Continental-shelf progradation by sediment-drift accretion

CRAIG S. FULTHORPE* ) n „,.„ . , ROBERT M CARTER ) DePartment °f Geology, James Cook ity, Townsville, Q 4811 Australia

ABSTRACT drift morphology is inhibited in areas affected by frequent downslope sediment mass-transport, and the North Atlantic drifts therefore formed in Multi-channel seismic profiles from the Canterbury Basin on the deep water on the lower slope or detached in basinal settings, where their eastern margin of the South Island of New Zealand reveal the impor- formation required millions of years at accumulation rates of a few tens of tance of current activity in shaping a Neogene shelf sediment prism. meters per million years (McCave and Tucholke, 1986). The shelf prism prograded across a broad, near-horizontal platform in The smaller sediment drifts that occur in shallow-shelf settings are water depths of 1,000 to 1,750 m. The platform was formed above a generally products of storm or tidal action; the role of permanent currents condensed section of late Eocene to late Oligocene limestones which is usually minor. The largest tidal sediment bodies are sand banks which overlie Cretaceous to Paleogene rift-fill and transgressive sediments. may be tens of kilometers long and several kilometers wide, but do not The Neogene sediment prism contains sediment drifts which are generally exceed 40 m in height (Belderson and others, 1982; Chang-shu as much as 25 km long and 15 km wide and extend up to 1,600 m Yang, 1989). The coarser grain size of shelf-sediment bodies reflects prox- (uncompacted) vertically. Individual drifts migrated westward and imity to sediment sources and relatively high current velocities. Shelf- can be traced between dip profiles, revealing that the long axes of sediment drifts are often only resolvable with high-frequency seismic most are subparallel to the present coastline and shelf-edge. Channel- profilers. like features at the landward edges of the drifts correspond to residual In the Canterbury Basin (Fig. 1), large Neogene sediment drifts de- space left between the landward-prograding off-shelf sediment drift veloped adjacent to the toe of a shelf-sediment prism which was and the adjacent shelf foreslope. Erosion or slow deposition character- prograding seaward across a peri-continental platform at water depths of ized the foreslope. Progradation of the shelf was by the accretion of 1,400 to 1,750 m. These deposits, therefore, formed in a bathymetric successive sediment drifts. Before ca. 11.5 Ma (= Pink Horizon), the setting intermediate between that of the shelf and that of the deep ocean, shelf-edge-parallel drifts were distributed across the central part of the and in which large sediment drifts have been rarely described (Pinet and basin, whereas subsequently they were concentrated to the northeast. Popenoe, 1985a, 1985b; Popenoe, 1985; Jacobs, 1989). Tectonic and The seismic architecture of the Neogene sediment prism results oceanographic events combined in the Canterbury Basin to create condi- from the interplay of an abundant western sediment source and an tions in which current deposition became the dominant mechanism of offshore boundary current system. Present-day ocean circulation in- shelf progradation. volves northward flow along the east coast of the South Island. The This study is based on the interpretation of an extensive grid of basin may have been subjected to a middle Miocene to late Pliocene high-quality, multi-channel seismic data, originally acquired for the oil phase of intensified flow, caused by local topographic enhancement industry consortium BP, Shell, Todd (Canterbury) Services Limited and/or global paleoceanographic events. Current activity has played a (BPST). Most of the seismic grid comprises 60-fold data, available in crucial role in the sedimentary evolution of the Canterbury Basin migrated form (Western Geophysical, 1982). A smaller data set of mi- Neogene shelf prism. grated, 24-fold data was also examined (Prakla Seismos, 1974; GECO, 1983). These data are now available on open file through the New Zea- INTRODUCTION land Geological Survey.

Current-deposited sediment bodies have been described previously in GEOLOGICAL SETTING two contrasting bathymétrie settings: the deep ocean (>2,000-m water depth) and the continental shelf (<200 m). Deposits in these two settings The eastern margin of the South Island of New Zealand is part of a differ in size and also in the type of current involved. The largest deposits continental fragment, incorporating the Campbell Plateau and Chatham are those of the deep ocean, and result from the steady flow, over long Rise, that rifted from Antarctica beginning at about 80 Ma (Molnar and periods, of major oceanic currents. In the western North Atlantic (Tu- others, 1975; Weissel and others, 1977). The Canterbury Basin lies at the cholke and Mountain, 1979, 1986; McCave and Tucholke, 1986), deep- landward edge of the resulting passive margin and underlies the present- ocean sediment drifts are from 200 to 2,000 m high, may be hundreds of day Canterbury Plains and continental shelf (Fig. 1). The most extensive kilometers long, and are therefore readily resolved on deep seismic pro- basement faulting and crustal thinning occurred seaward of the present- files. They were formed by the Western Boundary Undercurrent and other day shelf edge. Within this distal zone lies the Bounty Trough, a rifted deep currents, and comprise predominantly fine silt and muds with minor basin which formed at a high angle to the strike of the margin. Despite its coarse silt and sand (McCave and Tucholke, 1986). The development of proximity to a major plate boundary, now represented on the South Island by the Alpine fault, the Canterbury Basin has been a site of relative

•Present address: The University of Texas Institute for Geophysics, 8701 tectonic stability since rifting. Tectonic activity has been limited to subsi- Mopac Blvd., Austin, Texas 78759-8345. dence (Browne and Field, 1988), and seismic profiles show little or no

Geological Society of America Bulletin, v. 103, p. 300-309,9 figs., 1 table, February 1991.

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Figure 1. A. The South Island of New Zealand. The box shows the location of Figure IB. BP: Banks Peninsula. OP: Otago Peninsula. Bathymetric contours in meters. B. Offshore Canterbury Basin with distribution of seismically resolvable, current-deposited, sediment drifts. Thick lines delineate crests of shelf foreslopes (landward banks of the gutters) at the landward edges of sediment drifts. Broken lines indicate uncertain correlations. Approximate ages have been assigned to some drifts. Seismic profiles used in figures are shown as thin, broken lines and are labeled with figure numbers. The locations of the four exploration wells in the basin are also shown.

