2024 October LIP of the Month

Spatiotemporal link between the 1640 Ma LIPs and black shales and implications for the Statherian/Calymmian boundary*

Shuan-Hong Zhang^1,2*, Sandra L. Kamo³, Richard E. Ernst^4,5, Guo-Hui Hu^1,2, Qi-Qi Zhang¹, Hafida El Bilali⁴, Yue Zhao¹

1. Institute of Geomechanics, Chinese Academy of Geological Sciences, MNR Key Laboratory of Paleomagnetism and Tectonic Reconstruction, Beijing 100081, China

2. SinoProbe Laboratory, Chinese Academy of Geological Sciences, Beijing 100037, China

3. Jack Satterly Geochronology Laboratory, Department of Earth Sciences, University of Toronto, Toronto, Ontario M5S 3B1, Canada

4. Department of Earth Sciences, Carleton University, Ottawa, Ontario K1S 5B6, Canada

5. Faculty of Geology and Geography, Tomsk State University, Tomsk 634050, Russia

E-mail: tozhangshuanhong@163.com (Dr. S.-H. Zhang)

*This report is extracted and modified from the following paper:

Abstract

Phanerozoic boundaries and the base of the Ediacaran in theinternational chronostratigraphic time scale are defined by Global Boundary Stratotype Sections and Points(GSSPs). However, the pre-Ediacaran geological time scale is formally subdivided by approximate absolute ages not tied to any geological units. Here we present a high-precision zircon U–Pb CA–ID–TIMS age of 1641.7±1.2Ma for a tuff layer within the black shales of the Chuanlinggou Formation in the northern North China that preserves the world’s earliest multicellular eukaryotemicrofossils. The new age is similar to those obtained for black shales from the Cuizhuang Formation in the southern North China, and the Barney Creek and Fraynes formations in the North Australia, indicating synchronous deposition of voluminous black shales across the North China, North Australia and other cratons at ca.1640Ma. Global correlations and analysis ofthe spatial distribution of ca.1640Ma black shales and large igneous provinces (LIPs) and associated magmatic rocks in paleogeographic reconstruction reveal a spatiotemporal link between the ca.1640Ma LIPs and black shales. The global-scale geological event represented by the ca.1640Ma LIPs and coeval black shales can provide a natural marker for the Statherian/Calymmian boundary at ca.1640Ma in the international chronostratigraphic time scale.

1. Introduction

Black shales are sedimentary rocks containing >0.5wt.% of organic carbon and are the major source rocks for oil and gas deposits (Parviainen & Loukola-Ruskeeniemi, 2019). Deposition of marine black shales occurred mainly after the Great Oxidation Event (GOE) started approximately after 2450Ma (Condie et al., 2001; Jin et al., 2023) and has commonly been interpreted as having involved two processes:1)a high level of marine phytoplankton production that promoted high settling rates of organic matter through the water column and 2) high burial fluxes on the seafloor or anoxic (sulfidic) water-column conditions that led to high levels of preservation of deposited organic matter (Piper & Calvert, 2009). Therefore, black shales have important environmental and economic significance (Condie et al., 2001; Kennedy et al., 2002; Piper & Calvert, 2009; Jin et al., 2023).

The Phanerozoic boundaries and the base of the Ediacaranin theinternational chronostratigraphic time scale are each defined by a basal GSSP (Gradstein & Ogg,2020), and some boundaries are temporally correlated with LIPs and ocean anoxic events (OAEs) represented by global scale black shale depositions (Percival et al., 2015; Ernst & Youbi, 2017).However, the pre-Ediacaran timescale isdefined by Global Standard Stratigraphic Ages (GSSAs) with approximate ages (mostly to the nearest 100 Myr) due to a lack of stratigraphic constraints from global geologic events (Gradstein & Ogg,2020; Strachan et al., 2020). Recent spatiotemporal correlation of Precambrian LIPs and black shales suggest a potential way for using coeval LIPs and black shales as natural markers for boundaries in the pre-Ediacaran timescale (Ernst & Youbi, 2017;Zhang et al., 2018, 2021; Ernst et al. 2021). The Statherian/Calymmian boundary is currently regarded as the boundary between the Paleoproterozoic and Mesoproterozoic eras(Strachan et al., 2020), although some recent research has proposed the Orosirian/Statherian boundary as the Paleoproterozoic/Mesoproterozoic boundary (Shields et al., 2022; Wang et al., 2022). The Statherian/Calymmian boundary is defined by a approximate age of 1600 Ma, but lacks a natural stratigraphicmarker and constraints from global geologic events (Strachan et al., 2020).

