Spatial and Temporal Distribution and Characteristics of Tuff Intervals from Yanchang Formation in the Southeastern Margin of Ordos Basin

This study focuses on the tuff intervals involving zircon U-Pb geochronology, which are located at the bottom of Yanchang Formation in Yonghe outcrop section, Shanxi Province. In conjunction with the field exploration and microscopic observation, they show obvious regional differences. They mainly occur in the Yonghe and Daning counties, the southeast margin of Ordos Basin, and in the Ningwu Basin, and show a trend of thinning from west to east. The weighted average ages of the dominant component in the two Yonghe tuff samples are (241±3) Ma (n=29) and (238±3) Ma (n=29), respectively. Combined with the different well data in Shaanxi Province (227~241 Ma), these results indicate that the eruption of tuff is dated at the late Early Triassic to early Middle Triassic.