阴山—燕山地区晚侏罗世—早白垩世土城子组地层、沉积特征及盆地构造属性分析

Stratigraphy,sedimentology and tectonic background of basin evolution of the Late Jurassic-Early Cretaceous Tuchengzi Formation in Yinshan-Yanshan,North China.

阴山—燕山地区广泛分布晚侏罗世—早白垩世土城子组,保存了土城子组形成时期的古气候、古地理及大地构造演化的重要信息。然而,对于土城子组的沉积特征、盆地属性和大地构造背景仍存很大争议。文中对前人关于土城子组的相关研究成果进行了总结,在依据实测资料的基础上,以盆地分析方法为手段,从地层、沉积相、物源和古水流入手,研究和分析了土城子组盆地演化规律和构造属性。研究表明,土城子组垂向上经历了冲积扇—三角洲—湖泊—三角洲—河流—冲积扇的沉积环境变化,沉积相平面展布特征显示土城子组演化过程中出现了包头—呼和浩特、北京延庆、凌源3个沉积中心,古水流流向呈现分区性,分别向3个沉积中心汇聚,表明晚侏罗—早白垩世时期,阴山—燕山地区存在大青山、冀北和辽西3个泛盆。物源分析显示砾石成分呈现明显的分带性,辽西地区以火山岩和沉积岩为主,冀北地区主要为火山岩,其次为花岗岩和碳酸盐岩,大青山地区变质岩为主。从各地基底岩性可以看出,辽西地区土城子组下伏髫髻山组火山岩发育,古生代沉积岩出露较广,冀北地区则主要为前寒武系碳酸盐岩、花岗岩和晚古生代花岗岩,以及中生代火山岩,大青山地区以前寒武系变质岩为主,以上各地区盆地基底的岩性与砾石成分的分区性形成了很好的对应。砾石中老地层物质组分向上增加、新地层物质组分向上减少的特征则反映了揭顶剥蚀的特点,二者共同说明土城子组是在盆地基底差异性隆升的条件下剥蚀沉积的产物。广泛分布于冀北—辽西地区土城子组上部的巨型-大型板状-槽状交错层理厘定为风成沉积,这一特殊沉积环境下形成的产物也属于盆地基底隆升的结果。阴山—燕山与周边地区晚侏罗—早白垩世变质核杂岩的发现说明,土城子组为伸展背景下形成的陆内裂谷盆地,与同期岩石圈减薄作用耦合,它的形成可能主要受太平洋板块向西北的俯冲作用有关。鉴于土城子组盆地的构造属性,遂将晚侏罗世—早白垩世定位为华北北缘陆内裂谷的起始时间。提出一个新的土城子组盆地演化动力学模型,能够很好地解释晚侏罗世—早白垩世阴山—燕山地区土城子组地层发育、沉积演化以及与岩浆岩关系、盆地动力机制、盆地属性和土城子组与张家口组在不同区域所表现的不同接触关系等问题。

The Tuchengzi Formation is the most widely distributed deposits of the Late Jurassic to the Early Cretaceous in Yinshan-Yanshan in North China,which contains a lot of information on paleoclimate,paleogeography,basin and tectonic evolution.Vast distribution and a variety of sediments of the Tuchengzi Formation,however,restrain the interpretations of basin nature and tectonic background,and there are plenty of controversies about the Tuchengzi Formation as well.This paper has briefly reviewed those previous studies of the Tuchengzi Formation in Yinshan-Yanshan.Furthermore,on the basis of geological survey,the paper focuses on the stratigraphy,sedimentary facies,provenances and paleocurrents of the Tuchengzi Formation.Current study indicates that the Tuchengzi Formation was involved in an evolution of sedimentary facies in space-time,from alluvial fan to delta,lacustrine and river or alluvial fan in ascending order.Palaeo depositional centres of the Tuchengzi Formation were recognized to be located in Baotou-Huhehaote of Inner Monggolia,Yanqing of Beijing and Lingyuan of western Liaoning Province respectively,which are related to the distribution of paleocurrents and prove the existence of a giant lacustrine basin in Yinshan-Yanshan,extending from Daqingshan,Northern Hebei and Western Liaoning Province in the Late Jurassic-Early Cretaceous.Provenances analysis of conglomerates in the Tuchengzi Formation shows obvious zonations.Pebbles of volcanic and sedimentary rocks are mainly in Western Liaoning Province;pebbles of volcanic rocks and some granite and limestone dominate Northern Hebei Province;and pebbles of metamorphic rocks are chiefly in Daqingshan.In term of regional distribution of basements of the Tuchengzi Formation,provenances of this formation display a good concordance with diverse basements of the Tuchengzi Formation in Yinshan-Yanshan,and furthermore suggest that the Tuchengzi Formation were deposited by an erosion of basements in descending order.The widely distributed large scale plate-trough cross-beddings within the top part of the Tuchegnzi Formation in Western Liaoning Province and Hebei Province are interpreted as aeolian deposits resulting from basin＇s basement uplift of the Tuchengzi Formation.Metamorphic core complex in Yinshan-Yanshan or nearby areas in the Late Jurassic-Early Cretaceous previously reported suggests an extensional tectonic background at that time,and it is probable that the coupling of lithospheric thinning was impacted mainly by Pacific plate subduction northwestward in the Late Jurassic-Early Cretaceous.This research suggests that the Tuchengzi Formation is the sediments of rift basin and deposited in an extensional tectonic background of the Late Jurassic-Early Cretaceous.A new dynamic model of the evolution of basins of Tuchengzi Formation was proposed finally.