华北北部侏罗纪大型推覆构造带前缘盆地沉积特征和成因机制初探

A PRELIMINARY RESEARCH ON SEDIMENTARY FEATURES AND GENETIC MACHANISM OF FRONTAL BASINS BEFORE JURASSIC LARGE-SCALE NAPPE IN THE NORTHERN REGION OF NORTH CHINAc

华北北部的晚侏罗世沉积盆地群产生于自北向南的挤压构造背景下，为燕山—阴山大型推覆构造带的前缘拗陷盆地。盆地中以充填巨厚的红色粗碎屑堆积为特征，沉积环境主要包括网状河流、冲积扇、干化湖泊及沙漠。碎屑沉积物中普遍含有大量的非稳定组分，是在高沉积速率下形成的近源堆积产物，与这一时期曾存在强烈的构造隆升活动有关。初步分析表明，一些地段的逆冲系具有向南推进的特点，并形成背驮式盆地。

The northern region of North China is a birthplace of the famous 'Yanshanian Orogeny'. In the early Mesozoic(T 3-J 3),intense structure-magma actions occ urred and a series of volcano-sedimentary basins were formed. The late Jurassic sedi m entary strata ( the Tuchengzi Formation and Daqingshan Formation) distribute fro m the west of Liaoning province, cross the Yanshan region, to the Yinshan region in the southern of Inner Mongolia, constituting a nearly E-W basin group abou t 12 00km in length and 50 80km in width. The purpose of this paper is to determine the structural nature and relative tectonic event of the late Jurassic basins based on sedimentation responsed to regional tectonism. The preliminary r esults of the research are as follows: 1. Braidrivers,dry lakes,alluvial fans and desert environments were developt ed in Late Jurassic in the northern region of North China. Thick coarse cla s tic sediments(1000-4500m), contianing a nember of unstable clastic components, are products of proximal deposition related to nappe structure belt. Their accum ulation with high sedimentary rate (>133-437m/Ma) reflected that an intense tect onic uplift occured in this time and the main erosional area formed on the north parts of the basins. 2. The characteristcs of the asymmetric fillment, E-W distributions of sedi m entary facies and palaeocurrentwater, and coarsening-upward sequences indicate t hat the Late Jurassic deposition was controlled by synsedimentary thrusting on the north sides of the basins. The late Jurassic basins were generated under th e compressitve condition from the north to the south, belonging to the frontal d epressional basins of the large-scale Yanshan-Yinshin nappe structure belt. 3. According to the analysises of spatail relations among the basins, and sedime ntary marks , synsedimentary thrusting and others, the author preliminaryly infers that some of the Late Jurassic basins of this region are piggyback-basins in which thrust system moved forward south.