摘要:
昌图凹陷是燕辽造山带东北缘山前一次级裂陷构造单元,其中充填了白垩纪火山—沉积岩系。根据构造活动与沉积演化特征,地层层序划分为裂陷期 (SSⅠ )、断陷期 (SSⅡ )和坳陷期 (SSⅢ )三个超层序,断陷期 (SSⅡ )超层序可进一步细分为断陷早期 (SⅡ1)和断陷晚期 (SⅡ2 )两个层序。裂陷期超层序发育巨厚的溢流相、爆发相火山岩。断陷早期 (SⅡ1)层序发育低位与湖盆扩张两个体系域,低位体系域总体反映为一套近物源稀性泥石流沉积特征 ;湖盆扩张体系域反映水体相对较深,有扇三角洲及半深湖—深湖相沉积 ;岩屑砂岩发育,岩屑成分以火山岩为主。断陷晚期凹陷水体变浅,SⅡ2 层序中下部低位及湖盆扩张体系域总体发育特征类似SⅡ1层序,上部发育的湖盆收缩体系域,演化为扇三角洲、河道、滨浅湖及沼泽相 ;SⅡ2 层序砂岩以长石岩屑砂岩为主,岩屑成分除火山岩外,见有混合花岗岩及变质岩岩屑。坳陷期 (SSⅢ )超层序发育冲积平原相,砂岩中混合花岗岩及变质岩岩屑成分进一步增加。由于受区域构造环境控制,凹陷沉积沉降中心不断北移,表明松辽盆地南缘燕辽造山带在白垩纪不断隆升、向北扩展 ;凹陷中发育三期低位体系域、对应了三期断陷活动,指示造山带在造山后作用阶段经历了三期幕式快速隆升过程。
Abstract:
Changtu sag is Iocated in the northeast of Yan-Liao orogen as an important part of SongIiao Basin. It is fiIIed with voIcanic and sedimentary rocks, which can be grouped into three superseguences：SS!（corresponding to the rifting stage）, SS "（fauIt-rifting stage）and SS#（depressionaI stage）. During the earIy rifting, the sag was fiIIed with a huge of overfIowing and expIoding voIcanics. In the earIier fauIt-rifting stage, there deveIoped the Iowstand systems tract in Iower Jiufotang Formation that totaIIy refIects sedimentary characteristics of a thinner mudstone fIow and the expansion systems tract at the top of Jiufotang Formation that corresponded to a deep-water environment, incIuding fan-deIta and haIf-deep-Iake and deep- Iake. During the Iater fauIt-rifting stage, the depth of water became shaIIower with the predominant sedimentary facies of aIIuviaI apron, fan-deIta, swamp and offshore and shaIIow Iake and so on. The Jiufotang Formation was mainIy composed of muIti-compositionaI congIomerates and sandstones, of which the detritaI composition was dominated by voIcanic cIastics, suggesting their derivation from the earIier erupted voIcanic seguences which was rapidIy unroofed during the upIifting of the orogen. The Shahai Formation was mainIy composed of congIomerates, sandstones, interbeddings between sandstones and mudstone. DetritaI anaIysis shows that at the upper Shahai Fm., there occurred some proportion of metamorphic and granitic components, indicating the increase in the amount of upIifting and unroofing of the entire orogen whiIst the Iowering of upIifting rate. As the sedimentation continued, the centre of sediment and depression moved northwards, refIecting that Yan-Iiao orogen was continuousIy upIifted and separated during Iate Cretaceous. The emergence of the three Iowstand system tracts and three rifting activities demanded that the orogen had undergone at Ieast three rapid upIifting episodes during Cretaceous.
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