13
m) between predicted and actual depths to
boundaries/horizons (Table T20).The seismic sequences intersected on the upper continental rise at Site 1128 (Fig. F38) could not be tied to the regional Cenozoic seismic stratigraphy established on the slope and shelf (sequences defined in Feary and James, 1998, reprinted as Chap. 2) before drilling because of the lack of appropriate seismic tie lines. Instead, a local seismic stratigraphy was established for the presumed Cenozoic portion of the continental rise succession using the high-resolution site-survey seismic data (Fig. F39). These data shows that the succession extending down to Horizon 1C_2 was deposited in a "ponded" upper continental rise basin overlying a major unconformity/hiatus surface. Viewed on a regional basis, it is clear that this small basin was formed by local compression of the underlying succession between Horizons 1C_2 and 1C_3a (Fig. F39). Sequence geometry and the presence of folds and dislocation surfaces indicate that this local compression resulted from large-scale downslope movement. The dislocation surfaces show that movement must have occurred on a décollement surface or surfaces within the succession underlying Horizon 1C_3a, most probably in the vicinity of 300 mbsf.
Because
of the considerable time required to carry out a check-shot
survey in this water depth, the planned deployment of the
WST was abandoned. As a result, the time-depth relationship
for sequence boundaries and horizons located on seismic data
has been estimated principally on the basis of the
integrated sonic trace (Fig. F40),
derived from interval transit-time data. Therefore, the
time-depth relationship must be considered to be an
approximate estimate only. Because the integrated sonic
trace does not extend to the sediment surface, its location
in the time domain cannot be determined accurately. However,
the strong likelihood that Horizons 1C_2 and 1C_3a correlate
respectively with the top Oligocene (base debrite at 70 mbsf)
and Eocene-Oligocene (at ~260 mbsf) unconformities provides
an increased level of confidence for the proposed
correlation (Fig. F41).
This correlation shows that the best-fit time-depth
relationship defined by the integrated sonic trace (Fig. F40A)
falls within and toward the upper limit of the envelope
defined by the six stacking velocity curves from the
immediate vicinity of Site 1128, with a relatively small
difference (13
m) between predicted and actual depths to
boundaries/horizons (Table T20).
The data collected at Site 1128, particularly lithostratigraphic and biostratigraphic information, offer the opportunity to interpret the site data within a more regional context and also allow a description of the characteristics of seismic sequences intersected at this site (see "Lithostratigraphy" and "Biostratigraphy"). Downhole and seismic data can be correlated (Fig. F41) using the high-resolution site-survey seismic data collected by the Australian Geological Survey Organisation (AGSO) in 1996 (Feary, 1997); therefore, it is possible to describe and interpret the sequences intersected at Site 1128 in terms of the seismic stratigraphy established on the slope and shelf (Feary and James, 1998, reprinted as Chap. 2).
The base Sequence 2 boundary correlates with continental rise Horizon 1C_1, present as an unconformity surface within the upper continental rise ponded basin. Onlap patterns at the base of this sequence indicate that this sequence boundary should represent a hiatus surface. Compared with the >500-m thickness of Sequence 2 at the shelf edge, this sequence is extremely thin (25 m) at Site 1128. Reflections within this interval are evenly stratified with low to moderate amplitude. Biostratigraphic data indicate that the succession overlying Horizon 1C_1 is of Pleistocene-late Pliocene age, and lithostratigraphic data indicate that this interval consists of pelagic nannofossil ooze containing abundant spicules, interbedded with thin calciturbidites (below seismic resolution) consisting of foraminiferal packstone and grainstone. The basal hiatus surface corresponds to ~1.2 m.y.
Biostratigraphic and lithostratigraphic data indicate that the interval between Horizons 1C_1 and 1C_2 consists of a 30-m-thick coherent interval, equivalent to slope and shelf seismic Sequence 3, overlying a 15-m-thick debrite interval. The coherent interval correlates to the lower portion of lithostratigraphic Subunit IA, of late Miocene age, and the debrite interval to lithostratigraphic Subunit IB. The evenly stratified, low-moderate amplitude reflection character of the Sequence 3 interval is identical to the overlying Sequence 2 succession, and lithostratigraphic data indicate that it similarly consists of pelagic nannofossil ooze interbedded with thin calciturbidites. There is no indication on seismic data of the hiatus surface at the upper Miocene/lower Pliocene boundary, toward the middle of Sequence 3 (Fig. F41).
