川东南地区下志留统等深流沉积特征

Characteristics of contourites in the Lower Silurian in southeastern Sichuan Basin,China

现代海洋地质调查表明在碳酸盐岩台地边缘斜坡附近存在等深流沉积体。然而,保存在地层中台地边缘的等深流沉积报道较少。通过野外露头宏观观察、实测及岩石显微观察,认为在川东南地区下志留统小河坝组和石牛栏组中存在等深流沉积。在南川三泉、石柱金竹、武隆黄草场剖面中,小河坝组发育对称递变层理及具周期性变化的条带状构造;在桐梓白杨树、坡渡剖面中,石牛栏组下部发育瘤状灰岩、泥质条纹—条带状灰岩,上部见薄板状砂岩。通过对小河坝组和石牛栏组的沉积特征与现代巴哈马台地、马尔代夫碳酸盐岩台地边缘等深流沉积特征对比,并参考经典深水等深流沉积特征,认为川东南地区小河坝组存在等深流沉积,并在南川三泉地区发育等深积岩丘;石牛栏组上部薄板状细—粉砂岩沉积体亦可能为等深积岩,其下部的条纹状灰岩也可能由等深流(底流)迁移有关的牵引流沉积作用而形成。上述认识为古代等深流沉积研究提供了实例,对探讨川东南地区古地理演化及油气储集层性质有积极意义。

Modern marine geological surveys show that there are contourites along the slopes of carbonate platforms.However,it is difficult to identify contourite drifts preserved in the strata.In this study,we document contourites from the Lower Silurian Xiaoheba and Shiniulan Formations in the southeastern Sichuan Basin based on the field investigation and microscopic observation.Symmetrical graded beddings and banded structures with periodic variations are found from the Xiaoheba Formation at such sections as Sanquan in Nanchuan County,Jinzhu in Shizhu County,Huangcaochang in Wulong County.Nodular limestones and argillaceous strip-banded limestones are developed in the lower Shiniulan Formation,while thin-bedded sandstones are found in the upper Shiniulan Formation at Baiyangshu and Podu sections in Tongzi County.Compared with modern carbonate platforms at Bahama and Maldivian,and with consider of the typical deep-water isobathic sedimentary features,it is suggested that contourites predominate the sandstones of Xiaoheba Formation.Likewise,the thin-bedded sandstones yielding from the upper Shiniulan Formation might be the contourites controlled by periodic changes of sea level.Meanwhile,the striped limestones in the lower Shiniulan Formation are recognized as the deposits controlled by tractive current(i.e.,contour current).This study provides an actualistic example for ancient contourites which might enhance our understanding of the Lower Silurian palaeogeographic evolution and of oil/gas reservoirs in the southeastern Sichuan Basin.