Cenozoic faulting, although localized igneous centers of late Eocene to ter, 1985), a regional mid-Oligocene unconformity at the base of the Oligocene and Miocene age are present (Milne and others, 1975; Coombs Kekenodon Group which formed on a broad, near-horizontal platform and others, 1986). The modern Canterbury Basin is bounded by the Mio- above the Cretaceous to Paleogene transgressive sediments (Fig. 2). The cene volcanic centers of the Banks Peninsula (7.5-12 Ma) to the northeast Kekenodon Group sediments, glauconitic at the base, accumulated slowly and the Otago Peninsula (9.6-12.9 Ma) to the southwest (Watters, 1978). on this platform, blanketing and preserving its morphology. The Neogene basin sediments thin toward these features and also onshore, Consequent to the late Oligocene to early Miocene increase in the where at the western edge of the basin they become involved in faulting supply of sediment from the west, a Neogene sediment wedge (the Otakou associated with the mid-Cenozoic development of the Southern Alps. The Group) prograded across the mid-Cenozoic platform. Offshore explora- thickest Neogene sedimentary section lies approximately midway between tion wells Clipper-1 (Hawkes and Mound, 1984), Galleon-1 (Wilson, the Banks and Otago Peninsulas near the present-day shelf edge. 1985), and Endeavour-1 (Wilding and Sweetman, 1971) reveal the sedi- At the largest scale, the post-rift sedimentary history of the Canter- ments of the Otakou Group to be predominantly terrigenous silts, variably bury Basin can be characterized as a single, tectonically driven, argillaceous, with intermittent intervals of fine to very fine-grained sand transgressive-regressive cycle. The Onekakara, Kekenodon, and Otakou and mud. A 60-m-thick, very fine-grained, quartz sandstone overlies the groups (Carter and Carter, 1982) were deposited during the regional limestone and forms the base of the Otakou Group at Clipper-1 (Caver- transgressive, highstand, and regressive phases of this cycle, respectively sham Sandstone equivalent; Hawkes and Mound, 1984). At Resolution-1, (Fig. 2). Marine transgression accompanied post-rift, thermal subsidence the lithology is predominantly silty mudstone (Milne and others, 1975). during the Late Cretaceous and Paleogene, with maximum flooding of the Shell debris and glauconite also occur, and the sediment becomes increas- landmass during the Oligocene. The subsequent development of strike-slip ingly calcareous near the base of the Otakou Group. The Otakou Group motion along the Alpine fault resulted in an increased rate of sediment offshore exhibits lithology similar to that of the onland Bluecliffs Silt (Gair, supply (Norris and others, 1978) and progradation of the shelf, since 1959) and the equivalent Waikari Formation (Andrews, 1963). late Oligocene or early Miocene time, at a rate of between 1.5 and 4.9 km/m.y. REGIONAL SEISMIC STRATIGRAPHY OF THE Reduced terrigenous influx during maximum transgression in the KEKENODON AND OTAKOU GROUPS mid-Cenozoic led to the deposition of two basin-wide pelagic to hemipe- lagic, bioclastic limestones, the Amuri and Weka Pass Formations and The mid-Cenozoic, highstand limestone interval was penetrated by equivalent units. They are separated by the Marshall Paraconformity (Car- all four offshore exploration wells, demonstrating a general correlation

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SOUTHERN ALPS CANTERBURY PLAINS CANTERBURY BIGHT

E OTAKOU je KEKENODON X h- D. UJ ONEKAKARA o

BLUECLIFFS SILT/ CAVERSHAM SANDSTONE EQUIVALENT -YELLOW HZ WEKA PASS LIMESTONE o Çr KEKENODON GROUP CONCORD GREENSAND _ MARSHALL PARACONFORMITY AMURI LIMESTONE — GREEN

Figure 2. Post-rift stratigraphy of the Canterbury Basin. The sediments record tectonically controlled transgression and regression. The Onekakara, Kekenodon, and Otakou Groups were deposited during the regional transgressive, highstand, and regressive phases, respectively. Also shown is the detailed stratigraphy, based on outcrops onshore, in the vicinity of the mid-Oligocene Marshall Paraconformity, which forms the base of the Kekenodon Group. Green, Yellow, and Pink are regional seismic reflections (see also Fig. 3).

between the base of the lower (Amuri) limestone and a regionally trace- 1989). One particular reflection was designated "Pink Horizon" in reports able seismic reflection (Green Horizon in BPST reports; Figs. 3 and 4A). for BPST (Hawkes and Mound, 1984; Wilson, 1985; Figs. 3 and 4B). Green Horizon is a little lower than, but approximate to, the mid-Cenozoic Numerous channel-like features are present within the Otakou Group marginal platform that existed prior to the deposition of the Kekenodon sediment prism (Fig. 5A), mostly below Pink in the central part of the Group. Offlapping reflections within the overlying shelf sediment prism basin, both below and above Pink to the northeast. The features lie near downlap onto this reflection near shore. Farther offshore, however, they the seaward ends of paleo-foresets and were therefore interpreted by Car- downlap onto reflections at higher stratigraphic levels. One such promi- ter and Carter (1987) as representing slope and foot-of-slope canyons/ nent reflection above the limestone (Yellow Horizon in BPST reports) is of channels related to earlier Miocene shelf-slopes for which the topset and wide extent. It lies near the contact between the Kekenodon Group and upper foreset parts lay considerably farther west. It was assumed that these the overlying Otakou Group (Fig. 3). canyons/channels were conduits for the transport of sediment from the Seaward-dipping reflectors within the Otakou Group illustrate the shelf into deeper water, and were oriented normal to the strike of the progradation of the shelf since early Miocene time (Fig. 3). Eustatic sea- coastline and shelf edge. This study, however, indicates that these features level variation, perhaps combined with changes in the rate of sediment are oriented subparallel to the shelf edge, and they are reinterpreted here as supply, created high-frequency, stacked sequences (Fulthorpe and Carter, the landward ends of large off-shelf sediment drifts.

