Facies architecture, seismic stratigraphy and development of a high-latitude basin: the Powell Basin (Antarctica)

Abstract

Ten seismic facies assemblages organized into architectural elements have been identified on the basis of multichannel seismic profiles in the Powell Basin. These facies include: sheet drapes with parallel high-amplitude reflectors, sheet drapes with cyclic reflectors, sheet drapes with chaotic and moderate-amplitude reflectors, broad lenses with central channel, slope front fills with hummocky reflectors, sheet drapes with wavy reflectors, sigmoid wedges, irregular bodies with contorted reflectors, lenses with chaotic reflections and trough fills with divergent reflectors. The vertical and horizontal distribution of these architectural elements define the syn-rift, syn-drift and three post-drift sequences, whose boundaries are characterized by unconformities indicating significant changes in the factors controlling sedimentation on the margins and basin plain. The allogenic factors that appear to be more influential on the margin growth patterns are: the evolution of subsidence mechanisms, the tectonic regime on each of the margins, and the distribution of bottom currents related to the Weddell Sea Gyre. Subsidence controls the evolution of the accommodation space, thus causing a trend in the sequence external geometry from wedges during the syn-rift phase, to sigmoids during the drifting, followed by flat-downward convex lenses at the beginning of the post-drift phase and finally sheet-like geometries. Tectonics has a considerable influence on the extension of the source areas and margin geometry, and also modifies the depositional cycles associated to climatic changes, processes of ice sheet advance and retreat, sea-level fluctuations and isostatic adjustments. Bottom currents influence the distribution of depositional bodies, controlling the development of sediment drifts that constitute an important proportion of the sequences on the northern margin and in the basin plain. The evolution of the depositional sequences of the Powell Basin clearly depicts the influence of global processes linked to glacio-eustatic changes and the tectonics controlling the different phases in the development of a rifted basin. This high-latitude basin may be considered as a good example of a semi-isolated basin, which serves as a model for the stratigraphic reconstruction of margins and basins of high latitudes.