Seismic-stratigraphic framework of the forearc basin off central Sumatra, Sunda Arc

Earth and Planeta~ Science Letters, 54 (1981) 17-28 Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands

17

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Seismic-stratigraphic framework of the forearc basin off central Sumatra, Sunda Arc Desiree Beaudry and Gregory F. Moore Geological Research Division, A -015, Scripps Institution of Oceanography La Jollu, CA 92093 (U.S.A.) Received August 18, 1980 Revised version received February 25, 1981

New multichannel seismic reflection data provide information on the stratigraphic framework and geologic history of the forearc basin west of central Sumatra. We recognize six seismic-stratigraphic sequences that reflect the Cenozoic history and development of the outer continental shelf and forearc basin southeast of Nias Island. These sequences indicate several episodes of uplift of the subduction complex and filling of the forearc basin. Early in the development of this margin, Paleogene slope deposits prograded onto the adjacent basin floor. Onlapping this assemblage are two units interpreted as younger Paleogene(?) trough deposits. Uplift associated with rejuvenation of subduction in the late Oligocene led to erosion of the Sumatra shelf and formation of a regional unconformity. The early Miocene was a period of significant progradation. A Miocene limestone unit partly downlaps and partly onlaps the older Paleogene deposits. It is characterized by shallow shelf and oblique progradational facies passing into basin floor facies. A buried reef zone occurs near the shelf edge. The cutting of an erosional unconformity on the shelf and slope in late Miocene/early Pliocene time culminated this episode of deposition. In the late Pliocene, a large flexure developed at the western boundary of the basin, displacing the outer-arc ridge upward relative to the basin. Over 1 km of Pliocene to Recent sediment was deposited as a wedge in the deep western portion of the basin landward of the outer-arc ridge. These deposits are characterized by flat-lying, high-amplitude, continuous reflections that overstep the late Miocene unconformity. Up to 800 m of shallow-water limestone have been deposited on the shelf since mid-Pliocene time.

1. Introduction

The stratigraphic evolution of modern forearc basins remains largely speculative. Although the nature of the sedimentary fill of ancient forearc basins can be examined in the field [1-3], forearc basins in active arc systems can only be studied using seismic reflection and drilling data. The single-channel profiles available to the academic community generally are inadequate to decipher the details of the seismic stratigraphy of forearc basins, and only a few industrial multichannel profiles have been released (e.g. [4]). One of the better studied forearc basins is west of central Sumatra, where Karig et al. [5,6] have used industry seismic data to outline the structural framework of the basin. To learn more about the

structural and stratigraphic development of this forearc basin, the Scripps Institution of Oceanography (S.I.O.) collected a multifold common depth point (CDP) seismic reflection profile along a traverse southeast of Nias Island (Fig. 1). In this paper, we present a preliminary interpretation of the seismic stratigraphy and depositional histor3~ of the forearc basin west of central Sumatra. We have used data from an exploration well drilled by Union Oil Company to constrain lithologic and age interpretations. Our seismic reflection profile was collected on board the R.V. "Thomas Washington" in March, 1977 as part of the CCOP/SEATAR Sumatra Transect Program [7]. The source was an array of three 2000-psi airguns totaling 7.54 liters (460 cu. in.), and the data were recorded digitally on high-

0012-821X/81/0000-0000/$02.50 © 1981 Elsevier Scientific Publishing Company

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density tape. Initial processing through demultiplex was performed at S.I.O. Final processing was done commercially and consisted of deconvolution, velocity analysis (average of 1 every 2 km), normal moveout (NMO), stack, time-variant filter (8-80 Hz to 8-45 Hz), scale and display. The CDP stacking appears to have eliminated most

multiples although some internal multiples may still obscure deep reflectors. Interval velocities calculated from 21-channel CDP velocity data and from seismic refraction data [8] were used to construct the depth section. For additional details of the S.I.O. multichannel seismic system, see Moore and Curray [9].

