m thick, is mainly composed of alternations of siltstone and sandstone. The alternations form fining-upward sequences overlying the coarse-grained lowest formation of shallow tempestite facies which exhibit hummocky crossstratification, ripples, load casts, and so on. Many tuff beds were intercalated in the Morozaki Group and provide good key beds for correlation of the strata (Doi, 1983; Yamaoka, 1984; Kondo and Kimura, 1987; Yamaoka, 1993). Shibata and Itoigawa (1980) pointed out, in their comprehensive palaeogeographic study, that the mid-
CHITA PENINSULA
m
dark
m
&2j
sandstone dominated alternation (storm -induced) wavy or cross laminated or contorted sandstone mostly massive or faintly laminated sandstone conglomerate
m
tuff
IF
fault
B a
grey
siltstone
layer
Fig. 2. Lithological map showing major distribution of the Tsubutcgaura 1-6 = measured columnar sections shown in Fig. 4.
tsunami conglomerate:
I-V = numbers of conglomeratic
bodies;
178
T. Shiki, T. Yamazaki/Sedimenrary
--
_
-
- _ _-
Fig. 3. Schematic profile of the tsunami sediment bodies along the well exposed part of the Tsubutegaura coast: A, C = tempestitic sand-silt alternation; B = tsunami-induced sediments (tsunamiite); B-l = conglomerate lens; B-2 = widely extended sandstone.
dle part of the Morozaki Group was deposited in the center of a southwardly opening palaeobay (PalaeoIse Bay) at a water depth of more than 200 m (Fig.
r
w
Geology
104 (1996)
175-188
The Morozaki Group yields many fossils of mollusts, fishes, crustaceans, asteroids, whale bones, and others (The Tokai Fossils Editing Committee, 1993). Benthonic molluscs are common in every horizon of the Morozaki Group. The benthonic mollusts from mudstone of the middle Morozaki Group consist of an Acilanu assemblage characterized by the predominance of Acilana tokunagai. Many species in this assemblage have a close affinity with living molluscs that inhabit the sea bottom at depths from 200 to 400 m deep or more. The other benthonic fauna (crustaceans, asteroids and echinoids) also indicate similar uppermost bathyal environments (Yamaoka, 1985, 1987; Takeda et al., 1986; Hachiya et al., 1993; The Tokai Fossils Editing Committee, 1993). Besides these, the nektonic fossil communities from the mudstone consist mainly of a deep-sea fauna. In particular, the occurrence of small fishes having luminous organs is noteworthy.
U-I
Fig. 4. Detailed columnar sections of the Tsubutegaura tsunamiik Upper coupled unit: U-2 = widely extended tuffaceous upper sandstone layer; U-Z = upper conglomerate layer. Lower coupled unit: L-2 = calcareous lower sandstone layer; L-l = lower conglomerate layer.
T, Shiki, T. Yamazaki/ Sedimentary Geology 104 (1996) 175-188
179
sedimentation and synsedimentary tectonics of the Morozaki Group took place coeval with intermittent movements of the Median Tectonic Fault Line, which is the most important major strike slip fault in the geotectonic development of the Japanese Islands, and its conjugate fault systems. The left-lateral slip phase of the Atsumi Fault, one of the conjugate faults, occurred concurrently with the deposition of the middle Morozaki Group.
Fig. 5. The largest and exceptionally rounded boulder in the first conglomeratic body sticking out into the overlying tuffaceous sandstone
layer.
