台北凹陷梧桐沟组沉积期古地貌及其对沉积作用的控制

长期以来,对吐哈盆地台北凹陷二叠系梧桐沟组沉积体系的认识存在分歧,其中古地貌未能准确确定是关键问题所在。基于印模法和高分辨率层序地层学,确定了基准面旋回的转换界面,利用残余地层厚度和古水深校正恢复了台北凹陷二叠纪梧桐沟组沉积期古地貌。研究表明,梧桐沟组一段沉积期顶部发育的深灰色泥岩可以作为标志性的最大湖泛面和古地貌恢复的上标志层;梧桐沟组一段沉积期古地貌具有低凸—沟槽相间的特征,可进一步划分为两大凹槽和多个次级沟槽发育区,梧桐沟组二段和梧桐沟组三段沉积充填后,古地貌变得较缓;继承性的古凸起和同沉积断层共同控制扇体的发育,隆中沟控制古水流方向及砂体展布,台北凹陷南部的同沉积断层七深断层和火深断层加剧了隆起与洼陷的坡折程度,构造脊限制了扇体向前推进的距离;低凸—沟槽相间控制扇体的展布;砂体主要沿着古沟槽向前推进,并充填沟槽。梧桐沟组沉积期,研究区总体上呈由早期坡陡水深断陷转向晚期坡缓水浅拗陷的古地貌和沉积演化特征。

Paleo-Geomorphology and Its Controls on Sedimentation of the Permian Wutonggou

For a long time, researchers have divergent views on the sedimentary system of the Permian Wutonggou formation in the Taibei sag, Tuha basin, and the key issue lies in the failure to accurately determine the paleo-geomorphology. In this paper, based on the moldic method and high-resolution sequence stratigraphy, the conversion interfaces of the base level cycle are determined, and the residual stratum thickness and paleo-water depth correction under the interfaces is used to recover the paleo-geomorphology of the Permian Wutonggou formation in the Taibei sag. The study results show that the dark gray mudstones developed at the first member of the Wutonggou formation can serve as the maximum lake flooding surface and the upper marker layer for recovering paleo-geomorphology; the Permian paleo-geomorphology was characterized by “low convex interbedded with groove” during the deposit period of the first member of the Wutonggou formation, which can be further divided into two major grooves and several secondary grooves, while paleo-geomorphology became more gentle after the sediment filling of the second and third members of the Wutonggou formation. Inherited paleo-uplifts and the syn-depositional faults controlled the development of fan deposits. The grooves in the paleo-uplifts are the main factors controlling the direction of paleocurrent and sand body distribution. Huoshen and Qishen syn-depositional faults in the south of the Taibei sag intensify the slope break level between uplift and sag, and the tectonic ridges limit the forward distance of the fan bodies. The “low convex interbedded with groove” control the distribution of the fan bodies. Sand bodies mainly advanced along the paleo-grooves and filled the grooves. During the deposition of the Wutonggou formation, the paleo-geomorphology transformed from the early steep-slope and deep-water fault depression to the late gentle-slope and shallow-water depression.