SHEET GRAIN-FLOW-DOMINATED GRAVEL FAN DELTAS OF THE TERTIARY DAGZHUKA FORMATION IN THE XIGAZÊ AREA,XIZANG(TIBET)

The Tertiary molasse in the Xigazê area, Xizang (Tibet) is distributed mainly along the Yarlung Zangbo River suture zone, where lacustrine gravel fan-delta complexes are well developed. Six types of conglomerate lithofacies may be classified for these fan-delta complexes. Types A, B and C as the density-modified sheet grain-flow deposits rich in cohesionless sand matrix and lacking in cohesive clay matrix, are poorly sorted, free of internal fabric and grain-to matrix-supported. The bed thickness may be constant laterally within some distance. The basal surfaces of the rock beds are relatively smooth. Type A as fan-delta plain deposits is characterized by poor stratification, huge thickness and shortage of lacustrine mudstone intercalations. Type B as fan-delta front deposits is well-stratified and interbedded with lacustrine mudstones. Type C as profan-delta deposits is thinbedded and intercalated with lacustrine mudstones. Type B as fan-delta front deposits resulted from sheet tractional flows is characterized by grain-supported mechanism, unimodal grain size distribution, large-scale tabular cross-bedding and well-developed stratification. The bed thickness may be constant laterally. The basal surfaces of the rock beds tend to be planar, free of the form of downcutting channels, and interbedded with lacustrine mudstones. Type E as fan-delta plain channel deposits consists of the conglomerate lenses with grain-supported mechanism and imbricated structure. The basal surfaces have the form of down-cuttint channels. Type F which is cohesive matrix (clay)-supported and free of internal fabric is thought to be the high-viscosity debris flow deposits of the fan-delta plains and low-viscosity debris flow deposits of the fan-delta fronts. Sandstone lithofacies are uncommon in the fan-delta complexes. There are two types of sandstones:(1) sandstone lenses as the scour channel deposits of fan-delta fronts, and (2) thin-bedded, fine-grained sandstones with Bouma sequences as profan-delta turbidity current deposits.
Spasmodic sheet grain flows and sheet tractional flows were responsible for the formation of the above-mentioned fan deltas. The fan-delta plain deposits comprise principally Type-A conglomerate and a small amount of Type-E and Type-F conglomerates formed by high-viscosity debris flows. The fan-delta front deposits are composed dominantly of Type-D and Type-B conglomerates, subodinately of Type-F conglomerate formed by low-viscosity debris flows and a small amount of sandstone lenses. The profan-delta deposits consist chiefly of turbidity sandstone and Type-C conglomerate. The distribution of these lithofaeies may imply that as the flood waned, the coarser fragments were laid down; and when it entered into the fan-delta front and profan-delta environments, due to the addition of plenty of lake water, it is possible for the density-modified sheet grain flows to evolve into sheet tractional flows and then into low-density tubidity currents. Similarly, the highviscosity debris flows may also evolve into the low-viscosity debris flow, and then into the low-density turbidity currents. The sheet grain flows may owe their origin to the reactivation of the proximal alluvial fan deposits.
This molasse basin is located in the Yarlung Zangbo River suture zone. It was formed during the early stage of collision between Eurasian and Indian plates and overlain upon the sediments of the vanished Tethyan plate from which the sediments filled into the molasse basin were chiefly derived. Therefore the molasse basin was believed to be a peripheral basin of collision orogens (Dickinson, 1974).