INTRODUCTION

The East Antarctic Ice Sheet is presently the largest ice mass on Earth and it is the longest lived, having been initiated in the Paleogene (Barron et al., 1991; O'Brien, Cooper, Richter, et al., 2001). Since its formation, it has played a central role in global climate and in higher-order sea level change. Although global ice volume/temperature changes can be estimated using low-latitude ¹⁸O records, the distribution of ice on the continent and the evolution of the Antarctic Ice Sheet through glacial-interglacial cycles requires direct evidence from Antarctica. Prydz Bay, East Antarctica, was drilled during Leg 188 to sample the record left by the Lambert Glacier-Amery Ice Shelf drainage system that flows from the interior of the East Antarctic Ice Sheet. The results from Leg 188, when combined with results from Leg 119, also drilled in Prydz Bay, provide a record of the extensive erosion of the shelf by ice advances, starting in the late Eocene to early Oligocene but becoming intense from the middle Miocene onward (Barron et al., 1991; O'Brien, Cooper, Richter, et al., 2001). Erosion has removed large parts of the record in places, but the erosion surfaces can be traced into their correlative conformities on the continental slope, where the debris carried by the grounded ice has been deposited in trough mouth fans (Cooper et al., 1991; Kuvaas and Leitchenkov, 1992; Bart et al., 2000). These fans form where fast-flowing ice streams reach the shelf edge during episodes of extreme ice extent and should contain a record of intervals not represented on the shelf (Vorren and Laberg, 1997; Boulton, 1990). They should consist of thick mass flow deposits formed from debris released from the basal ice at the shelf break interbedded with thin hemipelagic and pelagic intervals deposited when the ice was shoreward of the shelf edge.

Trough mouth fans might hold the answer to a problem that has become apparent with the greater understanding of the position of the Antarctic ice grounding zone during the Last Glacial Maximum (LGM) (18-12 ka). There is evidence for grounding of the ice sheet at the shelf edge in many places (e.g., Mac.Robertson Land [Harris et al., 1996], eastern Ross Sea [Shipp et al., 1999; Domack et al., 1999], and Antarctic Peninsula [Pusdsey et al., 1994]). LGM grounding zone deposits have been identified in the western Ross Sea well back from the shelf edge (Licht et al., 1999; Shipp et al., 1999; Domack et al., 1999). The East Antarctic continental shelf displays abundant evidence of ice advance to the edge of the continental shelf (Vanney and Johnson, 1985; O'Brien and Leitchenkov, 1997; Bart et al., 2000). However, Anderson et al. (2002) reviewed the literature on East Antarctic Ice Sheet LGM grounding zone positions and concluded that around most of the margin, the LGM grounding zone is in a mid-shelf position or near its present location. In the case of Prydz Bay, Domack et al. (1998) and O'Brien et al. (1999) mapped LGM grounding zone wedges around Prydz Channel more than 130 km from the continental shelf edge (Fig. F1). Seaward of these wedges, the floor of Prydz Channel lacks flutes and other subglacial features and is blanketed by thick glaciomarine clays and oozes (O'Brien et al., 1999; Domack et al., 1998) indicating that the Lambert Glacier did not ground there during the last glacial cycle. This raises several questions:

1. Why did large parts of the East Antarctic Ice Sheet including some large interior-derived ice streams not reach the shelf edge during the LGM?
2. When did the ice sheet last reach the shelf edge if not during the LGM?
3. What sort of glacial episode is required to make the ice advance to the shelf edge?

Domack et al. (1998) suggested that relatively long periods of low sea level and increased ice volume might be needed for the interior ice sheet to respond and advance to the shelf edge. Site 1167 of Ocean Drilling Program (ODP) Leg 188 was intended to investigate these questions by sampling a record through the trough mouth fan at the mouth of Prydz Channel that would provide a history of Pleistocene advances.

In spite of their potential value as records of past glaciation, trough mouth fans were not drilled in the Antarctic margin until ODP Leg 188. Site 743 was drilled during Leg 119 on the Prydz Bay slope but to the east of the Prydz Channel Fan on a steep, eroded part of the slope (Barron, Larson, et al., 1989; O'Brien and Leitchenkov, 1997).

