Late Neogene geomorphological and glacial reconstruction of the northern Victoria Land coast, western Ross Sea (Antarctica)

Marine Geology 355 (2014) 297–309

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Late Neogene geomorphological and glacial reconstruction of the northern Victoria Land coast, western Ross Sea (Antarctica) Chiara Sauli ⁎, Martina Busetti, Laura De Santis, Nigel Wardell Istituto Nazionale di Oceanografia e di Geofisica Sperimentale (OGS), Borgo Grotta Gigante 42/c, 34010 Sgonico, Trieste, Italy

a r t i c l e

i n f o

Article history: Received 26 June 2013 Received in revised form 16 June 2014 Accepted 19 June 2014 Available online 27 June 2014 Keywords: Antarctica western Ross Sea Neogene seismic stratigraphy

a b s t r a c t This study is a contribution to the reconstruction of the geomorphology and the glacial history of the northern Victoria Land coastal glaciers. High-resolution single-channel reflection seismic lines were collected in 2002 within the framework of the Italian Antarctic programme (PNRA), in Wood Bay and Lady Newnes Bay, north to Cape Washington (western Ross Sea, Antarctica). The data provide evidence of overdeepened marine subglacial valleys, more than 1 km deep and 1–2 km wide, formed along the seaward extension of the Tinker, Aviator, Fitzgerald and Icebreaker glaciers and converging into the major SW–NE ice stream system. The spatial distribution and the geometry of the seismic facies, as well as the direct correlation with the seismic sequences in the Northern Basin, are interpreted to document 1) the depositional activity of a coastal glacial system seaward of northern Victoria Land (NVL) after 18 Ma (based on the seismic correlation with the base of DSDP 273) and possibly in the early Pliocene, in coalescence with expanded ice streams coming from the south along the Drygalski Basin, possibly draining from the WAIS as documented at AND-1B in the McMurdo Sound (Naish et al., 2007, 2008, 2009), followed by 2) the development of TAM tidewater glaciers that carved sea valleys near the Victoria Land coast, onto the shelf. The transition from a dynamic thick ice sheet covering the coastal area of NVL to the NVL valley glaciers advancing and retreating up to about 100 km from the coast in the middle Pliocene would represent a significant environmental change, possibly from interglacial conditions more temperate than today and gradually cooling to a cold and dry coastal regime. © 2014 Elsevier B.V. All rights reserved.

1. Introduction The Ross Sea is one of the key areas where the response of past Antarctic Ice Sheets to climate, tectonic and sea level change can be directly extracted from the thick sedimentary record. It is one of the largest embayments of the present Antarctic margin into which both the marinebased West Antarctic Ice Sheet (WAIS) and the terrestrial East Antarctic Ice Sheet (EAIS) waxed and waned since the onset of glaciations (Barron et al., 1991; Bartek et al., 1991; Denton et al., 1991; Hambrey and Barrett, 1993). Geophysical data document the evidence of ice streams crossing the Ross Sea continental shelf since the mid to late Miocene times (Brancolini et al., 1995a; De Santis et al., 1995; Anderson, 1999). Recurrent episodes of ice grounding up to the shelf edge, with widespread erosion, subglacial deposition and the deposition of trough mouth fans (TMFs), occurred in the late Miocene–early Pliocene in the eastern Ross Sea (Alonso et al., 1992; De Santis et al., 1995) and in the north western Ross Sea (Brancolini et al., 1995a; Bart et al., 2000, 2011).

⁎ Corresponding author. E-mail address: [email protected] (C. Sauli).

http://dx.doi.org/10.1016/j.margeo.2014.06.008 0025-3227/© 2014 Elsevier B.V. All rights reserved.

A general cooling trend is documented by the Cenozoic sedimentary and biostratigraphic record collected in the central and eastern Ross Sea by DSDP leg 28 sites and near the Victoria Land coast by CRP and ANDRILL drill sites (Cape Robert Science Team, 1998, 1999, 2000; Naish et al., 2007, 2008, 2009). The late Cenozoic record is discontinuous, and its interpretation provides controversial hypotheses: one is suggesting that polar and dry condition were established in Antarctica since the mid Miocene (Denton et al., 1991; Sugden et al., 1993; Lewis et al., 2006), the other suggests that temperate, wet glaciers persisted up to the mid Pleistocene, at least in some coastal areas (Webb and Harwood, 1991; Hambrey and McKelvey, 2000; Rebesco et al., 2006). Glacial marine but still temperate conditions were well documented by the Oligocene and early Miocene sections in the Ross Sea (Hayes and Frakes, 1975). Open marine conditions, with limited sea ice and marine water much warmer than today characterised interglacial intervals, in the Pliocene, as indicated by diatom associations in drill core AND-1B in the Southern McMurdo Sound (Naish et al., 2007, 2008, 2009). In the northern Victoria Land (NVL) and in the Terra Nova Bay region, onshore geologic and geomorphological investigations record the occurrence of a predominant polar glacial regime in the mid to late-Miocene (Baroni et al., 2005, 2008; Baroni and Fasano, 2006; Di Nicola et al., 2009). The persistence of dynamic conditions in the

