The Na Duong Basin (North Vietnam): A key for understanding Paleogene basin evolution in relation to the left-lateral Cao Bang-Tien Yen Fault

[Display omitted] •The Na Duong Basin was developed in the Early Oligocene in warm temperate-to-subtropical climate.•Sedimentation was originally dominated by fluvial and peat-forming processes followed by lake deposition.•Studied basins belonged to the long-distance terrestrial fluvial-lacustrine system track.•The fluvial-lacustrine system is proposed as a main Oligocene feeder system for the Beibuwan Basin. The Cao Bang-Tien Yen Fault (northern Vietnam) is subparallel to the Red River Fault Zone. The fault-controlled Cao Bang, That Khe, Lang Son and Na Duong Basins along the Cao Bang-Tien Yen Fault are filled with thick series of terrestrial deposits. The investigations conducted in the present study focused on the sedimentary infill of the Na Duong Basin. Sedimentological observations were made from four geological sites in this basin, including an open-air coalmine, and samples collected for standard palynological, petrographical and geochemical analysis. Seven lithofacies were recognized, and their respective depositional mechanisms were interpreted. Sedimentation in the Na Duong Basin was initially dominated by fluvial and peat-forming processes (Na Duong Formation), followed by lacustrine deposition (Rinh Chua Formation). These formations were deposited in the Early Oligocene, under a warm temperate-to-subtropical climate. The sedimentary infill of the basin is petrographically mature and was deposited in a relatively long-lasting fluvial-lacustrine sedimentary system. The Na Duong Basin was compared to the Cao Bang Basin. The Na Duong Basin is filled with mature sandy and silty deposits of fluvial and lacustrine origin, while the Cao Bang Basin is filled with a wide variety of immature or submature clastic deposits, from coarse-grained alluvial-fan through sandy fluvial to fine, organic-rich lacustrine deposits. Paleogene basins along the Cao Bang-Tien Yen Fault zone are interpreted as belonging to an associated extensive terrestrial drainage system. This system is proposed as a main Oligocene feeder system for the Beibuwan Basin (South China Sea).