Glacially related and bottom current controlled sedimentation processes on the West Antarctic continental margin –Interpretations derived from seismic reflection investigations Carsten Scheuer Dissertation zur Erlangung des Doktorgrades der Naturwissenschaften vorgelegt dem Fachbereich Geowissenschaften (FB 5) der Universität Bremen Bremerhaven, März 2006 _________________________________________________________ Alfred-Wegener Institut für Polar- und Meeresforschung ABSTRACT The advances and retreats of grounded ice on the Antarctic continental shelf during glacial- interglacial cycles led to the deposition of large sediment deposits. Depositional patterns on the continental slope and rise reflect interactions between the effects of ice sheet fluctuations, mass transport processes and bottom currents. The central element of this thesis is the study of the late Cenozoic glacial history of the South Pacific continental margin of West Antarctica by interpreting the record of marine sediments there, with focus on the sedimentary successions of the outer continental shelf, slope and rise. The thesis presents the results of seismic stratigraphic analyses of multi-channel and single channel seismic reflection profiles collected on the continental margin during the course of several cruises since 1986. Seismic stratigraphic patterns are interpreted with reference to recent sedimentation models in order to differentiate between pre-glacial and glacially-influenced sediment units. In contrast to previous studies, the production and interpretation of one previously unpublished along slope seismic profile enabled the correlation of seismic units along the entire continental margin of the western Antarctic Peninsula and the Bellingshausen Sea and, thus, an evaluation of terrigenous sediment supply and distribution. Furthermore, correlation of reflectors with downhole results at the ODP Leg 178 drill sites allows the development of an age related sedimentation model and estimates of sediment deposition rates. Glacial advances and retreats appear not to have been synchronous along the continental margin. In the Bellingshausen Sea, high Pliocene and Quaternary sedimentation rates imply an increase of sediment supply to the continental rise due to frequent advances of grounded ice on the continental shelf. This is in contrast to the situation on the Antarctic Peninsula margin where increasing late Miocene and decreasing post-Pliocene sedimentation rates are observed. The modelling of isopach grids in the Southern Pacific off West Antarctica enables assessment of local variability in sediment supply from continental margins. The greatest thicknesses and deposition rates of glacial sediments are observed in front of major glacial drainage outlets, i.e. Marguerite Trough on the western Antarctic Peninsula shelf, Belgica Trough in the Bellingshausen Sea, and the trough off Pine Island Bay in the Amundsen Sea. Seismic data show differences in the width, elevation, symmetry and structure of sediment mounds and depocentres that developed on the continental rise. The characteristics of contourite drifts observed off the western Antarctic Peninsula and in the western Bellingshausen Sea can be interpreted in terms of interaction between turbidites and a westward flowing bottom current. In addition to sediment depositions, seismic profiles provide insights into the oceanic basement structure. The topography of the ocean floor suggests that the morphology of the oceanic basement influences the shape and structure of sediment deposits. Correlations with magnetic seafloor spreading anomalies enable the verification of suspected fracture zones in the basement, which contributes to the improvement of knowledge about South Pacific plate kinematics. TABLE OF CONTENTS CHAPTER 1..................................................................................................................................... 1 INTRODUCTION 1.1 Aims of this thesis.....................................................................................................................................1 1.2 Structure of this thesis .............................................................................................................................3 CHAPTER 2 .................................................................................................................................................4 2.1 STATE OF THE ART ....................................................................................................................................4 2.1.1 Geologic and morphologic overview ...............................................................................................4 2.1.2 Tectonic setting of the oceanic basement ........................................................................................6 2.1.3 Southern ocean currents...................................................................................................................8 2.1.4 Glacial development and transport mechanisms ...........................................................................9 2.2 USED METHODS .......................................................................................................................................13 2.2.1 Seismic method................................................................................................................................13 2.2.2 Additional methods.........................................................................................................................14 REFERENCES .................................................................................................................................................15 CHAPTER 3................................................................................................................................... 19 VARIABILITY IN CENOZOIC SEDIMENTATION ALONG THE CONTINENTAL RISE OF THE BELLINGSHAUSEN SEA 3.1. Abstract..................................................................................................................................................19 3.2 Introduction............................................................................................................................................20 3.3 Tectonic-sedimentary background .......................................................................................................20 3.3.1 Basement evolution .........................................................................................................................21 3.3.2 Glacial history and sediment transport ........................................................................................22 3.3.3 Physiographic expression ...............................................................................................................23 3.4 Magnetic database..................................................................................................................................23 3.5. Seismic database and observations......................................................................................................24 3.5.1 Basement structure .........................................................................................................................24 3.5.2 Sediment units .................................................................................................................................27 3.5.2.1 Eastern section (Profiles AWI-94003, AWI-20010001)........................................................27 3.5.2.2 Middle section (Profiles AWI-94030, AWI 20010001) .........................................................32 3.5.2.3 Western section (Profile 92324)..............................................................................................32 3.6 Discussion................................................................................................................................................32 3.6.1 Tectonic implications......................................................................................................................32 3.6.2 Sedimentation processes.................................................................................................................34 3.6.2.1 Geometric expression ..............................................................................................................34 3.6.2.2 Nature of sedimentary units ...................................................................................................36 3.6.2.3 Age of sediment units ..............................................................................................................37 3.6.2.4 Sediment deposition rates .......................................................................................................38 3.6.2.5 Faults in Unit Be1 ....................................................................................................................39 3.7 Conclusions .............................................................................................................................................40 Acknowledgements.......................................................................................................................................41 REFERENCES .................................................................................................................................................42 CHAPTER 4..................................................................................................................................