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Seismic stratigraphy and evolution of the Raggatt Basin, southern Kerguelen Plateau
Seismic stratigraphy and evolution of the Raggatt Basin, southern Kerguelen Plateau J. B. Colwell, M. F. Coffin, C. J. Pigram, H. L. Davies, H. M. J. Stagg and P. J. Hill Bureau of Mineral Resources, G e o l o g y and Geophysics, Canberra, ACT, Australia
Received 16August 1987; revised5 September 1987 Six major seismic stratigraphic sequences in the Raggatt Basin on the southern Kerguelen Plateau overlie a basement complex of Cretaceous or greater age. The complex includes dipping reflectors which were apparently folded and eroded before the Raggatt Basin developed. The seismic stratigraphic sequences include a basal unit F, which fills depressions in basement; a thick unit, E, which has a mounded upper surface (volcanic or carbonate mounds); a depressionfilling unit, D; a thick unit C which is partly Middle to Late Eocene; and two post-Eocene units, A and B, which are relatively thin and more limited in areal extent than the underlying sequences. A mid or Late Cretaceous erosional episode was followed by subsidence and basin development, interrupted by major erosion in the mid Tertiary. Late Cenozoic sedimentation was affected by vigorous ocean currents. Keywords: Southern Indian Ocean; Kerguelen Plateau; Raggatt Basin; Cretaceous; seismic stratigraphy; tectonic evolution; ocean currents
Introduction The Kerguelen Plateau, in the southern Indian Ocean, is 2100 km long and generally 500 km across (Figure I). The northern part of the plateau, which includes the volcanic Kerguelen Islands and Heard Island, lies for the most part less than 1000 m below sea level. The southern plateau, south of 54°S, lies deeper at 10003000 m. The first multichannel seismic (MCS) investigation of the southern plateau, by the Australian Bureau of Mineral Resources, Geology and Geophysics (BMR) in 1985, outlined a major sedimentary basin, the Raggatt Basin, with up to 4000 m of sediment (Ramsay et al., 1986a, b). These data, supplemented by data from a subsequent cruise of the N/O Marion Dufresne (R. Schlich et al., in press), have provided the basis for the selection of sites for the 1987-88 Ocean Drilling Program investigation of the southern plateau. In this paper we summarize the seismic stratigraphy of the Raggatt Basin, interpret the seismic data in the light of geological sampling (coring and dredging), and develop a scheme for the evolution of the plateau.
Seismic stratigraphy Characteristics of the six major seismic stratigraphic sequences (Figure 2) which make up the Raggatt Basin are summarized in Table 1. These sequences overlie a basement complex which includes low-frequency dipping reflectors (Figure 2). In places, for example along the eastern margin of the plateau (Figures 2b and 3), the reflectors appear to be gently folded. The dipping reflectors are truncated upwards by an angular unconformity, at the interface between the basement complex and the overlying sediments (Figure 3). Dredging of scarps at the western edge of the Raggatt Basin yielded Lower Cretaceous basaltic lava and lava breccia, of ocean island chemical affinity, apparently from
within the basement complex (Leclaire et al., 1987). However these sites were located along major faults that may have been the loci of intrusive activity. Sequence F, the oldest of the seismic stratigraphic sequences, is confined to the deepest part of the basin, where it fills lows in pre-existing basement complex topography. It is overlain unconformably by a relatively thick unit, sequence E, the upper surface of which is characterized by mounds of probable volcanic or carbonate origin. MCS data show that the depocentre for sequences F and E was situated to the west of the depocentre of sequences C and D. Sequence D generally onlaps and drapes E at the western flank of the basin, and thins and wedges out on the eastern margin. Part of the unit in the vicinity of the mounds downlaps against the unconformity at the base of the sequence; these sediments were either derived locally from erosion of the mounds or deposited (possibly as contourites) against the mounds. Most of the faulting within the basin is confined to sequences F-D, with only rare growth faults extending into or through the overlying sequences (Figure 3). Sequence C is relatively thick, and is truncated by the seafloor on the basin's eastern flank. Sequences A and B occur only in the central, shallower part of the basin, and are truncated to the east and west due to erosion or non-deposition.