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Miocene is found within the volcanic eruptions (c. 13–5 Ma), of sedimentary sequences of both wet and dry-based glaciers (Smellie et al., 2011a,b). At present the NVL is characterised by a network of sinuous valleys with a dendritic pattern of coastal and outlet glaciers (Baroni et al., 2008). Onshore studies concluded that the Tinker, Aviator, Parker, Fitzgerald and Icebreaker, Mariner and Borchgrevink valley glaciers were isolated from the EAIS by the Transantarctic Mountains until the late Miocene (Baroni et al., 2005), and they have preserved typical polar geomorphological features with negligible erosional power, since 8.2–7.5 Ma (Armienti and Baroni, 1999). A grid of high-resolution reflection seismic data, collected by the Italian programme PNRA/VILD in 2002, provides for the first time new insights into the morphological setting and the record of depositional systems of glacial sea valleys and ridges in the offshore Wood Bay and Lady Newnes Bay (Fig. 1). The results of this work contribute to the reconstruction of the evolution of the NVL valley glaciers in their offshore area during the Neogene and show how they responded to coastal conditions rather than to the EAIS volume changes. The scope of this work is to identify the most suitable sites for future shallow and deep sediment drilling in the two coastal bays in order to date the evolution of the highly dynamic, small NVL valley glaciers and their interactions with the WAIS. 2. Regional setting The Ross Embayment and the Transantarctic Mountain (TAM) are the result of the continental crustal extension and rift shoulder uplift (Fitzgerald et al., 1986; Fitzgerald and Stump, 1997), which occurred during the Late Cretaceous and Cenozoic West Antarctic Rift System (Behrendt et al., 1991; Rocchi et al., 2002, 2005). As a consequence, in the Ross Sea several rift basins were formed, the westernmost of which, close to the Victoria Land coast, are the Victoria Land and Northern basins. The extensional tectonics, together with fluvial, marine and glacial processes, favoured the deposition in these basins of several

kilometres of sediments (Brancolini et al., 1995b,c; Cooper et al., 1987). Since the Eocene, right-lateral strike-slip tectonics overprinted both the extensional basins offshore and the NE–SW fault systems in the northern Victoria Land (Salvini et al., 1997), and a pervasive magmatic activity affected the western Ross Sea/northern Victoria Land area (Kyle, 1990; Armienti and Baroni, 1999; Rocchi et al., 2002; Rossetti et al., 2006). Glacial processes shaped the physiography of the Ross Sea: the loading of the ice caps on the continent and the glacial erosion overdeepened and foredeepened the continental shelf (ten Brink et al., 1995), and the erosion of the north-directed ice streams produced a basin and bank topography. In the western Ross Sea, the morphology is dominated by troughs parallel to the coast, specifically the N–S Nordenskjöld Basin, from McMurdo to the Drygalski Ice Tongue, limited eastward by a series of volcanic islands (Ross, Beaufort and Franklin islands) and submarine volcanoes, and the NE–SW Drygalski Basin, north of the Drygalski Ice Tongue up to the continental margin and limited eastward by the Crary and Mawson banks (Fig. 1). Water depths of almost 1 km are often present in the sea-valleys perpendicular to the coast offshore of the Victoria Land glaciers, for example in front of the Mackay Glacier in the south western Ross Sea and in the Wood and Lady Newnes bays, and exceed 1.5 km north of the Drygalski Ice Tongue (Fig. 1). The drainage basin of the NVL glaciers lies onshore and in the coastal area where mountain peaks exceeding 3500 m are present less than 80 km away from the coast. The NVL dendritic valley network are thought to be shaped by fluvial erosion after the TAM uplift, since at least 55 Ma, and developed along the main NW–SE regional fault systems (Baroni et al., 2005). A significant portion of the valley system trends N–S to NNW–SSE following transtensional faults that developed in post Eocene time (Salvini et al., 1997; Baroni et al., 2005). The glacial morphology is superimposed on the previous fluvial valleynetwork (Baroni etal.,2005). In the northern Victoria Land the magmatic activity has been almost continuous since middle Eocene, with the emplacement of alkaline rift-related plutons, dikes, swarms and volcanoes of the McMurdo Volcanic Group (Rocchi et al., 2002 and references therein). The last

Fig. 1. North-western Ross Sea location map with present bathymetry (contour lines every 100 m) (Davey, 2004), track lines of interpreted seismic lines (multichannel cruises from SDLS: IT88, IT90, USGS, and single channel cruises IT02 and PD90 profiles). The well DSDP 273 site is also shown. The single-channel seismic lines IT02 (Figs. 3–7, 11a), discussed in the text, are indicated with the red bold lines. The IT90AR57 and IT89AR15 multichannel profiles are displayed with dotted red lines. The ages of the volcanic rocks in NVL are displayed in the red boxes (Armienti et al., 1991; Wörner et al., 1989).

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volcanic event was the emplacement of the Melbourne Volcanic Province, belonging to the McMurdo Volcanic Group, N14.5 Ma to the present. This was widely diffused in the coastal area of the northern Victoria Land (Kyle, 1990; Rocchi et al., 2002) with a volcanic cycle that took place 3 Ma ago mainly along the southern coastal belt of Wood bay, between Cape Washington and the Tinker Ice Tongue (Armienti et al., 1991). The timing of magmatic activity is younger from north to south: north of Baker Range the basanites are dated 3.0–2.6 Ma (Armienti et al., 1991), Mount Melbourne stratovolcano has been active since 2.0 Ma (Armienti et al., 1991), and at Cape Washington volcanic outcrops range in ages from 2.7 to 1.67 Ma (Wörner et al., 1989). The total magnetic field anomaly map reveals several positive maxima related to the magmatic bodies of the Cape Washington, Mount Melbourne and the Random Hills, and offshore in Wood Bay from Cape Washington to the Aviator Ice Tongue (Ferraccioli et al., 2000). 3. Data During the summer season 2001–2002, on-board the M/V Italica, as part of the Programma Nazionale di Ricerca in Antartide (PNRA), 1520 km of high resolution single-channel seismic reflection lines was recorded in the Wood and Lady Newnes bays (Fig. 1 and Table 1). The following processing sequence was applied to the data: quality control, amplitude recovery, predictive deconvolution (operator of 60 ms and 5% of white noise), and 30–60 Hz Hi-pass filter. Multichannel seismic data, previously collected by BGR (Germany), MAGE (Russia), OGS (Italy) and USGS (United States), were integrated with the single channel seismic dataset (Fig. 1). The vertical resolution of the datasets varies from 5 to 60 m. There are no drill holes in the study area and indirect age control of the seismo-stratigraphic units comes from site DSDP 273 located about 200 km eastward. DSDP 273 recovered 340 m of glaciomarine mudstone from early Miocene to Pleistocene (Hayes and Frakes, 1975; Savage and Ciesielski, 1983; Hambrey and Barrett, 1993). Three unconformities of late Pleistocene (2.8–0.65 Ma) at 0.8 meter depth, mid-Miocene (14.7–4.0 Ma) at 42.5 meter depth, early Miocene (18.2–16.2 Ma) at 272.5 meter depth, and the well bottom (18.34 Ma) at 340 meter depth (Savage and Ciesielski, 1983) have been correlated to seismic horizons. However, only the horizon correlated to the base of the well (WB) has been continuously traced from the well DSDP 273 to the study area. The path is composed of the single-channel lines PD90-37/ PD90-40 (Anderson and Bartek, 1992), which cross the drill site, segments of the multichannel lines IT88-06 and USGS-411, and the single channel lines IT02-02 and IT02-03 (Fig. 2).

