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Acoustic stratigraphy, structure, and depositional history of the Nicobar Fan, eastern Indian Ocean
Marine Geology, 26 (1978) 269--288 © Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands
ACOUSTIC STRATIGRAPHY, STRUCTURE, AND DEPOSITIONAL HISTORY OF THE NICOBAR FAN, EASTERN INDIAN OCEAN
FREDERICK
A. B O W L E S ' , W I L L I A M F. R U D D I M A N
2 and W A L T E R
H. J A H N '
'Naval Oceanographic Laboratory, Naval Ocean Research and Development Activity, Bay St. Louis, Miss. 39529 (U.S.A.) 2 Lamont-Doherty Geological Observatory, Columbia University, Palisades, N.Y. 10960 (U.S.A.)
(Received September 20, 1976;revised and accepted March 10, 1977)
ABSTRACT
Bowles, F.A., Ruddiman, W.F. and Jahn, W.H., 1978. Acoustic stratigraphy, structure, and depositional history of the Nicobar Fan, eastern Indian Ocean. Mar. Geol., 26: 269--288. The Nicobar Fan is a topographically isolated segment of the Bengal Deep-Sea Fan. Seismic reflection data indicate that the Nicobar Fan may have begun forming in late Miocene to early Pliocene time, but considerably later than the Bengal Fan. Since then, the original basement relief has been gradually inundated and buried by fan sediments, although numerous basement peaks still pierce the surface of the fan. Sediment thicknesses in excess of 1.6 km occur in the northern part of the fan. The fan surface is marked by turbiditycurrent channels and numerous faults which, in some cases, may have acted as channels. The faults are apparently related to tensional stresses associated with a descending lithosphere and to intraplate seismic activity. Relatively few channels are encountered on the fan, and they are noticeably absent on the northern part. Most of the sediment filling the northern Java Trench consists of depressed fan sediments, although deposition from local sources is evident. Plate motion has gradually isolated the fan from its major source of sediment to the extent that a buried "fossil" fan surface exists in the southeastern portion of the survey area, and much, if not all, of the remaining Nicobar Fan is now receiving only pelagic sediments.
INTRODUCTION T h e f l o o r o f the B a y o f Bengal and a d j a c e n t s o u t h e r n area is o c c u p i e d by a massive c o n e o f intensely stratified s e d i m e n t s called the Bengal Deep-Sea F a n ( C u r r a y and M o o r e , 1 9 7 1 ) , a n d also called the Ganges C o n e (Ewing et al., 1 9 6 9 ) . The eastern p o r t i o n o f this fan b o r d e r i n g the Java T r e n c h is isolated f r o m the m a j o r b o d y o f s e d i m e n t s b y N i n e t y e a s t Ridge, f o r m i n g a smaller sub-fan called the N i c o b a r F a n (Curray and M o o r e , 1971). N o r t h o f a p p r o x i m a t e l y 2 ° N , t h e sediments o f the N i c o b a r F a n fill a n a r r o w gap, 1 0 0 - - 1 5 0 k m wide, b e t w e e n N i n e t y e a s t Ridge and the l a n d w a r d wall o f the Java T r e n c h ( F i g . l ) . T o the s o u t h , the fan b r o a d e n s c o n s i d e r a b l y ,
270 !ffi
5~ S
90'E
95 °
100 °
Fig. 1. Ship's tracks superimposed upon bathymetric map by Curray and Moore (1971 ). Locations of seismic reflection profiles (Figs.4, 5, and 7 ) and 3.5-kHz echogram profiles (Figs.8 and 9) are indicated by heavy track lines and indexed by letters. Piston cores were taken at stations 11, 13, and 18; bottom photographs were taken at all stations; and short "punch cores" were taken at all stations except 5, 7, and 9. Probable turbidity-current channels are indicated by the dotted lines. e x t e n d i n g s o u t h w a r d t o a b o u t 10°S (Ewing et al., 1 9 6 9 ) , w i t h a l o n g i t u d i n a l g r a d i e n t o f a p p r o x i m a t e l y 1 : 1 4 5 0 . A l t h o u g h N i n e t y e a s t Ridge a n d t h e t r e n c h diverge t o w a r d the s o u t h , the fan is w i d e s t ( a b o u t 9 2 0 k m ) at a b o u t 2 ° S. As later s h o w n , rugged t o p o g r a p h y still u n b u r i e d b y fan s e d i m e n t s o c c u p i e s a large p o r t i o n o f t h e sea f l o o r s o u t h o f 2 ° S. B o t h t h e N i c o b a r a n d Bengal F a n s are c o m p o s e d p r i m a r i l y o f erosional debris carried f r o m the H i m a l a y a n M o u n t a i n s t o t h e Bay of Bengal b y t h e G a n g e s - - B r a h m a p u t r a River s y s t e m s . T h e c o m b i n e d flow o f b o t h rivers delivers a p p r o x i m a t e l y o n e billion t o n s of s e d i m e n t a n n u a l l y ( m o s t l y silts a n d
271 sands) to the Bay of Bengal, where it is distributed southward by turbidity currents (Coleman, 1969}. Ewing et al. (1969) have shown that the unconsolidated sediments in the Bay of Bengal exceed 2.5 sec in thickness. This is in agreement with earlier work by Soviet investigators (Neprochnov et al., 1964), who reported maximum thicknesses of 3.0 km. More recently, however, Curray and Moore (1971) and Moore et al. (1974} have shown that well over 3 km of sediment exist over most of the Bengal Fan, and that beneath the northern shelf, the total accumulation of sediments (in the Bengal geosyncline) probably exceeds 12 km. Sediment thicknesses up to 1 km are indicated for the Nicobar Fan (Ewing et al., 1969; Curray and Moore, 1971). Little information has been published about the Nicobar Fan. In this paper, we use geophysical data, sediment cores, and b o t t o m photographs to delineate the structural, morphological, depositional, and historical character of the Nicobar Fan. METHODS In October and November 1971, the USNS "Bartlett" conducted a geological and geophysical survey of the Nicobar Fan. Magnetic, 3.5-kHz, and seismic reflection data were continuously collected along survey lines shown in Fig.1. Reflection profiles were obtained with a 30,000-J "sparker" sound source fired at 10-sec intervals. In addition, sediment cores and/or b o t t o m photographs were collected at 14 stations on the fan. Normal survey speed was a b o u t 8 knots, and the ship's position was determined by satellite navigation. Additional seismic reflection data were provided by " V e m a " cruise 29 and "Conrad" cruise 9. SEDIMENT THICKNESS AND DISTRIBUTION The accumulation of sediment in the survey area is shown in Fig.2 as isopachs of sediment thickness above acoustic basement in seconds of two-way travel time. Acoustic basement is defined as the deepest reflector in the seismic profiles. A major portion of the area surveyed is covered by thicknesses of 1.0 sec or greater, with thickest sediments toward the north. Acoustic basement is not visible in any of the northern profiles except adjacent to Ninetyeast Ridge where the deepest acoustic penetration observed is 1.6 sec. A consistent, b u t usually weak, basement reflection is generally visible beneath the thinner sediments south of the equator where sediment accumulations of 1.0 sec are more limited. Except for the thick sediment fill in the Java Trench, accumulations greater than 1.0 sec occur mostly to the west of 95°E, marking roughly north--south-trending troughs in the acoustic basement. In contrast, the region east of 95°E is characterized by an unusually thin sediment cover, usually less than 0.3 sec.
272
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..... il
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95"
100 o
Fig.2. Isopach map of Nicobar Fan. Contours are in tenths of seconds of two-way travel time (0.1 sec approximates 100 m). Sediment thickness was read every 15 minutes, or about every 2 kin. Stippled areas indicate basement outcrops. For practical reasons, the contours along the landward (east} wall of the trench have been omitted.
Present fan sediments T h r o u g h o u t m o s t of the area surveyed, acoustic b a s e m e n t is buried under a highly stratified, flat-lying s e q u e n c e o f turbidite layers. The strong, reverberant reflections from these layers delineate the sediments of the N i c o b a r Fan; on this basis, the area presently o c c u p i e d b y fan s e d i m e n t s is s h o w n in Fig.3. In profiles crossing the trench north of the equator, the parallel reflectors o f the fan dip toward, and can be traced c o n t i n u o u s l y into, the
273 90"
95°
100°
5"
O"
5°
9OOE,
Fig.3. Sediment distribution map. show approximate dispersal paths represent outcrops and/or regions tracklines indicate the location of
95 °
leo o
Shaded area indicates Nicobar Fan turbidites; arrows as indicated by present gradients. Unshaded areas of pelagic sedimentation. Short "tick" marks on the turbidity-current channels.
trench (Fig.4, profiles A and B). Thus, the fan deposits have clearly been carried downward, and are n o w part of the trench. Assuming that these layers were once flat-lying, they have been depressed by as much as 1.0 sec in relation to adjacent horizontal portions of the fan. South of the equator, outcrops and high-standing topography prevent the fan sediments from being traced directly into the trench (Fig.4, profiles C and D), although the fan deposits can be recognized as part of the trench fill to about 5 ° 30'S (Fig.3).
274
p
5 SEC
B
C
D
~ KM
'
Fig.4. Seismic profiles A, B, C, and D showing transverse crossings of fan and trench.
