Ages and Interpretation of the S3 Depositional Environment

Age assignments based on shipboard observations (smear slides of core-catcher sediment) are given in Fig. 2.1. Lithological features observed within the sediments recovered at Site 1103 (seismostratigraphic Unit S3) are consistent with an environment of very rapid deposition (dewatering stmctures in Fm and absence of bioturbation and insitu marine biota) on a glacially (poor sorting, size of clasts) influenced slope (tilt of beds, energy required to transport and Support clasts). Transport mechanisms for the observed diamictites are debris flows and turbidites or muddy debris flows for the sand- and silt/mudstones. Common features within the matrix (e.g. colour) and clast types (e.g. the mud rip of clasts) suggest

68

CHAPTER 2: The West Antarctic Shelf

differentiation of sand and mudstones out of the diamict facies during transport or a close-by cornmon source for all three facies.

Fig. 2.1. Age constraints - Biostratigraphic Summary of shelf sites (Holes 1097A (Fig. 1.2), 1100D, and 1103A (Fig. 2.1-Fig. 2.1)), based 011 shipboard observations and seismic col~elation. Light gray horizons are material with very rare diatom occurrence. Age assignment of S3 (4.5-4.6 Ma, Camerlenghi et al., in press) at Hole 1097A is based on benthic foraminifers, diatoms, and radiolarian biostratigraphy. Age assignment of the base of SI at Hole 1103A is based on diatom biostratigaphy. Dark g a y horizons show intervals barren of microfossils.

Note seismic unit S2 is rnissing at Site 1103 and the top of S3 at Site 1103 (early late Miocene) may be older than at Site 1097 (4.5-4.6 Ma). Noteworthy for later considerations: sediments above the unconformity S11S3 contain the top of Thalassiosira vulr~ifica giving a minimum time constraint for the age of the unconformity.

The relative dominance of diamicts vs. muds and sands may be an indicator of the proximity of the source or of the higher potential for this facies to be preserved. Muds were probably deposited On diamict bed tops between flow events and were subsequently reworked and incorporated in successive flows (Eyles et al., 2001). Mud and silt clasts were imported from lower energy marine settings with high biological productivity. The induration of these clasts (angular shape) indicates that their source was stable over a considerable time (e.g. an interglacial). Two specific scena~ios have been proposed by Eyles et al. (2001) and the ODP Leg 178 Shipboard Scientific Party (1999):

Unit S3 may represent the upper continental slope forsets of the prograding shelf with a paleo shelf edge close to the midshelf high (see chapter 2.1).

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CHAPTER 2: The West Antarctic Shelf

Alternatively, the facies of Unit S3 may be representatives o f the ,,till deltas" argued to be forming at the grounding line of ice streams (described for the ROSS Ice Shelf by Alley et al., 1989). Similar features have been proposed for the shelf o f the Antarctic Peninsula based on deep-tow boomer data (Vanneste and Larter, 1995).

2.5 Seismostratigraphic Correlation, Interpretation (SI-S3) and Shelf Model

2.5.1 Log to Seismic Correlation ( S l , S3) and Seismostratigraphic Interpretation (S 1-S3) Even though these are no samples o f topset Unit 1, FMS images and the logging data (e.g. the magnetic susceptibility log o f the GHMT Tool) allow evaluation o f downhole clast distribution and some information on the orientation of single larger clasts. Igneous rock fragments have high magnetic susceptibility values due to the magnetic minerals they carry, and they have low electrical resistivity due to the lack o f pore-space with free movable ions (bright spots in the FMS record). In our logs high magnetic susceptibility (magsus) values correspond to FMS intervals with abundant clearly detectable clasts (Fig. 2.1-Fig. 2.3).

Therefore the magsus profile will be used for an attempt to identify lithological introduced amplitude variations in the seismic record o f S 1.

For direct comparison the magnetic susceptibility values have been converted to time using the time-depth function established in chapter 2.2 and plotted at the same scale as part o f line 195-152 (Fig. 2.1). Logging units with high magnetic suceptibility values are marked with yellow, lower susceptibility units with light gray (Fig. 2.1). The apparently weak impedance contrast between the gravel rich and gravel poor units (the latter possibly related to interglacials) is represented in the seismic section by low amplitude, discontinuous reflectors (Fig. 2.1, Fig. 2.1, and Fig. 2.2). Commonly changes from high to low (Ic-11, IIIb-IIIc and IV- V ) and low to high impedance contrasts (Ia-Ib, 11-UIa and within the upper third o f Unit V ) Start with a positive reflector. However, the low to high impedance change between Unit IIIc and IV correlates within a weak negative reflector. The last ,,regularC', gravel rich lodgement till deposit o f the topsets (seismic unit S 1, logging unit IV, Fig. 2.5, Fig. 2.1, and Fig. 2.1) rests 011 a thin low magnetic susceptibility Zone followed by a section with the in'egular high magnetic susceptibility values o f logging unit V . Together with other logging data and the FMS images (Fig. 2.1B) this Zone may represent a clay rich ,,gougeL' type layer on top o f a brecciated reworking horizon. No imbricate fabric has been found within this layer, and the clasts are small and oriented horizontally. Very high resistivity changes at the clast borders may be attributed to the angular shape o f these components. As discussed earlier (chapter 2.2

CHAPTER 2: The West Antarctic Shelj

and 2.3) the strong black reflector (negative amplitude) is the seismic representation of the shelfwide S 1lS3 OS S 1lS2 unconformity (Fig. 2.1). Overall, the match between logging units, subunits and reflector packages is excellent and will improve other seismostratigraphic studies and depositional interpretations.

