粘土矿物.pdf

Marine Geology 123 1995 187. Late Quaternary transgressive/regressive sequences from Taranaki continental shelf, western New Zealand Scott D. Nodder New Zealand Oceanographic Institute, National Institute of Water and Atmospheric Research Ltd., P. 0. Box 14.901, Kilbirnie, Wellington, New Zealand Received 16 May 1994; revision accepted 12 January 1995 Abstract Interpretation of seismic reflection profiles 3.5 kHz, airgun, and piston core and borehole stratigraphic logs, collected from mid-outer shelf depths 50-130 m on the Taranaki continental shelf, western New Zealand, have delineated at least 4 alternating late Quaternary seismic and lithologic couplets. Each couplet comprises an acoustically transparent, upper silty unit that overlies a highly reflective sandy unit, characterised by a channelised erosive base. Radiocarbon and biostratigraphic nannofloral dating of selected samples provides chronostratigraphic control for the succession, indicating that the entire sequence is probably late Quaternary in age 600 kyr B.P.. Based mainly on nannofossil paleoenvironmental interpretations and seismic reflection characteristics, it is proposed that the silty units represent interglacial highstand deposits and that the intervening sandy units were deposited either as glacial lowstand or transgressive nearshore deposits that developed under rising sea-level conditions. Erosive, irregular basal unconities were probably developed during relative sea-level fall and represent Type 1 sequence boundaries; there may also have been a component of wave planation during the subsequent transgression. High- amplitude channel-fill seismic units are possibly fluvial and/or paralic sediments, deposited during the sea-level lowstand and/or the ensuing transgression. Tentative correlation of late Quaternary units on the Taranaki shelf is made to the deep-sea oxygen isotope stage curve to provide temporal resolution of the stratigraphic section. Four possible sedimentation models are considered, based on different temporal interpretations of the observed litho- and seismic stratigraphy. Estimated sedimentation rates for each model are compared to existing data-sets as a validation tool. The data are inconclusive, but appear to support scenario 1 which is characterised by moderate average sedimentation rates over the past 300,000 years, typically 0.57 m/kyr, with an anomalous phase of slow deposition or non-deposition and/or erosion during stage 6. 1. Introduction The application of sequence stratigraphic concepts to high-resolution seismic reflection data provide a framework within which one can eluci- date, or at least make inferences about, the late Quaternary evolution of continental margins. The development and distribution of continental shelf facies and sequences are controlled primarily by the magnitude and rate of relative sea-level fluctu- ations which are, in turn, determined by the factors of eustasy, tectonism and sediment supply e.g. Vail et al., 1977; Posamentier et al., 1988; Vail et al., 1991; Schlager, 1993. This paper presents stratigraphic and geophysi- cal ination from the broad, low gradient Taranaki continental shelf, western New Zealand Figs. 1 and 2. This region provides an unique opportunity to study the development of shelf sequences in both time and space over successive 0025-3227/95/9.50 0 1995 Elsevier Science B.V. All rights reserved SSDZ 0025-32279500004-6 188 S. D. Nodder/Marine Geology 123 1995 187-214 1iV AUSTRALIAN ski’ PLATE 15”E SYSTEM Chatham Rise A Active volcanoes WP Western Plat STG South Taranaki Graben NTG North Taranaki Graben SWG South Wanganui Sash Y Walnui-1 Fig. 1. Main structural elements within the greater Cook Strait area the Taranaki and Wanganui basins and the spatial relationship of this region to tectonic components of the active Hikurangi subduction system after Anderton, 1981; Palmer and Bulte, 1991; King and Thrasher, 1992. glacial-interglacial cycles since it is inferred that the high frequency and magnitude of glacio- eustatic sea-level fluctuations during the Plio- Pleistocene and Quaternary resulted in the Taranaki shelf being completely exposed during glacial periods e.g. Fleming, 1962; Beu and Edwards, 1984; Kamp and Turner, 1990. The present study of Quaternary marine deposits beneath the Taranaki shelf also has important implications for the well known Plio-Pleistocene marine deposits and lands exposed in onshore South Wanganui Basin e.g. Fleming, 1953; Pillans, 1990; Kamp and Turner, 1990; Carter et al., 1991. 