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Abstract: Geometry of Turbidite Deposits in a Precambrian Foreland Brazilian Basin

Abreu, C. J. and Carrasco, B. N. - Petrobras/Cenpes

Four excellent outcrops of a Precambrian-Eopaleozoic turbiditic sequence of the Itajaí Basin, southern Brazil, are being investigated to quantify the geometry, lateral continuity, thickness and internal textural variation of the sedimentary units. The outcrop lengths range from 60 to 300m, with heights up to 17m. Because the regional structural dip is 22° the stacked sections can reach tens to hundred of meters thick.

The outcrops show facies from channel complexes to lobe fringes. The Pedreira outcrop is an example of proximal facies, consisting of a channel complex approximately 60m thick. Clast-supported conglomerates form amalgamated, fining-upward bodies from 3 to 21m thick. The size of pebbles ranges from millimeters to 6cm. Intercalated pebbly mudstones and shales, potential flow barriers, were partially eroded, allowing vertical connection of the reservoir rocks. The shale clasts range from a few centimeters to lm and occur mainly in beds at the base of the sequence.

The Bergamota outcrop can be divided into three sections, comprising 45m thick. In the basal part predominates pebbly/sandy mudstones and clast-supported channelized conglomerates, with pebble sizes averaging 3cm; in the middle part, there are laterally continuous tabular beds of massive medium-grained sandstone, with individual thickness ranges from 0.15 to 3.5m forming lobe facies. Another common structures of the tabular beds are traction carpets; in the upper part, thin bedded turbidites with Tbcd Bouma sequence occur as lobe fringes. The thinning and fining-up Bergamota outcrop constitute a 4th order sequence and is overlain by the Pedreira outcrop, a sequence of the same order.

The outcrop Apiúna I, 145m thick, is a classic example of lobe facies association, with laterally extensive tabular beds of massive fine/medium-grained sandstones with rippled tops (Bouma) and non-erosive base. Intercalated thin- bedded turbidites are common, with packages ranging from 0.4 to 2.9m in thickness, have good lateral extension whereas individual beds of 2 to 10cm have lateral extensions of a few meters. The thicknesses of the tabular sandstone beds range from 0.15 to 3.7m, averaging 0.53m. Considering the thickness of individual sets, which form the beds, the average is 0.28m. The persistent lateral continuity of the shale beds with thickness ranging from 0.4 to 2.9m, in the Apiúna I, represents a vertical barrier to flow.

The Apiúna II outcrop, represents channel-lobe transition facies which progrades over the Apiúna I. It comprises 45m of massive coarse-grained sandstone, with moderately irregular base due to erosion of intercalated shales. This erosion enhances the vertical connection of the reservoirs, contrasting with the Apiúna I. Covering this sequence, there is a pebbly/sandy mudstone with shale blocks, and wavy structure on its top.

This variety of information in the outcrops can improve the management of turbiditic Brazilian reservoirs, and elsewhere.

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