Introduction

Most work in the Argentine Basin up to now has concentrated on the tectonic evolution during and following break-up (e.g., Tankard et al. 1995; Thomson 1998; Hinz et al. 1999; Macdonald et al. 2003; Franke et al. 2006, 2007). The exceptionally thick accumulation of sediments present in the Argentine Basin (Ewing et al. 1964) was concluded to have largely been derived from the continent, and transported to their present-day position by turbidity currents (Lonardi and Ewing 1971). Only in the last decade was it found that the uppermost sedimentary column of the southern Argentine slope and rise is characterized by a complex seismic reflection pattern including evidence of mound facies, giant sedimentary drifts, and numerous unconformities (Hinz et al. 1999). Additionally, a significant contourite depositional system (CDS) at the Argentine margin has been identified even more recently by Hernández-Molina et al. (2009). Such CDSs are generally attributed to along-slope processes (Stow et al. 2002; Rebesco and Camerlenghi 2008); in the present case, this indicates that contour-following currents generated by the thermohaline circulation have played an important role in shaping the seafloor in the Argentine Basin. Between 44°S and 48°S a set of continental slope terraces characterizes the CDS, and reveals both erosional (e.g., channels) and depositional (e.g., sediment drifts) features of contourites (Gruetzner et al. 2010; Hernández-Molina et al. 2010).

Today, the thermohaline circulation in the Argentine Basin is characterized by the interaction of northward-flowing Antarctic water masses (Antarctic Intermediate Water, AAIW; Circumpolar Deep Water, CDW; Antarctic Bottom Water, AABW) and southward-flowing North Atlantic Deep Water (NADW; Arhan et al. 1999, 2002a, 2002b; Carter et al. 2008). The transfer of heat and energy via these water masses constitutes an important component in maintaining the global ocean conveyor belt. Thus, the sedimentary deposits of the Argentine Basin represent an archive of profound changes in deep ocean circulation.

This study investigates the internal seismic structure of a continental slope terrace in 4,100 m water depth, the Valentin Feilberg Terrace, thereby refining an earlier Neogene seismic stratigraphy for the site (Hernández-Molina et al. 2009). Terrace evolution is evaluated in terms of variability in Southern Ocean paleoceanography, climate, and tectonics during the last 17 Ma.

Regional setting

Argentine margin morphology

The Argentine Basin extends over approx. 3.4 million km2 and is bounded by the Rio Grande Rise in the north, the Falkland Plateau in the south, the continental margin in the west, and the Mid Atlantic Ridge in the east (Fig. 1). It has been suggested that the Argentine margin can be subdivided into at least four segments (I to IV) bounded by the Falkland, Colorado, Ventana, and Salado transfer fracture zones (Franke et al. 2007). Each of these large tectonic segments contains thick (>8,000 m) sediments constituting the Argentine sedimentary basin located along the slope and rise (Hinz et al. 1999; Franke et al. 2007; Hernández-Molina et al. 2010).

Fig. 1
figure 1

Regional bathymetric map of the Argentine Basin, with the general circulation of deep water masses: ACC Antarctic Circumpolar Current, AABW Antarctic Bottom Water, LCDW Lower Circumpolar Deep Water, NADW North Atlantic Deep Water, UCDW Upper Circumpolar Deep Water; barriers (yellow) and passages (orange) of deep circulation: DP Drake Passage, IOR Islas Orcadas Rise, MEB Maurice Ewing Bank, M/F Malvinas-Falkland, M/F P Malvinas-Falkland Passage, NGP Northeast Georgia Passage, SRP Shag Rocks Passage (modified after Piola and Rivas 1997, and Arhan et al. 1999). Red lines World Ocean Circulation Experiment (WOCE) profiles along the Argentine margin (17) and across the continental slope terrace sector (11)

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An extensive and complex CDS is located in the southern part (44–48°S) of the Argentine margin between the Malvinas/Falkland and Bahía Blanca transfer fracture zones (Hernández-Molina et al. 2009). The CDS itself consists of two major sectors (Fig. 2a): a submarine canyon and channel sector in the north, shaped mainly by down-slope processes, and an escarpment and terrace sector in the south, dominated by along-slope sediment deposition and erosion (Lorenzo and Wessel 1997; Hernández-Molina et al. 2010).

