Biomarker stratigraphy of the lower cretaceous of Espirito Santo Basin, Brazil

Abstract

This study involves the use of molecular parameters (steranes and terpanes) and carbon isotopes as stratigraphic correlation markers in a Lower Cretaceous lacustrine sequence from the Espirito Santo Basin, southeast Brazil. The biomarker variations revealed changes in sedimentary environment that had not been detected by sedimentological, paleontological and seismistratigraphic methods. Additionally, the observed variations, assumed to have been contemporary for the whole Basin, could also be used as chronostratigraphic markers. The carbon isotope data led to the recognition of two sedimentary sequences. The upper sequence ( δ 13 C around − 26‰ ) begins with very low amounts of gammacerane and of a novel C 24 tetracyclic alkane, apparently with a des-A-hopane structure. A sharp increase in concentration of the C 24 compound divided the upper sequence into two stratigraphic subunits, the second of which shows an increasing gammacerane concentration with depth. The lower sequence ( δ 13 C around − 28‰ ) starts with a distinct drop in abundance levels of the C 24 tetracyclic alkane and of methyl-steranes. Gammacerane concentrations attain a maximum at the top of this sequence, showing subsequently a decreasing trend with depth, but remaining reasonably high throughout the sequence. However, a second sharp decrease of the C 24 tetracyclic alkane shows a modification in sedimentary environment downwards. Combined, all these stratigraphic changes delimitate four depositional cycles which could be easily recognized in wells from the eastern part of the basin showing gradual erosion updip to the west. Such sedimentary changes could be correlated with independent geochemical information on neighbouring lacustrine sequences, of the same age, suggesting a dominant freshwater influence in the basal part of the sequence, contrasted with a more saline environment in the upper levels. The use of these markers has been of great value in redefining stratigraphic correlations in the Espirito Santo Basin and may prove useful to resolve similar difficulties in vicinal areas.