Table 1 Acquisition parameters. Acquisition parameters Vessel Time period Source type Operating pressure Depth Shot interval Streamer Streamer length Hydrophones number Hydrophones distance Record length Sampling rate

M/V Italica 2001/2002 Single G.I. Gun of 105 cu. in. (for 210 cu. in. in Harmonic) and of 250 cu. in. (for 355 cu. in. Harmonic) 140 bar (2000 psi) 6m 8s Single-channel analogic streamer by COL.MAR 3.8 m 20 20 cm 4s 2000 Hz

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4. Results The seismic data grid combined with the bathymetric map shows that the shelf of the Wood and Lady Newnes bays are presently cut by sea valleys, named valleys A, B and C, seaward of the Tinker, Aviator, Parker, Fitzgerald and Icebreaker valley glaciers (Fig. 1). The bathymetric map and the seismic sections perpendicular to the axis of the valleys show that valley A in its proximal segment, close to the coast (Figs. 3, 4 and 8) has gentle symmetric flanks, while valleys B and C show asymmetric flanks, with the steeper one on the south-western side of valley B and on the north-eastern side of valley C respectively (Fig. 8). The valleys are oriented WNW–ESE, perpendicular to the coast, while in the distal segment their orientation changes to WSW–ENE, parallel to the axis of the Drygalski trough (Fig. 1). The valleys are delimited by the south western plateau and the north eastern bank, and separated by the central ridges 1 and 2 (Figs. 1 and 8). The valleys are 2–10 km wide, about 60 km long, with a maximum depth of more than 1000 meter bsl, shallowing to 700 m south eastward. The elevation of the bounding ridges, plateau and bank from the valleys axis is about 440 m (depth conversion of 600 ms, with a water velocity of 1480 m/s). Ridge 1 and Ridge 2 are about 6 km wide (Fig. 1). Ridge 1 and the north eastern bank appear in seismic profiles mostly flat topped and with a terraced-like shaped flank, while Ridge 2 is mound shaped (Figs. 3–6, 8). Three main seismic units, named from the bottom upward, Unit 3, Unit 2, and Unit 1, are identified in the seismic profiles. The seismo-stratigraphic correlation, from drill site DSDP 273 (Fig. 2) to the study area, allows the WB reflector, which lies below the base of the lowermost interpreted seismic Unit 3, to be dated at 18.34 Ma (Fig. 6). The sediments underlying the Unit 3 show, in general, subparallel reflectors (Figs. 5–9) and appear to be locally truncated by the unconformity B (Figs. 6, 7, 9). Unit 3 is bounded at its base by the generally flat erosional surface B and at its top by the irregular erosional surface T. The Unit 3 has a maximum thickness of about 500 ms two-way travel time (TWTT) (Figs. 3– 9), and thins in correspondence of the three valleys A, B and C that are incised into Unit 3 strata. This unit is essentially composed of obliquetangential reflectors, typically interpreted as large scale clinoform sets, downlapping on subhorizontal reflectors, and topped by the landward dipping truncational surfaces. The direction of the Unit 3 clinoform dip on each seismic profile is shown in Fig. 12, where the dashed line marks the location in a plan view of the most seaward clinoform offlap break. In Wood Bay, on the two seismic sections perpendicular to the coast, the foreset beds show apparent SE dip (profiles IT02-14 and IT02-18 in Fig. 9). On the seismic sections parallel to the coast, Unit 3 shows a sub-horizontally stratified and locally massive and chaotic facies (Figs. 3–5 and 8). On two strike lines (IT02-15 and IT02-16 in Figs. 3, 4) inclined reflectors are present along the SW flank of Ridge 1. In Lady Newnes Bay the foresets in Unit 3 have an apparent SW dip (see for example the NE end of line IT90AR57 in Fig. 8). Clinoforms with NE dips are observed in seismic profiles south of Coulman Island (e.g. line IT02-13 and IT89AR15 in Fig. 10a,b), and up to the shelf edge (e.g. line IT88AR06 in Fig. 11). Unit 2 was deposited above the unconformity T and constitutes the uppermost part of the ridges, plateau and bank. This unit is not continuous across the whole study area, as it is interrupted laterally by the intervening troughs (Figs. 3–5, 8). Unit 2 is thickest in the proximal area (200 ms thick) and it thins seaward (Figs. 6, 8 and 9). On the dip seismic section IT02-14 (Fig. 6) Unit 2 appears to be formed by three sub-units (Units 2.1, 2.2 and 2.3), bounded by landward dipping reflectors, truncating the strata below. Clinoforms dipping SE occur in the deepest sub-unit 2.3. The three sub-units have a terrace-like morphology with the youngest terrace being the uppermost and the closest to the coast (Fig. 6). On strike sections, parallel to the coast (Figs. 4–5, 8), Unit 2 is characterised by more complex internal