Fig. 2
figure 2

a Bathymetric map of the Argentine Basin (extracted from Smith and Sandwell 1997) showing the locations of CDS sectors, transfer fracture zones (TFZ), tectonic segments (based on Franke et al. 2007), and the study area (red box). b Shaded relief bathymetry of the study area, with the locations of continental slope terraces (black), other morphologic features (white) and seismic lines reported in this study (red lines)

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The terraces (from west to east) named Nágera, Perito Moreno, Piedra Buena, and Valentin Feilberg (Fig. 2b) are separated by deep channels and moats. Another prominent feature of the terrace sector is a partially buried giant drift (Fig. 2b) that creates a bathymetric step at the base of the slope (Hinz et al. 1999; Hernández-Molina et al. 2009), possibly formed by the southward branch of a confined large loop of AABW from the Eocene–Oligocene boundary (coevally with the Drake Passage opening) until the Middle Miocene (Hernández-Molina et al. 2010).

Oceanographic setting

The abyssal circulation in the Argentine Basin (Klaus and Ledbetter 1988; Reid 1989) is characterized by the complex interplay of various water masses (Fig. 3a). The present-day depths of each of these water masses coincide with the locations of specific sedimentary terraces on the continental slope (Fig. 3b; Hernández-Molina et al. 2009).

Fig. 3
figure 3

a Salinity (color coded) and neutral density (isolines) section along the Argentine continental margin (WOCE line 17, see Fig. 1 for location). AABW Antarctic Bottom Water, AAIW Antarctic Intermediate Water, LCDW Lower Circumpolar Deep Water, NADW North Atlantic Deep Water. b Salinity section across the terrace sector at the Argentine margin (WOCE line 11, see Fig. 1 for location). Terraces: N. Nágera, P.B. Piedra Buena, P.M. Perito Moreno, V.F. Valentin Feilberg (compiled with Ocean Data View; Schlitzer 2010)

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Deep and bottom waters of southern origin enter the Argentine Basin through three main north–south-trending openings (Fig. 1): (1) the saddle of the Falkland Plateau (sill depth 2,500 m), located between the Falkland Islands and the Maurice Ewing Bank (1,200 m), (2) the Falkland Passage between the Falkland Ridge and Islas Orcadas Rise at 35–40°W, with the 5,100-m-deep narrow Falkland Gap at 36°W, and (3) the >4,500-m-deep region extending east of the Islas Orcadas Rise. The net volume of this transport (~100 Sv; Arhan et al. 2002a) is derived from various components (Gordon 1975; Reid 1989; Arhan et al. 1999, 2002b; Orsi et al. 1999; Carter et al. 2008): (1) Weddell Sea Deep Water (WSDW), the deepest water mass in the basin (water depths >4,200 m), (2) South Pacific Deep Water (SPDW, ~3,500–4,200 m), flowing via the Antarctic Circumpolar Current toward the basin after exiting the Scotia Sea through the Shag Rocks Passage, (3) Lower Circumpolar Deep Water (LCDW, ~2,000–3,500 m), and (4) Upper Circumpolar Deep Water (UCDW, ~1,000–2,000 m).

LCDW, SPDW, and WSDW contribute to AABW, which flows northward along the Argentine margin as an intensified western boundary current and enters the Brazil Basin through the Vema and Hunter channels (Speer et al. 1992; Hogg et al. 1999). A considerable part of AABW turns eastward at the Rio Grande Rise to form an anti-cyclonic gyre in the Argentine Basin (Fig. 1). NADW enters the Argentine Basin from the north (Fig. 3) at depths of ~2,000 to 3,000 m. The core of NADW flow today occurs seaward of the AABW flow, slightly detached from the margin (Fig. 1). In the terrace sector at 45°S (World Ocean Circulation Experiment (WOCE) section 11, Fig. 3b), the presence of diluted NADW is detected by a local salinity maximum (34.8 psu) at 57°W.

