西秦岭徽县-成县早白垩世盆地沉积特征及其构造意义

徽县-成县(徽成)盆地是西秦岭造山带内一个具有代表性的早白垩世走滑拉分盆地。沉积相分析结果显示,盆地内部发育不同的沉积相组合,且呈现明显的时空变化特征。盆地充填序列分析表明,徽成盆地的沉积演化可划分为4个阶段:田家坝组沉积时期、周家湾组沉积时期、鸡山组沉积早期和鸡山组沉积晚期。田家坝组沉积时期,盆地南部以冲积扇砾岩和辫状河砂、砾岩沉积组合为主;周家湾组沉积时期,盆地西部以冲积扇砾岩和辫状河砂、砾岩沉积组合为主;鸡山组沉积时期,盆地北部和南部以冲积扇砾岩和辫状河砂、砾岩沉积为主。在整个沉积过程中,盆地中心表现为湖泊(前三角洲)相细粒沉积,而河流和三角洲体系则分布于冲积扇和深水湖泊(前三角洲)沉积之间。古流向和物源恢复结果证明,盆地沉积物主体来自于盆地北部、南部的花岗岩和前侏罗纪地层。盆地构造沉降和沉积充填过程主要受控于盆地北缘徽凤断裂,盆地南部抬升与盆地边界断层的活动密切相关,是盆地的主要物源区。

Depositional characteristics of Early Cretaceous Huixian-Chengxian basin in western Qinling Mountains and their structural significance

The Huixian-Chengxian (Huicheng) basin is a well-preserved pull-apart basin in the western Qinling orogenic belt. There exist various depositional sedimentary lithofacies that show obvious spatial variations. The sedimentary evolution of Huicheng basin can be divided into four stages. The first stage was characterized by minor sediments of Tianjiaba Formation, and the second stage by basin expansion of Zhoujiawan Formation; the third stage of early Jishan Formation was a maximum basin and the fourth stage had features of sedimentary infilling. In the sedimentary period of Tianjiaba Formation, alluvial fan and braided river dominated proximal areas of the southern basin margins; in the sedimentary period of Zhoujiawan Formation, coarse-grained sediments dominated the western basin margin; and in the sedimentary period of early Jishan Formation, alluvial fan and braided river dominated proximal areas of the southern and northern basin margins. Fine-grained lacustrine (prodelta) sediments were prevalent in the basin center. The middle basin was mostly fluvial and delta sandstone and conglomerate facies associations. Sediments were mainly derived from granite and pre-Jurassic rocks in the north and the south, as indicated by restored paleo-current directions and provenance. Tectonic subsidence and sedimentation were primarily controlled by the Huifeng border faults on the northern margin of the basin. The uplift of pre-Jurassic rocks resulted from the tilting of border faults, and this area became the main provenance area.