Ediacaran extension along the northern margin of the Yangtze Platform, South China: Constraints from the lithofacies and geochemistry of the Doushantuo Formation

Journal Pre-proof Ediacaran extension along the northern margin of the Yangtze Platform, South China: Constraints from the lithofacies and geochemistry of the Doushantuo Formation Han Wang, Zhiwu Li, Shugen Liu, Bo Ran, Jinmin Song, Jinxi Li, Yuehao Ye, Nan Li PII:

S0264-8172(19)30485-4

DOI:

https://doi.org/10.1016/j.marpetgeo.2019.104056

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JMPG 104056

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Marine and Petroleum Geology

Received Date: 9 July 2019 Revised Date:

17 September 2019

Accepted Date: 21 September 2019

Please cite this article as: Wang, H., Li, Z., Liu, S., Ran, B., Song, J., Li, J., Ye, Y., Li, N., Ediacaran extension along the northern margin of the Yangtze Platform, South China: Constraints from the lithofacies and geochemistry of the Doushantuo Formation, Marine and Petroleum Geology (2019), doi: https://doi.org/10.1016/j.marpetgeo.2019.104056. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.

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Ediacaran extension along the northern margin of the Yangtze Platform, South China:

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Constraints from the lithofacies and geochemistry of the Doushantuo Formation

3 Han Wanga, Zhiwu Lia , Shugen Liua , Bo Rana, Jinmin Songa, Jinxi Lia, Yuehao Yea, Nan Lib

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*

*

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a

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Chengdu, 610059, China

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b

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*Corresponding author.

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E-mail address: [email protected] (Zhiwu Li); [email protected](Shugen Liu)

State Key Laboratory of Oil and Gas Reservoir Geology and Exploration, Chengdu University of Technology,

CNPC Logging Company Limited Technical Center, Xi’an, 710077, China

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Abstract

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The Ediacaran Doushantuo Formation (Fm) of the Yangtze Platform provides excellent

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sedimentary records of the Precambrian tectonic evolution of the South China Block related to the

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final breakup of the Rodinia supercontinent. However, little is known about its sedimentary

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structural evolution on the northern margin of the Yangtze Platform. This paper presents a detailed

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lithofacies description and geochemical analysis on the Doushantuo Fm in the vicinity of

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Chengkou at the northern margin of the Yangtze Platform, in order to understand its tectonic

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setting and paleogeography. Abrupt variations in stratal thickness and lithofacies over a narrow

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area, abundant gravity-driven sedimentary structures, and distributions of phosphorite and

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manganese deposits suggest that both sides of the Chengkou sub-basin were dominated by

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shallow-water to slope environments, while the center of this sub-basin featured a deep-water

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environment. Geochemical indicators reveal a clear influence of hydrothermal fluid on the

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Doushantuo Fm in the center of the Chengkou sub-basin, implying strong synsedimentary

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extensional activity. These characteristics define a southwest-oriented deep-water subsidence zone

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(Chengkou sub-basin) that is approximately perpendicular to the strike of the northern margin of

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the Yangtze Platform. Combined with previous studies, our results indicate that the northern

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margin of the Yangtze Platform experienced significant extension during the Ediacaran period,

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which is suggested to be related to the latest breakup of the Rodinia, resulting in the occurrence of

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alternating uplifts and depressions in a pattern like that of a horst-graben suite. Such widespread

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extensional activity might have affected the whole Yangtze Platform, with sub-basins like 1

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Chengkou extending into its interior, which is of great significance to hydrocarbon exploration.

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Keywords: Lithofacies; Geochemistry; Extension; Doushantuo Formation; Northern margin of the

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Yangtze Platform

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1. Introduction

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The dramatic environmental changes from the Neoproterozoic Marinoan glaciation to the late

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Ediacaran and related biotic evolution are all associated with the fundamental dynamics of the

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evolution of the Earth system during this period (Li et al, 2015; Zhu and Li, 2017), which was

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dominated by the final breakup of the Rodinia supercontinent (Li et al., 2013). The Yangtze

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Platform, as the core of the South China Block, is considered to be one of the best regions in the

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world for investigating the Ediacaran (equivalent to Sinian in china) strata, because of its great

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significance in the reconstruction of the Neoproterozoic global supercontinent, paleogeography

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and ecosystem (Li et al., 1995; Jiang et al., 2003, 2011; Zhu et al., 207; Wang et al., 2014; Wei et

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al., 2018a; Daley et al., 2018). The uniqueness of the Precambrian earth system has aroused great

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interest in the stratigraphic, sedimentological and paleogeographic framework of the Ediacaran

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Doushantuo Formation (Fm) of the Yangtze Platform (Zhu et al., 2003, 2007; Vernhet et al., 2006;

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Vernhet, 2007; Vernhet et al., 2007; Vernhet and Reijmer, 2010; Jiang et al., 2011). It has therefore

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been correlated globally by biostratigraphy, chronostratigraphy, chemostratigraphy, and sequence

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stratigraphy, and its sedimentary succession has been relatively well established (Jiang et al., 2006;

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Zhu et al., 2007; Xiao et al., 2012; Lan et al., 2019). However, there remain disparate

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understandings of the sedimentary environments and tectonic framework of the Ediacaran

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Doushantuo Fm throughout the Yangtze Platform (Zhu et al., 2007; Vernhet and Reijmer, 2010;

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Jiang et al., 2011).

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The South China Block (SCB) was strongly involved in the multiphase breakup of the

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Rodinia supercontinent during the middle to late Neoproterozoic, which was initiated around 820

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Ma and continued until the end of Ediacaran with a weakening trend (Li et al., 2008, 2013; Wang

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et al., 2013). The Yangtze Platform was gradually transformed from a rift area prior to Marinoan

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glaciation to a sag area in the earliest Ediacaran characterized by regionally stable subsidence with

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a wide coverage of carbonate rocks, known as the cap carbonates (Jiang et al., 1996; Wang and Li, 2

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2003; Jiang et al., 2003). This transient tectonic quiescence was likely followed by widespread

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extension throughout the Yangtze Platform through the Ediacaran in response to the final breakup

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of the Rodinia. Based on numerous good-quality sections with a complete sequence and

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exceptionally well-preserved fossils, the stratigraphy, lithofacies and sedimentary environments of

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the Ediacaran Doushantuo Fm in the southern Yangtze Platform have been well documented

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(Wang and Li, 2003; Vernhet et al., 2006; Vernhet and Reijmer, 2010; Zhu et al., 2007; Jiang et al.,

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2011). The Doushantuo Fm there was characterized by an irregular seafloor bathymetry with

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numerous shallow water areas and a deep intra-shelf basin (sub-basin) in the southern Yangtze

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Platform (Vernhet, 2007, Vernhet et al., 2007; Vernhet and Reijmer, 2010). Such unexpectedly

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sharp variations in thickness and lithofacies may relate to the reactivation of underlying inherited

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rift (Wang and Li, 2003; Vernhet, 2007; Jiang et al., 2011). Therefore, it is suggested that there

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were intense syndepositional extensional tectonisms in the southern Yangtze Platform during the

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Ediacaran (Vernhet et al., 2006, Vernhet and Reijmer, 2010; Jiang et al., 2011). However, progress

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in the investigation of the Doushantuo Fm on the northern margin of the Yangtze Platform has

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been limited by a lack of complete sections, as they were severely destroyed by later faulting since

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the late Triassic (Li et al., 2018). From this perspective, the understanding of the tectonic

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evolution and paleogeography of the whole Yangtze Platform during the Ediacaran (Doushantuo

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Fm) remains incomplete.

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On the other hand, the black shales within the Doushantuo Fm are effective hydrocarbon

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source rocks (Liu et al., 2019), which is of great significance for both traditional and shale gas

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exploration in the Sichuan basin and adjacent regions. The Doushantuo Fm was overlain in turn by

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the dolomites of the Ediacaran Dengying Fm as a high-quality reservoir and then by the black

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shales of the early Cambrian Qiongzusi Fm as both source and seal rocks. These three

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stratigraphic units constitute an excellent petroleum system, within which the Anyue giant gas

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field in the central Sichuan basin was recently discovered (Liu et al., 2017b; Shi et al., 2018). The

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Doushantuo Fm shale was also considered to have important contribution to hydrocarbon

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accumulation in the Dengying Fm dolomites (Yang et al., 2018; Liu et al., 2019). The formation of

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such petroleum system was controlled by an extensional feature developed within the Yangtze

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Platform, named as the Mianyang-Changning intracratonic sag (Liu et al., 2013, 2017a) or the

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Deyang-Anyue intra-platform rift(Du et al., 2016; Zhou et al., 2017; Wei et al., 2018). 3

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Nevertheless, the mechanism responsible for this extensional feature is still an open question,

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which, to some extent, limits further hydrocarbon exploration of the deep-seated

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Ediacaran-Cambrian reservoirs in Sichuan basin (Liu et al., 2017a; Yang et al., 2018). Moreover,

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the recent success of shale gas exploration in the Doushantuo Fm in western Hubei Province

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highlighted its importance as a potential target throughout the Yangtze Platform (Wang et al., 2017;

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Wang et al., 2019). However, little is known about controls on the distribution of high-quality

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shales within the Doushantuo Fm.

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For the above two considerations, this study conducted a detailed lithofacies description and

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geochemical analysis of the Doushantuo Fm on the northern margin of the Yangtze Platform. By

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doing this, we can better understand the sedimentary environments and tectonic settings of the

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Doushantuo Fm throughout the Yangtze Platform. Furthermore, synsedimentary tectonism on the

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northern margin of the Yangtze Platform during the Ediacaran and its implications for the

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paleogeographic reconstruction, final Rodinia breakup and hydrocarbon exploration were

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discussed.