The fossil molluscs from the intercalated sandstone layers in the same formation, on the other hand, are composed of both the Lucinoma-Cyclocur& and Aciluna assemblages. These assemblages indicate shallow sea bottom environments, 100 to 200 m deep. The palaeontological data thus show that the intercalated sandstone layers were transported from a shallow sea bottom to the uppermost bathyal environment (Yamaoka, 1985, 1993; Yamazaki et al., 1988). Inorganic sedimentary structure in the Morozaki Group include sandstone dikes, diapiric foldings, convolute laminations, and ball-and-pillow structures (Hayashi, 1985). Hayashi (1987) clarified that the
3. Sedimentary tsunamiites
features of the conglomeratic
3.1. Occurrence The conglomeratic tsunamiites (i.e., the Tsubutegaura Conglomerates and associated sandstones) occur along the Tsubutegaura beach over a distance of about 150 m. They form some horizons in a stratigraphic interval of several metres, intercalated in the tempestitic sandstone-siltstone alternation (Figs. 2 and 3). They are characterised by aligned lenticular conglomeratic bodies filling erosional troughs. Only one of the horizons of the aligned conglomeratic bodies is well exposed since most of the others are usually under water or buried by beach sands. The occurrence of conglomeratic bodies in this horizon was described preliminarily by Yamazaki et al. (1989). The bodies are 10 to 40 m across and a few SE
NW Land
Fig. 6. Sketch of the fourth conglomeratic body viewed from the sea-side: A = underlying pate bluish gray siltstone; B = conglomerate layer of the lower unit; C = calcareous sandstone layer of the lower unit; D = conglomerate layer of the upper unit; E = tuffaceous laminated sandstone layer of the upper unit; F = overlying sand-silt alternation. The stratigraphic relation between the layers D and E is not obvious from this view.
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T. Shiki, T. Yamuzaki/Sedimentary
metres high. The distance between individual conglomeratic bodies is several metres (Figs. 2 and 3). They are succeeded by a widely extended white tuffaceous sandstone layer in which conspicuous lamination develops, and underlain by a pale bluish or purplish grey siltstone bed with eroded upper surface. Where the conglomeratic body or even pebble lags are absent, the tuffaceous sandstone layer succeeds directly the pale bluish or purplish grey siltstone bed (Figs. 3 and 4). 3.2. Lithologic composition of the conglomerates and the coupled sandstones One of the most conspicuous features of the Tsubutegaura conglomeratic tsunamiites is their more or less monomictic composition (Yamazaki et al., 1989). Almost all the clasts are gneiss or gneissose igneous rocks from the Ryoke metamorphic belt which is one of the most remarkable geological belts
Geology 104 (1996) 175-188
of the Japanese Islands. Rip-up clasts from the subjacent siltstones are minor constituents. Coaly wood shreds and fragments of gastropods are accessories. The associated sandstones are either tuffaceous or calcareous. Sand grains in the tuffaceous sandstone are mainly composed of intermediate volcanic rock fragments, volcanic glass shards, feldspars, and quartz. Gneissose rock fragments, metamorphosed pelitic and siliceous rock fragments, pyroxenes, biotite and siliceous fossil fragments are the minor constituents. Sand grains in the calcareous sandstone are similar to those in the tuffaceous sandstone, except for the predominance of calcareous fossil fragments. The predominance of a carbonate matrix, and the calcification of the effusive rock fragments and feldspars characterise this sandstone. 3.3. Internal structure of the conglomeratic bodies The conglomerates of the Tsubutegaura tsunamiites are mainly composed of pebbles and boulders
Fig. 7. lmbricated cobbles in the conglomerate of the upper unit in the fourth body. Palaeocurrents the photo). Note the very angular shapes and well sorting of the cobbles.
