华南地区珠江三角洲的形成和演变历史是国内外高度关注的重要科学问题,其中,全新世与更新世界线附近"花斑粘土"形成的沉积环境及气候条件是最有兴趣的问题之一.本研究利用珠江三角洲陆地西北部的广东省佛山市西樵山XQS-ZK7钻孔岩芯(全长16 m),对花斑粘土及其邻近6.1 m特定层位岩芯进行了 AMS 14C和OSL测年,同时进行了系统的环境磁学参数测量和粒度分析,并对代表性样品进行了岩石磁学参数测量.其中磁学参数主要包括xlf、Xfd％、S-300、HIRM、IRM、κ-T曲线等.将610 cm岩芯自下而上划分为4个层位,第一层年龄约67～60 kaB.P.,第二层年龄约60～59 ka B.P.,第三层(花斑粘土层)年龄约59～49 ka B.P.,这3层分别相当于深海氧同位素MIS 4后半期、MIS 4/MIS 3 过渡期和MIS 3早期的沉积层;第四层年龄小于11 ka B.P.,对应于MIS 1早期.缺失MIS 3中后期至MIS 2时期的沉积物.研究结果显示:本地花斑粘土与其下伏沉积物具有一种自下而上从粗到细的"正粒序层理"沉积构造,花斑粘土层中夹有3层灰色砂、粉砂、粘土并含有贝壳碎片,其原始沉积为水成成因类型;从花斑粘土到下伏地层,沉积物颜色由红黄杂色→灰白色→青灰色逐渐变化,与经典的风化作用程度自上而下逐渐减弱的规律一致;花斑粘土层下伏第一层灰色沉积层中的磁性矿物以磁铁矿为主并含部分碎屑赤铁矿,而花斑粘土层则含有较多的成土作用新生成的赤铁矿和针铁矿以及超细磁铁矿;花斑粘土中的亚铁磁性超顺磁(SP)颗粒矿物与高矫顽力不完全反铁磁性矿物同时形成,即原始水成沉积在后期高温高湿环境下经历了强烈的化学风化作用而产生了磁性矿物的转变.据此认为:由于华南地区湿热气候导致的强烈风化成土作用,本地花斑粘土经历了多期水下沉积→暴露地表→差异风化的气候-沉积环境变化过程,其最终形成是在M1S3升温阶段(尤其是中后期)遭受了强烈风化作用的基础上叠加了 MIS 2末次盛冰期海平面大幅度下降的暴露风化作用.这一认识说明珠江三角洲花斑粘土的形成时代与演化过程尚需做进一步深入研究.
A CASE STUDY OF MAGNETIC PROPERTIES AND SEDIMENTARY ENVIRONMENT OF THE SPOTTED CLAY IN THE PEARL RIVER DELTA
The formation and evolution of the Pearl River Delta in South China is an important scientific issue of great concern at both home and abroad,among which the sedimentary environment and climatic conditions of"the spotted clay"developed near the boundary of Pleistocene and Holocene is one of the most interesting issues.The Xiqiaoshan area of Foshan City,Guangdong Province is located in the northwest land part of the Pear River Delta.The XQS-ZK7 borehole investigated in this study is located in the delta plain area in the south of the Xiqiaoshan,about 100 km away from the coast of the South China Sea.Its geographical coordinates are 22°54′25.2″N,112° 58′52.1″E,with an altitude of 2 m and core length of 16 m.In this study,AMS 14C and OSL dating have been performed for specific horizons and representative samples of the 610 cm long core of the spotted clay and its adjacent horizons(the borehole depth is 188～798 cm).The grain size of 142 groups of samples was analyzed.The environmental magnetic parameters of 321 groups of samples were tested systematically.The rock magnetic parameters of 19 representative samples were measured.The magnetic parameter mainly include x1f,xfd％,S-300,HIRM,IRM,κ-T curve,etc.The XQS-ZK7 borehole is divided into four layers from bottom to top:the first layer,the interbedding of gray-black clay silt and red-gray fine sand,age about 67～60 ka B.P.;The second layer,gray silty clay layer,age about 60～59ka B.P.;The third layer,spotted clay with some thin layers of black gray sand and silt,age about 59～49 ka B.P.;The fourth layer,dark black-yellowish-brown clay,is less than 11 ka B.P.The geological age from the first to the third strata is late Pleistocene,and the fourth strata is Holocene.Compared with the marine oxygen isotope stages,the sedimentary ages of each layer are equivalent to the latter half of MIS4,the MIS4/MIS 3 transition period,the early MIS 3 and the early MIS 1,respectively.The sediments from the mid-late MIS 3 to MIS 2 are missing.From the first layer to the third layer,the average median grain size of the sediments is 12.51 μm,10.36 μm and 5.16 pm,respectively,indicating a"positive graded bedding"sedimentary structure from coarse to fine.The weathering degree of the spotted clay layer is uneven,with thin layers of gray-black gray sand,silt and clay in the upper,middle and lower parts,which contain carbonized plant debris and shell debris,indicating that the original deposition of the spotted clay and its underlying layer