Deep seismic stratigraphy

The Argentine continental slope is characterized by seaward-dipping reflector sequences (SDRs), which developed during seafloor spreading. The upper boundary (reflector AR1) of the SDRs (minimum age Hauterivian, 125 Ma) occurs as a distinct erosional surface along the Argentine outer shelf (Hinz et al. 1999), and marks the top of the oceanic crust in the deep sea. The emplacement of the SDRs was followed by the deposition of an early-drift succession in Hauterivian to Albian times. The top of this sequence, reflector AR2 (Fig. 4), is considered to represent the equivalent of reflector AII from the conjugate Cape Basin, which there has been drilled and found to be of late Aptian age (Ryan et al. 1978). The sedimentation following this period is topped by erosional surface AR3 (Hinz et al. 1999), named the Pedro Luro equivalent by Franke et al. (2007; Fig. 4). Based on a comparison with well data from the shelf (Bushnell et al. 2000), this reflector is interpreted to mark the Cretaceous/Tertiary boundary. Two seismic reflectors identified by Hinz et al. (1999) are interpreted to result from the variable deep water current system leading to the formation of drifts and winnowing during the Paleogene (reflector AR4, Late Eocene), and a renewed cooling and major expansion of the Antarctic ice sheet in Middle Miocene times (reflector AR5).

Fig. 4
figure 4

Multichannel seismic reflection profiles (lines BGR2004-06 and BGR2004-07) across the southern Argentine slope and rise (see Fig. 2 for location), showing the series of terraces at different water depths, and numerous channels and sediment drifts. Regional seismic reflectors AR1–AR5 are based on Hinz et al. (1999), and seismic units on Hernández-Molina et al. (2009). Arrows indicate the extent of present-day water masses: AABW Antarctic Bottom Water, LCDW Lower Circumpolar Deep Water, NADW North Atlantic Deep Water, UCDW Upper Circumpolar Deep Water

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Materials and methods

The multichannel seismic (MCS) reflection profiles evaluated here (Fig. 2b) were collected by the Bundesantalt für Geowissenschaften und Rohstoffe during cruise BGR04 (Neben and Schreckenberger 2005). These profiles are located in tectonic segment I defined by Franke et al. (2007), proximal to the source of deep and intermediate water masses (AAIW, AABW, CDW) that enter the Argentine Basin from the south.

Seismic signals were generated every 50 m using airguns in six groups with a total capacity of 3,124 inch3. Data were recorded for 14,336 ms at a sampling rate of 4 ms using a 3-km-long streamer (Neben and Schreckenberger 2005). Processing routines included pre-stack deconvolution, wave-number and frequency filtering and multiple attenuation, velocity analysis, stacking, deconvolution after stack, and FK or Kirchhoff time migration (Franke et al. 2007). Seismic sections are shown in two way traveltime (TWT). Depth estimates and thickness calculation of seismic units are based on a velocity–depth model derived from sonobuoy data by Houtz (1977).

The ages of regional reflectors above AR6 were roughly estimated for four representative stratigraphic sections selected on the MCS profiles BGR2004-06 and BGR2004-08 across the terrace sector. These sections (Table 1) were chosen according to the seismic characteristics of the deposits in areas of continuous, predominantly hemipelagic deposition, and without major erosional features. The age estimates were calculated using the mean sedimentation rate between AR6 and the seafloor, based on the calculated reflector depth in meters and the inferred age of AR6 (~14 Ma).

Table 1 Average sedimentation rates and reflector ages for four representative stratigraphic sections
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Results

In the southernmost Argentine Basin, typical margin-parallel morphosedimentary features are, from west to east, (1) a set of four terraces on the slope and rise, (2) channels separating the terraces, (3) a giant buried drift, and (4) sheeted drifts on the abyssal plain that are marked by flat sea-bottom morphology (Fig. 4; Hernández-Molina et al. 2009).

The ~100 km wide plateau of the Valentin Feilberg Terrace (Fig. 4) coincides with today’s LCDW–AABW interface at depths of ~3.8–4.2 km. Its topography is rather flat, with slope gradients between 0 and 0.6°. A margin-parallel channel is located at the western boundary of the terrace, formed by the steep (2.9°) eastern flank of the adjacent Piedra Buena Terrace. This channel (channel 1 of Hernández-Molina et al. 2009), with a ~130 m incision at 47.5°S, deepens northward to 920 m at 44.5°S. The wide (70 km) eastern flank of the terrace dips seaward (1.2°) down to the crest of a giant drift body at 5,300 m water depth (SP 1,500 in BGR2004-07, Fig. 5). Numerous unconformities, channels, and mound facies characterize the sedimentary sequence following formation of horizon AR4 in the terrace sector (Fig. 5). The discontinuities can be traced at the scale of the Valentin Feilberg Terrace as a whole, and are commonly accompanied by a change in seismofacies that can be linked to modifications in oceanographic conditions.