Age control Age control for the seismic stratigraphic sequences is limited to samples obtained by piston coring aboard USNS Eltanin cruises 47 and 54 (Markl, 1973; Quilty, 1973; Kennett and Watkins, 1976), and coring and dredging aboard N/O Marion Dufresne cruise 48 (Leclaire et al., 1987). Cores 54-7, 48-701, and 48-703 (Table 2; Figure 3) recovered pelagic carbonates of Cenomanian, Maastrichtian, and undifferentiated Cretaceious age, respectively, from the basement complex
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Seismic stratigraphy and evo/ution of Raggatt Basin: J. B. Co~well et al. 60 °
70 °
80 °
%o
)ZET
50 °
\
\
ENDERBY
BASI
60 °
(_____J/t
500kmj
Raggatt Basin 23/02/86-2
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Figure 1 The Raggatt Basin on the southern Kerguelen Plateau. Bathymetry is from Coffin eta/. (1986). Contour interval is 500 m. Heavy lines indicate 1985 RE Rig SeismicMCS profiles; light lines indicate analogue single channel seismic lines, mainly recorded by USNS Eltanin in 1971/72
or from thin, discontinuous layers ot sediment immediately overlying the basement complex. Cores 48704, 54.3, 54-4, 48-698, and 48-699 sampled Eocene pelagic carbonates from unit C (Figure 3). Pliocene pelagic carbonates were encountered in cores 54-9, 54-10, and 54-11 from unit A and possibly unit B, and Pleistocene and younger pelagic oozes from unit A were sampled by cores 47-15 and 48-697. Chert dredged during Marion Dufresne cruise 48 from the basement complex or unit E contains Late Cretaceous radiolaria (Leclaire, et al., 1987). Thus we interpret seismic sequences F-C to be ?Late Cretaceous to Eocene, and sequences A and B to be post-Eocene, and possibly entirely Neogene and Quaternary, in age. Cores 48-700 and 48-702 sampled thin Pleistocene sediments overlying the basement complex adjacent to the eastern boundary of the basin, and core 54-8 recovered Pliocene sediments from a similar position. 76
Comparison with other seismic stratigraphic studies The only previous study of the seismic stratigraphy of the southern Kerguelen Plateau identified three major reflectors on single channel seismic reflection data (Houtz et al., 1977). The upper reflectors, ~A' and 'B', were interpreted as Eocene and Cenomanian respectively, and the deepest reflector, 'C', only rarely observed, was interpreted as basement. The prominent unconformity between our sequences D and E is equivalent to the 'A' reflector, and the strong unconformity at the top of our basement complex correlates with 'B'. Layering within the basement complex is equivalent to 'C'. More is known of the seismic stratigraphy of a basin on the northern plateau, where Munschy and Schlich (1987) have defined five seismic sequences from ex-
M a r i n e a n d P e t r o l e u m G e o l o g y , 1988, V o l 5, F e b r u a r y
Seismic stratigraphy and evo/ution of Raggatt Basin," J, B. Co/we//et al.
A
A
v
V.E. at seob0tt0m: ~
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B
A
V.E. at sea bottom: ~
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F i g u r e 2 Uninterpreted and interpreted 48-channel migrated seismic reflection records (time displays with 12-fold stacking). The seismic system consisted of a 1.2 km hydrophone streamer and 16.4 litre airgun array. See Figure 1 for profile locations. (A) Profile across the central Raggatt Basin. Note mound development; (B) profile across the eastern flank of the Raggatt Basin. Note the dipping strata in basement complex
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Seismic stratigraphy and evolution of Raggatt Basin: J. B. Colwell et al. Table 1 Seismic sequences in the Raggatt Basin Sequence
:~imum ~icKness
Upper
Lower
Seismic characteristics
boundary
boundary
Configuration
Continuity
Amplitude
Frequency
(seconds twoway time)
seafloor, commonly erosional
concordant, erosional at basin margins
parallel
high
high
moderate to high
0.3
Recent?Pliocene
concordant, erosional at basin margins
erosional, onlap
parallel
moderate
low
moderate
0.2
Pliocene - ?
concordant, erosional at basin margins
erosional, onlap
parallel
moderate to high
low to moderate
moderate to high
0.6
concordant to erosional; erosional at basin margins
downlap adjacent to mounds; concordant elsewhere
highly variable
low to moderate
low to moderate
concordant, erosional at basin margins
mounds or erosional, onlap
[upper] chaotic
low
moderate
[lower] parallel
0.4
1.0 (total)
Eocene-?Late Cretaceous
high, but high with some abrupt character changes
F
concordant
erosional, onlap
parallel
basement complex
erosional
?
highly variable character ranging from reflection-free to high amplitude, low frequency dipping reflectors
moderateto poor
amination of MCS data. They define two upper units, $1 and $2, which we tentatively correlate with our sequences A and B, and three lower units I1 to I3 (Figure 4). The latter are presumably equivalent to our sequences C to F, but available data do not permit more precise correlation. Sequences $1 and $2 have a maximum thickness of 1400 m, and are thus much thicker than A and B. They are characterized by thickening and thinning of sediment packages, and rolling reflector surfaces - - presumably both features the effect of strong bottom currents. A prominent unconformity between $2 and I1 is referred to as the 'acoustic discordance'. It is more prominent than the equivalent unconformity between sequences B and C on the southern plateau. Geological sampling has demonstrated that the 'acoustic discordance' of the northern plateau represents an erosional interval that lasted from after the Middle Eocene to Late Oligocene or Early Miocene (Wicquart and Frohlich, 1986). The upper units, S1 and $2, are calcareous and siliceous ooze, giving way upwards to diatomite with much ice-rafted debris. The lower units, I2 and I 1, are calcareous ooze and chalk of Late Cretaceous to Middle Eocene age, with volcanic detritus and argillite immediately below the 'acoustic discordance' (Wicquart and Frohlich, 1986). Sequence I3 was not sampled but the base is probably Early
78
Age
moderate
0.5
Cretaceous, 100 to 120 Ma, if sediments accumulated at a constant rate (Wicquart and Frohlich, 1986).