are from northeast (from right to left in
T. Shiki, T. Yamazaki/Sedimentaty
but carry also some giant boulders as large as the thickness of its bed. The largest boulders in the conglomerates, reaching 3 m in diameter, appear in the extreme northwestern body which is rather small in size (Fig. 5). The fourth body from the northwestern end of the well exposed horizon shows the most conspicuous structure (Fig. 6) and was mentioned in our previous paper (Yamazaki et al., 1989). The conglomeratic body is composed of three layers: (1) a lower thin conglomerate layer, (2) an intercalated calcareous laminated sandstone layer, and (3) an upper conglomerate layer. The lower two layers form a coupled sedimentary unit, and the upper conglomerate layer forms a subunit coupled with the above-mentioned overlying tuffaceous sandstone layer. The lower coupled unit is found only in the fourth and fifth conglomerate bodies filling small channels in the underlying siltstone, but does not occur in the other bodies. Two-coupled units in the other bodies were amalgamated by some sedimentary process which took place in the narrow channels. The lower conglomerate layer, if present, consists of subangular pebbles of gneissose rocks and is only 50 cm thick at maximum. Reverse grading is obvious in some basal parts, and a crude concentration of larger gravels is seen in the upper half. The fabric of the conglomerate is clast-supported, and parallel orientation and imbrication of gravels are observed in some parts. The voids between gravels are filled with the detritus of the overlying calcareous sandstone. The lower conglomerate layer is covered and coupled with the calcareous sandstone layers. This ranges in thickness from several to 50 cm in the fourth body and is thinner in the fifth body. Parallel lamination and dish structures develop in some parts of the layer. The parallel lamination is contorted, though rarely, by the loading of cobbles in the overlying upper conglomerate layer as reported by Hayashi (1985, 1987). As stressed by Yamazaki et al. (19891, the most significant part of the conglomeratic deposits is the upper conglomerate layer which contains the giant boulders. The giant boulders tend to lie in the northwestern part of the upper conglomerate layer in each body, where it is also thicker. The conglomerate layer gradually thins out southeastward. The conglomeratic bodies therefore have the form of a bunch
Geology 104 (1996) 175-188
181
of grapes, with giant boulder gravels at their northwestern ends. The fabric of the upper conglomerate layer has been described in detail by Yamazaki et al. (1989). Gravels in this layer are clast-supported, and most of the clasts are angular to subangular and bladed in form. Voids among the framework gravel are filled with tuffaceous sand of the overlying layer. Shapes of gravels are usually asymmetrical around their longest axis as seen in fluvial gravels. As a matter of interest, northeastward imbrication develops in many parts, where the gravels are relatively well sorted (Fig. 7). The longest axis of the imbricated gravels is transverse to the palaeocurrents. The boulders and the smaller clasts accumulate like gravel clusters as seen in alluvial channels.
_:
/: ::’
,..’
. ..
,_....I
Fig. 8. High-energy current structures developed in the widely distributed tuffaceous sandstone layer (scale 5 cm): A = supercritical parallel lamination at the lowest part of the layer; B = antidune cross lamination with chute and pool structure; C = large convolute lamination, faintly visible.
T. Shiki, T. Yamazaki/ Sedimentary Geology 104 (1996) 175-188
.._ ,. ,,_,-, ._._._,__.._
.._,._.. - ..._
,, ...__......_.,...... ..-. ,__.... _ n. _..
:
,,;..
.. _. .- _....
i
.“:.
,.....
..___
. “....I
._
..._ _ _..._..... .. . ._. ..__.......-. . . ..
.._._
,/
‘ 1 I I/ M
B
1
t// Fig. 9. Diastasis cracks and horizontally expanded fractures in the siltstone. Wall-derived angular siltstone clasts (shaded) and sandstone fill the diastasis cracks (arrow I). Note the sand-filled horizontal fractures (arrow 2) due to expansion. Scale is 5 cm. A = fine laminated sandstone layer (part of tsunamiite) with water-escape pillar structures; B = siltstone layer.
T. Shiki, T. Yamazaki/Sedimenrary
Obstacle and wake fabrics, common among recent stream gravel clusters (Dal Cin, 1968; Brayshaw, 1984), are also present in the conglomerate layer. On the northeast side of the giant boulders, relatively well sorted smaller gravels are concentrated, with no arrangement such as seen on the stoss side of a boss formed by unidirectional current deposits. The random fabric parts are covered and armoured by coarser-grained cobbles. These features show that the gravels have been selectively transported and deposited by southwestward-flowing traction currents. Palaeocurrents from the northeast are indicated also by loaded flute casts found at the northwestern end of the fourth body. The southwestward palaeocurrent direction deviate significantly even at right angle from that of the surrounding storm-induced sediments which reveal westward to northwestward palaeocurrents in the middle part of the Morozaki Group. The upper conglomerate layer does not scour the underlying sediments except in the fifth lenticular body. Here, the underlying calcareous sandstone layer was contorted and eroded out leaving thin lenses and fragments of the well laminated sandstone. Contrary to the lower sandstone layer which is calcareous, the laminated sandstone layer of the upper couple is tuffaceous, similar to those of the general storm-induced turbidite or tempestite sandstone in the Morozaki Group. The layer is graded from medium sand to silt and attains 1.5 m in total thickness. Cross-lamination of the antidune type and chute and pool structures with convolutions in the division of the graded sandstone indicate deposition from high-energy currents in the upper flow regime. Giant convolute structures are also seen in the bed just above the antiduned sandstone (Fig. 8). Many current shadows behind gravels, sticking out from the underlying conglomerate layer, are seen in the lower part of the laminated sandstone layer. The palaeocurrent directions shown by both the current shadows and the laminations in the sandstone layer are from northeast to southwest and coincide with that of the upper conglomerate layer. 3.4. The underlying bed of the tsunamiite The underlying bed is pale bluish or purplish gray siltstone. Diastasis cracks (Cowan and James, 1992)
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Fig. 10. Thixotropic deformations in the siltstone underlying the conglomeratic tsunamiite.
