准噶尔盆地东北缘石炭系年代—地层格架:对卡拉麦里洋闭合时限的启示

Carboniferous chronostratigraphic framework in the northeastern margin of the Junggar Basin: Implications for the closure time of Karamaili Ocean

卡拉麦里地区洋—陆转换是中亚增生造山演化的重要地质过程,其对准噶尔石炭纪盆地发育与演化以及油气地质条件具有重要的控制作用;厘定卡拉麦里洋的闭合时限是揭示洋—陆转换的关键。但卡拉麦里地区的石炭纪地层划分尚未统一,严重制约了对该洋盆演化过程的认识。本文在回顾总结卡拉麦里地区的石炭纪地层划分沿革的基础上,基于岩石地层、古生物地层以及同位素年代学资料,对卡拉麦里断裂南侧的石炭纪地层进行了重新梳理,建立了卡拉麦里地区石炭系年代—地层格架。研究结果显示卡拉麦里地区石炭纪地层层序在结构上具有东西分段差异。卡拉麦里东段(双井子地区)和西段(白碱沟地区)下石炭统底部均为滴水泉组碎屑沉积,时代为359~350 Ma;其上覆地层均为松喀尔苏组a段,是一套火山—沉积序列,其顶、底火山岩年龄将其形成时代限定为350~332 Ma;松喀尔苏组b段碎屑沉积仅发育在卡拉麦里西段,其时代为332~323 Ma;西段上石炭统为巴塔玛依内山组,是一套火山—沉积序列,其测得最老和最年轻的火山岩年龄将其时代限定为323~306.5 Ma;而东段上石炭统为石钱滩组碎屑沉积,在盆内偶见火山碎屑岩及火山岩,时代为308~299 Ma。同时结合前人在卡拉麦里缝合带北侧地层古生物学及沉积学,断裂两侧火山岩以及南侧的地震地层学研究结果,认为卡拉麦里洋闭合于早石炭世晚期(332~323 Ma)。

The ocean-continental transition in the Karamaili region is an important geological process during the evolution of Central Asian Orogenic Belt, because it plays important roles in controlling the tectonic evolution of the Junggar Basin and related Carboniferous hydrocarbon geological conditions. Determining the closure time of the Karamaili Ocean is the key to reveal the ocean-continental transition. However, the division of Carboniferous strata in this region remains unclear, which hampers our understanding of the evolution of the Karamaili Ocean. This paper re-defined the Carboniferous of the Karamaili region based on lithological, paleontological and geochronological data, and established the Carboniferous chronostratigraphic framework in the Karamaili region. The results show that the Carboniferous sequence in the Karamaili region has differences between the eastern and western segments in stratigraphic structure. The bottom of the Lower Carboniferous in both the segments are the Dishuiquan Formation clastic sequences with the depositional age of 359~350 Ma. The overlying Songkaersu Formation(Part a)clastic-volcanic sequences are formed at 350~332 Ma based on the age constraint from its top and bottom volcanic rocks. The Songkaersu Formation(Part b)clastic rocks in the western segment formed in 332~323 Ma. The Upper Carboniferous in the western segment is represented by the Batamayineishan Formation clastic-volcanic sequences that may form at 323~306.5 Ma based on the volcanic ages.The Upper Carboniferous Shiqiantan Formation in the eastern segment consist of clastic, pyroclastic and volcanic rocks, having a depositional age of 308~299 Ma. Combining previous studies, it is suggested that the Karamaili Ocean closed during the late period of the Early Carboniferous(332~323 Ma).