Figure 3. Dip profile CB-82-54 (migrated) showing regional seismic reflections Green, Yellow, and Pink. Note the prograding clinoforms of the Otakou Group above reflection Yellow, which form high-frequency seismic sequences above Pink Horizon, and the seismically transparent, post-drift section beneath the modern slope. Reflection ages were estimated by extrapolation to the Clipper-1 well.

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Figure 4. Depth contours, in me- ters below mean sea level, to regional seismic reflections: A, Green; B, Pink. Depths are in meters below sea level. The distribution of those sediment drifts which had been deposited by Pink Horizon time is shown in B, using the same conventions as for Figure IB. The strike of the contours became more parallel to that of the present-day coastline and shelf edge between A and B (modified from Mound and Pratt, 1984).

SEISMIC CHARACTER AND DISTRIBUTION OF THE of each drift onlap a seaward-dipping reflection which is interpreted as the SEDIMENT DRIFTS contemporary paleo-shelf foreslope. The shelf foreslope can be a continu- ous surface, over which the drift migrated upslope (Fig. 5A); shelf-derived The morphology of the drifts is most clearly displayed on seismic sediment bypassed the slope and/or was eroded from it. In other cases, the profiles oriented in the dip direction (that is, oriented northwest-southeast, slope itself was a site of slow deposition and prograded seaward, although orthogonal to the present-day shelf edge). The drifts are characterized by much more slowly than the landward-migrating drift. Many drifts exhibit a convex-upward, mounded seismic reflections (Fig. 5). The landward ends lower zone of low-amplitude seismic reflections, with higher amplitudes

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/103/2/300/3381237/i0016-7606-103-2-300.pdf by guest on 24 September 2021 Figure 5. Appearance of current-deposited sediment drifts on dip profiles. A, Profile CB 82-50 (migrated): early, pre-Pink drift. Note the mounded reflection morphology, prominent gutter, and the continuous shelf-foreslope reflection (landward edge of the gutter). This profile forms the basis for the reconstruction in Figure 7. B, Profile CB-82-22 (migrated): late, post-Pink drifts near present-day shelf edge, showing the less compact morphology than the pre-Pink drifts in 5A. The seismically transparent zone within 0.2 s of the sea floor represents the post-drift phase of sedimentation in the Canterbury Basin. Slumps are apparent on the modern slope.

LINE CB-82-19 2 km sw NE

LU 2

ui >

i- > <

i o

h

Figure 6. Appearance of current-deposited sediment drifts on shelf-parallel profiles. Profile CB-82-19 (migrated) showing generally less-coherent drift morphology than dip profiles. Onlap is to the southwest.

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higher in the mound and particularly at their landward ends (Figs. 5A and FORMATION OF THE SEDIMENT-DRIFT SYSTEM 5B); other drifts display strong reflections throughout. On shelf-parallel profiles (Fig. 6), the direction of onlap and migration is to the southwest. Three pieces of evidence are crucial to the current-deposited Uniform northwesterly (landward) migration on dip profiles and south- sediment-drift interpretation for the seismic features described above. westerly migration on shelf-parallel profiles indicate a resultant overall (1) Features that can be traced between seismic profiles are subparal- westerly migration. Distinct shelf foreslopes are commonly absent on lel to the paleo-shelf edge and the present coastline. shelf-parallel profiles because of the acute angle between the profiles and (2) The consistent onshore (northwest) onlap on dip profiles and the shelf edge (Fig. 6). southwest onlap on shelf-parallel profiles (resultant westward onlap) indi- Except for rare instances in the extreme northeast of the grid, no drift cate that the sediment bodies in all cases migrated landward. morphology occurs below Yellow Horizon. Some drift mounds rest di- (3) The mounded seismic reflection morphology and the seismic rectly on this reflection (Figs. 5A and 5B), but in other cases, drift mor- character of the channel-like landward edges of the features, particularly phology was initiated above it; the latter configuration typifies the most the less compacted, post-Pink features (Fig. 5B), are similar to those of landward (oldest) drifts. Yellow Horizon represents the mid-Cenozoic migrating sediment drifts described in the North Atlantic (Tucholke and platform (draped with Kekenodon Group sediments) for the period of Mountain, 1979; McCave and Tucholke, 1986). formation of seismically resolvable drifts. Alternative mechanisms, such as channel and canyon formation by Each dip profile may display several drifts (Figs. 5B and IB). Down- downslope sediment transport, or scarp formation by slope failure, cannot dip (seaward), they extend progressively higher in the section. Pre-Pink account for both the shelf-edge parallel orientation and the mounded drifts are distributed across the central part of the basin, whereas those reflection morphology. It is inferred that deposition of the drifts took place above Pink, near the present-day shelf edge, are concentrated at the north- under the influence of a current, subparallel to the eastern margin of the eastern end (Fig. IB). The onlapped, seaward-dipping reflections South Island. The current-deposited sediment formed separate, often (paleo-shelf foreslopes) at the landward edges of the drifts can commonly mounded sediment bodies which onlapped the foreslope of the shelf sedi- be correlated between several adjacent dip profiles (Fig. IB). Correlations ment prism as a number of broad sediment banks or drifts. Figures 7 and 8 of this type are clearest in the central part of the basin. They are more provide examples of the evolution of the outer edge of the shelf-sediment difficult to make with certainty in the northeastern part of the seismic grid prism and adjacent, current-deposited, off-shelf sediment drifts. where the drifts are more numerous. There appear to be several discrete Drifts that directly overlie Yellow Horizon record current deposition paleo-shelves, each of which can be traced for some distance across the at, or near, the toe of the slope of the shelf-sediment prism. As a sediment seismic grid, beside which sediment drifts developed (Fig. IB). drift migrated landward and onlapped the foreslope of the shelf-sediment The most recent, post-Pink, drifts (Fig. 5B) appear considerably larger than the older, pre-Pink, drifts (Figs. 5A and 6). The seismic expres- sions of both pre- and post-Pink drifts extend up to 15 km horizontally, downdip, but differ significantly in their apparent heights. The older drifts exhibit a flatter, less mounded profile; and their landward edges have a more compact, channel-like appearance than do the post-Pink features. Decompaction calculations (method of Falvey and Deighton, 1982) indicate that the increase in drift height through time paralleled an increase in the relief of the paleo-shelf edge above the mid-Cenozoic platform at the toe of the slope. The decompacted paleo-shelf edge relief increased from 850 m for the earliest measurable shelf edge to 1,350 m for the Pink Horizon shelf edge. Most of the pre-Pink drifts formed during a period in which paleo-shelf edge relief increased from 1,250 to 1,350 m. The large, post-Pink drifts developed when decompacted paleo-shelf relief may have been as great as 1,600 m. The present-day shelf-edge relief is only 1,000 m. The less-mounded appearance of the oldest drifts is probably the result of additional burial compaction undergone by the oldest drifts. The crests of the oldest drifts are at depths of the order of 1 km below the present sea floor, whereas the crests of the most recent drifts are less than 400 m beneath the present sea floor. We conclude, therefore, that the less deformed, post-Pink features probably best reveal the appearance of active drifts. The decompaction calculations allow estimation of the variation with time of water depth over the mid-Cenozoic platform. Assuming paleo- water depths at the shelf edge similar to that of the present (150 m), the water depth over the mid-Cenozoic platform, at the toe of the slope, increased from 1,000 to 1,400 m during the period of pre-drift prograda- Figure 7. Interpreted evolution of sediment drift on Profile CB- tion recorded on the seismic profiles (ca. 25-12 Ma) and from 1,400 to 82-50 (see Fig. 5A), showing the development of the gutter as a rem- 1,750 m during the period of sediment-drift development (ca. 12-3 Ma). nant low between the landward-migrating drift and the shelf For comparison, the water depth at the toe of the modern shelf slope is foreslope. The shelf foreslope is predominantly erosive: dashed lines 1,150 m. indicate inferred, eroded sediment.