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2. Regional framework

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Lithology

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The Sunda Arc of central Sumatra is a continental margin arc-trench system with a ridged forearc [10]. The Sunda Arc has been the site of continuous subduction since at least the late Oligocene [5,11,12], and there is evidence that it has formed part of a convergent plate margin from Permian time [13]. Accretion of the thick sediments of the Bengal and Nicobar Fans during the late Miocene and early Pliocene has led to rapid uplift and outbuilding of the outer-arc ridge, creating a wide forearc basin [14]. Sediments derived from Sumatra are trapped in the basin. Seismic profiles and drill-hole information on the basin margins [5,6] demonstrate that the Paleogene continental margin was emergent during the late Oligocene, producing an angular unconformity on the shelf. During the Neogene, the western margin of Sumatra subsided and a thick section of Miocene to Recent strata transgressed the shelf unconformity and buried the paleo-shelf edge [5,6].

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Our reflection profile (Fig. 2) crosses a Union Oil Co. exploration well (Pandjang-1) located on the shelf margin (Fig. 1) at approximately shotpoint number (SPN) 2225. The well log (Fig. 3) provides lithologic and age data on part of the sedimentary section at the shelf edge. The well bottoms at a depth of 2300 m in a hard, pyritic sandstone containing Nummulites of Eocene age [6]. This horizon is overlain by lower Miocene strata, indicating the existence of a hiatus. The hiatus corresponds to a regional erosional unconformity on the shelf that has been interpreted by Karig et al. [5,6] as being late Oligocene in age. Above the unconformity are nearly 1000 m of lower Miocene strata (Fig. 3). The lower part of this section consists of 290 m of marginal marine sandstones, siltstones, and shales. The upper part contains nearly 700 m of shallow-water limestone. This vertical facies change probably represents an early Miocene marine transgression and an increase in carbonate production on the shelf. Another hiatus can be recognized at the

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Miocene/Pliocene boundary in the Pandjang well, where 200 m of mid-Pliocene sandstones, shales, and conglomerates overlie lower Miocene carbonates. Upper Miocene and lower Pliocene strata are absent. Mid-Pliocene clastics are conformably overlain by approximately 800 m of upper Pliocene to Recent coralgal detrital limestone.

4. Seismic stratigraphy To facilitate discussion of the basin stratigraphy and to provide a stratigraphic framework for fur-

21 ther study, we have subdivided the sedimentary section of the forearc basin into six seismic sequences (Fig. 2). The sequences are separated by surfaces which mark recognizable changes in mode or style of deposition. Sequence boundaries are defined according to the criteria outlined by Mitchum et al. [15]. The most common lower boundaries observed are onlap, indicative of fill in

a subsiding trough, and downlap, indicative of progradation and outbuilding of the shelf and slope. The most common upper sequence boundaries are revealed by truncation and thinning of reflectors by erosion and nondeposition. The seismic and geologic criteria used for comparison among the sequences are summarized in Table I. Each sequence is assigned a letter designation for

TABLE I Forearc Basin Seismic Stratigraphy

Seismic Interval

Seismic Facies Unit

Depositional Setting

External Form

Reflection Geometry at Boundaries

Reflection Configuralion

Reflection Character

Inferred Age

Obliqueprogradational

shelf margin and prograded slope

bank

concordant with truncation at top; downlap at base

parallel oblique

moderate to high amplitude

Upper Pliocene to Recent

Shelf

shallow shelf (undaform)

sheet or wedge

concordant at top; gentle onlap or downlap at base

parallel wavy

low continuity; variable amplitude

Onlap fill

basin floor

wedge

concordant with erosional truncation at top; onlap at base

parallel even

high continuity; moderate to high amplitude

Shelf

shallow shelf (undaform)

sheet or wedge

erosional truncation at lop; onlap at base

parallel wavy

low continuity; low to high amplitude

Basin slope and floor

basin floor (fondoform) basin slope (clinoform)

