Abstract:

With regard to the position of the lower boundary of the Quaternaryin the Fenwei Basin,there have been two different opinions:one advocatesthat this lower boundary should be located at the bottom of the Sanmen for-mation,while the other proposes that it should be placed lower,i.e.atthe bottom of the Zhangjiapo formation.Recent determinations of the paleomagnetic polarity epochs for drill holes CH₃ and CH₃₄ indicate that the upper part of the Qinchuan group corres-ponds to the Brunhes normal epoch,and the lower part to the Matuyama reversal epoch,whereas the Sanmen formation corresponds to the Gaussnormal epoch.A comparison with A.Cox's Standard Polarity Time Scaleshows that the ages of the lower boundaries of the Qinchuan group andthe Sanmen formation are 2.34 m.y.and 3.32 m.y.respectively.If the Quaternary lower boundary is delimited at the bottom of the Sanmen for-mation,its age is exactly between 3.00-3.50 m.y.,which is consistent with the latest view concerning the age of the lower limit of the Quaternary.Integrating rock assemblages,paleontological and paleo-climatological cha-racteristics and tectonic movements,the author holds that it is currentlymore reasonable to delimit the lower boundary of the Quaternary at thebottom of the Sanmen formation.