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

The Central Bransfield Basin is a deep narrow trough between the northern t ip of the Antarctic Peninsula and the South Shetland Islands. Analyses of s ingle-channel, high-resolution seismic reflection data are used to characte rise the seismic stratigraphy of the Central Bransfield Basin. The tectonis ed acoustic basement is overlain by a 1-s-thick sedimentary cover composed of two main sedimentary sequences. The Lower Sequence, which shows synsedim entary deformation, has only been identified on the Antarctic Peninsula mar gin. The Upper Sequence is a complex sedimentary package composed of eight seismic units whose distribution, geometry and seismic facies allow two typ es of seismic units to be distinguished: slope and basinal units. The slope units, constituted by progradational stratified seismic facies, form a sed imentary wedge extending from the shelf edge. The basinal units fill the ba sin floor showing chaotic and undulated seismic facies that change basinwar d into stratified seismic facies. Both types of seismic units display an in terfingering pattern at the base of the slope, suggesting an alternating sh ift of the sedimentary depocentre, from the slope to the basinfloor and vic e verse. This alternate pattern indicates that the sedimentary processes re sponsible for the infilling of the Central Bransfield Basin followed a cycl ic pattern, which has likely been associated with the advance and retreat o f 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 movi ng grounding line, resulting in a progradational sedimentary wedge on the s lope. At the end of glacial periods, coinciding with the retreat of the ice sheets, extensive sediment failures affected the continental margin. Durin g interglacial periods the ice sheets remained restricted to coastal locati ons and glacial troughs, where processes of meltwater formation might have been significant. Sediment-laden underflows are generated within these trou ghs, from where they flow and spread over the shelf and down the slope to t he basinfloor as sediment gravity flow deposits. The combined effect of the se processes is a progradational build up of the shelf and an aggradational infilling of the basin floor, together with the development of the interfi ngering pattern at the base of the slope. (C) 1999 Elsevier Science B.V. Al l rights reserved.