wedge or bank

mounded and truncation at top; concordant or gentle downlap at base

parallel even

high continuity; high amplitude

Sigmoid and oblique progradational

shelf margin and prograded slope

wedge or bank

erosional truncation at top with internal toplap; downlap at base

parallel oblique

low continuity; low amplitude

Shelf

back reef and shelf

sheet or wedge

truncation at top; onlap at base

divergent

low to high amplitude

Shelf margin build-up

shelf edge

pinnacle with velocity pull-up

concordant at top; downlap at base

convex upward

high amplitude

D

Divergent fill

basin floor

trough

concordant at top; onlap at base

parallel divergent

low continuity; variable amplitude

Lower Miocene or older

E

Onlap fill

basin floor

wedge

concordant at top; onlap at base

parallel wavy

low continuity; variable amplitude

Oligocene(?)

F

Slope front fill(?)

base of slope

lens or wedge

concordant or erosional truncation at top; downlap at base

parallel divergent

low continuity; variable amplitude

Eocene or older

A

B

C

Early to Mid-Pliocene

Lower Miocene

22 ease in discussion. The sequences are most easily identified in the deep portion of the basin where sedimentation has been more continuous. Correlation across the shelf edge is difficult due to rapid lateral changes in seismic facies and coalescence of unconformities. Reflection patterns and depositional environments are interpreted from seismic facies analysis as outlined by Mitchum et al. [16] and Sangree and Widmier [17]. Shelf, prograded slope, and onlap-fill seismic facies units commonly characterize the shelf, shelf margin, and basin floor environments, respectively, Seismic facies and sequence boundary determinations were made on large-scale seismic sections at both 3 and 6 times vertical exaggeration. Considerable detail has been lost in the small-scale reproduction shown in Fig. 2. Also, the ambiguity of the reflection data increases with increasing travel time on the seismic record due to loss of energy by attenuation and interference from multiples. Thus, the identification of seismic facies becomes less reliable with depth and age. 4.1. Seismic sequence A

Seismic sequence A is the uppermost sequence in the seismic-stratigraphic framework. It thickens across the shelf edge between SPN 2000 and 2150 where it contains westward-dipping clinoform reflections of variable amplitude and frequency (Fig. 2). These are interpreted as oblique progradational facies forming a talus slope deposit derived from the adjacent carbonate bank (Table 1). Seaward of the shelf break (SPN 1800 to 2000), the clinoform reflections downlap a flat-lying, highamplitude reflection, thus defining the lower sequence boundary. The strong reflection which marks the lower boundary of seismic sequence A can be traced westward across the basin at 0.8 to 1.0 seconds (two-way travel time). Sequence A thins near the center of the basin (SPN 1700). There are some diffraction hyperbolae within this interval which may be produced by small channels on the sea floor or buried erosional surfaces. Between SPN 1200 and 1300 the interval thickens slightly over the rear edge of the accretionary complex that forms the western

margin of the basin. Sediments within this portion of the sequence may be derived in part from the outer-arc ridge exposed at Nias (Fig. 1). On the outher shelf margin between SPN 2150 and 2500, a shallow carbonate platform produces a high-amplitude water bottom reflection. The acoustic character of the underlying reflectors is masked by this highly reflective surface. Shoreward of the margin, wavy, discontinuous, variable amplitude reflections typical of shallow-water (sand-prone) shelf facies characterize sequence A. 4.2. Seismic sequence B