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Fig. 2. Correlation path from the well DSDP 273 to the study area using the MCS profiles of the Antarctic Seismic Data Library System and single channel PD90 lines. In the site DSDP 273 the two unconformities of Late Pleistocene and Mid-Miocene ages, 14.7–4 Ma (RSU2) and 18.2–16.2 Ma (RSU4), and the well bottom (WB) time line of 18.34 Ma are identified (Savage and Ciesielski, 1983). The single channel lines PD9037-40, that cross the drill site, and segments of the lines USGS-411, IT88AR06, IT90AR57, and the IT02 single channel seismic sections, have been used to trace continuously the well bottom time line (WB) from the DSDP site 273 to the studied glacial valley system in Wood Bay. The B and T reflectors, shown in the figure, are base and top of Unit 3.

configurations, discontinuous sub-parallel internal reflectors and clinoforms with apparent NE dip. Unit 1, overlying Unit 2, is identified in the bottom of the three valleys where it also embodies the most recent infill, with reflectors onlapping the valley walls (Figs. 3, 4, 5 and 8). The isopach map (Fig. 12) of the seismic sequence between surface B and the seafloor (constant interval velocity of 1700 m/s for all units above B) shows the thickening of the sequence, and in particular of Unit 3 on the ridges in the valley system and in the Northern Basin at the shelf edge. The present thickness of the seismic sequence between surface B and the seafloor ranges from zero to 655 m. Vertical normal faults with negligible displacements affect the preUnit 3, Unit 3 and its basal unconformity B and cut only the youngest Units 2 and 1 (Figs. 3–9). The acoustic facies of Unit 3 is characterised by sub-vertical blank zones and diffuse micro-fractures that disrupt the lateral continuity of seismic reflectors. The south-western sector of the study area, between valley A and the Cape Washington coast, is characterised by seismically opaque, domeshaped features, 1–3 km wide, outcropping at the sea floor (Figs. 4–5, 8), deforming both Unit 3 and Unit 2, and in some cases draped by Unit 2 (e.g. in Fig. 8, profiles IT02-16 and IT02-01). South east of Cape Washington, a 25 km wide cone-like feature, comparable in size to the Mount Melbourne Volcano, rises for about 600 m from the sea floor (seen in the seismic profile IT90AR57 in Fig. 8). These features are interpreted as belonging to the Melbourne Volcanic Province that, along the southern coast of Wood Bay, is dated from 3.0 Ma to the present (Armienti et al., 1991) and is correlated to the main positive anomalies in the total magnetic field anomaly map of Ferraccioli et al. (2000). 4.1. Seismic facies interpretation The interpretation of units 3, 2 and 1 is based solely on their seismic facies, geometry and spatial distribution, due to the lack of direct stratigraphic constraints.

The sigmoidal shape of seismic Unit 3, thinning landward and seaward and the orientation of its clinoforms on dip profiles (Figs. 6 and 7), its upper and lower erosional boundaries, and the more complex internal configuration on strike profiles with basal reflectors predominantly subhorizontal and overlying reflectors laterally dipping, could resemble a similar seismic facies to that already described on seismic profiles across the Ross Sea (Hayes and Frakes, 1975; Bartek and Anderson, 1991; De Santis et al., 1995; Bart et al., 2000; Powell and Cooper, 2002; Howat and Domack, 2003), and near the coast of Victoria Land (Brancolini et al., 1995a), and interpreted as grounding-line wedges or ice-proximal fan delta (Alley et al., 1989). A similar seismic facies was interpreted in the southern Victoria Land Basin as a river-deltaic system, in unit Ri–Rj (Henrys et al., 2007; Fielding et al., 2008; Levy et al., 2012). Here, the direct stratigraphic control from sites AND-1B documents that such a prograding wedge formed in a coastal non-glacial open-water setting, during the early Pliocene. Until direct sampling is provided in the northern Victoria Land, we cannot determine if the coastline advance in Wood Bay and Lady Newness Bay, documented by the prograding wedge in Unit 3, was driven by fluvial or glacial processes, or both. Unit 3 covers the shelf area in front of the present northern Victoria Land coastline from the Drygalski Tongue to the Northern Basin, and the direction of the clinoform sets, as observed in the seismic profiles and drawn on the isopach map (Fig. 12), indicates the prevailing Unit 3 deposition paths. These paths are perpendicular to the coast, from shallow to deeper areas: toward SE in Wood Bay; toward SW–SE–NE in Lady Newnes Bay and toward NE (Fig. 11) in the Northern Basin, up to the shelf edge, inside a wide glacial trough that was carved during the B event. The wide and uniform distribution of clinoforms up to the shelf edge and their directions suggest a deposition seaward of a linesediment source, such as at the front of an ice cap, rather than a point-sediment source, such as, at a river mouth or at a sub-glacial outflow tunnel. The geometry of Unit 3 would be consistent with a broad,

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Fig. 3. Seismic line IT02-15 parallel to the coast (see Fig. 1 for the profile location). This is one of the strike lines nearest to the coast in which the south-western plateau and the two valleys A and B are clearly shown. The seismic facies, internally stratified, show the sedimentary origin of the coastal ridges. The SW flanks of valleys A and B are characterised by sharp truncation of Unit 3 sedimentary strata. Inclined, undulated reflectors downlapping onto Unit 3 strata characterise the NE side of valleys A and B and Ridge 2.

grounding-line, subglacial system prograding from the coast eastward and northward. This interpretation also explains the convex external shape of the youngest prograding wedges of Unit 3, its greatest

thickness just seaward the shelf edge, and the landward deepening of the erosional surfaces, within the unit, near the shelf edge (Fig. 11). The direct correlation of the youngest part of Unit 3, with 3–9 units of

Fig. 4. Seismic line IT02-16 parallel to the coast and to the previously described line of Fig. 3 (see Fig. 1 for the profile location). This profile crosses the study area from SW to NE, throughout the southern plateau, the Wood Bay ridges and valleys, up to the north eastern bank. The seismic facies of Unit 3 is opaque or consists of sub-horizontal reflectors. NE dipping reflectors characterise Unit 2 in ridges 1 and 2 and in the bank.

