塔里木盆地西北部震旦系—寒武系不整合面成因——来自沉积学的证据

Genesis of the Sinian-Cambrian Unconformity in the Northwestern Tarim Basin:Evidence from sedimentology

【目的】塔里木盆地西北部阿克苏地区广泛发育震旦系—寒武系平行不整合面,目前对其形成机制还存在争议。【方法】以沉积学方法为手段,基于上震旦统奇格布拉克组顶部的沉积相及其岩溶现象,通过详细的野外观察和镜下鉴定,对上述不整合面的成因开展研究。【结果】阿克苏地区奇格布拉克组顶部主要发育于台地边缘和斜坡环境。台地边缘包括颗粒滩和台缘礁,岩相为厚层—块状鲕粒白云岩、中—厚层状砂屑白云岩以及块状叠层石礁白云岩。斜坡环境包括的岩相有滑塌角砾白云岩、浊积岩以及薄层状粒泥白云岩。震旦纪晚期,阿克苏地区东部的沉积水体更深,岩溶发育程度也明显强于西部,垂向上以潜流带为主。此外,寒武纪早期沉积的玉尔吐斯组保持了与奇格布拉克组顶部相一致的沉积趋势,两者自西向东均表现为水体加深的地层序列。【结论】该不整合面由震旦纪末期台缘构造活动诱导的掀斜运动形成,导致阿克苏西部发生抬升,并形成了向东流动的大气淡水水文系统,使得东部台缘带岩溶作用强烈。该研究可为该区域震旦系白云岩储层的油气勘探提供参考。

[Objective]A Sinian-Cambrian parallel unconformity has been widely developed in the Aksu area of the northwestern Tarim Basin,and controversy remains over its formation mechanism[.Methods]Based on sedimentological research,the genesis of the above unconformity was studied through detailed field observation and microscopic identification,focusing on the sedimentary facies and karst phenomenon at the top of the Upper Sinian Qigebrak Formation[.Results]The results show that platform margin and slope facies are developed at the top of the Qigebrak Formation in the Aksu area.The platform margin includes grain beach and microbial reef,consisting of thick-to massive bedded dolo-oolite,medium-to thick-bedded intraclast dolopackstone and massive dolostromatolite reef.Lithofacies of the slope include:dolobreccia,turbidite,and thin-bedded dolowackestone.In the Late Sinian,the sedimentary water bodies in the eastern Aksu area were deeper,and the degree of karst development was significantly stronger than that in the western areas,which was dominated by horizontal phreatic zone vertically.In addition,in the Early Cambrian,the Yuertusi Formation maintained the same sedimentary trend as the top of the Qigebrak Formation,and both showed stratigraphic sequences of deepening water bodies from west to east.[Conclusions]This unconformity was formed by tilting movement induced by tectonic activity at the platform margin in the Late Sinian,which resulted in the uplift of the western Aksu area.In addition,this movement led to the atmospheric freshwater hydrological system flowing eastward,resulting in strong karstification in the eastern platform margin zone.This study could provide guidance for oil and gas explorations of the Sinian dolomite reservoirs in the region.