Evolution of the plateau From our interpretation of the seismic stratigraphy we develop a model for the evolution of the Raggatt Basin. The numbers below correspond to those in the sequence of cross-sections (Figure 3). 1. The basement complex developed during or before the Late Cretaceous. It may be either (a) primarily a volcanic complex, or (b) rifted, thinned continental crust with associated volcanics (Leclaire et al., 1987). Dipping reflectors within basement may be alternating lavas and volcaniclastic sediments, as has been demonstrated to be the case in the Rockall and Voring Plateaus (Roberts et al., 1984; Eldholm, Thiede, et al., 1986), or strata within the continental crust. Unlike the dipping reflectors of the Rockall and Voring Plateaus, those within the Kerguelen Plateau do not have consistent dips. 2. The reflectors within the basement complex appear to have been folded, but it remains possible that the dips are primary depositional phenomena, or are due to tilting by block-faulting. The basement complex was probably partly emergent, and was certain-
Marine and Petroleum Geology, 1988, Vol 5, February
Seismic stratigraphy and evolution of Raggatt Basin: J. B. Co/well
MD 48-697
I WEST
ELT 54-3.4
ELT 47-15
et al.
EAST
I ELT 54-9,10,11 ;-700,702 54-8 4D 48-701, 7O3 ELT 54-7
7 PRESENT DAY
5
? LATE CRETACEOUS
23/02/109; Figure 3 Evolutionary cartoon for the Raggatt Basin showing approximate stratigraphic position of piston cores, See Table 2 for core details
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Seismic stratigraphy and evolution of Raggatt Basin: J. B. Co/well et al. final stages of deposition of sequence E, probably Table 2 Location, length, water depth, and bottom age for piston cores located in the area of the Raggatt Basin towards the end of the Cretaceous, included the development of carbonate or volcanic mounds. This Water Core was possibly a time of uplift (Leclaire et al., 1987). Core Latitude Longitude depth length Bottom age . Continuing subsidence permitted the deposition of (S) (E) (m) (cm) sequence D, partly as in-fill between mounds of E. 48-704 57° 17' 77 ° 48' 1890 245 Late Eocene . Sequence C, which includes sediments as young as 54-3 57° 26' 77 ° 50' 1890 22 Late Eocene Eocene pelagic carbonates (Kennett and Watkins, 54-4 57° 26' 77 ° 53' 1828 82 Middle Eocene 1976; Leclaire et al., 1987) was deposited as the 47-15 57° 17' 78 ° 48' 1603 498 Pleistocene basin subsided further. 48-697 57° 23' 79 ° 39' 1680 735 Pleistocene . Probable erosion after the end of the Eocene was 48-698 57° 15' 80 ° 10' 1980 627 Late Eocene 54-9 57° 44' 80 ° 16' 1634 452 Pliocene followed by deposition of the post-Eocene sequ54-10 57° 45' 80 ° 40' 1706 513 Pliocene ences, A and B. These sequences are only a few 48-699 57° 08' 80 ° 57' 2625 375 Late Eocene hundred m thick, and are of limited areal extent. 54-11 57° 47' 81° 01' 1800 322 Pliocene The thin and incomplete cover of post-Eocene sedi54-7 55° 53' 81 ° 07' 4021 483 Cenomanian 48-700 57° 06' 81 ° 11' 3130 134 Pleistocene ments is almost certainly due to the onset of the 54-8 56o52 , 81 ° 11' 4158 582 Pliocene Antarctic Circumpolar Current in the Oligocene 48-702 57006 ' 81 ° 13' 3115 605 Pleistocene (Kennett and Watkins, 1976). Other effects of vigor48-701 57° 04' 81 ° 23' 3850 788 Maastrichtian ous bottom currents are scouring of sediment at the 48-703 57° 04' 81 ° 24' 3800 1203 Cretaceous base of fault scarps, and the deposition of scour-fill sediments and possible contourites within the Late ly partly eroded, before the development of the Cenezoic section. Raggatt Basin. 3. The development of the Raggatt Basin began with local subsidence to form a trough which filled with Regional implications sediments (sequence F). The sediments were deThe evolution of the Kerguelen Plateau and its conjurived, at least in part, from adjacent highs. Timing gate feature, Broken Ridge, dates from at least the of this event is uncertain, but was probably mid- or Early Cretaceous (Leclaire et al., 1987), and conseLate Cretaceous. quently the thick sediments and underlying basement 4. Subsidence continued and a relatively-thick sequcomplex of the Raggatt Basin record much of the ence of sediments (sequence E) was deposited. The history of the Indian Ocean. The Cretaceous and older tectonic framework of the region is poorly understood SEISMIC SEQUENCE AGE ia due to the lack of age control for the Enderby and 0Wharton Basins; however seismic reflection data from PLEISTOCENE both the northern (Munschy and Schlich, 1987) and S1 PLIOCENE southern plateau indicate that the plateau subsided 10? steadily through the Late Cretaceous. Subsequently Broken Ridge emerged prior to the Middle Eocene MIOCENE (Davies, Luyendyk, et al., 1974), and the northern part $2 20of the Kerguelen Plateau emerged for a time after the Middle Eocene. It is probable that the southern part emerged after the end of the Eocene, coinciding with 30the onset of rapid seafloor spreading at the Southeast OLIGOCENE Indian Ridge (Cande and Mutter, 1982). ~ACOUSTICDISCORDANCE' 40-
Acknowledgements EOCENE
We thank the master, crew, and technical staff aboard the R/V Rig, Seismic for their skillful assistance and cooperation, and Phil Symonds, Barry Willcox, and Paul Williamson for their constructive comments. We are grateful to Marc Munschy (IPG-Strasbourg) for fruitful discussions. Publication is by permission of the Director, Bureau of Mineral Resources, Geology and Geophysics, Canberra.
