西藏日喀则地区第三系大竹卡组砾质扇三角洲——片状颗粒流沉积

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

摘要:

1987和1988年仲夏,我们中国和联邦德国的地质同行联合对西藏雅鲁藏布江缝合带进行了地质考察,对日喀则地区所出露的第三系磨拉石进行了比较详细的沉积学工作,现将初步研究成果作一个简略介绍。西藏日喀则地区第三系磨拉石主要出露于雅鲁藏布江一带,是一套砾岩、砂岩、泥岩和少量泥灰岩的沉积组合,作为碰撞造山带山前或山间盆地的沉积产物,已被许多学者所认识。本文主要讨论始新—渐新世大竹卡组(钱定宇等,1985)磨拉石沉积,尤其是砾岩的沉积机制,并认为湖相砾质扇三角洲沉积是该磨拉石盆地的最主要沉积类型之一,扇三角洲沉积相带发育齐全,在德日剖面和南卡堆剖面(图1)部有完好的扇三角洲层序,是一种独特的、以突发性片状颗粒流和片状牵引流沉积为代表的扇三角洲类型。在青藏高原羌塘地区(余光明等,1986)和柴达木盆地(邓宏文等,1987)都报道过类似的第三纪沉积。研究此类发育于特定大地构造背景的扇三角洲的沉积特征和形成机制具有重要的理论意义。

Abstract:

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).