are observed just beneath the erosion surface of the bed. The cracks are filled with elastics of the overlying conglomerate and/or sandstone (Fig. 9). Thixotropic deformation of typical shock wave-induced type without any gravel loading is also observed in the siltstone bed. Where the conglomeratic bodies are absent, the tuffaceous sandstone layer directly covers the siltstone bed (Fig. IO> with local small erosional signs as mentioned above. Conglomerate lags are found in many places between the tuffaceous sandstone layer and the underlying siltstone bed.
4. Discussion 4.1. Transportation mechanism On the basis of the sedimentary records given above, we can assume that earthquakes, collapses of fault scarp, and tsunamis played important roles in the sedimentary processes of the Tsubutegaura Conglomerates and associated sandstones. As shown in the previous section, the Tsubutegaura Conglomerates occur in the storm-related systems deposited in 200 to 400 m or deeper submarine environments. Their sedimentary fabrics, however, are very exceptional to the system. In monomictic lithology, the very angular shape of many gravels and the lack of mud matrix, the upper conglomerate layer of the Tsubutegaura conglomeratic bodies resemble washed talus deposits on a bench at the foot of a coastal cliff. The sedimentary facies and fauna
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Such an assumption, however, is supported neither by the sedimentary features of the conglomerates, nor by palaeocurrent directions. The head or uppermost part of some debris flow deposits may have clast-supported, bedded and/or imbricated fabrics (Hirano and Iwamoto, 1981; Suwa and Okuda, 1983; Suwa et al., 1984), but such fabrics usually develop only poorly and change into matrix-supported massive textures. The Tsubutegaura Conglomerates, however, do not have any matrix-supported massive parts. As described in the previous chapter, the most significant part of the Tsubutegaura Conglomerates is the conglomerate layer in the upper couple. In this layer, the traction-current features, such as imbrica-
of the Middle Morozaki Group, however, argue against the coastal topography for the formation of the conglomerate layer. Soft siltstone clasts, picked up from the underlying strata, also testify against a simple origin as a talus deposit. The conglomeratic bodies seem to resemble the tongues of some debris flow. In fact, deposition of the conglomerates by submarine debris flows or submarine slides was suggested by a few workers (Hayashi, 1987; Kondo and Kimura, 1987). If the debris flow origin is adequate for the Tsubutegaura Conglomerates, concentrations of giant boulders and the thickening tendency of the lenticular bodies in the northwestern part of each body, might suggest that the debris flows were driven from the southeast.
1
Storm-induced current
gneissose carbonate
gravels muds
an
-N
tsunami
ebris
flow
collabse
3 back
tSunami:worked
gravel
flow
of tsunami
wave
layer
Fig. I I. Depositional scenario of the Tsubutegaura tsunamiites near the foot of a bank in the Palaeo-Ise Bay (see text). (1) Storm-induced current deposited on the bay floor (upper bathyal environment). Calcareous sands and giant boulders formed on top of a neighboring shallow bank, which is bordered by a fault scarp. (2) Severe earthquakes and resultant tsunamis attacked the palaeobay. The fault scarp partly collapsed and calcareous sand and boulders were trausported to the deeper basin floor. (3) Torrential tsunami ebb currents carrying debris swept the sea floor and deposited conglomeratic and successive sandy sediments.
