第四纪以来天津北部燕山隆升与山前盆地的形成*

文中报道了天津市北部燕山山前地带共4个高岩心采取率钻孔的沉积学、地层学、年代学和微体古生物综合研究。420个热退磁样品的磁倾角曲线显示,具147 m厚松散地层的标准孔Z3孔下部地层属于2.58 Ma的松山(Matuyama)极性带。随着地壳沉降,只是从早更新世,冲积扇侧缘泥质沉积物才开始充填了底砾岩层之上的沉积空间,形成广义的山前盆地中的泛滥平原;与传统上视为上新统特征沉积的红棕色黏土类似的亮黄橙-黄橙色-亮红棕色黏土,延续出现到早更新世中期,当时区域沉降中心位于距离山前断裂不足4 km处。0.78 Ma以来的布容(Brunhes)极性带厚57.8 m。谨慎地依据120 ka的布莱克(Blake)亚时确定上更新统后,可见中更新统大部缺失,与天津市区沧县隆起和北京永定河冲积平原南部所见此时的地层间断十分相近。3个钻孔的7个¹⁴C测年数据证实,距离山前断裂不足2 km的钻孔45 ka以来的地层最厚达41 m;显示自MIS3初始期,沉降中心向北迁移更靠近山前断裂,且在构造断裂作用下形成了新的狭义的山前盆地。研究区可见15 ka富有机质黏土及上覆泥炭,表明末次冰消期开始的气候转暖首先影响到华北平原山前地区。此项工作为研究晚新生代以来“源到汇”过程中渤海湾西岸的沉降提供了一个基准点。

The uplift of Yanshan Mountain and the formation of piedmont basin in northern Tianjin since the Quaternary

The present paper reported the integrated study on the sedimentology,stratigraphy,chronology,micropaleontology of four boreholes with high core recovery located on the piedmont zone of Yanshan Mountain in northern Tianjin. The inclination curve consisting of 420 hot demagnetizm samples from Borehole Z3 (unconsolidated deposits with 147 m thickness) indicated that the lower strata belong to the Matuyama polarity chron—dated at 2.58 Ma. Following the crustal subsidence,just from Early Pleistocene the mud sediments of alluvial fan fringe started packing the sediment space above the basal conglomerate layer in studied area,and fluvial plain formed in the generalized piedmont basin. Bright yellowish brown-yellow orange-bright reddish brown clay was continuously developed during middle Early Pleistocene. The regional subsidence center situated at the site from the piedmont fault less than 4 km at that time. The Brunhes polarity zone—dated 0.78 Ma is 57.8 m thick. On the basis of the fact that the bottom of Late Pleistocene had determined prudently based on the Blake subchron—dated 120 ka at Borehole Z3,it can be found that a larger part of Middle Pleistocene series absented. This feature is very like to the stratal hiatus found on the Cangxian Uplift in Tianjin area and in the south part of the Yongdinghe River fluvial plain in Beijing at the same time. A total of 7 ¹⁴C dating collecting from three boreholes confirmed that the thickest stratum since 45 ka has 40.7 m occurred at the borehole from the piedmont fault less than 2 km. It represents that under the tectonic fault effect since the initial period of MIS(Marine Isotope Stage)3,the subsidence center northward migrated to toe of the piedmont fault,a new narrow sense piedmont basin formed. In the study area, the earliest 15 ka organic-clay and overlaying peat occurred, suggesting that the warming since Last Deglacial stage had affected the piedmont area in the North China Plain firstly. The present study provided a datum mark for study on subsidence of west coastal plain of Bohai Bay in the source-to-sink processes since Late Cenozoic.