Regional Seismic Stratigraphic Variations Similarities and Differences

Stratigraphers have long recognized sedimentary sequences and bounding regional unconformities in seismic-reflection data across the Antarctic continental margin (e.g. Hinz and Block, 1984; Wannesson et al., 1985; Hinz and Kristoffersen, 1987; Cooper et al., 1995). These sequences, principally of inferred Cenozoic age, commonly have similar seismic geometries around Antarctica (e.g. Cooper et al., 1991b; Anderson, 1999). Some geometries are unique to polar continental margins, whereas other geometries are like those of low-latitude non-polar margins (e.g. Hinz and Block, 1984; Bartek and Anderson, 1991; Table D-1). A unified circum-Antarctic seismic stratigraphy does not exist, but an International effort to compile one via the CASP project is in progress (Davey and Cooper, 2007). Numerous separate and sometimes different seismic stratigraphies exist for various localities.

Many seismic features have been cited to suggest intermittent ice on the Antarctic margin (Table D-1), but those giving the strongest evidence for ice are the regional seismic unconformities, the broad erosional troughs and depositional banks on the continental shelf, and the large-scale fans and

Table D1-1: Common large-scale geomorphic and seismic stratigraphic features of the Antarctic continental margin (listed by location and decreasing inferred age).

Feature Regions Processa Timingb

Continental shelf

A. Deep regional PB, RS seismic unconformities on the shelf (lower ~1/3 of sedimentary section)

B. Shallow regional All seismic unconformities on the shelf (upper ~2/3 of section)

C. Prograded and All aggraded sequences under the outer shelf

D. Mound features with All chaotic seismic facies

E. Broad cross shelf All troughs and adjacent banks

F. Overdeepened and All foredeepened continental shelf

Continental slope

G. Regional seismic All unconformities

H. Massive sediment PB, WS fan on the slope at the outlet of a broad cross-shelf trough

I. Steep slopes and All migrating high-relief channels

J. Variable size sediment WL, RS, fans on the upper AP

slope at mouths of seafloor and buried troughs.

Continental rise

K. Regional seismic All unconformities

Eustacy

Ice-sheet erosion

Sediment carried to the shelf by glaciers Ice-sheet deposition

Ice-stream erosion

Ice-sheet erosion

Bottom currents

Sediment deposited at shelf edge by one broad ice stream Bottom currents with coarse sediment Sediment deposited at shelf edge by multiple ice streams

Bottom currents and diagenesis

Cretaceous to late Oligocene (?)

late Oligocene (?) and younger

Oligocene and younger late Oligocene and younger early Miocene and younger mid-Miocene and younger

Cretaceous and younger early Oligocene and younger (WS); early Pliocene and younger (PB) Oligocene and younger late Miocene and younger

Cretaceous and younger

Table D1-1: (Continued).

Feature

Regions

Processa

Timing

L. Large sediment drift All features

M. An up-section All landward shift of depocenters from rise to slope.

Bottom currents and downslope sediment supply Reduction of sediment supply to rise early Miocene and younger mid-Miocene and younger aThe principal process is listed - others such as lithospheric loading, sediment loading, paleoceanographic processes, diagenesis, etc. are also commonly involved. bInitiation time of features varies in different regions.

prograding deposits on the continental slope at the mouths of the shelf troughs. These features are observed on all sediment-covered segments of the Antarctic margin, in East and West Antarctica, and principally occur in the upper part of the stratigraphic section. These features are increasingly common up-section, indicating more abundant glacial events more recently. The ubiquitous overdeepened and foredeepened depth profile of the continental shelf is the ultimate evidence of sustained strong glacial erosion of the entire Antarctic margin.

Ten Brink et al. (1995) modelled the geometries of the seafloor and stratigraphic sections on the continental shelf and upper slope, incorporating lithospheric, glacial and eustatic processes in the models. They showed that multiple advances and retreats of grounded ice sheets across the continental shelf, coupled with redistribution of sediment from onshore and shelf areas to the continental slope, are required to match the observed geometries. Eustatic and paleoceanographic processes are important for sediment redistribution, especially on the continental slope and rise, but are not sufficient by themselves to explain the shelf erosion and prograding geometries beneath the outer continental shelf. Bartek et al. (1991) illustrated that the stratal signatures of the Oligocene and younger sections in the eastern Ross Sea (i.e. unconformities and prograding sections) are similar to those of low-latitude non-polar margins, indicating that the Neogene stratal signature results from glacio-eustatic fluctuations.

Seismic data provide a relative history of increasing circum-Antarctic glacial events, as noted above, but drilling and seafloor coring provide the only absolute age control and ground truth of glacial lithologies and processes. Definitive ages are limited due to the small number of cored sites, which in turn are biased toward sampling of shallow younger sections.