2. The Chuanlinggou Formation in the Yanliao Basin in the northern North China Craton

The Yanliao Basin in the northern North China Craton (NCC) is the location of the standard sections for late Paleoproterozoic–Mesoproterozoic stratigraphy in China and the total thickness of the late Paleoproterozoic–Mesoproterozoic strata is over 10km (BGMRHP, 1970; Lu et al., 2008). Black shales are common in several stratigraphic units including Chuanlinggou, Gaoyuzhuang, Hongshuizhuang and Xiamaling formations. The Palaeo- to Mesoproterozoic Greater McArthur Basin in the North Australian Craton (NAC) contains an unmetamorphosed and relatively undeformed succession of sedimentary and minor volcanic rocks with a preserved thickness of up to 15km (Munson, 2014, 2023). Previous investigations have shown that the northern–northeastern margin of the NCC was connected to, or at least a near neighbor to, the northern margin of the NAC during the Mesoproterozoic period (Zhang et al., 2017; Wang et al., 2019; Nixon et al., 2022). Furthermore, the two 1.40-1.35Ga hydrocarbon-rich black shale units including the Xiamaling Formation in the NCC and the Velkerri Formation in the NAC were deposited and matured in the same shared basin of Columbia, which has been proposed to have a similar setting to the Gulf of Mexico which formed during the breakup of Pangea (Mitchell et al., 2021). The connection between the northern–northeastern margin of the NCC and the northern margin of the NAC lasted for at least 500 Myr from ca.1800Ma to 1300Ma (Zhang et al., 2022).

The Chuanlinggou Formation in the Yanliao Basin hosts the earliest black shales and granular iron formation (GIF)after final stabilization of the NCC atca.1.85Ga. Black shales within the Chuanlinggou Formation are the earliest hydrocarbon source rocks known in China (Zhao et al., 2019) and they host the world’s earliest multicellular eukaryotes (Miao et al., 2024). The Chuanlinggou Formation is mainly distributed in the western and southern Yanliao Basin and the central and southern Taihangshan region (Fig. 1). Oolitic to stromatolitic ironstone,termed the Xuanlong-type ironstone, is common at the lowermost part of this formation (Fig. 2) in the westerm margin of the Yanliao Basin (Du et al., 1992), and represents the earliest GIF in China (Bekker et al., 2010). The Chuanlinggou Formation consists mainly of siltstone and shale with minor sandstone and argillaceous dolostone. The black shales are common within the middle–upper part of the Chuanlinggou Formation, and are mainly found near Jixian to Kuancheng in the southern Yanliao Basin (Figs.2 and 3). They have a cumulative thickness of 30-200m and TOC concentrations of 0.1-2.6wt.% (Zhao et al., 2019). Multicellular eukaryote microfossils (Qingshania magnifica) have recently been discovered from the dark gray shales about 100m below the top of the Chuanlinggou Formation in the Wengjiazhuang section in Kuancheng County (Miao et al., 2024). Tuffaceous ash beds have been identified in black shales within the upper part of the Chuanlinggou Formation near Kuancheng County (Fig. 4). They are yellowish- to grayish-white in color and of 0.3-3cm thick. The tuff beds from the Shenxianling–Wengjiazhuang section (GPS position N40°35'02.4", E118°32'15.4") have been previously dated by the SHRIMP method, with zircon U–Pb ages of 1621±12Ma (Sun et al., 2013) and 1634.8±6.9Ma (Liu et al., 2019). Although the above two tuff samples were collected from a same location, they exhibit a wide age range of 13.8Myr and have large 2σ errors of 6.9-12Myr. Moreover, these ages are younger than a zircon U–Pb age (1641±4Ma) for a potassium-rich volcanic rocks in the overlying Tuanshanzi Formation (Zhang et al., 2013). Therefore, the two tuff ages of black shales within the Chuanlinggou Formation are inconsistent with stratigraphic relationships.