Seismic sequence B is a wedge-shaped unit of onlapping basin fill restricted largely to the trough seaward of the shelf break (Fig. 2). It is characterized by low- and high-amplitude, horizontal, continuous reflections that show strong basal onlap against a prominent erosional unconformity marking the lower sequence boundary. On the basis of internal reflection geometry and external form, this interval is interpreted as a shale-prone, trough -fill facies unit which attained a thickness of 1000 m on the west side of the basin. West of SPN 1300, the strata, are folded and tilted eastward by the flexure forming the seaward margin of the basin. Toward the center of the basin, the reflections thin below the resolution of the data and pass laterally into a zone with numerous diffraction hyperbolae. Seismic sequence B thins eastward across the shelf edge and passes into a shallow-water shelf facies east of SPN 2000 which is mostly conformable with the overlying sequence A. On the shelf, some of the reflections show evidence of thinning and erosional truncation along the upper sequence boundary. The lithology on the shelf (SPN 2225) is coralga! limestone as determined from well data (Fig. 3). However, there is a dramatic facies change seaward of the shelf edge. On the basis of reflection character and depositional setting (Table 1), the lithofacies in the basin is inferred to be a terrigenous turbidite deposit. 4.3. Seismic sequence C

Several seismic facies are recognizable within seismic sequence C (Fig. 2). Oblique prograda-

23 tional facies of variable amplitude and frequency occur on the shelf margin and prograded slope. West of SPN 1600, prograded slope facies merge downdip with a deeper-water basin floor seismic facies unit characterized by high-amplitude, continuous, parallel reflections. The seismic facies units on the shelf are characterized by low- to high-amplitude, wavy reflections. This lateral assemblage of seismic facies units may have formed through progressive lateral development of clinoforms. On the shelf, the strata are truncated updip by a regional unconformity correlated with the late Miocene/early Pliocene hiatus in the Pandjang well. A local pinnacle interpreted as a shelf-margin carbonate build-up with a velocity pull-up occurs at SPN 2225 at about 1.5 seconds (two-way travel time). This facies is correlated with a lower Miocene reefal zone in the well. A low-amplitude zone between SPN 2500 and 2650 may represent more mud-rich back-reef facies landward of the reef zone. These reflectors grade eastward into strong, continuous reflectors more typical of clastic sediments deposited in a marginal marine environment. The top of sequence C is delineated by a strong reflection which appears to be a major unconformity. Reflectors below this unconformity show evidence of erosional truncation, channeling, and mounded topography. Overlying reflectors onlap the unconformity in a landward direction. A broad mound occurs near the top of this sequence between SPN 1600 and 1875. The upper surface of this mound is highly reflective and irregular. The internal reflection pattern is hummocky. It is uncertain whether this is an erosional feature or a submarine fan deposit. 4.4. Seismic sequence D

Seismic sequence D is a basin-restricted interval. The upper sequence boundary is defined by downlap above and minor truncation below the interface. This surface is nearly flat and probably formed a smooth, narrow basin floor. The lower sequence boundary is defined by basal onlap against an irregular surface which is concave upward. The internal reflection geometry is subparal-

lel to wavy. Seismic cycles are fairly continuous within the thicker parts of the interval, but exhibit loss of continuity and amplitude toward the margins of the basin where they thin by internal convergence. The seismic facies of sequence D is interpreted to be a divergent basin fill. The axis of maximum sediment thickness occurs near the center of the basin. The interval pinches out in the east at SPN 1900 just seaward of the shelf edge, and terminates against the accretionary complex to the west. 4. 5. Seismic sequence E

Seismic sequence E is another restricted interval that is 2.5km thick at SPN 1300 where it is apparently truncated by the flexure that forms the rear edge of the subduction complex (Fig. 2). The interval thins abruptly by basal onlap toward the center of the basin and pinches out at SPN 1580 against the westward sloping surface of the underlying sequence. The upper sequence boundary is traced along a reflection which dips eastward relative to the overlying reflections. This slope may be of a depositional nature, although we believe that the western margin of sequence E has been tilted landward by uplift of the outer-arc ridge. On the basis of external form and restricted areal distribution this sequence is interpreted as a thick basin fill confined on the west and slightly deformed by a growing structural high. 4. 6. Seismic sequence F

