古地貌对断陷盆地沉积体系的控制作用:以青藏高原伦坡拉盆地始新统牛堡组为例

Control of Tectonic-Paleogeomorphology on Deposition System of Faulting-Subsiding Basin: A Case from the Eocene Niubao Formation in Lunpola Basin,Central Tibet

青藏高原的形成是亚洲乃至世界历史上重大的地质事件之一,伦坡拉盆地作为西藏地区唯一发现含工业油流的新生代陆相盆地,研究其成因及演化对西藏地区的油气勘探具有重要的意义。利用二维地震和钻井等资料,阐明了始新世牛堡组沉积时期伦坡拉盆地古地貌特征,揭示了古地貌对于沉积体系的控制作用。牛堡组沉积时期,伦坡拉盆地构造单元可划分为3个:北部逆冲带,中央凹陷带以及南部冲断隆带。断陷盆地发育经历了湖盆形成、扩张和萎缩阶段,对应于牛堡组层序地层划分中的牛一段、牛二段及牛三段。在恢复伦坡拉盆地古地貌的基础上,指出盆地存在断控坡折型和挠曲坡折型两种控制盆地沉积体系分布的地貌,分别位于盆地的南部和东部。在不同地貌的控制下,盆地北部发育冲积扇和湖底扇组合沉积体系,盆地东缘、东南缘以及南缘则发育辫状河三角洲及扇三角洲组合。盆地中部发育浅湖-半深湖沉积,周缘发育滨湖亚相。盆地内的许多微地貌也对盆地内砂体起到了阻挡与疏导的作用,从而控制了众多水道的规模。

The formation of the Tibetan Plateau is a crucial geological process in the earth. The Lunpola Basin is of great geological significance as it is the only terrestrial basin discovered of Cenozoic in Tibet so far and hence, the study of its genesis and evolution is not only significant for oil and gas exploration in Tibet, but also provides insights for the research of the Tibetan Plateau. Based on the seismic date and well logs, this paper elaborates the tectonic and geomorphological characteristics of the Lunpola Basin within the accumulation of the Eocene Niubao Formation, and reveals the interrelation between tectonic geomorphology and the sedimentary system. During the deposition of the Niubao Formation, the Lunpola Basin can be divided into three tectonic-paleogeomorphological units: the northern thrust belt and the southern thrust belt. While the development of the rifted basin is accompanied by the formation, expansion and contraction of the lacustrine. Corresponding to the above three stages, the first member, the second member and the third member of the Niubao Formation can be subdivided in the stratigraphic framework. By restoring the geomorphology of the Lunpola Basin, we propose that two types of flexure slope break zone and faulted slope zone depositional models that control the distribution of the basin sedimentary features, which are located in the southern and eastern part of the basin respectively. Constrained by different landforms, a composite sedimentary system of alluvial fan and the sublacustrine fan are developed in the northern of the basin, the braided river and delta system are developed to the eastern and the southeast margin of the basin. In the central part of the basin, shallow lacustrine and semi-deep lacustrine facies is developed, and the peripheral areas developed shore lacustrine subfacies. There are many microtopographical structures in the basin which also play a role in inhibiting and dredging the sand body in the basin, thus controlling the size of submerging channels.