西秦岭南部白龙江隆起中三叠统光盖山组岩相及盆地分析意义

Lithofacies of the Upper Middle Triassic Guanggaishan Formation in Both Sides of the Bailongjiang Uplift,Southwestern Qinling Orogenic Belt:Implications to the Basin Analysis

对西秦岭南部白龙江隆起南北两侧的中三叠统上部光盖山组岩相进行了研究,按照沉积作用方式识别划分出碎屑流、颗粒流、浊流、牵引流、平坦床砂静水5种沉积作用及岩相类型,并区分各种亚相类型.位于隆起北侧迭部飞仙剖面的岩相可识别出9个相序旋回,单个相序基本结构为砾质碎屑流相-鲍玛序列组合相,反映构造和沉积作用的旋回性和阶段性.总结发现,隆起南北两侧岩相的共同点是斜坡碎屑流和浊流型重力流较为发育,滑塌型重力作用少见,物源区较近且以灰岩为主;差别是不同剖面岩相结构、相序旋回有所不同,被侵蚀的物源地层时代不一样.进一步研究表明,光盖山组属于较缓的边坡沉积,是大陆碰撞阶段残留海盆大陆一侧的边缘相,盆地的形成与构造挠曲作用产生的沉降有关.鉴于洮河盆地晚古生代和早-中三叠世存在深海和残留洋盆,若尔盖盆地与之共用一个基底,因此,推测这2个盆地的中-上三叠统复理石之下可能没有台地相上古生界和中-下三叠统,不利于在该区开展油气勘探.

The lithofacies of the upper Middle Triassic Guanggaishan Formation outcropping along both northern and southern sides of the Bailongjiang uplift （composite anticline）, southwestern Qinling orogenic belt were studied in this article. By sedimentation and lithofacies, the Guanggaishan Formation is identified and classified into five types： debris flow, grain flow, tur bidity current, traction current, and still-water bedding current, which can be subdivided into more sublithofacies in this paper. Nine cycles of lithofacies sequence have been recognized from the Feixian section, Diebu in northern side of the Bailongjiang up lift, of whicl4 a single cycle is composed of gavel debris facies to Bouma sequence, indicating cyclicity and periodicity of tecton- ism and sedimentation. It is proposed that there are some similarities and discrepancies for lithofacies in two sides of the Bailongjiang uplift. The similarities are well developed slope debris and turbidity flow facies with little slumping and collapsed facies, and proximal sources comprising limestone. The discrepancies are different texture and cycle number of facies sequence from different sections, different aged strata in source regions. The assemblage of lithofacies indicates that the GuanggaishanFormation is of relatively ramp continental slope facies within remnant sea basin, where the basin had been formed by flexing during continent collision. According to deep sea basins and remnant basin in the Late Paleozoic Early Middle Triassic for the Taohe basin, north to the Bailong）iang uplift, and to the sharing of the same basin basement for the Ruo＇ergai flysch basin, south to the uplift, it is inferred that there could be no shallow platform carbonate facies under the both flysch basins, which means poor prospective for oil gas resources exploration in the region.