Mesoproterozoic Biogenic Thrombolites from the North China Platform
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Int J Earth Sci (Geol Rundsch) (2013) 102:401–413 DOI 10.1007/s00531-012-0817-9 ORIGINAL PAPER Mesoproterozoic biogenic thrombolites from the North China platform Dongjie Tang • Xiaoying Shi • Ganqing Jiang Received: 10 March 2012 / Accepted: 11 August 2012 / Published online: 23 September 2012 Ó Springer-Verlag 2012 Abstract Thrombolites are abundant in the subtidal stages: (1) organomineralization of the cores through dolostones of the Mesoproterozoic Wumishan Formation replacement of organic matter by minute organominerals (ca 1.50–1.45 Ga) in the North China platform. Three resulting from anaerobic degradation of bacteria and EPS major components are identified within the thrombolites: and (2) inorganic precipitation of the outer layers fostered irregular mesoclots, micritic matrix and spar-filled voids. by an increase in carbonate alkalinity in micro-environ- The mesoclot generally comprises a relatively organic-rich ment due to organic matter decomposition. The thrombo- micritic core and a microsparitic outer layer that consists of lites from the Mesoproterozoic Wumishan Formation may fibrous aragonite (pseudocrystals) with less organic matter. have formed through complex interactions between In the core of mesoclots, abundant fossilized organic microbes and environments and represent the earliest remnants, such as putative coccoidal and filamentous known Precambrian biogenic thrombolites. bacteria and mucus- to film-like extracellular polymeric substance (EPS), are closely associated with organomi- Keywords Mesoproterozoic Á Protogenetic thrombolite Á nerals including nanoglobules and submicron-scale poly- Organomineral Á North China platform hedrons. In exceptionally well-preserved mesoclots, their outer layers commonly contain micropores displaying as bacterial molds and filamentous bacteria fossils. The matrix Introduction of mesoclots consists mainly of micropeloids (20–30 lmin diameter) and minor terrigenous detritus. Some mesoclots Thrombolites are a type of microbialites (Aitken 1967; have denticulate edges and their matrix shows growth Shapiro and Awramik 2006) that are formed through laminations that envelope the outlines of mesoclots. These complex interactions of benthic microbial communities features indicate that the mesoclots are primary and they with environments by trapping and binding of sediment were mineralized earlier than the surrounding matrix. The grains, and/or in situ mineral precipitation (Burne and mineralization of mesoclots may have proceeded in two Moore 1987). They are characterized by having mottled and clotted internal fabrics (Aitken 1967) that distinguish them from laminated stromatolites (e.g., Kennard and D. Tang Á X. Shi (&) James 1986). Opinions regarding the origin of thrombo- School of Earth Sciences and Resources, China University lites, however, are divergent. Earlier study suggested that of Geosciences, Beijing 100083, China the mottled/clotted thrombolitic structures may have e-mail: [email protected] resulted from destruction of laminations in stromatolites by X. Shi metazoans or diagenesis (e.g., Hofmann 1973; Walter and State Key Laboratory of Biogeology and Environmental Heys 1985). Thus, abundant occurrence of thrombolites Geology, Beijing 100083, China since the late Neoproterozoic was often ascribed to the onset of metazoans (e.g., Garrett 1970; Walter and Heys G. Jiang Department of Geoscience, University of Nevada, 1985). A number of subsequent studies, however, showed Las Vegas, NV 89154-4010, USA that primary thrombolites are widespread, not resulting 123 402 Int J Earth Sci (Geol Rundsch) (2013) 102:401–413 from disturbed or bioturbated stromatolites (e.g., Kennard formation (Dupraz et al. 2004, 2009; Benzerara et al. 2010; and James 1986). Some researchers further argued that Spadafora et al. 2010; Perri et al. 2012). mesoclots in thrombolites might have derived from in situ In this paper, we report thrombolites from the Mesopro- mineralization of discrete microbial colonies, predomi- terozoic Wumishan Formation (ca. 1.50–1.45 Ga) in Yes- nantly coccoidal microbes (e.g., Kennard and James 1986; anpo area, Hebei Province, China, with focus on their Burne and Moore 1987; Kahle 2001). Study of modern internal microfabrics and organominerals. Ultrastructures in microbialites suggested that the clotted textures in mesoclots and other microbial remains of thrombolites were thrombolites may form in environments with high pro- studied by optical microscopy and Field Emission Scanning portion of eukaryotes such as metaphytes and metazoans, Electronic Microscope (FESEM). Our study confirms that and they are not specifically related to coccoidal microbes the Mesoproterozoic thrombolites are of biogenic origin