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Fig. 5. Seismic lines IT02-01 and IT02-17 parallel to the coast, across the glacial valleys and ridges of the Wood Bay system. NE dipping reflectors characterise Unit 2 in ridges 1 and 2 and in the displayed flank of the north eastern bank. The sediments of Unit 1 filling the SW part of valley A are interpreted as recent glacial and hemipelagic deposits and particularly sediments that slid along the valley flanks.

Bart et al. (2000, 2011) (Fig. 11) in the Northern Basin, would favour the hypothesis of a glacial origin. Nevertheless given the lack of direct samples and of clear evidence for glacial scouring in this section, the hypothesis of a broad shelf-margin delta deposition in an open sea environment cannot be definitively discounted.

The internal sub-vertical diffuse micro-fractures that disrupt the lateral continuity of seismic reflectors in Unit 3 could be linked, considering the glacial origin of the unit, to the occurrence of dynamic processes such as glacial dewatering and fluid-escape in rapidly loaded sediments (Weigelt et al., 2012).

Fig. 6. Seismic line IT02-14 perpendicular to the coast showing the sigmoidal Unit 3 made of clinoforms dipping toward the SE, downlapping onto subhorizontal reflectors, above unconformity B. In the right side of the line the tapering of the bottomset in horizontal reflectors and the downlap of clinoform onto stratified sub-horizontal reflectors are shown. In the left side three back stepping grounding zone wedges (sub-units 2.1, 2.2 and 2.3) are displayed in Unit 2. They are interpreted as documenting advances and retreats of the coastal glaciers: the highest and innermost wedge is the youngest. The unconformity T truncates the clinoform reflectors of Unit 3, the unconformity B underlies the tapering of bottomset of the horizontal reflectors, and WB is the well bottom reflector from DSDP 273 dated b18.4 Ma.

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Fig. 7. Seismic line IT02-18 is perpendicular to the coast and shows the typical facies in dip lines of Unit 3, made of foreset beds dipping seaward with its external sigmoidal, wedge shape.

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The distribution, the shape and the internal geometry of Unit 2 suggest that a subglacial and ice proximal deposition, tabular till strata and prograding wedges accumulated on the ridges, flanking the glacial sea valleys A, B and C. All the valleys and ridges in Wood Bay have a NW–SE orientation, extending for almost 100 km from the present coast. The position, the width and the orientation of the sea valleys imply that they were incised during the seaward expansion of the coastal glaciers Tinker, Aviator, Parker, Fitzgerald and Icebreaker (Fig. 1), after the deposition of Unit 3. The deposition of Unit 2 over the ridges flanking the glacial valleys occurred mainly on their NE side while the other steep flank was erosional as testified by the apparent direction of clinoforms on ridges 1 and 2 (Fig. 8). The landward dipping sub-units 2.1, 2.2, 2.3, deposited over the Ridge 2 (in profile IT02-14, Fig. 6), are interpreted as retreating grounding-zone wedges which, according to Bart and Anderson (1995), would show stepwise retreats of the ice front toward the coast. The Unit 2 may be of Pliocene–Pleistocene age on the basis of the inferred age of the volcanic features intruding both Units 3 and 2 and lying on the top unconformity (T) of Unit 3 (profiles IT02-16, IT02-01, Fig. 8) suggesting an equal or older age of 3.0 Ma. The volcanic intrusions in the southernmost sector of the study area are not dated directly, but their age is probably coeval to the nearby onshore volcanic outcrops, dated from 3.0 Ma to the present (Armienti et al., 1991). The Unit 1 is formed by the most recent glacio-marine and marine sediments that partially filled the three valleys (Figs. 3, 4, 5 and 8). It is likely the result of deposition during grounding-ice retreat, hemipelagic settling, bottom-current-activity, and recent valley slope failures. Two short piston cores, ANTA02-AV44 and ANTA02-AV45 (Colizza et al., 2004) and subbottom profilers (SBP 3.5 kHz, Finocchiaro et al., 2002)

Fig. 8. Line drawings of seismic profiles IT02-15, IT02-16, IT02-17/01, and IT89-57, parallel to the present coastline. The figure shows the main characters of the glacial valley and ridge system and the main interpreted glacial seismic Units 3, 2, and 1, in the Wood and Lady Newnes bays. Volcanic intrusive and effusive cone-like features are shown in the south western plateau. In light blue are illustrated also the section view of glaciers that were crossing the valleys at the time of deposition of Unit 2. The cartoon shows: thickest glaciers in the valley axis, sharply eroded steep SW flanks of valleys A and B and deposited subglacial prograding fans over the NE ridges.

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Fig. 9. Line drawings of seismic profiles IT02-14 and IT02-18, perpendicular to the coast, and showing the main interpreted seismic units: lower Unit 3 with its seismic facies of prograding laminated, frequently spaced, oblique-tangential clinoforms; Unit 2 generally of sub-horizontal strata, shown on the ridges with different colours, to show their subunit nature and nondirect correlation due the dividing valleys. The shape of the advancing ice front at the stepwise grounding zone wedges (subunits 2.1, 2.2 and 2.3) are drawn in dark blue.

collected along the axis and on the western flank of the valley A (Fig. 1), document that the upper part of Unit 1 is made of laminated biosiliceous mud and glaciomarine sediments overlying thin sandy volcano-clastic silt and subglacial diamictons. This is consistent with the, typically a few meters thick, post-LGM sediments in the area (Shipp et al., 1999). The upward passage from the diamicton to the overlying sandy sediments at 10.3 ka marks the last time when the grounding ice detached and retreated, in agreement with data from the Drygalski area (Cunningham et al., 1999; Domack et al., 1999).

