Seismic stratigraphy of the Central Bransfield Basin (NW Antarctic Peninsula): interpretation of deposits and sedimentary processes in a glacio-marine environment

Abstract

The Central Bransfield Basin is a deep narrow trough between the northern tip of the Antarctic Peninsula and the South Shetland Islands. Analyses of single-channel, high-resolution seismic reflection data are used to characterise the seismic stratigraphy of the Central Bransfield Basin. The tectonised acoustic basement is overlain by a 1-s-thick sedimentary cover composed of two main sedimentary sequences. The Lower Sequence, which shows synsedimentary deformation, has only been identified on the Antarctic Peninsula margin. The Upper Sequence is a complex sedimentary package composed of eight seismic units whose distribution, geometry and seismic facies allow two types of seismic units to be distinguished: slope and basinal units. The slope units, constituted by progradational stratified seismic facies, form a sedimentary wedge extending from the shelf edge. The basinal units fill the basin floor showing chaotic and undulated seismic facies that change basinward into stratified seismic facies. Both types of seismic units display an interfingering pattern at the base of the slope, suggesting an alternating shift of the sedimentary depocentre, from the slope to the basinfloor and vice versa. This alternate pattern indicates that the sedimentary processes responsible for the infilling of the Central Bransfield Basin followed a cyclic pattern, which has likely been associated with the advance and retreat of the ice sheets over the margins during glacial and interglacial episodes. During glacial periods, the ice sheets advanced, eroded the shallower sea floor areas and deposited diamicton and debris flow deposits along the moving grounding line, resulting in a progradational sedimentary wedge on the slope. At the end of glacial periods, coinciding with the retreat of the ice sheets, extensive sediment failures affected the continental margin. During interglacial periods the ice sheets remained restricted to coastal locations and glacial troughs, where processes of meltwater formation might have been significant. Sediment-laden underflows are generated within these troughs, from where they flow and spread over the shelf and down the slope to the basinfloor as sediment gravity flow deposits. The combined effect of these processes is a progradational build up of the shelf and an aggradational infilling of the basin floor, together with the development of the interfingering pattern at the base of the slope.