Seismic sequence F occurs at the base of the stratigraphic section. Well control suggests that, on the shelf, the top of this sequence is Eocene in age. The upper sequence boundary is defined by the overlapping relationship of the overlying reflections. The boundary itself is characterized by a strong, discontinuous, westerly dipping reflection. The seismic facies can not be confidently interpreted at this depth due to loss of resolution. The external form and base-of-slope position are indicative of a slope front fill seismic facies unit (Table 1). However, new seismic data across other parts of the basin suggest that it may be a carbonate facies. On the shelf, at SPN 3100, the upper

24

boundary of sequence F merges updip with the Mio-Pliocene unconformity. Seismic sequence F appears to downlap a strong reflection between SPN 1450 and 1800 (Fig. 2). The reflection is characterized by a long-period, high-amplitude wavelet. It underlies the stratigraphic section at 4.0 seconds (two-way travel time) indicating that a strong impedance contrast exists at depth. 4. 7. B a s e m e n t

The nature of the substratum underlying the Sunda forearc basin is problematical. It has been variously interpreted as a pre-Miocene accretionary complex [5,6], as an oceanic crustal remnant [12], and as thinned and subsided continental crust [8]. Beneath the continental shelf, basement occurs much shallower and is believed to be Paleozoic/Mesozoic igneous and metamorphic rocks [5]. Seismic refraction measurements indicate crustal velocities of 6.5-6.8 k m / s beneath the basin and layers of 4.3, 5.3, and 6.0 k m / s near the Sumatran coast [8]. The multichannel reflection data presented in this paper are inadequate to directly resolve the nature of the deeper crustal material beneath the forearc basin. However, the interpretation of seismic sequence F suggests that the older sediments were deposited on a continental slope with deeper water to the southwest. Such an environment of deposition is compatable with Hamilton's [11,12] interpretation that the oldest sediments were deposited on a continental slope and out onto oceanic crust.

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5. Depositional history Two depositional cycles are evident from our seismic data. Major transgressions occurred in Miocene and Quaternary times. Unconformities separating the transgressive sequences represent the regressive phases of these cycles. Fig. 4 is a depth section showing how the overall sedimentation patterns change with time. The unconformity cut into seismic sequence F correlates with the regional shelf unconformity of Oligocene age [5,6]. Karig et al. [6] attribute the unconformity to uplift associated with renewed rapid subduction. Wood [18] recognizes an unconformity of similar age in many basins peripheral to the Sunda shield and attributes it to a period of uplift and erosion of the volcanic arc at the end of early Oligocene time. We cannot rule out a mid-Oligocene eustatic sea level drop [19] as a major factor in this erosional event. The late Paleogene regression was accompanied by erosion and intense basin sedimentation. These conditions produced the large fill deposits, seismic sequences E and D, which occupy the western portion of the basin and pinch out across the slope and shelf. These pre-Miocene units (Fig. 4) show strong basal onlap in a shoreward direction indicative of rapid subsidence accompanied by filling of a deep basin or trough. By early Miocene time, marine sedimentation encroached on the shoreline and thick limestone deposits transgressed marginal marine sandstones and shales in the Pandjang well (Fig. 3). The seismic section (Fig. 2) indicates development of a carbonate bank on the paleo-shelf break between SPN 2200 and 2300. The thickness, morphology, and internal geometry of the Miocene interval suggests that the early Miocene was a period of

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Fig. 4. Depth section showing stratigraphy of forearc basin. The top of seismic sequence F is correlated with an unconformity of Oligocene age. The top of seismic sequence C is correlated with an unconformity at the Miocene/Pliocene boundary.

