狮子洞铀系测年与百色盆地第Ⅲ级阶地的年代

布兵盆地是广西百色盆地东南缘一小型附属盆地,狮子洞位于其南侧喀斯特峰林中,其内堆积由上部砂质粘土与下部砾石层构成,类似河流相二元结构。狮子洞与其周围分布广泛的河流第III阶地海拔高度相当,表明洞穴与阶地堆积时代的一致性。本文报道狮子洞内夹生于砂质粘土层次生碳酸盐岩样的铀系测年结果。位于砂质粘土与砾石层界面处的钙板,高精度质谱铀系年代为366±19ka。其左上1.4m贴附于洞壁突岩的石幔,质谱年代为517±42ka。鉴于具明确层位意义、纯净致密次生碳酸盐岩质谱铀系年代高度可信,并基于狮子洞堆积与布兵盆地第Ⅲ级阶地同时、附属盆地与主盆地同步发育的认识,这二个盆地第Ⅲ级阶地应在366～517ka间形成。本文结果对系统研究百色盆地地貌和旧石器文化演化具重要性。

U-Series Dating of the Shizi Cave and Age of the Third Terrace of the Bubing and Baise Basins,Guangxi,China

The Bubing Basin is a small basin subordinate to the larger one of Baise in Guangxi Zhuang Autonomous Region,southern China.The Shizi Cave is located among the karstic peaks which constitute the former basin’s southern border.A season of trial excavation in the cave in 2007 led to the discovery of a number of mammal fossils,including Cervus sp.,Sus sp.,Macaca sp.and now extinct Stegodon orientalis.The cavern’s depositional sequence can be divided into two layers: upper fossiliferous sandy-clay and lower weathered sandstone gravels,which resembles the dualistic structure of fluvial terraces.Such a phenomenon is rare in limestone caves in Guangxi.The sandy-clayey deposits in the Shizi Cave are intercalated with flowstone layers and other speleothem formations,from which four relatively pure and dense calcite samples were taken.Sample SZD-3,taken from a thin flowstone layer at the boundary of the two depositional layers,gives a high precision MC-ICP-MS U-series date of 366±19 ka,which coincides well with the dates determined by alpha spectrometry(332+199-69).Another sample ca.1.4 m higher,SZD-4,taken from calcite drapery adhered to a projecting rock on the cave wall,yielded a date to 517±42 ka,representing the maximum age of the deposits inside the cave.Being situated at the same elevation of 140~150 m above sea level,the deposits inside the Shizi Cave should be coeval with the third terrace of the Bubing Basin.Based on the highly reliable mass spectrometric U-series dating of pure and dense cave calcites with definite stratigraphic provenience,and on the recognition that the evolution of the Bubing Basin has been contemporaneous with the Baise Basin,the age of the third terrace of both basins may be within the range of 366~517 ka.This represents the first reliable radioisotopic age assignment to this terrace and marks an important step forward in systematic geomorphological and archaeological studies in this region.