宣化地区古老的潮汐沉积

大约在十八亿年前左右,在经过长期风化剥蚀后基本上处于准平原状态的华北地台上,沿着北东向的几条基底断裂,形成一个沿朝阳、蓟县、石家庄、长治一线展布的北东向长形断陷沉积盆地。初期,可能仅在局部地区(如蓟县附近)沉积河流相砂砾岩。

THE OLD TIDAL DEPOSITS IN THE XUANHUA AREA,HEBEI PROVINCE

About 1.8 b.y. ago, the marine transgression taking place in the Xuanhua area, Hebei Province transformed this area into a bay opening toward east and extending EW. The marine terrigenous clastic sediments with a thickness of approximately 200 m were deposited there. The various types of primary sedimentary structures marking clastic tidal environment are well developed in these sediments, such as herringbone cross-bedding, lenticular bedding, wavy bedding, flaser bedding, beach eross-bedding, tidal channel and tidal channel fill of cross-bedding, reactivation surfaces, interference ripple marks, mud cracks, and penecontemporaneous deformation structure. This paper makes a brief discussion on the origin and distribution of these sedimentary structures.On the basis of lithological characters and succession, the primary sedimentary structures and their sequences as well as the grain size distribution of sediments, the late Precambrian tidal depositional environment in this area can be subdivided as follows: tidal flat (high-tidal flat, mid-tidal flat and low-tidal flat), sandy beach, algal flat and oolitic bank, and shallow sea. Moreover, the main sedimentary features of these sub-environments are described and two types of depositional models for the area studied are proposed. Finally, paleocurrent regime is also briefly investigated.