25

progradation and asymmetrical filling of the basin from the arc side. The unconformity between seismic sequences C and B is correlated with a hiatus in the Pandjang well (Fig. 3). In the well, mid-Pliocene basal conglomerates overlie lower Miocene carbonates, representing a hiatus of about 10 m.y. On the seismic section (Fig. 2), the unconformity is recognized by truncated reflections and erosional features such as mounds and channels. The regional extent of this surface has been confirmed by correlation within a grid of Union Oil Co. profiles on the shelf and on submerged portions of the outer-arc ridge. Upper Pliocene and Pleistocene sediments overstep the Mio-Pliocene unconformity and represent an overall Quaternary transgression. The basal units of seismic sequence B show strong marine onlap in a shoreward direction, and the geometry of seismic sequence A indicates that progradation has built the shelf break basinward approximately 10 km to its present position while deep-water sedimentation rates have been reduced. Two mechanisms may be invoked to explain these cycles of deposition and erosion: global fluctuation of sea level and regional tectonic events. The depositional cycles observed on our seismic section can be correlated with first order cycles on the chart of Cenozoic relative changes of sea level [19]. The late Oligocene unconformity observed on this and many other seismic profiles in the region corresponds to a major regression on the sea level cycle chart. Another major regression occurs in late Miocene/early Pliocene time. Global transgressions occurred in early Miocene and late Pliocene times coincident with marine transgressions documented by well data on the Sumatra shelf. From these observations it may be argued that sedimentation patterns in the forearc basin have been influenced by eustatic sea level changes. However, structural evidence [5,6,20] indicates that regional tectonic events also occurred during these times. Lowered sea levels undoubtly accented the unconformities. The geometry of the seismic sequences and their thickness distribution patterns (Fig. 5) show that the basin depocenter has oscillated with time, primarily in response to uplift of the outer-arc ridge. For example, during the deposition of

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seismic sequence E, the locus of maximum sedimentation shifted to the extreme western margin of the basin. This coincides with a period of increased subduction activity and formation of an incipient subduction complex [6]. The rising outerarc ridge began to act as a dam, trapping sediment in a narrow structural trough. As the ridge became elevated, the sediment in the basin was tilted landward and the depocenter migrated eastward away from the outer-arc ridge during the deposition of sequences C and D. Another abrupt westward displacement of the depocenter occurred at the beginning of seismic sequence B. The cause of this abrupt displacement is unknown, but may be re-

26 lated to relative subsidence of the outer-arc ridge. Renewed uplift of the outer-arc ridge led to nearly uniform sedimentation in the basin during sequences B2, B1, and A. These oscillations imply that regional tectonics are an important factor controlling sedimentation patterns in forearc basin settings. During the interim between tectonic events, a progressive landward shift of the depocenter is observed (fig. 5). The new shelf-to-basin profile is primarily controlled by the relative rates of subsidence and sedimentation in the basin.

crust, continental crust, or accretionary complex) and the amount of sediment accreted at the trench. More information for different arc systems is needed before useful comparisons can be made. It is now becoming clear that there is such a great diversity of forearc types and evolutionary histories that one tectonic model cannot explain the development of all convergent margins. Data such as that presented here allows us to document the similarities and differences among arcs and to develop more suitable tectonic models.

6. Comparison to other forearc basins

7. Conclusions

The wealth of new convergent margin data that have recently become available allow recognition of similarities and differences among forearc regions. For example, the Japan, Guatemala, and Sunda margins all exhibit shelf unconformities that have been attributed to erosion at sea level during the Oligocene [4,5,6,21]. Whether the erosion is due to tectonic uplift of the margin in response to renewed subduction [6], or to eustatic lowering of sea level [4] remains unresolved. The forearc basins of these regions have all experienced post-Oligocene subsidence. Tectonic erosion of the Japan margin has been proposed to explain the subsidence there [21], although documented subsidence of the actively accreting Sunda margin [6,9] suggests that margin subsidence is not dependent upon tectonic erosion. The Neogene structural evolution of the seaward margins of the Sunda, Japan, Guatemala, and Peru forearc basins has been very different. The structural high off Japan has been rapidly subsiding [21], and the highs off Guatemala and Peru have been fixed or slowly rising relative to the forearc basin [4,22], while the high off Sumatra has experienced several periods of uplift. Correspondingly, the depocenter off Japan has been shifting seaward [21], and the depocenters off Guatemala [4] and Peru [22] have been progressively shifted landward, while the depocenter off Sumatra has oscillated back and forth through time. The differences in the subsidence histories of these margins are undoubtedly related to differences in forearc basin basement types (oceanic