4.2. Correlation with other seismo-stratigraphic studies in the western Ross Sea Two main Plio-Pleistocene seismic sequences (RSS-7 and RSS-8), diachronous and with different thicknesses and ages in different areas, were mapped (Brancolini et al., 1995a,b,c) in the western Ross Sea on the basis of their relative chronostratigraphic position (see Table 2) and their dominant seismic facies consisting of prograding foreset wedges.

Fig. 10. Seismic profiles IT02-13 (a) and IT89AR15 (b) showing the seaward prograding wedge of ice proximal sediments in Unit 3, south of Coulman Island. The interpretation of base and top (B, T) of Unit 3 is shown.

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Table 2 Seismic units in this study, in the Wood and Lady Newnes bays. Correlation to DSDP 273 (Savage and Ciesielski, 1983) and their correspondence to seismic sequences interpreted previously in the western Ross Sea (northern and southern VL coastal area) and in the Northern Basin (NB). DSDP 273 Savage and Ciesielski (1983)

Anderson and Bartek (1992)

Brancolini et al. (1995a,b,c)

Bart et al. (2000, 2011)

Levy et al. (2012)

This study

Central Basin

Northern Basin

Unit 1

1–8 units (undifferentiated) (Plio-Pleistocene)

Northern and Southern VL coast (VLB) and Northern Basin RSS-8 RSU1–Present (2.5–0 Ma) RSS-7 RSU2–RSU1 (4.0–2.5 Ma) RSS-6 RSS-5 (14–10 Ma) RSS-4 (19–16 Ma)

Northern Basin edge continental shelf and upper slope 1–2 units (0.71–Present) 9–3 units (2.08–0.71 Ma)

Southern VL coast (VLB) Rj–Present (3.4–0 Ma) Ri–Rj (~4.75– 3.4 Ma)

Wood Bay and Lady Newnes Bay Unit 2

Hiatus (4.0– 14.7 Ma)

Unit 2

Unit 10

Offshore Victoria Land, the 500 ms thick wedges of the lower RSS-7 sequence were interpreted as glacial fans deposited in front of the TAM wet-based glaciers during interglacial periods in environmental conditions more temperate than today and gradually cooling in PlioPleistocene times (Brancolini et al., 1995a,b,c). The RSS-7 was also mapped in the Northern Basin as an ice-grounding prograding wedge, with an overall northward direction (Brancolini et al., 1995a; Bart et al., 2000). High resolution seismic profiles allowed 7 units (Units 9– 3) to be identified in the two palaeo-troughs of Drygalski and Joides basins and in the adjacent Mawson and Pennel banks (Anderson and Bartek, 1992; Bart et al., 2000, 2011), whose ages span from the minimum age of 0.71 to 2.08 Ma, on the basis of diatom analysis from piston cores (Bart et al., 2011). Along the south western margin of the Victoria Land Basin, the prevalent interval of eastward clinoform sets of Ri–Rj seismic package of Fielding et al. (2008) is interpreted to be equivalent to the RSS-7 sequence. The correlation of the basal reflector Ri to AND-1B (Naish et al., 2008), whose age is constrained to early Pliocene, and the updated analysis of the drill-core sediments (Levy et al., 2012) from McMurdo Sound (CIROS-2, DVDP-11, DVDP-10, AND-1B) suggest open marine conditions of deposition. This sequence was dated at ~ 4.75–3.4 Ma and interpreted as a thick terrigenous clastic succession that prograded into standing water or falling relative sea-level, possibly via deltas (Fielding et al., 2008; Levy et al., 2012). In the western Ross Sea, RSS-8 sequence has been described by Brancolini et al. (1995a) as made up of an isolated set of units, deposited during the most recent episodes of advance and retreat of the Victoria Land glaciers and the ice streams over the continental shelf. In the Pleistocene section till sheets and erosional unconformities, indicative of shelf-wide advances of grounded ice, have been recognised (Alonso et al., 1992; Bart et al., 2000). In the south western VLB the late Pliocene to early Pleistocene (~ 3.4–1.5 Ma) and Pleistocene (1.5 Ma–Present) thinner seismic units indicate changes in sediment distribution linked to a significant palaeo-environmental change and to variation in a glacial regime that accompanied the shift to colder and arid conditions and the onset of modern polar environment (Fielding et al., 2008; Golledge and Levy, 2011; Levy et al., 2012). In the Northern Basin RSS8 comprises the top-most Units 1 and 2 of Bart et al. (2000), is dated younger than 0.71 Ma, and documents some of the last repeated WAIS grounding events on the outer shelf of the western Ross Sea (Bart et al., 2011). The correlation of the seismic Units 3 and 2 from Wood Bay and Lady Newnes Bay to the unit Ri–Rj of the southern Victoria Land Basin is prevented by the erosion and the intrusion of volcanic features and faults, while the correlation of the seismic Unit 3 and its lower boundary from Wood Bay and Lady Newnes Bay to the Northern Basin is direct. The youngest prograding foreset wedges of Unit 3 correspond to Units 9–3 of Bart et al. (2000, 2011) and are part of the RSS-7 sequence. The basal unconformity B of Unit 3 is directly correlated to the RSU2 (equivalent to Unconformity 10 of Bart et al., 2000, 2011) and indirectly

