Site 1134 (Fig. F22) penetrated Cenozoic seismic Sequences 2, 3, 4, 6A, and 7 (sequences defined in Feary and James, 1998, reprinted as Chap. 2). It is unclear whether the basal Cenozoic unconformity was intersected before unstable hole conditions terminated coring. Postdrilling correlation indicates significant differences between the predrill seismic stratigraphy (Fig. F23) and the actual succession intersected at Site 1134. The base of Sequence 2 was predicted to occur at a much shallower depth than was the case, with the slumped interval at the base of Sequence 2 erroneously assigned to Sequence 3. In addition, a previously unrecognized sequence assignable to regional Sequence 7 (Feary and James, 1998, reprinted as Chap. 2) was intersected toward or at the base of Hole 1134A. It appears likely that the base Cenozoic unconformity either occurs immediately below the base of hole or more probably was intersected in the final two cores (from which no sediment was recovered). The high-resolution site-survey seismic data (Fig. F23), together with the regional seismic database, indicate that significant hiatuses should occur at all sequence boundaries and also at intrasequence horizons within Sequences 3 and 6A.
A check-shot survey using
the single-channel WST was undertaken at this site to determine the time-depth
relationship for the Cenozoic succession. The parameters and procedures
undertaken during the check-shot survey at this site are described in "Downhole
Measurements" and the seven time-depth tie-points established
by the check-shot survey are presented in Fig. F24.
These points were plotted on a depth to two-way-traveltime graph (Fig. F25A)
to (1) determine the relationship between depths encountered at Site 1134 and
sequence boundaries and horizons located on seismic data and (2) to compare the
check-shot-corrected time-depth relationship with predictions based on stacking
velocities. This plot shows that the actual time-depth relationship defined by
the check-shot survey falls at the lower limit of the envelope (defined by the
six stacking velocity curves from the immediate vicinity of Site 1134), with a
relatively small difference (
11
m) between predicted and actual depths to boundaries/horizons (Table T18).
The plot of check-shot data and velocity (Fig. F25B)
shows a reasonably good correlation, and the integrated sonic trace (Fig. F25),
derived from interval transit-time data, is in excellent agreement with stacking
velocities.
The data collected at Site 1134, particularly lithostratigraphic and biostratigraphic information, offer the opportunity to view the site data within a more regional context and to describe the characteristics of the seismic sequences intersected at this site (see "Lithostratigraphy" and "Biostratigraphy"). Downhole data were correlated with seismic stratigraphy (Fig. F26) based both on the regional moderate-resolution multichannel seismic data collected by the Japan National Oil Corporation (JNOC) in 1990 (Feary and James, 1998, reprinted as Chap. 2) and on the high-resolution site-survey seismic data collected by the Australian Geological Survey Organisation (AGSO) in 1996 (Feary, 1997).
Sequence 2 is relatively thin (66 m) at Site 1134, with seismic data indicating that the basal sequence boundary is a marked unconformity/hiatus surface containing erosional channels that cut into the underlying Sequence 3 succession. Reflections within the upper part of Sequence 2 (corresponding to the bioturbated nannofossil and foraminifer ooze of lithostratigraphic Unit I) are low amplitude, evenly stratified, and conformable. Reflections within the lower part of Sequence 2 (corresponding to the slumped nannofossil and foraminifer ooze, unlithified wackestone, packstone, and rudstone of lithostratigraphic Unit II) are irregular and discontinuous. Biostratigraphic datums indicate that Sequence 2 is of Pleistocene age, although probably containing a very thin Pliocene interval at the base (see "Biostratigraphy"). The predrill estimate of the depth to the base of Sequence 2 was at variance with the intersected succession because the base Sequence 2 unconformity was misidentified as occurring above instead of below the poorly stratified and irregular reflections corresponding to the slumped facies of lithostratigraphic Unit II.
