Depositional history, contact relationships, and characterization of Upper Miocene Baogeda Ula Formation in central Nei Mongol with a description of fossil skunks

doi:10.19615/j.cnki.2096-9899.251029

Abstract

Abstract:

In addition to its well-known fossils, the Upper Miocene Baogeda Ula Formation is coupled with multiple basaltic lava flows, creating an ideal setting for studying its depositional history, geochronology, and associated vertebrate fossils. This paper, in honor of Prof. Qiu Zhanxiang for his widely admired emphasis on the geological contexts of vertebrate fossils, attempts to synthesize known contact relationships, existing K-Ar dates, and capping basalt elevations to reframe our concept of the lithostratigraphy, magnetostratigraphy, and lateral distribution of the Baogeda Ula Formation. Within this new framework, the Baogeda Ula Formation is defined by unconformable contact with the underlying Tunggur Formation at the lower boundary and a capping basalt at the upper boundary. In many sections, two or three layers of basalts are interbedded within sedimentary strata, with the capping basalts typically belonging to the top two basalts. The newly defined Baogeda Ula Formation includes a lower member as exemplified by Halajin Hushu section and Ulan Hushuyin Nur section, and an upper member at Baogeda Ula section. This expanded concept of the Baogeda Ula Formation includes two major faunas, i.e., the Bahean Halajin Hushu Fauna and Baodean Baogeda Ula Fauna, both falling within the Upper Miocene. We also describe rare fossil skunks (Promephitis) found in recent years that support the age assessments presented herein.

Key words: Nei Mongol, Miocene, Baogeda Ula Formation, stratigraphy, skunks

摘要：

内蒙古上中新统宝格达乌拉组不仅出产丰富的脊椎动物化石，它还与若干层玄武岩流互层，为探讨其沉积历史、接触关系及分布特征提供了理想条件。能够应用地层中出产的生物化石、玄武岩同位素年龄及磁性地层的相互制约进行综合年代学研究更为中国乃至东亚少有。结合过去30年来积累的地质观察对玄武岩与沉积岩的接触关系、火山岩盖板的海拔高度并综合近年来发表的K-Ar测年提出一个新的岩石地层框架。除了重新定义宝格达乌拉组外，还试图估算玄武岩风化速率并还原其原始分布。在此新的框架下，宝格达乌拉组被定义为以不整合方式上覆于中中新统通古尔组之上及以整合或假整合方式下伏于阿巴嘎玄武岩盖层之下。除玄武岩盖层外，在不同剖面上可以观察到一层或一层以上玄武岩夹层。这些夹层不仅显示同沉积时的火山活动，也提供了探讨玄武岩夹层测年对地层沉积年代的有效控制。重新定义的宝格达乌拉组包括两段：下段的哈拉津胡舒剖面，或许还包括乌兰呼舒音剖面的上部砾岩，该段出产晚中新世早期(灞河期)的哈拉津胡舒动物群；而上段则由宝格达乌拉剖面代表，其中出产晚中新世晚期(保德期)的宝格达乌拉动物群。但上、下两段之间的准确界限目前还不清楚。此外，还描述了宝格达乌拉组中出现的臭鼬化石，其对年代的估计也略有帮助。本文为庆祝邱占祥先生90寿辰专辑献稿。邱先生是内蒙古化石研究的早期领导之一，也是宝格达乌拉组化石的最初鉴定者。

关键词: 内蒙古, 中新世, 宝格达乌拉组, 地层, 臭鼬

CLC Number:

P534.621

WANG Xiao-Ming, SUN Lu, LI Lu, LI Qiang, QIU Zhu-Ding, Zhijie Jack TSENG. Depositional history, contact relationships, and characterization of Upper Miocene Baogeda Ula Formation in central Nei Mongol with a description of fossil skunks. Vertebrata Palasiatica, 2026, 64(1): 100-124.

王晓鸣, 孙蕗, 李录, 李强, 邱铸鼎, 曾志杰. 2026, 64(1): 100-124, 内蒙古上中新统宝格达乌拉组沉积历史、接触关系、分布特征及臭鼬化石的描述. 古脊椎动物学报.