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1 2 km SE NW _1 J

Figure 8. Interpreted evolution of sediment drifts on Profile CB-82-26, illustrating shelf progradation by accretion of two successive sediment drifts. The landward (northwest) drift was apparently initiated above Yellow Horizon. Note the slight deposition on the shelf foreslope during the growth of the seaward (southeast) drift. This contrasts with the erosional slope of Figure 7. Dashed lines indicate inferred eroded sediment.

prism, the junction between the two became a topographic low; the con- A Model of Shelf-Edge Drift Accretion junction of these sediment bodies necessarily created a gutter parallel to the existing shelf edge. U-shaped seismic reflections occur within the gutters. Figure 9 summarizes a model of shelf progradation by sediment-drift The smooth, continuous reflection, which can form the seaward slope of accretion as a five-stage process, based on our observations in the Canter- the shelf-sediment prism, indicates erosion of this surface during landward bury Basin. In the pre-drift stage, the current initially followed the toe of migration of the drift and filling of the gutter. Typically, during filling of the shelf-sediment prism (Fig. 9A). Deposition of a sediment drift began the gutter, the shelf-sediment prism aggraded, whereas erosion (Fig. 7) or seaward of the high-velocity core of the current, but at the same time slow sediment accumulation (Fig. 8) characterized its foreslope. erosion may have occurred beneath the core (Fig. 9B). Growth of the The evidence for erosion or slow deposition at the paleo-shelf fore- mature drift progressively confined the current within the gutter, which slopes, coupled with the presence of U-shaped reflections within the adja- may be on the order of 300 m deep at this stage (Fig. 9C). Landward drift cent gutters, suggests intensified flow velocities in the gutters. The migration indicates an oblique component of flow (Figs. 9C and 9D). As high-amplitude reflections there may therefore represent sands deposited as the gutter narrowed and aggraded (Figs. 9C and 9D), an increasing pro- a result of such increased flow strength. portion of the flow followed the seaward edge of the drift which ultimately

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A PRE-DRIFT

•hm TH B EARLY DRIFT

Figure 9. Model of drift evolution in response to a current subparallel to the shelf edge. Approximate water depths at the toe of the shelf foreslope and the crest of a mature drift illustrate the aggradation of the drift to shelf depths. See text for discussion.

