Elsevier

Marine Geology

Volume 228, Issues 1–4, 30 April 2006, Pages 93-116
Marine Geology

Miocene reversal of bottom water flow along the Pacific Margin of the Antarctic Peninsula: Stratigraphic evidence from a contourite sedimentary tail

https://doi.org/10.1016/j.margeo.2005.12.010Get rights and content

Abstract

A Fossil Mounded Sedimentary Body (MB) has been identified in the sedimentary record on the central continental rise west of Adelaide Island, on the Antarctic Peninsula Pacific margin. The growth patterns of the MB are defined through a detailed regional stratigraphic analysis using multichannel seismic reflection profiles. The MB has an elongated NE trend. It overlaps and continues to the NE of an extensive cluster of seamounts, and it developed between two non-depositional troughs. Nine seismic units have been identified: Unit 9 (the pre-MB stage), Unit 8 (MB growth stage), Units 7 and 6 (MB maintenance stage), Units 5 and 4 (transitional stage), and Units 3, 2 and 1 (inactive stage). We interpret the MB as a patch drift plastered against the NE, lee side of an obstacle, as a long Contourite Sedimentary Tail (CST), within a deep current that flowed northeastward. This segment of the rise is, however, affected at present by a SW-flowing branch of the Lower Circumpolar Deep Water (LCDW) from the Weddell Sea. The depositional patterns of the MB growth and maintenance stages, which are attributed an early Miocene age on the basis of regional correlation of MCS profiles with DSDP Site 325 and ODP Leg 178 drill sites, provide the first evidence that bottom currents on the central continental rise flowed towards the NE at that time, probably as part of the Lower Circumpolar Deep Water (LCDW) of the Antarctic Circumpolar Current (ACC). We suggest that significant palaeocirculation and palaeoceanographic changes occurred in this area, and probably more widely, during the middle Miocene or at the Miocene/Pliocene boundary. Although these results do not modify the regional stratigraphy of the major sediment drifts found on the continental rise of the Antarctic Peninsula's Pacific margin, they do indicate that the bottom current regime controlling the development of contourite deposits may have changed over time and also that more than one water mass has probably affected their distribution.

Introduction

Bottom contour currents in modern ocean basins often generate large sedimentary bodies (contourite deposits or drifts), comparable in size to turbidite fans (Stow et al., 1986, Stow et al., 2002, Rebesco, 2005). Several drift classification systems have recently been proposed, based mainly on morphologic, sedimentologic and seismic characteristics (McCave and Tucholke, 1986, Faugères and Stow, 1993, Faugères et al., 1999, Rebesco and Stow, 2001, Stow et al., 2002, Rebesco, 2005). All drifts are related to the regional oceanographic conditions and the physiographic domains where they developed. Thus, it is possible to decode, from their morphologic, stratigraphic and sedimentary characteristics, the pathway and approximate flow velocity of the water mass that was responsible for their development. This is particularly relevant when buried contourite drifts and erosional discontinuities are found in the sedimentary record of a basin, because it is then possible to reconstruct it palaeoceanographic conditions.

We have recently completed a morphologic and stratigraphic interpretation of the region between 68° and 74°W and 65°–67°30′S (Fig. 1A), using multichannel seismic profiles and multibeam echo sounder data collected by several projects and research groups. In this area, on the central continental rise off Adelaide Island, we found a large Fossil Mounded Sedimentary Body (MB) (Hernández-Molina et al., 2004). In this paper we give a more complete description of the development of the MB, present new multibeam echo sounder data that reveals additional seamounts within the MB area, and discuss the palaeoceanographic implications of these observations.

Section snippets

Geological and oceanographic setting

There was continuous subduction at the Pacific margin of the Antarctic Peninsula at least from early Cretaceous time until the early Tertiary (Storey et al., 1996). During the Tertiary period, subduction stopped along most of the margin, as ridge-crest segments of the Antarctic–Phoenix spreading centre migrated into the trench (Barker, 1982, Larter and Barker, 1991a). Ridge-crest segments arrived first at the southwestern part of the margin during the Palaeocene or Eocene, then progressively

Methodology

The present study is based on the analysis of multichannel seismic reflection (MCS) profiles collected by the British Antarctic Survey (BAS) and the Istituto Nazionale di Oceanografía e di Geofisica Sperimentale (OGS). However, this work is part of a more extensive regional work (Hernández-Molina et al., in preparation) which also utilises seismic reflection data from the Brazilian Antarctic Program, Rice University of Texas, and Spain (Project ANT99-0817), as well as multibeam echo sounding

The fossil mounded sedimentary body (MB): morphosedimentary features and seismic stratigraphic analysis

We have identified a MB on the central continental rise offshore from Adelaide Island between 65°S and 65°30′S and 71°45′W–72°45′W (Fig. 1B). The MB (Seismic Units 8 to 6 in Fig. 3, Fig. 4) is buried 300 to 150 m below the sea floor, which is at a water depth of 3600 m, and at which it has no morphological expression. It has a mounded, elongated shape, overlapping and continuing to the NE of an extensive cluster of seamounts. Buried seamounts are identified on line IT110 and line BAS20,

Chronostratigraphic constraints

The age of the seismic units can be assessed by correlation with DSDP Site 325 (Hollister et al., 1976), through which lines BAS19 and IT110 both pass approximately 60 km NW of the MB (Fig. 1, Fig. 4A). Reflections on line 48 can also be traced to Site 325, via intersecting line IT89049 (Fig. 1B). In addition, a regional stratigraphic study of the continental margin off Adelaide Island has been carried out (Hernández-Molina et al., in preparation), taking into consideration chronostratigraphic

Effect of seamounts on an impinging flow

Knowledge of how seamounts and seamount chains interact with ocean circulation is important from a geological point of view. Akin to island mass effects (Heywood et al., 1990), seamounts generate seamount effects, with important effects on oceanographic processes (Roden, 1987), marine biota (Rogers, 1994), sedimentation and erosion rates (Davies and Laughton, 1972, Roberts et al., 1974), and hence palaeoceanographic interpretations (Roden, 1987). The streamline distortion around obstacles is

Conclusions

The analysis and interpretation of three parallel MCS profiles and multibeam echo sounding data collected over the continental rise on the Pacific margin of the Antarctic Peninsula can be summarised with the following conclusions:

  • a)

    A Fossil Mounded Sedimentary Body (MB) has been identified in the Early Miocene sedimentary record over the oceanic crust on the central continental rise offshore from Adelaide Island.

  • b)

    The MB has an elongated NE trend, overlapping and continuing to the NE of an

Acknowledgements

This research has been partially supported by Spain's Inter-ministerial Science and Technology Committee (CYCIT), through Project ANT99-0817 We thank the officers, crew, technical support staff and scientists who sailed on RRS Discovery cruise 172 (1988) and R/V OGS-Explora cruises in 1989 and 1992. On the OGS-Explora cruises, seismic data were collected as part of Italy's PNRA (Programma Nazionale di Ricerche in Antartide). PNRA supported Michele Rebesco's contributions through the SEDANO

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