Beyond this point, the sediments in the trench are apparently younger than the fan deposits. Obviously younger sediment fill can be identified in profiles north of 5 ° 30'S where the fan deposits can be seen dipping beneath a wedge of younger trench sediments (Fig.4, profile D). The fact that the surface and sub-bottom layering within the wedge slope upward toward the trench wall indicates local downslope transport of sediment from the adjacent island arc. A similar sediment wedge is not obvious in profiles north of a b o u t 2°N where the trench fill appears to consist entirely of depressed fan sediments. Thus, transport of sediment down the trench wall is apparently less important along this portion of the trench. This floor of the trench does, however, have a tendency to be relatively flat, suggesting possible axial transport and deposition by turbidity currents from the north. This, in turn, could obscure the effects of whatever local downslope transport might occur.
275
As shown by Fig.3 (and Fig.2), the continuity of the fan is broken in numerous places where the underlying basement (volcanic) topography protrudes above the fan surface. In particular, the region of thin (0.3 sec) sediment cover east of 95°E (Fig.2) is most conspicuous because of the absence of fan sediments (shown in Fig.3 by absence of shading). The area represents a broad, tectonically complex portion of basement largely covered by pelagic sediments. The contrast between the rugged topography and the topographically flat surface of the fan is shown in Fig.5, profile E.
Fig.5. Seismic profile E across Nicobar Fan and adjacent area of high, rugged topography. The fan sediments begin on the left and end at the center of the profile; to the right of center, the sediments are dominantly pelagic.
Unfortunately, the general fabric in the area cannot be determined because of the broad spacing of the survey lines. Sclater and Fisher (1974), however, show ridges and intervening basins trending north-northeast--south-southwest. As shown by profile E, ponding of the fan sediments behind outcrops (ridges) has caused the fan surface to build considerably higher than the sheltered basin floors. The high topography has apparently prevented the spread of fan sediments to the south and east into this area. A few of the trench profiles show evidence of possible intrusive tectonic activity or folding in the axial portion of the trench (Fig.4, profiles Band C). Some of this activity is obviously quite recent as in profile B where the fan turbidites are shown uplifted into a prominent anticlinal structure. The structures in the trench may represent local, isolated intrusions or crustal material of perhaps short discontinuous ridges (since they do not appear in adjacent profiles). Axial ridges have been observed in other trenches, notably, the Peru--Chile Trench where they are thought to form by imbricate thrusting (Prince and Kulm, 1975). There seems to be no evidence of imbricate thrusting in our trench profiles where basement is visible. Such thrusting, however, may well not apply to this segment of the Java Trench where there is major strike-slip motion between the plates (Fitch, 1972). The contacts between sediments and rock outcrops can be indicative of their relative ages. In all cases where the fan sediments terminate abruptly against the outcrops, the reflectors show little or no deformation; in other places, they have gradually encroached over more gently rising basement
276 with the fan reflectors sometimes overlapping more transparent (pelagic} sediments. Aside from some broad, gentle upwarping of deeper reflectors, there are relatively few instances where the horizontal layering of the fan has undergone major deformation as a result of basement material having been thrust upward through the sediments. Thus, most of the major underlying and exposed topographic features were in existence before the fan sediments were deposited.
Relict fan sediments Ewing et al. (1969) have placed the southern limit of the Nicobar Fan at approximately 10°S. Drilling at sites 211 (09°46.53'S, 102°41,95'E) and 213 (10 ° 12.71'S, 93 ° 53.77'E) by the Deep-Sea Drilling Project (DSDP) has confirmed this boundary. No turbidites, for example, were encountered at site 213, southward penetration of the fan sediments having been prevented by a large ridge north of the site. Farther east at site 211, however, a 200-mthick unit of pelagic ooze and interbedded terrigenous sands and silts was cored. Overlying this unit was approximately 100 m of siliceous ooze of late Pliocene and Quaternary age. Thompson (1974} has shown that the sands and silts are mineralogically similar to turbidite deposits of the Bengal Fan. The occurrence of buried turbidites at site 211 (Fig.6) is of particular interest because of the isolated location of the site with respect to the present distribution of fan sediment (Fig.3}. Reflection profiles along the track lines shown in Fig.6 indicate no evidence of fan sediments ever having penetrated from the west into the region of site 211. Evidence suggests, instead, that the fan sediments reached the site by a more northerly route when the broad, rugged region of pelagic deposition (Fig.3) was located farther west than at present. At that time, a narrow, b u t flat-lying, segment of fan may have existed between the pelagic region and the trench, reaching as far south as site 211. Evidence of a turbidite surface buried beneath a later pelagic cover is indicated in the seismic records by a reflective layer beneath transparent sediments around site 211. The overlying transparent cover varies in thickness from being barely perceptible to a b o u t 0.2 sec. The layer is most prominently displayed in profiles immediately to the northeast (Fig.7, profile H) of site 211 and north (Fig.7, profile F) at a b o u t 7°S, where, in b o t h cases, it appears as a strikingly flat, reflective surface. Elsewhere, the layer appears intermittently (Fig.7, profile G}, suggesting an interfingering of turbidites within the topography east and south of site 211. In these areas, the layer is generally less reflective and also somewhat distorted by crustal movement. The last major orogeny in the sediment source area which resulted in the modern Himalayas, was latest Pliocene and Pleistocene in age, and correlates with the age of turbidites drilled at site 211 (Curray and Moore, 1971; Moore et al., 1974}. Although the energy of the transporting turbidity
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F i g . 6 . M a p s h o w i n g d i s t r i b u t i o n o f b u r i e d t u r b i d i t e s u r f a c e ( s t i p p l e d a r e a ) . S h a d e d a r e a in n o r t h i n d i c a t e s d e p r e s s e d f a n s e d i m e n t s in t r e n c h . H e a v y t r a c k l i n e s i n d e x e d b y l e t t e r s r e p r e s e n t s e i s m i c p r o f i l e s s h o w n in F i g . 7 . S o l i d c i r c l e s h o w s l o c a t i o n o f D S D P s i t e 2 1 1 .
currents was probably intense at this time {Pimm, 1974), the well-defined nature of the single buried reflector in Fig.7 suggests that the episode of turbidite deposition for this distal portion of the fan was brief, with isolation of site 211 from further turbidite deposition occurring relatively soon after the turbidites began reaching the area. It has been suggested that the entire Nicobar Fan may have been isolated from its source of supply either by the convergence of the northern end of Ninetyeast Ridge against the Indonesian Trench system, or the formation of a pre-trench rise in the northern area associated with the buckling of the lithosphere as it descended into the trench (Pimm, 1974). Isolation of the entire fan at this time (Pliocene) seems unlikely, however, since reflection
278
6 SEC
BURIED
G
6 SEC
RI~FL~CTO~
I'~Dl~lEl I
RE~ZLECTOJ{
F
Fig.7. Seismic profiles F, G, and H showing buried turbidite surface. Profile F is also indexed in Fig.1.
profiles across the present fan reveal only the slightest hint of a surface layer of pelagic sediment. Alternatively, isolation of site 211 may have occurred while turbidite deposition continued over the major portion of the fan. As shown by Fig.6, a buried turbidite surface is not visible in the profiles north of a b o u t 7 ° S. Thus, a portion of the route inferred to have been traveled by the turbidity currents has already been subducted. As plate motion carried the high-standing pelagic area toward the trench, early buckling of the lithosphere b e t w e e n the trench and pelagic area could eventually have confined the flow of turbidity currents in this region entirely to the trench axis, thereby isolating the area of site 211 from its sediment supply b u t not the remainder of the fan. STRUCTURE AND AGE OF FORMATION
Seismic reflection and refraction data across the Bengal Fan show that it consists of three major stratigraphic units, each separated at their boundaries by pronounced seismic velocity discontinuities. Cores from the Deep-Sea Drilling Project (Moore et al., 1974} have shown that the sediments below the " l o w e r " discontinuity (which represents acoustic basement) are largely of pelagic origin. The overlying, intermediate stratigraphic unit is middle Eocene to upper Miocene in age, and represents the earliest fan turbidites. These locally deformed turbidites are, in turn, overlain by another sequence of relatively u n d e f o r m e d turbidites representing the t o p m o s t stratigraphic unit and the m o d e m Bengal Fan. The boundary b e t w e e n the early and m o d e m turbidites is marked by a second, or " u p p e r " , discontinuity. Profiles of the Bengal Fan show that acoustic basement (i.e., lower discontinuity) rises as it approaches Ninetyeast Ridge and appears to be continuous with the basement reflector on the ridge (fig.3d in Curray and Moore, 1971). An analogous situation is observed on the Nicobar Fan
279 (Fig.4, profile A), where acoustic basement also appears to coincide with a ' reflector beneath the pelagic sediments on Ninetyeast Ridge. Despite the similarity, it is our opinion that although acoustic basement under the Nicobar Fan may be lithologically similar to that under the Bengal Fan (i.e., boundary between pelagic and turbidite deposits), it is coincident in time with the upper and not the lower discontinuity of the Bengal Fan. This interpretation is consistent with Curray and Moore (1975), who maintain that the Nicobar Fan developed later than the Bengal Fan. The major piece of evidence leading to our interpretation is the apparent absence of an upper discontinuity in the Nicobar Fan profiles. As noted, the upper discontinuity divides the Bengal Fan turbidites into two distinct stratigraphic units categorized as "W" (modern fan) and "Y" (early fan) by Curray and Moore (1971). South of about 11°N, seismic profiles show the discontinuity to be approximately 0.75--0.50 sec, or less beneath the fan surface. Assuming a similar depositional history for the Nicobar Fan, we would expect a similar discontinuity to appear in our profiles (spanning roughly the same latitudes) at approximately the same depth of burial. No discontinuity is ever observed above acoustic basement, indicating, instead, a significant difference in the depositional history of the Nicobar Fan. The "Y" turbidites of the Bengal Fan (bounded by the lower and upper discontinuities) are approximately early Eocene to late Miocene in age. The lack of the upper discontinuity in the Nicobar Fan sediments indicates that these early fan deposits are missing on the east side of Ninetyeast Ridge. Curray and Moore (1975) suggest that prior to the Oligocene, the region east of Ninetyeast Ridge was sealed off by an active spreading ridge. Thus, early fan deposition took place to the north and west of Ninetyeast Ridge until the spreading ridge became extinct in the early Oligocene. At that time, sediments of the Bengal Fan began to extend east of Ninetyeast Ridge, forming the Nicobar Fan. Deposition of Nicobar Fan sediments during Oligocene through middle Miocene time would, however, constitute "Y" sediments (i.e., pre-upper discontinuity). The absence of these sediments in our profiles suggests two possibilities: (1) that the spreading ridge remained a major barrier to the dispersal of fan sediments east of Ninetyeast Ridge long after its extinction, or (2) that although turbidites began breaching the spreading ridge shortly after its extinction, deposition of these early fan sediments did not achieve significant proportions (i.e., they did not extend very far south or become very thick). In the latter case, Curray and Moore {1975) note that during the Oligocene and early Miocene, the Himalayas apparently were not yet high enough to provide much detritus. Moreover, because of plate motion, most of the deposited "Y" sediments would have probably long since disappeared into the Java Trench. Curray and Moore (1971) correlate the upper discontinuity in their seismic profiles with renewed turbidity-current activity on the Bengal Fan resulting from major uplift of the Himalayan Mountains in the middle-to-late Miocene.