became the new toe of the shelf-sediment prism and the new location of the current core (Fig. 9E). A similar pattern of inferred behavior has been attributed to the Gulf Stream where it crosses the northern Blake Plateau (Pinet and Popenoe, 1985a, 1985b). Most of the Blake Plateau lies between water depths of 700 and 1,000 m, and it may therefore be analogous to the Canterbury strike, the drifts may terminate against one another, or against the shelf Basin current-swept platform. Pinet and Popenoe (1985a) noted shifts of foreslope. the current axis from an inner-plateau location to an offshore, deeper Shelf progradation, then, was an episodic process, with the position of location and inferred that the shifts were controlled by falls in eustatic the shelf edge stepping seaward by 5 to 15 km with each successive sea level. The rise in base level caused by individual drift aggradation in accretion of a current-deposited sediment drift. The offshore end of each the Canterbury Basin would be expected to produce the same offshore drift became the new toe of the shelf-sediment prism and subsequently the migration of the current axis as a fall in sea level. site of onlap of the next drift and of a new gutter. Landward migration of a Although the gutters, in one sense, simply define the landward- new drift in some instances began before the preceding gutter was filled. migrating edges of sediment drifts, their role as a site of onlap and drift migration makes them probably the most dynamic part of the drift system. Current Direction and Sediment Supply The paleo-shelf crest reflectors, used for correlation between profiles, are the crests of the landward banks of the gutters. They represent time hori- The Southland Current flows northeast along the east coast of the zons immediately prior to filling of the gutters and accretion of the South Island at present (Heath, 1973), but the detailed history of this and sediment drifts to the shelf. In the central part of the basin, pre-Pink gutters other oceanic currents around New Zealand is poorly known. The Coriolis that share a common landward bank top reflector were apparently filled effect deflects currents in the Southern Hemisphere to the left, and there- simultaneously along their length. They can be traced for as much as 25 fore a northward-flowing current would tend to follow bathymetric con- km along strike (Fig. IB). The post-Pink drift/gutter systems appear to tours and hug the toe of the shelf-sediment prism, as required by the correlate for similar distances along strike; however, with seismic line proposed model of drift formation. The upslope migration displayed on spacings on the order of 5 km, seismic coverage proved inadequate for the dip profiles may have been a consequence of this deflection. A component unambiguous delineation of the drifts (Fig. IB) and, most importantly, for of up-current migration of the drifts may also have occurred, as known to the recognition of along-strike drift terminations. The model of drift evolu- occur for deep-ocean mud waves (McCave and Tucholke, 1986). tion presented here, therefore, remains essentially two-dimensional; along Large volumes of sediment must have been transported to the mid-

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Cenozoic platform at the toe of the shelf foreslopes to form the drifts. At TABLE 1. NEOGENE, CURRENT-DEPOSITED SEDIMENT IN THE CANTERBURY BASIN present, little sand or coarser sediment spills over the shelf edge and onto Ref. Approximate ag Scale the slope in the Canterbury Basin; most is carried northeast by shelf cur- Feature e (Ma)

rents, augmented by tidal and wave action (Carter and Heath, 1975; Platform scour 1 Recent 0-•3 No deposition (for example, Chatham Rise) Herzer, 1979). During the period of progradation by drift accretion, oce- Post-Pink drifts 2 Mio-Pliocene 3- 11.5 15 km» 1,600 m anic currents around New Zealand may have been of significantly greater Pre-Pink drifts 2 Middle Miocene 11.5--12 15 km x 1,400 m Isolated drift 2 Early Miocene 17- 19 1,200 m thick strength than at present, sufficient to transport silt and perhaps fine-grained Weka Pass Limestone 3 Earliest Miocene - 19 -24 5 km x 25 m sand at platform depths (1,400 to 1,750 m) from sources outside the basin. Concord Greensand 4 Oligocene 24--28 up to 5 m thick

