Unconformity-bounded Upper Paleozoic megasequences in the Beishan Region (NW China) and implications for the timing of the Paleo-Asian Ocean closure
Graphical abstract
Introduction
The Central Asian Orogenic Belt (CAOB) is one of the largest accretionary orogenic collages in the world, and is generally considered to have formed as a result of the consumption and closure of the Paleo-Asian Ocean between the Siberia Craton to the north and the Tarim and North China cratons to the south (Sengör et al., 1993, Li et al., 2009; Xiao et al., 2015, Xiao et al., 2017, Yakubchuk, 2017). The CAOB extends eastward from the Urals through Kazakhstan, Tianshan–Xingmeng orogens in North China and Mongolia, to the Okhotsk Sea (Fig. 1). The Beishan Region is located in the southernmost part of the CAOB, which forms a key region of the final attachment of the Tarim and North China cratons to the southern accretionary orogenic collages of the CAOB (Zuo et al., 1991, Xiao et al., 2010, Xiao et al., 2015, Wang et al., 2014).
Extensive studies of the CAOB have provided many models regarding the duration and architecture of its orogenesis (e.g., Zuo and He, 1990, Carroll et al., 1995, Carroll et al., 2001, Windley et al., 2007, Xiao et al., 2015, Safonova et al., 2017, Yakubchuk, 2017). It is a long-lived accretionary orogenic collage involving multiple accretion of continents, microcontinents, arc systems and accretionary complexes within the Paleo-Asian Ocean (Li et al., 2009, Xiao et al., 2010, Liu et al., 2015, Zhang et al., 2016, Zhou et al., 2017). But questions still remain with respect to the exact timing and processes of the final amalgamation of the Paleo-Asian Ocean in the Late Paleozoic (Sengör et al., 1993, Wu and He, 1993, Li et al., 2009, Xiao et al., 2006b, Qin et al., 2017). Concerning the Beishan Region, a diverse range of views exists with regard to the timing of the closure of the Paleo-Asian Ocean. These widely varying interpretations can be summarized into three main views. (1) The branches of the Paleo-Asian Ocean in the Beishan Region closed prior to the Silurian–Devonian, evidenced by a major regional unconformity and Devonian molasse deposits (Zuo and He, 1990, Zuo et al., 1991, He et al., 2005, Xu et al., 2008). (2) The accretionary orogenesis was completed in the Permian–Triassic because of the Liuyuan “ophiolitic mélange”, arc-related magmatism and Triassic deformation (Xiao et al., 2010, Mao et al., 2012b, Song et al., 2013, Tian et al., 2015, Tian et al., 2016, Zheng et al., 2016). (3) The Paleo-Asian Ocean was closed in the latest Carboniferous in view of the geochronology of ductile shear zones (Liu and Wang, 1995, Li et al., 2009, Cai and Xu, 2012).
Notably, all these interpretations have heavily relied on geochronology and geochemistry of magmatic rocks (Zuo et al., 1991, Li et al., 2009, Xiao et al., 2010, Wang et al., 2017a), with little reference to stratigraphic sequences. On the other hand, megasequences bounded by extensive, but not global, major unconformities represent the stratigraphic packages deposited during distinct phases of plate motion (Sloss, 1963, Hubbard et al., 1985, Allen and Allen, 2013). Consequently, study of megasequence integrated with a well-defined chronostratigraphic framework is a powerful approach to improve understanding of the regional tectonic evolution, as has been well demonstrated in studies concerning the opening process of the North Atlantic (Falvey, 1974, Sinclair et al., 1994), or the amalgamation history of the CAOB in NW China (Carroll et al., 1995, Carroll et al., 2001). Upper Paleozoic stratigraphic sequences in the Beishan region provide an excellent record of the final amalgamation of the CAOB in this region (Fig. 2). However, to date, they have not been synthesized to serve this purpose. In regards to the Upper Paleozoic biostratigraphy of the Beishan Region, this has not been updated since the geological mapping conducted during the 1960–1970 s (Jin, 1974, Zhu and Shen, 1977, Zhu, 1983). Although these early biostratigraphic schemes have proved crucial for geological mapping and regional stratigraphic correlation, they are inadequate for high-resolution interpretation of the tectonic events that affected basin development and geological evolution of the Beishan Region. This paper presents a new interpretation of the Upper Paleozoic megasequences of the Beishan Region, as well as several new isotopic ages, primarily based on our extensive fieldwork conducted in the past five years (2012–2016). Specifically, we aim (1) to provide an updated review of the Upper Paleozoic lithostratigraphy, biostratigraphy and chronostratigraphy across the Beishan Region, (2) to establish a tectonostratigraphic framework defined by major unconformities and megasequences, and (3) to discuss the tectonic development of this region during the Late Paleozoic as well as regional correlations with coeval megasequences in NW and NE China.