T. Shiki, T. Yumnzaki/Sedimentwy
tion and gravel cluster fabrics, are most conspicuous. They formed by southwestward flow, namely, almost at right angle to the suggested debris flow. Very rapid transportation with little grain collision and abrasion is confirmed by the presence of nonresident rip-up clasts and high angularity of the gneissose gravels in the upper conglomerate layer. The lower conglomerate rests on scours in the underlying siltstone. The fabrics of the conglomerates suggest that the lower conglomerates are principally of debris-flow origin. Sedimentary features of some parts of the conglomerate layer, however, such as clast-supported fabrics, crude bedding and imbrications of gravels, though not clear enough to determine the current direction, show superimposition of traction currents on the debris flow deposits. The supercritical bed forms of the overlying calcareous sandstone layer support the idea that rapid traction currents followed the debris flow. The traction current transported boulders in the uppermost bathyal sea floor, deeper than 200 m, cannot be a tidal current. Needless to say that seasonal storm-induced waves and the resultant currents at such depths are too weak to move giant boulders. A tsunami and its related currents are the only possible agent. If submarine avalanche deposits are reworked at the foot of a submarine cliff by a very powerful traction current and transported to the shallow bathyal sea floor, or if newly derived debris-flow deposits on the sea floor are reworked by such traction currents, the resultant deposits can have fabrics similar to those of the upper conglomerate layer. The foregoing discussions are mainly concerned with the deposition of the conglomerate layers. The overlying sandstone layers also reveal remarkable characters of the successive stage of the sedimentary processes. The supercritical bed forms of the sandstones are evidence that rapid traction currents followed the conglomerate deposition. Such high power current structures have never been observed in the storm-induced turbidites in the Morozaki Group. In turn, some severe earthquake tremors and quick water pressure changes just before the deposition of the conglomerates are evidenced by the diastasis cracks and thixotropic deformations in the underlying siltstone as described above. Needless to say that a tsunami wave is a more powerful agent than a
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is5
storm wave for formation of the diastasis cracks suggested by Cowan and James (1992). Since a tsunami induced by an earthquake has several pulses, the tsunami-induced ebb currents also act in several pulses. It must also be pointed out that tsunamis induced very close to the hypocenter of an earthquake generally have only a few pulses. The two coupled conglomerates and overlying sandstone layers were possibly produced by two big pulses, though there were some differences in the depositional processes between the two. Seemingly an alignment of conglomerate bodies may also have resulted from these pulses. 4.2. Possible tectonic and hydrodynamic origin of the processes
Large fault movements on both east and west sides of the Chita Peninsula are evident (Hayashi, 1987). With submarine fault movements, the fault scarp should collapse and produce an amount of debris at the foot of the scarp. A subaqueous slump leaving a vertical cliff can occur as a result of earthquakes and succeeding tsunamis, as illustrated by the example of the Flores Island tsunami of December 12, 1992 (Yeh et al., 1993). It is quite possible that most of the very angular gravels in the conglomerate layers represent a collapse breccia at a fault scarp. On the other hand, the rounded giant boulder, the largest in the Tsubutegaura conglomeratic deposits, must have been originally on a shallow bottom surface, such as a shallow bank, where seasonal storm waves could round the boulder. Although seasonal storm-induced currents could not remove the boulder, exceptionally high energy currents, such as tsunami-induced currents, could bring down the largest boulder from the surface of the bank. Big boulders occasionally can roll down further and greater distance than smaller gravels when the water is deep enough and/or the role of gravity is effective (Kimura, 1954, 1956). The shallow water derivation is supported by the intercalated calcareous sandstone layer. We suggest that some strong tsunami trains with a large amount of debris can transport cobbles on the upper-bathyal sea floor. In particular, the ebb current of tsunamis is powerful enough to remove the cob-
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186