280
Results from the Deep-Sea Drilling Project (Moore et al., 1974) show that this period is indeed represented in the sediments by increased deposition of terrigenous sands and silts. At this time, then, both fans began receiving large volumes of terrigenous sediment. The greater sediment thickness on the Nicobar Fan (relative to the thickness of the "W" sediments on the Bengal Fan) indicates that most of this sediment was channeled down the east side of Ninetyeast Ridge. This is also supported by the fact that the minimum Pliocene sedimentation rate at DSDP site 211 is nearly double the m a x i m u m rate at site 218 located on the Bengal Fan (Pimm, 1974). Thus, formation, or at least major development, of the Nicobar Fan appears to have started during late Miocene time. FAULTING
Bathymetric profiles collected along the survey lines shown in Fig.1 show that the surface layers of the fan are locally offset by faulting. Unfortunately, the survey lines are not close enough to determine the strike of the faults. In most cases, the offsets appear to be faulted folds, although the folding may be a secondary drag effect. Usually, several faults occur together, sometimes forming graben and step-like arrangements (Fig.8, profiles I and J). Faulting similar to this has been observed on the seaward slopes of other deep-sea trenches such as the Japan Trench (Ludwig et al., 1966). Although the vertical displacements along the faults are minor (the largest measuring 98 m), the slopes of the fault surfaces are often too steep to be resolved by the profiler. The displacements do not all show the same general attitude, that is, the trench side of the fracture is not usually downfaulted as in Fig.8, profiles I and J. An a t t e m p t to map the faults produced no discernible pattern in either their attitude or frequency of occurrence (relative to distance from the trench). None of the profiles show any indication of
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Fig.8. E c h o g r a m profiles I and J at 3.5 kHz showing surface expression o f faults o n the Nicobar Fan. Trench is to the right in both profiles.
281 depositional smoothing of the faults, thus, they all have the appearance ' of having been recently formed. The larger offsets and step faults generally occur near the trench, usually where the fan sediments slope toward the trench. Often, the faulting carries into the trench b o t t o m itself, although a few profiles show relatively little disturbance in the trench. Across the central and western portions of the fan, gentle warps or undulations of the fan surface are more c o m m o n ; the faults that occur here are usually small (similar to that shown at extreme left in profile J). Some of the larger faults can be recognized in the seismic profiles as offsets in deeper layers of the fan and also by the occurrence of a dark, nearly vertical line, suggesting acoustic focusing associated with offsets in reflectors along a steeply dipping fault plane (Fig.4, profiles A and B). Although the offsets are not always obvious, a comparison of corresponding seismic and bathymetric profiles shows that these vertical traces occur directly beneath most of the surface faults. In some cases, the offsets appear to increase with depth, indicating that once formed, the faults remain active (thus explaining the fresh appearance of the faults at the surface). The faulting on the fan, particularly the graben and step-like faults, suggests tensional stresses. Such stresses are predicted across the top (convex) side of the lithosphere where it bends downward as it passes beneath the trench (Isacks et al., 1968). The faulting that occurs some distance from the trench, then, presumably cannot be explained by such local stresses. Zones of folded and faulted sediments have been observed, however, near the southern margin of the Bengal Fan where they are thought to relate to a wide belt of seismicity stretching between Ceylon and the Cocos Islands (Eittreim and Ewing, 1972). Earthquake mechanism solutions indicate strike-slip motion for this region (Sykes, 1970). This intra-plate seismicity may also account for much of the faulting across the Nicobar Fan. CHANNELS Although 22 potential channel crossings are indicated in Fig.l, only seven channel crossings actually occurred along the entire ship's track. Some of these are probably crossings of the same channel, however, the broad spacing of the track does not warrant connecting them in most cases. Echogram profiles show that the channel dimensions range from approximately 11 to 70 m in depth to several kilometers in width; most have small, natural levees and/or terraced sides (Fig.9, profile K). The channels have a very "fresh" appearance, showing no signs of being filled in with sediment. In a few cases, the channel b o t t o m is highly reflective, indicating deposits of coarse-grained sediment. All the crossings are of single channels, with one exception (Fig.9, profile L), where a tri-channel system suggesting braiding was crossed.
282
,
',
K
Fig.9. E c h o g r a m profiles K and L at 3.5 kHz showing channel crossings.
At least five other possible channels were crossed, but these show none of the channel characteristics mentioned above. Morphologically, they resemble some of the small distributary channels on the Amazon cone shown by Damuth and Kumar (1975). The chief difficulty in recognizing these "channels" is distinguishing them from the many small faults which mark the surface of the fan. This, in turn, raises the possibility that some faults have proxied as channels at some time. It is d o u b t f u l that increasing the survey coverage on the fan would reveal the existence of a complex network of channels. Curray and Moore (1975) note that turbidity currents on the medial, and especially distal, portions of the Bengal Fan tend to be broad-fronted, moving as sheet-flow between channels rather than within specific channels. Those channels which pass through these fan areas are small and widely spaced. The entire Nicobar Fan is largely distal with respect to its primary source of sediment to the north. Most interesting, however, is the apparent absence of channels passing down the narrow, northern portion of the fan. Of the five tracklines crossing this region (Fig.3), a single channel was encountered on the southernmost crossing, but there was no sign of its continuation to the north. Similarly, an absence of channels in the northern portion of the fan has also been indicated by Curray and Moore (1975, Fig.l). In addition, they also indicate the appearance of channels roughly where we encountered our northernmost channel. Although present-day sediments passing east of Ninetyeast Ridge are thought to be transported down the axis of the trench (Curray and Moore, 1975), one could expect some abandoned channels to still exist on this part of the fan. Convergence of Ninetyeast Ridge with the Java Trench has most likely carried once-existing channels into the trench, but this does not explain the fact that our channel cannot be traced as far north as even the
283 next profile. Convergence of the ridge and trench also raises the possibility that deformation of the fan may have obscured the channels. Seismic and 3.5-kHz profiles show, however, that deformation of the fan sediments is no more severe here than on some areas of the fan where channels exist. Thus, neither the absence of abandoned channels in the narrow, northern portion of the fan nor the abruptness with which channels appear to the south can be fully accounted for at this time. RECENT DEPOSITION B o t t o m photographs and sediment cores taken at stations along the survey track provide primary evidence relating to the present environment of deposition on the fan. The photographs (229) of the fan are m o n o t o n o u s l y similar in appearance, nearly all conveying the impression of a flat, soft b o t t o m {Fig.10A). Stations 10 and 12 occur on topographic highs and show a b o t t o m strewn with angular rock fragments and/or large, rounded {pillowed?) outcrops (Figs.10B and 10C). Without exception, the photographs show no sign of the sedimentary microrelief associated with b o t t o m current activity. The implication, then, is that the environment at all stations is quiescent. This is also supported, in part, by the deposition of pelagic sediment on rock outcrops {Fig.10C) and by the presence of well-defined lebensspuren (Figs.10D and 10E). The lebensspuren are of particular interest. Moderate to abundant lebensspuren and fecal debris are displayed in the photographs, suggesting a large animal population. Turbidity currents are thought to affect abyssal life in t w o ways: (1) obliterating life by rapid burial of organisms under thick sediment layers, and {2) enhancing life b y delivering nutrient-rich organic material to great depths {Heezen et al., 1955). Studies of the Congo River (Heezen et al., 1964) and Cascadia Channel (Griggs et al., 1969) seem to substantiate the latter effect. Studies in the Bay of Bengal, however, show a very weak development of the b o t t o m population on the Bengal Fan {Sokolova and Pasternak, 1962). Similarly, a very sparse benthos is observed in the photographs of the Nicobar Fan despite the abundance of lebensspuren. The general scarcity of epifauna suggests a nutrient deficit, perhaps caused by the absence of b o t t o m currents which tend to stir up and replenish available nutrients, or more significantly, an absence of nutrientdelivering turbidity currents. Owens et al. {1967) have noted that in areas of low sedimentary deposition, animal signs may be preserved, hence, b o t t o m photographs may show the sum o f animal activity over a very long period of time. Since, in. general, lebensspuren on the fan are plentiful while biota is sparse, many of the lebensspuren may be quite old. Close inspection of the b o t t o m photographs shows that some lebensspuren are well-defined, while others have a smoothed appearance. Because such selective smoothing by b o t t o m currents is unlikely, it is reasonable to conclude that many of the lebensspuren are old and have been gradually s m o o t h e d b y slow pelagic deposition.
