藏南卧龙剖面下侏罗统海相地层的岩石磁学特征及其古环境意义

Rock Magnetism of the Lower Jurassic Marine Strata in Wolong Section,Southern Tibet and Its Environmental Significance

侏罗纪早托阿尔期的大洋缺氧事件(TOAE)是中生代一次重要的全球性古海洋和古气候事件。以往的研究主要集中于西特提斯域的记录,而对其在东特提斯域的响应研究很少。此次研究了位于东特提斯域的西藏定日卧龙剖面下侏罗统普普嘎组—聂聂雄拉组海相地层,初步厘定了卧龙剖面TOAE层位,并对包含TOAE层位的20 m厚的地层开展了详细的岩石磁学研究。岩石磁学参数的变化显示出四个演化阶段,Ⅰ、Ⅱ、Ⅲ和Ⅳ。其中阶段Ⅰ和阶段Ⅳ分别记录了TOAE事件之前和之后的沉积过程。阶段Ⅱ记录了TOAE的早、中期,表现为细粒磁性矿物含量减少及之后的铁硫化物含量的增加,很可能是对海侵引起的大洋低氧带(OMZ)扩张的响应。阶段Ⅲ记录了TOAE的晚期,表现为高矫顽力磁性矿物含量的显著增加,可能指示沉积区海洋环境由还原环境转变为氧化环境。总之,岩石磁学参数的变化可以为理解东特提斯域TOAE的演化提供重要的启示。

The Toarcian Oceanic Anoxic Event （ TOAE） , which occurred in the early Jurassic, is one of the most important global paleoclimatic events in the Mesozoic. Previous studies had focused on TOAE records in Western Tethys. However, little is known about this event in Eastern Tethys. In this study, we have carried out a detailed rock magnetic investigation of the Lower Jurassic marine strata of the Wolong section in Tingri, southern Tibet, which was situated in the eastern Tethys in the early Jurassic. The studied section consists of the Pupuga Formation and the overlying Nieniexiongla Formation. The T O A E interval was tentatively delineated in the Wolong section based on stratigraphic correlations with those in the Western Tethys. High-resolution rock magnetic data from the 20-m thick section containing the T O A E interval show four stages of environmental evolution, i. e.,Stages I , II, H I,and IV. Variations in rock magnetic data in Stages I and IV document the deposition processes before and after the T O A E event, respectively. Stage II is characterized by a decrease in the abundance of fine-grained magnetic mineral and the subsequent occurrence of iron sulfides. These features likely represent responses to the vertical and lateral expansion of Oceanic Minimum Zone （ O M Z ） associated with the transgression during the early and middle TOAE. Stage III saw a pronounced increase in the abundance of high-coercivity minerals （hematite） and corresponded to the recovery stage C3 of the TOAE. This feature may indicate a rapid change in the marine depositional environment from a reducing to an oxidizing state . Overall , changes in rock magnetic parameters could provide important insight into understanding the evolution of TOAE in Eastern Tethys .