Sea levels during late marine isotope stage 3 (or older?) reported from the Red River delta (northern Vietnam) and adjacent regions

摘要

A 60m-long sediment core from the lower Red River delta in northern Vietnam contains three depositional sequences consisting mostly of regressive shallow-marine sediments bounded by unconformities. The youngest unit represents the modern Red River delta. The second unit, a facies succession comparable to the overlying deltaic succession, formed during an earlier sea-level highstand, as indicated by the marine isotope stage (MIS) 3 age obtained by radiocarbon dating. The third, undated unit might reflect earlier high sea levels of the middle to late Pleistocene. The MIS-3 ages, derived from shoreline-related sedimentary facies, are supported by records from adjacent mega-deltas and shelf sites with paleoshoreline facies. Proximal sites from the central Indochina craton show high sea levels during late MIS 3, whereas distal sites from the open continental shelf indicate much lower sea levels at the same time. Independent, globally relevant sea-level reconstructions suggest that both positions have been affected by later vertical movement. This discrepancy between sea-level records from central cratonic sites and the continental margin can be explained by a balanced response to locally differentiated sediment loading and compaction on the shelf, as is supported by published seismic data. However, if the complete set of radiocarbon dates is contaminated by younger material, then the second recorded sea-level highstand may be associated with MIS 5, as no marine deposits have yet been reported in any of the three Indochinese mega-deltas for this time interval.