Drilling at Site 1126 penetrated Cenozoic seismic Sequences 2, 3, 4, and 6A (sequences defined in Feary and James, 1998, reprinted as Chap. 2) and bottomed in Cretaceous siliciclastic sediments. A direct seismic tie to Jerboa-1 23.4 nmi to the west (Fig. F30) indicates that these Cretaceous sediments are of Cenomanian age. It is possible that a thin interval of Sequence 7 sandstones may also be present below seismic resolution. The high-resolution site-survey seismic data (Fig. F31), 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 establish the time-depth
relationship within the Cenozoic succession and to assess
the degree to which stacking velocities (derived from
high-resolution site-survey seismic data for common depth
points adjacent to sites) could be used for preliminary
depth estimates. The parameters and procedures undertaken
during the check-shot survey at Site 1126 are described in "Downhole
Measurements". The nine time-depth tie
points derived from the check-shot survey are presented in
Figure F32.
These points were plotted on a depth to two-way-traveltime
graph (Fig. F33A)
to (1) show the relationship between depths encountered at
Site 1126 and sequence boundaries and horizons located on
seismic data and (2) compare the check shot-corrected
time-depth relationship to 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 1126,
with a relatively small difference (
17
m) between predicted and actual depths to
boundaries/horizons (Table T19).
The relatively poor match between check-shot data and the
velocity log (Fig. F33B)
is possibly a response to poor contact between the sonic
tool and borehole wall. The integrated sonic trace (Fig. F33)
derived from interval transit-time data is in excellent
agreement with stacking velocities, although it required
check-shot data to anchor the uppermost point.
The data collected at Site 1126 allow a description of the characteristics of the seismic sequences intersected at this site (see "Lithostratigraphy" and "Biostratigraphy"). Downhole data were correlated with seismic stratigraphy (Fig. F34) based 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 the high-resolution site-survey seismic data collected by the Australian Geological Survey Organisation (AGSO) in 1996 (Feary, 1997).
Regional seismic data indicate that the thickness of Sequence 2 varies greatly, from a maximum of >500 m through the spectacular sigmoidal clinoforms beneath the modern shelf edge and uppermost slope (see "Seismic Stratigraphy" in the "Site 1127" chapter) thinning to <100 m landward beneath the outer shelf and seaward down the upper and middle slope. Sequence 2 is relatively thin (60 m) at Site 1126, with seismic data indicating that the basal sequence boundary is a marked unconformity/hiatus surface containing erosional channels cut down into the underlying Sequence 3 interval. Reflections within Sequence 2 are evenly stratified and conformable. This sequence correlates with the bioturbated nannofossil and foraminifer ooze of lithostratigraphic Unit I, and biostratigraphic datums indicate a Pleistocene age.
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 1126 intersects Sequence 3 beneath the upper slope where it is 105 m thick. The high-resolution site-survey seismic data show that Sequence 3 includes one definite unconformity/hiatus surface displaying significant erosional downcutting (Fig. F31, Horizon A) and two other prominent horizons that may represent hiatuses (Fig. F31, Horizons B and C). Lithostratigraphic Subunit IIA (of latest Miocene-Pliocene age) correlates with the sub-sequence above Horizon A, the upper Miocene Subunit IIB correlates with the sub-sequence between Horizons A and B, and the middle Miocene Unit III correlates with the sub-sequence underlying Horizon B (Fig. F34). No lithostratigraphic break is recognized that coincides with Horizon C within the middle Miocene.
Lithostratigraphic data indicate that the Pliocene sequence (above Horizon A) consists of bioturbated, white-gray nannofossil ooze wackestones and light gray nannofossil ooze packstones, with two slumped intervals. The erosional unconformity at the top of the upper Miocene sequence (between Horizons A and B) is marked by a firmground. The sub-sequence between Horizons A and B consists of bioturbated calcareous nannofossil ooze interbedded with irregular to nodular chert/porcellanite layers corresponding to Subunit IIB. The middle Miocene Unit III, which forms the remainder of Sequence 3 (below Horizon B) consists of bioturbated calcareous nannofossil ooze with wackestone to mudstone texture, together with interbedded silicified layers.
Within the Eucla Basin seismic Sequence 4 is a relatively thin, aggradational unit characterized by conformable internal reflections (Feary and James, 1998). This sequence is only 41 m thick at Site 1126, corresponding to the uppermost portion of lithostratigraphic Unit IV. Biostratigraphic data indicate an early Miocene age. Because of limited recovery, it is difficult to fully describe the lithologic characteristics of this sequence. Available data indicate that it consists of a diverse array of rock types dominated by pervasively bioturbated chalks and oozes, together with interbeds of silicified calcareous pelagic limestone and porcellanite/chert. The upper sequence boundary is a bioclastic packstone containing planktonic and benthic foraminifers, ostracodes, and echinoderm debris.
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). The 190-m section through Sequence 6A intersected at Site 1126 was first encountered at 206 mbsf. Despite their differences in overall seismic geometry, Sequence 6A consists of a diverse lithologic suite essentially identical to that encountered in Sequence 4, to the extent that these units are grouped into the single lithostratigraphic Unit IV. Biostratigraphic data indicate that this sequence ranges from the middle Eocene through to the late Oligocene. Although expected to occur as a hiatus surface representing the Eocene/Oligocene boundary, no apparent break was detected coinciding with the boundary between the Eocene Lobe 1 (oldest) and Oligocene Lobe 3 (youngest) at ~330 mbsf (Fig. F31). However, postcruise work may clarify the detailed biostratigraphy within this poorly recovered section.
A thin interval of brown quartz sandstones (Core 182-1126D-27R-CC) occurs between the middle Eocene nannofossil-foraminifer chalk of Sequence 6A and the dark greenish black to black sandstones of presumed Cretaceous age (see "Cretaceous Sequence"). Available data indicate that these brown sandstones are probably of middle Eocene age (P12; see "Biostratigraphy"), although the sample for age dating was taken from the very top of the sandstone and may represent contamination from the overlying Sequence 6A chalks. One core fragment contains an irregular coating of calcareous sandstone, inferred to represent a cavity infill, that contains sparse bioclasts among the predominantly quartz grain clastic component. We suggest that either this cavity infill or both cavity infill and brown sandstone be tentatively assigned to Sequence 7 and speculate that irregular thicknesses of Sequence 7 sandstones may have infilled depressions and irregularities in the eroded Cretaceous surface.
The base of the Cenozoic section throughout the Eucla Basin is a dramatic angular erosional unconformity corresponding to a hiatus of some 50 m.y. (Fig. F34). At Site 1126 this unconformity is present as a boundary between overlying middle Eocene facies and the underlying 60-m succession of black to green siltstone, sandstone, and granule conglomerate lithologies, inferred to be of Cenomanian age on the basis of the direct seismic tie to exploration well Jerboa-1.
