秦岭东部李官桥盆地玉皇顶组生物地层划分

ON BIOSTRATIGRAPHICAL SUBDIVISION OF YUHUANGDING FORMATION IN LIGUANQIAO BASIN OF EASTERN QINLING REGION

长期以来,李官桥盆地始新统玉皇顶组的时代一直被认为是属早始新世。笔者通过野外追索,使盆地中原来分散的化石地点有了层位归属,并于玉皇顶组上部发现了新的哺乳动物化石群。本文将玉皇顶组分为上、中、下三段,并根据各层位哺乳动物化石性质,提出玉皇顶组的时代范围应为早始新世至中始新世早期。

Liguanqiao Basin, located in eastern Qinling Mountain region, is a small intermontane Mesozoic-Cenozoic down-faulted basin. Deposits in the basin are predominantly clastic rocks, mudstone and marl formations of late Cretaceous-Tertiary fluvial and lacustrine facies. Among them the Eocene deposits are mostly developed, in which a large number of copious mammalian fossils was found. It provides a scientific basis for biostratigraphical subdivision of the deposits. The versionson the Eocene stratigraphical subdivision are still disagreeable with the imcreasing fossil-localities and fossil mammals, especially the age of lower part of the Eocene Yuhuangding Formation has not finally determined.The age of Yuhuangding Fm had long been supposed to be Early Eocene. Our field investigation in this area permits us to correct several scattered fossil-localities with some certain horizons. Meanwhile, a new mammalian assemblage was found at the top part of the formation. The minly identifiable fossil mammals include Asiocoryphodon progressivus, As-iocoryphodon sp., Gobiatherium minutus, Eomoropus? zhanggoueensis, Lophialetes? primus, Yimengia sp. and Forstercooperia? sp.. Biostratigraphical revision and study of Yuhuangding Fm show that the formation can be subdivided into three members (Lower, Middle and Upper), and the horizons of fossil mammal at different localities can be reasonably incorporated into three 'fossiliferous horizons' (Lower, Middle and Upper). Comparison between Yuhuangding faunas and the approximatly equivalent faunas in China and North America indicates that Yuhuangding Fm is of Early Eocene and early Middle Eocene in age.