Biostratigraphy and palaeoenvironments of early Pleistocene deposits in the southern part of the Gulf of Tonkin, Vietnam

Abstract

This study investigates the biostratigraphy and palaeoenvironments of the early Pleistocene deposits in the southern part of the Gulf of Tonkin, Vietnam. Biostratigraphic data, sediment cores, well-logs, and seismic data were used to analyse and construct the chronostratigraphy and paleo-depositional environments of the study area. The microfossil assemblages were recognized using new datasets from five exploration wells, including the biomarker calcareous nannofossils (Gephyrocapsa oceanica, Pseudoemiliania lacunosa, Helicosphaera sellii,Calcidiscus macintyrei, and Discoaster brouweri); the foraminifera (Globigerinoides obliquus, Globorotalia truncatulinoides, and Globorotalia tosaensis); and the palynomorph biomarkers (Phyllocladus spp. and Podocarpus imbricatus). By integrating these assemblages with the well-logs and seismic data, a detail chronostratigraphic framework for the early Pleistocene deposits of the study area was established. The early Pleistocene deposits can be subdivided into four calcareous nannofossil biozones, including CNPL7, CNPL8, CNPL9, and lower CNPL10, with the top of boundaries dated at 1.60 Ma, 1.24 Ma, 1.06 Ma, and 0.739 Ma, respectively. The results also provide useful information about depositional environments that are interpreted as shallow marine environment in the lower part of early Pleistocene, which has been identified as the calcareous nannofossil zones (CNPL7 and CNPL8). These zones are characterized by a change from a delta front to prodelta, with prograding seismic reflectors. The upper part of the early Pleistocene includes the calcareous nannofossil zones (CNPL9 and lower CNPL10) that correspond to a widespread area of middle-outer neritic environment with parallel to subparallel seismic reflections. In addition, the Pliocene/Pleistocene boundary in the southern part of the Gulf of Tonkin is dated at 1.93 Ma. This boundary is also equivalent to the base of the Ledong Formation in the Yinggehai Basin and the Qiongdongnan Basin, to the top of Wanshan Formation in the Pearl River Mouth Basin, in offshore China.