ABSTRACT

Coniacian to Lower Maastrichtian coarse-grained turbidites fill intra-slope, fault-controlled canyons in the Campos Basin, off-shore Brazil. They form part of an Upper Albian to Lower Paleocene transgressive succession characterized by onlapping, deepening-upward sedimentation throughout the eastern Brazilian margin. In the Carapeba and Pargo (CRP-PG) oil fields the turbidites consist mainly (> 95%) of graded beds without any other sedimentary structures. Individual beds are up to 12 m thick and are composed of small pebble(< 2 cm) to granule-rich sandstones, and medium to very coarse sandstones. The finer-grained portions of these graded beds show better sorting and higher porosity, and these characteristics can be recognized in density logs. Use of 92 density logs calibrated w th 13 cores permitted the mapping of 198 coarse-grained turbidites in the CRP-PG turbidite system. The turbidites form eight thinning- and fining-upward facies successions, some bounded by regional unconformities or local erosion surfaces. The successions are 27-140 m thick and contain 7-58 turbidites. Their durations are estimated to be between 0.4-0.9 m.y. They form 1-12 km wide, tabular or linguoid sandstone bodies in which the younger or more distal turbidites become finer-grained, thinner-bedded, and more discontinuous upsection and downcanyon. The successions are stacked in an overall retrogradatioual pattern for at least 20 km, recording the backfilling of the CRP-PG canyon.

Turbidites in the CRP-PG area were probably deposited during falls of relative sea level that punctuated the overall transgressive setting of the late Cretaceous and early Tertiary. There is a mismatch between the number of successions (eight) and the number of time-equivalent third-order fluctuations of relative sea level (four and part of a fifth; Haq et al 1988), suggesting that eustatic sea level fluctuation was not the dominant control on most of the successions. Cross sections and isopach maps of the successions do not suggest internal channeling, and a channel-fill origin for the fining- and thinning-upward successions seems unlikely. We therefore suggest that episodic tectonic reactivation in the uplifted Precambrian source area, and faulted basin margin, led to relatively abr pt increases in the volume of sediment supplied to the head of the canyon across a narrow shelf. Steady denudation and decreasing supply of sediment led to formation of the fining- and thinning-upward successions.