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Vertebrata Palasiatica ›› 2009, Vol. 47 ›› Issue (2): 111-134.

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A NEW EARLY TO LATE MIOCENE FOSSILIFEROUS REGION IN CENTRAL NEI MONGOL: LITHOSTRATIGRAPHY AND BIOSTRATIGRAPHY IN AOERBAN STRATA

WANG Xiao-Ming, QIU Zhu-Ding, LI Qiang, Yukimitsu TOMIDA, Yuri KIMURA, Zhijie Jack TSENG, WANG Hong-Jiang   

  • Online:2009-06-15 Published:2009-06-15

内蒙古中部敖尔班地区的岩石及生物地层

王晓鸣,邱铸鼎,李强,富田幸光,木村由莉,曾志杰,王洪江   

Abstract: Vertebrate fossil localities of Early Miocene age are rare in north China in general and in Nei Mongol in particular. Here we report a recently discovered, richly fossiliferous Early Miocene through Late Miocene sequence in central Nei Mongol. The new Aoerban strata from Sonid Zuoqi represent one of the longest exposures in the region, spanning up to 50 m in total thickness, and produce fossils along much of the section with distinct lithologies, resulting an easily correlated in situ fossil collection with excellent stratigraphic documentation. Three lithologic units are recognized. We formally name the Aoerban Formation at the lower part of the section, which contains three discrete members of fine-grained sediments altered by paleosols: 1) Lower Red Mudstone Member, 2) Middle Green Mudstone Member, and 3) Upper Red Mudstone Member. Resting disconformably above the Aoerban Formation is the Balunhalagen bed, which is characterized by basal conglomerates followed by orange-colored mudstones and siltstones. Cutting into the Balunhalagen bed is the Bilutu bed, which is another unit of channel gravel sediments and overbank deposits. Paleontologically four discrete faunal units are clearly recognizable, mostly based on small mammals obtained from five wash sites. We distinguish two Early Miocene faunas. The Lower Aoerban Fauna and Upper Aoerban Fauna, produced from the Lower Red Mudstone and Upper Red Mudstone Members, respectively, contain characteristic Early Miocene elements of north China and/or Europe. The Balunhalagen Fauna, derived from the Balunhalagen bed, is characteristic of the Middle to Late Miocene transition. The Bilutu Fauna from the capping Bilutu bed features a mixed composition of Middle and Late Miocene taxa. Both faunal and sedimentological evidence suggest that some of the earlier faunal elements have been reworked into the later Bilutu deposits; thus the Bilutu Fauna is mostly likely of Late Miocene age.

