Lithofacies and depositional environments of Miocene deposits from tectonically-controlled basins (Red River Fault Zone, northern Vietnam)

The Lao Cai, Yen Bai and Bao Yen Basins connected with the Red River Fault Zone (RRFZ) are filled with over 1000 m thick Neogene (Miocene) clastic deposits of different type and origin. Coarse-grained alluvial deposits predominate along the south-western margin of the Lao Cai and Yen Bai Basins, while different types of conglomerates and sandstones, even siltstones occur in their central and along their north-eastern parts. Such asymmetry in the cross-section basin fill points to the south-western active margin of these basins.Three main facies associations composed of 12 lithofacies have been recognized in the Miocene succession filling the studied basins: (1) debris-flow dominated alluvial fan association, (2) sheetflood dominated alluvial fan association, and (3) lacustrine association. Both alluvial fan facies associations are broadly spread in the studied basins, especially along their south-western margins. The debris-flow dominated deposits are represented by decimetre to several meters thick, sand- and mud-supported disorganized pebble- to cobble-size, occasionally boulder breccia and conglomerate beds. The sheetflood dominated deposits are represented by alternating planar-bedded pebble and cobble gravel couplets. The occurrence of conglomerate bodies within and overlapping fine deposits of the lacustrine facies association may suggest deposition in a debris-flow and sheetflood dominated alluvial-fan setting associated with narrow, elongated basins that could be occupied by lakes and bordered by active fault scarps.The facies pattern of the particular basins is connected with the evolution of the RRFZ. The origin of these basins may be correlated with a sinistral transtensional regime which occurred in the Early to Middle Miocene. Moreover, the high-grade metamorphic core of the Day Nui Con Voi Massif, presently the main structural element of the RRFZ, was probably not exposed during sedimentation of most of the strata in the studied basins.