High-resolution Palaeogene sequence stratigraphic framework for the Cuu Long Basin, offshore Vietnam, driven by climate change and tectonics, established from sequence biostratigraphy

Abstract

The chronostratigraphy of the Cuu Long Basin, offshore Vietnam to the south of the Mekong Delta, is poorly understood, especially for the Palaeogene succession. This paper utilises a comprehensive biostratigraphic database from 46 exploration wells to establish a high-resolution sequence biostratigraphic framework for the Oligocene and upper Eocene based on the identification of 36 transgressive-regressive depositional cycles. They are shown to be climate-driven and correlate to 406 kyr ‘heartbeat of the Oligocene’ eccentricity cycles. Their occurrence has been rigorously cross checked against seismic across the basin. This re-evaluation requires the repositioning of the Oligo-Miocene boundary in the Cuu Long Basin from the C seismic pick to the BI.1 seismic pick.By integrating sequence biostratigraphic and seismic datasets, the Oligocene can be seen as characterised by two types of depositional sequence. Firstly, thin and repetitive depositional packages, here termed ‘Sequences’ but perhaps better visualised as parasequences, the deposition of which was controlled by cyclically changing climate. Secondly, much thicker packages, each comprising a succession of parasequences separated by unconformities, and which tie to the main seismic divisions within the Oligocene/upper Eocene and were driven by tectonics. The latter are visualised as 3rd order sequences, with the parasequences forming 4th order sequences.Correlating the succession of parasequences to the 406 kyr ‘heartbeat of the Oligocene’ isotope cycles allows accurate ages to be proposed for each parasequence boundary, and in doing so an accurate time framework is established for the basin. The first sediments may have been deposited during the latest middle Eocene (Seismic group F/G) at the same time as or just prior to the initial stage of faulting prior to the opening of the South China (East Vietnam) Sea, with deposition of seismic group E commencing at about 36.6 Ma during the late Eocene. Unconformities reflecting 3rd order sequence boundaries are dated at 33.4 Ma, 29.8 Ma and 27.4 Ma.