Abstract
AbstractThe Luoquan Formation provides a record of the Ediacaran‐Cambrian glaciation in the North China Craton. The sedimentary record is well expressed in the Henan Province along the central China orogen, and includes a rich archive of striated pavements, diamictites, and dropstone‐bearing laminites. A reappraisal of the sedimentological evolution of the Luoquan Formation notes the following features. First, striated pavements with crosscutting striations do not necessarily record multiple phases of glacial (re)advance, but more likely originate through the development of sticky spots in a palaeo‐ice stream setting. The development of obstacles, basal adfreezing, or porosity variations in the subglacial substrate resulted in curvilinear and bifurcating striae, which can superficially be mistaken for crosscutting striae in isolated sections. Second, “massive” diamictites as previously described are in fact commonly weakly stratified, and there is a continuum from dropstone‐bearing rhythmically bedded shales and siltstones, through stratified diamictites to massive diamictites. This continuum is believed indicative of deposition by rain out from debris rich ice for those diamictites with less pronounced stratification, in contrast to the mass flow hypothesis previously suggested. Third, the presence of large‐scale, recumbent folds with associated thrusts is described at the type section. The suite of large‐scale deformation structures, measuring >30 m in amplitude, is sealed by undeformed diamictites. The deformation structures are interpreted to reflect soft‐sediment deformation produced through ice bulldozing. Integrating these observations, it is proposed that the Luoquan Formation was deposited in a large proglacial lake setting, with a range of ice contact to ice distal environments recognized.
Highlights
The Neoproterozoic is arguably most famous for its severe glacial episodes which dominated the Cryogenian interval, and which may have been global in extent (Evans & Raub, 2011; Hoffman, Kaufman, Halverson, & Schrag, 1998; Li, Evans, & Halverson, 2013)
Integrating the observations and interpretations from each of the study sections, a general depositional model is presented for the Luoquan Formation (Figure 10)
It is proposed that the ice masses feeding the Luoquan basin were warm‐based, and given the wide spatial distribution of striated pavements across the Henan Province (Guan et al, 1986), it is proposed that the region was covered by an ice sheet rather than small valley glaciers
Summary
The Neoproterozoic is arguably most famous for its severe glacial episodes which dominated the Cryogenian interval, and which may have been global in extent (Evans & Raub, 2011; Hoffman, Kaufman, Halverson, & Schrag, 1998; Li, Evans, & Halverson, 2013). Hoffman et al, 1998, 2017; Li et al, 2013; Rooney, Strauss, Brandon, & Macdonald, 2015; Zhao & Zheng, 2010; Zhou et al, 2004) Many workers consider these to be discrete global Snowball Earth glaciations, poor global age control allows them to be viewed as diachronous (Spence, Le Heron, & Fairchild, 2016). Given the value of diamictite‐bearing deposits in revealing insights into past climates, including both the severity of glaciations in the context of snowball Earth (Spence et al, 2016), new perspectives and a re‐evaluation of the Luoquan Formation is long overdue. The new work forms the framework for future investigations, across the southern flank of the NCC
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