Acoustic Stratigraphy

The approximately 1,500 km long Wilkes Land segment of the continental margin (Fig. WL-1) formed during the Cretaceous separation of Australia and Antarctica (Cande and Mutter, 1982; Veevers, 1987; Sayers et al., 2001; Colwell et al., 2006; Leitchenkov et al., 2007; O'Brien and Stagg, 2007).

The stratigraphy of the margin is known mainly from the seismic stratigraphic interpretation of numerous MCS surveys (Sato et al., 1984; Wanneson et al., 1985; Tsumuraya et al., 1985; Eittreim and Hampton, 1987; Ishihara et al., 1996; Tanahashi et al., 1997; Brancolini and Harris, 2000; Stagg et al., 2004a, b); complemented by surface sediment cores (Domack et al., 1980; Payne and Conolly, 1972; Domack, 1982; Tsumuraya et al.. 1985; Hampton et al., 1987; Ishihara et al., 1996; Tanahashi et al., 1997; Brancolini and Harris, 2000; Leventer et al., 2001; Escutia et al., 2003; Michel et al., 2006); and limited deep geological sampling recovery at DSDP Sites 268 and 269 (Hayes and Frakes, 1975). The best-surveyed area is the eastern Wilkes Land margin (EWL) from the Adelie Coast to George V Land. West of this area (the western Wilkes Land margin-WWL), Japan and Russia collected widely spaced seismic lines that were then augmented during the 2001-2002 Australian Antarctic and Southern Ocean Profiling (ASSOPP) Project (Stagg et al., 2004a, b; Leitchenkov et al., 2007).

5.3.1.1. Pre-ice-sheet stratigraphy

Along the Wilkes Land margin syn- and post-rift sedimentary rocks reach thicknesses of more than 7 km (Stagg et al., 2004a, b). Pre-Eocene syn-rift strata are about 3 km thick and are highly variable in seismic character, with discontinuous, faulted, and tilted strata onlapping the flanks of the acoustic basement (Eittreim and Smith, 1987; Eittreim, 1994; De Santis et al., 2003; Stagg et al., 2004a, b; Leitchenkov et al., 2007).

The thickest (at least 9 km) depocentre of post-rift sedimentary rocks is located in the WWL off the Bud Coast (Close et al., 2007). In the EWL postrift strata are up to 5 km thick across the Wilkes Land continental shelf, slope and rise (Eittreim and Smith, 1987; Hampton et al., 1987; Wannesson, 1990; Tanahashi et al., 1994; De Santis et al., 2003). These strata are well-layered on the continental rise and become less stratified and more discontinuous landward (Eittreim and Smith, 1987; Eittreim, 1994;

Wilkes Land

Terra Adelie

Budd Coast

DSDP Site 268

Surveys

France (IFP) 1982 US Geological Survey 1984 Japan (JNOC) 1983-1996 Italian-Australian 2000 Australian 2000-2002 Russia (RAE) 200!>2g0Z--

W21 - Seismic Line 269 - DSDP Site

Bathymétrie contour (Interval 500 m)

ANTARCTICA

Figure WL-1: Location of multichannel seismic-reflection profiles collected on the Wilkes Land margin. Location of the seismic sections shown in Figs. 2 and 3 is indicated.

Figure WL-2: Multichannel seismic line WEGA W21 and line drawings of multichannel seismic profiles IFP 107 and WEGA W35 showing the overall architecture of the Wilkes Land margin from the continental shelf to the continental rise (modified from Escutia et al., 2005). See Fig. WL-1 for location of the seismic lines.

De Santis et al., 2003). A prominent regional unconformity (WL-U3) within the Cenozoic post-rift section beneath the continental shelf (Fig. WL-2) is believed to be due to erosional processes related to the first advance of grounded ice sheets onto the continental shelf (Eittreim and Smith, 1987; Tanahashi et al., 1994; Eittreim et al., 1995; Escutia et al., 1997; Escutia et al., 2005). The pre-ice-sheet strata below unconformity WL-U3, where resolvable, are flat-lying and less stratified than glacial strata above the unconformity.

