Danghe area (western Gansu, China) biostratigraphy and implications for depositional history and tectonics of northern Tibetan Plateau
Danghe area (western Gansu, China) biostratigraphy and implications for depositional history and tectonics of northern Tibetan Plateau
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DOI：
10.1016/s0012-821x(03)00047-5
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发表时间：
2003-03
影响因子：
5.3
通讯作者：
Xiaoming Wang;Banyue. Wang;Zhanxiang Qiu;Guangpu Xie;Junyi Xie;W. Downs;Z. Qiu;T. Deng
中科院分区：
文献类型：
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作者：
Xiaoming Wang;Banyue. Wang;Zhanxiang Qiu;Guangpu Xie;Junyi Xie;W. Downs;Z. Qiu;T. Deng
甘肃西部党河地区处于阿尔金左行断裂东北端与沿着青藏高原北方缘的党河南山逆冲断层相互作用的焦点。党河地区拥有一套厚厚的第三纪沉积物和相关的化石记录，是青藏高原上少数几个能够提供构造历史、沉积环境和生物记录（包括脊椎动物和植物化石）综合研究的地方之一。然而，过去的研究未能利用古生物学数据，或者经常被过时的区域年表概念所误导。这些研究中不正确的年龄估计往往对构造解释产生深远的影响。我们提出了新的地层和古生物证据党河地区，并展示了一个新的年代学方案，表明一个年轻得多的年龄为大多数的沉积物。党河地区可识别出三套沉积物：（1）渐新世跑牛泉（新名称）地层，基底，主要为细粒深紫色泥岩和粉砂岩，包括经典的Yindirte动物群，形成了Tabenbulukian哺乳动物时代的基础;（2）中新世早期至中新世晚期早期铁匠沟（新名称）在一个向上变粗的序列中包含一个新的扁齿龙类长鼻兽的地层，终止于一个巨大的砾岩;（3）一个未命名的晚第三纪地层，包含另一个向上变粗的层序。根据我们新的地层格架，我们重新解释了先前发表的西水沟中部层序的磁柱，其代表的年代为C_6n ~ C_4Ar，对应于20 ~ 9.3Ma的时间跨度，比迄今所认识的要年轻得多。这一新的年代框架表明党河南山的沉积历史至少跨越早渐新世至晚中新世，可能上新世。渐新世至中新世中期的早期沉积物以细粒沉积物为主，表明沉积物来源较远。古生物学资料显示，当时的环境相对干燥，这是今天中国北方的典型环境。中新世晚期（约9-12 Ma），党河地区开始接受粗粒沉积物，并更接近山前。新的地层格架表明，沉积作用的开始比一般认为的要早，但也表明沉积物的存在比许多人意识到的要年轻得多。
The Danghe area in western Gansu Province is at the focal point of interaction of the northeastern end of the left-lateral Altyn Tagh Fault and growing ramp thrusts of the Danghe Nanshan Mountains along the northern rim of the Qinghai–Tibetan Plateau. With a thick sequence of Tertiary sediments and associated fossil records, the Danghe area is one of few places on the Tibetan Plateau that can offer integrated studies of its tectonic history, depositional environment, and biological records, including vertebrate and plant fossils. Past studies, however, have not been able to capitalize on the paleontological data or were often misguided by an outdated notion of regional chronology. Incorrect age estimates in these studies often have profound effects on tectonic interpretations. We present new stratigraphic and paleontologic evidence from the Danghe area and demonstrate a new chronologic scheme that indicates a much younger age for the majority of the sediments. We recognize three packages of sediments in the Danghe area: (1) Oligocene Paoniuquan (new name) Formation, basal, predominantly fine-grained, dark purple mudstones and siltstones that include the classic Yindirte Fauna, which forms the basis of the Tabenbulukian mammal age; (2) early Miocene to early late Miocene Tiejianggou (new name) Formation that contains a new platybelodont proboscidean in a coarsening-upward sequence terminating in a massive conglomerate; and (3) an unnamed late Neogene formation that contains another coarsening-upward sequence. In light of our new stratigraphic framework, we reinterpret a previously published magnetic column for the middle sequence in Xishuigou as representing chrons C6n through C4Ar corresponding to a span of about 20–9.3 Ma, much younger than has been realized so far. This new chronological framework suggests a depositional history of the Danghe Nanshan that spans at least the early Oligocene through late Miocene, to possibly Pliocene. Early depositions during the Oligocene through middle Miocene are dominated by fine-grained sediments indicating a distant source of sedimentation. Paleontological data suggest a relatively dry environment, as is typical of northern China today. By late Miocene (around 9–12 Ma), the Danghe area began to receive coarse sediments and was much closer to the mountain front. The new stratigraphic framework indicates an earlier onset of sedimentation than has generally been assumed, but also suggests the presence of sediments much younger than many have realized.