U-PB Zircon Ages and Provenance of Upper Cenozoic Sediments from the Da Lat Zone, SE Vietnam: Implications For an Intra-Miocene Unconformity and Paleo-Drainage of the Proto–Mekong River

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Abstract The present-day Mekong River is the twelfth longest river in the world. It drains the Tibetan Plateau and forms a large delta in south Vietnam. Remnants of upper Cenozoic fluvial to marginal marine proto-Mekong sediments are exposed in the Da Lat Zone in southeast Vietnam and are likely the on-land equivalents of large sediment packages offshore in the Cuu Long and Nam Con Son Basins. Provenance studies are used here to identify sources that allow reconstruction of sediment pathways. The Oligo-Miocene Di Linh Formation has a main Cretaceous zircon age population and subordinate Paleoproterozoic (c. 1.8-1.9 Ga) zircons, sourced mainly from Cretaceous plutons. In contrast, the early Pliocene to Pleistocene Song Luy Formation includes additional Permian–Triassic and Ordovician–Silurian age populations which are interpreted to be sourced from basement rocks in central and northern Vietnam. There is a significant change in provenance between these formations, interpreted as an intra-Miocene unconformity. The late Pliocene to Pleistocene Ba Mieu Formation shows a much greater abundance of Precambrian zircons, including a small age peak at c. 2.5 Ga, interpreted to be derived from old basement exposed in South China. Abundant Cretaceous zircons and a lack of Jurassic zircons in all three formations contrast with the present-day Mekong River delta. Therefore, the data are proposed to show a development of the proto–Mekong River progressing northward from the Da Lat Zone, to the Kontum Massif and northern Vietnam, to capturing of reversed rivers and/or reworking of sediments from old cratons in South China.