藏南日喀则弧前盆地帕达那组沉积相分析

Sedimentary Facies of the Padana Formation in the Xigaze Forearc Basin,South Tibet

藏南桑桑地区帕达那组是上白垩统日喀则弧前盆地浅水沉积。基于野外实测剖面对帕达那组进行了详细的沉积相研究。帕达那组岩相类型主要有五类:砾岩相、砂岩相、粉砂岩相、页岩相和灰岩相。砂岩中发育大量楔状和板状交错层理、平行层理、粒序层理及泥砾、潜穴、生物扰动等沉积构造,页岩中发育潜穴及大量钙质结核。根据岩相和沉积构造特征识别出陆棚相和三角洲相。三角洲相包括前三角洲、三角洲前缘及三角洲平原三个亚相,可进一步划分为水下分支河道、河口砂坝、支流间湾、分流河道、洪泛平原、沼泽等微相。帕达那组地层自下向上发育陆棚相和三角洲相(依次为前三角洲、三角洲前缘及三角洲平原亚相),整体显示出古水深变浅的趋势。这一沉积环境的变化代表着日喀则弧前盆地由欠充填到过充填阶段的演化。

The Padana Formation represents the shallow marine stage of the Xigaze forearc basin in the south Tibet during the Late Cretaceous.This paper focused on the study of the sedimentary facies of the Padana Formation in the area of Sangsang in the southern Tibet.Five types of lithofacies were recognized in the Padana Formation,including conglomerates,sandstones,siltstones,shales and limestones.Abundant sedimentary structures were developed in sandstones and siltstones,including wedge cross bedding,planar cross bedding,grading beds,parallel lamination,mud interclasts,biotic burrows and bioturbations.Plenty of carbonate concretions and some biotic burrows occurred in shales.Based on the studies of lithofacies and sedimentary structures,we interpret that continental shelf and delta are the sedimentary facies of the Padana Formation.Furthermore,the delta facies include prodelta,delta front and delta plain subfacies.Delta front can be further divided into subaqueous distributary channel,mouth bar and interdistributary bay,while delta plain is composed of distributary channel and floodplain.The evolution of the sedimentary environments from continental shelf to delta upwards in the Padana Formation suggests shallowing of the Xigaze forearc basin which may reflect the filling history of the Xigaze forearc basin from underfilled to overfilled stages.