bles deposited by previous processes and to produce sedimentary fabrics like the conglomerates of Tsubutegaura. Many oceanographers have reported on the propagation, wave length and wave height of tsunamis, and coastal disasters caused by the run-up of tsunamis (Miyazaki, 1971; Miyoshi, 1987; Abe and Ishii, 1987). Although the properties of an ebb current of a tsunami have not yet been observed, the energy transport of tsunami waves in deep water is widely accepted because of its long wave length. By the run-up of tsunamis, a large water mass is piled up on the coast and therefore generates a resultant rapid ebb current (return current). The offshore velocity of a rip-current produced even by seasonal storms can reach to about 1 m/s at its maximum (Nagata, 1971). It is quite possible that a tsunami-induced ebb current, with a large amount of debris in suspension may reach a velocity of several metres per second. In general, even a normal traction current of a velocity of several metres per second can transport giant boulders 2 or 3 m in diameter as shown by Baker (1984). Transportation of giant limestone boulders by the ebb current of the Yaeyama Earthquake tsunami has been suggested by Kato and Kimura (1983). Rather recently, Pickering et al. (1991) suggested that back-flow surges of a tsunami could be of sufficient magnitude to move all normal available grain-sizes. Together with the tsunami-induced current, the initial moment due to the collapse of a submarine scarp may have caused the movement of the giant boulders in the Tsubutegaura Conglomerates. All the sedimentary features of the Tsubutegaura Conglomerates and associated sandstones discussed above can be explained better by the combined effect of the collapse of submarine scarp and tsunami-induced currents. The hydrodynamic circumstances of the deep-sea tsunami-worked coarse-grained sediments will be discussed in more detail in a separate paper. 4.3. Depositional
scenario
of
the
Tsubutegaura
tsunamiite The sedimentary basin of the Morozaki Group of the Middle Miocene was a southerly facing bay with a shallow bank cut by fault scarps (see Fig. 2).
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Intermittent storm-induced turbidity currents were the major sedimentary agencies of the middle formations of the Morozaki Group which were deposited in the upper bathyal environment of 200 to 400 m deep or deeper (Fig. 111. Severe earthquakes and tsunamis took place when one of the faults bordering the center of the basin and the bank moved. A submarine fault scarp in the Ryoke Gneiss collapsed. Newly collapsed debris and older talus deposits which has been at the foot of the scarp, formed debris flows and were transported down to the deeper part of the basin. In turn, the talus debris was caught by the tsunami ebb currents, and some kind of sediment-gravity flow with a large amount of gravels was initiated. The gravel-rich flow scoured the floor and was followed by its sandy tail, which flowed down from the shallow bank surface, carrying the calcareous materials. Soon after the deposition of the lower unit sediments, gravels of the upper conglomerate were brought down very rapidly by a succeeding severer tsunami pulse. Much of the collapsed and previously deposited debris may have been carried some tens of metres toward the shallower places by the up-rush of tsunamis. Gigantic ebb currents, however, could catch a considerable amount of newly collapsed gravels and carry them back to the deeper floor. The second current was more powerful than the preceding current. It carried away smaller debris, and imbricated larger gravels of the first tsunamiite. Following the deposition and rearrangement of the upper conglomerate layer, the tail of the powerful ebb current of the same pulse containing a large amount of sorted sand grains came from the shallow area in the bay and deposited the tuffaceous laminated sandstone layer on top of the conglomerates. It covered directly a wide area of the siltstone bed where no conglomerate bodies had been deposited. During these processes, previously deposited silty muds, namely, the bottom sediments, experienced seismic tremor and rapid pressure change due to the passage of the high-amplitude tsunami wave. Thixotropic deformations took place first with the seismic tremor. Succeeding compaction and depression due to the passage of the high-amplitude tsunami wave caused diastasis cracks and associated horizontal tear-off cracks. These cracks were filled with sand grains and small pebbles.
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Acknowledgements We are grateful to Prof. H. Okada (Kyushu University) for his critical reading of the manuscript. We wish to thank Dr. T. Hayashi for drawing our attention to the interesting sediments, the Tsubutegaura Conglomerates. We are also indebted to Prof. J. Itoigawa (Nagoya University) and Mr. M. Yamaoka (Kita Junior High School) for providing palaeontological information, and to Mr. H. Komori for assistance in field work. Thanks are extended to Profs. S. Okunishi, H. Suwa and S. Nakamura (Disaster Prevention Institute, Kyoto University) and their colleagues for their valuable discussions. The text figures were kindly prepared by Mr. K. Irino (Kyoto University).
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