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2. Geological background

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The SCB was formed by the amalgamation of the Yangtze and Cathaysia blocks (Fig 1a) during

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the early Neoproterozoic Jinning orogeny (1.0–0.8 Ga), and is an integral part of the Rodinia

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supercontinent (Wang and Li, 2003; Li et al., 2003, 2005). It is generally agreed that the SCB

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experienced intense rifting during the late Neoproterozoic that was associated with the breakup of

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the Rodinia supercontinent due to superplume events (825–750 Ma) and subsequent continental

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rifting and drifting (750–550 Ma), leading to a prolonged Rodinia supercontinent break-up process

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(Li et al., 2003; Li, 2008, 2013).

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The Yangtze Platform, also referred to as the Yangtze Block or Yangtze Craton, is located

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between the Qinling Orogen to the north and the Jiangnan Orogen to the south (Figs. 1a, b), and is

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a stable continental core (crystalline basement) that was finalized also during the early

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Neoproterozoic Jinning orogeny (Wang and Li, 2003). The Precambrian crystalline basement of

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the Yangtze Platform was subsequently overlain by continuous rift-drift successions (820–520 Ma)

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comprising, in younging order, the Banxi Group (Liantuo Fm), Jiangkou Fm, Datangpo Fm,

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Nantuo Fm, Doushantuo Fm, Dengying Fm and Qiongzhusi Fm (Jiang et al., 2003), which

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recorded a complete rifting history of the SCB in response to the breakup of the Rodinia. The 4

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Yangtze Platform itself was characterized by weak extension with some intracratonic sub-basins

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(rifts or sags) in its interior, but strong extension along its margins from the late Neoproterozoic to

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early Cambrian (Liu et al., 2017a, 2017b; Li et al., 2019).

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Based on the analysis of the Neoproterozoic bimodal magmatism (830–750 Ma) and related

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syn-rift deposition, it has been proposed that there were several major rift basins resulting from the

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breakup of Rodinia along the southeastern margin (Nanhua rift), northern margin (Qinling rift)

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and western margin (Kangdian rift) of the Yangtze Platform (Li et al., 2003; Wang and Li, 2003).

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Meanwhile, a series of northeast-trending and approximately east-west-trending minor rifts/sags

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also developed within the Yangtze Platform under such extensional setting (Dong et al., 2013; Gu

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and Wang, 2014). The Cryogenian glacial and interglacial successions (750–635 Ma) including the

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Jiangkou Fm, Datangpo Fm and Nantuo Fm recorded the rift-drift transition (Jiang et al., 2003).

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The Nantuo Formation (ca. 654–635 Ma; Marinoan glaciation) marks the latest stage of rifting,

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and is characterized by abrupt lateral variations in thickness and widespread subaerial exposure

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unconformity on its top (Wang and Li, 2003; Jiang et al., 1996). The first transgression linked all

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the individual sub-basins of the Yangtze Platform, which is represented by widespread deposits of

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Ediacaran Doushantuo Fm (Ca. 635–551 Ma) that is predominantly composed of dolomites and

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black shales, and thus suggested a change in the subsidence regime of the basin (Jiang et al., 1996,

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2003; Cui et al., 2014; Condon et al., 2005). The widespread occurrence of shallow-marine

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carbonate rocks in the overlying Ediacaran Dengying Fm (Ca. 551–541 Ma) symbolized a

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relatively stable platform environment (Jiang et al., 1996, 2003; Condon et al., 2005; Chen et al.,

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2015). Therefore, the transition from predominantly mechanical, locally-compensated subsidence

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(rift stage) to thermal, regionally-compensated subsidence (post-rift to sag stage) likely

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corresponds to the glacial deposits of the Nantuo Fm (Jiang et al., 1996; Wang and Li, 2003; Jiang

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et al., 2003). However, the extension throughout the Yangtze Platform did not come to an end,

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albeit not as strong as before. This can be demonstrated by many previous studies on the southern

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Yangtze Platform (e.g., Vernhet et al., 2006; Vernhet et al., 2007; Vernhet and Reijmer, 2010; Jiang

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et al., 2011) and several recent articles proposing that there were a series of nearly

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south-north-trending or northeast-trending weak extensional sub-basins on the north margin of the

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Yangtze Platform during the Ediacaran to early Cambrian, which could relate to the final breakup

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of the Rodinia supercontinent (Yang et al., 2016, 2018; Hou et al., 2017; Zhao et al., 2017; Li et al., 5

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2019).

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The northern margin of the Yangtze Platform extends from west to east over 1500 km along

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the southern edge of the Qinling-Dabie Orogen. Our study area involves its western part that is

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usually referred to as the Upper Yangtze Platform (Fig. 1). Following the long-term passive

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continental margin with the opening of small oceanic basin (the Mianlue Oceanian of paleozoic)

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from the late Neoproterozoic to early Triassic, the northern margin of the Yangtze Platform was

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strongly involved in the Mesozoic fold-and-thrust deformation caused by continuous convergence

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between the South China and North China blocks, which gave birth to the Qinling Orogen (Zhang

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et al., 2001). As a result, a thrust system (the Daba Shan thrust system) and a foreland basin (the

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Sichuan basin) were successively generated along the southern flank of the Qinling Orogen (Fig.

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1b). The present northern margin of the Yangtze Platform is separated from the Qinling Orogen by

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the large-scale Chengkou fault, which also divides the Daba Shan thrust system (DBS) into the

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northern Daba Shan thrust-nappe belt (NDBS) and the southern Daba Shan fold-and-thrust belt

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(SDBS). The Chengkou fault has been proposed as the tectono-stratigraphic boundary between the

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Yangtze Platform and the South Qinling rift basin (Fig. 1b; Zhang et al., 2001, 2010; Wang et al.,

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2011; Dong and Santosh, 2016; Li et al., 2018) from the late Neoproterozoic to early Paleozoic.

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This implies that the Chengkou fault was a north-dipping synsedimentary normal fault prior to

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structural inversion. During the middle to late Triassic, NE-directed compression caused by the

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collision of the North and South China blocks resulted in the Chengkou fault being thrust

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southwards over the Yangtze block, forming a series of southwest-vergence thrust nappe structures

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(Fig. 1b; He et al., 2011; Li et al., 2018). During the Middle Jurassic to Early Cretaceous, this belt

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was reactivated by intracontinental orogeny characterized by dextral transpressional thrusting, so

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that it curved further southwestwards and was emplaced at its present location (Li et al., 2018).

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This curved belt is limited by two Precambrian domes at its ends, that is, the Hannan-Micang Shan

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dome to the west and the Shennongjia-Huangling dome to the east (Fig. 1b). For the convenience

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of discussion, we divided the northern margin of the Yangtze Platform in the study area into three

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segments, i.e., the northwestern area to the northwest of Maliuba Town, the southeastern area to

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the east of Dong’an Town and the intervening central area (Fig. 1).

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Fig. 1. (a) Simplified map of China showing the location of the northern margin of the Yangtze Platform (blue box

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represents the study area), modified from Jiang et al. (2011). (b) Simplified geological map showing the exposures

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of the Ediacaran strata and main tectonic units of the northern margin of the Yangtze Platform (modified from Li et

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al., 2019). Triangles (1–8) show the locations of the major sections used for lithofacies and geochemical analysis

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of the Ediacaran Doushantuo Fm, among them, all the sections except section 5 (Wang, 2015) and section 7

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(BGMRSP, 1974) are comes from this study; (c) Simplified stratigraphic units with major marker beds and age

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constraints from the Cryogenian Nantuo Fm (NT), Ediacaran Doushantuo Fm (DST) and DengyingFm (DY) to

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early Cambrian QiongzhusiFm (QZS) in the Yangtze Platform, modified from Jiang et al. (2011) and Cui et al.

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(2016). Radiometric ages are from Condon et al. (2005), Xing et al. (2018) and Chen et al. (2015).

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The Cryogenian to early Paleozoic rocks on the northern margin of the Yangtze Platform

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outcrop as a serial of thrust sheets along the Daba Shan thrust belt or around the Precambrian

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cores of the Hannan-Micang Shan and Shennongjia-Huangling domes (Fig. 1b). The stratigraphic

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units involved in this study include the Cryogenian Nantuo Fm, Ediacaran Doushantuo Fm and

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Dengying Fm, and early Cambrian Qiongzhusi Fm (Fig. 1c). The Nantuo Fm is typically a

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sequence of glacial tuffaceous siliciclastic rocks with thicknesses varying from > 800 m in the

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vicinity of Chengkou County to < 300 m to the northwest and southeast further, and there is a

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regionally subaerial exposure unconformity on its top (BGMRSP, 1974; Wang et al., 2019). The 7

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Ediacaran Doushantuo Fm was informally divided into four distinct members (Members I-IV) in

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the Yangtze Platform (Fig. 1c) (Zhou and Xiao, 2007). Overlying the Cryogenian Nantuo Fm

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tillites is known as the cap dolomite (Member I), which has similar lithological characteristics

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over the Yangtze Platform so that it functions as a lithostratigraphic marker (Jiang et al., 2006,

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2011). Member II is predominantly composed of black organic-rich shale intercalated with

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thin-bedded muddy dolomite and with minor chert, and the overlying Member III consists mainly

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of thin-medium bedded dolomite and limestone (Vernhet and Reijmer, 2010). Member IV is

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similar to Member II in lithology but contains more dolomites in its upper part and ends with

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manganese- and phosphorite-bearing rocks. Thus, the Doushantuo Fm combine with the the lower

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Dengying are made up of two upward-shallowing cycles following the cap dolomite: the lower

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one consists of Member II and III; the upper one is composed of Memver IV and the Ediacaran

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Dengying Fm. Among them, the Dengying Fm generally overlay conformably on the Doushantuo

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Fm except on some paleohighs, and is marked by the first occurrence of light grey thin-medium

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bedded dolomite in low-energy environments (Jiang et al., 2011) or thick bedded to massive

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dolomites in high-energy environments. The Dengying Fm dolomites in the platform environment

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(equivalent to Liuchapo Formation in the basin setting) underwent extensive subaerial exposure

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before being overlain by the early Cambrian Qiongzhusi Fm that is predominantly composed of

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black organic-rich shale with minor siltstone. It should be noted that the boundary between the

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Doushantuo Fm and the overlying Dengying Fm is the most important regional lithostratigraphic

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marker, which is traceable from the shallow water platform to the deep basin and provides

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important tie-points for correlation between the sections explored in this study.

