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Three sedimentary systems tracts (LST, TST, and HST) have been identified in Nha Phu Bay and the adjacent shelf, Khanh Hoa Province, Vietnam. These have been deposited since the Last Glacial Maximum (LGM) and interpreted in this study via the analysis of high-resolution acoustic data and sediment cores. The lowstand systems tract (LST) comprises sandy sediments exposed at seabed on the outer shelf as sand ridges, and underlying the transgressive systems tract (TST) on the middle shelf. The LST developed under shallow-marine settings during the sea-level lowstand and early sea-level rise before 14.6 cal kyr BP. The TST comprises primarily fine-grained sediments deposited during the transgression between 14.6 and 8.0 cal kyr BP in an incised trough of the outer shelf, on the middle shelf, and in Nha Phu Bay. Erosion of the seaward portion of the TST on the middle shelf might occur during the late transgression (9.0–8.0 kyr BP). The highstand systems tract (HST) comprises sediments deposited during the sea-level highstand between 8.0 and 0.0 cal kyr BP and is distributed widely in the bay, on the inner-middle shelf, and partially on the outer shelf. A paleo-coast zone, located in 100–120 m present-day water depth, is recognized through the appearance of sand ridges and sandy sediments. The seafloor sediment distribution is mapped via four major acoustic echo types, combined with seabed sediment samples. The coarse-grained sediment distribution, subaqueous outcrops, and thin deglacial sediment thickness on the inner shelf indicate erosion-dominated processes under vigorous hydrodynamic conditions. Nha Phu Bay and the middle shelf are interpreted as the depocentres. The estimated sediment load deposited in Nha Phu Bay is 0.18 Mt./yr, accounting for 15–35% of the annual sediment discharge from the Dinh River. The seabed morphology, muddy sediment distribution, and deglacial sediment thickness reveal a dominant sediment-transport pathway, across the inner shelf to the middle shelf, near the northern shoreline of Nha Phu Bay under the influence of the southwest monsoon. • Sequence-stratigraphic framework for the study area has developed under global sea-level changes since the LGM. • Sedimentary processes were interpreted on sequence stratigraphy, sediment distribution, seabed morphology, and hydrodynamics. [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]