The Hoanh Bo Trough-a landward keyhole to the syn-rift Late Eocene-Early Oligocene terrestrial succession of the northern Song Hong Basin (onshore north-east Vietnam)

Located on the northern Vietnam onshore/offshore transition, the Hoanh Bo Trough is an excellently exposed terrestrial Palaeogene sedimentary sequence that may be treated as an analogue for regional interpretations of the sedimentary and structural evolution of the northern Song Hong Basin. The Hoanh Bo Trough lies to the north of the northern Song Hong Basin and to the west of the Beibuwan Basin, the origin and evolution of which are linked with Palaeogene South China Sea rifting. Field and archival well sedimentological observations were made throughout the Palaeogene succession of the Hoanh Bo Trough, and samples were collected for palynological, petrographical, and geochemical analysis. Based on the coexistence of particular lithofacies, proximal alluvial fan, distal alluvial fan, fluvial alluvial plain with channels, alluvial plain and/or lake margin, and lacustrine facies associations were distinguished. Palynological analyses suggest the sedimentary infill of the Hoanh Bo Trough is of the Late Eocene-Early Oligocene age and was deposited in a very warm tropical/subtropical climate. In turn, geochemical results demonstrate that the deposits have intermediate chemical maturity and were probably reworked from older sedimentary rock sources. Moreover, it is suggested to combine the Dong Ho and Tieu Giao formations and synonymize them as the Dong Ho Formation. The sedimentary pattern, age, climatic conditions, and structural evolution of the Hoanh Bo Trough align well with the rift initiation, rift development, and rift termination tectonic system tracts. Moreover, the Hoanh Bo Trough could be treated as a landward keyhole for the offshore basins: for instance, the Kien An Basin in the northern Song Hong Basin.