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. 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 CONTINENTAL-SHELF PROGRADATION 301 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.