陕西蓝田地区新第三纪地层划分的几点讨论

蓝田地区灞河组和蓝田组的时代归属是我国三趾马动物群分期的关键问题之一。通过与府谷老高川三趾马动物群的对比及古地磁测定，该区新第三纪地层可分为上新统静乐组（原蓝田组上部）、蓝田组（原蓝田组下部）和灞河组．蓝田组的地质时代可归保德期庙梁亚期，地质年龄约为距今５．３—５．０Ｍａ期间．灞河组的地质时代可与喇嘛沟动物群的对比，为保德早期或稍早．

DISCUSSUS ON SUBDIVISION OF LATE MIOCENE IN LANTIAN AREA,MIDDLE OF SHAANXI

abe Formation and Lantian Formation were named by Liu Dongshen et al. firstly, theformer is a suit of interbeds with gray--green, light--yellow clay, sandy and sandy clay, thelatter is a SLlit of red clay contained abundant carbonate--node. Both of them contain a lot offossils of HI'PParion fauna. According to the fossil assemblage, the Babe Formation was putto lower Pliocene, correlating with Baode Formation and Lantian Formation was correlatedwith Jingle Formation, the geological age of which is Late Pliocene. In 1984, Li Chuankiu etal. correlated it with Vallesian of Europe, and the Lantian Formation was put into Baodean,contrasting with Turolian of Europe. The fossils in Lantian Formation is not abundant, theidentified species are HiPParion plocodus (being considered there are H. fossatum at least), Cltl'IOtherl'um habereri, Cervavi'tus novorossiae, Dicoryphochoerus madius, HOnanotheriumsp. Chleuastochoerus sp. etc. All of those fossils are common sorts in Baodean, so its age belongs to Baodean undoubtedly. However, it is worth to discuss the position of the LantianFormation in BaodeanThe authors of this paper found lots of fossils of HiPParion fauna in Laogaochuan, FuguCounty,North Shaanxi in 1985,preliminary studies showed that the age of HI'PParl'On faunafound in Laogaochuan could be divided into two periods, the Miaoliang Subage and LamagouSubage. The paleomagnetic ages are 5.5-5. 0 Ma and 7. 4-5. 5 Ma respectively. Comparedwith Miaoliang and Lamagou HiPParion fauna, the fossil assemblage in Lantian Formation ismore similar to the former evidently. The most species in the Formation have homologues inthe Miaoliang fauna. The age of Miaoliang fauna was considered as late Baodean becausethere are some animals that mainly lived in late period and its paleomagnetic age is about5. 3 Ma. The Paleomagnetic determination shows that the geological age of the Lantian Formation is about 5. 2-2. 5 Ma in the Duanjiapo section, Lantian County, the boundary ofPlio. /Mio. (about 5. 0 Ma) lies the 12m above sub face, the record of 3. 6 Ma lies 49m abovethe subface. This indicates that the middle and upper part of original Lantian Formationshould include Pliocene, the thickness of real Miocene is only about 10m (the fossils in Lantian Formation lies 6m above subface), paleomagnetic age of this part is 5. 0-5. 3 Ma. Thegeological age of Lantian Formation is Miaoliang Subage, Late Baodean. The Baodean represent a geological periods as long as 10. 0-5. 0 Ma, although Babe Formation lies under Lantian Formation, it cannot rule out the former was excluded from Baodean completely. Thereare two fossilferous layers in Babe Formation, the most of them exited in local faunas ofBaode district. there are also some ancient components or local elements. The different paIeoenvironment between Lantian district and Baode maybe result the emergence of some localelements, furthermore, some of those species are found in other faunas gradually.Compared with Lamagou HI'PParion fauna, there are some similarities between them besides the common elements in Babe Formatioll, others,such as Hipparion chiai,Dinocrocutaetc.were found in Lamagou fauna, and they are characterized by yielding some ancient elements. Paleomagnetic age of Lamagou is about 7. 4 Ma, so its geological age should beBaodean also.