藏北雁石坪地区中侏罗统玛托组高频层序及海平面变化分析

西藏北部雁石坪地区晚巴柔—早巴通期玛托组是一个以砂岩、泥岩为主夹少量灰岩组成的混积型陆棚环境的沉积。含有介壳的凝缩段、下超面及沟蚀面,它们是划分体系域的关键界面。体系域具有二元结构特征,即海侵—高水位体系域,且TST沉积旋回厚度>HST,准层序类型有3种,分别是以砂岩为主的准层序、以泥岩为主的准层序和以潮坪体系向上变浅的准层序,准层序叠置构成进积型和退积型准层序组。采用沉积体系分析方法,初步建立研究区玛托组相对海平面变化曲线,并与藏南及全球海平面曲线进行对比分析,结合碳、氧同位素和磁化率资料,探讨研究区晚巴柔—早巴通期玛托组海平面变化控制因素。研究认为全球海平面变化控制了雁石坪地区晚巴柔早期海平面变化,而班公湖—怒江逢合带向北俯冲构造活动引起的区域洋盆容积变化是晚巴柔晚期—早巴通期海平面变化的主要因素。

Analysis of high-frequency sequences and sea-level change of the Middle Jurassic Matuo Formation in the Yanshiping area, northern Tibet

The Middle Jurassic late Bajocian-early Bathonian Matuo Formation in Yanshiping,northern Tibet,is represented by mixed continental shelf deposits composed mainly of sandstone and mudstone with minor limestone.The condensed interval with shells,downlap and ravinement surface constitute the critical boundaries of the systems tracts of the Matuo Formation.The systems tracts are characterized by a binary structure,i.e.transgressive-highstand systems tracts,and the sedimentary thickness of TST is greater than that of HST.The parasequences are divided into three types,i.e.parasequences dominated by sandstones,parasequences dominated by mudstones,and upward-shallowing parasequences dominated by the tidal flat system.The prograding sequence and retrograding parasequence set is formed by parasequence stacking.A curve of relative sea-level change of the Matuo Formation has been established in the study area by using the depositional system method,and the constraints on the sea-level change in the late Bajocian-early Bathonian Matuo Formation in the study area have been studied based on a comparative study of the curve of sea-level changes in southern Tibet and the globe,combined with the data of oxygen and carbon isotopes and magnetic susceptibility.The eustatic sea-level changes control the early Late Bajocian sea-level change in the Yanshiping area,while the volume change of the regional oceanic basin caused by northward subduction of the Bangong Co-Nujiang suture is the primary factor responsible for the sea-level change during the latest Bajocian-early Bajthonian.