长江三角洲QDQ2钻孔晚更新世以来沉积物粒度特征及其古环境意义

Grain sizes characteristics of sediments from QDQ2 borehole in the Yangtze River Delta since the Late Pleistocene and their paleoenvironmental significance

文章以江苏省南通市启东地区QDQ2钻孔沉积物为研究对象,通过沉积物的粒度分析,探讨晚更新世以来长江三角洲的沉积演化过程及其对古环境的指示。QDQ2钻孔沉积物主要由粉砂和黏土组成,总体粒径偏细。用加速器质谱(AMS)14C方法测得埋深约117 m、86 m和40 m处的沉积物年龄分别为40057 cal yr B.P.、36105 cal yr B.P.和10112 cal yr B.P.。结合长江三角洲地区其他钻孔的岩性、粒度特征以及测年数据,将QDQ2钻孔晚更新世以来的地层自下而上划分为上更新统(128.2~50.56 m)和全新统(50.56~0 m)。上更新统按沉积物粒度特征进一步划分为3段:下段(128.2~108.85 m)以粉砂为主,分选性良好,粒度频率曲线多为单峰式;中段(108.85~73.45 m)以粗砂为主,粒度频率曲线整体为双峰式或多峰式;上段(73.45~50.56 m)以粉砂为主,呈现以单峰式或双峰式为主的频率曲线。全新统(50.56~0 m)以粉砂为主,砂级含量较少。基于沉积特征分析,认为该地区晚更新世以来至少发生过两次明显的海平面波动:晚更新世中期(108.85~73.45 m)的第一次海退,随后快速海侵;晚更新世晚期(末次冰盛期)(52.56~50.56 m)发生区域海退至最低海平面,全新世的海侵至中期达最高海平面。在近现代滩涂围垦等人类活动的影响下,研究区逐渐发展至现今的地貌。该研究为恢复长江三角洲地区晚更新世以来的沉积环境演化提供了重要信息。

This study analyzes the samples from QDQ2 borehole of the Qidong area,Jiangsu Province,to in-vestigate the sedimentary evolution of the Yangtze River Delta since the Late Pleistocene and its implications for the paleoenvironment.Grain size analysis was performed on the borehole sediments to reconstruct their evolution-ary characteristics.The results indicate that the sediments predominantly consist of silt and clay,with fine-grained size and poor sorting.AMS14C dating suggests that the sediment at the depth of 117 m,86 m and 40 m is approxi-mately 40057 cal yr B.P.,36105 cal yr B.P.and 10112 cal yr B.P.years old,respectively.Based on comparisons with the lithology,grain size characteristics,and dating results of other boreholes in the region,the strata of QDQ2 borehole include,from bottom to top,Upper Pleistocene strata(128.2~50.56 m)and Holocene strata(50.56~0 m).The lower part of the Upper Pleistocene(128.2~108.85 m)is primarily composed of well-sorted silt,with a unimodal grain-size frequency curve.The middle part(108.85~73.45 m)contains medium to coarse sand,ex-hibiting predominantly bimodal or multimodal frequency curves.The upper part(73.45~50.56 m)is dominated by silt,showing unimodal or bimodal frequency distributions,while the Holocene strata(50.56~0 m)consist mainly of silt with low sand content.The study identifies at least two significant fluctuations of regional sea-level.The first regression occurred during the early Late Pleistocene(108.85~73.45 m),followed by a rapid transgres-sion(75~50.56 m).A subsequent regional regression took place at the latest of Late Pleistocene(LGM)(52.56~50.56 m),leading to the present geomorphic pattern.These findings provide valuable insights into the sedimentary evolution of the Yangtze River Delta since the Late Pleistocene.