The presence of a thick, early Miocene sand layer at the Clipper-1 well 1. Gtasby and Summerhayes (1975). supports this, as does the similar lithology of a possible drift deposit 2. This paper. 3. Ward and Lewis (1975). exposed onshore, the early Miocene Caversham Sandstone of the Dunedin 4. Gair (1959). district (Benson, 1967). Alternatively, current velocities may have been only locally en- hanced, and sediment-transport distances along strike correspondingly The earliest drifts were shallower and narrower than later features, short; that is, sediment was supplied from rivers within the Canterbury and not all of them extended down to reflector Yellow (Fig. 8, NW drift). Basin itself, and downslope sediment movement was volumetrically im- The interpretation of drift development presented here indicates that in portant. It is possible that drift formation was the result of the reworking of these cases, either (1) drift morphology developed on the slope of the material from the adjacent shelf, delivered to the platform indirectly by shelf-sediment prism, rather than at its toe, or (2) landward progradation tidal and wave action, or directly during eustatic lowstands. Periods of low of the drift proceeded for some distance up the slope of the shelf-sediment sea level would have enhanced sediment bypassing of the shelf, perhaps to prism by gentle onlap before any significant mounded morphology devel- feed the sediment drifts at the toe of the slope. Such an interpretation might oped. Both of these interpretations are consistent with an increasing imply that each drift was the product of a single sea-level cycle. Lack of current strength during the period of drift development. precise age control, however, does not permit the correlation of individual The development of drift accretion may have involved the enhance- drifts with the eustatic curve of Haq and others (1987). ment of current activity by local tectonic events such as changes in the bathymetry of the highs surrounding the basin. The estimated age of the INITIATION OF THE SEDIMENT-DRIFT oldest continuous sediment drift coincides with a late Miocene pulse of DEPOSITIONAL SYSTEM volcanic activity surrounding the Canterbury Basin, on the Banks and Otago Peninsulas and offshore (Herzer, 1975; Watters, 1978; Lewis and The locations of the boreholes in the Canterbury Basin do not allow others, 1985; Herzer and others, 1989). determination of the exact timing of initiation of the drift accretion deposi- In conclusion, therefore, it seems likely that a dominant southern tional system. Thinning of the section toward the offshore wells and the ocean circulation pattern, associated with the Antarctic Circumpolar Cur- discontinuity of reflectors in the interval of interest degrade the accuracy of rent, was well established by early Miocene time and that the submerged age estimates. Tracing the landward bank top reflector of the oldest con- eastern margin of the South Island lay in the path of this current system. tinuous drift/shelf channel to the Endeavour-1 exploration well, however, There may have been some intensification of this current during the mid- indicates a late middle Miocene age (Waiauan; 10.5 to 12 Ma), based on dle Miocene. Drift development, however, was only able to occur when the biostratigraphy presented by Wilding and Sweetman (1971). A large volumes of sediment were introduced to the region. Like the Gulf Waiauan age (11.5 Ma) was assigned to Pink Horizon by Fulthorpe and Stream, the core of the current in the Canterbury Basin may have been Carter (1989) based on a tie to the Clipper-1 exploration well and the some distance offshore, in a location dictated by water depth (compare biostratigraphy of Hawkes and Mound (1984). Apparently the initiation with Pinet and Popenoe, 1985a). The development of seismically resolva- of the drift system preceded Pink by a relatively short period, but one dip ble drifts may have been delayed until the shelf had prograded into the profile contains an isolated, possible drift preceding the first continuous path of the current. feature. The estimated age of this feature is early Miocene (Altonian; 16.5 to 19 Ma). TERMINATION OF DRIFT ACCRETION Outcrops onshore in the eastern South Island reveal that currents controlled the deposition of the regional, Oligocene to early Miocene Recent slope sedimentation differs from that of the Tertiary (Figs. 3 highstand deposits within the Kekenodon Group, which display large-scale and 5B). A thin, seismically transparent layer overlies the last drifts, and cross-bedding (Table 1). Ward and Lewis (1975) inferred that cross- there is abundant evidence of recent slumping (Herzer, 1979). In addition, bedding in the Arno Limestone (= Weka Pass Formation) was the result of the relief of the shelf above the toe of slope is now less than that during the a semi-permanent, eastward-flowing current and that intermittent latest stage of drift accretion. Cores show that recent upper-slope sediments northward-flowing storm surge currents created contemporaneous scour comprise muds with sandy turbidites (Griggs and others, 1983). Based on channels. Carter and Landis (1972) had previously suggested that the average accumulation rates at the Clipper-1 exploration well, an approxi- Marshall Paraconformity and overlying cross-bedded limestone were re- mate estimate of the age of the base of this layer is latest Pliocene to early lated to increased erosional activity associated with the initiation of the Pleistocene. Antarctic Circumpolar Current, near the Oligocene-Miocene boundary The regional change in sedimentation may equate with the growth of (Barker and Burrell, 1977). This evidence relates to current activity land- the abyssal Bounty Fan, at the mouth of the Bounty Trough, where an ward of the seismic profiles that preceded the oldest seismically resolved aggradational sediment wave system was initiated at ca. 3 Ma (Carter and drifts. The late Miocene occurrence of drift-accretion progradation did not, others, in press). Both the growth of this fan and the mantling of the therefore, reflect the initiation of current activity on the margin. It may, eastern South Island slope with terrigenous mud can be attributed to the however, reflect an intensification of such activity, or a reorientation of the influx of a large volume of sediment from the Southern Alps, associated flow. In this regard, Glasby and Summerhayes (1975) reported evidence, with the onset of glaciation and high-frequency, glacio-eustastic, sea-level including nondeposition and phosphatization, indicating that bottom- variation. current activity intensified over the Campbell Plateau during late Miocene A reorganization of global current systems may also have accompa- time. nied the late Neogene climatic deterioration and, concomitantly, this