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3. Lithofacies and depositional environments of the Doushantuo Fm

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Totally six sections were investigated in this study besides two from previous publications to

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explore the lithofacies variation of the Ediacaran Doushantuo Fm along the northern margin of the

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Yangtze Platform (Fig. 1b). Most of them are located in the SDBS and along the Chengkou fault.

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It should be pointed out that there are significant sedimentary variations for the Doushantuo Fm

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from different sections across the Chengkou fault that separated the Yangtze Platform from the

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South Qinling rift basin. For instance, for the Kangjiaping section (Fig. 1b) which is located to the

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south of the Chengkou fault, the Doushantuo Fm is dominated by dolomite and shale with an

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apparent four-member structure (BGMRSP, 1974). However, for the Heyu section (Fig. 1b) just 8

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north of the Chengkou fault and not far from the Kangjiaping section, it is dominated by shale and

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chert without the typical four-member structure and thus fails to correlate with that in the Yangtze

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Platform. In this sense, the inter-section lithofacies correlation of the Doushantuo Fm was aimed

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largely at those sections to the south of the Chengkou fault. On the other hand, the lower part of

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the Doushantuo Fm in the vicinity of Chengkou County is usually incomplete due to severe

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destruction by thrusting. We therefore focused our lithofacies analysis largely on the Member IV

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of the Ediacaran Doushantuo Fm.

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3.1. Lithofacies observations

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Five out of six sections along the Chengkou fault were surveyed in detail to disclose the

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thickness, lithology and depositional sequence of the Ediacaran Doushantuo Fm on the northern

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margin of the Yangtze Platform (Fig. 2). They are the Fucheng and Lianghekou sections in the

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northwestern area, the Dazhu and Mingyue sections in the central area, and the Gaozhu section in

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the southeastern area (Figs, 1b and 2). The Fucheng and Mingyue sections were selected for

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correlation between the margin of the northern Yangtze Platform and its interior extension

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respectively. For each section, detailed petrographic analysis through thin section examination

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integrated with field-based sedimentary structure observation was employed to constrain

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lithofacies. This also lays the foundation for sedimentary environment interpretation and

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geochemistry analysis that will be discussed later.

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Fig. 2. Stratigraphic and lithofacies correlation for the Ediacaran Doushantuo Fm from different sections (see Fig.

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1b for locations) on the northern margin of the Yangtze Platform. Note that the lateral distance is not to scale.

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Except for the Gaoyan section (BGMRSP, 1974; Wang, 2015; Chen et al., 2017), all data is from this study.

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Stratigraphic subdivision and correlation are based on Jiang et al. (2011).

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3.1.1. Fucheng section

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The Fucheng section is located at the southern flank of the Micang Shan dome that represents

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the interior of the northern Yangtze Platform. The Micang Shan dome behaved as a paleo-high

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during the late Neoproterozoic, and little or no Cryogenian to early Ediacaran deposition occurred

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in this area. Accordingly, the Doushantuo Fm of the Fucheng section unconformly overlay

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Precambrian basement metamorphic rocks and is only about 12 m thick. It is predominantly

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composed of alternating light grey thin-bedded sandy dolomites and dolomitic granule-bearing

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sandstones. The lithology graduates upwards into medium-bedded to massive dolomites without

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perceptible terrigenous clasts, which are typical of the Ediacaran Dengying Fm. Planar

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stratification of upper flow regime is typical of dolomitic sandstones that often contain subangular

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dolomitic intraclasts derived from intercalated dolomite strips. Obviously, the Doushantuo Fm 10

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here does not have the typical four-member structure and is likely incomplete in stratigraphy. It is

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similar to the lithofacies association of Gaoshi-1 Well and Nuji Well in the Yangtze Platform

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(Wang et al., 2019), and suggests a relatively stable shallow water environment.

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3.1.2. Lianghekou section

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The Lianghekou section is located at the northwestern SDBS and about 75 km southeast of

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the Fucheng section, where the Doushantuo Fm is about 125 m thick and shares the four-member

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structure but with varying lithology. The Doushantuo Fm conformably overlies the Nantuo Fm

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which is dominated by greenish gray tillites. The cap dolomite (Member I) is about 6 m thick and

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is mainly composed of brownish-red thin-bedded muddy dolomite with mud content increasing

273

upwards. Overlying the cap dolomite is an approximately 37 m-thick interval (Member II)

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containing two parts. The lower part is dominated by grayish thin-medium bedded siltstone and

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fine-grained sandstone. The upper part consists of a serial of rhythmic layers, each with a similar

276

thickness of a dozen centimeters (Fig. 3a). Each rhythmic layer is a combination of grayish

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coarse- to medium-grained sandstone with normal graded bedding at the bottom, grey fine-grained

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sandstone and siltstone in the middle, and dark gray mudstone at the top, exhibiting a fining

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upward sequence (Fig. 3b). Abrupt contact always occurred between two adjacent layers, and

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scoured surfaces can also be observed at the base of each layer. These observations suggest each

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rhythmic layer has a sequence similar to the Bouma sequence (Fig. 3b), implying they were

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deposited by turbidity currents (Zhang, 2014). Above the turbidite interval is an approximately 71

283

m thick interval (Member III) of dark gray laminated shale and siltstone interbedded with

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fine-grained sandstone. Hummocky cross-stratification is typical of this interval and generally has

285

a wave length of a dozen centimeters, probably as a result of tempestite deposition. It should be

286

noted that the Members II and III of the Doushantuo Fm in the Lianghekou section are

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predominantly clastic rocks due to the proximity to the Hannan-Micang Shan paleo-high to the

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northwest, resulting in difficulty in correlation with other sections. The Member IV is about 11 m

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thick and dominated by light grey thin-bedded sandy dolomites with terrigenous clast content

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increasing upwards, intercalated with black bedded and banded phosphorite, sandy intraclastic

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phosphorite and phosphoritic sandstone. Its base shows a sharp contrast in lithology and an abrupt

292

contact with the underlying Member III (Fig. 3c), and its top exhibits a transition from sandy

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dolomites to the light gray medium-bedded to massive oolite dolomites of the Dengying Fm. 11

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Abundant gravity-driven and storm-driven sedimentary structures are typical of this interval. In

295

the lower part, the thin-bedded dolomite is characterized by meter-scale slump folds as a result of

296

soft sediment deformation (Fig. 3d), or locally formed the gravity slide collapse block which is a

297

mixture of many kinds of breccia-like fragments (Fig. 3e). The intercalated phosphoritic rocks

298

occur throughout this interval and are usually present as a mixture of terrigenous clasts, angular

299

dolomite fragments, and phosphoritic granules and pebbles. The phosphatic pebbles are oval or

300

long strip in shape, poorly sorted and locally have a radial arrangement (Fig. 3f). While the

301

terrigenous clasts, generally as matrix, are largely well-sorted, sub-rounded, coarse- to

302

medium-grained quartz sands (Fig. 3g). Scoured surface at the base of sandy intraclastic

303

phosphorite can also be observed in the upper part (Fig. 3h). All these sedimentary structures

304

robustly suggest that the Lianghekou section was dominated by slope environment during the

305

deposition of the Doushantuo Fm.

306

12

307 308

Fig. 3. Photographs of typical sedimentary structures and lithofacies of the Lianghekou section. (a) Rhythmic

309

layers of normal grading from coarse sandstone to mudstone. The dotted box represents Fig b; (b) High

310

magnification of classical Bouma turbidite sequences, Ta-to Td represent the massive sandstone with scour surface

311

unit, parallel lamination, ripple cross-lamination and horizontal lamination, respectively; (c) Lithofacies abrupt

312

variation surface between Members IV and III of the Doushantuo Fm; (d) Thin-bedded dolomite with slump folds; 13

313

(e) Gravity slide collapse block, orange dash line represents the dolomite, white dash lines represent phosphorite

314

pebbles and the yellow dash line represent the interbedding of phosphorite and dolomite; (f) The phosphorite

315

pebbles is arranged in a radial pattern; (g) Mixed sedimentation of oval/long strip-shaped phosphatic pebbles and

316

terrigenous quartz; (h) Scoured/erosion surface between the phosphorite pebbles and underlying sandy dolomite.

317 318

3.1.3. Dazhu section

319

The Dazhu section is located at the central NDBS and about 2 km north of the Chengkou

320

fault. The Doushantuo Fm at this site is over 500 m thick (BGMRSP, 1974), which represents its

321

maximum deposition thickness in the study area in spite of significant repetition of strata by

322

folding. The Members I, II, and III of the Doushantuo Fm were unfortunately not outcropped due

323

to civil construction. The partially exposed Member IV is about 140 m thick and predominantly

324

consists of black organic-rich shale intercalated with muddy and silty dolomites and bedded chert.

325

The uppermost part of Member IV is likely lost due to thrust faulting on our section, and thus an

326

abrupt contact is present between the Doushantuo Fm and the overlying Dengying Fm that is

327

dominated by grey medium- to thin-bedded dolomitic limestone. Nonetheless, it is generally

328

believed that there is a transitional relationship between them in this area (BGMRSP, 1974) and no

329

subaerial exposure unconformity has been reported so far. It should be noted that albeit the Dazhu

330

section is located north of (but very close to) the Chengkou fault at present, the lithology and

331

succession of the Doushantuo and Dengying Fms here are similar to those of Mingyue section

332

discussed below. This means that the Dazhu section was situated at the transition zone between the

333

Yangtze Platform and the South Qinling rift basin but much closer to the Yangtze Platform during

334

the Ediacaran.

