Lithofacies and depositional environments of the Paleogene/Neogene sediments in the Hoanh Bo Basin (Quang Ninh province, NE Vietnam)

The Hoanh Bo Basin is a multiple period formed basin which is associated with the tectonic mechanism due to the activity of the Chi Linh-Hon Gai and Trung Luong Faults during the late Paleogene – early Neogene time. The basin is filled with continental sediments, comprising mainly polymictic conglomerates, sandstones, siltstones, claystones and shales. Coarser-grained polymictic deposits predominate along the northern and western margins of the basin, while different types of siltstones, claystones, even sandstones occur in its center, as well as along its southern and eastern parts toward the Ha Long Bay. The source rocks for sedimentary materials are mainly originated from rocks of the Hon Gai and Ha Coi Formations. Based on the dominant grain-size class, texture, stratification, degree of clast rounding and sorting, 17 lithofacies were determined in the Hoanh Bo Basin infill. The basin is filled by gravelly, sandy and fine-grained lithofacies just covers a modest area but it represents a full of succession of 4 depositional systems including alluvial fan, fluvial, deltaic and lacustrine environments. Alluvial fan and fluvial environments are characterized by an assemblage of lithofacies such as sand- and mud-supported disorganized breccias (Gb), clast- to sand-supported conglomerates (Gmm), clast-supported inverse-graded conglomerates (Gig), planar-cross-bedded conglomerates (Gp), trough cross-bedded conglomerates (Gt), horizontally stratified conglomerates (Gh), planar cross-bedded sandstones (Sp), trough cross-bedded sandstones (St), ripple cross-laminated sandstones (Sr), horizontally bedded sandstones (Sh), massive sandstones (Sm), massive mudstones, siltstones (Fsm), laminated siltstones and mudstones (Fl), massive claystones (Fm) and coaly lithofacies (C); the deltaic assemblage is dominated by planar-cross-bedded conglomerates (Gp), horizontally stratified conglomerates (Gh), planar cross-bedded sandstones (Sp), horizontally bedded sandstones (Sh), low-angle cross-bedded sandstones (Sl); while the lacustrine assemblage consists of ripple cross-laminated sandstones (Sr), horizontally bedded sandstones (Sh), normally graded sandstones (Sng), massive mudstones, siltstones (Fsm), laminated siltstones and mudstones (Fl), massive claystones (Fm) and coaly lithofacies (C).