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sediment influx. Late Neogene cooling commenced in the Northern Hemi- Carter, L., and Heath, R. A„ 1975, Role of mean circulation, tides, and waves in the transport of bottom sediment on the New Zealand continental shelf: New Zealand Journal of Marine and Freshwater Research, v. 9, p. 423-448. sphere ca. 3.1 Ma (Raymo and others, 1989); the first evidence of major Carter, L., Carter, R. M., Nelson, C. S., Fulthorpe, C. S., and Neil, H. L., 1990, Evolution of Pliocene to Recent abyssal sediment waves on Bounty Channel levees, New Zealand: Marine Geology (in press). ice-rafting in the North Atlantic is dated at ca. 2.4 Ma (Shackleton and Carter, R. M., 1985, The mid-Oligocene Marshall Paraconformity, New Zealand: Coincidence with global eustatic fall or rise?: Journal of Geology, v. 93, p. 359-371. others, 1984). As a late Pliocene change in deposition of slope sediment Carter, R. M., and Carter, L., 1982, The Motunau fault and other structures at the southern edge of the Australian-Pacific also occurred in the North Atlantic, where increased sedimentation rates plate boundary, offshore Marlborough, New Zealand: Tectonophysics, v. 88, p. 133-159. 1987, The Bounty Channel system: A 55 million-year-old sediment conduit to the deep sea, southwest Pacific accompanied slope valley cutting (Piper and Normark, 1989), the 2.4 and Ocean: Geo-Marine Letters, v. 7, p. 183-190. Carter, R. M., and Landis, C. A., 1972, Correlative Oligocene unconformities in southern Australasia: Nature (Physical 3.1 Ma events may prove to be globally recognizable in continental margin Science), v. 237, p. 12-13. facies, as well as in pelagic sediment. Coombs, D. S., Cas, R. A., Kawachi, Y., Landis, C. A., McDonough, W. F„ and Reay, A., 1986, Cenozoic volcanism in north, east, and central Otago, in Smith, I.E.M., ed., Late Cenozoic volcanism in New Zealand: The Royal Society of New Zealand Bulletin 23, p. 278-312. Cullen, D. J., 1980, Distribution, composition and age of submarine phosphorites on Chatham Rise, east of New Zealand. CONCLUSIONS in Bentor, Y. K-, ed., Marine phosphorites—Geochemistry, occurrence, genesis: Society of Economic Paleontolo- gists and Mineralogists Special Publication 29, p. 139-148. Falvey, D. A., and Deighton, I., 1982, Recent advances in burial and thermal geohistory analysis: Australian Petroleum The Neogene sediment drifts in the Canterbury Basin resulted from a Exploration Association Journal, No. 22, p. 65-81. Fulthorpe, C. S., and Carter, R. M., 1989, Test of seismic sequence methodology on a Southern Hemisphere passive rapid sediment influx onto a broad, current-swept platform in water depths margin: The Canterbury Basin, New Zealand: Marine and Petroleum Geology, v. 6, p. 348-359. of 1,000 to 1,750 m, above a sequence of Cretaceous to Paleogene trans- Gair, H. S., 1959, The Tertiary geology of the Pareora district, south Canterbury: New Zealand Journal of Geology and Geophysics, v. 2, p. 265-296. gressive sediments. Current-deposited sediment formed large drifts at the GECO, 1983, Seismic acquisition survey—Canterbury Bight. PPL 38202/203 shot by Prakla Seismos 1974 and reproc- essed by GECO 1983, for BP, Shell, Todd (Canterbury) Services Limited: New Zealand Geological Survey toe of the slope of the shelf-sediment prism. The drifts migrated landward Open-File Petroleum Report No. 1053. up and along the shelf foireslope, in the process forming remnant gutters Glasby, G. P., and Summerhayes, C. P., 1975, Sequential deposition of authigenic marine minerals around New Zealand: Paleoenvironmental significance: New Zealand Journal of Geology and Geophysics, v. 18, p. 477-490. between each drift and the adjacent shelf foreslope. Filling of the gutter Griggs, G. B., Carter, L., Kennett, J. P., and Carter, R. M., 1983, Late Quaternary marine stratigraphy southeast of New Zealand: Geological Society of America Bulletin, v. 94, p. 791-797. completed the accretion of a drift to the overall shelf-sediment prism. The Haq, B. U., Hardenbol, J., and Vail, P. R., 1987. Chronology of fluctuating sea levels since the Triassic: Science, v. 235, seaward face of the drift became the new shelf foreslope; its toe became the p. 1156-1167. Hawkes, P. W., and Mound, D. G., 1984, Clipper-1 geological completion report, for BP, Shell, Todd (Canterbury) location of the new current flow axis. Services Limited. New Zealand Geological Survey Open-File Petroleum Report No. 1036. Heath, R. A., 1973, Present knowledge of the oceanic circulation and hydrology around New Zealand—1971: Tuatara, The sediment drifts of the Canterbury Basin provide new information v. 20, p. 125-140. Herzer, R. H., 1975, Uneven submarine topography south of Mernoo Gap—The result of volcanism and submarine on Neogene current activity on the eastern margin of the South Island. sliding (Note): New Zealand Journal of Geology and Geophysics, v. 18, p. 183-188. Outcrop evidence indicates current activity on the eastern margin as far 1979, Submarine slides and submarine canyons on the continental slope off Canterbury, New Zealand: New Zealand Journal of Geology and Geophysics, v. 22, p. 391-406. back as the Oligocene, probably associated with the origin of the Antarctic Herzer, R. H., Challis, G, A., Christie, R.H.K., Scott, G. H., and Watters, W. A., 1989, The Urry Knolls, late Neogene alkaline basalt extrusives, southwestern Chatham Rise: Royal Society of New Zealand Journal, v. 19, p. 181-192. Circumpolar Current. Regional geological evidence, including the seismi- Jacobs, C. L., 1989, Sedimentary processes along Flemish Pass, offshore eastern Canada: A GLORIA and high-resolution cally resolvable drifts, indicates that the eastern margin of the South Island seismic reconnaissance: Canadian Journal of Earth Sciences, v. 26, p. 2177-2185. Lewis, K. B., Bennett, D. J., Herzer, R. H., and von der Borch, C. C., 1985, Seismic stratigraphy and structure adjacent to was swept by currents throughout most of the Neogene, as indeed are the an evolving plate boundary, western Chatham Rise, New Zealand, in Kennett, J. P., von der Borch, C. C., and others, Initial reports of the Deep Sea Drilling Project, Volume 90: Washington, D.C., U.S. Government Priming equivalent modern platforms of the Chatham Rise (Cullen, 1980) and the Office, p. 1325-1337. Campbell Plateau (Glasby and Summerhayes, 1975). The initiation of McCave, I. N., and Tucholke, B. E., 1986, Deep current-controlled sedimentation in the western North Atlantic, in Vogt, P. R., and Tucholke, B. E., eds., The geology of North America, Volume M, The western North Atlantic region: drift-accretion progradation during the middle Miocene may have been a Geological Society of America, p. 451-468. Milne, A. D., Simpson, C., and Threadgold, P., 