Seismic stratigraphy and sediment distribution on the Wilkes Land and Terre Adelie margins, East Antarctica

摘要

Seismic reflection data show the existence of two major sedimentary basins along the continental margin of Wilkes Land and Terre Adelie, East Antarctica, that contain more than 5 s TWT (>9 km) of sediments. Four seismic megasequences are identified (MS4 to MS1) that are bounded by: basement, unconformities of interpreted Turanian, Maastrichtian and early Middle Eocene age, and the seafloor. The 4-5 km thick rift and pre-rift sediments are concentrated in a margin-parallel basin (Sabrina Basin). On the basis of seismic correlation with the Australian margin, this basin is interpreted to be of Late Jurassic to mid-Cretaceous age. The post-rift sediments are generally thick along the margin and in the adjacent deep-ocean basin, but are particularly thick in a major depocentre off west Wilkes Land, named here the Budd Coast Basin (BCB). The BCB contains a maximum observed thickness of 5 s TWT (approx 9 km) of post-rift sediments and its location suggests that the sediments were largely derived from a sub-glacial basin currently occupied by the Totten Glacier. Within the post-rift sediments, a major regional unconformity is identified that is interpreted to be of early Middle Eocene age (approx 45 Ma). While some previous interpretations have suggested that this unconformity marks the onset of the glaciation of East Antarctica, the unconformity is interpreted here to be the product of the major global plate reorganisation in the Middle Eocene and the coincident sharp increase in the sea-floor spreading rate between Australia and Antarctica. A seismic unconformity that can be correlated with the onset of glaciation is not identified, and the position of glacial onset within the sedimentary section can only be estimated through the extrapolation of sedimentation rates from the underlying section and from Deep Sea Drilling Project Site 269. The post-Middle Eocene section is much thicker than the underlying post-rift section and this is interpreted to be largely due to the very high sediment fluxes associated with the period of polythermal glaciation from approx 34-9 Ma. Sediment isochron and isopach maps derived from a regional seismic grid show that the depocentre of the BCB trends approximately north-south beneath the continental slope but is deflected to the east beneath the deep ocean basin. This distribution indicates that deep-marine processes were important in lateral, along-margin sediment transport. A west-to-east direction of movement is inferred by the sediment distribution. This direction is opposite to the present-day marginal currents along the continental slope and rise.