Flooding surfaces, depositional elements, and accumulation rates : characteristics of the Lower Cretaceous tectonosequence in the Recôncavo Basin, Northeast Brazil
The Lower Cretaceous tectonosequence constitutes a major chronostratigraphic interval in the Recôncavo Basin, Northeast Brazil. It encompasses a time span of ca. of 24 Ma and forms a continental sedimentary package that exceeds 5 km in thickness within its depocenters. The tectonosequence is bounded by two basinwide unconformities, the rift onset unconformity at its base and the breakup unconformity at its top. The basal deposits of the tectonosequence characterize transitional rift conditions. The bulk of the succession constitutes the record of the rift phase in the basin. Biostratigraphy in the Recôncavo Basin is based on nonmarine ostracods. The tectonosequence encompasses ostracod zones NRT 002 through NRT 009. Tectonic, sedimentologic, and paleoecologic characteristics permit the recognition of six tectono-depositional intervals within the tectonosequence. Tectono-depositional intervals cover a time span between 2 Ma and 7.5 Ma, and their boundaries are distinct marker beds. Lithologic, paleontologic, and geochemical characteristics of the marker beds suggest low sediment input at the time of their deposition. Tectonic and/or paleoclimatic changes may account for phases of marker bed creation. Within tectono-depositional inteivals II, IV, and V sedimentary cyclicity occurs. Sedimentary cycles spanning tens to hundreds of thousands years are termed tectono-depositional episodes. Such an episode is comparable in scale with a parasequence, and is also bounded by marker beds reflecting flooding events. Depositional systems within the tectonosequence show a change from lacustrine-dominated sedimentation at the base to fluvial-dominated deposition at the top. Reservoir potential increases toward the top of the tectonosequence, source rock and sealing conditions, however, follow the opposite path. The bulk of known hydrocarbon accumulations in the Recôncavo Basin occurs at the prerift/rift interface. Accumulation rate estimates show increasing values toward the top of the tectonosequence. This may be related to a better integration of the drainage system that fed into the basin and removal of possible structural barriers located along the basin's margins. Analysis of the sedimentary record of the tectonosequence suggests, besides tectonism, paleoclimate was an important mechanism controlling facies distribution and evolution.