滦平盆地大北沟组-大店子组沉积和地层格架及陆相层型意义

The Stratigraphic Framework of the Dabeigou-Dadianzi Formations in Luanping Basin, Northern Hebei and Its Stratotype Significance

依据大量实测剖面资料初步建立了滦平盆地侏罗 白垩系大北沟 大店子组地层格架 ,证明盆地在两组沉积期为西南高、东北低的簸箕形 ,两组地层从西南向东北在地层厚度、沉积旋回组成、相序与环境和标志层的分布等方面均发生规律性变化。地层格架不仅有助于研究盆地形成演化、构造应力条件和火山活动 ,而且可以确定盆地沉降中心 ,确定建立地层层型的候选剖面。位于盆地沉降中心的张家沟剖面大北沟组 大店子组以扇三角洲前缘半深湖 深湖相深水沉积为主 ,由 5个完整的沉积旋回 (SDC)组成。旋回沉积相序大北沟组由滨浅湖相 前扇三角洲半深湖、深湖相 扇三角洲前缘相构成 ;大店子组明显水体加深 ,沉积环境相对单一 ,地层厚度增大 ,旋回相序由半深湖、深湖相泥页岩、粉砂岩夹砂岩组成 ,部分层位为扇三角洲前缘的砂、砾岩层。两组沉积较厚 ,达 665m。并且排除了其它剖面上“蜂窝梁砾岩”(辫状河道 分流河道砾岩 )对下伏层侵蚀造成的大北沟组顶部沉积间断 ,以及“玄武安山岩”喷发活动造成的大店子组顶部地层缺失。张家沟剖面沉积连续、暴露完全、化石极为丰富 ,是建立侏罗 白垩系临界阶陆相层型和研究界线划分的理想剖面。

A stratigraphic framework of Late Jurassic Dabeigou Formation and Early Cretaceous Dadianzi Formation has been set up recently on the basis of data obtained from facies analogy, fossil fauna and mark-beds of many sections in Luanping basin, northern Hebei. The stratigraphic framework is very important in recognizing the depression center of the basin and determining the stratotype sections, in addition to studying basinal development, tectonic conditions and volcanic activities. In the depositional period of the two formations, the basin was faulted down like a dustpan with the deeper part in the southwest. And from southwest to northeast, these sections gradually change in stratigraphic thickness, number of sedimentary cycles and facies sequences. The Zhangjiagou section discovered recently is situated at the deepest center of the basin. The Dabeigou-Dadianzi Formations in the section show a successive deposition and is characterized by mudstone, shale and siltstone interbedded with sandstone and conglomerate of semi-deep lake facies and deep lake facies in front of fan-shaped deltas. They have a large stratigraphic thickness (665 m) and consist of 5 big depositional cycles. Most of the cycles are composed of transitional facies sequences, i.e. the beach-lake facies, the fan-shaped delta facies, the fore-fan-shaped delta facies and the semi-deep lake and deep lake facies. Moreover, the section avoids the stratigraphic absence caused by volcanic eruptions and sedimentary interruptions resulting from erosion by the 'Fengwoliang Conglomerate'. So the Zhangjiagou section is an ideal candidate section for erecting the terrestrial stratotype and studying the Jurassic-Cretaceous boundary in China.