Elsevier

Precambrian Research

Volume 342, 15 June 2020, 105668
Precambrian Research

Iron-rich carbonate tidal deposits, Angepena Formation, South Australia: A redox-stratified Cryogenian basin

https://doi.org/10.1016/j.precamres.2020.105668Get rights and content

Highlights

  • Robust sedimentological constraints on geochemical proxy data are used to constrain the environment-specific redox state of Cryogenian shallow marine settings.

  • Strong negative to slightly positive cerium (Ce) anomalies (Ce/Ce*) suggest intermittent syn-sedimentary iron-oxide precipitation in a Cryogenian oxidized peritidal environment.

  • An oxic, peritidal setting is chronostratigraphically equivalent to the ferruginous subtidal marine setting of the Balcanoona Reef complex.

Abstract

The Cryogenian Angepena Formation (ca. 650 Ma) of South Australia records deposition in a peritidal environment equivalent to, and landward of the Balcanoona reef complex, and offers valuable insights into shallow marine chemistry during the nonglacial interlude between two global ice ages. The sedimentary facies comprise an iron-rich marine red bed succession and include red mudcracked dolomitic shales, tepee dolostone, crossbedded intraclastic ooid grainstone, and tepee-related sheet cavities with marine cements. These facies are interpreted to have been deposited in a Cryogenian tidal flat to subtidal setting that is laterally equivalent to the reefal and back reef environments of the Balcanoona Formation. Marine cements from this environment are characterized by well-preserved, non- and bright-cathodoluminescent zoning, and strong negative to slightly positive cerium (Ce) anomalies (Ce/Ce*). We suggest that the Angepena Formation is indicative of intermittent synsedimentary iron oxide precipitation in an oxidized peritidal environment in an otherwise strongly ferruginous and anoxic oceanic setting. Large negative Ce anomalies develop over a narrower depth range when compared to modern oxic marine settings, highlighting the importance of dissolved Mn and Fe concentrations in controlling the magnitude of the negative Ce anomalies. Overall, coupled sedimentological and geochemical evidence suggest a Neoproterozoic shallow marine system at the interface of an oxidizing atmosphere and an anoxic ferruginous marine system.

Introduction

The Neoproterozoic Era is characterized by some of the most dramatic environmental changes in Earth’s evolutionary history, encompassing global glaciation (e.g. Harland, 1964, Hoffman et al., 2017), the accumulation of significant atmospheric oxygen (e.g. Och and Shields-Zhou, 2012, Lyons et al., 2014) and the radiation of complex life (e.g. Narbonne, 2005, Love et al., 2009, Knoll and Sperling, 2014). Several independent geochemical proxies have been used to support the idea of a stepwise increase in atmospheric pO2 during this time, which is referred to as the Neoproterozoic Oxygenation Event (e.g. Canfield et al., 2007, Fike et al., 2006, Scott et al., 2008, Och and Shields-Zhou, 2012, Lyons et al., 2014). However, the timing and magnitude of this oxygenation event remains poorly constrained. Recent evidence suggests that while the oxygenation of the atmosphere and surface ocean environments may have been underway by ~800 Ma (Thomson et al., 2015, Turner and Bekker, 2016, Cole et al., 2016), the deep oceans may not have become pervasively oxygenated until the Ediacaran (e.g. Canfield et al., 2007) or as late as the middle Paleozoic (e.g. Sperling et al., 2015, Wallace et al., 2017). Our understanding of the redox state of Neoproterozoic surface environments is complicated by spatial and temporal variability in marine conditions during this dynamic interval (e.g. Li et al., 2015, Jin et al., 2018), necessitating robust sedimentological constraints on geochemical proxy data in order to constrain the environment specific redox state. Given strong spatial variability, a detailed facies analysis coupled with paleoredox proxy work is essential to move forward our understanding of Neoproterozoic environmental evolution.

Marine chemical sediments—carbonates and ferruginous sediments—can serve as robust archives for paleoredox proxies when coupled with sedimentological and petrographic work. Nearshore depositional settings such as tidal flats record deposition at the interface of the atmosphere and marine settings and can therefore provide insights into both systems. The Cryogenian Angepena Formation (northern Adelaide Fold Belt, South Australia; Fig. 1) provides an opportunity to study a Neoproterozoic intertidal environment (Giddings et al., 2009, Wallace et al., 2015, Hood and Wallace, 2012). The Angepena Formation represents the peritidal facies of the Balcanoona reef complex, which developed during the nonglacial period between the Cryogenian ‘Snowball Earth’ ice ages (Giddings et al., 2009, Wallace et al., 2015). Peritidal sediments of the Angepena Formation, as well as laterally equivalent Balcanoona Formation back reef and reef margin facies, contain well-preserved marine cements that are ideally suited for tracking the chemical composition of Neoproterozoic seawater.