Fig. 5
figure 5

Detailed seismic stratigraphy (lines BGR2004-08 and BGR2004-13) across the Valentin Feilberg Terrace. Arrows indicate the extent of present-day water masses: AABW Antarctic Bottom Water, LCDW Lower Circumpolar Deep Water, NADW North Atlantic Deep Water

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The mound facies comprises asymmetric mounded, elongated sediment bodies, which show a steep and a less inclined flank (e.g., SP 5,150–5,450, BGR2004-08, Fig. 5). These mounds show discontinuities at their bases and, in places, internal discontinuities. The internal reflectors are sub-parallel with mostly moderate amplitudes. They show onlap and downlap onto the discontinuities. In general, the seismic units forming these mounds are lenticular and upward convex. The vertical stacking of the seismic reflectors shows a lateral migration. Furthermore, sheeted sedimentary bodies are plastered to the slope with sub-parallel internal reflectors (e.g., SP 3,500–5,500, BGR2004-08, Fig. 5), and channel-related mounds (e.g., SP 16,000–17,000, BGR2004-06, Fig. 6a) exhibiting chaotic reflectors. The mounded features exhibit only minor lateral variations in seismic reflection characteristics (Fig. 6b). Following published classifications (Faugères et al. 1999; Stow et al. 2002; Shanmugam 2006; Rebesco and Camerlenghi 2008), these features are interpreted as mounded drifts, plastered sheeted drifts, and channel-related drifts all shaped by bottom currents.

Fig. 6
figure 6

Detailed seismic stratigraphy a across (lines BGR2004-06 and BGR2004-07) and b along (line BGR2004-14) the Valentin Feilberg Terrace. Arrows in a indicate the extent of present-day water masses: AABW Antarctic Bottom Water, LCDW Lower Circumpolar Deep Water, NADW North Atlantic Deep Water

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Overall, these depositional features and the fact that they are oriented parallel, rather than oblique, to the slope (Figs. 2b, 6b) indicate that contour-following current transport dominates over down-slope (turbiditic) processes at the southern Argentine margin. The depositional style changes further northward where across-slope sedimentary processes are indicated by submarine canyons (Hernández-Molina et al. 2009; Krastel et al. 2011; Fig. 2b).

Above reflector AR5 another reflector showing regional extent and good lateral continuity was recently identified in the Argentine Basin (Hernández-Molina et al. 2009). This stratigraphic key horizon, here named AR6 (Fig. 4), marks the top of seismic unit IU that shows a weak to transparent acoustic response. Throughout the Argentine Basin, unit IU exhibits an aggradational stacking pattern with little variation in thickness. During deposition of the approx. 220–270 m thick unit IU at the Piedra Buena and Valentin Feilberg terraces (Fig. 4), the giant drift in the abyssal plain also underwent a vertical growth stage (Fig. 5).

Fig. 7
figure 7

Summary of depositional sequences above reflector AR4 on the Valentin Feilberg Terrace. * Hinz et al. (1999), ** Hernández-Molina et al. (2009)

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The present-day Valentin Feilberg Terrace developed during deposition of the sedimentary sequence bounded by AR6 and the seafloor, unit UU. This unit is further subdivided here based on variations in reflection characteristics. Four high-amplitude discontinuities (named VF1 to 4) separate five subunits named UUa to UUe (from bottom to top) that can be regionally identified across the terrace. The derived ages for reflectors and stratigraphic units averaged over the area are listed in Table 1.

Unit UUa (bounded by AR6 and VF1) is deeply eroded under the eastern edge of the terrace plateau and the uppermost drift flank (~SP 1–1,000 in profile BGR2004-07, Fig. 6a). Depositional structures exhibiting internal reflectors of moderate strength onlap AR6 in the west (~SP 3,600–4,800 in profile BGR2004-08, Fig. 5), and form a mounded drift attached to the western flank of the giant drift in the east (~SP 2,950–3,300 in profile BGR2004-13, Fig. 5).

