Upper Triassic Sediments In West Sichuan Area: Implication For Paleoclimate

The Late Triassic Maantang period was a special interval during which it turned from carbonatite to clasolite in the western part of Sichuan basin. Understanding this transition correctly will be significant for studying the paleoclimate evolution in Western Sichuan Area. From Late Triassic Norian to Rhaetian, it turned from transitional environment to terrestrial environment in western Sichuan. And there were few research of the paleoclimate about this region before.Based on detailed field investigationss, sampling, contrast and slice analysis of six typical sections and the core of well Longshen 1, Maantang Formation were divided into four members. The deposition of the first member was gray oomicrite. Above the first member was sponge reef limestone of the second member, part of which was micrite. The third member was dark grey Calcium shale with testas, which had led to the death of sponge reef. The fourth member was alternating beds of aleurolite and mudstone. The first, second and third members were belong to carnian, and the fourth member was belong to Norian.On Contrasting the deposition of Carnian of the study area with that of south Alps, Europe, and Himalayan spiti basin, it's found that the transition from carbonatite to clasolite developed all around the Tethys, which was named as"Carnian carbonate crisis"or"Carnian pluvial event."According to the MS (mean sensitivity ) and AS (annual sensitivity) of the rings of three fossil woods sampled collected in upper Triassic Xujiahe Famation in western Sichuan Area, the MS was 0.263, 0.238, and 0.242, all were less than 0.3, which indicated that the annual water supplying of the growing environment of the woods was adequate. With the slice study of the fossil woods, it was regarded that there was a strong monsoonal climate in the late Triassic west Sichuan basin and the rainfall of different seasons in a year changed greatly.The Carnian pluvial event was explained to be affected by active tectonic movement and megamonsoon system in the late Triassic. During this time interval strong tectonic movement caused Qinling orogenic belt upheaval to be supply sources. Adding with the influence of megamonsoon prevailed in Triassic, the weathering rate in the study area were accelerated, which caused mass of terrigenous siliciclasts and clay material to input into carbonate depositional area, which triggered the stagnation of carbonate. The megamonsoon in Paleotethys maybe at least continued to the middle Xujiahe period, late Triassic Rhaetian.