The mid-Proterozoic strata, widely deposited in the North China, has recorded the real geological history of the earth's evolution in the ancient world from 1.8 Ga to 0.8 Ga (Ying et al., 2011; Tang et al., 2016), especially the widely distributed and well-preserved outcrops of the Mesoproterozoic Wumishan Formation, has reserved abundant natural information including paleogeography, paleoclimate, paleontology, palaeotectonics, paleomagnetism and provide excellent research materials for geologists from all over the world. (Zhang, 1985; Chen et al., 2013; Li et al., 2013; Mei and Maurice, 2013; Tang et al., 2014 and 2015; Shen et al., 2018). The Wumishan Formation is celebrated for its large thickness (the thickest outcrop > 3000 m) of dolomitic and various microbialitic deposits whose main types include various forms (laminated, wavy, cone-shaped, columnar, domal) of stromatolites, thrombolites, and oncolites (Cao, 1991; Mei et al., 2008; Tang et al., 2013b; Lu et al, 2019). Moreover, it contains a variety of different types of soft-sediment deformation structures (SSDS) and microbial mat structures (MMS) which called attention of geoscientists in the last three decades.
Since the earliest record of SSDS in the Wumishan Formation was found in Xishan section which is located on the northwest of Beijing (Song, 1988), more than 15 types of SSDS (e.g., liquefied mixed layers, diapirs, liquefied veins, liquefied convolute beddings, plate-spine breccia structures, thixotropic diapirs, thixotropic wedges, thixotropic veins, mound-and-sag structures, ball-and-pillow structures, loop-beddings, undulated deformations, deformed stromatolites, load casts, flame structures, chrysanthemum structures) were identified in the Wumishan Formation and the major triggering mechanisms are earthquakes and storms (Xie et al., 1997; Zhang et al., 2007; Qiao and Gao, 2007; Qiao and Li, 2009; Ettensohn et al., 2011; Su and Sun, 2011 and 2012; van Loon and Su, 2013; Qiao et al., 2017) which are also the majority perspectives of the triggering mechanisms of SSDS in the world (van Loon, 2009; Shanmugam, 2017). However, with the development and deepening of researches of SSDS, the microbial mats and their related structures have usually been observed associated with the SSDS. Especially in the Proterozoic, the microbial mats could be treated as one key factor for influencing soft-sediment deformation directly, due to they were the only biological agents at that time and their growth could enhance or limit the formation of SSDS (Obermeier, 1996; Harazim et al., 2013). Even though they were not the major mechanism, they definitely can be a special proxy for representing and recording the SSDS in carbonate and siliciclastic rocks (van Loon and Su, 2013; Hill and Corcoran; 2018). Thus, in the Mesoproterozoic Wumishan Formation, the microbial mats have contributed to the morphologies and perhaps formations of some SSDS.
Due to the flourishing of microbial communities (Walter, 1976; Knoll, 2003; Cao and Yuan, 2009), the microbial mats structures (MMS) were well-known in the Precambrian carbonates, but the microbially-induced sedimentary structures (MISS: Noffke et al., 2001), which were formed by the microbes baffled, trapped and bound siliciclastic sediments and formed the various structures (Banerjee et al., 2014; Suarez-Gonzalez et al., 2019), were commonly identified in in siliciclastic-dominated environments (Noffke, 2009; Eriksson et al., 2012; Lan, 2015; Hill et al., 2016), and the typical MISS in the Mesoproterozoic formations include wrinkle structures, levelled ripple marks, organic carbonaceous laminae, microsequences, polygonal sand cracks, polygonal sand crack fills, and gas domes (Lan et al., 2013). It is precisely because the differences of depositional environments, the reports of such structures from carbonate deposits are rare and limited. Thus, we use the term “MMS”, for its broader connotation, to demonstrated the microbial associated problematic structures which we found in the Wumishan Formation.
Despite the earliest report on the loop beddings can be retrospected to Bradley (1931), which was about the lacustrine oil-shale deposits displaying loop beddings from the Eocene Green River Formation, western USA. And also, a large number of records on the loop beddings had been marked on the different periods and lithologies in the following decades, such as the Devonian and Miocene shales, the Paleogene siltstones and mudstones, and the Ordovician carbonates (Gibling et al., 1985; Trewin, 1986; Tian et al., 2006; Yuan et al., 2006). But until Qiao et al., (2007) the first record of the loop bedding in the Wumishan Formation had been reported, additionally, the other records about the loop beddings in the Wumishan Formation were reported only in Qiao and Li (2009) and Su and Sun (2011). The origin of the loop beddings is interpreted as the extensional deformations induced by weak earthquake when the entire laminated soft deposits have not yet been liquefied in a lentic environment (Cavlo et al., 1999; Rodríguez-Pascua et al., 2000). Its cross-sectional view contains closed pseudoconcentric annular layers, and the long axis is parallel to and the short axis is perpendicular to the strata. In addition, the loop beddings usually appear in a chain shape; moreover, the joints present a X-shaped fracture which is caused by tensions.
In this paper, we report a problematic annulate structure, which is identified in alternately “bright and dark” laminites of the Wumishan Formation, revealing the similar morphological features and forming material basis of the loop bedding, whose laminations are composed of dark layers with high organic matters and bright layers of carbonate particles. However, it usually appears alone and keeps a good shape of cross-sectional pseudoconcentric cylinder with no X-shaped fractures on the joints. Meanwhile, it also comprises a core which wasn’t reported in the loop bedding, and its formation is related to the microbial activities. Such as the different mechanisms of SSDS and MMS, one is the “external” force such as earthquake, tsunami and storm, the other one is the “internal” force which is the cohesiveness from itself, the essential difference between the loop bedding and the problematic annulate structure is the mechanism. Therefore, we named this annulate structure as “loopite” and tried to dig into the origin of this structure to determine if it’s a special type of the loop bedding or a new type of MMS. Meanwhile, to study the origin of the loopite will provide new insight into the sedimentation of microbial communities in the Mesoproterozoic formations.