大巴山构造带东段秭归盆地侏罗纪沉积充填过程及其构造演化

位于大巴山构造带东段秭归盆地较为完整地保存了侏罗纪地层,其沉积和构造演化过程可划分两个不同阶段,即早侏罗世受西北缘断裂走滑控制的伸展阶段和中、晚侏罗世受东部黄陵地区抬升挤压消亡的阶段。下侏罗统香溪组底部砾岩表明侏罗纪盆地开始形成。盆地北缘的砾岩较厚,主要形成于冲积扇和辫状河体系,并在其砾石中发现化石砾石,显示距离物源区较近。盆地东南部和南部砾岩较薄,形成于曲流河沉积环境。古水流和碎屑物源分析表明,盆地早期沉积物主要来自于盆地北部神农架地区。中侏罗统泄滩组和陈家湾组以湖泊相沉积为主。上沙溪庙组和下沙溪庙组为较成熟的曲流河沉积,沉积砾岩中无灰岩和燧石,几乎全部为泥岩砾石。上侏罗统遂宁组和蓬莱镇组虽然也为曲流河沉积,但其河床砂岩底部再次出现了灰岩、燧石等砾石。另外,中侏罗统上部和上侏罗统古水流方向为自东向西,指示物源区变为盆地东部的黄陵背斜地区。盆地的沉积充填特征和盆地西北缘具左行走滑运动的高桥断裂,以及北部南倾的多条逆断层表明断层对盆地形成的主控作用。黄陵背斜地区后期的不断抬升对秭归盆地有重大影响,持续的抬升可能导致盆地消亡。秭归盆地的沉积充填和演化过程,是对大巴山构造带东段“燕山运动”的响应。

Jurassic Sedimentary Filling Process of Zigui Basin in the Eastern Section of Daba Mountain Tectonic Belt and Its Structural Evolution

The Zigui basin is a well preserved Jurassic sedimentary basin that lies in the eastern section of Daba Mountain tectonic belt. Its sedimentary filling process and structural evolution can be divided into two distinct stages. The first stage is featured by extensional subsidence controlled by a northwest strike slip fault in early Jurassic, while the second stage is characterized by contractional uplift in response to the rise of Huangling area in middle late Jurassic. The conglomerate at the bottom of lower Juassic Xiangxi Formation marks the beginning of Zigui basin. The thick conglomerate layer distributing in the northern basin deposited in alluvial fan and braided river system. Some fossils preserved in limestone can be found as gravels, and this indicates deposition proximal to source area. The thinner conglomerate deposited in the meandering river is distributed around south and southeast of the basin. The restored paleo currents and provenance analyses show that sediments were mainly from the Shennongjia area at the early stage. Mid Juassic Xietan Formation and Chenjiawan Formation are composed mainly of sandstone and mudstone, formed in lacustrine system at the early stage of middle Jurassic. The Shangshaximiao Formation and Xiashaximiao Formation are characterized by deposition in the steady or mature meandering river at the late stage of middle Jurassic, because the conglomerate contains no limestone and chert gravels but only mudstone gravel. The Suining Formation and Penglaizhen Formation were dominated by meandering fluvial facies, but the gravels preserved at the bottom of channel sandstone differ from those in middle Jurassic. The occurrence of limestone and chert gravels indicated the change of provenance area. In addition, the paleo current was westward at the late stage of middle Jurassic and late Jurassic, which shows the Huangling area maybe provide much clastics. Sedimentary filling characteristics and the northwestern margin fault of the basin with sinistral strike slip, as well as a number of south dipping reverse faults on the north of basin margin show that the Zigui basin was controlled by these faults. The uplifting of Huangling anticline strongly affected the evolution of Zigui basin in late Mesozoic, and resulted in the death of Zigui basin in late Jurassic. The sedimentary filling process and structural evolution of Zigui basin was in response to the "Yanshan Movement" in the eastern section of Daba Mountain tectonic belt.