塔里木盆地西南凹陷古新统巨厚海相石膏岩沉积特征及环境意义

Sedimentary characteristics of Paleocene huge marine gypsum rock in southwestern depression of Tarim Basin and its environmental significance

塔里木盆地(简称塔,下同)西南凹陷古新世阿尔塔什组发育巨厚层海相石膏岩,夹薄层泥岩、粉砂岩及灰岩,是塔西南凹陷断续海侵环境下多期次蒸发沉积的产物。野外调查显示,该层海相石膏岩出现于皮拉里、阿尔塔什、麻扎塔格及大山口地区的阿尔塔什组露头剖面。石膏岩在凹陷内分布广泛,在西昆仑山前、南天山山前及麦盖提斜坡带均有发育且沉积厚度比较稳定。石膏岩中主要盐类矿物为石膏、硬石膏。扫描电镜分析发现,石膏岩中尚含石盐、钙芒硝及含钾镁的硫酸盐等;石膏岩样品中石盐、石膏、硬石膏等多呈细晶或自形微晶,推测阿尔塔什组沉积期,古盐湖曾出现过富钾卤水;通过对皮拉里剖面石膏岩样品进行地球化学分析,揭示该地区古新世古盐湖演化过程中出现2个相对富钾峰值。在古盐湖演化过程中,由于多期次特提斯海水的侵入,凹陷内部阿尔塔时期发育了碎屑岩—巨厚层石膏岩—碎屑岩的沉积韵律,古盐湖卤水表现为淡—咸—淡的变化规律。伴随着阿尔塔时期4次大规模的海侵,石膏岩沉积从西昆仑山前扩展到麦盖提斜坡地带,海侵范围也逐渐扩大。在阿尔塔什组顶部发育中厚层灰岩,显示在阿尔塔什组沉积晚期,塔西南凹陷沉积环境从●湖相逐渐向浅海相环境演变。

Huge-thickness marine gypsum rock were deposited in southwestern Tarim Basin during the Aertashi For- mation of the Paleocene. Which is composed of marine gypsum rock and intercalated with thin mudstone, silt- stone and limestone beds, represents cycling transgression and regression in southwestern Tarim Basin from the Paratethys Ocean to the west. Through field survey, large amounts of marine gypsum rocks in the Aertashi For- mation were found in Pi｜ali, Aertashi, Mazartag, and Dashankou areas. In this depression, gypsum rock withstable deposition thickness is widely distributed and developed in the West Kunlun and South Tianshan piedmont as well as on the Maigaiti slope. Major minerals in gypsum rock are gypsum and anhydrite. SEM analysis shows that the gypsum rock contains halite, glauberite, potassium-bearing magnesium sulfate, and clay minerals that are mainly finely crystalline or microcrystalline, indicative of the potassium-rich brine entry into the depression during the deposition of the Aertashi Formation. Geochemical analysis of gypsum rock of the Pilali section re- veals that two peaks relatively rich in potassium occurred during the ancient saline lake evolution in Paleocene. Sedimentary rhythms of clastic rock-thick gypsum rock-elastic rock were developed in the depression during the Aertashi period because of multi-stage transgression by the Paratethys seawater, accompanied by variation of sea- water salinity from brackish to saline and to brackish. Gypsum rock of the Aertashi Formation expanded from west Kunlun Mountains to Maigaiti slope due to four large-scale transgressions, indicating gradually widening transgression by that time. Medium-thick limestone beds developed in the upper part of the Aertashi Formation suggests that the sedimentary environment had evolved from the lagoon to the shallow sea in southwestern de- pression of Tarim Basin by the end of the generation of the Aertashi Formation.