Lacustrine Tempestite Sediment Feature and Significance in Second and Fourth Members of Xujiahe Formation,Upper Triassic in Middle of Sichuan Basin

Based on the field section and the core observation,this paper presents the characteristics of lacustrine elastic tempestites in the second and fourth members of Xujiahe formation,Upper Triassic in the middle of Sichuan basin,sets up the storm deposition model in this area,including the conditions and significance of the tempestite formation. The tempestites are composed of unconsolidated arenaceous sediment and boulder clay,which were from the sediments of mere corcagh and shoreside swamp churned up by storm wave,and transported to delta and beach bar of shore shallow lake. A plenty of the sedimentary structures of typical tempestite appears with less of mono-layer storm sediment,and the superposed half-baked tempestites formed by polygenic storm wave are often observed. The existence of tempestites is helpful to in-depth understanding of sedimentary facies in this area,indicating that the Upper Triassic was subjected to influence of the paleopacific climate in this area,and the hydrodynamic force of lacustrine basin was correspondingly intensified. Parts of original sandy sediments were vulnerable to be reconstructed,shaping new beach bar of shore shallow lake. It is suggested that the tempesite sand body can be as a favorable reservoir rock and could form prospective lithologic or structural-lithologic hydrocarbon reservoir.