335

3.1.4. Mingyue section

336

The Mingyue section is located at the central SDBS and about 20 km southeast of the Dazhu

337

section, where the Doushantuo Fm is relatively complete in succession and about 193 m thick.

338

The typical cap dolomite is absent at this site, and replaced by dolomitic limestone interbedded

339

with black mudstone directly conformably overlies the Nantuo Fm tuffaceous tillites. The Member

340

II is about 49 m thick and is dominated by dark organic-rich shale intercalated with dark gray

341

thin-bedded siltstone, silty and muddy dolomite, muddy limestone and bedded chert. Silicification

342

is common throughout this interval. Overlying Member II is a nearly 29 m-thick interval (Member 14

343

III) of interbedded light grey thin- bedded dolomite and dark gray thin-bedded limestone. Member

344

IV is about 113 m thick and contains two lithological units. The lower-middle unit is largely

345

composed of black organic-rich shale and dark gray thin-bedded muddy dolomite with

346

intercalations of black bedded chert, showing a sharp contrast in lithology with the underlying

347

Member III (Fig. 4a). Its lithology is similar to that of Member II but with more dolomite

348

intercalation that increase upwards. Apart from chert beds, silicification is also widely present in

349

this unit as diffused silica or scattered authigenic euhedral quartz crystals in dolomite (Fig. 4b)

350

The upper unit is an about 19.5 m thick interval of dark greenish gray thin-bedded muddy

351

dolomite and shows a transitional relationship with the overlying Dengying Fm that is dominated

352

by interbedded gray thin-bedded dolomite and limestone. However, closer inspection reveals that

353

these interbedded dolomites and limestones actually constitute a serial of rhythmic layers with

354

Bouma sequence. This can be clearly observed both on outcrops and in thin section from other

355

sites in the vicinity, which exhibit obvious normal graded bedding and locally basal scoured

356

surface and cross bedding (Fig. 4c). Though such features were not observed in the dolomites of

357

the Doushantuo Fm of the Mingyue section because of extremely small grain size, the occurrence

358

of carbonate clasts in dolomitic mudstone suggests a mechanical deposition (Fig. 4d). Thus, the

359

coeval of black shale, bedded chert and carbonate turbidite jointly suggests the Mingyue section

360

was dominated by deep water environment during the Ediacaran.

361

3.1.5. Gaozhu section

362

The Gaozhu section is located at the southeastern SDBS and about 80 km southeast of the

363

Mingyue section, where a continuous succession containing the Cryogenian Nantuo Fm, the

364

Ediacaran Doushantuo Fm and Dengying Fm is well exposed. The Doushantuo Fm is about 58 m

365

thick and shows the typical four member structure, unconformably overlying the brownish red and

366

greenish gray clastic rocks of Nantuo Fm. The cap dolomite is about 3 m thick and composed of

367

dark gray thin- to medium-bedded dolomite and silty dolomite. Above the cap carbonate is a 26

368

m-thick sequence (Member II) of black shale interbedded with dark gray thin-bedded muddy

369

dolomite and bedded or banded phosphrite (Fig. 3e). Member III is approximately 20 m thick and

370

largely composed of light grey thin- to medium-bedded dolomites with abundant decimeter to

371

meter-scale hummocky and swaley cross-stratification (Fig. 3f), indicating a storm-dominated

372

depositional environment. Member IV is about 9 m thick and represented by the occurrence of 15

373

gravity-driven sliding of soft dolomitic sediments in its lower part. Its upper part is composed of

374

dark gray dolomitic mudstone intercalated with grey thin-bedded dolomite, and shows a

375

transitional relationship with the overlying light grey thin-bedded dolomites with a few

376

gravity-driven sliding structures of the Dengying Fm. The base of this interval is rugged and cuts

377

into the underlying thin- to medium-bedded dolomites of Member III (Fig. 4g). Those dolomitic

378

sediments are present as decimeter- to meter-sized olistoliths embedded in dolomitic mudstone as

379

matrix (Fig. 4g). The olistoliths are subangular to subrounded in shape and themselves contain a

380

lot of angular fragments of dark grey dolomitic mudstone (Fig. 4h) that were likely produced by

381

storm-related activity. The dolomitic mudstones surrounding these olistoliths were also deformed

382

following their outline (Fig. 4h). Taken together, these structures suggest that the Gaozhu section

383

was dominated by slope environment during the deposition of the Ediacaran Doushantuo Fm.

16

384 385

Fig. 4. Photographs of typical sedimentary structures and lithofacies of Mingyue and Gaozhu sections. (a)

386

Macroscopic photographs of lithological changes between the member III and member IV, Mingyue section; (b)

387

The authigenic quartz crystals in dolomitic mudstone from the member IV of Doushantuo Fm, Mingyue section; (c)

388

The classical Bouma (Ta-Td) turbidite sequences, which comes from the Dengying Fm, Mingyue section; (d) The

389

carbonate clasts in the organic-rich mudstone, and these clasts have the characteristics of being transported. The 17

390

photomicrograph comes from the member IV of Doushantuo Fm, Mingyue section; (e) Black thin Muddy dolomite

391

intercalations with banded phosphatics from the member II of Doushantuo Fm, Gaozhu section; (f) Hummocky

392

cross-stratified beds on the member III of Doushantuo Fm, Gaozhu section; (g) Sliding erosive surface between the

393

underlying thin-bedded dolomite (Member III) and overlying debris (Member IV), come from the boundary of

394

Members III/IV of the Doushantuo Fm, Gaozhu section; (h) The soft sediment deformation of black mudstone

395

resulting from the overlying allochthonous carbonate olistoliths. The fig. h is the left part of the fig. g, and the

396

photo comes from the top of the Doushantuo Fm, Gaozhu section.

397 398

3.2 Manganese and phosphorus deposits distribution

399

The coeval occurrence of manganese and phosphorus deposits largely in the upper part of the

400

Doushantuo Fm on the northern margin of the Yangtze Platform provides favorable information

401

for its depositional environment interpretation.

402

Mn2+ ions are generally stable in aqueous solutions within reducing conditions below the

403

oxic-anoxic (O2/H2S) interface in deep basin environments, but prone to precipitate near the

404

redox-reduce interface as ferromanganese (Mn4+) oxyhydroxide (Force and Cannon, 1988;

405

Burdige and Kepkay, 1983; Cronan et al., 1991). Elevated Mn concentrations in sediments have

406

also been linked to increased submarine hydrothermal activity (e.g., Corbin et al., 2000; Jach and

407

Dudek, 2005). Therefore, abundant manganese can be formed at the transitional zone of

408

oxidation-reduction in slope environments when affected by distal hydrothermal vents, which is

409

commonly known as the “bathtub ring model” (Force and Cannon, 1988; Frakes and Bolton,

410

1992). In addition, manganese is also formed near hydrothermal vents at the center of anoxic

411

deep-water basins which is predominantly influenced by episodic oxidation events with

412

oxygenated density flows, representing “episodic ventilation” (Fig. 5a; Huckriede and Meischner,

413

1996; Yu et al., 2016). However, phosphorus deposits are mainly related to the biogeochemical

414

processes in the oxygenated shallow depositional environment (Glenn et al., 1994; Muscente et al.,

415

2015; Cui et al., 2016). With the enhance of chemical weathering and upwelling currents, the input

416

of phosphorus gradually increased in the proximal marine setting, and stimulated high

417

productivity and followed by precipitation of oxide particulates, apatite and phosphatic skeletons

418

on the seafloor (Mort et al., 2007; Halverson et al., 2007; Filippelli, 2001; Cui et al., 2016;

419

Muscente et al., 2015; Compton and Bergh, 2016). As the Lianghekou section, which is close to 18

420

the paleo-high (oldland), it is characterized by mixed sedimentation of phosphatic pebbles

421

(intraclasts) and terrigenous.

422

In the study area, manganese and phosphorus are predominantly enriched at the top of the

423

Doushantuo Fm (Fan et al., 1999; Wang et al., 1999; Zhu et al., 2018). The most interesting

424

phenomenon is that most of the manganese deposits are concentrated in the central part of the

425

NW-SE-trending northern margin of the Yangtze Plaform along the Chengkou fault (Tan and Wan,

426

2006), while phosphorus deposits are predominantly located at both sides (see Fig. 5 for exact

427

locations). What’s more, geochemical analysis on typical manganese deposits at Gaoyan Town

428

near Chengkou County revealed that they were formed near the redox-reduce interface and

429

significantly affected by hydrothermal activity (Wang, 2015; Chen et al, 2017).

430

Therefore, it is reasonably inferred that the manganese deposits in the central area near

431

Chengkou were formed in a relatively lower-slope to deep-water environment influenced by

432

hydrothermal activity. However, the phosphorus deposits at both sides were formed in a relatively

433

shallow-water to upper-slope environment. Such a model has been proposed for phosphogenesis

434

in the Doushantuo Fm at the southern margin of the Yangtze Platform (Muscente et al., 2015; Cui

435

et al., 2016). This inference is also in agreement with our lithofacies observations described above

436

and definitely contributes to our depositional environment interpretation below.

437 438

Fig. 5. (a) Schematic map of mineral distribution, modified from Tan and Wan (2006). (b) Inferred metallogenesis

439

model of Mn-P deposits in the Ediacaran Doushantuo formation along the northern margin of the Yangtze Platform,

440

modified from Force and Cannon (1988) and Huckriede and Meischner (1996). See Fig. 5a for the mineral

441

locations.