2. Modern physiographic and oceanographic setting The broad Taranaki continental shelf has a 150 km wide opening to the Tasman Sea and s the western approach to the constricted narrows of Cook Strait Fig. 2. The shelf area occupies approximately 30,000 km2 and slopes gently towards the west with an overall gradient of less than 0.1” and locally less than 0.5”. Prominent morphological features include the 100-200 m deep, NW-SE-trending D’Urville Sea-valley, and the Farewell Rise, a 150 km long, gently sloped submerged high, located between Capes Farewell and Egmont, that separates mid- and inner shelf from outer shelf environments Lewis and Eade, 1974; Nodder and Baldwin, 1992 Figs. 2 and 3. Circulation in the greater Cook Strait region is dominated by storm-induced, wave-generated cur- rents and flows e.g. Lewis, 1979. The wave regime is characterised by locally-generated waves, derived predominantly from the southwest-west, with typical wave heights of l-3 m and wave periods of 6-8 s, superimposed upon a southwesterly back- ground swell with period of 9-12 s Pickrill and Mitchell, 1979 Fig. 4. A northeasterly-directed longshore littoral drift system, induced by the dominant wave approach direction west- southwest, operates along the Westland and northwestern Nelson coasts of the South Island at the southern margin of the Taranaki shelf Chiswell and Bowman, 1985. A similar nearshore circula- tion system also exists within the South Taranaki and Wanganui Bight areas, actively moving sedi- ment in a general southeasterly direction across a shallow nearshore sill Lewis, 1979; Kibblewhite, 1982 Fig. 2. There is only minor from tides and semi- permanent ocean currents to the modern hydraulic regime on the Taranaki shelf Carter and Heath, 1975; Heath, 1982. Tidal currents on the open shelf typically have speeds of less than 0.3 m/s Proctor and Carter, 1989, while the Westland Current, and its easterly-directed offshoot, the D’Urville Current Fig. 2, both have low mean surface speeds, ranging between 0.01 and 0.1 m/s Carter and Heath, 1975. The distribution of surficial sediments broadly reflects variations in the hydraulic regime across Legend Mean circulation patterns / Shelf edge Quatemary gravel, sand, silt and mud m Quatemary volcanic deposits m Tertiary sandstone, siltstone, mudstone and limestone Paleozoic-Mesozoic greywacke and argillite, possibly with coal measures N.W. Nelson loumull Paleozoic quattzite limestone, argilllte, conglomerate and shale Schist and gneiss Granite r - 1 Fig. 3 -- Fig. 2. Location map showing western approaches to Cook Strait. Bathymetric contours are shown in 25 m intervals after Lewis and Eade, 1974 and mean circulation patterns are summarised from Heath 1982 and Kibblewhite 1982. The distribution of main rock types exposed onland and potential sediment sources for the western continental shelf are also depicted modified from Officers of the New Zealand Geological Survey, 1972. 190 S. D. Nodder/Marine Geology 123 1995 187-214 Mud and silt .._ . _ ,. _ .._ -._.--_..--,. - Sand _ . _ _ . ..- --.. -.c -- Gravel -Y .‘-“- .‘ .,_ ; . ,, ,, ,; ; _..._..___...__..__... ,,p _‘y;,_ ._.., .z _ _ .., .__*_r-.7. -. --. Calcareous gravel .-I -_--- and sand _- , Proctor and Carter, 1989; Nodder et al., 1992. the Taranaki shelf with coarse-grained sediments dominating in areas of high current and/or wave activity and fine-grained deposits accumulating in zones of relatively sedate circulation Lewis and Eade, 1974; Carter and Heath, 1975; Proctor and Carter, 1989 Fig. 3. The Farewell Rise com- prises sandy sediments, and separates inner shelf calcareous gravelly muds and sands from terrige- nous silts in the Central Mud Belt. Shelf mud accumulation also occurs in the vicinity of Tasman and Golden Bays and off the Manawatu coast, where currents are generally weakest Lewis and Eade, 1974; Proctor and Carter, 1989. Bouldery, laharic deposits submarine extensions of the onshore Mt Taranaki ring-plain, and silty sedi- ments characterise the Taranaki continental shelf, out past the shelf-break Nodder et al., 1992. 3. Tectonics and subsidence The Taranaki shelf lies in the back-arc region, a considerable distance behind the active Australian- Pacific plate boundary, located along the eastern margin of the North Island Fig. 1. Nonetheless, the evolution of the plate boundary during the Neogene has had marked impact on the geological development of the Taranaki region e.g. Palmer and Bulte, 1991; King and Thrasher, 1992. Increased rates of oblique plate convergence since about 10 Ma have resulted in the uplift and subsequent erosion of the Southern Alps Kamp and Tippett, 1993, which have provided a ready source of sediment to the Taranaki shelf. Enhanced sediment availability has been concomitant with the progressive subsidence of the South Taranaki S.D. Nodder/Marine Geology 123 1995 187-214 191 Maui-A Plat A5 Tasman-i I I I I I , 50 40 30 20 10 0 Frequency Scales for surface wind roses Scale of frequency 0 5 10 I_8 , I I I I a I I I No. in centre shows percentage calms A Winds Original data grouped as recorded 12345678 Height m A Wave height c5 6 1214 Period s A Wave period L Fig. 4. Representative wind and wave data from