冀西北尚义盆地对晚侏罗世构造活动的沉积响应

Sedimentary response of the Shangyi basin,northwestern Hebei,to the Late Jurassic tectonism

尚义盆地位于走向近东西的中生代燕山冲断带前缘盆地群的西段,在晚侏罗世土城子时期堆积了大量的粗碎屑。盆地中土城子组沉积相变明显,自北向南从粗砾质变为粉砂质沉积,具比较典型的非对称相带分布特征。土城子早一中期,从北向南总体上形成砾质冲积扇(包括泥石流)-辫状河流-洪泛平原或干化湖泊等古环境布局。晚期则以砂砾质辫状河流沉积为主,可能夹有发育大型风成交错层理的砂质沙丘沉积。土城子组碎屑成分以变质岩和花岗质岩石为主.物源主要来自盆地北侧的"内蒙地轴",沉积充填具有同构造砾岩的性质。尚义盆地北缘发育一系列呈叠瓦状排列的逆冲断裂。断裂与盆地耦合关系的初步分析显示.北缘逆冲断裂的构造载荷是控制尚义盆地形成和演化的主要因素。靠近北部冲断带一侧沉积相序的中—下部具有向上变粗的特点.可能指示了土城子组沉积受前进式的冲断载荷和岩石圈挠曲控制的过程。根据尚义盆地与其北侧相关断裂的空间配置、沉积碎屑北粗南细的变化、盆地横断面北厚南薄的楔状体特征等分析.盆地属于陆内的前陆式盆地,推测是在自北向南的近水平挤压构造背景下形成的.

The Shangyi basin is located in the west segment of the frontal basin group of the Mesozoic nearly E- W-striking Yanshan thrust belt, where voluminous coarse fragments were accumulated during the deposition of the Late Jurassic Tuchengzi Formation. The Tuchengzi Formation in the basin shows a pronounced lateral sedimentary facies change from coarse gravelly deposits in the north to silty deposits in the south, displaying typical characteristics of asymmetric facies belt distribution. In the early-middle stages of deposition of the Tuchengzi Formation, there generally occurred conglomeratic alluvial fans （including mudfiow deposits）, braided streams and flood-plains and/or dried lakes from north to south. In the late stage of deposition of the Tuchengzi Formation, sandy and gravelly braided-river deposits were predominant, possibly intercalated with sandy dune deposits with large-scale eolian cross-bedding. Metamorphic rocks and granitoid rocks are dominant clastic components of conglomerate, indicting that the provenance was mainly the ＂Inner Mongolian axis＂ to the north of the basin. The sedimentary fillings have the nature of syntectonic conglomerate. A series of imbricate thrusts are developed at the northern margin of the basin. Preliminary analysis of the coupling relation between the faults and basin shows that the structural load of the thrust on the northern margin was an important factor for controlling the formation and evolution of the basin. The middle and lower parts of the sedimentary facies series near the side of the thrust to the north have the coarsening-upward characteristics, probably implying that the process of deposition of the Tuchengzi Formation was controlled by progressive thrust loading and lithosphere flexuring. Based on the relations between the basin and its related faults to the north, south-fining clastic size change and south-thinning, wedge-shaped section of the sedimentary basin, it is inferred that the basin formed in the continental interior in a N-S subhorizontal compressional setting.