The Ilhas and Sao Sebastiao Formations of Cretaceous age represent a part of the Mesozoic section in the Reconcavo Basin, Bahia, Brazil. This basin is an asymmetrical graben. The two formations form a thick sequence of interbedded lutites and very fine-grained arenites. Conglomerates are locally prominent and carbonates are present as concretions and cement.

Sedimentary structures include lamination, asymmetrical ripple marks, cross-lamination, and cross-stratification on several scales, directional and non-directional substratal markings, including flute casts, groove casts, and load casts, and convolute bedding. Interstratal crumpling, pull-aparts, and distortion are locally prominent. In general, the sediments and structures resemble those found in turbidite sequences, but graded bedding is lacking.

The orientation of fold axes in slumped beds, the areal geology, and the geometry of the formations indicate that the paleoslope dipped west. The most abundant paleocurrent indicators, ripple marks and cross-stratification, indicate a southerly paleocurrent which flowed parallel with the contours of the paleoslope. Substratal markings such as flute casts and groove casts trend parallel with the paleocurrent direction and do not indicate the dip of the paleoslope.

Whereas many structures, particularly substratal markings, are identical to those in turbidite sequences, the lack of graded bedding and the textural, compositional, and structural similarities of these rocks to those of the modern Mississippi Delta indicate that the Reconcavo Basin rocks were of deltaic, rather than of turbidity-current, origin.

Review of the turbidity-current literature suggests, to the writers, that graded bedding may be the only valid criterion for differentiating turbidites from non-turbidites. The indiscriminate application of the term "turbidite" to sand-shale sequences where graded bedding is indistinct or lacking only serves to divert the geologist from seeking another interpretation which may be more valid. It is important that the distinction between turbidites and non-turbidites be made carefully and repeatedly in every study of bedded shales, siltstones, and sandstones, because inferences of slope direction and other paleogeographic interpretations depend for their validity on this distinction.