284
Fig.10. Bottom photographs on Nicobar Fan and on outcrop areas. The tilt of the camera in photograph A has produced the false impression of a slight preferred orientation to the microrelief.
01
t'O
286 With one exception, sediment cores taken at several stations confirmed the existence of a surface veneer of pelagic sediment covering the Nicobar Fan. Piston cores were taken at stations 11, 13, and 18 (Fig.l). Although layers of terrigenous micaceous, quartz sands were cored at these places, the top 284 cm of sediment at station 11 and the 155 cm at station 13 consisted of olive-gray and yellowish-brown lutite, respectively. A white (silicic) volcanic ash layer also occurred near the surface at station 13. Short (62 cm or less} " p u n c h cores" were taken simultaneously with the camera drops at stations 13 through 18, 22, and 23. Although such cores are unreliable for stratigraphic studies because they represent repeated penetrations of the sediment, t h e y are indicative of the type of sediment at the surface of the fan. Significantly, none of the cores contained any terrigenous sand or silt layers. Station 18 provided the single exception to the above observations. The top 4 cm of the core taken here consisted of quartz sand, indicating recent turbidity-current deposition, at least in the northern portion of the fan. It is likely, however, that this sand does not represent the true surface. A camera punch core containing all olive-gray lutite was also taken at station 18. Thus, it appears that pelagic sediment overlying the sand layer was not recovered in the piston core. Curray and Moore (1975} suggest that the Nicobar Fan was cut o f f from its sediment source in middle Pleistocene time by the convergence of Ninetyeast Ridge with the Java Trench. Unfortunately, we cannot pinpoint the ages of the topmost turbidites in our fan cores. The evidence, however, clearly indicates that turbidity currents have been largely inactive for some time, and that present deposition is now primarily pelagic. CONCLUSIONS {1) Seismic reflection profiles of the Nicobar Fan north of the equator show sediment thickness consistently greater than 1.0 sec. The deepest acoustic penetration observed in this area was 1.6 sec. Sediments thin toward the south where numerous basement peaks still rise above the surface of the fan. A broad, raised portion of rugged sea floor south of 2 ° S bordering the Java Trench remains unburied by the fan turbidites. (2 } The fan reflectors, in general, terminate abruptly against basement outcrops with no apparent sign of deformation, or gradually onlap more gently sloping topographic highs. Thus, the major exposed and buried topographic features pre-date the formation of the Nicobar Fan. (3) Seismic reflection profiles show the sediments in the northern Java Trench to consist predominantly of fan deposits which have been depressed to form the trench. Recent trench deposition is represented by a wedge of horizontal or westward-dipping sediments overlying the older, eastwarddipping fan sediments. (4) The southeastern extremity of the Nicobar Fan between 6 and 10°S is a relict feature. Evidence of buried fan turbidites now cut off from the larger
287
b o d y of fan sediments is indicated in this area by an acoustically reflective horizon beneath transparent sediments. This interpretation is reinforced b y the nearby presence of buried turbidites at DSDP site 211. (5) Unlike the Bengal Fan, seismic reflection profiles do not show the existence of a discontinuity separating the Nicobar Fan turbidites into two distinct stratigraphic units. This absence of a lower or early sequence of turbidites indicates that the formation of the Nicobar Fan began considerably later than the formation of the Bengal Fan. (6) Echogram profiles show that in comparison to the Bengal Fan, relatively few turbidity-current channels are incised into the surface of the Nicobar Fan. For undetermined reasons, no channels are observed on the fan north of about 3 ° N. (7) Echogram profiles show the surface of the fan to be marked with numerous faults which, in seismic profiles, appear to offset subsurface reflectors. Near the trench, the faulting is apparently the result of tensional stresses caused by the downbending of the lithosphere as it passes beneath the trench. Farther o u t on the fan, away from the trench, the faulting may be related to intra-plate seismic activity observed between Ceylon and the Cocos Islands. (8) Convergence of Ninetyeast Ridge with the landward wall of the Java Trench has virtually isolated the Nicobar Fan from its northern source of sediment. B o t t o m photographic evidence and sediment cores indicate that present deposition over the fan is primarily pelagic. ACKNOWLEDGEMENTS
We thank the officers, crew, and other scientific party of the USNS "Bartlett" for their assistance in the collection of this data; H. Eppert and T. Holcombe for critical reviews; P. Michalko and B. Grosvenor for illustrations; G. Garner and S. Madosik for data preparation; and L. Thigpen for typing.
REFERENCES Coleman, J.M., 1969. Brahmaputra River: channel processes and sedimentation. Sediment. Geol., 3: 129--239. Curray, J.R. and Moore, D.G., 1971. Growth of the Bengal Deep-Sea Fan and denudation in the Himalayas. Geol. Soc. Am. Bull., 82: 563--572. Curray, J.R. and Moore, D.G., 1975. Sedimentary and tectonic processes in the Bengal Deep-Sea fan and geosyncline. In: C.A. Burke and C.L. Drake (Editors), The Geology of Continental Margins. Springer, New York, N.Y., pp.617--627. Damuth, J.E. and Kumar, N., 1975. Amazon Cone: morphology, sediments, age, and growth pattern. Geol. Soc. Am. Bull., 86: 863--878. Eittreim, S.L. and Ewing, J., 1972. Mid-plate tectonics in the Indian Ocean. J. Geophys. Res., 77: 6413--6421. Ewing, M., Eittreim, S., Truchan, M. and Ewing, J.I., 1969. Sediment distribution in the Indian Ocean. Deep-Sea Res., 16: 231--248.
288 Fitch, T.J., 1972. Plate convergence, transcurrent faults, and internal deformation adjacent to Southeast Asia and the western Pacific. J. Geophys. Res., 77 : 4432--4460. Griggs, G.B., Carey, A.G. and Kulm, L.K., 1969. Deep-sea sedimentation and sediment-fauna interaction in Cascadia Channel and on the Cascadia Abyssal Plain. Deep-Sea Res., 16: 157--170. Heezen, B.C., Ewing, M. and Menzies, R.J., 1955. The influence of submarine turbidity currents on abyssal productivity. Oikos, 6: 170--182. Heezen, B.C., Menzies, R.J., Schneider, E.D., Ewing, W.M. and Granelli, N.C.L., 1964. Congo Submarine Canyon. Bull. Am. Assoc. Pet. Geol., 48: 1126--1149. Isacks, B., Oliver, J. and Sykes, L.R., 1968. Seismology and the new global tectonics. J. Geophys. Res., 73: 5855--5899. Ludwig, W.J., Ewing, J.I., Ewing, M., Murauchi, S., Den, N., Asano, S., Hotto, H., Hayakawa, M., Asanuma, T., Ichikawa, K. and Noguchi, I., 1966. Sediments and structure of the Japan Trench. J. Geophys. Res., 71: 2121--2137. Moore, D.G., Curray, J.R., Raitt, R.W. and Emmel, F.J., 1974. Stratigraphic--seismic section correlations and implications to Bengal fan history. In: Initial Reports of the Deep-Sea Drilling Project, 22. U.S. Gov. Printing Office, Washington, D.C., pp.403-412. Neprochnov, I.P., Korylin, V.M. and Mikhno, 1964. The results of seismic research on the structure of the earth crust and sedimentary mass in the Indian Ocean. Int. Geol. Congr., 22nd Sess., Rep. Soy. Geol., pp.52--61. Owens, D.M., Sanders, H.L. and Hessler, R.R., 1967. Bottom photography as a tool for estimating benthic populations. In: J.B. Hersey (Editor), Deep-Sea Photography. Johns Hopkins, Baltimore, Md., pp.229--234. Pimm, A.C., 1974. Sedimentology and history of the northeastern Indian Ocean from Late Cretaceous to Recent. In: Initial Reports of The Deep-Sea Drilling Project, 22. U.S. Govt. Printing Office, Washington, D.C., pp.717--804. Prince, R.A. and Kulm, L.D., 1975. Crustal rupture and the initiation of imbricate thrusting in the Peru--Chile trench. Geol. Soc. Am. Bull., 86: 1639--1653. Sclater, J.G. and Fisher, R.L., 1974. Evolution of the east central Indian Ocean, with emphasis on the tectonic setting of the Ninetyeast Ridge. Geol. Soc. Am. Bull., 85: 683--702. Sokolova, M.N. and Pasternak, F.A., 1962. Quantitative distribution of Bottom Fauna in the northern part of the Arabian Sea and in the Bay of Bengal. Doklady, 3: 15--18. Sykes, L.R., 1970. Seismicity of the Indian Ocean and a possible nascent Island Arc between Ceylon and Australia. J. Geophys. Res., 75: 5041--5055. Thompson, R.W., 1974. Mineralogy of sands from the Bengal and Nicobar Fan, Sites 218 and 211, eastern Indian Ocean. In: Initial Reports of the Deep-Sea Drilling Project, 22. U.S. Govt. Printing Office, Washington, D.C., pp.711--714.
