Patterns of sedimentation in the passage from the Marfim to the Pojuca Formation (Rio da Serra Stage/Aratu Stage) and the stratigraphic significance of Marker 15, lower Cretaceous, Recôncavo Basin, Bahia

Abstract

In order to understand the sedimentary evolution in the passage from the Late Rio da Serra Stage to the Aratu Stage (Early Cretaceous) in the Recôncavo basin, the sedimentary packet lying between the top of the Marfim Formation and Marker 15 (base of the Pojuca Formation) was analyzed regionally. Emphasis was placed on sedimentation of the Catu and Biriba sandstones, believed to make up the substratrum during sedimentation of the ostracode calcarenites of Marker 15. Compared with the Middle Rio da Serra Stage, this was a period of mild diastrophism, during which the basin is believed to have acted much like a ramp, with sha/low-water sedimentation occurring throughout. The 8iriba sandstones were deposited over this practically flat substratum molded by the delta plain of the Marfim Formation. These sandstones are characterized by their lobed external geometry, which pinches out upstream. and by sedimentarv structures typical of deltaic sedimentation. The proposed depositional model was strongly controlled by relative variations in the lake level, much as occurs with lowstand wedges in marine basins. The graphoeletríc feature that characterizes Marker 15 is a result of the occurrence of grain-supported ostracode biocalcarenites, with ostracode carapaces filled in wíth micrite, locally neomorphosed to sparry calcite. These deposits came from the reworking and transport of carapaces and valves deposited earlier and are characteristic of condensation zones. The chronostratigraphic significance of the marker derives from its location inside the Paracypridea obovata obovata ostracode zone (NRT 005), paracypridea elegans elegans subzane (NRT 005.2), always intermediary between the tops of subzanes NR T 005.2 and NRT 005.1.