Descriptive Notes of Site-Survey Seismic Profiles3

Acquisition parameters for the profiles displayed in Figure AF1 (an oversized figure that accompanies this volume) and others used for Ocean Drilling Program Leg 178 site locations are given in Tables T9 and T10, both in the "Explanatory Notes" chapter. Figure F10, in Barker and Camerlenghi (Chap. 2, this volume), is a track chart of all the seismic profiles available in the ANTOSTRAT Antarctic Peninsula Regional Working Group database (see also Cunningham et al., 1995), including those displayed in Figure AF1. The profiles in Figure AF1 are located with respect to seafloor morphology in the two maps in the lower right corner of Figure AF1, side 1. All times referred to are two-way traveltime (TWT).

Profiles I95-135, I95-135A, I95-137, and I95-130

The Programma Nazionale di Ricerche in Antartide (Italy) (PNRA) profiles on Drift 7 (Sites 1095 and 1096) are 60-fold stacks of common depth point (CDP) gathers spaced 12.5 m apart. Traces have been deconvolved before stack with a 160-ms operator and 4-ms predictive interval. Velocity analyses were performed every 4 km. The display sections were produced with time-variant three-trace lateral mix, time-variant band-pass filter, and trace equalization using a variable window.

These profiles illustrate the three-dimensional structure of Drift 7: profile I95-135 is a dip section from the continental shelf to the lower continental rise along the axis of elongation of Drift 7. The drift is separated from the steep (up to 20°) continental slope by an area highly incised by gullies and incipient deep-sea channels. The sediments rest on oceanic basement and thin from the drift crest seaward. The limited energy provided by the two Generator Injector guns prevents unequivocal detection of oceanic basement below the thicker and rougher sediment section at the base of the slope, although the main shallow sedimentary reflectors can be traced from the continental rise into the slope. Note the transition from a basin-fill geometry, for sediments directly overlying oceanic basement, to a drape geometry in overlying units. This transition is considered to represent a change from turbiditic sedimentation, on young basaltic seafloor approaching the paleosubduction zone, to mainly pelagic, alternating with contouritic and distal turbiditic deposition during the drift growth and maintenance stages of Rebesco et al. (1996, 1997).

Profile I95-130A is a proximal strike section through the drift crossing Site 1096. The drift is asymmetric, with a steep side and a gentle side, and is bounded to the northeast and southwest by large deep-sea channel systems, each displaying local and asymmetric levee deposits. Drift growth is identified by reflector divergence below the present-day steep side of the drift and by progressive migration of the paleodrift crest to the northeast. The bottom-simulating reflector at ~700 ms below seafloor, particularly evident under the steep side of the drift, is believed to be a silica diagenetic front (see also "Physical Properties" in the "Site 1096" chapter). Profile I95-137 is a distal strike section through Drift 7, crossing Site 1095. Note that the relief of the drift is considerably reduced with respect to the proximal drift. This allows more silt turbidite deposition on the drift, which inverts its sense of asymmetry: its steeper side is to the northeast, and it assumes the character of an overbank deposit of the channel system to the northeast. The loss of seismic energy below this channel system, evident on profile I95-130A, is the result of the presence of coarse sediments of irregular geometry within the channels. The two cross sections show that the channel systems are long-lived features that have never migrated through the position of the present-day sediment drift.

Profile IT92-114

Profile IT92-114A (Site 1101) is a 30-fold stack of CDP gathers spaced 25 m apart. Although this line was acquired with a streamer configuration similar to that of the lines collected in 1995 (above), the shot spacing was wider (50 m), reducing coverage. Compared to the 1995 profiles, profile IT92-114A has less vertical and horizontal resolution (4-ms sampling and 25-m CDP spacing) but larger total source volume (72 L) and thus greater penetration. Traces were deconvolved before stack with a 160-ms operator and 16-ms predictive interval. Velocity analyses were every 5 km. The display section was produced with time-variant three-trace lateral mix, time-variant band-pass filter, and trace equalization using a 300-ms window.

