河北滦平东杨树沟剖面砂岩中的发散状层理及其成因探讨

Radiational bedding in sandstone and analysis of its origin in Dongyangshugou section of Luanping,Hebei Province

河北省滦平县下白垩统断陷盆地东杨树沟剖面,发育砂岩—泥岩互层序列,在砂岩急剧变厚的部位见到一种"发散状"层理。在这种层理中,纹层从砂体较薄的部位向砂体变厚的方向发散和倾斜,且纹层面与岩层面的夹角从上往下逐渐增大,最上部纹层面与岩层面的夹角接近0°,最下部纹层面与岩层面的夹角接近90°,由于岩层层面是近于水平的,也就是最下部的纹层的倾角接近90°,如此高角度的层理不可能是正常水流作用下沉积的。关于这种层理类型,文献中没有找到记录。前人对该剖面的研究成果中曾经把它当作交错层理,认为这个急剧变厚砂岩是河道沉积。根据这个成因分析,可以预测砂体的展布方向是垂直于岸线的。经仔细观察,发现这种"发散状"层理是小型同生正断层控制下的同生层理。该套砂岩不是河道砂,砂层局部变厚是由于同生正断层引起的,其沉积环境被认为是扇三角洲前缘的舌状体沉积,根据这种成因分析,可以预测砂体的展布方向垂直于断层走向,根据区域资料,断层是平行于湖岸线分布的。

A radiational bedding was found in the Lower Cretaceous mudstone and sandstone section located at Dongyangshugou village in Luanping County of Hebei Province, where the sandstone is suddenly thickened. In this kind of bedding, the laminas radiate and incline from the thin side toward the thick side of the strata, the angle between the lamina and the interface of the strata is increasing from top to bot- tom. At the top, the angle between the lamina and the interface of the strata is near 0°. At the bottom, the angle between the lamina and the interface of the strata is about 90°. Interface of the strata is nearly horizontal, that is to say, there is no record in reference about this kind of radiational bedding. In the earlier study result in this area, the incline angle of the lamina at the bottom is 90°, which can not beformed by normal fluid flow. This kind of bedding was considered was cross bedding, and the locally thicken sandstone with this kind of bedding was thought as river channel, so the distribution of the thick sand was considered as perpendicular to the lake shore. By careful observation and study, this kind of ra- diational bedding is a kind of syngenetic bedding controlled by the growth normal fault. The sandstone with this kind of bedding is not river channel sand, the phenomenon of locally sudden change in thickness is caused by growth fault. The sedimentary environment of the sandstone is lobe of delta front, according to this model, the distribution of the thick sand is considered as parallel to the growth fault and the lake shore.