Figures/Tables 11

Fig. 1 Map of central Nei Mongol showing major Neogene fossil sites and locations of Fig. 2 (black rectangle) Modified from Qiu et al. (2021:fig. 1)

Fig. 2 Map of Baogeda Ula area showing major fossil localities, distribution of Neogene basalts (Abag Volcanic Field), basalt dating sites, Cenozoic sediments capped by the basalts, and estimated former distribution (see section 3.7) of Baogeda Ula Formation Black rectangle shows the location for Fig. 6

Fig. 3 Thematic relationships of sedimentary and volcanic strata in the Baogeda Ula area See straight red lines in Fig. 2 for orientations of how the individual sections are located Elevation (m above sea level) at top of local topographic heights are measured from Google Earth Pro (2024) and vertical exaggerations are approximately 232:1

Fig. 4 Three sections of the Baogeda Ula Formation showing lithological variations A. unit-stratotype section at small mammal sampling site (IM0702) (the relatively high appearance of basalts on left hill is due to photographic perspective; it is substantially lower as shown in Fig. 9); B. section at IM0703 loc; C. a key reference section at Halajin Hushu. Photos by Wang Xiaoming

Fig. 5 Lithological section at unit-stratotype section of the Baogeda Ula Formation Modified from Sun et al. (2018:fig. 2). Due to a down-slope flow along an erosional topography, the lower basalts are exposed about 300 m apart and at markedly different elevations (see Fig. 9) These two capping basalts are starting points of the measured sections NA and NEB, terminology follow Sun et al. (2018)

Fig. 6 Map of Ulan Hushuyin Nur area (previously known as the “Road Mark 482 locality”) and fossil sites Fine-grained red beds of Middle Miocene Tunggur Formation and capping light grey and whitish gravel beds of Upper Miocene Baogeda Ula Formation are shown along the shoreline of the ephemeral lake bed of Ulan Hushuyin Nur Google Earth satellite image (Google Earth Pro (Version 7.3.6.9796), 2024) was downloaded on August 9, 2023

Fig. 7 Lower boundary-stratotype section in the Ulan Hushuyin site at an unconformable contact (white wavy line) between red mudstones of Middle Miocene Tunggur Formation in the lower part of the section (red arrows) and capping light grey and whitish gravel beds as the basal gravel beds of the Upper Miocene Baogeda Ula Formation (white arrows) A. close up of erosional contact relationship; B. water-worn postcranial fossils of large mammals found in the basal gravels of the Baogeda Ula Formation; C. field photo of a Hipparion cheek tooth within the upper white gravel beds; D. a broader view of the main section along the north shore of Ulan Hushuyin Nur Person in background for scale Photos taken by Wang Xiaoming on August 29, 2023 at N43°56′4.54″ E114°29′48.96″

Fig. 8 Upper boundary-stratotype section of Baogeda Ula Formation, 6 km northeast of Baogeda Ula Sumu showing two distinct basalts The lower basalt is 15-20 m below the local capping upper basalt and can be seen to taper off toward the right side of photo. Field tracing as well as on Google Earth images suggest that the lower basalt became the capping basalt at the unit-stratotype section of Baogeda Ula Formation, as also concluded by Sun et al. (2018) We think this relationship matches with the section at Bayin Mende described by Luo and Chen (1990:fig. 6B), in which the lower basalt was dated to 7.11 Ma±0.48 Ma (sample B48), providing a key age constraint within Baogeda Ula Formation Photo looking toward the north by Wang Xiaoming on July 9, 2007 at N44°06′26.5″ E114°39′04.2″

Fig. 9 Estimates of capping basalt lateral erosion rate at unit-stratotype section Based on photo in Sun et al. (2018:fig. 1C) with vertical exaggeration of 4:1

Fig. 10 Promephitis cf. P. parvus from Baogeda Ula Formation A. IVPP V34184, occlusal view (in stereo) of left M1 from IM0702 locality; B. V25146, occlusal view (in stereo) of left dentary fragment with m1 and m2 alveolus from Ulan Hushuyin locality (IM0710); C. V25174, occlusal view (in stereo) of right dentary fragment with m1 and m2 alveolus from Ulan Gangga locality; D. lingual and E. labial views of V25146; F. lingual and G. labial views of V25174

Fig. 11 Bivariate plot of lower m1 length (horizontal axis) and width (vertical axis) for select Chinese species of Promephitis and North American early skunks

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