442 443 444

3.3 Depositional environment interpretation Significant variations in thickness, lithofacies and Mn- and P-deposits distribution of the 19

445

Doushantuo Fm along the northern margin of the Yangtze Platform imply it was developed at

446

diverse environments. Three typical environments, i.e., shallow-water platform environment,

447

slope environment and deep-water basin environment were recognized based on the following

448

considerations: (1) the thickness shows a dramatic change and ranges from >300 m in the central

449

area to <50 m in the northwestern and southwestern areas, or the succession is partially or even

450

entirely missing at local paleo-highs; (2) the lithology exhibits a sharp shift from black

451

organic-rich shale dominant in the central area to carbonate-rich rocks spread on both sides; (3)

452

abundant gravity-driven and/or storm-related sedimentary structures are common in the

453

northwestern and southeastern areas; and (4) manganese is mainly enriched in the central area,

454

while phosphorite is predominantly deposited on both sides. Taken lithofacies marks and

455

succession completeness into account, the correlating and differentiating of depositional

456

environments are largely focused on the Member IV of the Doushantuo Fm as below.

457

3.3.1. Shallow-water platform environment

458

The shallow-water platform environment is represented by the Fucheng section in the

459

northwestern area close to the Hannan-Micang Shan paleo-high (oldland). With increasing

460

distance away from the center of the Hannan-Micang Shan paleo-high, the Doushantuo Fm in this

461

area show a gradual increase in thickness and a decrease in coarse-grained siliciclastic content,

462

implying there was locally no deposition of Doushantuo Fm near the core of oldland (BGMRSP,

463

1974). The Doushantuo Fm of the Fucheng section is dominated by thinly interbedded dolomitic

464

and siliciclastic rocks, and is greatly influenced by high siliciclastic influx due to its proximity to

465

oldland. To the west and north further, the dolomitic rocks are usually missing and replaced by

466

siliciclastic rocks, and subaerial exposure unconformity is often present at the top, indicative of a

467

shallow-water environment. Planar stratification produced from the upper flow regime is

468

widespread throughout the Doushantuo Fm of the Fucheng section, and intercalary dolomitic

469

strips were often broken into angular fragments or subangular pebbles that were mixed and

470

deposited with coarse siliciclastic grains. This means that the Doushantuo Fm in this area was

471

deposited in a shallow-water high-energy environment, likely a tidal shoreline (Tamura and

472

Masuda, 2003).

473

3.3.2. Slope environments

474

Slope deposition is the most prominent feature of the Doushantuo Fm in the study area and 20

475

diagnostic lithofacies indicator for environment reconstruction, and can be further divided into

476

upper and lower slopes.

477

The upper slope environment is dominated by abundant gravity-driven and/or storm-related

478

sediments and represented by the Lianghekou and Gaozhu sections in the northwestern and

479

southeastern areas, respectively (see Fig. 1b for locations). Meter-scale slump folds in the

480

thin-bedded dolomites of the Member IV of the Doushantuo Fm at Lianghekou (Fig. 3d) are

481

typical of gravity-driven sliding structures in the upper slope environment (Krajewski et al., 2014).

482

The mixed deposition of phosphatic pebbles with terrigenous siliciclasts and large angular to

483

subangular dolomitic fragments (Figs. 3e, g) implies that they were originally formed in a

484

relatively shallow-water environment and transported to the upper slope environment where

485

thin-bedded dolomites were deposited. Chaotic and radial arrangement of bad-sorted phosphatic

486

pebbles (Fig. 3f) also indicates they were severely influenced by high-energy storm activity

487

typical of upper slope environment. Similar slope deposition was also found in the Member IV of

488

the Doushantuo Fm at Gaozhu in the southeastern area, which is characterized by light gray

489

allochthonous dolomitic olistoliths of decimeter- to meter-scale embedded in dark gray dolomitic

490

mudstone as a background rock (Fig. 4g). These olistoliths themselves are brecciated rocks

491

containing angular fragments of both light gray dolomite and dark gray dolomitic mudstone,

492

which are in chaotic arrangement (Fig. 4h), implying they were likely subjected to storm-related

493

reworking. The brecciated rocks ware then re-transported and re-deposited in background

494

mudstone by gravity-driven sliding in upper slope environment (Figs. 4g, h). The background

495

mudstone was simultaneously subjected to soft sediment deformation (Fig. 4h), likely indicative

496

of larger water depth at Gaozhu than that at Lianghekou. Apart from those characteristic

497

sedimentary structures, the occurrence of phosphatic deposits in the Member IV of the

498

Doushantuo Fm at Lianghekou and Member II at Gaozhu (Fig. 5) also suggests a relative

499

shallow-water upper slope environment.

500

Owing to a lack of coarse-grained sediments, the lower slope environment is not so easy to

501

be recognized as the upper slope environment. Nonetheless, high black organic-shale

502

concentration and the occurrence of rhodochrosite deposit in the Member IV of the Doushantuo

503

Fm at Gaoyan (Wang et al, 2015; Chen et al, 2017) give a strong signal of relatively deep-water

504

environment. There was a similar environment at Mingyue but with larger water depth, which can 21

505

be illustrated by the coexistence of thinly interbedded black organic-rich shale, bedded chert and

506

muddy dolomite (Fig.4a). Most importantly, the muddy dolomites and dolomitic shales contains

507

silt- to fine sand-sized dolomitic detritus (Fig. 4d), which, together with the typical Bouma

508

sequence in the overlying Dengying Fm (Fig. 4c), implies that they are actually calciturbidites

509

formed in the lower to basal slope environment or even basin environment.

510

3.3.3. Deep-water basin environments

511

Deep-water basin is generally characterized by low-energy suspension-settling sedimentation

512

in an anoxic/reducing depositional environment (Flügel 2004; Jiang et al., 2011). This kind of

513

environment is limited in the study area for the Member IV of the Doushantuo Fm and can be

514

represented by the Dazhu section in the central area. At Dazhu, the predominance of thick black

515

organic-rich shale containing horizontal lamination produced from lower flow regime, the

516

occurrence of intercalary bedded chert, and the absence of perceivable carbonate sediment jointly

517

indicate a low-energy, anoxic and deep-water basin environment for the Member IV of the

518

Doushantuo Fm.

519

3.3.4. Summary

520

The correlating of depositional succession and environment above makes it possible to reveal

521

the tempo-spatial variability of paleogeography for the Doushantuo Fm. Spatially, the thickness,

522

lithofacies and depositional environment of the Doushantuo Fm vary significantly from northwest

523

to southeast along the trend of the northern margin of the Yangtze platform (Fig. 6). In the

524

northwestern area west of Lianghekou, the Doushantuo Fm has minimum thickness of <50 m

525

generally or is entirely missing, and is composed of mixed deposition of coarse-grained

526

siliciclastic rocks and dolomite typical of shallow-water platform environment. To the southeast, it

527

gradually thickens to >50 m but <130 m, and is characterized by abundant gravity-driven and

528

storm-related deposition and phosphatic deposit in upper slope environment, such as that at

529

Lianghekou. To the southeast further in the central area, the Doushantuo Fm is dominated by black

530

organic-rich shale with minor bedded chert, calciturbidite and manganese deposit representative of

531

deep-water lower slope and basin environments. Its thickness rapidly increases to over 300 m at

532

Dazhu and sharply decreases to approximately 50 m at Gaoyan. In the southeastern area east of

533

Dong’an, the Doushantuo Fm again features gravity-driven and storm-related deposition and

534

phosphatic deposit in the upper slope environment similar to that at Lianghekou, and has a 22

535

thickness of around 50 m or less. Thus, the depositional environments for the Doushantuo Fm

536

along the northern margin of the Yangtze Platform laterally change from shallow-water platform

537

and upper slope environments in the northwestern area to deep-water lower slope and basin

538

environments in the central area and then to a relatively shallow-water upper slope environment

539

again in the southeastern area. Such paleogeographic configuration suggests a pattern of

540

alternating uplift and subsidence zones in paleogeomorphology along the northern Yangtze

541

Platform margin. Violent variations in thickness and lithofacies in a relative narrow area, for

542

example between the Mingyue and Gaoyan sections, imply that the Doushantuo Fm was deposited

543

in a horst/graben setting that oriented perpendicular to the trend of the northern Yangtze Platform

544

margin. A graben-related subsidence zone was accordingly generated in the central area near

545

Chengkou, which we referred to as the Chengkou sub-basin. Abundant gravity-driven sedimentary

546

structures in the slope environments on both sides of the Chengkou sub-basin indicate that there

547

was significant synsedimentary normal faulting associated with regional extensional activity (e.g.,

548

Montenat et al. 2007; Krajewski et al. 2014).

549

550 551

Fig. 6. Schematic diagram illustrating the tectonic-sedimentary model. a. Schematic map of the lithofacies

552

paleogeography of the Ediacaran Doushantuo Fm, modified from Yang et al. (2016), Jiang et al. (2011) and Zhu et

553

al. (2007). b. Extension model of the Ediacaran Doushantuo Fm within the northern margin of the Yangtze

554

platform that is similar to that of a horst/graben. 23

555 556

Temporally, the Ediacaran on the northern Yangtze Platform margin show an overall

557

shallowing upward succession (Wang et al. 2019). There is no typical cap dolomite at Mingyue,

558

instead of interbedded dolomitic limestone and black mudstone, which indicates that there is

559

weakly paleo-geomorphologic differentiation during the deposition of the cap dolomite. Following

560

the cap dolomite, two secondary shallowing upward successions were recorded in the rest of the

561

Doushantuo Fm and the lower Dengying Fm. The first succession consists of the Members II and

562

III of the Doushantuo Fm, and is represented by black shale in the lower part (Memer II) and

563

thinly interbedded limestone and dolomite in the upper part (Member III) in the Chengkou

564

sub-basin. On the northwestern side, it is characterized by clastic turbidites in the lower part

565

(Memer II) and storm-related fine-grained clastic rocks in the upper part (Member III) at

566

Lianghekou. On the southeastern side, it features black phosphate-bearing shale in the lower part

567

(Memer II) and storm-related dolomites in the upper part (Member III) at Gaozhu. The second

568

succession comprises the Member IV of the Doushantuo Fm and the lower Dengying Fm, is

569

symbolized by black shale and muddy dolomite with manganese deposit in the lower part and

570

light gray thin-bedded dolomite in the upper part in the Chengkou sub-basin, displaying a distinct

571

trend of gradually upward increase in dolomite content. On both sides, it is characterized by

572

gravity-driven dolomitic sediments in the lower part and light gray medium-bedded to massive

573

dolomites in the upper part.

