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Shelf-Slope-Basin Depositional Systems of the Santos Basin Eocene Stratigraphic Sequence (Brazil Deep Seds—Deep-Water Sedimentation in the Southeast Brazilian Margin Project)


Moreira, J.L.P., E.T.C. Abdalla, A.Z.N. Barros, L.F. Rodrigues, Petrobras, Rio de Janeiro, Brazil


A 3rd order depositional sequence from the passage of lower to middle Eocene of Santos basin, southeastern Brazilian continental margin, offers a good opportunity to understand transport and sedimentation mechanisms on a shelf-slope-basin during a spe­cific phase of relative sea level cycle. Seismic reflection termination is the best criterion to identify the key stratigraphic surfaces in a depositional sequence (SB, TS and MFS). RMS seismic amplitude extractions surrounding these surfaces have shown an extraordi­nary illustration of a complete suite of depositional systems for each system tract. Shelf­break evolution is also important to reconstruct physiography and to recognize shallow and deep marine siliciclastic depositional systems. A river-dominated shelf characterizes the earlier stages of lowstand system tract. Four canyons, developed in successive 4th order sequences, are responsible for individual episodes of sediment transfer to the basin. The canyons were created by river discharge onto the slope, from a 4th order eastward­migrating fluvial-deltaic system. Depositional system characteristics of the slope basin regions are: canyon, canyon-confined lag deposits, a radial unconfined sand-rich basin fan, intra-canyon meandering channels, and upper-slope sandy deposits fed by linear gul­lies; on the coastal-shelf regions, fluvial-deltaic system and beach-ridges are observed. A wave-dominated shelf characterizes both transgressive and highstand systems tracts. The linearity of shelf-break, canyons absence, and steep slope are others particularities. The depositional system characteristics of the coastal-shelf regions are fluvial systems with lateral sand-ridges and lagoons. Slope instability gravity flows produce chaotic deposits on basinal regions and comprise mud-matrix para-conglomerates with thinly interbedded shales, siltstones and fine-grained sandstones.