50-
PALEOCENE
60-
Maestrichtian
70- u)
12
O
~
Campanian
~
Santonian
<
References
80-
90-
~ l Coniacian u.l! Turonian
Cenomanian
13
Albian 1(]0
23/02/114
Figure 4 Seismic stratrigraphic sequences of a basin on the northern Kerguelen Plateau (after Munschy and Schlich, 1987)
80
Cande, S. C. and Mutter, J. C. (1982) A revised identification of the oldest sea-floor spreading anomalies between Australia and Antarctica Earth Planet. Sci. Lett. 58, 151-160 Coffin, M. F., Davies, H. L. and Haxby, W. F. (1986) Structure of the Kerguelen Plateau province f r o m Seasat altimetry and seismic reflection data Nature 324, 134-136 Davies, T., Luyendyk, B., et al. (1974) Initial Reports of the Deep Sea Drilling Project 26, US G o v e r n m e n t Printing Office, Washington, D.C.
Marine and Petroleum Geology, 1988, Vol 5, February
S e i s m i c s t r a t i g r a p h y a n d e v o l u t i o n o f R a g g a t t B a s i n : J. B. C o / w e l l et al. Eldholm. O., Thiede, J., eta/. (1986) Formation of the Norwegian Sea Nature 319, 360-361 Houtz, R. E. Hayes, D. E. and Markl, R. G. (1977) Kerguelen Plateau bathymetry, sediment distribution, and crustal structure Marine GeoL 25, 95-130 Kennett, J. P. and Watkins, N. D. (1976) Regional deep-sea dynamic processes recorded by late Cenozoic sediments of the southeastern Indian Ocean GeoL Soc. Am. Bull. 87, 321-339 Leclaire, L., Bassias, Y., Denis-Clocchiatti, M., Davies, H., Gautier, I., Gensous, B., Giannesini, P. J., Morand, F., Patriat, P., Segoufin, J., Tesson, M. and Wannesson, J. (1987) Lower Cretaceous basalt and sediments from the Kerguelen Plateau Geomarine Lett. 7 (4), in press Markl, R. G. (1973) USNS Eltanin Cruise 54 to the Kerguelen Plateau and Southeast Indian Rise Antarct. J. U. S. 8, 6-8 Munschy, M., and Schlich, R. (1987) Structure and evolution of the Kerguelen-Heard Plateau (Indian Ocean) deduced from seismic stratigraphy studies Marine Geol. 76, 131-152 Quilty, P. G. (1973) Cenomanian-Turonian and Neogene sediments from northeast of Kerguelen Ridge J. Geol. Soc. Aust. 20, 361-368
Ramsay, D. C. Colwell, J. B., etaL (1986a) RIG SEISMIC research cruise 2: Kerguelen Plateau - initial report Bur. Min. Resour. Aust. Report 270, 41 pp. Ramsay, D. C., Colwell, J. B., Coffin, M. F., Davies, H. L., Hill, P. J., Pigram, C. J. and Stagg, H. M. J. (1986b) New findings from the Kerguelen Plateau Geology 14, 589-593 Roberts, D. G., Backman, J., Morton, A. C., Murray, J. W. and Keene, J. B. (1984) Evolution of volcanic rifted margins: synthesis of Leg 81 results on the west margin of the Rockall Plateau Initial Reports of the Deep Sea Drilling Proiect 81, US Government Printing Office, Washington, D. C., 883-911 Schlich, R., Munschy, M., Boulanger, D., Cantin, B., Coffin, M. F., Durand, J., Humler, E., Li, Z. G., Savary, J., Schaning, M. and Tissot, J.D. (1987) Resultats preliminaires de la campagne oceanographic de sismique reflexion multitraces MD47 dans le domaine sud du plateau de kerguelen C.R. Acad. ScL Paris (in press) Wicquart, E. and Frohlich, F, (1986) The sedimentation on the Kerguelen-Heard Plateau. Relationships with the evolution of the Indian Ocean during the Cenozoic Bull. Soc. Geol. France 11 (4), 569-574
