An Upper Aptian saline pan/lake system from the Brazilian equatorial margin: integration of facies and isotopes JACKSON D. S. PAZ*, DILCE F. ROSSETTI   1 and MOACIR J. B. MACAMBIRA* *Universidade Federal do Para ´ , Centro de Geocie ˆncias, Campus do Guama ´ S/N Bele ´m-PA, Brazil  Museu Paraense Emı ´lio Goeldi, Av. Magalha ˜ es Barata, 376, C.P. 399, CEP 60040-170 Bele ´m-PA, Brazil (E-mail: rossetti@dsr.inpe.br) ABSTRACT 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 87 Sr/ 86 Sr- and d 34 S-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 87 Sr/ 86 Sr ratios between 0Æ707824 and 0Æ709280, which are higher than those from Upper Aptian sea water (i.e. between 0Æ70720 and 0Æ70735). The d 34 S value varies from 16Æ12 to 17Æ89& 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 87 Sr/ 86 Sr values respond to changes in expansion and contraction episodes of saline pan/lake systems. Keywords Brazil, evaporite, isotope, sedimentary facies, strontium, sulphur. INTRODUCTION Distinguishing between marine and non-marine evaporites has been always challenging (Hardie, 1984; Brookins, 1988; Hovorka et al., 1993; Playa ` et al., 2000), especially in places where the associated deposits lack diagnostic features to help in the reconstruction of depositional envi- ronments. This problem increases when evapor- ites evolve from hybrid brines, which is probably the most likely in many cases (Denison et al., 1998). Lack of sedimentological parameters has led to the use of Sr and S isotopes to help investigate the genesis of many evaporite deposits worldwide (e.g. Hovorka et al., 1993; Denison et al., 1998; Playa ` et al., 2000; Schreiber & El Tabakh, 2000). Both elements are abundant and uniformly distributed in sea water, displaying isotope ratios that vary through time in a known manner (e.g. Burke et al., 1982; Bralower et al., 1997). Once the age is well established, these isotope tracers might provide reliable information on brine sources. Deviations from the established seawater curve are taken as indicators of contin- ental or hybrid brines (Claypool et al., 1980; 1 Present address: Instituto Nacional de Pesquisas Espaciais-INPE, Centro de Observac¸a ˜o da Terra (OBT/ DSR), Rua dos Astronautas, 1758 CP 515, CEP 12245- 970 Sa ˜o Jose ´ dos Campos-Sa ˜o Paulo Brazil. Sedimentology (2005) 52, 1303–1321 doi: 10.1111/j.1365-3091.2005.00744.x Ó 2005 International Association of Sedimentologists 1303