The Proterozoic deposits of Ouricuri do Ouro Formation (OROF) in NW of Chapada Diamantina/BA characterize an alluvial fan system composed by four facies associations:
1. Proximal deposits non-cohesive debris-flow dominated: It consists of polymictic conglomerates with thin lenticular interbedded pebbly sandstones. Conglomerates are 1-6 m thick, massive, poorly sorted, clast-supported and sand-matrix-supported, locally normal-graded or inverse-graded. Clasts are mostly sub-angular to sub-rounded, pebble to cobble sized, locally boulder grade conglomerates (< 40 cm). The amalgamated bodies occur right on the basement, with slightly irregular lower boundaries and lenticular bodies (<1 m) with massive, low-angle or trough cross-stratified sandstones. These deposits occur at proximal areas of alluvial-fans and amalgamated bodies suggest the aggradation of individual floods. The sandstones indicate waning-deposits, produced by low energy tractive currents after debris-flow;
2. Proximal sheetfloods: comprise massive, low-angle and, poorly sorted, very coarse-sandstones, with pebble and cobble dispersed, intercalated with conglomerates, locally forming thick fining-upward packages (<5 m). Their deposits alternate with debris-flow deposits. The weakly channelized to unconfined sandstone domain and tractive structures suggest high-energy ephemeral currents;
3. Intermediate sheetfloods: comprised by superimposed thick tabular bodies (<2 m) of medium- to coarse-sandstones. The bodies are internally compound by thin (<0.5 m) repetitive fining-upward cycles of low-angle cross-stratified, horizontal laminated and ripple cross laminated sandstones. Laminated siltstones locally superimpose this cycles, representing the suspension processes on final flow conditions;
4. Distal sandy flood plains: bodies composed by centimetric intercalation of very-fine sandstones and siltstones. The sandstones are well sorted with horizontal-lamination and/or ripple-cross stratification, while siltstones locally show mud cracks. These deposits indicate alternations of tractive currents and decanted sediments on final flow stage. The mud cracks suggest a sub aerial exposure and ephemeral features of floods. Thereby, these deposits represent distal flooding plains associated to ephemeral sheetfloods.
The altenance of thick debris-flow deposits and proximal sheetfloods characterizes the sedimentation in the lower part of OROF. However, intermediate sheetfloods and distal sandy flood plains characterize the upper portion. This vertical stacking of OROF indicates a retrogradation of alluvial system, which suggests a regional gradient decrease and should be a product of tectonic activity decline or backfaulting of basin margin. In contrast to progradational coarsening-upward cycles commonly described in succession of alluvial fans facies, the occurrence of high frequency fining-upward cycles are results of the progressive waning of flow capacity and competence, associated to the runoff decrease across time.