ACOUSTIC STRATIGRAPHY, STRUCTURE, AND DEPOSITIONAL HISTORY OF THE NICOBAR FAN, EASTERN INDIAN OCEAN
FREDERICK
A. B O W L E S ' , W I L L I A M F. R U D D I M A N
2 and W A L T E R
H. J A H N '
'Naval Oceanographic Laboratory, Naval Ocean Research and Development Activity, Bay St. Louis, Miss. 39529 (U.S.A.) 2 Lamont-Doherty Geological Observatory, Columbia University, Palisades, N.Y. 10960 (U.S.A.)
(Received September 20, 1976;revised and accepted March 10, 1977)
ABSTRACT
Bowles, F.A., Ruddiman, W.F. and Jahn, W.H., 1978. Acoustic stratigraphy, structure, and depositional history of the Nicobar Fan, eastern Indian Ocean. Mar. Geol., 26: 269--288. The Nicobar Fan is a topographically isolated segment of the Bengal Deep-Sea Fan. Seismic reflection data indicate that the Nicobar Fan may have begun forming in late Miocene to early Pliocene time, but considerably later than the Bengal Fan. Since then, the original basement relief has been gradually inundated and buried by fan sediments, although numerous basement peaks still pierce the surface of the fan. Sediment thicknesses in excess of 1.6 km occur in the northern part of the fan. The fan surface is marked by turbiditycurrent channels and numerous faults which, in some cases, may have acted as channels. The faults are apparently related to tensional stresses associated with a descending lithosphere and to intraplate seismic activity. Relatively few channels are encountered on the fan, and they are noticeably absent on the northern part. Most of the sediment filling the northern Java Trench consists of depressed fan sediments, although deposition from local sources is evident. Plate motion has gradually isolated the fan from its major source of sediment to the extent that a buried "fossil" fan surface exists in the southeastern portion of the survey area, and much, if not all, of the remaining Nicobar Fan is now receiving only pelagic sediments.
INTRODUCTION T h e f l o o r o f the B a y o f Bengal and a d j a c e n t s o u t h e r n area is o c c u p i e d by a massive c o n e o f intensely stratified s e d i m e n t s called the Bengal Deep-Sea F a n ( C u r r a y and M o o r e , 1 9 7 1 ) , a n d also called the Ganges C o n e (Ewing et al., 1 9 6 9 ) . The eastern p o r t i o n o f this fan b o r d e r i n g the Java T r e n c h is isolated f r o m the m a j o r b o d y o f s e d i m e n t s b y N i n e t y e a s t Ridge, f o r m i n g a smaller sub-fan called the N i c o b a r F a n (Curray and M o o r e , 1971). N o r t h o f a p p r o x i m a t e l y 2 ° N , t h e sediments o f the N i c o b a r F a n fill a n a r r o w gap, 1 0 0 - - 1 5 0 k m wide, b e t w e e n N i n e t y e a s t Ridge and the l a n d w a r d wall o f the Java T r e n c h ( F i g . l ) . T o the s o u t h , the fan b r o a d e n s c o n s i d e r a b l y ,
270 !ffi
5~ S
90'E
95 °
100 °
Fig. 1. Ship's tracks superimposed upon bathymetric map by Curray and Moore (1971 ). Locations of seismic reflection profiles (Figs.4, 5, and 7 ) and 3.5-kHz echogram profiles (Figs.8 and 9) are indicated by heavy track lines and indexed by letters. Piston cores were taken at stations 11, 13, and 18; bottom photographs were taken at all stations; and short "punch cores" were taken at all stations except 5, 7, and 9. Probable turbidity-current channels are indicated by the dotted lines. e x t e n d i n g s o u t h w a r d t o a b o u t 10°S (Ewing et al., 1 9 6 9 ) , w i t h a l o n g i t u d i n a l g r a d i e n t o f a p p r o x i m a t e l y 1 : 1 4 5 0 . A l t h o u g h N i n e t y e a s t Ridge a n d t h e t r e n c h diverge t o w a r d the s o u t h , the fan is w i d e s t ( a b o u t 9 2 0 k m ) at a b o u t 2 ° S. As later s h o w n , rugged t o p o g r a p h y still u n b u r i e d b y fan s e d i m e n t s o c c u p i e s a large p o r t i o n o f t h e sea f l o o r s o u t h o f 2 ° S. B o t h t h e N i c o b a r a n d Bengal F a n s are c o m p o s e d p r i m a r i l y o f erosional debris carried f r o m the H i m a l a y a n M o u n t a i n s t o t h e Bay of Bengal b y t h e G a n g e s - - B r a h m a p u t r a River s y s t e m s . T h e c o m b i n e d flow o f b o t h rivers delivers a p p r o x i m a t e l y o n e billion t o n s of s e d i m e n t a n n u a l l y ( m o s t l y silts a n d
271 sands) to the Bay of Bengal, where it is distributed southward by turbidity currents (Coleman, 1969}. Ewing et al. (1969) have shown that the unconsolidated sediments in the Bay of Bengal exceed 2.5 sec in thickness. This is in agreement with earlier work by Soviet investigators (Neprochnov et al., 1964), who reported maximum thicknesses of 3.0 km. More recently, however, Curray and Moore (1971) and Moore et al. (1974} have shown that well over 3 km of sediment exist over most of the Bengal Fan, and that beneath the northern shelf, the total accumulation of sediments (in the Bengal geosyncline) probably exceeds 12 km. Sediment thicknesses up to 1 km are indicated for the Nicobar Fan (Ewing et al., 1969; Curray and Moore, 1971). Little information has been published about the Nicobar Fan. In this paper, we use geophysical data, sediment cores, and b o t t o m photographs to delineate the structural, morphological, depositional, and historical character of the Nicobar Fan. METHODS In October and November 1971, the USNS "Bartlett" conducted a geological and geophysical survey of the Nicobar Fan. Magnetic, 3.5-kHz, and seismic reflection data were continuously collected along survey lines shown in Fig.1. Reflection profiles were obtained with a 30,000-J "sparker" sound source fired at 10-sec intervals. In addition, sediment cores and/or b o t t o m photographs were collected at 14 stations on the fan. Normal survey speed was a b o u t 8 knots, and the ship's position was determined by satellite navigation. Additional seismic reflection data were provided by " V e m a " cruise 29 and "Conrad" cruise 9. SEDIMENT THICKNESS AND DISTRIBUTION The accumulation of sediment in the survey area is shown in Fig.2 as isopachs of sediment thickness above acoustic basement in seconds of two-way travel time. Acoustic basement is defined as the deepest reflector in the seismic profiles. A major portion of the area surveyed is covered by thicknesses of 1.0 sec or greater, with thickest sediments toward the north. Acoustic basement is not visible in any of the northern profiles except adjacent to Ninetyeast Ridge where the deepest acoustic penetration observed is 1.6 sec. A consistent, b u t usually weak, basement reflection is generally visible beneath the thinner sediments south of the equator where sediment accumulations of 1.0 sec are more limited. Except for the thick sediment fill in the Java Trench, accumulations greater than 1.0 sec occur mostly to the west of 95°E, marking roughly north--south-trending troughs in the acoustic basement. In contrast, the region east of 95°E is characterized by an unusually thin sediment cover, usually less than 0.3 sec.
272
90 o
95 °
P
100 ~
tl i
..... il
"l OO°E
95"
100 o
Fig.2. Isopach map of Nicobar Fan. Contours are in tenths of seconds of two-way travel time (0.1 sec approximates 100 m). Sediment thickness was read every 15 minutes, or about every 2 kin. Stippled areas indicate basement outcrops. For practical reasons, the contours along the landward (east} wall of the trench have been omitted.
Present fan sediments T h r o u g h o u t m o s t of the area surveyed, acoustic b a s e m e n t is buried under a highly stratified, flat-lying s e q u e n c e o f turbidite layers. The strong, reverberant reflections from these layers delineate the sediments of the N i c o b a r Fan; on this basis, the area presently o c c u p i e d b y fan s e d i m e n t s is s h o w n in Fig.3. In profiles crossing the trench north of the equator, the parallel reflectors o f the fan dip toward, and can be traced c o n t i n u o u s l y into, the
273 90"
95°
100°
5"
O"
5°
9OOE,
Fig.3. Sediment distribution map. show approximate dispersal paths represent outcrops and/or regions tracklines indicate the location of
95 °
leo o
Shaded area indicates Nicobar Fan turbidites; arrows as indicated by present gradients. Unshaded areas of pelagic sedimentation. Short "tick" marks on the turbidity-current channels.
trench (Fig.4, profiles A and B). Thus, the fan deposits have clearly been carried downward, and are n o w part of the trench. Assuming that these layers were once flat-lying, they have been depressed by as much as 1.0 sec in relation to adjacent horizontal portions of the fan. South of the equator, outcrops and high-standing topography prevent the fan sediments from being traced directly into the trench (Fig.4, profiles C and D), although the fan deposits can be recognized as part of the trench fill to about 5 ° 30'S (Fig.3).
274
p
5 SEC
B
C
D
~ KM
'
Fig.4. Seismic profiles A, B, C, and D showing transverse crossings of fan and trench.