and (Feldmann and McKenzie 1998). It has been also empha- their internal microfabrics are transitional between those sized that taphonomic factors, rather than the presence of found in Phanerozoic and Paleoproterozoic thrombolites eukaryotes, may control the formation of thrombolitic (Kah and Grotzinger 1992), similar to those ‘‘hybrid texture in microbialites (Turner et al. 2000; Planavsky and microbialites’’ proposed by Riding (2008, 2010). Ginsburg 2009). Other researchers further pointed out that different microbial communities with particular metabo- lisms may be responsible for the clotted fabrics in modern Geological setting microbial mats (Myshrall et al. 2010; Mobberley et al. 2012). For late Neoproterozoic thrombolites, variable Stratigraphic succession and age constraints microbial groups and environmental conditions have been suggested as the major controls for the clotted fabrics The Mesoproterozoic succession in the North China plat- (Harwood and Sumner 2011). form was deposited in rift and post-rift basins associated with Despite their debatable origin, modern thrombolites the tectonic evolution from the break-up of Columbia to the certainly represent a kind of microbial mat ecosystems assembly of Rodinia. The stratigraphic succession comprises (Myshrall et al. 2010; Jahnert and Collins 2012). Their two large tectono-sedimentary cycles (Fig. 1). The lower ancient analogs, however, have been seldom confirmed to cycle (Jixian Group) is dominated by carbonates, and the be biogenic origin (e.g., Monty 1976; Thompson et al. upper cycle (Qingbaikou Group) is composed of siliciclas- 1990; Soudry and Weissbrod 1995), especially for those tics and black shales. Both the Jixian and Qingbaikou Groups older than late Neoproterozoic (Harwood and Sumner are bounded by regional unconformities. A number of zircon 2011). The oldest thrombolites were reported from the U–Pb SHRIMP ages (Lu and Li 1991; Li et al. 1995, 2010; Paleoproterozoic (ca. 1.92 Ga) strata in NW Canada (Kah Gao et al. 2007, 2008a, b; Lu et al. 2008; Su et al. 2010) from and Grotzinger 1992), but they were interpreted as inor- the succession provide reasonable geochronologic con- ganic sea-floor carbonate precipitates under carbonate straints for the stratigraphic units (Fig. 1). According to the supersaturated and anoxic ocean chemistry (Kah and available age data, the top and base of the Gaoyuzhuang Grotzinger 1992). So far, no thrombolite has been reported Formation are at ca. 1.53 and ca. 1.60 Ga, respectively, and from Mesoproterozoic successions (Grotzinger and James those for the Xiamaling Formation are at ca. 1.30 and ca. 2000; Harwood and Sumner 2011). Therefore, the study of 1.40 Ga (Qiao et al. 2007; Gao et al. 2009, 2010). So far, no Mesoproterozoic thrombolites and their genesis would direct age has been obtained from the Wumishan Formation. provide insights for understanding the interactions between However, stratigraphic relationship and radiometric ages microbial communities and environments in early Earth from underlying and overlying strata roughly constrain history and the relationship between the secular evolution the Wumishan Formation as early Mesoproterozoic (the of ocean chemistry and change in microbial ecosystems. Calymmian Period) between ca. 1.50 and ca. 1.45 Ga (Gao Revealing the genesis and organomineralization mech- et al. 2009). The Changlongshan Formation is likely younger anisms of ancient thrombolites is challenging, because than 1.0 Ga in age (Gao et al. 2009, 2010) and is early mesoclots, the key feature and framework of thrombolites, Neoproterozoic. Therefore, the upper Mesoproterozoic is have been largely preserved as sparitic or micritic car- very likely absent in most parts of the North China platform: bonate with rare recognizable fossils. It is often difficult to the unconformity between the Changlongshan and Xiamal- identify fossilized bacterial remnants and their related ing Formations represents probably a stratigraphic gap organominerals at the microscopic resolution (Soudry and lasting for *300 Ma. Weissbrod 1995). Recent studies indicate that microfabrics The Wumishan Formation is one of the most widespread and organominerals at submicron and nanometer scales Mesoproterozoic lithostratigraphic units in the North China are critical for understanding the microbial activities and platform. Lithologically, it is largely composed of dolo- their interactions with environments during microbialite stone, with abundant siliceous bands and a variety of 123 Int J Earth Sci (Geol Rundsch) (2013) 102:401–413 403 Erathem Formation thins to *1,800 m and unconformably contacts Formation Lithology Age (Ma) and references /System with the underlying sandy dolostone of the upper Fujunshan <520 Ma Jingeryu upper cycle Gaoyuzhuang Formation and the overlying black shale of Changlongshan