Units UUb and UUc both show a vertical stacking pattern with sub-parallel reflectors, and can be classified as sheeted drifts. These units onlap AR6 at the western terrace margin (~SP 17,000–20,000 in profile BGR2004-06, Fig. 6a), and are separated by the strong unconformity VF2. While unit UUb is rather transparent, unit UUc exhibits moderate- to high-amplitude internal reflections that migrate upslope. In the east, unit UUb exhibits a downlap termination to unit UUa and the giant drift (~SP 1,000–1,400 in profile BGR2004-07, Fig. 6a), whereas reflectors in the distal part of unit UUc are truncated along the terrace flank (~SP 1,000–1,500 in profile BGR2004-07, Fig. 6a).

Unit UUc is terminated by reflector VF3, which marks the base of an elongated mounded drift (SP 5,100–5,300 in profile BGR2004-08, Fig. 5), the formation of which began at the eastern edge of the Valentin Feilberg Terrace during deposition of unit UUd. The drift mound has a width of ~12 km, a height of ~300 m, and exhibits a steep eastern slope and a gentler western slope.

The strong internal reflectors within unit UUd (SP 4,500–5,300 in profile BGR2004-08, Fig. 5) indicate an eastward drift crest migration until it became partly eroded during the formation of subunit UUe (reflector VF 5). In the center of the Valentin Feilberg Terrace, the sediments of unit UUd bounded by VF4 and the seafloor fill an erosional trough (SP 4,500–4,900 in profile BGR2004-08, Fig. 5) that developed during deposition of UUc.

The uppermost unit UUe also exhibits internal reflectors prograding seaward into the mounded drift. The drift crest is partly eroded within this unit. Toward the western margin, reflections become wavy and finally terminate in a mounded structure with a chaotic internal reflection pattern that comprises the proximal part of units UUd and UUe at the flank of the Piedra Buena Terrace.

Discussion

Mid-Miocene climatic optimum

In Fig. 7, a stratigraphy for the last ~17 Ma is developed based on the modification and extension of the regional seismostratigraphic framework provided by previous investigations (Franke et al. 2007; Hernández-Molina et al. 2009). Above horizon AR5 (estimated age 15 Ma, Hinz et al. 1999), the aggradational unit (IU) terminated by reflector AR6 is prominent in the Argentine Basin (Hernández-Molina et al. 2009). In contrast to units LU and UU that exhibit lateral thickness variations due to alternating depositional and erosional features, unit IU reveals only minor thickness variations and does not show any indication of erosion (Figs. 4, 5, and 6) over the terraces. It is therefore possible that this aggradational unit has been deposited during the Mid-Miocene climatic optimum (MMCO), a time of relative sea level highstand (Haq et al. 1987; John et al. 2004), minimum ice volume (Holbourn et al. 2005), and high mid-latitude temperatures (Zachos et al. 2001). The timing of the MMCO is relatively well constrained from orbitally tuned oxygen isotope stratigraphies to ~17 to 14 Ma (Holbourn et al. 2007). A similar aggradational unit tentatively dated between ~18.7 and 12.1 Ma was found in the Jane Basin and Weddell Sea (unit 4 of Maldonado et al. 2005). Hence, a refined age of ~17 Ma (onset of the MMCO) is suggested here for reflector AR5, while for reflector AR6 an age of ~14 Ma (termination of the MMCO) can be assigned (cf. below). The sediment thickness above reflector AR5 at SP 4,500 in profile BGR04-08 is calculated to be ~1.5 km. With an inferred age of ~17 Ma for reflector AR5, the average sedimentation rate over the Valentin Feilberg Terrace since the Middle Miocene would have been >8 cm/1,000 years.

The rather transparent acoustic response of unit IU suggests steady hydrographic conditions, while the absence of erosional features could indicate moderate to low bottom current activity. Despite this inferred sluggish circulation, sedimentation rates during deposition of unit IU are estimated to be high (approx. 8–9 cm/1,000 years). However, high continental erosion rates due to the presence of a climatic (e.g., temperature and precipitation)–continental silicate weathering feedback mechanism have been postulated for the Middle Miocene (Wan et al. 2009); through enhanced riverine sediment supply, this would have resulted in higher sedimentation rates in the deep sea. Furthermore, regional subsidence (Van Andel et al. 1977) combined with down-slope transport processes at the steep eastern flank of the Piedra Buena Terrace may have contributed to the apron-like appearance of unit IU.