Preliminary interpretations of the seismic data indicate that two distinct modes of deposition contribute to the sedimentary fill within the basin. During episodes of uplift of the subduction complex, sediments tend to accumulate in the rapidly subsiding trough landward of the outer-arc ridge. Onlapping fill units restricted to the deeper, trenchward side of the basin probably represent trough fill deposits with longitudinal dispersal patterns. When the structural configuration is relatively stable, or if sediment supply :increases, deposition is capable of overwhelming the effects of subsidence and sediments are prograded into the basin from the arc side. Well-developed shelf and prograded-slope deposits accumulate in association with carbonate buildups on the shallow margins of the basin. The Cenozoic development of the forearc basin off Sumatra is characterized by at least two episodes of uplift of the subduction complex accompanied by filling of the basin. In the late Oligocene, subduction was renewed [5,12], and regional uplift and/or eustatic sea level lowering led to erosion of the Sumatra shelf and the development of a regional unconformity [6]. An accretionary prism began to form, building a small outer-arc ridge behind which sediments began to accumulate. In late Miocene/early Pliocene time, accretion of thick Nicobar Fan sediments caused rapid growth and enlargement of the outer-arc ridge. Sediments delivered to the forearc basin at this time were ponded behind the ridge. During the interim between structural or

27

tectonic events, the basin steadily subsided and sedimentation occurred mainly on the shelf and slope. In the early Miocene, reef growth was established and maintained on the shallow margins of the basin as the shelf subsided. By late Pliocene time, the flexure forming the western margin of the basin had developed. Upper Pliocene to Recent sediments prograded across the shelf edge and downlapped the early Pliocene ponded trough deposits. The Sumatra shelf and forearc basin have continued to subside and the basin remains unfilled.

Acknowledgements Acquisition and processing of the data presented here were made possible b l financial support of the Scripps Industrial Asscciates program and through the efforts of P.J. Crampton, J.L. Abbott, P.C. Henkart and others in the S.I.O. Shipboard Geophysical and Comt,uter Groups. Their contributions are gratefully ~cknowledged. Dr. Fred Hehuwat, Director of the Indonesian National Institute of Geology and Mining, and Drs. G.G. Shor, J.R. Curray, and D.E. Karig organized and planned the data acquisition program. Discussions with T.H. Shipley clarified our thinking on seismic stratigraphy. Shipley, Curray, Karig, J.S. Watkins, D.R. Seely, and R.T. Buffler provided reviews of the manuscript. This research was supported by the International Decade of Ocean Exploration Program of the National Science Foundation through grants OCE76-24101 and OCE79- 18185.

References 1 W.R. Dickinson, Sedimentary basins developed during evolution of Mesozoic-Cenozoic arc-trench system in western North America, Can. J. Earth Sci. 13 (1976) 1268. 2 R.V. Ingersoll, Paleogeography and paleotectonics of the late Mesozoic forearc basin of northern and central California, in: Mesozoic Paleogeography of the Western United States, D.G. Howell and K.A. McDougall, eds., Soc. Econ. Paleontol. Mineral., Pacific Section, Pacific Coast Paleogeography Symposium (1972) 471. 3 R.V. Ingersoll, Evolution of the Late Cretaceous forearc basin, northern and central California, Geol. Soc. Am. Bull. 90 (1979) 813.