Beyond this point, the sediments in the trench are apparently younger than the fan deposits. Obviously younger sediment fill can be identified in profiles north of 5 ° 30'S where the fan deposits can be seen dipping beneath a wedge of younger trench sediments (Fig.4, profile D). The fact that the surface and sub-bottom layering within the wedge slope upward toward the trench wall indicates local downslope transport of sediment from the adjacent island arc. A similar sediment wedge is not obvious in profiles north of a b o u t 2°N where the trench fill appears to consist entirely of depressed fan sediments. Thus, transport of sediment down the trench wall is apparently less important along this portion of the trench. This floor of the trench does, however, have a tendency to be relatively flat, suggesting possible axial transport and deposition by turbidity currents from the north. This, in turn, could obscure the effects of whatever local downslope transport might occur.
275
As shown by Fig.3 (and Fig.2), the continuity of the fan is broken in numerous places where the underlying basement (volcanic) topography protrudes above the fan surface. In particular, the region of thin (0.3 sec) sediment cover east of 95°E (Fig.2) is most conspicuous because of the absence of fan sediments (shown in Fig.3 by absence of shading). The area represents a broad, tectonically complex portion of basement largely covered by pelagic sediments. The contrast between the rugged topography and the topographically flat surface of the fan is shown in Fig.5, profile E.
Fig.5. Seismic profile E across Nicobar Fan and adjacent area of high, rugged topography. The fan sediments begin on the left and end at the center of the profile; to the right of center, the sediments are dominantly pelagic.
Unfortunately, the general fabric in the area cannot be determined because of the broad spacing of the survey lines. Sclater and Fisher (1974), however, show ridges and intervening basins trending north-northeast--south-southwest. As shown by profile E, ponding of the fan sediments behind outcrops (ridges) has caused the fan surface to build considerably higher than the sheltered basin floors. The high topography has apparently prevented the spread of fan sediments to the south and east into this area. A few of the trench profiles show evidence of possible intrusive tectonic activity or folding in the axial portion of the trench (Fig.4, profiles Band C). Some of this activity is obviously quite recent as in profile B where the fan turbidites are shown uplifted into a prominent anticlinal structure. The structures in the trench may represent local, isolated intrusions or crustal material of perhaps short discontinuous ridges (since they do not appear in adjacent profiles). Axial ridges have been observed in other trenches, notably, the Peru--Chile Trench where they are thought to form by imbricate thrusting (Prince and Kulm, 1975). There seems to be no evidence of imbricate thrusting in our trench profiles where basement is visible. Such thrusting, however, may well not apply to this segment of the Java Trench where there is major strike-slip motion between the plates (Fitch, 1972). The contacts between sediments and rock outcrops can be indicative of their relative ages. In all cases where the fan sediments terminate abruptly against the outcrops, the reflectors show little or no deformation; in other places, they have gradually encroached over more gently rising basement
276 with the fan reflectors sometimes overlapping more transparent (pelagic} sediments. Aside from some broad, gentle upwarping of deeper reflectors, there are relatively few instances where the horizontal layering of the fan has undergone major deformation as a result of basement material having been thrust upward through the sediments. Thus, most of the major underlying and exposed topographic features were in existence before the fan sediments were deposited.
Relict fan sediments Ewing et al. (1969) have placed the southern limit of the Nicobar Fan at approximately 10°S. Drilling at sites 211 (09°46.53'S, 102°41,95'E) and 213 (10 ° 12.71'S, 93 ° 53.77'E) by the Deep-Sea Drilling Project (DSDP) has confirmed this boundary. No turbidites, for example, were encountered at site 213, southward penetration of the fan sediments having been prevented by a large ridge north of the site. Farther east at site 211, however, a 200-mthick unit of pelagic ooze and interbedded terrigenous sands and silts was cored. Overlying this unit was approximately 100 m of siliceous ooze of late Pliocene and Quaternary age. Thompson (1974} has shown that the sands and silts are mineralogically similar to turbidite deposits of the Bengal Fan. The occurrence of buried turbidites at site 211 (Fig.6) is of particular interest because of the isolated location of the site with respect to the present distribution of fan sediment (Fig.3}. Reflection profiles along the track lines shown in Fig.6 indicate no evidence of fan sediments ever having penetrated from the west into the region of site 211. Evidence suggests, instead, that the fan sediments reached the site by a more northerly route when the broad, rugged region of pelagic deposition (Fig.3) was located farther west than at present. At that time, a narrow, b u t flat-lying, segment of fan may have existed between the pelagic region and the trench, reaching as far south as site 211. Evidence of a turbidite surface buried beneath a later pelagic cover is indicated in the seismic records by a reflective layer beneath transparent sediments around site 211. The overlying transparent cover varies in thickness from being barely perceptible to a b o u t 0.2 sec. The layer is most prominently displayed in profiles immediately to the northeast (Fig.7, profile H) of site 211 and north (Fig.7, profile F) at a b o u t 7°S, where, in b o t h cases, it appears as a strikingly flat, reflective surface. Elsewhere, the layer appears intermittently (Fig.7, profile G}, suggesting an interfingering of turbidites within the topography east and south of site 211. In these areas, the layer is generally less reflective and also somewhat distorted by crustal movement. The last major orogeny in the sediment source area which resulted in the modern Himalayas, was latest Pliocene and Pleistocene in age, and correlates with the age of turbidites drilled at site 211 (Curray and Moore, 1971; Moore et al., 1974}. Although the energy of the transporting turbidity
277 5°
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F i g . 6 . M a p s h o w i n g d i s t r i b u t i o n o f b u r i e d t u r b i d i t e s u r f a c e ( s t i p p l e d a r e a ) . S h a d e d a r e a in n o r t h i n d i c a t e s d e p r e s s e d f a n s e d i m e n t s in t r e n c h . H e a v y t r a c k l i n e s i n d e x e d b y l e t t e r s r e p r e s e n t s e i s m i c p r o f i l e s s h o w n in F i g . 7 . S o l i d c i r c l e s h o w s l o c a t i o n o f D S D P s i t e 2 1 1 .
currents was probably intense at this time {Pimm, 1974), the well-defined nature of the single buried reflector in Fig.7 suggests that the episode of turbidite deposition for this distal portion of the fan was brief, with isolation of site 211 from further turbidite deposition occurring relatively soon after the turbidites began reaching the area. It has been suggested that the entire Nicobar Fan may have been isolated from its source of supply either by the convergence of the northern end of Ninetyeast Ridge against the Indonesian Trench system, or the formation of a pre-trench rise in the northern area associated with the buckling of the lithosphere as it descended into the trench (Pimm, 1974). Isolation of the entire fan at this time (Pliocene) seems unlikely, however, since reflection
278
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RE~ZLECTOJ{
F
Fig.7. Seismic profiles F, G, and H showing buried turbidite surface. Profile F is also indexed in Fig.1.
profiles across the present fan reveal only the slightest hint of a surface layer of pelagic sediment. Alternatively, isolation of site 211 may have occurred while turbidite deposition continued over the major portion of the fan. As shown by Fig.6, a buried turbidite surface is not visible in the profiles north of a b o u t 7 ° S. Thus, a portion of the route inferred to have been traveled by the turbidity currents has already been subducted. As plate motion carried the high-standing pelagic area toward the trench, early buckling of the lithosphere b e t w e e n the trench and pelagic area could eventually have confined the flow of turbidity currents in this region entirely to the trench axis, thereby isolating the area of site 211 from its sediment supply b u t not the remainder of the fan. STRUCTURE AND AGE OF FORMATION
Seismic reflection and refraction data across the Bengal Fan show that it consists of three major stratigraphic units, each separated at their boundaries by pronounced seismic velocity discontinuities. Cores from the Deep-Sea Drilling Project (Moore et al., 1974} have shown that the sediments below the " l o w e r " discontinuity (which represents acoustic basement) are largely of pelagic origin. The overlying, intermediate stratigraphic unit is middle Eocene to upper Miocene in age, and represents the earliest fan turbidites. These locally deformed turbidites are, in turn, overlain by another sequence of relatively u n d e f o r m e d turbidites representing the t o p m o s t stratigraphic unit and the m o d e m Bengal Fan. The boundary b e t w e e n the early and m o d e m turbidites is marked by a second, or " u p p e r " , discontinuity. Profiles of the Bengal Fan show that acoustic basement (i.e., lower discontinuity) rises as it approaches Ninetyeast Ridge and appears to be continuous with the basement reflector on the ridge (fig.3d in Curray and Moore, 1971). An analogous situation is observed on the Nicobar Fan
279 (Fig.4, profile A), where acoustic basement also appears to coincide with a ' reflector beneath the pelagic sediments on Ninetyeast Ridge. Despite the similarity, it is our opinion that although acoustic basement under the Nicobar Fan may be lithologically similar to that under the Bengal Fan (i.e., boundary between pelagic and turbidite deposits), it is coincident in time with the upper and not the lower discontinuity of the Bengal Fan. This interpretation is consistent with Curray and Moore (1975), who maintain that the Nicobar Fan developed later than the Bengal Fan. The major piece of evidence leading to our interpretation is the apparent absence of an upper discontinuity in the Nicobar Fan profiles. As noted, the upper discontinuity divides the Bengal Fan turbidites into two distinct stratigraphic units categorized as "W" (modern fan) and "Y" (early fan) by Curray and Moore (1971). South of about 11°N, seismic profiles show the discontinuity to be approximately 0.75--0.50 sec, or less beneath the fan surface. Assuming a similar depositional history for the Nicobar Fan, we would expect a similar discontinuity to appear in our profiles (spanning roughly the same latitudes) at approximately the same depth of burial. No discontinuity is ever observed above acoustic basement, indicating, instead, a significant difference in the depositional history of the Nicobar Fan. The "Y" turbidites of the Bengal Fan (bounded by the lower and upper discontinuities) are approximately early Eocene to late Miocene in age. The lack of the upper discontinuity in the Nicobar Fan sediments indicates that these early fan deposits are missing on the east side of Ninetyeast Ridge. Curray and Moore (1975) suggest that prior to the Oligocene, the region east of Ninetyeast Ridge was sealed off by an active spreading ridge. Thus, early fan deposition took place to the north and west of Ninetyeast Ridge until the spreading ridge became extinct in the early Oligocene. At that time, sediments of the Bengal Fan began to extend east of Ninetyeast Ridge, forming the Nicobar Fan. Deposition of Nicobar Fan sediments during Oligocene through middle Miocene time would, however, constitute "Y" sediments (i.e., pre-upper discontinuity). The absence of these sediments in our profiles suggests two possibilities: (1) that the spreading ridge remained a major barrier to the dispersal of fan sediments east of Ninetyeast Ridge long after its extinction, or (2) that although turbidites began breaching the spreading ridge shortly after its extinction, deposition of these early fan sediments did not achieve significant proportions (i.e., they did not extend very far south or become very thick). In the latter case, Curray and Moore {1975) note that during the Oligocene and early Miocene, the Himalayas apparently were not yet high enough to provide much detritus. Moreover, because of plate motion, most of the deposited "Y" sediments would have probably long since disappeared into the Java Trench. Curray and Moore (1971) correlate the upper discontinuity in their seismic profiles with renewed turbidity-current activity on the Bengal Fan resulting from major uplift of the Himalayan Mountains in the middle-to-late Miocene.
