成都盆地晚新生代地层划分与对比中存在的问题探讨

Problems Existing in the Division and Comparison of Late Cenozoic Strata in the Chengdu Basin

成都盆地为龙门山推覆造山带与龙泉山褶断带之间的陆相盆地,晚新生代时期沉积了厚度较大的松散砂砾石层,过去对其地层划分与对比主要依赖于盆地周围的阶地剖面。笔者通过野外调查及室内综合研究,认为这些阶地剖面存在如下问题:(1)长度较短、地层连续性差,沉积物的颜色、成分、结构构造及剖面序列等相似;(2)缺少年代地层与生物地层资料;(3)缺少地层划分对比的标准剖面和标志层,进而导致依据阶地剖面建立的岩石地层单元时代归属存在不确定性,年代地层与岩石地层存在矛盾,生物地层还没有起到地层划分对比的支撑作用。建议在盆地坳陷区实施地质调查井,通过钻井的岩石地层组合及充填序列、年代数据的系统测试和孢粉组合的系统分析等,建立成都盆地地层划分的标准剖面和辅助对比剖面,从而完善该区晚新生代地层划分与对比,为成都市地下空间的三维地质结构建模提供依据。

The Chengdu basin is a continental basin located between Longmenshan nappe orogenic belt and Longquanshan fold fault belt,which has deposited a thick layer of loose gravel in Late Cenozoic.In the past,the stratigraphic division and comparison of Late Cenozoic in Chengdu basin was mainly depended on the terrace sections around it.Through field investigation and indoor comprehensive research,it is considered that there are some defects about these terrace sections:(1)short length,poor stratigraphic continuity,similar in color,composition,structure of sediments and section sequence;(2)lack of chronostratigraphic and biostratigraphic data;(3)lack of standard sections and marker layers for stratigraphic division and comparison.As a result,the age attribution of lithostratigraphic units established according to terrace sections is uncertain,chronostratigraphy and lithostratigraphy are irreconcilable,and biostratigraphy has not played a supporting role in stratigraphic division and comparison.It is suggested to implement geological survey wells in the basin depression area,and establish the standard profile and auxiliary correlation section for stratigraphic division of Chengdu basin through systematic test of rock stratigraphic assemblage,filling sequence and chronological data,and systematic analysis of sporopollen assemblage.This can promote the stratigraphic division and comparison of Late Cenozoic in this area,which provides a basis for the three-dimensional geological structure modeling of underground space in Chengdu City.