Section snippets
Geological setting
Beishan Region is located in the middle of the CAOB, possibly connecting the Tianshan sutures to the west and the Solonker Suture to the east (Sengör et al., 1993, Wang et al., 2007, Xiao et al., 2010, Xiao et al., 2015, Zhang et al., 2016). This region is bounded by the Mongolian collage system to the north and the Tarim–Dunhuang Craton to the south (Li et al., 2009, Xiao et al., 2010, Zhou et al., 2017). It also marks the southern margin of the Turfan Basin and the western margin of the
Analytical techniques
Zircon grains were sorted by heavy liquid and magnetic techniques after sample crushing. Random zircons were selected and mounted in epoxy resin by hand-picking under a binocular microscope and then polished at the Institute of Regional Geology and Mineral Resources Survey, Langfang, Hebei Province, China. CL images of zircon grains were taken by a Quanta 400FEG scanning electron microscope to recognize their internal structure at Northwest University, Xi’an, China.
Zircon U-Pb dating was
Results
Sample 13DHL-16TW1 was collected from rhyolitic tuffs in the upper part of the Ganquan Formation (Section A in Fig. 4, Fig. 5). Two age clusters were found evident from 28 spots (from a total of 30 spots analyzed). The first age cluster came from 12 spots which indicate a 206Pb/238U weighted mean age of 419 ± 13 Ma, while the second age cluster, derived from 16 spots, give a mean age of 301.8 ± 3 Ma (MSWD = 1.16) (Fig. 6a, 6b and 7a). The age of the younger cluster was regarded as the age of
Update of stratigraphy
Upper Paleozoic biostratigraphy of the Beishan Region has been summarized and reviewed by several workers, including the Devonian (Zhai, 1981), Visean (Jin, 1974), Bashkirian and Moscovian (Zhang and Guo, 1985), Cisuralian and Guadalupian (Zhu, 1983, Zhang, 1995), and Lopingian (Zhu and Shen, 1977). More recently, a number of radiometric ages have been published (Fig. 3; Li et al., 2011b, Guo et al., 2014, Jia et al., 2016, Wang et al., 2017b). However, discrepancies have been found between
Unconformities
Megasequences are bounded by extensive, but not global, major unconformities with huge sedimentary gaps (Hubbard et al., 1985, Allen and Allen, 2013). Three major unconformities have been recognized by some geologists as a testament to the final amalgamation of the CAOB and the closing of the Paleo-Asian Ocean in the Beishan Region (Liu and Wang, 1995, Zuo et al., 2003, Tian et al., 2015). The first one is between the Mississippian–Guadalupian and its underlying strata, the second between the
Tectonostratigraphic framework and implications
The first step in creating a tectonostratigraphic framework is the identification of megasequence boundaries (Hubbard et al., 1985, Vail et al., 1991, Miall, 2010). Sloss (1963) introduced three types of unconformities: biostratigraphic hiatus, local unconformity, and interregional unconformity. The last of these refer to unconformities that can be traced over large areas from the margins into the interior of a sedimentary basin. For instance, several such interregional unconformities subdivide
Conclusions
1. Isotopic dating combined with biostratigraphy have provided a new set of ages for Upper Paleozoic formations in the Beishan Region. They are, in an ascending order, latest Silurian–Middle Devonian Sangejing Formation (Queershan Formation), Late Devonian Dundunshan Formation, Visean Baishan Formation, Serpukhovian –Moscovian Shibanshan Formation, Kasimovian–Sakmarian Ganquan Formation, Artinskian–Wordian Shuangbutang Formation, Wordian Jushitan Formation, latest Wordian–early Wuchiapingian
Acknowledgments
Our research is supported by the National Science Foundation of China (41402097 and 41402195) and China Geological Survey (12120115002301 and 121201011000150012). We gratefully acknowledge Xiumian Hu for his encouragement to this study and Franz T. Fürsich for his advice about trace fossils. G.R. Shi acknowledges supports from the Australian Research Council and Deakin University. An anonymous reviewer has provided insightful comments on the earlier version, which greatly improve this
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