Seismic expression of depositional elements associated with a strongly progradational shelf margin: northern Santos Basin, southeastern Brazil

Expressão sísmica de elementos deposicionais associados a uma margem de plataforma fortemente progradante: Eoceno do norte da bacia de Santos, margem sudeste brasileira

ABSTRACT:

Seismic facies analysis and seismic geomorphology are important tools for the analysis of depositional elements in subsurface. This paper aimed to investigate the character and genesis of depositional elements and erosive features associated with an Eocene progradational shelf margin in northern Santos Basin. Identified seismic facies are interpreted as shelf-margin deltas/shoreface deposits, tangential (oblique) clinoforms, sigmoidal clinoforms, topset reflectors, mass-transport deposits and turbidites. These facies are grouped into four associations representing periods of relatively constant environmental conditions. Association 1 is composed of shelf-margin deltas/shoreface deposits, tangential clinoforms and extensive sand-rich turbidites disposed as submarine channels and frontal splays. A progressive increase in clinoform angle within this association has been identified, culminating in high-relief sigmoidal clinoforms with less voluminous turbidites of facies association 2. Association 3 is composed by subparallel to divergent topset reflectors, interpreted as continental to shelfal deposits placed during base-level rises. These are always truncated basinward by slump scars, formed as a consequence of sediment overload at the shelf margin during aggradations. Association 4 is composed of sigmoidal clinoforms, mass-transport deposits and turbidites. Early clinoforms are steeper as a consequence of the topography of the slump scars. Subsequently, dip angles become progressively gentler as the system approach to the equilibrium profile. The steep physiography was favorable for canyon incision, which played an important role in turbidite deposition. Mass-transport deposits, formed subsequent to slope collapse, are composed of mud-rich diamictites, and show strong internal deformation.

KEYWORDS:
Seismic facies; Seismic geomorphology; ­Deep-marine deposition; Mass-transport deposits; Turbidite systems