Sequence biostratigraphy of the late Oligocene to Miocene succession in the northern Song Hong Basin, offshore Vietnam

Abstract

The Cenozoic sedimentary succession of the Song Hong Basin, offshore North Vietnam is poor in marine microfossils, especially in the early Miocene and Oligocene and subsequently is poorly dated. However, it is rich in terrestrially derived pollen, spores, and freshwater algae. Biostratigraphic assemblages from two wells, termed Well A and B, have been evaluated using the approach of sequence biostratigraphy. The Oligocene fluvio-lacustrine succession through the Dinh Cao Formation displays multiple transgressive-regressive cycles comparable with those reported from the Cuu Long Basin to the south. By applying the local Cuu Long Oligocene palynological zonation (CLO zones), these cycles can be correlated directly with the cycles from the Cuu Long Basin, showing that sedimentation began during sequence 19, at about 29 Ma or a little earlier off-structure, over basement. The deepest interval within Well A yielded abundant pollen of the montane tree Alnus, and with the position of the basin within the northern tropics at 20oN, this suggests that in the mid Oligocene, the basin was intermontane, with Alnus widespread in swamps after the lake shallowed, at an altitude of at least 1000 m. The Miocene succession has been interpreted by reference to SEA (‘Southeast Asia’) depositional cycles following Morley et al. (2021). The evaluation suggests a complete early Miocene succession, varying from fluvial with low sedimentation rates for the youngest part of the Dinh Cao and Phong Chau Formations to shallow marine with increased sedimentation rates after 17 Ma through the Phu Cu Formation which correlates to the middle Miocene climatic optimum. There was a significant period of non-deposition from 13.8 to 9.5 Ma in Well A and from 13.5 to 11.8 Ma in well B, corresponding to the middle Miocene unconformity seen in other nearby basins.