Abstract: Feeder Canyon Evolution Model Deduced from the Stacking Pattern of a Tertiary Turbidite System in Campos Basin, Brazil

Castro, R. D. ; P. M. Magalhães; A. M. C. Moliterno; V. S. S. Santos; M. Blauth; L. F. G. Caddah - Petrobras/E&P

In the central-northern part of Campos Basin, an Oligocene-Miocene canyon-mouth is filled with oil-rich deep-water sandstones. This depositional system was studied using a gradual up-scaling transposition analysis, integrating the available data of cores, well-logs, seismic lines and seismic amplitude maps. The facies succession shows a general backstepping style (1), evolving from pebbly mudstones, passing through coarse- to fine-grained turbidites, sandy to muddy contourites, until being sealed by highstand hemipelagites. The facies associations can be divided into five evolutive phases, probably related to eustatic changes (2). Based on the facies stacking patterns (1), the morphologic evolution of the feeder canyon with its associated processes was deduced, as follows:

- EMBRYONIC CANYON: Initial process of slope failure with the development of a sub-regional decametric package of pebbly mudstone, interlayered with metric sandstones segregated from the debris flows. The sliding process was aborted resulting in an immature canyon, probably restricted to the lowermost part of the slope;

- INACTIVE CANYON: Passive muddy background deposition (hemipelagites), draping erosional scars of the previous phase;

- MATURE CANYON: Revitalization of retrograding slides that projected the canyon-head landward, until it breaches the shelf-edge. The shelf-indenting canyon captured a huge amount of shelf-sands which was transferred to the basin-floor by turbidity currents, forming depositional lobes. The deposits show a typical fining and thinning upward pattern. As long as grain-size and thickness decrease, traction currents record increases. Bottom currents (of contour or tidal origin?) are inferred as the background process. The preservation potential of traction structures grows with the waning of turbidity currents. The following facies sequence can be observed: amalgamated massive medium- to coarse-grained sandstones (turbidites); fine- to medium-grained sandstones with some traction structures and increasing interbedded shales to the top (slightly modified turbidites) and, finally, centimetric interlaminations of sand and shale, with abundant climbing-ripple laminations and cross-stratification (thin-bedded turbidites modified to contourites?);

- REJUVENATED CANYON (Dissection Phase): Reactivation of strong turbidity currents that cut the previous sandy-lobes, isolating sand-bodies. The incised channels are arranged in a divergent pattern, irradiating from the mouth of the feeder canyon. The dissection over the top of sandstones became an important paleogeomorphic oil-trap for a large area around;

- SENILE CANYON (Hemipelagites): Quiescence of turbidity currents and deposition of shales and marls. Fossilization of the paleogeomorphic surface.

AAPG Search and Discovery Article #90933©1998 ABGP/AAPG International Conference and Exhibition, Rio de Janeiro, Brazil