An Upper Aptian saline pan/lake system from the Brazilian equatorial margin: integration of facies and isotopes.

Facies and isotope (Sr and S) data were combined in order to discuss the genesis and palaeohydrology of an Upper Aptian evaporite succession formed during the initial development of an intra-continental rift from northern Brazil. Comparison of facies distribution between two investigated areas shows: (i) stable, well-stratified and hypersaline lakes with periods of anoxia and closure in the eastern margin of the basin, where there was evaporite precipitation only in central lake environments; and (ii) more ephemeral conditions in the eastern margin of the basin, where a saline pan complex developed and evaporite precipitation took place mainly in marginal salinas and surrounding mudflats. In both areas, expansion/contraction cycles developed as sedimentation took place, accompanied by a decrease and then an increase in ⁸⁷Sr/⁸⁶Sr- and δ³⁴S-isotope values. This, combined with the wide distribution of Sr- and S-isotope data within individual depositional cycles, as well as petrographic and scanning electronic microscopic (SEM) images, led to the conclusion that diagenesis was not enough to obscure the primary geochemical signature. A non-marine brine source is suggested by ⁸⁷Sr/⁸⁶Sr ratios between 0·707824 and 0·709280, which are higher than those from Upper Aptian sea water (i.e. between 0·70720 and 0·70735). The δ³⁴S value varies from 16·12 to 17·89‰ in the eastern margin of the basin, which is in disagreement with Upper Aptian marine values (13 to 16‰). Both geochemical tracers were influenced by facies characteristics, and thus a model is provided, where ⁸⁷Sr/⁸⁶Sr values respond to changes in expansion and contraction episodes of saline pan/lake systems. [ABSTRACT FROM AUTHOR]