甘肃董湾晚新近纪地层及中新统/上新统界线

LATE MIOCENE-EARLY PLIOCENE BIOSTRATIGRAPHY AND MIOCENE/PLIOCENE BOUNDARY IN THE DONGWAN SECTION,GANSU

古地磁测年表明,甘肃董湾晚新近纪剖面代表了7.3～3.5 Ma间的连续风成古土壤堆积。在74.8 m厚的剖面上,发现了小哺乳动物化石层位40个,共产出小哺乳动物39种。根据小哺乳动物组合特点,董湾剖面可以划分出4个生物地层带,Ⅰ+Ⅱ带为晚中新世晚期,相当于保德期;Ⅲ+Ⅳ带为上新世早期,相当于榆社期早期。中新统/上新统界线(5.33 Ma)位于剖面距顶部35.5 m处,比以鼢鼠Mesosiphneus最早出现为标志的生物地层Ⅲ带的底界低2 m。由于榆社阶底界直接对应于中新统/上新统界线,该剖面因其良好的地层和生物条件可被推荐为榆社阶界线层型的候选剖面。

The Dongwan section is located near Dongwan Village,Qin'an County,Gansu Province (34°58′N,105°47′E).The section is a continuous eolian red clay sequence and is dated from 7.3 to 3.5 Ma by paleomagnetic study.The 74.8 m long section is divided into 21 layers(0 to 20)and 39 species of small mammals are found from 40 fossil levels.Based on faunal composition of small mammals,especially the myospalacines,four biozones are recognized. Biozone I covers layers 0~7,ranges magnetic polarities from upper part of C3Br.1n to the lower part of C3An.2n,and spans from 7.3 to 6.5 Ma.The mammals found in this biozone include Quyania chowi,Ochotonoma n.sp.1,O.primitiva,Ochotona minor,O.lagreli,Sicista sp.,Prospermophilus orientalis,Sinocricetus zdanskyi,? Kowalskia sp.,Prosiphneus licenti,P. tianzuensis,Lophocricetus grabaui,Pseudomeriones abbreviatus,and Hansdebruijnia pusillus. BiozoneⅡcovers layers 8～14.It ranges from the lower part of polarity zone C3An.2n to the base of C3n.4n,and has a time span from 6.5 to 5.24 Ma.The fossils found in this biozone include Sulimskia sp.,Parasoriculus sp.,Paenepetenyia cf.P.zhudingi,Ochotonoma n. sp.2,O.primitiva,Ochotona minor,O.gracilis,O.plicodenta,Prospermophilus orientalis, Sinocricetus zdanskyi,Prosiphneus eriksoni,Pliosiphneus lyratus,Chardina n.sp.,C.sinensis, Pseudomeriones abbreviatus,Sicista sp.,Apodemus sp.,Hansdebruijnia pusillus,Chardinomys sp.,and Allorattus sp.The beginning of biozoneⅡis marked by the first appearance of Prosiphneus eriksoni in the depth of 59 m. BiozoneⅢcovers layers 15～17,ranges from the base of C3n.4n to the upper part of C3n. 2n(Nunivak),and spans the time from 5.24 to 4.55 Ma.Fossils found in this biozone include Erinaceus sp.,Parasoriculus sp.,Ochotonoma n.sp.2,Ochotona minor,O.lingtaica,Trischizolagus mirificus,Sinotamias n.sp.,Sinocricetus zdanskyi,Cricetinus mesolophidus,Mimomys teilhardi, Pliosiphneus lyratus,Chardina sinensis,C.truncatus,Mesosiphneus sp.,Pseudomeriones abbreviatus,P.complicidens,Chardinomys sp.,C.yusheensis,and Apodemus sp.The beginning of biozoneⅢis marked by the first appearance of Mesosiphneus sp.at 35.5 m. BiozoneⅣcovers layers 18～20,ranges from the upper part of C3n.2n to the top of the section,and has a time span from 4.55 to 3.5 Ma.The small mammals found in this biozone include Ochotonoma n.sp.2,Pliosiphneus lyratus,Mesosiphneus praetingi,Chardina truncatus, and Chardinomys nihowanicus.The first appearance of Mesosiphneus praetingi at the depth of 20.4 m marks the beginning of this biozone. BiozoneⅠandⅡare correlated to the late Late Miocene,equivalent to Baodean,while biozoneⅢandⅣare correlated to the early Pliocene,equivalent to early Yushean.According to the paleomagnetic study,the boundary of Miocene/Pliocene(5.33 Ma)is located at the depth of 35.5 m of the section,and coincides with the last appearances of Prosiphneus eriksoni and Ochotonoma primitiva.The Miocene/Pliocene boundary is 2 meters lower than the boundary between biozoneⅡandⅢ,which is marked by the earliest appearance of Mesosiphneus sp.Except Mesosiphneus sp.,the earliest records of Paenepetenyia cf.P.zhudingi,Apodemus sp.and Ochotona lingtaica are also found closely above the boundary(1～2.5 meters).About 3 meters below the boundary,Parasoriculus sp.,Sulimskia sp.,Pliosiphneus lyratus,Chardina sinensis and Chardinomys sp.appear,and O.plicodenta and Sicista sp.disappear.The continuous sequence and high density of fossil levels qualify the section as a candidate stratotype section of Yushean Stage.