The transition from Sequence 3 ooze down into the debrite unit correlates with a poorly defined disconformity surface on seismic data, and the hiatus surface toward the base of Sequence 3 is probably also represented by this indistinct discontinuity. Reflections are very indistinct within the debrite unit, although there are subtle indications of low amplitude reflections downlapping to the south onto the Horizon 1C_2 surface. By analogy with the clastic sediments intersected at the base of Site 1126, clasts of terrigenous clastic sediment within the debrite are of probable Cretaceous age. Their presence indicates derivation of at least some of the debrite material from Cretaceous strata shown on seismic data to outcrop at the seafloor on the middle and lower continental slope. The source of the Oligocene and middle and upper Miocene clasts also present within the debrite unit is unknown, although the absence of abyssal paleodepth indicators within the benthic foraminiferal assemblage in this unit is consistent with derivation from upslope.
Biostratigraphic data indicate that the intervals between Horizons 1C_2 and 1C_3 (104 m thick) and between Horizons 1C_3 and 1C_3a (86 m thick), corresponding to the lower Oligocene, should be correlated with slope and shelf seismic Sequence 6 (Fig. F41). In addition, the 28-m-thick upper Eocene interval immediately underlying Horizon 1C_3a is also equivalent to the basal part of Sequence 6. This sequence therefore includes a diverse range of lithofacies from several units and subunits, including the spicule-dominated clay and nannofossil-dominated ooze alternations of lithostratigraphic Subunit IC; the variably calcareous, spicule-dominated clay, claystone, and clayey chalk of Unit II, interbedded with calciturbidites of planktonic foraminiferal and nannofossil ooze; and the glauconitic sandstone turbidites of Unit III. Horizon 1C_3a, correlated with the Eocene/Oligocene boundary at 260 mbsf, corresponds to the transition from moderately consolidated clay down into lithified claystone.
Horizon 1C_2, forming the boundary between the lithostratigraphic Subunit IB debrite and the underlying succession correlated to Sequence 6, is one of the most dramatic surfaces visible on seismic data on the continental rise. On the upper continental rise, this surface displays relief of as much as 70 m, principally as a result of faulting and folding of the underlying units between Horizons 1C_2 and 1C_3a (Fig. F39). As noted above, seismic geometry indicates that this faulting and folding is a reflection of large-scale downslope movement that appears to have affected the entire 190-m interval between these two horizons, as well as some undetermined component of the succession beneath Horizon 1C_3a that may extend to the base of Sequence 6.
The upper part of Sequence 6 at Site 1128, between Horizons 1C_2 and 1C_3, consists of relatively evenly stratified, continuous to semicontinuous reflections, with mostly low amplitudes except for two higher-amplitude, continuous reflectors estimated to occur at ~95-105 and ~135-140 mbsf. This upper reflector may correlate with a grainstone interval at 95 mbsf, and the lower reflector may correlate with grainstone or chert layers at 135-137 mbsf (see "Lithostratigraphy"). The uniform mudstone interval between Horizons 1C_3 and 1C_3a is relatively featureless on seismic data, consisting of low-amplitude, semicontinuous reflections. By contrast, the interval underlying Horizon 1C_3a is characterized by high-amplitude, semicontinuous to continuous, evenly stratified reflections. This may result from some combination of variable lithification, the presence of grainstone calciturbidites, or the presence of chert layers resulting from preferential silicification.
The thick middle Eocene interval at Site 1128 (>163 m), corresponding to lithostratigraphic Unit IV, correlates with seismic Sequence 7 on the shelf and slope. As is the case upslope, Sequence 7 in the continental rise setting consists of a siliciclastic facies suite. In this environment, Sequence 7 is dominated by silty clay to sandy siltstone, coarsening downward to a silty sandstone intersected at the deepest extent of Site 1128 penetration. Unlike the markedly progradational seismic geometry of Sequence 7 on the shelf and slope, the equivalent sequence on the upper continental rise is characterized by evenly stratified, continuous to semicontinuous, low- to moderate-amplitude reflections.