The shales and carbonates of the Angepena Formation are strongly enriched in disseminated iron oxides, and represent the oldest marine red bed succession of the Adelaide Fold Belt. Marine red beds (iron-rich strata) are marine sedimentary strata containing iron (oxyhydr)oxides that impart a characteristic red weathering color (Wagreich, 2009, Wang et al., 2011, Cai et al., 2012, Hu et al., 2012). As iron is highly insoluble in its oxidized (ferric) form under circumneutral conditions, marine iron-rich strata have been used to draw important inferences about marine redox evolution throughout Earth’s history (e.g. Song et al., 2017). However, diagenetic iron enrichment or remobilization can potentially complicate the interpretation of the paleoredox significance of marine iron-rich strata. This paper presents a combined process-based sedimentologic and geochemical analysis of the Angepena Formation to constrain the redox state and depositional setting of this iron-rich Cryogenian tidal flat system.

Section snippets

Geological setting

The Angepena Formation forms part of the Cryogenian Umberatana Group in the northern Adelaide Fold Belt, South Australia (Fig. 1, Fig. 2). The Umberatana Group comprises strata deposited during the Sturtian glaciation, the Marinoan glaciation, and the intervening nonglacial period (Coats and Preiss, 1987), and therefore includes all strata of Cryogenian age in the Adelaide Fold Belt. Age constraints for the Umberatana Group are few, but include U-Pb zircon dates from a tuffaceous horizon in the

Methods

Stratigraphic sections were measured using a Jacob’s staff. Stratigraphic sections were correlated by walking out contacts and by using high-resolution air photos coupled with stratigraphic logs to trace distinctive stratigraphic beds. Samples collected from within measured sections and in laterally equivalent beds include marine cements, depositional micrites, dolomitic shales, ooid grainstones and ferruginous mudstones and siltstones. Transmitted light and cathodoluminescence (CL) petrography

Angepena Formation

The Angepena Formation in the Balcanoona reef complex features a thick and laterally extensive marine iron-rich succession. The Angepena Formation can be divided into four main facies, including: red dolomitic mudcracked shale (referred to herein as F1); laminated dolostone with tepees (F2); carbonate-rich facies with oolitic and intraclastic grainstones (F3); and carbonate sheet cavity facies (F4).

The most carbonate-rich facies (F3, F4) are proximal to, and interbedded with the Balcanoona reef

Geochemical results

Based on sedimentological analysis and stratigraphic sections of the Angepena Formation, the northernmost section (A1, Fig. 1) was selected for geochemical-sedimentological analysis to provide information on redox conditions in Cryogenian nonglacial nearshore settings. This stratigraphic section was selected based on its paleogeographic location (i.e. the most oceanward facies of the Angepena tidal red beds) as well as the comparative abundance of marine cemented lithologies, which are regarded

Discussion

The significant iron oxide enrichment of the Angepena Formation (shales 3–6%; up to 10%, Coats and Blissett, 1971; FeO concentrations in ooid laminae up to 46.5 wt%)—and the sedimentary and petrographic evidence for synsedimentary iron oxide deposition—is potentially indicative of marine oxidation processes. This stands in stark contrast to the deeper water equivalent Balcanoona Formation carbonates, in which iron is predominantly present as ferrous iron bound within the carbonate crystal

Conclusions

The Cryogenian Angepena Formation is an iron-rich peritidal sedimentary succession deposited during the Cryogenian nonglacial interval.

  • 1.

    This study provides evidence for the lateral equivalence of the Angepena Formation and the Balcanoona Formation in the Northern Flinders Ranges. This work develops the stratigraphic relationships documented in some previous studies (Coats and Blissett, 1971, Fromhold and Wallace, 2011, Wallace et al., 2015), and establishes four facies. Facies include F1) red

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

We thank constructive reviews from Lizzie Trower and an anonymous reviewer. This study benefitted from discussion with Maurice Tucker, Murray Gingras. This study also benefitted from assistance by Hayden Dalton and Eric Duda. Brennan O’Connell acknowledges funding from APPEA, AAPG, and IAS. Ashleigh Hood acknowledges funding from the Australian Research Council DECRA (DE190100988) and the Baragwanath Geology Research Fund. Maxwell Lechte acknowledges funding from the Fonds de Recherche du

Data availability

The model code (.m file) is available upon request. Geochemical data is provided in Supplemental Tables 1–3.

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