The sediment recovered from Sequence Group S l at Sites 1100 and 1103 (Leg 178 Shipboard Scientific Party, 1999), logging data, resistivity images, and the detailed cosselation agree with the scenario that a grounded ice sheet regularly extended across the continental shelf during the development of S 1. Individual prograding sequences, recognized by truncation and downlap of reflectors in sequence Group S 1, also suggest repeated episodes of ice advance and retreat. The internal discontinuity of reflectors within S l represents erosional removal of a formerly continuous till or interglacial biogenic deposit. Logging Units 111 a and b at Site 1103 consist of two gravel sich tills. The depositional model presented in Chapter 2.5.2 assumes that a glacial cycle deposit consists of a basal till layer with "iceberg- turbated" and RD-sich finer interglacial On top, however here this interglacial layer is missing. Further seaward in the seismic record (Fig. 2.1A) a new reflector package appears at the proper seismostratigraphic location. It may be speculated that prior to deposition of Unit Ia (and related erosion during ice advance) this ,,interglacial" layer might also have been present at Site 1103. Alternatively to the interpretation of the low magnetic susceptibility layers as being associated to interglacials, the topsets could consist of stacked lodgement tills with interglacial deposits having been removed OS being too thin to be detected as independent seismic packages.

The top of sequence group S3 cossesponds roughly to an improvement in sediment core recovery at 247 mbsf. The upper part of S3, above the three strong reflectors C, d, and e (Fig. 2.1 and Fig. 2.1), consists dominantly of massive diamict with low internal impedance contrasts (Fig. 2.1). Below these weak reflectors the strong seaward dipping reflectors (C, d, e) represent impedance contrasts within the slope-related diamict, sandstone, mudstone succession recovered in the cores 31R to 38R. The proper scaling and plotting of density and velocity derived impedance indicators reveal that previous ideas concerning the cause of acoustic impedance changes related to lithological changes used to explain the reflector origin are not entirely valid (Leg 178; Shipboard Scientific Party, 1999). The first strong acoustic contrast related to lithologic changes actually is seen recovered between the massive and stratified diamictite (-274- 286 mbsf) and laminated mudstone (Fl). Tentatively this impedance change is related to reflector C (Fig. 2.1, Fig. 2.1, and Fig. 2.1). The next major impedance change occurs within a diamictite unit, where diamictites with dominantly

CHAPTER 2: The West Antarctic Shelf

crystalline clasts (33R) overlie a diamictite unit containing silt rip-up clasts. As described in chapter 2.2. large crystalline components are contribute significantly to the average P-wave velocity of a sediment. Tentatively this impedance change is related to reflector d (Fig. 2.1, Fig. 2.1, and Fig. 2.1). The lithological origin of the weakest of the three reflectors e (Fig. 2.1, Fig. 2.1, and Fig. 2.1) is less well constrained. Possible candidates for the reflector origin are a deformed till horizon with fewer clasts recovered in core 36R, or the thin sandlayers of core 37R (Fig. 2.1). The observed seaward increase in amplitudes of this reflector package (see seismostratigraphic description chapter 2.1.4) may be due to better preservation of interglacial related muds and sands further down the slope. Alternatively, if the idea of facies separation out of a single diamict during mass wasting is valid (see chapter 2.4.2), longer downslope transport would enhance the separation of the facies S and F. The transitions observed between the three facies are not gradual enough to support the theory that facies D, S, F originale from a single mass wasting event. In any case it must be recalled that only 34% of the Unit S3 record has been recovered.

Drilling results from Sites 1100, 1102, 1103 (Leg 178; Shipboard Scientific Party, 1999) indicate that the three Sequence Groups S l , S2, and S3 were deposited under a glacial regime. Foresets (S3) and topsets have their origin in a periodically advancing grounded ice sheet to reaches the shelf edge in some cycles. Previously the glacial section drilled in the shelf transect of the Antarctic Peninsula had been inteipreted as changing upward from deposits representing a proximal glaciomarine environment (sequence group S3) to principally subglacial Strata deposited beneath the base of a grounded ice sheet on the shelf (topsets of sequence group S l and S2, Leg 178; Shipboard Scientific Party, 1999; Eyles et al., 2001). This interpretation is not entirely valid - conclusions regarding the depositional environment (more glacial, less glacial) should not be drawn across different settings (slope foresets, topsets). To observe changes in these settings time equivalent cores representing the seismostratigraphic packages of topsets and foresets of S l and S2 would have been required.

Leg 178 failed to provide these records.

During deposition of S2 and S I , glacial sequences are characterized by low-angle topsets and steepening foresets that prograde the margin by 20 km (along sesimic line 195-152 or AMG845-08) and aggradate the outer shelf by 200 to 300 m.

CHAPTER 2: The West Anrarctic Shelf

Fig. 2.1. Detailed graphic correlation of the time converted magnetic susceptibility profile (B, logging data) to reflector packages of the S l topset Unit at Site 1103 ( C , SP (shot points): 1700-1710). The section is composed of discrete intervals with higher (light gray) and lower (dark gray) gravel abundante. Within logging unit 111, ontop of IIIb an interval of lower magnetic suceptibility is missing (due to erosion) further seaward of Site 1103 the rnissing section appears in the seisrnic profile (A). Anomalous values in the magnetic susceptibility log near 0.78 sbss (1 17 mbsf) may be caused by the APS bow spring, which had been lost in the hole during logging.

CHAPTER 2: The West Aniurctic Shelf