Journal of Paleontology, 91(3), 2017, p. 369–392 Copyright © 2017, The Paleontological Society 0022-3360/15/0088-0906 doi: 10.1017/jpa.2016.163


Silicified microbiota from the Paleoproterozoic Dahongyu Formation, Tianjin, China


Min Shi,1 Qing-Lai Feng,1 Maliha Zareen Khan,1 Stanley Awramik,2 and Shi-Xing Zhu3


1StateKey Laboratory ofGeological Processes and MineralResources,China University of Geosciences,Wuhan, 430074, China ⟨shimin@cug.edu.cn⟩, ⟨qinglaifeng@cug.edu.cn⟩, ⟨mallu786@hotmail.com⟩ 2Department of Earth Sciences, Preston Cloud Research Laboratory, University of California, Santa Barbara, CA 93106, United States


⟨awramik@geol.ucsb.edu⟩ 3State Key Laboratory of Biogeology and Environmental Geology, China University of Geosciences,Wuhan, 430074, China ⟨1404063125@qq.com⟩


were identified in open nomenclature. One new taxon of microfossil was described: Xiaohongyuia sinica Shi and Feng new genus new species.


Introduction


The so-called “boring billion” (~1.8–0.8 Ga) was characterized by environmental stability and its conditions are thought to have delayed eukaryotic diversification (Holland, 2006; Planavsky et al., 2014). Tectonically, this period was far from boring, since it involved the activity of the Columbia and Rodinia super- continent (Zhang et al., 2012). Moreover, the evolution of Earth’s biosphere, atmosphere, and hydrosphere is thought to tie with the continental movements (Campbell and Allen, 2008; Santosh, 2010; Young, 2013). Correspondingly, recent studies have revealed that obvious chemical and geological changes occurred during the “boring billion” (Guo et al., 2012). Javaux et al. (2013) proposed that the “boring billion” was actually an “exciting time” for early eukaryotes, but direct taxonomic evi- dence is not very common in the early “boring billion” (Knoll et al., 2006; Butterfield, 2007). Based on the study of micro- fossils from ca. 1600 Ma strata, Yin (1985) proposed that an obvious morphological differentiation had occurred. However, subject to photographic limitations, demonstrating character- istics of morphological differentiation are blurred. In order to portray the biological community during this critical era, the unmetamorphosed and classic Jixian section was selected for a systematic study of microfossils from the Dahongyu Formation. This involved the preparation of 914 thin sections of chert


Abstract.—Cherts and silicified dolostones of the ca. 1600 Ma Dahongyu Formation (uppermost Changcheng Group) from North China contain well-preserved microfossils. Cyanobacteria filaments and coccoids dominate the studied Dahongyu microbiota in the Jixian section. These microfossils show the characteristics of endobenthic, epibenthic, and allochthonous microfossils, which illustrated well a vertical distributional view of an intertidal microbiota. Large num- bers and size of allochthonous microfossils are the distinguishing characteristics of this new microbiota. A few of these possess ornaments or odd morphology, which suggest their potential eukaryotic nature. Through regional comparisons, it is revealed that there was significant regional disparity within the Yanshan Basin of microbiotas during deposition of the Dahongyu Formation. The newly studied Dahongyu microbiota is similar to the microbiota from the underlying Changcheng Group, with only slight differences. The absence of typical eukaryotes and the emergence of unique micro- fossils (especially small fusiform microfossils) make the Dahongyu Formation and the subsequent strata of the Jixian Group distinct from contemporaneous eukaryote-bearing strata. In the Dahongyu assemblage, 19 species were recognized, six species were identified informally and seven species


samples for examination. Consequently, a large number of microfossils were recovered.


Geological setting, stratigraphy, and age


The Yanshan Basin in North China is one of the rift basins that formed during the break-up of the Columbia supercontinent (Lu et al., 2002, 2004; Zhang et al., 2012). Unmetamorphosed successions of Proterozoic sedimentary rocks are widely exposed in this basin. The studied Jixian section is located in the middle of the Yanshan Basin (Fig. 1) and exceeds 10,000m in thickness. In ascending order, it has been divided into the Changcheng, Jixian, Huailai, and Qingbaikou groups, with ages ranging from 1670 to 850Myr (Li et al., 2011) (Fig. 2). In the lower part of the Changcheng Group, clastic rocks are dominant. Carbonates initially emerged in the upper part of the Chang- cheng Group and dominated the sediments in the Jixian Group. Clastic rocks appeared again in the upper part of the Jixian section, dominating the Huailai Group and the Qingbaikou Group.


The Dahongyu Formation was the last sedimentary


formation of the Changcheng Group. During Dahongyu deposition, wide-spread and long-standing volcanic activities occurred (Ren, 1987). Volcanic materials and terrestrial deposits are ubiquitous in the Yanshan Basin and exhibit obvious


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