Prydz Bay is the downstream end of the Lambert Glacier-Amery Ice Shelf ice drainage system, which drains ~16% of the East Antarctic Ice Sheet (Allison, 1979; Fricker et al., 2000). The Lambert Glacier-Amery Ice Shelf system responds to mass balance fluctuations in the interior of the East Antarctic Ice Sheet that are then reflected in the sediments of Prydz Bay and the adjacent slope and rise (Fig. F1). Prydz Bay is typical of the Antarctic shelf in having its deepest areas inshore, near the front of the Amery Ice Shelf in the Amery Depression (Fig. F1). On the eastern side of the bay, the seafloor shoals to depths as shallow as 200 m in Four Ladies Bank. Four Ladies Bank is separated from the Princess Elizabeth Land coast by a series of deeps and saddles collectively known as the Svenner Channel. The Amery Depression is linked to the shelf edge by Prydz Channel, which occupies the western part of the bay and has depths from 700 meters below sea level (mbsl) at its inshore end to ~500 mbsl at the shelf edge.

Prydz Channel Fan can be seen in bathymetric contours as a smooth seaward bulge directly north of the mouth of the Prydz Channel and in isopach contour as locally thick sediment overlying the surface (pp-12) that defines the base of the fan (Fig. F2) (O'Brien, Cooper, Richter, et al., 2001). The fan extends to water depths of ~2400 m and has a surface slope of ~2°.

O'Brien and Harris (1996) inferred that the Prydz Channel and Fan developed in the early to mid-Pliocene, when the amount of ice accumulating in Princess Elizabeth Land on the southeastern side of Prydz Bay increased to the point where it deflected the flow of the main Lambert-Amery system westward. The system then formed a fast-flowing ice stream that cut Prydz Channel and deposited debris on the upper slope.

Data

The Prydz Channel Fan is imaged by a number of seismic surveys; however, the most comprehensive network of data is Australian Geological Survey Organisation (AGSO) Survey 149, which consisted of lines collected in 1995 using a single generator-injector (GI) gun (45-in³ generator chamber) and a four-channel, 25-m streamer (Fig. F3) (O'Brien et al., 1995). The lines were arranged with dip lines normal to fan contours and tie lines at the base of the fan and on the shelf. Two lines were shot on the slope west and east of the fan to investigate contrasting settings. Ice conditions made the collection of along-slope lines in midfan impossible in 1995, and again in 1997, a large iceberg restricted the length of along-slope lines that could be shot (Harris et al., 1997).

Reflection Geometry

In dip section, the Prydz Channel Fan slopes seaward in a continuous concave-upward curve with an average slope of 2° (cf. Laberg and Vorren, 1995) and with only small steps in the profile (Fig. F4). The fan sediments extend >50 km from the shelf edge. The steps on the upper fan are probably small slump scars or, near the shelf break, ice keel scours. Farther down the fan, the steps are down to basin and are probably the noses of debris flows that have halted on the slope. Internally, the fan displays clinoforms in packages that pinch out at the base of the fan or, in the case of the uppermost packages, in midfan where the pinchout corresponds to a step in the seafloor profile (Fig. F4). In sections across the slope in midfan, the fan forms a broad mound with smaller-scale mounds on its surface (Fig. F5). These vary from 0.5 to 30 km across. Small gullies with levees are present in a few places, but these are not common. At the toe of the fan, the overall mound shape is less obvious on strike sections, but the smaller mounds and channel-levee systems are more obvious. Mounds are up to a few kilometers across and 10 to 20 ms high and overlap in an irregular fashion (Fig. F6). The channel-levee systems are of similar scale but comprise levees arranged symmetrically about a channel with a floor that produces high-amplitude reflections (Fig. F6).

The fan foreset reflectors form moderately continuous packages 10-20 ms thick of high-amplitude reflectors separated by reflection-poor intervals (Figs. F4, F6). The reflector packages pinch and swell, and some channel structures are present. Reflection-poor intervals show faint reflectors parallel to the basal or mounded reflectors (Figs. F4, F6).

The foresets forming the upper continental slope seaward of Four Ladies Bank extend some 25 km from the shelf edge and are concave upward, sloping as much as ~4° (Fig. F7). Several mounded deposits are visible on the surface, suggesting slump deposits. Foreset reflections are less continuous than in the trough mouth fan seaward of Prydz Channel and show abundant mounding and channeling.