the Taranaki continental shelf modified after Pickrill and Mitchell, 1979. Graben and the South Wanganui Basin due to downwarping arising from inferred coupling of the crust with the subducting Pacific plate King and Thrasher, 1992; Stern et al., 1992 Fig. 1. Progressive subsidence to the south and uplift to the north Pillans, 1986 has led to the accumula- tion of 4-5 km of mainly shallow marine sediments in the South Wanganui Basin since 5 Ma Anderton, 1981. At the same time, to the west on the Western Plat of the Taranaki Basin, 192 S.D. Nodder/Marine Geology 123 1995 187-214 subsidence, induced partly by sediment loading, has resulted in the preservation of a 2-3 km thick sequence of progradational shelf-margin sedi- ments, known as the Giant Foresets ation Beggs, 1990. Regional tilting of 2”-3” to the southwest of an area that encompasses both onshore Taranaki Peninsula and offshore Taranaki shelf, began in the late Pleistocene e.g. Palmer and Bulte, 1991; King and Thrasher, 1992, and probably continues to present-day. Plio-Pleistocene subsidence rates, estimated from geohistory analysis of offshore exploration wells by Hayward and Wood 1989 range from 0.05-o. 14 m/kyr for Taranaki Basin to 0.3 m/kyr for South Wanganui Basin. An average long-term subsidence rate of approximately 1 m/kyr is suggested by Anderton 1981 and Stern et al. 1992 for the past 4 Myr in the South Wanganui Basin. Additionally, Norris and Grant- Taylor 1989 have inferred a subsidence rate for the western Taranaki shelf of between 2.3 and 2.8 m/kyr for the past 20,000 years. 4. Sediment supply Modern sedimentation patterns and rates suggest that the west coast of the South Island is presently a major source of sediments deposited on the western North Island shelf via a northeasterly- directed longshore drift sediment transport system Carter and Heath, 1975; Griffiths and Glasby, 1985 Fig. 2. Estimates of the contribution of suspended sediment to the continental shelf by west coast South Island rivers range from 21240 x lo6 Griffiths and Glasby, 1985 to 44OSO x lo6 tonnes/yr Adams, 1980, and have led to the generation of Farewell Spit Tahuroa since the end of the last glacial ca. 14 kyr B.P. Present-day estimates of sedimentation rates on the west coast South Island range from 0.5 to 2.3 mm/yr Van der Linden, 1969; Norris, 1978; Probert and Swanson, 1985. It is inferred that the influence of fluviatile of sediment to the western shelf by South Island rivers has also contributed to the Plio- Pleistocene progradation of the continental shelf Beggs, 1990 and to the inferred late Quaternary development of the Farewell Rise as a paleo-barrier or spit Nodder, 1991. On the west coast South Island, river-borne terrigenous detritus is derived predominantly from the erosion of the metamorphic and plutonic rocks of the Southern Alps Adams, 1980 Fig. 2. Additional onshore sources of sediment delivered to the Taranaki shelf include volcanic deposits associated with Mt Taranaki and central North Island Lewis, 1979 Plio-Pleistocene sedimentary rocks of the Wanganui Basin, and, to a lesser extent, the Mesozoic basement rocks of the North Island Axial Ranges Fig. 2. Contributions of suspended sediment to the western continental shelf, south of Cape Egmont, by North Island rivers is, however, considerably less than the amounts supplied by South Island fluvial systems i.e. approximately 9 x lo6 tonnes/yr; Griffiths and Glasby, 1985. Similarly, northern South Island rivers only supply a fraction of the suspended material delivered to the continental shelf by west coast South Island rivers i.e. approximately 4 x lo6 tonnes/yr; Griffiths and Glasby, 1985. 5. Eustatic sea-level change Determining the magnitude and timing of eustatic i.e. global sea-level fluctuations from shelf successions is usually confounded by the influences of subsidence and sediment supply, two factors that are generally local in extent. Nonetheless, there is compelling evidence to sug- gest that eustatic sea-level cycles during the late Neogene were both glacio-eustatic in origin and often exceeded 100 m in amplitude e.g. Imbrie et al., 1984; Chappell and Shackleton, 1986. Carter et al. 1991 and Abbott and Carter 1994 have proposed that fifth-order sea-level cycles are reflected within the onshore cyclothemic Plio- Pleistocene Wanganui succession and mimic the influence of orbital forcing, arising from Milankovitch frequencies, in particular, obliquity fifth-order; with a periodicity of ca. 100 kyr, tilt sixth-order; ca. 40 kyr, and precession seventh- order; ca. 20 kyr. The ongoing subsidence of the Taranaki-Wanganui region suggests that correla- tive successions are likely to be preserved in off- shore areas. SD. NodderJMarine Geology 123 1995 187-214 193 zlBB Parallel even reflectors 3.5 kHz - - - - - - Airgun only CR2001 ----------- 3.5 kHz only 3.5 kHz survey tracks and