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Received 16August 1987; revised5 September 1987 Six major seismic stratigraphic sequences in the Raggatt Basin on the southern Kerguelen Plateau overlie a basement complex of Cretaceous or greater age. The complex includes dipping reflectors which were apparently folded and eroded before the Raggatt Basin developed. The seismic stratigraphic sequences include a basal unit F, which fills depressions in basement; a thick unit, E, which has a mounded upper surface (volcanic or carbonate mounds); a depressionfilling unit, D; a thick unit C which is partly Middle to Late Eocene; and two post-Eocene units, A and B, which are relatively thin and more limited in areal extent than the underlying sequences. A mid or Late Cretaceous erosional episode was followed by subsidence and basin development, interrupted by major erosion in the mid Tertiary. Late Cenozoic sedimentation was affected by vigorous ocean currents. Keywords: Southern Indian Ocean; Kerguelen Plateau; Raggatt Basin; Cretaceous; seismic stratigraphy; tectonic evolution; ocean currents
Introduction The Kerguelen Plateau, in the southern Indian Ocean, is 2100 km long and generally 500 km across (Figure I). The northern part of the plateau, which includes the volcanic Kerguelen Islands and Heard Island, lies for the most part less than 1000 m below sea level. The southern plateau, south of 54°S, lies deeper at 10003000 m. The first multichannel seismic (MCS) investigation of the southern plateau, by the Australian Bureau of Mineral Resources, Geology and Geophysics (BMR) in 1985, outlined a major sedimentary basin, the Raggatt Basin, with up to 4000 m of sediment (Ramsay et al., 1986a, b). These data, supplemented by data from a subsequent cruise of the N/O Marion Dufresne (R. Schlich et al., in press), have provided the basis for the selection of sites for the 1987-88 Ocean Drilling Program investigation of the southern plateau. In this paper we summarize the seismic stratigraphy of the Raggatt Basin, interpret the seismic data in the light of geological sampling (coring and dredging), and develop a scheme for the evolution of the plateau.
Seismic stratigraphy Characteristics of the six major seismic stratigraphic sequences (Figure 2) which make up the Raggatt Basin are summarized in Table 1. These sequences overlie a basement complex which includes low-frequency dipping reflectors (Figure 2). In places, for example along the eastern margin of the plateau (Figures 2b and 3), the reflectors appear to be gently folded. The dipping reflectors are truncated upwards by an angular unconformity, at the interface between the basement complex and the overlying sediments (Figure 3). Dredging of scarps at the western edge of the Raggatt Basin yielded Lower Cretaceous basaltic lava and lava breccia, of ocean island chemical affinity, apparently from
within the basement complex (Leclaire et al., 1987). However these sites were located along major faults that may have been the loci of intrusive activity. Sequence F, the oldest of the seismic stratigraphic sequences, is confined to the deepest part of the basin, where it fills lows in pre-existing basement complex topography. It is overlain unconformably by a relatively thick unit, sequence E, the upper surface of which is characterized by mounds of probable volcanic or carbonate origin. MCS data show that the depocentre for sequences F and E was situated to the west of the depocentre of sequences C and D. Sequence D generally onlaps and drapes E at the western flank of the basin, and thins and wedges out on the eastern margin. Part of the unit in the vicinity of the mounds downlaps against the unconformity at the base of the sequence; these sediments were either derived locally from erosion of the mounds or deposited (possibly as contourites) against the mounds. Most of the faulting within the basin is confined to sequences F-D, with only rare growth faults extending into or through the overlying sequences (Figure 3). Sequence C is relatively thick, and is truncated by the seafloor on the basin's eastern flank. Sequences A and B occur only in the central, shallower part of the basin, and are truncated to the east and west due to erosion or non-deposition.
