渝东北巫溪地区晚奥陶世五峰期元素地球化学特征及其对沉积环境的限制

Elemental geochemical characteristics and limit on sedimentary environment in the Late Ordovician Wufengian period in the Wuxi area,NE Chongqing

渝东北地区奥陶系五峰组沉积期广泛发育硅质岩及页岩等细粒沉积物,深化其海平面变化及沉积环境研究对页岩气勘探意义重大。综合全岩分析、主—微量及稀土元素地球化学指标及草莓状黄铁矿等系统研究表明,巫溪地区五峰组Al2O3/(Al2O3+Fe2O3)均值为0.78、Y/Ho均值为29.05,指示其属陆源碎屑影响的大陆边缘—远洋沉积环境。微量元素UEF—MoEF及V/Cr—U/Th交会特征反映五峰组沉积期水体属硫化—缺氧状态。稀土元素分析表明δEu值具明显负异常、由下至上ΣREE渐增且轻稀土由相对富集变为无富集或轻微亏损,指示正常海水沉积、受热液作用影响不明显。多参数氧化还原指标U/Th、Ni/Co及V/Cr等共同指示五峰组沉积期属厌氧—贫氧环境,对应草莓状黄铁矿平均粒径由下至上逐渐变大,反映氧化性逐渐增强。综合研究表明,渝东北巫溪地区五峰组沉积期历经完整海侵—海退三级海平面变化旋回并可细分为5个次级海平面变化旋回;早期属厌氧环境、晚期属贫氧环境,沉积环境由外陆棚向内陆棚过渡。

The deep-water rocks are confused to reflect fluctuation of sea level by the fine-grained sediments and occurrence of a thin succession.Upper Ordovician black siliceous rocks and siliceous shales(Wufeng Formation)from Wuxi(Northeastern Chongqing,Southwest China)are studied using integrated petrographic,and geochemical methods(major,trace elements and REE)and pyrite framboid in order to decipher depositional conditions during the sedimentation of siliceous rocks.Al2 O3/(Al2 O3 + Fe2 O3),SiO2/Al2 O3 and Al-Fe-Mn values indicate that the sediments of Wufeng Formation are deposited in a continental margin and influenced by detrital input.UEF-MoEFand V/Cr-U/Th cross plots suggest the Wufeng Formation was formed in dysoxic to anoxic conditions.In addition,redox proxies such as U/Th and V/Cr suggest that the lower part of Wufeng Formation was deposited in an anoxic environment,while the upper part was deposited in a dysoxic environment.Furthermore,the gradual increase of the average size of pyrite framboids(from 3.46μm to 4.21μm)shows that the oxidizability increased.During the Wufengian period deposition,sea level fluctuations can be divided into a third-order cycles and subdivided into five fourth-order cycles.U/Th,Ni/Co,V/Cr,and the size of pyrite framboid decrease gradually,which indicates that oxidizability(from anoix to dysoxic)and depth of the water gradually increased.The sedimentary environment was transformed from outside shelf to inner shelf.