280
Results from the Deep-Sea Drilling Project (Moore et al., 1974) show that this period is indeed represented in the sediments by increased deposition of terrigenous sands and silts. At this time, then, both fans began receiving large volumes of terrigenous sediment. The greater sediment thickness on the Nicobar Fan (relative to the thickness of the "W" sediments on the Bengal Fan) indicates that most of this sediment was channeled down the east side of Ninetyeast Ridge. This is also supported by the fact that the minimum Pliocene sedimentation rate at DSDP site 211 is nearly double the m a x i m u m rate at site 218 located on the Bengal Fan (Pimm, 1974). Thus, formation, or at least major development, of the Nicobar Fan appears to have started during late Miocene time. FAULTING
Bathymetric profiles collected along the survey lines shown in Fig.1 show that the surface layers of the fan are locally offset by faulting. Unfortunately, the survey lines are not close enough to determine the strike of the faults. In most cases, the offsets appear to be faulted folds, although the folding may be a secondary drag effect. Usually, several faults occur together, sometimes forming graben and step-like arrangements (Fig.8, profiles I and J). Faulting similar to this has been observed on the seaward slopes of other deep-sea trenches such as the Japan Trench (Ludwig et al., 1966). Although the vertical displacements along the faults are minor (the largest measuring 98 m), the slopes of the fault surfaces are often too steep to be resolved by the profiler. The displacements do not all show the same general attitude, that is, the trench side of the fracture is not usually downfaulted as in Fig.8, profiles I and J. An a t t e m p t to map the faults produced no discernible pattern in either their attitude or frequency of occurrence (relative to distance from the trench). None of the profiles show any indication of
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Fig.8. E c h o g r a m profiles I and J at 3.5 kHz showing surface expression o f faults o n the Nicobar Fan. Trench is to the right in both profiles.
281 depositional smoothing of the faults, thus, they all have the appearance ' of having been recently formed. The larger offsets and step faults generally occur near the trench, usually where the fan sediments slope toward the trench. Often, the faulting carries into the trench b o t t o m itself, although a few profiles show relatively little disturbance in the trench. Across the central and western portions of the fan, gentle warps or undulations of the fan surface are more c o m m o n ; the faults that occur here are usually small (similar to that shown at extreme left in profile J). Some of the larger faults can be recognized in the seismic profiles as offsets in deeper layers of the fan and also by the occurrence of a dark, nearly vertical line, suggesting acoustic focusing associated with offsets in reflectors along a steeply dipping fault plane (Fig.4, profiles A and B). Although the offsets are not always obvious, a comparison of corresponding seismic and bathymetric profiles shows that these vertical traces occur directly beneath most of the surface faults. In some cases, the offsets appear to increase with depth, indicating that once formed, the faults remain active (thus explaining the fresh appearance of the faults at the surface). The faulting on the fan, particularly the graben and step-like faults, suggests tensional stresses. Such stresses are predicted across the top (convex) side of the lithosphere where it bends downward as it passes beneath the trench (Isacks et al., 1968). The faulting that occurs some distance from the trench, then, presumably cannot be explained by such local stresses. Zones of folded and faulted sediments have been observed, however, near the southern margin of the Bengal Fan where they are thought to relate to a wide belt of seismicity stretching between Ceylon and the Cocos Islands (Eittreim and Ewing, 1972). Earthquake mechanism solutions indicate strike-slip motion for this region (Sykes, 1970). This intra-plate seismicity may also account for much of the faulting across the Nicobar Fan. CHANNELS Although 22 potential channel crossings are indicated in Fig.l, only seven channel crossings actually occurred along the entire ship's track. Some of these are probably crossings of the same channel, however, the broad spacing of the track does not warrant connecting them in most cases. Echogram profiles show that the channel dimensions range from approximately 11 to 70 m in depth to several kilometers in width; most have small, natural levees and/or terraced sides (Fig.9, profile K). The channels have a very "fresh" appearance, showing no signs of being filled in with sediment. In a few cases, the channel b o t t o m is highly reflective, indicating deposits of coarse-grained sediment. All the crossings are of single channels, with one exception (Fig.9, profile L), where a tri-channel system suggesting braiding was crossed.
282
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Fig.9. E c h o g r a m profiles K and L at 3.5 kHz showing channel crossings.
At least five other possible channels were crossed, but these show none of the channel characteristics mentioned above. Morphologically, they resemble some of the small distributary channels on the Amazon cone shown by Damuth and Kumar (1975). The chief difficulty in recognizing these "channels" is distinguishing them from the many small faults which mark the surface of the fan. This, in turn, raises the possibility that some faults have proxied as channels at some time. It is d o u b t f u l that increasing the survey coverage on the fan would reveal the existence of a complex network of channels. Curray and Moore (1975) note that turbidity currents on the medial, and especially distal, portions of the Bengal Fan tend to be broad-fronted, moving as sheet-flow between channels rather than within specific channels. Those channels which pass through these fan areas are small and widely spaced. The entire Nicobar Fan is largely distal with respect to its primary source of sediment to the north. Most interesting, however, is the apparent absence of channels passing down the narrow, northern portion of the fan. Of the five tracklines crossing this region (Fig.3), a single channel was encountered on the southernmost crossing, but there was no sign of its continuation to the north. Similarly, an absence of channels in the northern portion of the fan has also been indicated by Curray and Moore (1975, Fig.l). In addition, they also indicate the appearance of channels roughly where we encountered our northernmost channel. Although present-day sediments passing east of Ninetyeast Ridge are thought to be transported down the axis of the trench (Curray and Moore, 1975), one could expect some abandoned channels to still exist on this part of the fan. Convergence of Ninetyeast Ridge with the Java Trench has most likely carried once-existing channels into the trench, but this does not explain the fact that our channel cannot be traced as far north as even the
283 next profile. Convergence of the ridge and trench also raises the possibility that deformation of the fan may have obscured the channels. Seismic and 3.5-kHz profiles show, however, that deformation of the fan sediments is no more severe here than on some areas of the fan where channels exist. Thus, neither the absence of abandoned channels in the narrow, northern portion of the fan nor the abruptness with which channels appear to the south can be fully accounted for at this time. RECENT DEPOSITION B o t t o m photographs and sediment cores taken at stations along the survey track provide primary evidence relating to the present environment of deposition on the fan. The photographs (229) of the fan are m o n o t o n o u s l y similar in appearance, nearly all conveying the impression of a flat, soft b o t t o m {Fig.10A). Stations 10 and 12 occur on topographic highs and show a b o t t o m strewn with angular rock fragments and/or large, rounded {pillowed?) outcrops (Figs.10B and 10C). Without exception, the photographs show no sign of the sedimentary microrelief associated with b o t t o m current activity. The implication, then, is that the environment at all stations is quiescent. This is also supported, in part, by the deposition of pelagic sediment on rock outcrops {Fig.10C) and by the presence of well-defined lebensspuren (Figs.10D and 10E). The lebensspuren are of particular interest. Moderate to abundant lebensspuren and fecal debris are displayed in the photographs, suggesting a large animal population. Turbidity currents are thought to affect abyssal life in t w o ways: (1) obliterating life by rapid burial of organisms under thick sediment layers, and {2) enhancing life b y delivering nutrient-rich organic material to great depths {Heezen et al., 1955). Studies of the Congo River (Heezen et al., 1964) and Cascadia Channel (Griggs et al., 1969) seem to substantiate the latter effect. Studies in the Bay of Bengal, however, show a very weak development of the b o t t o m population on the Bengal Fan {Sokolova and Pasternak, 1962). Similarly, a very sparse benthos is observed in the photographs of the Nicobar Fan despite the abundance of lebensspuren. The general scarcity of epifauna suggests a nutrient deficit, perhaps caused by the absence of b o t t o m currents which tend to stir up and replenish available nutrients, or more significantly, an absence of nutrientdelivering turbidity currents. Owens et al. {1967) have noted that in areas of low sedimentary deposition, animal signs may be preserved, hence, b o t t o m photographs may show the sum o f animal activity over a very long period of time. Since, in. general, lebensspuren on the fan are plentiful while biota is sparse, many of the lebensspuren may be quite old. Close inspection of the b o t t o m photographs shows that some lebensspuren are well-defined, while others have a smoothed appearance. Because such selective smoothing by b o t t o m currents is unlikely, it is reasonable to conclude that many of the lebensspuren are old and have been gradually s m o o t h e d b y slow pelagic deposition.
