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

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 Sigma REE 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.