Age control Age control for the seismic stratigraphic sequences is limited to samples obtained by piston coring aboard USNS Eltanin cruises 47 and 54 (Markl, 1973; Quilty, 1973; Kennett and Watkins, 1976), and coring and dredging aboard N/O Marion Dufresne cruise 48 (Leclaire et al., 1987). Cores 54-7, 48-701, and 48-703 (Table 2; Figure 3) recovered pelagic carbonates of Cenomanian, Maastrichtian, and undifferentiated Cretaceious age, respectively, from the basement complex
0264-8172/88/010075-07 $03.00 ©1988 Butterworth & Co. (Publishers) Ltd Marine and Petroleum Geology, 1988, Vol 5, February
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Seismic stratigraphy and evo/ution of Raggatt Basin: J. B. Co~well et al. 60 °
70 °
80 °
%o
)ZET
50 °
\
\
ENDERBY
BASI
60 °
(_____J/t
500kmj
Raggatt Basin 23/02/86-2
63°
Figure 1 The Raggatt Basin on the southern Kerguelen Plateau. Bathymetry is from Coffin eta/. (1986). Contour interval is 500 m. Heavy lines indicate 1985 RE Rig SeismicMCS profiles; light lines indicate analogue single channel seismic lines, mainly recorded by USNS Eltanin in 1971/72
or from thin, discontinuous layers ot sediment immediately overlying the basement complex. Cores 48704, 54.3, 54-4, 48-698, and 48-699 sampled Eocene pelagic carbonates from unit C (Figure 3). Pliocene pelagic carbonates were encountered in cores 54-9, 54-10, and 54-11 from unit A and possibly unit B, and Pleistocene and younger pelagic oozes from unit A were sampled by cores 47-15 and 48-697. Chert dredged during Marion Dufresne cruise 48 from the basement complex or unit E contains Late Cretaceous radiolaria (Leclaire, et al., 1987). Thus we interpret seismic sequences F-C to be ?Late Cretaceous to Eocene, and sequences A and B to be post-Eocene, and possibly entirely Neogene and Quaternary, in age. Cores 48-700 and 48-702 sampled thin Pleistocene sediments overlying the basement complex adjacent to the eastern boundary of the basin, and core 54-8 recovered Pliocene sediments from a similar position. 76
Comparison with other seismic stratigraphic studies The only previous study of the seismic stratigraphy of the southern Kerguelen Plateau identified three major reflectors on single channel seismic reflection data (Houtz et al., 1977). The upper reflectors, ~A' and 'B', were interpreted as Eocene and Cenomanian respectively, and the deepest reflector, 'C', only rarely observed, was interpreted as basement. The prominent unconformity between our sequences D and E is equivalent to the 'A' reflector, and the strong unconformity at the top of our basement complex correlates with 'B'. Layering within the basement complex is equivalent to 'C'. More is known of the seismic stratigraphy of a basin on the northern plateau, where Munschy and Schlich (1987) have defined five seismic sequences from ex-
M a r i n e a n d P e t r o l e u m G e o l o g y , 1988, V o l 5, F e b r u a r y
Seismic stratigraphy and evo/ution of Raggatt Basin," J, B. Co/we//et al.
A
A
v
V.E. at seob0tt0m: ~
23/02/107
3:1
2 km J
B
A
V.E. at sea bottom: ~
3:1
0 [
2 km
23/O2/108
-5
J
F i g u r e 2 Uninterpreted and interpreted 48-channel migrated seismic reflection records (time displays with 12-fold stacking). The seismic system consisted of a 1.2 km hydrophone streamer and 16.4 litre airgun array. See Figure 1 for profile locations. (A) Profile across the central Raggatt Basin. Note mound development; (B) profile across the eastern flank of the Raggatt Basin. Note the dipping strata in basement complex
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Seismic stratigraphy and evolution of Raggatt Basin: J. B. Colwell et al. Table 1 Seismic sequences in the Raggatt Basin Sequence
:~imum ~icKness
Upper
Lower
Seismic characteristics
boundary
boundary
Configuration
Continuity
Amplitude
Frequency
(seconds twoway time)
seafloor, commonly erosional
concordant, erosional at basin margins
parallel
high
high
moderate to high
0.3
Recent?Pliocene
concordant, erosional at basin margins
erosional, onlap
parallel
moderate
low
moderate
0.2
Pliocene - ?
concordant, erosional at basin margins
erosional, onlap
parallel
moderate to high
low to moderate
moderate to high
0.6
concordant to erosional; erosional at basin margins
downlap adjacent to mounds; concordant elsewhere
highly variable
low to moderate
low to moderate
concordant, erosional at basin margins
mounds or erosional, onlap
[upper] chaotic
low
moderate
[lower] parallel
0.4
1.0 (total)
Eocene-?Late Cretaceous
high, but high with some abrupt character changes
F
concordant
erosional, onlap
parallel
basement complex
erosional
?
highly variable character ranging from reflection-free to high amplitude, low frequency dipping reflectors
moderateto poor
amination of MCS data. They define two upper units, $1 and $2, which we tentatively correlate with our sequences A and B, and three lower units I1 to I3 (Figure 4). The latter are presumably equivalent to our sequences C to F, but available data do not permit more precise correlation. Sequences $1 and $2 have a maximum thickness of 1400 m, and are thus much thicker than A and B. They are characterized by thickening and thinning of sediment packages, and rolling reflector surfaces - - presumably both features the effect of strong bottom currents. A prominent unconformity between $2 and I1 is referred to as the 'acoustic discordance'. It is more prominent than the equivalent unconformity between sequences B and C on the southern plateau. Geological sampling has demonstrated that the 'acoustic discordance' of the northern plateau represents an erosional interval that lasted from after the Middle Eocene to Late Oligocene or Early Miocene (Wicquart and Frohlich, 1986). The upper units, S1 and $2, are calcareous and siliceous ooze, giving way upwards to diatomite with much ice-rafted debris. The lower units, I2 and I 1, are calcareous ooze and chalk of Late Cretaceous to Middle Eocene age, with volcanic detritus and argillite immediately below the 'acoustic discordance' (Wicquart and Frohlich, 1986). Sequence I3 was not sampled but the base is probably Early
78
Age
moderate
0.5
Cretaceous, 100 to 120 Ma, if sediments accumulated at a constant rate (Wicquart and Frohlich, 1986).
