川北旺苍地区奥陶系地球化学特征及沉积环境分析

The geochemical characteristics and sedimentary environment analysis of Ordovician in Wangcang region,northern Sichuan Basin

四川盆地北部奥陶系出露良好,地层较全。研究中选取旺苍地区的两条剖面,采集12件样品,进行了常量、微量及稀土元素测试,结合野外露头观察将奥陶系沉积相划分为滨岸、陆棚、局限台地、台缘浅滩及台缘斜坡5种沉积相类型。从元素的地球化学性质、富集规律以及环境分析来看,随着水体加深和与陆地距离的增加,奥陶系碳酸盐岩中的Sr的含量呈上升趋势,且Sr/Ca增加,Ni增多,Mn富集,ΣREE也有增加的趋势,表明旺苍地区奥陶系自下而上水体总体加深,在西梁寺组沉积期达到极值,随后有变浅的趋势;Na/Ca比值反映从局限台地到台缘斜坡的盐度降低;Fe^(2+)/Fe^(3+)和V/(V+Ni)共同指示了赵家坝组至牯牛潭组沉积期为缺氧的还原环境,而宝塔组沉积时为相对富氧的环境,说明该时期海平面可能有一次相对下降的过程,也可能与此时水体富氧有关。

The Ordovician is well exposed with complete stratigraphy in northern Sichuan Basin. Two sections in Wangcang region were selected and twelve samples were collected, which were tested with major, trace and rare earth elements. Combined with field outcrop observation, the Ordovician were divided into five types sedimentary facies： shore, shelf, restricted platform, platform margin shoals and platform margin slope. Based on the geochemical characteristics of elements, the enrichment regularity and environmental analysis, the amount of Sr, Sr/Ca, Ni and Mn in the Ordovician carbonate rock were on the rise and ΣREE also had an upward trend with the increase of water depth and the distance from land. These data indicate that the Ordovician sea level were overall deepens. The highest sea level appeared in the Xiliangsi Formation sedimentary period, and then changed the trend to shallow. Na/Ca ratio suggested that the salinity was reduced from restricted platform to platform margin slope. Fe2＋/Fe3＋ and V/（V＋Ni） both indicated that the Zhaojiaba Formation and the Guniutan Formation sedimentary period were anoxic and reducing environment. But the Pagoda Formation sedimentary period were relative oxygen enriched environment, which suggested that the sea level may have a relative decline in this process, or may be associated with oxygen-rich sea water at this period.