284
Fig.10. Bottom photographs on Nicobar Fan and on outcrop areas. The tilt of the camera in photograph A has produced the false impression of a slight preferred orientation to the microrelief.
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t'O
286 With one exception, sediment cores taken at several stations confirmed the existence of a surface veneer of pelagic sediment covering the Nicobar Fan. Piston cores were taken at stations 11, 13, and 18 (Fig.l). Although layers of terrigenous micaceous, quartz sands were cored at these places, the top 284 cm of sediment at station 11 and the 155 cm at station 13 consisted of olive-gray and yellowish-brown lutite, respectively. A white (silicic) volcanic ash layer also occurred near the surface at station 13. Short (62 cm or less} " p u n c h cores" were taken simultaneously with the camera drops at stations 13 through 18, 22, and 23. Although such cores are unreliable for stratigraphic studies because they represent repeated penetrations of the sediment, t h e y are indicative of the type of sediment at the surface of the fan. Significantly, none of the cores contained any terrigenous sand or silt layers. Station 18 provided the single exception to the above observations. The top 4 cm of the core taken here consisted of quartz sand, indicating recent turbidity-current deposition, at least in the northern portion of the fan. It is likely, however, that this sand does not represent the true surface. A camera punch core containing all olive-gray lutite was also taken at station 18. Thus, it appears that pelagic sediment overlying the sand layer was not recovered in the piston core. Curray and Moore (1975} suggest that the Nicobar Fan was cut o f f from its sediment source in middle Pleistocene time by the convergence of Ninetyeast Ridge with the Java Trench. Unfortunately, we cannot pinpoint the ages of the topmost turbidites in our fan cores. The evidence, however, clearly indicates that turbidity currents have been largely inactive for some time, and that present deposition is now primarily pelagic. CONCLUSIONS {1) Seismic reflection profiles of the Nicobar Fan north of the equator show sediment thickness consistently greater than 1.0 sec. The deepest acoustic penetration observed in this area was 1.6 sec. Sediments thin toward the south where numerous basement peaks still rise above the surface of the fan. A broad, raised portion of rugged sea floor south of 2 ° S bordering the Java Trench remains unburied by the fan turbidites. (2 } The fan reflectors, in general, terminate abruptly against basement outcrops with no apparent sign of deformation, or gradually onlap more gently sloping topographic highs. Thus, the major exposed and buried topographic features pre-date the formation of the Nicobar Fan. (3) Seismic reflection profiles show the sediments in the northern Java Trench to consist predominantly of fan deposits which have been depressed to form the trench. Recent trench deposition is represented by a wedge of horizontal or westward-dipping sediments overlying the older, eastwarddipping fan sediments. (4) The southeastern extremity of the Nicobar Fan between 6 and 10°S is a relict feature. Evidence of buried fan turbidites now cut off from the larger
287
b o d y of fan sediments is indicated in this area by an acoustically reflective horizon beneath transparent sediments. This interpretation is reinforced b y the nearby presence of buried turbidites at DSDP site 211. (5) Unlike the Bengal Fan, seismic reflection profiles do not show the existence of a discontinuity separating the Nicobar Fan turbidites into two distinct stratigraphic units. This absence of a lower or early sequence of turbidites indicates that the formation of the Nicobar Fan began considerably later than the formation of the Bengal Fan. (6) Echogram profiles show that in comparison to the Bengal Fan, relatively few turbidity-current channels are incised into the surface of the Nicobar Fan. For undetermined reasons, no channels are observed on the fan north of about 3 ° N. (7) Echogram profiles show the surface of the fan to be marked with numerous faults which, in seismic profiles, appear to offset subsurface reflectors. Near the trench, the faulting is apparently the result of tensional stresses caused by the downbending of the lithosphere as it passes beneath the trench. Farther o u t on the fan, away from the trench, the faulting may be related to intra-plate seismic activity observed between Ceylon and the Cocos Islands. (8) Convergence of Ninetyeast Ridge with the landward wall of the Java Trench has virtually isolated the Nicobar Fan from its northern source of sediment. B o t t o m photographic evidence and sediment cores indicate that present deposition over the fan is primarily pelagic. ACKNOWLEDGEMENTS
We thank the officers, crew, and other scientific party of the USNS "Bartlett" for their assistance in the collection of this data; H. Eppert and T. Holcombe for critical reviews; P. Michalko and B. Grosvenor for illustrations; G. Garner and S. Madosik for data preparation; and L. Thigpen for typing.
REFERENCES Coleman, J.M., 1969. Brahmaputra River: channel processes and sedimentation. Sediment. Geol., 3: 129--239. Curray, J.R. and Moore, D.G., 1971. Growth of the Bengal Deep-Sea Fan and denudation in the Himalayas. Geol. Soc. Am. Bull., 82: 563--572. Curray, J.R. and Moore, D.G., 1975. Sedimentary and tectonic processes in the Bengal Deep-Sea fan and geosyncline. In: C.A. Burke and C.L. Drake (Editors), The Geology of Continental Margins. Springer, New York, N.Y., pp.617--627. Damuth, J.E. and Kumar, N., 1975. Amazon Cone: morphology, sediments, age, and growth pattern. Geol. Soc. Am. Bull., 86: 863--878. Eittreim, S.L. and Ewing, J., 1972. Mid-plate tectonics in the Indian Ocean. J. Geophys. Res., 77: 6413--6421. Ewing, M., Eittreim, S., Truchan, M. and Ewing, J.I., 1969. Sediment distribution in the Indian Ocean. Deep-Sea Res., 16: 231--248.
288 Fitch, T.J., 1972. Plate convergence, transcurrent faults, and internal deformation adjacent to Southeast Asia and the western Pacific. J. Geophys. Res., 77 : 4432--4460. Griggs, G.B., Carey, A.G. and Kulm, L.K., 1969. Deep-sea sedimentation and sediment-fauna interaction in Cascadia Channel and on the Cascadia Abyssal Plain. Deep-Sea Res., 16: 157--170. Heezen, B.C., Ewing, M. and Menzies, R.J., 1955. The influence of submarine turbidity currents on abyssal productivity. Oikos, 6: 170--182. Heezen, B.C., Menzies, R.J., Schneider, E.D., Ewing, W.M. and Granelli, N.C.L., 1964. Congo Submarine Canyon. Bull. Am. Assoc. Pet. Geol., 48: 1126--1149. Isacks, B., Oliver, J. and Sykes, L.R., 1968. Seismology and the new global tectonics. J. Geophys. Res., 73: 5855--5899. Ludwig, W.J., Ewing, J.I., Ewing, M., Murauchi, S., Den, N., Asano, S., Hotto, H., Hayakawa, M., Asanuma, T., Ichikawa, K. and Noguchi, I., 1966. Sediments and structure of the Japan Trench. J. Geophys. Res., 71: 2121--2137. Moore, D.G., Curray, J.R., Raitt, R.W. and Emmel, F.J., 1974. Stratigraphic--seismic section correlations and implications to Bengal fan history. In: Initial Reports of the Deep-Sea Drilling Project, 22. U.S. Gov. Printing Office, Washington, D.C., pp.403-412. Neprochnov, I.P., Korylin, V.M. and Mikhno, 1964. The results of seismic research on the structure of the earth crust and sedimentary mass in the Indian Ocean. Int. Geol. Congr., 22nd Sess., Rep. Soy. Geol., pp.52--61. Owens, D.M., Sanders, H.L. and Hessler, R.R., 1967. Bottom photography as a tool for estimating benthic populations. In: J.B. Hersey (Editor), Deep-Sea Photography. Johns Hopkins, Baltimore, Md., pp.229--234. Pimm, A.C., 1974. Sedimentology and history of the northeastern Indian Ocean from Late Cretaceous to Recent. In: Initial Reports of The Deep-Sea Drilling Project, 22. U.S. Govt. Printing Office, Washington, D.C., pp.717--804. Prince, R.A. and Kulm, L.D., 1975. Crustal rupture and the initiation of imbricate thrusting in the Peru--Chile trench. Geol. Soc. Am. Bull., 86: 1639--1653. Sclater, J.G. and Fisher, R.L., 1974. Evolution of the east central Indian Ocean, with emphasis on the tectonic setting of the Ninetyeast Ridge. Geol. Soc. Am. Bull., 85: 683--702. Sokolova, M.N. and Pasternak, F.A., 1962. Quantitative distribution of Bottom Fauna in the northern part of the Arabian Sea and in the Bay of Bengal. Doklady, 3: 15--18. Sykes, L.R., 1970. Seismicity of the Indian Ocean and a possible nascent Island Arc between Ceylon and Australia. J. Geophys. Res., 75: 5041--5055. Thompson, R.W., 1974. Mineralogy of sands from the Bengal and Nicobar Fan, Sites 218 and 211, eastern Indian Ocean. In: Initial Reports of the Deep-Sea Drilling Project, 22. U.S. Govt. Printing Office, Washington, D.C., pp.711--714.