Evolution of the plateau From our interpretation of the seismic stratigraphy we develop a model for the evolution of the Raggatt Basin. The numbers below correspond to those in the sequence of cross-sections (Figure 3). 1. The basement complex developed during or before the Late Cretaceous. It may be either (a) primarily a volcanic complex, or (b) rifted, thinned continental crust with associated volcanics (Leclaire et al., 1987). Dipping reflectors within basement may be alternating lavas and volcaniclastic sediments, as has been demonstrated to be the case in the Rockall and Voring Plateaus (Roberts et al., 1984; Eldholm, Thiede, et al., 1986), or strata within the continental crust. Unlike the dipping reflectors of the Rockall and Voring Plateaus, those within the Kerguelen Plateau do not have consistent dips. 2. The reflectors within the basement complex appear to have been folded, but it remains possible that the dips are primary depositional phenomena, or are due to tilting by block-faulting. The basement complex was probably partly emergent, and was certain-
Marine and Petroleum Geology, 1988, Vol 5, February
Seismic stratigraphy and evolution of Raggatt Basin: J. B. Co/well
MD 48-697
I WEST
ELT 54-3.4
ELT 47-15
et al.
EAST
I ELT 54-9,10,11 ;-700,702 54-8 4D 48-701, 7O3 ELT 54-7
7 PRESENT DAY
5
? LATE CRETACEOUS
23/02/109; Figure 3 Evolutionary cartoon for the Raggatt Basin showing approximate stratigraphic position of piston cores, See Table 2 for core details
Marine and Petroleum Geology, 1988, Vol 5, February
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Seismic stratigraphy and evolution of Raggatt Basin: J. B. Co/well et al. final stages of deposition of sequence E, probably Table 2 Location, length, water depth, and bottom age for piston cores located in the area of the Raggatt Basin towards the end of the Cretaceous, included the development of carbonate or volcanic mounds. This Water Core was possibly a time of uplift (Leclaire et al., 1987). Core Latitude Longitude depth length Bottom age . Continuing subsidence permitted the deposition of (S) (E) (m) (cm) sequence D, partly as in-fill between mounds of E. 48-704 57° 17' 77 ° 48' 1890 245 Late Eocene . Sequence C, which includes sediments as young as 54-3 57° 26' 77 ° 50' 1890 22 Late Eocene Eocene pelagic carbonates (Kennett and Watkins, 54-4 57° 26' 77 ° 53' 1828 82 Middle Eocene 1976; Leclaire et al., 1987) was deposited as the 47-15 57° 17' 78 ° 48' 1603 498 Pleistocene basin subsided further. 48-697 57° 23' 79 ° 39' 1680 735 Pleistocene . Probable erosion after the end of the Eocene was 48-698 57° 15' 80 ° 10' 1980 627 Late Eocene 54-9 57° 44' 80 ° 16' 1634 452 Pliocene followed by deposition of the post-Eocene sequ54-10 57° 45' 80 ° 40' 1706 513 Pliocene ences, A and B. These sequences are only a few 48-699 57° 08' 80 ° 57' 2625 375 Late Eocene hundred m thick, and are of limited areal extent. 54-11 57° 47' 81° 01' 1800 322 Pliocene The thin and incomplete cover of post-Eocene sedi54-7 55° 53' 81 ° 07' 4021 483 Cenomanian 48-700 57° 06' 81 ° 11' 3130 134 Pleistocene ments is almost certainly due to the onset of the 54-8 56o52 , 81 ° 11' 4158 582 Pliocene Antarctic Circumpolar Current in the Oligocene 48-702 57006 ' 81 ° 13' 3115 605 Pleistocene (Kennett and Watkins, 1976). Other effects of vigor48-701 57° 04' 81 ° 23' 3850 788 Maastrichtian ous bottom currents are scouring of sediment at the 48-703 57° 04' 81 ° 24' 3800 1203 Cretaceous base of fault scarps, and the deposition of scour-fill sediments and possible contourites within the Late ly partly eroded, before the development of the Cenezoic section. Raggatt Basin. 3. The development of the Raggatt Basin began with local subsidence to form a trough which filled with Regional implications sediments (sequence F). The sediments were deThe evolution of the Kerguelen Plateau and its conjurived, at least in part, from adjacent highs. Timing gate feature, Broken Ridge, dates from at least the of this event is uncertain, but was probably mid- or Early Cretaceous (Leclaire et al., 1987), and conseLate Cretaceous. quently the thick sediments and underlying basement 4. Subsidence continued and a relatively-thick sequcomplex of the Raggatt Basin record much of the ence of sediments (sequence E) was deposited. The history of the Indian Ocean. The Cretaceous and older tectonic framework of the region is poorly understood SEISMIC SEQUENCE AGE ia due to the lack of age control for the Enderby and 0Wharton Basins; however seismic reflection data from PLEISTOCENE both the northern (Munschy and Schlich, 1987) and S1 PLIOCENE southern plateau indicate that the plateau subsided 10? steadily through the Late Cretaceous. Subsequently Broken Ridge emerged prior to the Middle Eocene MIOCENE (Davies, Luyendyk, et al., 1974), and the northern part $2 20of the Kerguelen Plateau emerged for a time after the Middle Eocene. It is probable that the southern part emerged after the end of the Eocene, coinciding with 30the onset of rapid seafloor spreading at the Southeast OLIGOCENE Indian Ridge (Cande and Mutter, 1982). ~ACOUSTICDISCORDANCE' 40-
Acknowledgements EOCENE
We thank the master, crew, and technical staff aboard the R/V Rig, Seismic for their skillful assistance and cooperation, and Phil Symonds, Barry Willcox, and Paul Williamson for their constructive comments. We are grateful to Marc Munschy (IPG-Strasbourg) for fruitful discussions. Publication is by permission of the Director, Bureau of Mineral Resources, Geology and Geophysics, Canberra.