3. High-precision U-Pb geochronology of tuffs from the Chuanlinggou Formation

U–Pb CA–ID–TIMS dating on tuffs from the Chuanlinggou Formationin the Yanliao Basin was completed at the Jack Satterly Geochronology Laboratory at the University of Toronto, Canada.Four euhedral long prismatic zircon grains (with approximate weights of 1-4 g) from sample 18KC-S03-06 were selected for U–Pb CA–ID–TIMS dating. The results are plotted on the concordia diagram of Fig. 5A. Concordant results for the 4 grains overlap and have a weighted mean ²⁰⁷Pb/²⁰⁶Pb age of 1641.7±1.2Ma (2σ; MSWD=1.3, N=4). The zircons have high Th/U ratios of 0.39-0.43, indicating a magmatic origin. Therefore, we consider 1641.7±1.2Ma to best represent the time of deposition of the tuff bed within the black shales in the Chuanlinggou Formation in the Yanliao Basin.

4. Precise deposition age of black shales of the Chuanlinggou Formation in the NCC

The lower boundary age of the Yanliao Basin was previously considered as ca.1800Ma and the deposition age of the Chuanlinggou Formation was regarded as ca.1700Ma (Lu et al., 2008) or ca.1800Ma (Lamb et al., 2009). However, geochronological results from last ten years have shown that the lower boundary age of the Yanliao Basin must be younger than ca.1670Ma (Li et al., 2013).Our new U–Pb CA–ID–TIMS age for a tuff bed within the black shales in the upper part of the Chuanlinggou Formation of 1641.7±1.2Ma is older than the previously reported SHRIMP U–Pb zircon dates (1621±12Ma,Sun et al., 2013;1634.8±6.9Ma,Liu et al., 2019)on tuffaceous ash beds within the black shales in the upper part of the Chuanlinggou Formation. Our newly obtained age is also slightly older than the previous age obtained from the overlying Tuanshanzi Formation (1641±4Ma, Zhang et al., 2013; 1634±9 Ma, 1637±8 Ma, Ma et al., 2024), and is consistent with the stratigraphic sequences(Fig. 1B). Therefore, our high-precision U–Pb CA–ID–TIMS age of 1641.7±1.2Ma provides an improved timing constraint on the deposition of black shales within the Chuanlinggou Formation in the NCC. This new age also provides the timing of the world’s earliest multicellular eukaryotes (Miao et al., 2024), the earliest GIF in China (Bekker et al., 2010) and the astronomical rhythms and the chaotic behavior of the solar system recorded in the Chuanlinggou Formation (Shi et al., 2023). The new high-precision age clearly indicates that ages of the world’s earliest multicellular eukaryotes and the Xuanlong-type ironstone located below the black shales of the Chuanlinggou Formation are older than 1641.7 Ma.

5. Global correlations of black shales at ca. 1640Ma and implications

Black shales are indicative of low, or oxygen-absent, deep-ocean conditions and global scale black shale depositions are usually related to ancient OAEs (Jenkyns, 1988; Nozaki et al., 2013). Global correlations of black shales on different cratons are of great significance for identification of ancient OAEs and for subdivisions of the Proterozoic timescale (Zhang et al., 2018; 2021), because most boundaries in the pre-Ediacaran timescale lack precise age constraints (Strachan et al., 2020; Shields et al., 2022). Our new U–Pb age of 1641.7±1.2Ma for black shales in the Chuanlinggou Formation(replacing the 1621–1635 Ma, Sun et al., 2013;Liu et al., 2019) is similar to those of the black shales from the Cuizhuang Formation in the southern NCC (Lyu et al., 2022) and the Barney Creek Formation of the McArthur Group in the southern McArthur Basin in the NAC (Page & Sweet, 1998), indicating synchronous deposition of black shales within the Chuanlinggou and Cuizhuang formations in the NCC and the Barney Creek Formation in the NAC as previously suggested (Zhang et al., 2021, 2022).