The lithostratigraphic record from proximal drilling and dredging on the Antarctic margin 'establishes' a general history (at the drill sites) of no regional glaciers in Cretaceous and earlier times on the shelf (PB, RS, WL) and on the slope (WS). Evidence of glacial episodes is first seen in the late Eocene to early Oligocene as diamicts from grounded ice on the shelf (PB, RS) and from glacial erratics on the slope (WS). Upper Oligocene glacial marine deposits are sampled on the shelf (RS) and slope (WS). Lower Miocene sections show increasing evidence of ice and deep-ocean currents, with IRD and sediment drifts on the rise (PB, WL, AP, WS?) and glacial marine sediments and diamicts on the shelf (RS). The middle Miocene has an increased glacial hemipelagic signature on the rise (PB), and glacial marine deposits on the shelf (RS). The late Miocene and early Pliocene were times of enhanced glacial activity, as recorded by: (1) shelf deposits of glacial diamicts (PB, AP) and glacial marine sediments (RS, AP); (2) glacial marine sediments on the slope (WS, WL); and (3) rise deposits of glacial hemipelagics (PB, WL, RS, AP) and turbidites (WL, WS, AP). Upper Neogene deposits are principally glacial on the shelf (PB, RS, AP), slope (PB, WS) and rise (PB, WL, RS, AP, WS). Based on recovered drill cores and Antarctic Peninsula coastal geology, the general history of events for East and West Antarctica is essentially the same since the early Oligocene. However, the middle Miocene and Oligocene history for offshore West Antarctica is based on only two Ross Sea DSDP cores (Hayes and Frakes, 1975) and two tentatively dated SHALDRIL cores from the western Weddell Sea (Anderson et al., 2006; Shipboard Scientific Party, 2006).

The above general history at the sparse drill sites has been greatly expanded by many investigators who have traced seismic unconformities and seismic stratigraphic units from core sites and rare onshore sedimentary sections up to hundreds of km to infer ages and lithofacies for key stratigraphic features listed in Table D-1. The expanded seismic stratigraphic history includes inferences of the following (although uncertainties about these inferences are inherently greater than the uncertainties about ages at drillsites):

• A pre-ice-sheet to early-glacial period (Cretaceous to early Oligocene): On the inner and mid shelf, variable seismic facies (WRS, PB), narrow channel geometries (PB) and sea-floor dredged/cored rocks indicate the presence of pre-ice-sheet subaerial, fluvial and shallow marine environments in the Cretaceous. These are unconformably overlain (WRS, PB) by upper Eocene to lower Oligocene early glacial sediments. This transition, from pre-ice-sheet to early glacial conditions, has not been sampled elsewhere on the shelf. Beneath the slope and upper rise, thick sediment sections are observed (all areas), but are not yet well imaged and mapped in most areas.

• Aggrading shelf period (Oligocene): On the shelf, stratal geometries in drilled Oligocene glaciomarine sections mostly aggrade the PSEs in PB and ERS, but PSEs appear to prograde where the shelf is strongly uplifted and eroded (WWS). Aggrading PSE geometries occur in unsampled areas of the outer shelf (WL, AP). On the slope, geometries show high-relief paleo-slope-canyons (PB, WL, RS, WS), and in the WS, deposition of the massive Crary Fan (WS) began. On the rise, seismic facies indicate higher energy depositional environments with paleo-channel-levee systems first developing (PB, WL, RS, AP, WS). Where drilled (PB, WL, RS), lithologies from this period have glacial components indicating onshore glaciers. Backstripping calculations indicate normal shelf water-depths (PB, RS).

• Uniform prograding shelf (early and middle Miocene): In many regions, seismic sequences uniformly prograde the continental shelf edge, with an up-section increase in the dips of foreset-beds (glaciomarine deposits) and variable erosion of topset strata (with diamicton) (PB, WL, RS, AP, WS). Sea-level stratigraphic control began to shift to ice-dominated stratigraphic control, with documented cyclic shelf erosion by grounded ice sheets. Initial regional erosion and overdeepening of East Antarctic shelves commenced (PB, RS, WL). Slope geometries indicate canyon shifting and infilling (PB, WL, RS) and fan growth (WS), with rise geometries showing the construction of large drift mounds (WWS, PB, WL, AP) and channel-levees (WL, WS, AP, PB). Abundant contourite deposits (PB, AP) with some turbidites (AP, WL) are documented. Glacial and interglacial sediment volumes decreased on the rise (PB, AP), but increased on the slope (WS).

• Local and focused prograding shelf (late Miocene to Pleistocene): A prominent regional unconformity occurs in the late Miocene to early Pliocene across all margin segments (PB (A, PP12); WL (U8); RS (U2); AP (BGMS) WS (W5)). The unconformity marks a circum-Antarctic change from areas of uniform PSE progradation to local-arcuate and broadly focused PSE progradation into small and/or overlapping upper-slope fans (e.g. QML, WL, AP) and broad trough-mouth fans (e.g. PB, WS) lying at or near the end of cross-shelf troughs. Strong regional shelf erosion, by both narrow and wide ice streams early in this period, was followed by regional deposition of topset banks composed principally of glacial diamicton (PB, AP, WL, WS?) and glacial marine (RS) sediment. Periods of pelagic sedimentation on the shelf indicate open water and sea ice. On the slope, foreset dips steepened as fans formed above the unconformity. On the rise, sedimentation rates decreased (PB, AP) and depocentres shifted progressively landward, moving from the rise to beneath the slope (PB, WL, AP,

WS, AP?). Stratigraphic control was dominated by episodic grounded ice, with sediment deposition by ice and sediment distribution by ocean-currents.

• Sediment drape (late Pleistocene and Holocene): Thin well-layered acoustic units commonly infill shelf depressions and drape across the slope and rise (All regions). These units provide a record of pelagic sedimentation from the last few interglacial and glacial periods. Deep inner-shelf basins, in particular, trap Holocene biogenic sediments with that were deposited at very high sedimentation rates; these biogenic sediments yield an ultra-high-to high-resolution (i.e. decadal to millennial) record of climate variability (AP, WL, PB).

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