Profile IT92-114A is a dip section from the continental shelf to the lower continental rise along the axis of elongation of Drift 4 across Site 1101. Note the similar morphological and stratigraphic elements on this profile and profile I95-135 (the slope angle is 16.5°). The oceanic basement reflector can be followed here to the base of slope, where it is affected by velocity pull-up. The deep, discontinuous subhorizontal reflector at ~7.5 s TWT is the Mohorovicic Discontinuity (Moho). A strong reflector can be traced from the drift to the continental shelf that correlates with the uplift unconformity of Larter and Barker (1991b) and Larter et al. (1997).

Profile AMG 845-08

British Antarctic Survey (BAS) profile AMG 845-08 (shelf transect, Sites 1100, 1102, and 1103) is a 24-fold stack of CDP gathers spaced 25 m apart with 4-ms sampling. Traces were deconvolved before stack with a 300-ms operator and 24-ms predictive interval. Velocity analyses were every 3 km. The display section was produced with three-trace lateral mix, time-variant band-pass filter, and trace equalization using a 500-ms window. No attempt was made to remove the seafloor multiple for this display.

Part of profile AMG 845-08 lies along the same track as profile I95-152, which was obtained to improve vertical resolution. The two profiles are complementary. Profile AMG 845-08 extends from the continental rise to the mid-shelf basin. On the rise, it crosses from the small Drift 2 to the lower continental slope via the channel-dissected uppermost continental rise (this area has GLORIA coverage; see Rebesco et al., 1996). Oceanic crust here is aged ~11 Ma (see also Fig. F7, in Barker and Camerlenghi [Chap. 2, this volume]), and, beneath the rise, reflections from the Moho are seen at 7-7.5 s. Sediments overlying the ocean floor can be traced beneath the slope. The uppermost 2 s of the shelf section of the profile may be compared with the expanded and higher resolution profile I95-152. Below 2 s, Sequence Group S2 and Sequence S3 may be seen downlapping onto a strong reflector considered to be the top of the precollision accretionary prism. This strong reflector is common along the margin and forms part of the S4/S3 "uplift unconformity" of Larter et al. (1997). Its equivalent in an older collision segment farther to the southwest continues seaward to the hiatus in terrigenous sedimentation discovered at Deep Sea Drilling Project Site 325 and considered by Larter and Barker (1991a) to have been caused by collision-related uplift at that segment of the margin.

Profile I95-152

PNRA profile I95-152 (shelf transect, Sites 1100, 1102, and 1102) is a 30-fold stack of CDP gathers spaced 6.26 m apart with 2-ms sampling. Traces were deconvolved before stack with an 80-ms operator and 4-ms predictive interval. Velocity analyses were every 2 km. The seafloor multiple reflection was attenuated by the application of a median filter to CDP gathers corrected for multiple velocity. The display section was produced with time-variant three-trace lateral mix, deconvolution in the frequency domain, time-variant band-pass filter, and trace equalization using a variable window.

Profile I95-152 provides a high-resolution image of the geometry of the glacial prograding wedge and of the "preglacial" structure of the continental shelf, complementary to that of profile AMG 845-08, which follows the same track. Figure F2, in the "Shelf Transect" chapter, shows the boundaries of the major seismic Sequence Groups S1 through S4 of Larter and Barker (1991b), Larter and Cunningham (1993), and Larter et al. (1997). A distinctive feature of this profile and of AMG 845-08 (above) is the comparative strength of topset reflectors within Sequence Group S1, considered to result from ice loading and related shear, which is greatly reduced in related foresets beyond the paleoshelf edge. The low-profile near-surface wedge of sediment landward of shotpoint (S.P.) 950 (sampled at Site 1100) was interpreted by Vanneste and Larter (1995) as a prograding till body emplaced during the last glaciation but not reaching the continental shelf edge at that location (see also Fig. F3, in the "Shelf Transect" chapter).

Profile AISM-06

Most of Programa Nacional de Investigación en la Antártida (Spain) profile AISM-06 (Site 1097) is a 48-fold stack of CDP gathers 12.5 m apart from shots at 25-m intervals with 2-ms sampling (resampled to 4 ms), obtained from the Hesperide in December 1997. Traces were deconvolved before stack with a 250-ms operator and 8-ms predictive interval. Velocity analyses were every 6 km. The display section was produced with time-variant band-pass filter and trace equalization. The continental slope section of AISM-06 (S.P. >5480) was shot at 50-m intervals with 24-fold cover.