Subsequently, reflector AR6 can be related to the Middle Miocene climate transition, a significant reorganization of Earth’s climate system characterized by Southern Ocean cooling and expansion of the Antarctic cryosphere (Flower and Kennett 1994). That cooling took place between 14.2 and 13.8 Ma (Shevenell et al. 2004; Holbourn et al. 2007) is evidenced by a step-like increase (Flower and Kennett 1994; Zachos et al. 2001) in the long-term Cenozoic δ18O record, leading to the inception of continuous icehouse conditions in Antarctica (Billups and Schrag 2002; Shevenell et al. 2008).

Valentin Feilberg Terrace evolution

At the Argentine margin a new morphology of the terraces developed after deposition of unit IU, most likely due to profound changes in bottom flow patterns at ~14 Ma (Hernández-Molina et al. 2009). A stronger flow at the depth of the modern AAIW/UCDW boundary (today at ~2,000 m) is indicated by strong erosion on the Piedra Buena Terrace (Fig. 4). On the other hand, the Valentin Feilberg Terrace started growing due to the deposition of sheeted and mounded drifts, which at their distal ends partly covered the giant drift in the deep sea (Fig. 6a). These changes indicate that the strong return flow of AABW over the terrace postulated to have generated the giant drift (Hernández-Molina et al. 2010; Fig. 8a) became weaker or changed position after ~14 Ma (Fig. 8b). This could be due to a deepening of the Vema Channel, which before ~14 Ma was possibly too shallow to enable a deep connection between the Argentine and Brazil basins (Kennett 1982).

Fig. 8
figure 8

Evolutionary sketches for the development of the Valentin Feilberg Terrace in six stages at various water depths from 17 Ma to Recent. Symbols Inferred bottom water flow: circled cross northward flow, circled dot southward flow (size indicates current strength). D deposition, E erosion, gd giant drift

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Subunit UUa, deposited during the late Middle Miocene (~14–12 Ma), shows sheeted drifts in the west (SP 3,600–4,700, BGR2004-08, Fig. 5) and a small mounded drift in the east (SP 2,950–3,300, BGR2004-13, Fig. 5). The step-like morphology of the Valentin Feilberg Terrace initiated with the deposition of unit LU was leveled again into a gently dipping slope (Fig. 8c). A westward-shifted drift onlapping the western flank of the giant drift crest in combination with the small dimensions of the mound suggest that the AABW flow developed another (weaker) branch westward of the giant drift during the deposition of unit UUa (Fig. 8c). On the other hand, unit UUa is deeply eroded under the upper Valentin Feilberg Terrace flank in the east (SP 5,175–5,585, BGR2004-13, Fig. 5), indicating very strong bottom current activity. This could be due to a deepening of the interface between CDW and AABW caused by the emergence of LCDW during the Middle Miocene (Nisancioglu et al. 2003; Hernández-Molina et al. 2009). Unit UUa is topped by reflector VF1 that is tentatively dated to ~12 Ma, and thus coincides with reflector c of Maldonado et al. (2005), the most significant erosional surface within the Jane Basin. A major unconformity of similar age has also been reported for the Scotia Sea, where it is interpreted to represent the first incursion of AABW in form of WSDW into this basin (Maldonado et al. 2006). The event seems to be coeval with a major Miocene glaciation (Mi4) and a lowering of sea level (Ser3; Maldonado et al. 2006).

Unit UUb topped by high-amplitude reflector VF2 is a mainly aggradational subunit deposited between ~12 and ~9 Ma in the confined setting between the continental margin and the giant drift (Fig. 8c). It exhibits vertical growth in its distal part including the crest of the small drift mound (SP 2,900–3,100, BGR2004-13, Fig. 5), indicating reduced current strength. In contrast, truncated internal reflectors (SP 17,250–18,500, BGR2004-06, Fig. 6a) indicate sediment erosion proximal to the Piedra Buena Terrace flank likely due to higher bottom current energy of a more confined flow of LCDW. This is in agreement with evidence for a Middle Miocene increase in the deep water circulation in the Southern Hemisphere presented by Carter et al. (2004) and Joseph et al. (2004).