574

Lateral variation in lithofacies and upward shallowing in succession indicate notable effect of

575

paleo-geomorphologic inheritance on deposition throughout the Doushantuo Fm. However, the

576

occurrence of gravity-driven slump folds and olistoliths in the Member IV of the Doushantuo Fm

577

robustly suggest synsedimentary normal faulting (e.g., Montenat et al. 2007; Krajewski et al.

578

2014). This means that regional extensional activity along the northern Yangtze Platform margin

579

likely initiated following the cap dolomite, but significantly enhanced during the deposition

580

duration of the Member IV of the Doushantuo Fm.

581

4. Geochemical analysis

582

4.1. Samples and analytical methods

583

Totally 27 samples were collected from the Members II and IV of the Doushantuo Fm at

584

Mingyue for whole rock analysis of major and trace elements (including rare earth elements 24

585

(REEs) ) to explore hydrothermal activity related to regional extension on the northern margin of

586

the Yangtze Platform during the Ediacaran. All samples were collected from fresh outcrops of

587

black shale and muddy dolomite with variant siliceous content (Please refer to Supplements for

588

sample information). The samples were first cleaned with distilled water and then crushed and

589

powered to less than 200 meshes using an agate mill by hand. Major elements were measured by

590

automatic X-ray fluorescence spectrometer (XRF) at the Institute of Geology and Geophysics,

591

Chinese Academy of Sciences (IGGCAS), Beijing. The detection limit for all major oxides is less

592

than 0.01 wt. %, and the analysis error is less than 3%. Trace elements (including REEs) were

593

analyzed using inductively coupled plasma-mass spectrometry (ICP-MS) at the Institute of

594

Tibetan Plateau Research, Chinese Academy of Sciences (ITPCAS), Beijing. The precision of the

595

analysis were generally below 5% of the reported REEs.

596

Cerium anomaly (Ce/Ce*) was calculated as Ce/Ce* = Cen / (Lan × Prn) 1/2

1/2

, and europium

597

anomaly (Eu/Eu*) was calculated as Eu/Eu* = Eun / (Smn × Gdn)

(Taylor and McClennan,

598

1985), where “n” refers to concentration normalized against Post-Archean Australian Shale

599

(PAAS; McLennan, 1989; Taylor and McLennan, 1985).

600

4.2 Results

601

All results including major elements, partial trace elements, and REEs are listed in

602

supplements as Tables 1, and 2. For comparison, additional major, partial trace elements, REEs of

603

Gaoyan section analyzed by (Wang, 2015; Chen et al., 2017) are also given in supplements as

604

Tables 3, and 4.

605

4.2.1 Major elements

606

Loss on ignition (LOI) values are generally high due to high carbonate content and ranges

607

from 1.86–43.95 wt% (mean = 23.92 wt%). The abundance of major elements shows considerable

608

variation for different members of the Doushantuo Fm. SiO2 concentration ranges from 8.25 wt%

609

to 93.35 wt% with a mean of 59.52 wt% for Member II and from 6.2 wt% to 53.92 wt% with a

610

mean of 29.51 wt% for Member IV. MgO+CaO concentration ranges from 0.28 wt% to 42.08 wt%

611

with a mean of 13.05 wt% for Member II and from 18.76 wt% to 49.09 wt% with a mean of 34.29

612

wt% for Member IV. The Member II is marked by high TiO2 (0.06–0.79%, mean = 0.39%) and

613

Al2O3 concentrations (1.04–15.29%, mean = 6.66%). In contrast, the Member IV has relatively

614

low TiO2 (0.04–0.35%, mean = 0.16%) and Al2O3 concentrations (0.42–4.55%, mean = 2.23%). 25

615

4.2.2 Trace and rare earth elements

616

The ∑REE concentrations (not include Y) of all samples vary from 11.99 ppm to 193.31 ppm.

617

PAAS-normalized REE-Y distributions display flat to gently left-inclined patterns, with slight

618

depletion of light REEs, weak negative Ce anomaly, and weak positive Eu and Y anomalies (Fig.

619

7). As for the trace elements of interest in this paper, Th (0.48–11.41 ppm, mean = 3.55 ppm) and

620

U (0.37–18.59 ppm, mean = 3.35 ppm) show wide variations in concentration on the whole.

621

Still, significant difference in rare earth element composition exists between the Members II

622

and IV of the Doushantuo Fm due to lithological variation. The average total REE concentration

623

of Member IV is lower than that of Member II. The Ce/Ce* ratios for samples of Member IV vary

624

from 0.60 to 0.82 (mean = 0.71) and show a more obvious negative Ce anomaly, compared to

625

those for samples of Member II that range from 0.84 to 1.06 (mean = 0.95). The Eu/Eu* values

626

varied from 1.03 to 1.25 (mean = 1.10) for Member IV and from 0.93 to 1.98 (mean = 1.17) for

627

Member II, suggesting a weak to intermediate positive Eu anomaly as a whole.

628

4.3 Identification of hydrothermal activities

629

REEs consist of a geochemically coherent group of elements with similar chemical properties

630

(Murray et al., 1992). REEs have proven to be fairly stable during diagenesis and thus are valuable

631

in identifying the origin of fine-grained sediments (e.g., Murray et al., 1991; Murray et al., 1992).

632

REE concentrations, REE-Y patterns, and Ce and Eu anomalies provide important information on

633

discriminating the origin of different sediments (Thomson 2001). The characteristics of REE-Y

634

are influenced by a combination of seawater adsorption, contained terrigenous composition, and

635

metalliferous particles (Murray et al., 1992; Owen et al., 1999). Generally speaking, the

636

terrigenous detrital sediments show no negative or positive Ce anomalies and have flatter REE

637

patterns with no apparent fractionation of LREEs from HREEs (Edward, 1990). However,

638

hydrogenous marine sediments (seawater signatures) are primarily characterized by (1) uniform

639

LREE depletion and relatively abundant HREEs due to preferential removal of LREEs by

640

seawater (cf. Elderfield and Greaves, 1982); and (2) apparent negative Ce anomalies and positive

641

Y anomalies due to the variable solubility in different environments (Bolhar et al., 2004;

642

Elderfield and Greaves, 1982). By contrast, hydrothermal vent deposits are marked by more

643

pronounced positive Eu anomalies and enrichment of LREEs, without a pronounced negative Ce

644

anomaly (Murray et al., 1991, 1992). 26

645

Positive Eu anomalies are commonly found in extremely reducing hydrothermal fluids that

646

favor reduction of Eu3+ to Eu2+ (Michard et al., 1983, Michard, 1989) and are very common in

647

marine hydrothermal sediments (Douville et al., 1999; Murray et al., 1991; Owen et al., 1999).

648

This is the case for our samples from the Doushantuo Fm of Mingyue section near Chengkou and

649

published data, as shown in their PAAS-normalized REE-Y patterns compared with typical

650

modern seawater and hydrothermal fluid (Fig. 7). Samples from the Member IV of Doushantuo

651

Fm of the Mingyue section have relative HREE enrichment patterns with weak positive Eu

652

anomalies, apparent negative Ce anomalies and significant positive Y anomalies, which shows

653

similar profile to that of both seawater and low-T hydrothermal fluid (Figs. 7a), robustly

654

indicating a mixing of hydrothermal input and seawater. Samples form Member II, however,

655

exhibit flat REE-Y patterns without apparent negative Ce anomalies and positive Y anomalies,

656

implying a dominant contribution of terrigenous detrital input. Nevertheless, some samples have

657

apparent positive Eu anomalies that indicate, more or less, a significant influence of hydrothermal

658

fluid (Figs. 7b). Although terrigenous detrital feldspars can lead to positive Eu anomalies (Owen et

659

al., 1999), the absence of a significant correlation between Al (%) and Eu/Eu* for all samples

660

excludes that possibility. Apparent Eu anomalies may also result from interference with barium

661

oxides formed during analysis (Jarvis et al., 1989). However, there is no significant positive

662

correlation between Eu/Eu* and Ba, indicating that the interferences can be neglected. Based on

663

these considerations above, the positive Eu anomalies observed in our samples are definitely an

664

accurate reflection of low-T hydrothermal fluid.

665

For comparison and further appraisal of hydrothermal activity, we also plotted previously

666

published data (Wang, 2015; Chen et al., 2017) for samples from manganese deposits in the

667

Member IV of the Doushantuo Fm at Gaoyan that is very close to Mingyue (Figs. 7c, d). We

668

divided manganese deposits into Mn-rich and Mn-bearing rocks according to Mn contents. The

669

Mn-rich sediments (MnO2>20%) of Gaoyan section exhibit relative HREEs enrichment patterns

670

with more pronounced positive Eu anomalies, negative Ce anomalies and positive Y anomalies

671

(Fig. 7c). The REE-Y patterns of all samples show excellent similarity with low-T hydrothermal

672

fluid profile (Fig. 7c), except for a few samples with strong negative Ce anomaly, indicating the

673

Mn-rich fluid was derived from deep marine hydrothermal activity and admixed with seawater.