4 D.R. Seely, The evolution of structural highs bordering major forearc basins, in: Geological and Geophysical Investigations of Continental Slopes and Rises, J.S. Watkins, L. Montadert and P.W. Dickerson, eds., Am. Assoc. Pet. Geol., Mere. 29 (1979) 245. 5 D.E. Karig, Suparka S., G.F. Moore and P.E. Hehanussa, Structure and Cenozoic evolution of the Sunda arc in the central Sumatra region, in: Geological and Geophysical Investigations of Continental Slopes and Rises, J.S. Watkins, L. Montadert and P.W. Dickerson, eds., Am. Assoc. Pet. Geol., Mem. 29 (1979) 233. 6 D.E. K~ig, M.B. Lawrence, G.F. Moore and J.R. Curray, Structural framework of the fore-arc basin, N.W. Sumatra, J. Geol. Soc. London 137 (1980) 77. 7 CCOP-IOC, Metallogenesis, Hydrocarbons and Tectonic Patterns in Eastern Asia, U.N. Development Program, Committee for Co-ordination Of Joint Prospecting for Mineral Resources in Asian Offshore Areas-Intergovernmental Oceanographic Commission (UNESCO, Bangkok, 1974) 158 pp. 8 R.M. Kieckhefer, G.G. Shor, Jr., J.R. Curray, W. Sugiarta and F. Hehuwat, Seismic refraction studies of the Sunda Trench and forearc basin, J. Geophys. Res. 85 (1980) 863. 9 G.F. Moore and J.R. Curray, Structure of the Sunda Trench lower slope off Sumatra from multichannel seismic reflection data, Mar. Geophys. Res. 4 (1980) 319. 10 W.R. Dickinson and D.R. Seely, Structure and stratigraphy of forearc regions, Am. Assoc. Pet. Geol., Bull. 63 (1979) 2. 11 W. Hamilton, Subduction in the Indonesian region, in: Island Arcs, Deep Sea Trenches, and Back-Arc Basins, M. Talwani and W.C. Pitman, IV, eds. Am. Geophys. Union, Maurice Ewing Ser. 1 (1977) 15. 12 W. Hamilton, Tectonics of the Indonesian region, U.S. Geol. Surv. Prof. Paper 1078 (1979) 345 pp. 13 J.A. Katili, Geochronology of west Indonesia and its implication on plate tectonics, Tectonophysics 19 (1973) 195. 14 J.R. Curray and D.G. Moore, Sedimentary and tectonic processes in the Bengal deep-sea fan and geosyncline, in: The Geology of Continental Margins, C.A. Burk and C.L. Drake, eds. (Springer Verlag, New York, N.Y., 1974) 617. 15 R.M. Mitchum, Jr., P.R. Vail and S. Thompson, III, Seismic stratigraphy and global changes of sea level, 2. The depositional sequence as a basic unit for stratigraphic analysis, Am. Assoc. Pet. Geol., Mem. 26 (1977) 53. 16 R.M. Mitchum, Jr., P.R. Vail and J.B. Sangree, Seismic stratigraphy and global changes of sea level, 6. Stratigraphic interpretation of seismic reflection patterns in depositional sequences, Am. Assoc. Pet. Geol., Mem. 26 (1977) 117. 17 J.B. Sangree and J.M. Widmier, Interpretation of depositional facies from seismic data, Geophysics 44 (1979) 131. 18 P.A. Wood, Hydrocarbon plays in Tertiary S.E. Asia basins, Oil Gas J. 78, No. 29 (1980) 90. 19 P.R. Vail, R.M. Mitchum, Jr. and S. Thompson, III, Seismic stratigraphy and global changes of sea level, 4. Global cycles of relative changes of sea level, Am. Assoc. Pet. Geol., Mem. 26 (1977) 83. 20 B.G.N. Page, J.D. Bennett, N.R. Cameron, D.McC. Bridge, D.H. Jeffrey, W. Keats and J. Thaib, A review of the main

28 structural and magmatic features of northern Sumatra, J. Geol. Soc. London 136 (1979) 569. 21 R. Von Huene, M. Langseth, N. Nasu and H. Okada, Summary, Japan Trench transect, in: Scientific Party, Initial Reports of the Deep Sea Drilling Project, Legs 56 and

57, Part 1 (U.S. Government Printing Office, Washington, D.C., 1980) 473. 22 W.T. Coulbourn and R. Moberly, Structural evidence of the evolution of fore-arc basins off South America, Can. J. Earth Sci. 14 (1977) 102.