Shallow- and Deep-Marine Siliciclastic Depositional Systems of the Santos Basin: How to Recognize Them (Brazil Deep Seds—Deep-Water Sedimentation in the Southeast Brazilian Margin Project)

Moreira, J.L.P., S.F. Santos, R. S. F. D’Avila, C.V. Madeira, Petrobras, Rio de Janeiro, Brazil

Previous studies of Santos basin shallow and deep marine siliciclastic depositional sys­tems have systematically underrated their stratigraphic and physiographic context, wrongly describing them as classic turbidite deposits. Seismic stratigraphy analysis has been used to identify main stratigraphic surfaces, like unconformities. Unconformity surfaces mark sediment delivery timing from continental to basinal regions and, consequently, locate important overlying depositional systems. Careful identification, mapping and seismic attributes extraction of these surfaces provide a detailed imaging of the physiography and depositional systems characteristics. Physiography context establishment allows distin­guishing shallow from deep marine siliciclastic depositional systems (SMSDS and DMSDS). These systems are differentiated by the characteristics of their transfer and depositional zones. SMSDS transfer zones are characterized by several parallel fluvial channels, some of them connected. They are shallow, non-erosive, 0.6 km width, 30 m thick, filled by sandy facies. Depositional zones are formed by various amalgamated, elongated sand bodies up to 170 m total thickness. Highstand prograding pro-delta shales overlie those transgressive sandstones. DMSDS transfer zones are distinguished by a direct connection of the river channel to the canyons, delivering their sedimentary load in the basinal region. Basically, canyons are 100m deep and 3km wide sediment conducts, but can contain up to 30m of cemented conglomerates. Depositional zone is characterized by sand-rich basin fan com­prising amalgamated channel elongated lobes complexes reaching 170m thickness. Facies associations of both depositional systems are very similar despite their different physio­graphic positions, probably because both were strongly influenced by hyperpicnal flows induced by catastrophic river-floods.