Unit 3

associated to the Ri reflector of Fielding et al. (2008) (Table 2). The RSU2 in the western Ross Sea is a large hiatus spanning several millions of years (14.7–4.0 Ma) at the DSDP 273 (Savage and Ciesielski, 1983) and it represents therefore an amalgamation of different erosional surfaces. The sediments lying above the Ri reflector (of Fielding et al., 2008) in the McMurdo Sound were deposited during the early Pliocene global warming event, at the beginning of the 4.7–3.4 Ma interval, when a broad shelf-delta margin was constructed and the activity of dynamic wet-based valley glaciers was diffuse along the Victoria Land coast (Levy et al., 2012). The top unconformity T of Unit 3, near the coast of the NVL, is assumed to be 3.0 Ma old, the same as the volcanic features of the Melbourne Volcanic Province that appear to rest on it. The T unconformity may be possibly correlated to the Rj reflector of Fielding et al. (2008) marking the transition to cooling climatic conditions. We assume that the younger, thinner and discontinuous wedges of Unit 2 are part of RSS-8; its age is difficult to establish because, due to the erosion in the Ross Sea, it is found only locally and could therefore be diachronous. In the southern Victoria Land RSS-8 is equivalent to the Rj — sea-floor seismic sequences of Fielding et al. (2008) (Table 2). The Rj seismic reflector is suggested to correspond to the c. 2.0 Ma event (Esser et al., 2004; Fielding et al., 2008) that marks the modification of the sea-floor morphology to the present ramp configuration and the transition to the current cold, polar environment as recorded on Antarctic margins (Hambrey and McKelvey, 2000; Rebesco et al., 2006). Alternatively the Wood Bay Unit 3 could be equivalent to the earlymid Miocene Ross Sea Sequence RSS-4, mapped over most of the Ross Sea, spanning from about 19 to 16 Ma, sampled by DSDP 272 and 273.

Fig. 11. Line drawing of IT88AR06 seismic profile in the Northern Basin at the present shelf edge showing the basal and top unconformities of Unit 3 (B and T). It displays the convex external shape of Unit 3 and the landward deepening of the erosional surfaces within its upper part. The correlation with unconformities 3, 5 and 7 of Bart et al. (2000, 2011) is shown.

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RSS-4 shows a seismic facies very similar to Unit 3, that, in the Central Ross Sea and in the Northern Basin, is characterised by large and widespread prograding wedges, made of foreset beds (with height of about 300 ms TWTT), downlapping onto stratified sub-horizontal facies (De Santis et al., 1995). The correlation between the Wood Bay Unit 3 and RSS-4 would be consistent with the stratigraphic position of Unit 3, whose erosional base (the unconformity B) lies above reflector WB, dated b 18 Ma at DSDP 273. Another possible time frame for Unit 3 could be during the formation of RSS-5 (14–10 Ma). In the eastern Ross Sea glacial prograding wedges (De Santis et al., 1995), drilled at DSDP 272, with an age between 14.1 and 13.8 Ma (Hayes and Frakes, 1975), document ice expansion over the continental shelf at the end of the Mid-Miocene Climate Optimum and coincide with the overall global cooling that led to the main expansion of the Antarctic Ice Sheet at 13.9 Ma, that was associated with a global fall in sea level.

5. Discussion Unit 3 is distributed over all the study area and also further to the north up to the shelf edge (Fig. 12). It is lying above a generally flat, sub-horizontal erosional, surface (B) that is likely to be an amalgamation of several erosional events and does not show any evidence of typical glacial or subglacial tunnel troughs in the Wood Bay and Lady Newness Bay. The foreset dips of Unit 3, toward the SE in Wood Bay, toward SW– SE–NE in the Lady Newnes Bay and toward the NE in the Northern Basin (Fig. 12), suggest an overall, coeval, seaward progradation of a depositional coastal marine system with erosion and transport capacity much larger than the modern Antarctic coastal setting.

A transgressive delta extending the coastline about 100 km seaward after the early–mid Miocene (about 4.8–4.0 Ma) was deposited during a free-ice period in the southern Victoria Land Basin, as interpreted and documented in the McMurdo coastal area (Fielding et al., 2008; Levy et al., 2012). The same depositional setting can be inferred for the prograding wedge of the seismic Unit 3 and unit Ri–Rj of Fielding et al. (2008), for their coastal position and similar seismic facies. Direct correlation through seismic profiles is however prevented by erosion in the Drygalski Basin and the presence of volcanic features and faults. Conversely Unit 3 has been interpreted northward to the shelf edge in the Northern Basin and correlated directly with the ANTOSTRAT RSS-7 sequence and the Units 9–3 of Bart et al. (2000, 2011). A glacial origin for the Unit 3 prograding wedge could be explained by a coeval expansion of NVL coastal glaciers, or of a possible ice cap (as was previously suggested by Powell et al., 2001 from the CRP-3 Miocene sedimentary record), that responded to lower altitude coastal conditions with a dynamic behaviour of wet-based glaciers, while the WAIS oscillated frequently, sometimes reaching the outer shelf (early Pliocene glacial maximum) (Fig. 13, Phase I). A non-glacial origin for Unit 3 would imply, as in the Ri–Rj unit in the southern VLB, thick terrigenous wedges prograding eastward via deltas over the continental shelf, during an open-sea warm period when the WAIS advances occurred less often (early Pliocene global warming). Unit 2 is more certainly of glacial origin as previously described in the seismic lines (see grounding zone wedges on IT02-14, Fig. 6). The clinoform beds of this unit are thinner, stacked in packages and have different and less coherent orientation than those of Unit 3, meaning that they were likely to have originated by multiple phases of seaward expansion, in a SE direction, of the Tinker, Aviator, Parker, and Fitzgerald and Icebreaker glaciers. Each carved separate structural marine valleys

Fig. 12. The isopach map of the seismic sequence between surface B and the seafloor. The arrows show the direction of foreset dips on each profile, and the dashed line suggest the limit of the most seaward foreset's offlap break in plan view. Red lines show erosion limits. DSDP 273 site location and interpreted volcanic deposits on seismic lines are also displayed. The tracks of all shown seismic profiles in earlier figures are displayed with black bold lines.