Profile AISM-06 was obtained along virtually the same track as the PD-88 profile used to locate Site 1097 (see Fig. F1, in the "Site 1097" chapter). It crosses an interlobe region where the progradational Sequence Groups S1 and S2 thin to ~150 ms (at Site 1097), and access to Sequence S3 is therefore easier. Previously published analyses of shelf sedimentation (Larter and Barker, 1989, 1991b; Larter et al., 1997) have interpreted Sequence S3 as having an upper boundary (with S2) that was climate-related and thus essentially synchronous along the margin. Its lower boundary (with S4) was associated with ridge-crest subduction and therefore diachronous along the margin. Sequence S3 shows little variation in thickness or geometry along the margin, in contrast to the overlying Sequence Groups S2 and S1, which are focused into progradational lobes.

Profile I97H-221G

Single-channel line I97H-221G was obtained by stacking eight traces of a 10-hydrophone mini-streamer. Shot spacing was 12.5 m, and the sampling interval was 1 ms (high-cut filter 308 Hz). Traces were deconvolved with a 60-ms operator and 1-ms predictive interval. The display sections were produced with three-trace time-variant lateral mix, time-variant band-pass filter, and trace equalization using a variable window.

PNRA profile I97H-221G (Site 1099) shows the structural setting of Palmer Deep Basin III. The sediment fill is as much as 300 ms thick. It displays high-amplitude reflectors, with many hyperbolae produced by diffractions from both the seafloor and deeper lithologic or structural discontinuities. A reflector of unknown origin is seen between 1 and 1.5 s TWT below the seafloor. A prominent reflector with eastward apparent dip below the depocenter of Basin III has been interpreted by Rebesco et al. (1998) as the master fault of a half-graben structure producing the inferred active subsidence of the basin floor. Other, weaker reflectors on the footwall side of the master fault have been judged to be conjugate faults. Profile I97H-221G also shows a twofold division of the sedimentary fill: a low-reflectivity subunit above and a high-reflectivity subunit below. The boundary between the two, at 76 meters below seafloor (mbsf) at Site 1099, has been identified by drilling as a downward transition from massive, bioturbated clayey silts to thinly spaced alternations of massive diatom ooze and fine-grained graded turbidites.

Profiles LMG 98-2

National Science Foundation-United States Antarctic Program (NSF-USAP) profiles LMG 98-2 (Palmer Deep, Sites 1098 and 1099) were acquired from the L. M. Gould in Palmer Deep during drilling of Leg 178 and were used to refine the positions of the two sites. The Huntec deep-tow boomer was towed ~100 m below the sea surface; the firing interval was either 1000 or 750 ms, depending on water depth, and the record length was 250 ms. The displays were obtained with automatic gain control, with band-pass filters of 1.4-5.0 kHz (Basin I) and 2.0-4.0 kHz (Basin III).

The profile across Basin I shows the drape geometry of the thin sedimentary fill in the southwest corner of Basin I, where Site 1098 is located. The strong hyperbolic basal reflectors are hard rock, hit by the hydraulic piston corer at 46 mbsf at Hole 1098A but not sampled. The profile across Basin III shows only the upper part of the sediment fill. Acoustic basement of this profile is the high-amplitude mid-basin reflector, which corresponds to a clast-poor diamict layer within a mixed pelagic and turbiditic upper unit at 34 mbsf at Site 1099. (See also "Background and Objectives" and "Seismic Stratigraphy" in the "Palmer Deep" chapter, and Rebesco et al. [1998]).

3Signal processing was performed as follows: PNRA profiles by Claudio Pelos and Lorenzo Sormani, Osservatorio Geofisico Sperimentale, Trieste, Italy; AMG profile by Alex P. Cunningham, BAS, Cambridge, United Kingdom; and AISM profile by Emilia L. Gulmezova and J. Maldonado, Universidad de Granada, Spain. Signal acquisition and recording of NSF-USAP deep-tow boomer profiles were done by Michael Belliveau, Geoforce Consultants Ltd., Dartmouth NS, Canada.