1975, Well completion report Resolution-1, for BP, Shell, Todd response to an intensification and/or reorientation of this current activity, (Canterbury Services Limited: New Zealand Geological Survey Open-File Petroleum Report No. 648. but it was certainly associated with the delivery of substantial amounts of Molnar, P., Atwater, T., Mammerycx, J., and Smith, S. M., 1975, Magnetic anomalies, bathymetry and the tectonic evolution of the South Pacific since the Late Cretaceous: Royal Astronomical Society Geophysical Journal, v. 40, terrigenous sediment to the offshelf marginal platform. The Pliocene ter- p. 383-420. Mound, D. G., and Pratt, D. N., 1984, Interpretation and prospectivity of PPL 38202, Canterbury Basin, New Zealand, mination of drift activity correlates broadly with, and may have been for BP, Shell, Todd (Canterbury) Services Limited: New Zealand Geological Survey Open-File Petroleum Report caused by, sharp changes in global climate at ca. 3.1 and 2.4 Ma. No. 1021. Norris, R. J., Carter, R. M., and Turnbull, I. M., 1978, Cainozoic sedimentation in basins adjacent to a major continental Sediment drifts located on marginal platforms possibly comprise a transform boundary in southern New Zealand: Geological Society of London Journal, v. 135, p. 319-335. Pinet, P. R., and Popenoe, P., 1985a, A scenario of Mesozoic-Cenozoic ocean circulation over the Blake Plateau and its novel hydrocarbon play. The drifts, located at the base of a section pro- environs: Geological Society of America Bulletin, v. 96, p. 618-626. 1985b, Shallow seismic stratigraphy and post-Albian geologic history of the northern and central Blake Plateau: grading into a basin, are well placed to intercept hydrocarbons migrating Geological Society of America Bulletin, v. 96, p. 627-638. from basinal source beds. Furthermore, they have depositional closure and Piper, D.J.W., and Normark, W. R., 1989, Late Cenozoic sea-level changes and the onset of glaciation: Impact on continental slope progradation off eastern Canada: Marine and Petroleum Geology, v. 6, p. 336-347. may be capped by muds. If clean sands are present, drifts of this type have Popenoe, P., 1985, Cenozoic depositional and structural history of the North Carolina margin from seismic stratigraphic analyses, in Poag, C. W., ed., Stratigraphy and depositional history of the U.S. Atlantic margin: Stroudsburg, reservoir potential and may be worthwhile exploration targets in some Pennsylvania, Van Nostrand Reinhold, p. 125-187. basins. Prakla Seismos, 1974, Marine seismic reflection, gravity and magnetic surveys, Canterbury Bay, New Zealand, for BP, Shell, Todd (Canterbury) Services Limited: New Zealand Geological Survey Open-File Petroleum Report No. 626. Raymo, M. E., Ruddiman, W. F., Backman, J., Clement, B. M., and Martinson, D. G., 1989, Late Pliocene variation in ACKNOWLEDGMENTS Northern Hemisphere ice sheets and North Atlantic deep water circulation: Paleocea nography, v. 4, p. 413-446. Shackleton, N. J., Backman, J., Zimmerman, H., Kent, D. V., Hall, M. A., Roberts, D. G., Schnitker, D., and Baldauf, J., 1984, Oxygen isotope calibration of the onset of ice-rafting and history of glaciation in the North Atlantic region: We gratefully acknowledge the use of open-file seismic data supplied Nature, v. 307, p. 620-623. Tucholke, B. E., and Mountain, G. S., 1979, Seismic stratigraphy, lithostratigraphy and paleosedimentation patterns in the by the New Zealand Geological Survey, Lower Hutt. In particular, we North American Basin, in Talwani, M., Hay, W., and Ryan, W.B.F., eds.. Deep drilling results in the Atlantic Ocean: Continental margins and paleoenvironment: American Geophysical Union, Maurice Ewing Series, No. 3. thank Dr. Richard Cook (NZGS) and Mr. Anthony Shutes (BP New p. 58-86. Zealand Ltd.) for their invaluable assistance in the acquisition of seismic 1986, Tertiary paleoceanography of the western North Atlantic Ocean, in Vogt, P. R., and Tucholke, B. E., eds.. The geology of North America, Volume M, the western North Atlantic region: Geological Society of America, profiles. We also thank Dr. W. P. Dillon and Dr. J. B. Anderson for their p. 631-650. Ward, D. M., and Lewis, D. W., 1975, Paleoenvironmental implications of storm-scoured ichnofossiliferous mid-Tertiary thoughtful reviews of the manuscript. This research was financially sup- limestones, Waihao district. South Canterbury, New Zealand: New Zealand Journal of Geology and Geophysics, ported by the Australian Research Council and by James Cook University v. 18, p. 881-908. Watters, W. A., 1978, Tertiary volcanism—Miocene, in Suggate, R. P., Stevens, G. R., and Te Punga, eds., 1978, The of North Queensland. geology of New Zealand: Wellington, New Zealand, Government Printer, v. 2, p. 637-644. Weissel, J. K., Hayes, D. E., and Herron, E. M., 1977, Plate tectonic synthesis: The displacements between Australia, New Zealand and Antarctica since the Late Cretaceous: Marine Geology, v. 25, p. 231-277. Western Geophysical, 1982, Final operation report, Canterbury Bight. PPLs 38202 and 38203, for BP, Shell, Todd REFERENCES CITED (Canterbury) Services Limited: New Zealand Geological Survey Open-File Petroleum Report No. 898. Wilding, A., and Sweetman, I.A.D., 1971, Endeavour-1, for BP, Aquitaine and Todd Petroleum Development Limited: Andrews. P. B., 1963, Stratigraphic nomenclature of the Omihi and Waikari Formations, north Canterbury: New Zealand New Zealand Geological Survey Open-File Petroleum Report No. 303. Journal of Geology and Geophysics, v. 6, p. 228-256. Wilson, I. R., 1985, Galleon-1 geological completion report, for BP, Shell, Todd (Canterbury) Services Limited: New Barker, P. F„ and Burrell, J., 1977, The opening of the Drake Passage: Marine Geology, v. 25, p. 15-34. Zealand Geological Survey Open-File Petroleum Report No. 1146. Belderson, R. H„ Johnson, M. A„ and Kenyon, N. H„ 1982, Bedforms, in Stride, A, H., ed.. Offshore tidal sands: Yang, Chang-shu, 1989, Active, moribund and buried tidal sand ridges in the East China Sea and Southern Yellow Sea: Processes and Deposits: London, England, Chapman and Hall, p. 27-57. Marine Geology, v. 88, p. 97-116. Benson. W. N„ 1967, Geological map of Dunedin district (with explanatory notes): Wellington, New Zealand, New Zealand Department of Scientific and Industrial Research, scale 1:50,000. Browne, G. H„ and Field, B. D., 1988, A review of Crelaceous-Cenozoic sedimentation and tectonics, east coast. South MANUSCRIPT RECEIVED BY THE SOCIETY NOVEMBER 27, 1989 Island New Zealand, in James, D, P., and Leckie, D. A„ eds.. Sequences, stratigraphy, sedimentology: Surface and REVISED MANUSCRIPT RECEIVED JULY 2, 1990 subsurface: Canadian Society of Petroleum Geologists Memoir 15, p. 37-48. 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