Pre-ice-sheet rocks have been dredged from the Wilkes Land continental shelf and slope. On the inner shelf, Mesozoic sediments have been exposed via erosion by late Cenozoic glaciers near the Mertz ice tongue. Lignite was recovered (Mawson, 1940, 1942), and lower Cretaceous brecciated, carbonaceous siltstone was cored (Domack et al., 1980). Other dredge samples in the area, acquired by Leventer et al. (2001), include sedimentary clasts of Paleogene lignites with reworked Early Cretaceous flora. On the upper continental slope off Terre Adelie, Sato et al. (1984) dredged samples of locally derived Oligocene and Miocene limestone and undated sedimentary, metamorphic and igneous rocks of mostly ice-rafted origin.

5.3.1.2. Continental shelf glacial stratigraphy

Glacial sequences on the shelf thicken seaward in prograding wedges (Fig. WL-2). The sequences are deeply eroded by broad troughs that cross the shelf. The troughs are interpreted as the erosional paths of ice streams during times of glacial maxima (Eittreim et al., 1995). Foreset strata are commonly truncated at or near the seafloor beneath the troughs (Fig. WL-2). Topset strata form the banks adjacent to the troughs. Ice is inferred to have moved slowly over bank areas and rapidly in the troughs. Geometry of strata in buried troughs on the shelf suggests to some (Eittreim et al., 1995; Escutia et al., 2000) that the locations of ice streams and their erosional troughs and banks have shifted during consecutive glacial advances.

Regional glacial seismic sequences and unconformities defined by different workers (Table WL-1) in the EWL were renamed and in some cases redefined by De Santis et al. (2003). On the shelf, sequences are truncated by two

Table WL-1: Summary of the terminology assigned in previous publications to the inferred wilkes Land glacial sequences and their bounding unconformities (updated from Escutia et al., 2005). Unconformities (tied with lines) and sequences (in between these lines) are listed from younger at the top to older.

Wanneson

Eittreim

Tanahashi

Escutia

De

Close

Leitchenkov

(1991)

and Smith

et al. (1994),

et al.

Santis

et al.

et al. (2007)

(1987)

Eittreim

(1997,

et al.,

(2007)

et al. (1995)

2000)

2003

-WL-S9 -WL-U8 WL-S8 WL-U7 WL-S7 WL-U6 WL-S6 WL-U5 WL-S5 WL-U4-WL-S4 - WL-U3-

Unit 3

WL1-

Phase 2

UNCONF Al

-WL-S9 -WL-U8 WL-S8 WL-U7 WL-S7 WL-U6 WL-S6 WL-U5 WL-S5 WL-U4-WL-S4 - WL-U3-

regional unconformities, WL-U3 and WL-U8 (Wannesson et al., 1985; Eittreim and Smith, 1987; Hampton et al., 1987; De Santis et al., 2003), and the erosion is thought to result from grounded ice sheets moving across the continental shelf (Tanahashi et al., 1994; Eittreim et al., 1995; Escutia et al.. 1997; Escutia et al., 2005). Eittreim et al. (1995) calculated erosion of 300 to 600 m of strata below WL-U3. Sequences below WL-U8 are dominantly aggradational and sequences above are principally progradational. For unconformity WL-U8, Eittreim et al. (1995) estimated erosional truncation of 350 to 700 m of sediment. Unconformity WL-U8 marks changes in the geometry of the outer shelf progradating wedges, from shallower dips below WL-U8 to steeper dips above (foreset slopes up to about 10°).

