The seismic stratigraphic results from Sites 1127, 1129, and 1131 (Fig. F29) are considered together, as these sites represent a transect of three closely spaced sample points through the same sequences that form spectacular clinoforms beneath the modern shelf edge. The high-resolution site-survey seismic data (Fig. F30) show that Sites 1127 and 1129 penetrate thick Sequence 2 intervals and bottom in Sequence 3, whereas Site 1131 penetrates both Sequences 2 and 3 and bottoms in Sequence 4 (sequences defined in Feary and James, 1998, reprinted as Chap. 2). Seismic data indicate that the basal Sequence 2 sequence boundary should represent a significant hiatus, with the basal Sequence 3 sequence boundary probably representing a minor hiatus.
Check-shot surveys using the single-channel WST were undertaken at Sites 1129 and 1131 to establish the time-depth relationship along this shelf edge to uppermost slope location and to correct the integrated sonic curves from Sites 1127, 1129, and 1131 for drift and for the pipe intervals. The parameters and procedures undertaken during these check-shot surveys are described in "Downhole Measurements" in the "Site 1129" chapter and "Downhole Measurements" in the "Site 1131" chapter. The 10 time-depth tie points derived from the check-shot survey at Site 1129 are presented in Figure F31, and the eight time-depth tie points derived from the survey at Site 1131 are presented in Figure F32. The presence of high methane and H2S gas levels at Site 1127 caused deployment of downhole tools to be kept to a minimum until the effect of high gas levels on downhole equipment could be assessed. Accordingly, no check-shot survey was undertaken at this site. The establishment of a time-depth relationship (Fig. F33A) was dependent on use of the integrated sonic curve, constrained by results from the other two sites.
The time-depth tie points for Sites 1129 and 1131 were plotted on depth to two-way-traveltime graphs (Figs. F34A, F35A) to (1) determine the relationship between depths encountered at each site and sequence boundaries and horizons located on seismic data and (2) compare the check-shot-corrected time-depth relationships with predictions based on stacking velocities. These plots show that the actual time-depth relationships defined by the check-shot surveys fall toward the base of the envelopes defined by the stacking velocity curves for the immediate vicinity of each site. This is a result of stacking velocities providing an underestimate of true traveltimes. Based on this data, the integrated sonic curve for Site 1127 was also located toward the base of the stacking velocity envelope at this location. The velocity underestimates correspond to depth errors of (1) as much as 24 m between predicted and corrected depths to boundaries at Site 1127, (2) as much as 12 m between predicted and corrected depths to boundaries at Site 1129, and (3) as much as 15 m between predicted and corrected depths to boundaries at Site 1131 (Table T17, also in ASCII format). Plots of check-shot data and velocity for Site 1129 (Fig. F34B) and Site 1131 (Fig. F35B) show reasonably good correlations. Integrated sonic traces derived from interval transit-time data for both sites (Figs. F34, F35) are in excellent agreement with stacking velocities.
The data collected at each of these sites show the site data within a more regional context and also allow a description of the characteristics of seismic sequences intersected at these sites (see "Lithostratigraphy," and "Biostratigraphy"; "Lithostratigraphy" and "Biostratigraphy" in the "Site 1129" chapter; and "Lithostratigraphy" and "Biostratigraphy" in the "Site 1131" chapter). Correlations of lithostratigraphic and biostratigraphic data with seismic stratigraphy at Sites 1127 (Fig. F36), 1129 (Fig. F37), and 1131 (Fig. F38) are 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).
Comparison of the high-resolution site-survey seismic data along the shelf edge transect with regional seismic data indicates that the Sequence 2 interval intersected at all three sites should represent a relatively complete succession, with the 467-m thickness at Site 1127 representing a more expanded upper Sequence 2 interval (Fig. F36), the 532-m thickness at Site 1131 representing a more expanded middle Sequence 2 interval (Fig. F38), and the 556-m thickness at Site 1129 representing a more expanded lower Sequence 2 interval (Fig. F37). The site-survey seismic data indicate that there should be significant facies differences between the Sequence 2 successions intersected at these three sites (Fig. F30). Site 1127 and the deeper parts of Sites 1129 and 1131 are characterized by uniform, continuous, evenly stratified clinoform reflectors, whereas the uppermost part of Site 1131 and the upper one-third of Site 1129 are characterized by mounded reflector geometries.
Lithostratigraphic data indicate that the continuous, evenly stratified reflectors at all three sites represent bioclastic packstone-dominated facies, with minor bioclastic wackestone and thin grainstone beds. The succession at Site 1127 shows a subtle cyclicity with alternating bioclastic wackestone- and packstone-dominated packages passing up into thin capping grainstone beds, interpreted as shallowing-upward cycles (see "Lithostratigraphy"). Downhole logging data indicate that this cyclicity is also present within the intervals characterized by continuous, evenly stratified reflectors at Sites 1129 and 1131 (see "Downhole Measurements" in the "Site 1131" chapter, and "Downhole Measurements" in the "Site 1129" chapter). The intervals characterized by mounded reflector geometries at Sites 1129 and 1131 are dominated by bryozoan floatstone and rudstone, interpreted as bryozoan mounds (see "Lithostratigraphy" in the "Site 1131" chapter, and "Lithostratigraphy" in the "Site 1129" chapter). The floatstone intervals alternate with bioclastic packstone intervals, interpreted to represent times when oceanographic conditions were unsuitable for bryozoan mound development (as at the present time).
Biostratigraphic data indicate that almost the entire thickness of Sequence 2 at all three sites represents Pleistocene accumulation overlying a thin (10-16 m) basal Pliocene interval. The Sequence 2 shelf edge clinoform succession, therefore, represents a spectacularly high sediment accumulation rate. The sequence boundary at the base of Sequence 2 is an unconformity marking the transition from Pliocene- to middle Miocene-age sediments, representing a 10-12-m.y. hiatus.
Site 1131 penetrated the entire 52-m thickness of Sequence 3, whereas Sites 1127 and 1129 penetrated only the upper 44 and 48 m, respectively. Biostratigraphic data indicate that Sequence 3 is of middle Miocene age. Recovery was very poor at all three sites, primarily as a result of preferential silicification of the well-lithified, partially dolomitized, bioclastic, glauconitic packstone to grainstone lithofacies apparently forming most of this sequence throughout the transect. The only other facies recovered was nannofossil chalk in the upper part of the Sequence 3 succession at Site 1127; seismic data confirmed that Site 1127 intersects a thin interval not represented at Sites 1131 or 1129. Despite the relative lithofacies uniformity, the seismic character of Sequence 3 varies considerably across the transect, from (1) poorly stratified, discontinuous to semicontinuous reflectors at Site 1127, in contrast to (2) low-amplitude, more evenly stratified and more continuous reflectors at Site 1131, and passing upslope to (3) higher amplitude, evenly stratified and continuous reflectors at Site 1129. This variability cannot be correlated with lithofacies differences, possibly because of the very poor recovery.
Sequence 4 was only intersected in the deepest part of Site 1131, where biostratigraphic evidence indicates an ~20-m thickness of early Miocene age. Minimal recovery within this interval means that the sequence boundary was not observed. Lithostratigraphic data indicate that this sequence probably consists of the same well-lithified, partially dolomitized, bioclastic, glauconitic packstone to grainstone lithofacies forming the overlying Sequence 3. Similarly, the seismic character of the Sequence 4 interval intersected at this site is identical to the low amplitude, evenly stratified, continuous reflectors of the overlying Sequence 3.