674

But There is weaker negative Ce anomaly in some samples (Fig. 7c), which may be due to the 27

675

additional adsorption of Ce4+ on the surface of Mn-oxide (Bau and Dulski, 1996). The Mn-bearing

676

sediments (MnO2<20%) of Gaoyan section are marked by flat REE-Y patterns with significant

677

positive Eu anomalies and negative Ce anomalies as well (Fig. 7d). Compared with the Mn-rich

678

sediments, their relatively high REE concentrations with weak positive Y anomalies imply there

679

was a significant contribution of terrigenous detrital input. Still, the REE-Y patterns of

680

Mn-bearing sediments show similarity to low-T hydrothermal fluid profile (Fig. 7d), suggesting

681

they were influenced by a complex combination of hydrothermal fluid, seawater and terrigenous

682

detrital input.

683

28

684 685

Fig.7. PAAS-normalized REE-Y patterns for samples from the Doushantuo Fm compared with the average

686

compositions of hydrothermal fluids (Bau and Dulski, 1999) and modern seawater (Bolhar et al., 2004). (a) and (b)

687

are for samples from the Members IV and II of the Doushantuo Fm of Mingyue (MY) section (this study); (c) and

688

(d) are for Mn-rich and Mn-bearing sediments from the Member IV of the Doushantuo Fm of Gaoyan (GY)

689

section (Wang, 2015; Chen at al., 2017).

690 691

A hydrothermal origin of REE composition can also be confirmed through SiO2-Al2O3 and

692

U-Th discrimination diagrams (Fig. 8). The SiO2/Al2O3 ratio is a good indicator for the source of 29

693

sediment given that Al2O3 and SiO2 contents represent relative contribution of sedimentary

694

(detrital clay minerals) and hydrothermal sources (chert) respectively (Bonatti et al. 1972, 1975;

695

Crerar et al. 1982; Wonder et al. 1988; Nicholson 1992). Hydrothermal fluid can produce

696

additional Si and result in high SiO2/Al2O3 ratios in related sediments, while hydrogenous

697

sediments are typically marked by lower SiO2/Al2O3 ratios (Bonatti et al. 1975). As plotted on

698

SiO2-Al2O3 diagram (Fig. 8a), most of the samples from the Dousahntuo Fm of Mingyue and

699

Gaoyan sections fall in the field of hydrothermal origin, with a few in the hydrogenous field. This

700

is highly consistent with abundant banded to bedded cherts on outcrops and authigenic quartzs

701

observed in the thin-sections of organic-rich shale (Fig. 4b). A similar signature can also be found

702

in the U-Th discrimination diagram (Fig. 8b), which is usually employed to distinguish among

703

hydrothermal, hydrogenous, and pelagic sediments (Bonatti, 1975; Lottermoser, 1991). Almost all

704

Mn-rich and Mn-bearing samples from the Gaoyan section fall in the hydrothermal field, besides a

705

few Si-rich shale samples from the Member II of Doushantuo Fm of Mingyue section. Thus,

706

SiO2-Al2O3 and U-Th discriminations jointly revealed a remarkable effect of hydrothermal fluid

707

on the sediments of the Doushantuo Fm in the vicinity of Chengkou, which is consistent with their

708

REE-Y patterns discussed above.

709

710 711

Fig.8. Crossplots of SiO2 versus Al2O3 (a) and U versus Th (b) for samples from the Doushantuo Fm of Mingyue

712

and Gaoyan sections. In order to eliminate the influence of carbonates on discrimination result, we removed the

713

samples with high carbonate content (MgO+CaO>15 wt %) from (b). Fields of hydrothermal, hydrogenous, and

714

pelagic sediments are based on Bonatti (1975), Wonder et al., (1998) and Lottermoser (1991).

715 716

Based on our analysis above, it is concluded that there were significant hydrothermal 30

717

activities near the Chengkou area on the northern margin of the Yangtze platform during the

718

deposition duration of the Doushantuo Fm (especially the Member IV; Liu et al., 2019), which

719

may be associated with syn-sedimentary normal faults channeling hydrothermal fluids from depth

720

to the seafloor.

721

5. Discussion

722

5.1 Ediacaran extension along the northern margin of the Yangtze Platform

723

It is generally considered that the Nantuo Fm was deposited during the final rifting stage of

724

Rodinia and represents a rift-drift transition that was followed by a sag phase when the Ediacaran

725

Doushantuo and Dengying Fms were deposited (e.g. Wang and Li, 2003). The duration of the

726

Nantuo Fm, previously estimated as ca. 750-690 Ma (Wang and Li, 2003), was recently dated

727

between 654 Ma and 635 Ma (Zhang et al., 2008), implying that the South China Block and thus

728

the Yangtze Platform was substantially affected by the final break-up of Rodinia during the latest

729

Neoproterozoic (Vernhet et al., 2007; Li et al., 2008, 2013; Wang et al., 2013; Zhao et al., 2017).

730

Widespread subaerial exposure unconformity at the top of the Nantuo Fm overlain by ubiquitous

731

cap carbonate of the Doushantuo Fm (Jiang et al., 1996, 2003; Wang and Li, 2003; Zhang et al.,

732

2008; Wang et al., 2019) means that the relief due to the vertical movements associated with

733

Rodinia break-up might have been largely compensated by the Nantuo Fm diamictites. In spite of

734

this, the Doushantuo Fm sedimentation was influenced to some extent by the residual

735

rifting-inherited relief, as in the cases of the southern (Vernhet, 2007, Vernhet and Reijmer, 2010)

736

and northern (this study) margins of the Yangtze Platform. Some previous studies (e.g., Wang and

737

Li, 2003; Vernhet et al., 2007) suggested the Ediadaran Doushantuo Fm was deposited during the

738

thermal subsidence of the Yangtze passive margin following the rifting related to final Rodinia

739

breakup. However, significant variations in the thickness and lithofacies of the Doushantuo Fm

740

both on the southern (Vernhet, 2007; Vernhet and Reijmer, 2010) and northern margins of the

741

Yangtze Platfom, together with the geochemistry indicator, jointly imply a new episode of

742

extensional activity, although not so strong as before. Ediacaran sediments containing abundant

743

gravity-driven sedimentary structures dominated the slope environments both on the southern

744

(Vernhet et al., 2006, Vernhet et al., 2007; Zhu et al., 2007; Jiang et al., 2011) and northern

745

margins of the Yangtze Platform, marking the slope instability induced by syn-depositional normal

746

faulting. This stage of extension can be directly supported by numerous mafic-ultramafic intrusive 31

747

dikes (ca. 650–620 Ma) and coeval bimodal volcanic rocks in the South Qinling area, suggesting

748

an intense intra-plate rift setting related to the final breakup of the Rodinia supercontinent (Xue et

749

al., 2011; Wang et al., 2013, 2016; Zhu et al., 2015). Such rift-related magmatic activity definitely

750

exerted a more prominent influence on the northern Yangtze Platform margin than the southern

751

margin, and might have affected the interior of the Yangtze Platform.

752

New vertical movements might trace pre-existing structural weak zones and further enhance

753

topographic relief, which contributes to paleo-bathymetry effect on the Doushantuo Fm

754

sedimentation. The high/low geometry in topography determines the development of

755

shallow-water platform facies on topographic highs (horsts), basin facies in topographic lows

756

(grabens) and slope facies within the intervening transitional zones (fault zones) (Vernhet, 2007;

757

Vernhet and Reijmer, 2010). Such scenario is common at platform-basin direction in passive

758

margin settings. However, an intriguing phenomenon is the significant variations in thickness and

759

lithofacies of the Doushantuo Fm along the trend of the northern Yangtze Platform margin,

760

implying the occurrence of alternate uplifts (topographic highs) and depressions (topographic lows)

761

in a pattern similar to that of a horst-graben suite. Further to the southwest, newly discovered

762

Ediacaran paleo-uplift in the northeastern part the Sichuan basin based on seismic profiles (Gu et

763

al., 2016; Yang et al., 2016) suggests that the alternating uplift-depression configuration also

764

existed within the interior of the Yangtze Platform (Yang et al., 2016; Zhao et al., 2017; Li et al.,

765

2019). Based on these considerations and our lithofacies observation and geochemical analysis, it

766

is proposed that a SW-trending sub-basin, with its long axis oblique to the northern margin of the

767

Yangtze Platform at high angle, was developed in Chengkou area during the Ediacaran and hence

768

described as the Chengkou sub-basin (Fig. 6, 9). Though it is difficult or even impracticable to

769

reconstruct its original geometry due to severe destruction by later deformation, the proposal of

770

the Chengkou sub-basin can reasonably explain several features of the Doushantuo Fm on the

771

northern margin of the Yangtze Platform: (1) abrupt lateral variations in thickness and lithofacies

772

within a narrow area near Chengkou; (2) abundant gravity-driven sedimentary structures in slope

773

environments on both sides of the Chengkou sub-basin; (3) distinctive distribution of manganese

774

and phosphorus deposits; and (4) hydrothermal activity through the duration of the Doushantuo

775

Fm. In addition, the development of the Chengkou sub-basin also justifies the coexistence of

776

organic-rich high-quality source rocks and volcanic ash interlayers, which also suggests 32

777

significant hydrothermal activities in the Chengkou area during the Doushantuo Fm deposition

778

(Liu et al., 2019).

779

All these observations show that the northern Yangtze Platform margin experienced notable

780

extension associated with the final breakup of Rodinia, and the consequent Chengkou sub-basin

781

might extend into the interior of the Yangtze Platform (Fig. 9). Taking into account the slope

782

instability and more prominent hydrothermal activity, we consider the duration of the Member IV

783

of the Doushantuo Fm to be the peak period of syn-depositional normal faulting. And the inherited

784

basin may further affect the upper Ediacaran Dengying Fm, which could not be filled up until the

785

Early Cambrian Qiongzhusi Fm.