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The modification of depositional style, linked to climatic changes, would implicate the advance of tidewater valley glaciers still responding dynamically to coastal conditions and reflecting the transition into polar to subpolar glaciers, with reduced melt water and frozen basal conditions. The T unconformity between the Unit 2 and Unit 3 is dated, in Wood Bay and Lady Newnes Bay, at about 3.0 Ma in accordance with the Cape Washington volcanic features of the Melbourne Volcanic Province, and the equivalent Rj unconformity of Fielding et al. (2008) in the southern Victoria Land Basin that marks the worsening climatic conditions. 6. Conclusion

Fig. 13. Cartoon of the proposed evolution of the NVL coastal glacial system with NVL glaciers likely coalesced in a local ice cap advancing in the bays during Phase I, the SE expansion of the individual glaciers carving the marine valleys in Phase II and their NE deflection by WAIS ice streams coming from the south in Phase III. The Phase I in the cartoon shows the seaward advance of the NVL coastal glaciers, during a period of time with open sea condition and a reduced WAIS. During Phase I the WAIS, that was oscillating frequently across the continental shelf, did not often reach the outer shelf in the western Ross Sea. The growth of a glacial coastal system is the mechanism responsible for the deposition of glacial sediments in the north western Ross Sea during the Phase I.

on the sea floor, eroding into the sediments of Unit 3 (Figs. 8 and 13, Phase II) and locally depositing the sediments of Unit 2 on the flanking ridges. The enhanced sea-valley erosion and the WAIS retreat would be linked to the persistence of warm climatic conditions and of stilldynamic coastal glaciers during a period in which a cooling trend was establishing and leading to major climate/glacial transition (3.4 Ma). The polar to sub-polar tidewater glaciers developed from the early Pliocene dynamic sub-polar glaciers that were characterising the coastal margin (McKay et al., 2009). A possible explanation of the discrepancy between the direction of the glacialvalleys A, B and C and the apparentNEdip ofthe foresetbedsin Unit 2, on ridges 1 and 2 and in the north eastern bank (Fig. 8), could be that the coastal glaciers, while expanding toward the SE, were deflected toward the NE by an expanding WAIS (Fig. 13, Phase III). This hypothesis would also explain why the axis of valleys A and B are deviated in their distal segment, as shown by the present bathymetric map (Fig. 1). We infer that the deviation of the coastal glaciers toward the NE, implied a different rheology across the ice section that probably caused the erosion of the SW flank of each valley and the deposition over the ridges on the NE flank (Fig. 8). The seismic facies and spatial distribution of Unit 3 and Unit 2 are significantly different and the transition between the two units records a change in the local structural and/or environmental conditions.

High resolution seismic data interpretation and direct correlation with the glacial sequences of the Northern Basin allowed us to interpret the 500 ms thick clinoform sets as ice-proximal sediments deposited, certainly after 18 Ma on the basis of DSDP 273 core data, and possibly in early Pliocene time, in a wide marine area of Wood Bay and Lady Newnes Bay, offshore the NVL coast. A younger glacial sequence with different seismic characteristics was deposited locally on the top of the glacial marine delta probably after 3.0 Ma. On the seismic facies analysis of these units we have inferred a possible scenario for the evolution of the NVL coastal system in the two bays during Plio-Pleistocene: at 5–3.4 Ma the coalesced coastal glaciers of NVL, interacting with an oscillating WAIS, contributed to the deposition of broad prograding wedges eastward in the two bays, and northward in the Northern Basin (Phase I, Fig. 13); about 3 Ma individual still-dynamic NVL coastal tidewater glaciers carved the glacial seatroughs, during an interglacial period in a progressing cooling climatic conditions (Phase II, Fig. 13); the successive intervening northward WAIS re-advance deviated the NVL coastal glaciers, reflecting the increment of ice volume and transition to modern polar environment (Phase III, Fig. 13). The present work presents an opportunity to locate shallow and deep drilling sites in the framework of the ANDRILL or IODP programmes to verify the history of the NVL coastal glaciers, their interaction with a stable EAIS and an oscillating WAIS, and to time constrain the main climate events. Acknowledgements This work was funded by Italian PNRA (Programma Nazionale di Ricerca in Antartide) (PEA2010/A2.04 Sauli). The authors thank the crew of the R/V Italica and the PNRA logistic support for the acquisition of the high-resolution seismic data. The multichannel seismic data were made available throughout the Antarctic Seismic Data Library System (SDLS) and the single channel data were kindly provided by John Anderson and Lou Bartek. The authors thank IHS for providing an educational user licence of the Kingdom Suite that was used for the seismic data interpretation. The authors thank also Ross Powell and David Pollard for the useful advice as well as David Piper, John Anderson, Stuart Henrys and an anonymous reviewer for their helpful suggestions that contributed to the improvement of the manuscript. References Alley, R.B., Blankenship, D.D., Roney, S.T., Bentley, C.R., 1989. Sedimentation beneath ice shelves — the view from ice stream B. Marine Geology 85 (2–4), 101–120. Alonso, B., Anderson, J.B., Diaz, J.I., Bartek, L.R., 1992. Pliocene–Pleistocene seismic stratigraphy of the Ross Sea: evidence for multiple ice sheet grounding episodes. In: Elliot, D.H. (Ed.), Contribution to Antarctic Research Series III. Antarctic Research Series, 57, pp. 93–103. Anderson, J.B., 1999. Antarctica's glacial history. In: Antarctic Marine Geology, Cambridge University Press, pp. 207–248. Anderson, J.B., Bartek, L.R., 1992. Cenozoic glacial history of the Ross Sea revealed by intermediate resolution seismic reflection data combined with drill site information. In: Kennett, J.P., Warnke, D.A. (Eds.), The Antarctic Paleoenvironment: A Perspective on Global Change I. Antarctic Research Series, 56, pp. 231–263.

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