The Fraynes Formation of the Limbunya Group in the Birrindudu Basin in the northwestern NAC is considered another hydrocarbon reservoir in the NAC (Bullen, 2017; Subarkah et al., 2023). This formation is a laminated dolomitic siltstone with black shale intervals, with an increasing proportion of carbonate up-section (Bullen, 2017). The TOC contents of shales range from 0.04wt.% to 8.39wt.% and have an average TOC content of 2.09wt.% (Subarkah et al., 2023). Recent CA–ID–TIMS U–Pb zircon dating on a volcanogenic tuffaceous siltstone yields a weighted mean ²⁰⁷Pb/²⁰⁶Pb age of 1642.2±3.9Ma (Munson et al., 2020), indicating deposition of the black shales within the Fraynes Formation also occurred at ca.1640Ma. Apart from the NCC and NAC, the ca.1640Ma black shales may also exist in other cratons such as India and Siberia (see details in Zhang et al. 2024).

6. Spatiotemporal link between ca.1640Ma LIP volcanism and black shales and constraints on the Statherian/Calymmian boundary

Ca.1640Ma LIPs or intraplate mafic events interpreted as fragments of LIPs are widely distributed in Laurentia, Baltica, Siberia, Central Australia and Western Africa (Ernst, 2014; see details in Zhang et al. 2024).

Six samples of volcanogenic tuffaceous beds from the Chuanlinggou and Cuizhuang formations in the NCC and the Barney Creek and Fraynes formations in the NAC have a weighted mean ²⁰⁷Pb/²⁰⁶Pb age of 1641.5±2.1Ma (Fig. 5B) and exhibit a peak age of 1641.5Ma in a probability density plot (Fig. 5C). The ca.1640Ma LIPs and associated magmatic rocks in Laurentia, Baltica, Siberia, Central Australia and Western Africa exhibit two peak ages of 1640.9Ma and 1632.5Ma in probability density plot and the older peak age of 1640.9Ma is similar to the peak age of tuffaceous beds within black shales (Fig. 5C), suggesting a temporal link between ca.1640Ma LIP volcanism and black shales. In the paleogeographic reconstruction map of the Columbia supercontinent at ca.1600Ma (Li et al., 2023), cratons hosting ca.1640Ma LIPs and associated magmatic rocks were connected or near neighbors to those with ca.1640Ma black shales (Fig. 6), indicating a spatiotemporal link between ca.1640Ma LIP volcanism and black shale deposition in the Columbia supercontinent.

Previous researchers over last 10 years revealed a spatiotemporal and causal link between some LIPs and large volumes of black shale deposition (Ohkouchi et al., 2015; Percival et al., 2015, 2016; Ernst & Youbi, 2017; Zhang et al., 2018, 2019, 2021; Teixeira et al., 2020; Diamond et al., 2021; Ernst et al., 2021). The suggested mechanism for this causal link is that LIP volcanism enhances continental weathering and subsequently increases input of nutrients (P and Fe) from continents into the oceans, which enhances oceanic productivity; the enhanced oceanic productivity increases burial of organic matter leading to deposition of large volumes of black shales (Erba et al., 2004; Ohkouchi et al., 2015; Percival et al., 2015, 2016; Diamond et al., 2021; Xu et al., 2024; Zhang et al., 2024). Because eruption of LIPs can lead todeposition of large volumes of black shales, wide distribution of ca.1640Ma LIPs and black shales across different cratons (Fig. 6)suggest a probable causal link between them. We further propose that the ca.1640Ma LIPs and black shales represent a global-scale geological event and provide a natural marker for the Statherian/Calymmian boundary at 1640Ma in the international chronostratigraphic scale (Ernst & Youbi, 2017;Strachan et al., 2020; Shields et al., 2022).

Acknowledgement

This research was financially supported by the National Natural Science Foundation of China (41920104004, U2244213, 41725011).

References