786 787

5.2 Implications for the paleogeography of the the Yangtze Platform

788

Owing to data availability, previous paleogeographic reconstructions for the Yangtze Platform

789

during the Ediacaran and early Cambrian were largely focused on its southern margin, and

790

considered that its northern part was dominated by regular peritidal carbonate platform (or ramp)

791

environments (e.g., Zhu et al., 2003; Jiang et al., 2011). However, a growing body of research has

792

showed that such scenario was oversimplified, and is calling for a more rigorous and

793

comprehensive reconstruction.

794

During the Ediacaran, the southern Yangtze Platform margin is characterized by a rimmed

795

carbonate shelf with a large intra-shelf sub-basin (Guiyang-Yichang sub-basin, Fig. 9; Vernhet and

796

Reijmer, 2010; Jiang et al., 2011). Both the Doushantuo Fm and partial Dengying Fm exhibit

797

obvious lateral differences in thickness and lithofacies, which was thought to be dominated by

798

rifting-inherited paleo-bathymetry by Vernhet (2007). On the western margin of the Yangtze

799

Platform, a nearly SN-trending intracratonic rift/sag during the late Ediacaran and/or the early

800

Cambrian was recently discovered based on seismic and drilling data (Mianyang sub-basin, Fig. 9;

801

Liu et al., 2013, 2017a; Li et al., 2015; Du et al., 2016; Zhou et al., 2017; Wei et al., 2018; Yang et

802

al., 2018). Despite the controversy over the onset of extension, it is generally accepted that the

803

Mianyang sub-basin was resulted from extension tectonism and controlled by rifting-inherited

804

structures within the underlying Ediacaran Doushantuo Fm and in turn the basement rocks (Chen

805

et al., 2009, 2012; Liu et al., 2017a, 2017b; Zhao et al., 2017; Wei et al., 2018b). Similarly, our

806

result and several recent studies have revealed that a NE-SW-trending sub-basin (Chengkou 33

807

sub-basin, Fig. 9) was developed on the northern margin of the Yangtze Platform during the

808

Ediacaran (Yang et al., 2016, 2018, 2019; Hou et al., 2017; Zhao et al., 2017; Li et al., 2019; Liu

809

et al., 2019). Both the Mianyang and Chengkou sub-basins are oblique to the craton margin and

810

may be classified as epicratonic basin. In contrast, the Guiyang-Yichang sub-basin is

811

margin-parallel and similar to a continental rim basin (Allen and Allen, 2013). Regardless of their

812

geometry and orientation, they are margin-related extensional basins in origin and were generated

813

by new extensional activity during the Ediacaran coupled with rifting-inherited paleo-structures

814

and paleo-bathymetry (Vernhet, 2007; Vernhet and Reijmer, 2010; Wei et al., 2018b; Liu et al.,

815

2019). The occurrence of these sub-basins made the Yangtze Platform quite irregular during the

816

Ediacaran deposition, rather than a consistent platform environment over its northern part as

817

proposed before, which was represented as alternating uplifts (topographic highs) and depressions

818

(topographic lows) (Fig. 9). This geometry is a first order control on the deposition of the

819

Doushantuo Fm and even Dengying Fm. The sub-basins were dominated by deep-water basin and

820

slope facies that are marked by dark fine-grained rocks with abundant gravity-driven sedimentary

821

structures such as slump folds and olistoliths, while no deposition or shallow-water platform facies

822

represented by light carbonates was developed outside the sub-basins. The marginal zones

823

surrounding these sub-basins are usually the preferred areas for the development of high-energy

824

grainstone facies.

825

34

826 827

Fig. 9. Paleogeographic reconstruction of the Ediacaran Doushantuo Fm. Chengkou sub-basin is the study area,

828

modified from Yang et al. (2019), Liu et al. (2019) and Li et al. (2019). MianYang sub-basin is modified from Yang

829

et al. (2016, 2018) and Liu et al. (2017a). Guiyang-Yichang intra-shelf sub-basin is modified from Vernhet et al.

830

(2006) and Jiang et al. (2011). Wuxi sub-basin is modified from Zhu et al. (2007) and Zhou et al. (2007). Nanhua

831

rift basin is modified from Wang and Li, 2003.

832

5.3 Significance for hydrocarbon exploration

833

The coexistence of sub-basins and intervening highs or platforms throughout the northern

834

Yangtze Platform does favor the development of high-quality source and reservoir rocks in the

835

Ediacaran to early Cambrian sedimentary formations. Tectonic inheritance within and around

836

these sub-basins determines, to a great degree, the facies architecture of the Doushantuo Fm and in

837

turn the overlying Dengying Fm and Qiongzhusi Fm (Zhao et al., 2017; Yang et al., 2018; Li et al.,

838

2019). Laterally, the centers of sub-basins controlled the organic-rich source rocks of the

839

Doushantuo Fm and Qiongzhusi Fm, whihe the high-quality dolomite reservoirs of the Dengying

840

Fm were developed in the shallow environments on both flanks of the sub-basins (Fig. 10).

841

Vertically, the organic-rich source rocks of the Doushantuo Fm at the bottom (Liu et al., 2019), the 35

842

reservoir rocks of the Dengying Fm in the middle and the organic-rich source and seal rocks of the

843

Qiongzhusi Fm at the top constitute an excellent hydrocarbon assemblage similar to a sandwich

844

structure (Liu et al., 2017b; Yang et al., 2018). Therefore, high-quality dolomite reservoirs are

845

immediately adjacent to organic-rich source rocks both vertically and laterally, which, combined

846

with the effective migration channels along marginal synsedimentary normal faults and

847

unconformities, defines a quite efficient hydrocarbon accumulation mode (Fig. 10). And the

848

Doushantuo Fm may be the main source rock, due to the vertical migration of hydrocarbon was

849

easier than the lateral migration (Liu et al., 2019). Such a mode has been validated by the

850

discovery of the Anyue giant gas field bordering the Miangyan sub-basin in the central Sichuan

851

basin (Liu et al., 2017a; Yang et al., 2018; Shi et al., 2018; Du et al., 2016; Zhou et al., 2017; Wei

852

et al., 2018; Yang et al., 2018). This discovery also draws an inspiring prospect for the

853

hydrocarbon exploration of the Ediacaran to Early Cambrian suite in other similar areas in the

854

Sichuan basin, among which the area around the Chengkou sub-basin may be one of the most

855

promising. Recent progress in shale gas investigation of the Doushantuo Fm in eastern Yangtze

856

Platform (Wang et al., 2017) has also shown its great potential for both conventional and

857

unconventional hydrocarbon exploration (Wang et al., 2019; Liu et al., 2019). It is therefore

858

proposed that the margin-related extensional sub-basins such as the Chengkou sub-basin on the

859

northern Yangtze Platform margin during the Ediacaran are of great significance for hydrocarbon

860

accumulation and worthy of further exploration.

861

862 863

Fig. 10. (a) Schematic map of the lithofacies paleogeography of the Ediacaran Doushantuo Fm, modified from

864

Yang et al. (2016), Jiang et al. (2011), Zhu et al. (2007), and Liu et al. (2017a). (b) Supposed hydrocarbon

865

accumulation model from the Ediacaran to Early Cambrian in the vicinity of the sub-basin.

866 36

867

6. Conclusions

868

The lithofacies and geochemical analyses of the Doushantuo Fm along the northern Yangtze

869

Platform margin reveal that it experienced notable extension during the Ediacaran, likely in

870

response to the final breakup of Rodinia. Four main findings are summarized as follows.

871

(1) The Ediacaran Doushantuo Fm shows significant variations in lithofacies and thickness

872

along the northern margin of the Yangtze Platform. Three depositional environments were

873

distinguished: shallow-water platform, slope and deep-water basin. The slope can be further

874

divided into relatively shallow-water upper slope and relatively deep-water lower slope.

875

(2) The sediments of the Doushantuo Fm, especially the Member IV, were influenced

876

remarkably by hydrothermal fluids in the vicinity of Chengkou, which, together with slope

877

instability represented by abundant gravity-driven sedimentary structures, likely implies the peak

878

period of syn-depositional normal faulting during the Member IV deposition.

879

(3) A SW-trending sub-basin oblique to craton margin at high angle, the Chengkou sub-basin,

880

was developed in the vicinity of Chengkou during the Ediacaran, based on previous studies on

881

seismic data and our observations of abrupt variations in strata thickness and lithofacies over a

882

narrow area, hydrothermal activity, and manganese and phosphorus deposit distribution.

883

(4) The northern Yangtze Platform was quite irregular during the Ediacaran and even early

884

Cambrian, and characterized by alternating uplifts (topographic highs or platforms) and

885

depressions (sub-basins). This irregularity is of great significance for hydrocarbon accumulation in

886

the Ediacaran to Early Cambrian suite in the northern Sichuan basin, which deserves further

887

exploration.

888 889

Acknowledgments

890

This work was supported by the National Science and Technology Major Project

891

(2017ZX05005003-007), the National key basic research and development program (973)

892

(2012CB214805) and the National Natural Science Foundation of China (4160020869). We are

893

grateful to editors and experts for their constructive comments.

894 895

Appendix A. Supplementary data

896

Supplementary data associated with this article can be found, in the online version, at 37

897 898

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899

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Burdige, D.J., Kepkay, P.E., 1983. Determination of bacterial manganese oxidation rates in sediments using

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Chen, D.Z., Wang, J.G., Qing, H.R., Yan, D.T., Li, R.W., 2009. Hydrothermal venting activities in the Early

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[10] Chen, D.Z., Wang, J.G., Yan, D.T., Wei, H.Y., 2012. Tectono-depositional patterns and palaeogeography in

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47

Highlights Revised Highlights: 1. Abrupt variations in lithofacies and thickness of Doushantuo Formation, Yangtze Platform. 2. Obvious hydrothermal sediments in the center of sub-basin. 3. The extensional sub-basin might extend into the interior of the Yangtze Platform. 4. Extension is related to the breakup of Rodinia and is of significance for hydrocarbon exploration.