关中盆地新生界地层划分对比与第四纪下限

Cenozoic stratigraphic correlation and the lower limit of Quaternary in Guanzhong Basin

关中盆地巨厚的新生代沉积为研究区域构造、沉积环境演变与古气候变化研究提供了良好的地质记录,但目前对新生界地层划分和第四纪下限问题仍存在争论。基于钻孔和剖面资料,结合古地磁和生物地层,对关中盆地新生界地层进行了梳理和修订,就第四纪下限和新生界地层划分与对比问题进行了探讨。岩性地层、磁性地层、生物地层证据以及气候变化特征等都支持将第四纪下限定在传统三门组的黄三门与绿三门的交界处或风成黄土与红黏土的界线之处,即松山负极性向高斯正极性转换处(M/G),古地磁年龄为2.6 Ma。基于关中盆地钻孔和剖面的古地磁数据及前人资料,划分了新生界地层,重新厘定了各沉积地层的沉积年代,将新近纪和第四纪沉积分为风成相和河湖相两套同期异相沉积地层序列,风成红黏土的年代延伸到中中新世,建议将中新世的风成堆积命名为段家坡组。这对关中盆地沉积环境形成演化与古气候变化研究、盆地资源开发以及地质工程建设具有重要意义。

Thick Cenozoic sediments in Guanzhong Basin provide excellent geological records for the study of the regional tectonics,sediment environment evolution and paleoclimate change.However,there are still controversies about the Cenozoic stratigraphic division and the lower boundary of Quaternary in Guanzhou Basin.Based on paleomagnetism and paleontology strata from drilling cores and outcrop sections,we reappraise and reestablish the Cenozoic strata framework in Guanzhong Basin.According to fluvial-lacustrine deposits and eolian red clay-loess sequences,the authors discuss the lower limit of Quaternary in Guanzhong Basin.Evidence from lithology,magnetostratigraphy,biostratigraphy and climate change supporting the lower limit of Quaternary strata should be confined to the boundary of“Yellow Sanmen”and“Green Sanmen”in the traditional Sanmen Formation or the boundary of eolian loess and red clay sediments around M/G boundary,and the paleomagnetic age is 2.6 Ma.Based on palaeomagnetic data of drilling cores,outcrops and previous literature,we revise the Cenozoic chronostratigraphic chart in Guanzhong Basin and amend or reallocate the ages of different strata from Eocene to Holocene.We propose that Neogene and Quaternary deposits in the Guanzhong Basin can be generally classified into two sets of different genesis sediments e.g.eolian and fluvial-lacustrine sequences.The eolian deposits can be extended to the Miocene period,which is older than previous conventional knowledge,and this Miocene eolian red clay sequences can be named as Duangjiapo Formation.This study is helpful to understand the formation and evolution of the sedimentary environment,paleoclimatic change,basin resource exploitation and geological engineering construction in Guanzhong Basin.