50-
PALEOCENE
60-
Maestrichtian
70- u)
12
O
~
Campanian
~
Santonian
<
References
80-
90-
~ l Coniacian u.l! Turonian
Cenomanian
13
Albian 1(]0
23/02/114
Figure 4 Seismic stratrigraphic sequences of a basin on the northern Kerguelen Plateau (after Munschy and Schlich, 1987)
80
Cande, S. C. and Mutter, J. C. (1982) A revised identification of the oldest sea-floor spreading anomalies between Australia and Antarctica Earth Planet. Sci. Lett. 58, 151-160 Coffin, M. F., Davies, H. L. and Haxby, W. F. (1986) Structure of the Kerguelen Plateau province f r o m Seasat altimetry and seismic reflection data Nature 324, 134-136 Davies, T., Luyendyk, B., et al. (1974) Initial Reports of the Deep Sea Drilling Project 26, US G o v e r n m e n t Printing Office, Washington, D.C.
Marine and Petroleum Geology, 1988, Vol 5, February
S e i s m i c s t r a t i g r a p h y a n d e v o l u t i o n o f R a g g a t t B a s i n : J. B. C o / w e l l et al. Eldholm. O., Thiede, J., eta/. (1986) Formation of the Norwegian Sea Nature 319, 360-361 Houtz, R. E. Hayes, D. E. and Markl, R. G. (1977) Kerguelen Plateau bathymetry, sediment distribution, and crustal structure Marine GeoL 25, 95-130 Kennett, J. P. and Watkins, N. D. (1976) Regional deep-sea dynamic processes recorded by late Cenozoic sediments of the southeastern Indian Ocean GeoL Soc. Am. Bull. 87, 321-339 Leclaire, L., Bassias, Y., Denis-Clocchiatti, M., Davies, H., Gautier, I., Gensous, B., Giannesini, P. J., Morand, F., Patriat, P., Segoufin, J., Tesson, M. and Wannesson, J. (1987) Lower Cretaceous basalt and sediments from the Kerguelen Plateau Geomarine Lett. 7 (4), in press Markl, R. G. (1973) USNS Eltanin Cruise 54 to the Kerguelen Plateau and Southeast Indian Rise Antarct. J. U. S. 8, 6-8 Munschy, M., and Schlich, R. (1987) Structure and evolution of the Kerguelen-Heard Plateau (Indian Ocean) deduced from seismic stratigraphy studies Marine Geol. 76, 131-152 Quilty, P. G. (1973) Cenomanian-Turonian and Neogene sediments from northeast of Kerguelen Ridge J. Geol. Soc. Aust. 20, 361-368
Ramsay, D. C. Colwell, J. B., etaL (1986a) RIG SEISMIC research cruise 2: Kerguelen Plateau - initial report Bur. Min. Resour. Aust. Report 270, 41 pp. Ramsay, D. C., Colwell, J. B., Coffin, M. F., Davies, H. L., Hill, P. J., Pigram, C. J. and Stagg, H. M. J. (1986b) New findings from the Kerguelen Plateau Geology 14, 589-593 Roberts, D. G., Backman, J., Morton, A. C., Murray, J. W. and Keene, J. B. (1984) Evolution of volcanic rifted margins: synthesis of Leg 81 results on the west margin of the Rockall Plateau Initial Reports of the Deep Sea Drilling Proiect 81, US Government Printing Office, Washington, D. C., 883-911 Schlich, R., Munschy, M., Boulanger, D., Cantin, B., Coffin, M. F., Durand, J., Humler, E., Li, Z. G., Savary, J., Schaning, M. and Tissot, J.D. (1987) Resultats preliminaires de la campagne oceanographic de sismique reflexion multitraces MD47 dans le domaine sud du plateau de kerguelen C.R. Acad. ScL Paris (in press) Wicquart, E. and Frohlich, F, (1986) The sedimentation on the Kerguelen-Heard Plateau. Relationships with the evolution of the Indian Ocean during the Cenozoic Bull. Soc. Geol. France 11 (4), 569-574
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