兰州盆地永登剖面晚古新世—早渐新世沉积物中粘土矿物的特征及其古气候指示意义

采用X射线衍射、扫描电镜分析方法,对兰州盆地永登剖面晚古新世—早渐新世沉积物中粘土矿物的相对含量、伊利石结晶度、显微形貌及其古气候意义进行深入研究。结果表明:晚古新世—早渐新世沉积物中粘土矿物主要为伊利石、蒙脱石、绿泥石和少量的坡缕石,其中伊利石的含量最高,而且从晚古新世到早渐新世,沉积物中伊利石的含量呈明显升高的趋势,蒙脱石则呈明显减少的趋势;在大约55 Ma处伊利石含量急剧减小,而蒙脱石含量急剧增大。此外,可见到毛发状的自生坡缕石沿蒙脱石晶片边缘交代生长,而碎屑成因的坡缕石则主要呈破碎的短棒状。粘土矿物分布特征表明,兰州盆地晚古新世—早渐新世的古气候以干旱为主导,并且经历了由相对温暖湿润到半干旱半湿润或干旱炎热的转变,这种气候转型主要受全球气候的影响,即受行星风系的控制;而在大约55 Ma处伊利石、蒙脱石的含量变化则记录了全球性的PETM事件。

Clay Mineralogy and Its Palaeoclimatic Indicator of the Late Paleocene to Early Oligocene Sediments in Yongdeng, Lanzhou Basin

Clay mineralogy of the Late Paleocene to Early Oligocene sediments in Yongdeng,Lanzhou and their palaeoclimatic interpretations were investigated using X-ray diffraction(XRD) and scanning electron microscopy(SEM).Our results showed that clay mineral composition of the sediments was dominantly illite,smectite,and chlorite,with minor palygorskite.From the Late Paleocene to Early Oligocene sediments illite content showed an upward increase trend,while the smectite content exhibited an opposite trend to that of illite.There were a sharp decrease in illite content and a rapid increase in smectite content at ~55 Ma in the curves of clay mineral composition.SEM observation showed that smectite flakes were replaced by authigenic palygorskite along the particle edges.Palygorskite derived from smectite usually displayed a hair-like shape,while the detrital palygorskite exhibited a short club outline.Clay mineralogy of the sediments indicated that arid/semiarid climate prevailed over the Late Paleocene to Early Oligocene period,but it changed from relatively humid to arid/semiarid conditions in Lanzhou basin,which was controlled by the global climate system,i.e.,the planetary wind system,and the sharp change in illite and smectite contents at ~55 Ma indicated the global PETM event recorded in the area.