内蒙古中部通古尔台地及周边地区古地磁及磁性地层的最新研究

RECENT PALAEOMAGNETIC AND MAGNETOSTRATIGRAPHIC INVESTIGATIONS ON AND AROUND THE TUNGGUR TABLELAND,CENTRAL NEI MONGOL (INNER MONGOLIA)

通古尔(Tunggur)台地作为20世纪20—30年代美国自然历史博物馆第三中亚考察团研究过的一个经典的脊椎动物化石地点,在亚洲晚新生代陆相哺乳动物学和生物年代学研究中扮演着重要的角色。20世纪90年代中期以来,在通古尔地区开展了大量的野外工作,发现了数个以小型哺乳动物为主的动物群,建立了从早中新世至上新世哺乳动物演化序列的框架。这一框架有可能成为东亚新近纪陆相哺乳动物群对比的标尺。通古尔地区动物群的演替,对于深入认识这一地区古环境和动物地理的分化也十分重要。2005年8月的野外工作,主要在通古尔台地的两个地点——奥尔班和阿勒特希热进行,目的是寻找新生代中晚期的脊椎动物化石和根据古地磁极性年代标尺确定化石的时代。最厚的古地磁采样剖面在奥尔班(Aoerban,43°20′21．6″N,113°54′48．6″E)。取样点起自43°20′36．4″N,113°54′48．6″E,终于43°20′52．7″N,113°54′23．1″E。测试结果显示,剖面上50 m的粘土岩、粉砂岩和细砂岩的互层,除近底部5 m厚的褐棕色粘土岩外均为负极性时。负极性段大致可与国际标准古地磁柱的5Dr或6r对比,相当于18—20 Ma。该剖面上同时进行了小哺乳动物化石的采集;据初步鉴定,化石组合包括了晚渐新世和大量早中新世的成员。因此,奥尔班动物群极有可能代表内蒙古中部地区新近纪哺乳动物群序列中时代最早的一个新成员。阿勒特希热(Aletexire,43°47′21．6″N,113°05′25．0″E)采样剖面始于43°47′53．6″N, 113°04′51．1″E,终于43°47′52．9″N,113°04′42．2″E。剖面厚约13 m,以粉砂岩和细砂岩为主,含有与推饶木动物群相同的成员。测试结果显示,整个剖面记录为正极性时。这便与王晓鸣等于2003年把阿勒特希热剖面古地磁测试的结果与默尔根剖面的对比产生了一些矛盾。原因可能是阿勒特希热的采样间隔过大,或两剖面根本就不是同一时代的产物。同样的问题似乎也出现在与推饶木剖面的对比上。看来有必要对这一地区的古地磁测试作更深入的工作。

As a classic vertebrate fossil locality of the American Museum of Natural History Central Asiatic Expeditions in the 1920s and 1930s, the Tunggur Tableland in central Nei Mongol （Inner Mongolia） has played an important role in late Cenozoic terrestrial mammalian palaeontology and biochronology of Asia. Field work since the mid-1990s has greatly expanded the faunal representations to include mammal assemblages （ particularly those of small mammals） from Early Miocene to Pliocene （ Qiu et al., 2006 ）. The mammalian faunal sequence that has been established in Nei Mongol has the potential to become a standard of comparison for East Asia. Faunal changes in the Tunggur area are also important for gaining an in-depth view of regional differentiations in palaeoenvironment and zoogeography. In August 2005, field studies were done at two localities on the Tunggur Tableland with the objectives of locating and identifying middle to late Cenozoic vertebrate fossils and dating them on the basis of the palaeomagnetic polarity time scale. The thickest section sampled is at Aoerban （43°20′21.6”N, 113°54′48.6”E） where nearly 50 m of interbedded claystone, siltstone, and fine-grained sand are reversely magnetized except for about 5 m of brownish-tan claystone near the base that records normal polarity. The polarity for the Aoerban section suggests a correlation to the period near the Chron 5Dr or 6r, which is roughly in the 18 -20 Ma age range. The section also was sampled for small mammal fossils, which tentatively are assigned to Early Miocene age. The second locality sampled is Aletexire （43°47′21.6”N, 113°05′25.0”E） where about 13 m of siltstone and fine sandstone that contain probable Tairum Nor fauna （ Wang et al., 2003 ） record normal polarity.