Abstract
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 (CaCO3) 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.
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