should be of hydrogenetic sediments.Meanwhile,the sediment color gradually changes from variegated color of red-yellow to gray-white,which is consistent with the law that the weathering degree of the original sediment gradually weakens from top to bottom until it becomes unweathered original rocks.The absence of sediments from the mid-late MIS 3 to MIS 2 is also evidence of sedimentary discontinuity,exposure to the surface and weathering.Therefore,the spotted clay should have undergone a multi-stage sedimentation-weathering process.The magnetic characteristics and magnetic mineral components of the borehole sediments are as follows:The low frequency magnetic susceptibility(xlf)changes from high to low,the magnetic frequency dependant susceptibility(xfd％)is low,the S-300 ratio varies from high to low,the hard magnetic remanent(HIRM)is low,and the prominent magnetic minerals of the first layer of gray sediment under the spotted clay are magnetite(average contribution 68％),partial detrital hematite(average contribution 30％)and a small amount of ultra-fine magnetite(average contribution 2％).The low frequency magnetic susceptibility(xlf)of the second layer and the third layer of the spotted clay layer varied from low to high(up to 29.9×10-8 m3/kg),the percentage of frequency-dependent magnetic susceptibility(xfd％)increased from low to sudden rise(up to 28.5％),and the S-300 ratio gradually decreased(the lowest value reached 0.25～0.31).The hard magnetic remanent(HIRM)is higher(up to 987.56× 10-6 Am2/kg),the average contribution of magnetite decreased(36％～34％),the average contribution of ultra-fine magnetite content increased(4％～6％),and the average contribution of detrital hematite content slightly decreased(42％～26％),which is characterized by the emergence of more new pedogenic hematite and goethite.The combined average contribution of both can reach 34％.The general characteristics show that the ferromagnetic SP mineral grains and the high coercivity incomplete antiferromagnetic minerals are formed at the same time in the spotted clay,indicating that the original hydrogenic deposits underwent strong chemical weathering in the subsequent environment of high temperature and high humidity,and produced the transformation of magnetic minerals.The original deposition age of the local spotted clay is 59～49 ka B.P.in Late Pleistocene,corresponding to the early warming stage of the marine oxygen isotope MIS 3 stage,while the top deposit is 49 ka B.P.After that,no deposition occurred at the bottom of the Holocene,which corresponds to the middle of MIS3(dry and cool climate),late MIS 3(hot and humid climate)and MIS 2(dry and cold climate).Therefore,it is concluded that the final formation of the local spotted clay layer is based on the strong weathering in the warming stage of MIS3(especially in the middle and late stages),and the exposed weathering of the sea level decreased greatly in the Last Glacial Maximum of MIS 2,that is,multi-staged weathering superposed by climates of warming and wetting,cooling and drying,warming and wetting,and cooling and drying.Therefore,the climate-sedimentary environment of spotted clay should be multi-stage process of"underwater deposition,then exposed surface,and then differential weathering".This understanding shows that the formation age and evolution process of the spotted clay in the Pearl River Delta need further research.
magnetic propertiessedimentary environmentclimatic changespotted clayPearl River Delta
万方数据
维普