From a regional perspective, Sequence 3 is thickest (~240 m) beneath the modern shelf and thins downslope to a feather edge beneath the modern middle shelf. Site 1134 intersects Sequence 3 beneath the upper slope, where it is 86 m thick. Sequence 3 correlates with lithostratigraphic Unit III (of middle and late Miocene age), which consists of dominantly pelagic, calcareous nannofossil and nannofossil foraminifer ooze. The upper Sequence 3 boundary represents a significant unconformity surface, corresponding to a hiatus of ~3 m.y. The high-resolution site-survey seismic data show that Sequence 3 includes a surface at ~112-114 mbsf (intra-Sequence 3 horizon on Fig. F23) that is a definite unconformity/hiatus associated with significant erosional downcutting. Biostratigraphic and lithostratigraphic data (see "Biostratigraphy" and "Lithostratigraphy") indicate that, despite apparent lithostratigraphic uniformity, this surface represents a hiatus of between 2 to 5 m.y., corresponding to the middle-upper Miocene boundary.
Within the Eucla Basin, seismic Sequence 4 is a relatively thin, aggradational unit characterized by conformable internal reflections (Feary and James, 1998, reprinted as Chap. 2). This sequence is 62 m thick at Site 1134, corresponding to lithostratigraphic Unit IV (Fig. F26) of early Miocene age. Lithostratigraphic data indicate that the intensely bioturbated, unlithified or partially lithified wackestone/packstone, foraminifer chalk, and minor packstone of Sequence 4 are less calcareous than overlying sequences. The variable amplitude, evenly stratified reflections constituting Sequence 4 probably represent the foraminifer chalk to wackestone/packstone alternations characteristic of this unit.
Sites 1126 and 1134 offered the only opportunities during this leg to characterize the lithology and age of seismic Sequence 6A, shown on regional seismic data to consist of three deep-water sediment lobes derived from the north (Feary and James, 1998, reprinted as Chap. 2). The 154-m section through Sequence 6A intersected at Site 1134 was first encountered at 214 mbsf, corresponding to a lithostratigraphic and velocity boundary (see "Lithostratigraphy" and "Downhole Measurements"). Sequence 6A correlates with lithostratigraphic Unit V, which consists of strongly bioturbated, texturally variable, calcareous nannofossil chalk interbedded with silicified equivalents and ranges in age from late Eocene to late Oligocene. Although limited recovery prevented the formal identification of lithostratigraphic subunits, substantial lithologic variations were noted (see "Lithostratigraphy"). However, the poorly constrained lithostratigraphic boundaries do not appear to correlate with seismic stratigraphic boundaries. The downhole transition from wackestone to mudstone at 252 mbsf corresponds to a dramatic increase in gamma-ray values (see "Downhole Measurements") but correlates only with a pronounced reflection at 1190 ms two-way traveltime (Fig. F23). No apparent lithostratigraphic break was detected coinciding with the boundary between the lower Oligocene Lobe 2 and upper Oligocene Lobe 3 at ~270 mbsf (see Fig. F23), which was expected to occur as a minor hiatus surface. The transition from packstone into the underlying mudstone facies at ~305 mbsf (possibly corresponding to the Eocene-Oligocene unconformity located biostratigraphically at ~315 mbsf) does not correlate with any seismic stratigraphic horizon.
As noted above, intersection of an interval of Sequence 7 sandstones was unexpected at this site. Only 30 cm of limonitic, coarse-grained sandstone containing abundant skeletal grains was recovered and assigned to lithostratigraphic Unit VI. Biostratigraphic data show that this sandstone is of middle Eocene age and indicate a likely hiatus of ~3 m.y. at the top Sequence 7 boundary. Reinterpretation of the site-survey seismic data indicates that this previously unrecognized interval is a thin feather edge of Sequence 7 overlying the irregular basal Cenozoic unconformity. Although no Mesozoic material was recovered, it is likely that either of the two deepest cores in Hole 1134A (from which no material was recovered) may have penetrated into the Cretaceous sequence. Alternatively, the basal Cenozoic unconformity may be present immediately below the base of Hole 1134A.
