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The late Pleistocene Holocene stratigraphic architecture on the northeastern Brazilian continental shelf off the Parnaíba Delta has been explored by high-resolution seismic profiles. The seismic surveys reveal the widespread distribution of incised valleys of different size in offshore continuation of the present-day Parnaiba delta. According to morphology two channel types can be distinguished: U-shaped channels in the eastern part and V-shaped channels in the western part. The stratigraphic successions were grouped into four seismic units separated by different seismic boundaries. The characteristics of the seismic boundaries and internal reflectors of the seismic units were used to distinguish between marine and riverine deposits. The incised-valleys architectural elements were used to link sedimentation processes and variations in base level from late Pleistocene channel avulsion and channel infill in the lowermost course of the paleo-Parnaíba River to marine sediments of the present-day inner shelf. The change of the depositional environments in relation to deglacial sea-level rise is compared to incised valley infills of the Mekong River and Red River systems in Southeast Asia. [ABSTRACT FROM AUTHOR]

The Parnaíba River Delta (PRD) is the second major delta in NE Brazil and occupies approximately 100 km of the Brazilian equatorial coast. This delta is highly asymmetric due to a large tidal channel complex in the western portion that has at present time no natural connection to the Parnaíba River (PR) and the actual PR mouth in the eastern part of the PRD. A sedimentological, stratigraphic, and mineralogical study was performed on vibracores acquired in late Holocene sediments from the PRD to investigate how this environment developed during the Holocene. The core acquired in the western portion of the delta (VC-10/16) contains a medium sand layer with fluvial facies between depths of 1.50 and 0.52 m, dated by the radiocarbon method at 4.057 ± 22 BP. However, above a depth of 0.52 m, a transition to silt with tidal channel facies and higher organic matter contents (OM) and calcium carbonate (CaCO 3) is observed. Moreover, a fining-upward trend was also observed in the other four cores located farther west, and the mineral assemblage at the bases of these cores has strong similarities with cores collected in the eastern portion of the PRD (VC-07/16) in the main channel of the PR. However, the core-tops show significant variations in the assemblage of the key minerals (cordierite, cummingtonite, and garnet), indicating that in the past, the PR was connected to the western portion of the delta. The sedimentary provenance and the variations in the depositional environment indicate that a lobe switching process changed the main channel's course and caused the isolation of the delta's western portion during the late Holocene. During this period, the main channel ran through the western portion, and the large tidal channels of the delta are the ancient main channels that were abandoned during the lobe switching process probably caused by tectonics influence. • Mineral assemblages to identify delta lobe switching. • Parnaíba River Delta sedimentology and evolution during Holocene. • Vibrocores in shallow sediments. • Age dating, compositional and textural features of sediments. [ABSTRACT FROM AUTHOR]