The prominent surface at the base of the fan, surface A of Mitzukoshi et al. (1986), was mapped on both the 1995 data and on older, lower-resolution lines. It can be traced along the slope adjacent to the trough mouth fan and onto Four Ladies Bank. This surface marks the beginning of fan growth, as determined by tracing surface A beneath the continental slope to a paleoshelf break that is linear across Prydz Channel and parallel to the shelf on both sides of the fan. On the GI gun lines, 12 horizons that show some truncation or downlap could be seen; however, only 5 could be mapped confidently on most lines. These are surfaces pp-2, pp-4, pp-5, pp-7, and the fan base (surface A), herein designated pp-12 (Fig. F8). Close examination of an expanded display of the seismic data indicates that pp-12 intersects Site 739 within the early Pliocene interval that includes Cores 119-739C-13R to 15R (105.9-130.3 meters below seafloor [mbsf]) (Barron et al., 1991).

Topset-Foreset Relationships

The seismic stratigraphy of both the Prydz Channel Fan and the adjacent Four Ladies Bank provides some insights into sedimentation processes at the head of trough mouth fans. Line AGSO 149/0901 runs along the fan axis into Prydz Channel (Fig. F3). Prior to fan deposition, the shelf aggraded. The base of the fan (surface pp-12) shows as a high-amplitude reflector with a convex-upward seaward dip that passes landward into a horizontal, high-amplitude topset reflector (Fig. F9). Surface pp-12 passes offshore into steeply dipping, undulating reflectors of poor continuity and variable amplitude. From pp-12 time onward, shelf aggradation occurred on the edge of Prydz Channel but was minor in the center of the channel where progradation prevailed.

Seaward of the pp-12 paleoshelf edge, overlying foresets offlap against gently seaward-dipping topsets ~0.15 s two-way traveltime (TWT) thick (Fig. F9). This geometry is repeated but with the base of successive sequence-bounding topsets being progressively higher.

On Four Ladies Bank, away from the trough mouth fan, paleoshelf edges aggraded vertically by 0.25 s TWT until ~pp-4 time with only minor prograding of the paleoshelf edge between pp-12 and pp-7 (Fig. F10). From pp-7 time onward, the shelf edge prograded rapidly, forming an upper slope wedge with greatest thickness beneath the present shelf edge. Topsets display high amplitudes, especially those pp-12 and older. Surface pp-12 is onlapped by topsets to a point ~8 km landward of the pp-12 paleoshelf edge (Fig. F10).

Thickness Relationships

A series of isopach maps illustrates several major features of fan deposition. The total fan thickness (seafloor to pp-12) (Fig. F2) shows that the locus of fan deposition was beneath the present shelf break where the axis of Prydz Channel crosses the shelf break (Fig. F2). Prydz Channel has very thin post-pp-12 sediments, whereas Four Ladies Bank shows a relatively thick, horizontal layer of similar age sediments. On the slope, post-pp-12 sediments are slightly thicker on the eastern side of the Prydz Channel axis than on the western side. Also, sediments thin more rapidly on the western side than the eastern side.

The initial phase of fan sedimentation (pp-12 to pp-7) produced a low-relief wedge of slope sediments that was slightly thicker beneath the eastern side of the area seaward of Prydz Channel (Fig. F11). This suggests that a broad channel fed sediment to the shelf edge, although the contours are influenced by the thick topsets deposited on Four Ladies Bank prior to surface pp-7 (Fig. F10). The next phase (Fig. F12) (pp-5 to pp-7) saw deposition of a pronounced lobe seaward of the present Prydz Channel axis. Prydz Channel appears to become narrow during this phase of deposition, leading to an approximation of a point source for fan sediment and rapid progradation of the shelf edge along the channel axis. Subsequent sequences show seaward displacement of the locus of thickest deposition (pp-2 to pp-5).

The final stage of fan sedimentation (Fig. F13) (pp-2 to surface) consists of a relatively thin layer that drapes the fan and thickens to the west, with the thickest parts of the sequence on the middle to western side of the fan.