“鄂西地区一个含早期人类和巨猿化石的新地层单位——高坪组”的商榷

COMMENTS ON “THE GAOPING FORMATION——A NEW STRATIGRAPHIC UNIT WITH THE GIGANTOPITHECUS FOSSILS FROM WEST HUBEI”

程捷等人(2004)报道了鄂西建始地区高坪龙骨洞的发掘和研究情况,并试图建立一个新的岩石地层单位——高坪组,建组的主要依据是将龙骨洞A剖面获得的地磁极性柱与标准磁性年表的松山反极性世的早期进行对比,即龙骨洞极性柱上部和中部的正极性事件分别与奥都威和留尼旺极性亚时对比。根据这种对比方案,可以计算出龙骨洞A剖面的平均沉积速率,结果发现该剖面的沉积速率变化很大,特别是留尼旺亚时的沉积速率远远大于相邻时间段的沉积速率,这与龙骨洞的沉积环境稳定以及相对应的地层岩性一致等相矛盾,因此,这一极性对比方案值得商榷,高坪组的年代存在不确定性。如果将龙骨洞A剖面的两个正极性事件与布容期和贾拉米诺极性亚时相对比,可以获得较均匀的沉积速率,由此可推算出的龙骨洞古人类年龄大约在1.1—1.4Ma之间,其时代与中国其他大部分地区的古人类年龄也比较相近。

Cheng Jie et al. investigated the Longgudong Cave from Gaoping at Jianshi County, western Hubei, and created a new lithostratigraphic name--the Gaoping Formation. They recognized four magnetozones from the section A of Longgudong Cave and correlated the two normal magnetozones with the Olduvai subchron and the Reunion subchron. Based on their magnetostratigraphic correlation, we computed the average sedimentation rate of every magnetozone, and found that the calculated sedimentation rate varies rapidly in the whole section. In particular, the sedimentation rate in the Reunion subchron is much greater than that in the other magnetozones. However, this rapidly variable sedimentation rate is inconsistent with the steady depositional environment of Longgu Cave and the uniform lithology of the related strata. Cheng Jie and colleague＇s result should be reconsidered in light of this inconsistency, and we believe that the age of the Gaoping Formation remains uncertain. If the two normal magnetozones from the section A of Longgu cave are correlated with the Brunhes and Jaramillo subchrons respectively, then the sedimentation rate through the Gaoping Formation is relatively constant. If this correlation is correct, then the age of the Longgu Cave＇s hominins is between 1.1Ma and 1.4Ma, which is similar to the age of other hominin sites in China.