The confined sheeted drift comprising subunit UUc grew during the Late Miocene (~9 to ~6 Ma). Its well-stratified homogenous appearance suggests sedimentation under moderate bottom water flow over wide areas of the terrace (Fig. 8d). Continuous bottom flow is also indicated in areas displaying wavy reflectors (SP 4,000–4,300, BGR2004-08, Fig. 5). Locally, high-amplitude internal reflector sequences likely reflect a cyclic sedimentation pattern interpreted as variable current activity over time. Erosional features are extensive at the margins of subunit UUc: in the east, truncated reflectors at the distal terrace boundary indicate that a vigorous bottom current shaped the terrace flank into its present-day form (Fig. 8d). Focused northward flow along the Piedra Buena Terrace flank continued during deposition of UUc, as suggested by an evolving moat structure that truncates the reflectors onlapping AR6 (SP 3,100, BGR2004-8, Fig. 5). Taking into account paleo-water depths of >4.6 km for the eastern terrace flank and ~4.1 km for the western margin (Fig. 8d), erosional features were likely shaped by AABW and LCDW, respectively.

Another profound change in the terrace architecture is marked by Late Miocene reflector VF3, coeval with reflector b (~6.8 Ma) of Maldonado et al. (2005), an important unconformity in the northern Weddell Sea. Above reflector VF3, a mounded drift developed during the Late Miocene/Early Pliocene (unit UUd, 6–3 Ma) in the eastern sector of the terrace plateau (SP 5,100–5,400, BGR2004-8, Fig. 5). The drift exhibits a steep east side and a gently dipping, smooth west side. During deposition of unit UUd, the drift grew vertically and its eastern flank steepened, likely due to erosion through continued northward flow of AABW below ~4,300 m. High-amplitude internal reflections prograding westward in the upper part of UUd may be related to cyclic changes in current strength due to intensified glacial/interglacial climate variability during the Late Pliocene (Zachos et al. 2001). Stronger bottom flow (LCDW–AABW) during the Pliocene is also indicated by a moat under the central terrace plateau (Fig. 8e), and sediment waves that developed toward the west of the moat (SP 3,900–4,200, BGR2004-8, Fig. 5). At the western rim of the terrace, channelized northward flow (at a paleo-depth of ~4.0 km, likely LCDW) continued, forming a channel-related drift with chaotic internal structure (Fig. 8e). The overall indications for strengthening of bottom flow during the Late Miocene/Early Pliocene can be related to stronger thermohaline overturn induced by a gradual cooling of the water masses that were sinking in the southern oceans (Billups 2002). Furthermore, stronger production of NADW commencing around 6 Ma (Wright and Miller 1996) and an enhanced flow of NADW into the South Atlantic following the restriction of the Panamanian Isthmus at ~5 Ma (Haug and Tiedemann 1998) modified the general deep circulation pattern, although it is unlikely that NADW directly affected the sedimentation at the depth of the Valentin Feilberg Terrace.

The youngest high-amplitude reflector VF4 (~2.5 Ma) on the Valentin Feilberg Terrace marks the Late Pliocene/early Pleistocene boundary. A very thin sedimentary cover indicates strong erosion between SP 3,600 and 4,200 (BGR2004-8, Fig. 5), while the moat that developed during deposition of unit UUd was filled up again, indicating that the center of strongest erosion over the terrace plateau migrated westward during the Pleistocene (Fig. 8f). The well-stratified, horizontal, seafloor-parallel, high-amplitude reflectors of the moat fill (SP 4,350–5,000, BGR2004-8, Fig. 5) may be related to ponded turbidites (Faugères et al. 1999), and thus possibly indicate down-slope transport processes.

Conclusions

Seismic analyses of contourites on a southern Argentine continental slope terrace, the Valentin Feilberg Terrace, reflect variations in bottom current flow at 4,100 m water depth. Based on changes in depositional style during terrace growth, a sequence of flow variations for the last 17 Ma is deduced. Up to now no wells have been drilled in the southern Argentine Basin, and thus the dating of the major unconformities is quite speculative: (1) low bottom current activity during the Mid-Miocene climatic optimum (approx. 17–14 Ma); (2) current reorganization through the emergence of LCDW at approx. 14–12 Ma; (3) moderate flow speed of LCDW and AABW from approx. 12–6 Ma; (4) strengthening of bottom flow after 6 Ma due to a general change in deep water mass organization following the closure of the Panamanian gateway.

The available multichannel seismic profiles constitute a high-quality site survey for drilling sedimentary deposits on continental slope terraces in different water depths, which would reveal important aspects of deep water history and provide constraints for the chronostratigraphic framework. The inferred sedimentations rate of >8 cm/1,000 years would make a drill site on the Valentin Feilberg Terrace suitable for high-resolution paleoclimate studies of the Neogene.