摘要: 中国北方新近纪地层中,早中新世的哺乳动物化石地点比较稀少,化石面貌也不十分清楚。在内蒙古中部,尽管通古尔台地上有研究历史近百年的新近纪经典地点,但上世纪90 年代发现的嘎顺音阿得格,依然是内蒙古目前惟一产出早中新世动物群的小盆地。同内蒙古诸多其他新近纪哺乳动物群一样,嘎顺音阿得格动物群与相邻动物群彼此缺乏时空上的联系,还不易建立可靠的层序关系。 位于苏尼特左旗东南约60 km 的敖尔班(曾用名“奥尔班”,见Liddicoat et al., 2007),红色地层大面积出露,是2004 年发现的一个新的哺乳动物化石地点。敖尔班剖面总厚50余米,大、小哺乳动物化石共生,岩性特征易识别,有利层位对比,是研究生物地层学的理想地点。自2004 年以来,我们连续在敖尔班地区采集化石并进行地层工作,建立了一个较完整的哺乳动物序列,时代跨越早中新世晚期到晚中新世晚期。更加难得的是,敖尔班同一剖面上含有4 个哺乳动物化石层位,其上下层序关系一目了然,在内蒙古新近纪地层中仅此一例。敖尔班哺乳动物序列的建立,无疑将对整个内蒙古中部新近纪,尤其是早中新世动物群面貌的了解具有促进作用。本文着重对敖尔班岩石地层进行描述,并结合我们4 年来对脊椎动物化石的积累与认识,试图进一步完善内蒙古中部地区的生物地层层序,进而建立这一地区的年代地层框架。对化石的详细描述将适时发表。 根据岩性及接触关系,敖尔班剖面可分为三大段:敖尔班组、巴伦哈拉根层及必鲁图层。敖尔班组(新建组)由一套红色和绿色的泥岩及粉砂岩组成,发育有成熟的古土壤层,厚约42 m,时代属于早中新世中晚期。敖尔班组的层型剖面建立在敖尔班露头出露最厚的中部,下部未见底,顶部与上覆巴伦哈拉根层呈假整合或不整合接触。该组目前已知分布仅局限于敖尔班露头。敖尔班组可进一步划分出三段:下红泥岩段、中绿泥岩段及上红泥岩段。三段呈连续沉积。敖尔班组与巴伦哈拉根层之间的假整合所代表的沉积间断延续了中中新世的大部分时段。巴伦哈拉根层为一套橘红色砂岩、粉砂岩及底砾岩,时代大致是最晚中中新世至最早晚中新世。不整合于巴伦哈拉根层之上是必鲁图层。两层之间似乎缺失了晚中新世的大部分堆积。必鲁图层的底砾岩是一种切割与充填构造,其中所含钙质结核及相当数量的化石都可能是巴伦哈拉根层原生堆积物再沉积的结果。必鲁图层的分布还需做更多工作,其时代可能是晚中新世晚期。 主要依据小哺乳动物的组合,在敖尔班剖面中可建立4 个动物群。最早为敖尔班组下红泥岩段产出的下敖尔班动物群。该动物群的特征是,小哺乳动物中在渐新世十分兴旺的一些科,如Ctenodactylidae, Tachyoryctoididae, Aplodontidae 和Zapodidae 还相当繁荣;中新世出现的属,如Mioechinus, Keramidomys, Heterosminthus 和Democricetodon 等占动物群总量的半数以上;大哺乳动物中残留有Palaeogale 和裂爪兽。比下敖尔班动物群稍晚的是上敖尔班动物群,产出于敖尔班组的上红泥岩段。上敖尔班动物群的特征是,小哺乳动物在渐新世中占统治地位的一些科或完全绝迹,如Ctenodactylidae, 或在种类和数量上明显减退,如Aplodontidae和Zapodidae; 缺少在下敖尔班动物群中还相当常见的一些古老属,如Amphechinus, Tachyoryctoides 和Sinolagomys 等;出现了下敖尔班动物群中所没有的Megacricetodon, Cricetodon 和Alloptox 属;大哺乳动物中出现了长鼻类和柄杯鹿(Ligeromeryx / Lagomeryx)。经过相当长的一个沉积间断,敖尔班剖面的上部出现了巴伦哈拉根动物群。该动物群中渐新世常见的小哺乳动物科进一步衰落,同时出现了亚洲古北界晚中新世以后常见的跳鼠科(Dipodidae)和鼢鼠科(Siphneidae),具有明显的中中新世晚期或晚中新世早期生物组合的特点。最后是敖尔班剖面顶部必鲁图层中的必鲁图动物群。必鲁图动物群显然带有中中新世及晚中新世的混合特征,很可能是水流作用再沉积的结果。根据其中最进步分子的成分判断,估计必鲁图动物群的年代是晚中新世的晚期,其特征是,鼠科(Muridae)动物高度分化,而野兔科(Leporidae)尚未出现;含有晚中新世宝格达乌拉动物群或最晚中新世二登图动物群中大量出现的属种,如Lophocricetus grabaui, Paralactaga suni, Dipus fraudator 和Hansdebruijnia pusilla 等;但二登图动物群中很繁荣的一些属在必鲁图动物群中未被发现或者发现的个体数量很少,如Prospermophilus, Paralophicricetus 和Microtodon 等。