During the open-marine Holocene, thick laminated diatom mud and oozes were deposited in deep (> 1,000 m) inner shelf basins, such as for example the Adelie Drift (Costa et al., 2007). Based on AMS radiocarbon dates, this drift has accumulation rates on the order of 20-21 m/k.y. Opal, Ti and Ba time-series show decadal to century variance suggestive of solar forcing and El Nino Southern Oscillation (ENSO) forcing (Costa et al., 2007).

5.3.1.3. Continental slope glacial stratigraphy

Although partly obscured by seafloor multiples, the stratigraphy of the continental slope consists of seaward-dipping reflectors (Eittreim and Smith, 1987; Hampton et al., 1987; Eittreim et al., 1995). Prograding strata above the WL-U8 unconformity downlap and pinch out at the base of the continental slope, but deeper units (i.e. between WL-U8 and WL-U3) continue across the margin (Hampton et al., 1987; Eittreim et al., 1995; Escutia et al., 1997; De Santis et al., 2003) (Fig. WL-2).

Sediments forming prograding foresets were delivered directly to the outer shelf and upper slope as deforming tills at the base of ice streams at times of glacial maxima (Eittreim et al., 1995). Ice-stream delivery of a large volume of unconsolidated sediment to the steep slope resulted in sediment failures that led to the development of large chaotic deposits at the base of the paleoslope foresets (Eittreim et al., 1995; Escutia et al., 2000; De Santis et al., 2003; Escutia et al., 2007). More-recent slope strata are dissected by erosional submarine gullies (Eittreim et al., 1995) and slope canyons (Escutia et al., 2000).

Sea-floor sediment cores from the continental slope contain debris-flow units and numerous hiatuses. The oldest sediment has been dated as late Miocene in age, indicating that gravity flows have been a dominant slope process since at least this time (Escutia et al., 2003).

5.3.1.4. Continental rise glacial stratigraphy

On the EWL continental rise, strata above the WL-U3 unconformity include six glacial-related seismic units, WL-S4 to WL-S9 (De Santis et al., 2003; Donda et al., 2003) (Table WL-1, Fig. WL-3). The two deepest units, WL-S4 and WL-S5, consist of stratified and continuous reflectors that onlap at the base of the slope (Escutia et al., 1997; Donda et al., 2003). Acoustic signatures of isolated channel-levee complexes that characterize turbidite deposition are first observed up-section within unit WL-S5 (Escutia et al., 1997; Escutia et al., 2000; Escutia et al., 2002; Donda et al., 2003). During deposition of units WL-S6 and WL-S7, channel-levee complexes became widespread and turbidity flows were the dominant process building the sedimentary ridges on the rise. Wavy reflectors that are characteristic of bottom contour-current deposition occur on the lower rise in unit WL-S6 and on the upper rise in WL-S7. Within Unit WL-S8, there is evidence for bottom contour-current and turbidite flows, but WL-S8 mostly infills previous depressions (Escutia et al., 1997; Escutia et al., 2000; Escutia et al., 2002; Donda et al., 2003). Unit WL-S9 is a discontinuous unit on the rise, and, where present, comprises channel and levee complexes and layered reflectors (Donda et al., 2003). Recent studies on the WWL margin glacial strata show a similar evolution of the glacial sedimentary sequences (i.e. increased proximal turbidite facies up-section and influence of bottom-contour-current deposition) above unconformities 'eoc' (Close et al., 2007) and WL3 (Leitchenkov et al., 2007), which correlate with WL-U3 on the

4.0MPOtnt600 800 1000 1200 1400 16001^1800 2000 2200 2400 2600 2800 3000 3200 3400

4.0MPOtnt600 800 1000 1200 1400 16001^1800 2000 2200 2400 2600 2800 3000 3200 3400

Figure WL-3: High-resolution multichannel line WEGA W30 across the distal continental rise. Also shown is a line drawing interpretation of this profile after Donda et al. (2003). Location of seismic line is shown in Fig. WL-1.

EWL. Between 110° and 130° large debris-flow deposits are also reported forming throughout the Miocene (Donda et al., 2007a, b).

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