鄂尔多斯盆地西、南缘奥陶纪地质事件群耦合作用

Ordovician Geological Events Group in the West and South Ordos Basin

北祁连造山带和北秦岭造山带在早古生代经历了相似的洋陆转化过程,于奥陶纪时发育了汇聚板块边缘的沟-弧-盆体系,分别形成了北西向展布的北祁连造山带走廊南山北缘早古生代岛弧及弧后盆地和东西向展布的北秦岭造山带早古生代岛弧及弧后盆地。期间,秦岭-祁连洋的俯冲造山作用和鄂尔多斯盆地西南缘沉积类型和内陆湖盆的发展演化之间存在有机的联系,构成了盆-山耦合体系,引发一系列构造事件、火山喷发事件和多种类型的事件沉积等。它们之间存在着一系列成因机制上的联系,有着共同的宏观背景。鄂尔多斯盆地西、南缘在几乎相同时期存在一次构造背景的转变,由被动大陆边缘转化为主动大陆边缘,并诱发了多期火山喷发事件,在盆地西南缘奥陶系形成多套斑脱岩夹层,这些斑脱岩可能为同时期或者稍后的钾盐矿(包括含钾卤水)的形成提供了重要物源。同时,鄂尔多斯盆地南缘由浅水碳酸盐台地陷落为深水斜坡,在盆地西、南缘奥陶系有规律的集中发育重力流沉积(海底扇、浊积岩等)、滑塌沉积和震积岩等事件沉积。从形成机制上,华南板块向北俯冲触发了火山活动和地震,火山喷发在奥陶系集中沉积了多套凝灰岩夹层,地震活动导致同时期大套重力流沉积,并触发相对深水区沉积物向深水区移动,使得重力流沉积转化为浊流沉积,形成了具有良好储层的浊积岩。统计表明,上述事件发育的时间与秦岭地区构造活动相对最活跃的时期基本一致。因此这些分布稳定的凝灰岩薄层和中奥陶世集中有规律分布的重力流沉积砂体为华南板块向华北本快俯冲背景下形成的,它们之间存在耦合关系。

The North Qilian Orogenic belt and North Qinling Orogenic belt experienced a similar ocean-land transformation process in the early Paleozoic Era,and developed a trench-arc-basin system in the convergent margin in the Ordovician Era.Consequently,it formed the northwest-trending Early Paleozoic island arc and back-arc basin at the northern margin of Nanshan along the North Qilian Orogenic belt corridor,and EW-trending Early Paleozoic island arc and back-arc basin along the North Qinling Orogenic belt,respectively.During this time,it formed a basin-range coupling system between subduction-accretion orogeny in the Qinling-Qilian ocean and the southwestern margin and interior lake of the Ordos basin.Then,it triggered a series of tectonic events,volcanic eruptions events and various types of depositional events.There is a link among them on a series of genetic mechanisms, which have a common macroeconomic background.There is a tectonic shift-from passive continental margin into active continental margin,in the western and southern margin of Ordos Basin in the almost identical period.And then it induced multi-period volcanic eruption events in the Ordovician,and formed multiple sets of bentonite of Ordovician rocks in the southwestern margin of the Ordos Basin.Meanwhile,the southern margin of Ordos Basin turned into deepwater slope from shallow water carbonate platform,and developed,regularly and concentratedly,event deposits,such as gravity flow deposits（submarine fan,turbidite,etc.）,slumping deposits and seismites.On the formation mechanism,the northward subduction by the South China plate triggered volcanic activity and earthquakes,and volcanic eruptions made a series of bentonites concentrated in Ordovician sedimentary rocks,seismic activity leaded to large sets of gravity flow deposits in the same period,and triggered relatively deep water sediments moving into deeper water area,making gravity flow deposits to be turbidite with good reservoirs,which constitute the main reservoir of oil and gas in deep water.Statistics show that the development time of these event deposits is consistent with the relatively most active period of tectonic activity in Qinling region.Therefore,these stable-distribution thin beded bentonites and the Middle Ordovician gravity flow deposits with concentrated and regular distribution formed in the background of subduction by the South China plate,and there is a coupling relationship between them.