Elsevier

Sedimentary Geology

Volume 324, 1 July 2015, Pages 12-31
Sedimentary Geology

Enigmatic carbonates of the Ombombo Subgroup, Otavi Fold Belt, Namibia: A prelude to extreme Cryogenian anoxia?

https://doi.org/10.1016/j.sedgeo.2015.04.007Get rights and content

Highlights

  • The Ombombo Subgroup, Namibia contains unusual carbonates and copper mineralisation.

  • The dolomitic Devede Fm. contains curl breccias, sheet cavities, and tepees.

  • Syn-sedimentary dolomitisation suggests ocean anoxia in the Early Neoproterozoic.

  • Pre-Sturtian anoxia implies that glaciation isn't the sole cause of Cryogenian anoxia.

Abstract

The Ombombo Subgroup of the Otavi Fold Belt, Kaokoveld, Namibia preserves a succession of clastic and carbonate sediments with unusual sedimentary features. The stratigraphy of these units is discussed here in detail for the first time since their initial definition, with particular emphasis on the sedimentology of carbonate units. Early Neoproterozoic shales of the Beesvlakte Formation, equivalent to the Zambian Katangan Copperbelt's Lower Roan Formation, host evaporitic lithologies and minor copper mineralisation. The overlying, dolomitic ~ 760 Ma Devede Formation contains carbonate platformal lithologies which are in many ways dissimilar to Phanerozoic shallow-water carbonates. This includes unusual “curl breccias”, sheet cavities, carbonate shrubs, and tepee carbonate lithologies which contain large quantities of fibrous cements. “Curl breccias” are defined here as distinctive, curled intraclasts of laminated dolomite that often have shrinkage cracks in their margins, and are cemented by fibrous dolomite cements. Fibrous cements take on two forms: an early, length-fast fascicular-optic dolomite and a later length-slow phase with unit extinction. The presence of overlying internal sediments, the fibrous habit of these first-generation cements, as well as their preserved cathodoluminescent and optical character, suggests that these cements originally precipitated as calcite and dolomite marine cements respectively. After this initial marine calcite precipitation, all components of Devede Formation carbonates have been mimetically dolomitised, preserving original depositional fabrics. Combined with the presence of marine dolomite cements, this style of dolomitisation is suggested to be syn-sedimentary, similar to that of some Cryogenian dolomites, suggesting unusual ocean conditions during the Early Neoproterozoic. In particular, the presence of dolomite marine cements, which have been linked to ocean anoxia and high seawater Mg/Ca conditions, suggests that the onset of marine anoxia in the Neoproterozoic may have occurred during the growth of the Devede Formation carbonate platforms, prior to the Sturtian glaciation. This implies that glaciation may not be the sole cause for the development of marine anoxia during the Neoproterozoic. It is possible that marine anoxia in this southern African ocean basin may have contributed to a build-up of metals in seawater, perhaps pre-enriching basinal fluids for large-scale stratabound copper mineralisation.

Introduction

Neoproterozoic sedimentary successions across the world share several relatively unusual stratigraphic elements, including the presence of distinctive, thick glacial units and strange carbonate lithologies (e.g. Hoffmann and Prave, 1996, Hoffman et al., 1998, Preiss, 2000). The largest two episodes of glaciation during this time, termed the (earlier) Sturtian and (later) Marinoan glaciations are widespread stratigraphic marker units which can be globally correlated, defining an icehouse interval sometimes called “Snowball Earth” (e.g. Hoffmann and Prave, 1996, Hoffman et al., 1998, Preiss, 2000, Halverson et al., 2005, Rooney et al., 2014).

The Neoproterozoic Otavi Group of the Otavi Fold Belt, Namibia preserves a well-exposed, dominantly carbonate succession including sediments from both large-scale glaciations (Hoffmann and Prave, 1996) (Fig. 1, Fig. 2). In the last two decades, this Namibian stratigraphy has been the subject of much interest, largely due to the controversies surrounding these extreme glaciations (e.g. Hoffman et al., 1998). However, the pre-Sturtian (Tonian to early Cryogenian) Neoproterozoic succession, lacking extreme glacial episodes, remains relatively poorly understood. Developing an understanding of the paleo-oceanic and environmental conditions during this time is important, as this interval represents the lead-up to one of the coldest periods in Earth's history. Therefore a better understanding of conditions in these pre-glacial oceans, developed through sedimentology and stratigraphy, could help reveal why the Earth transitioned into such an extreme ice age.

In the Otavi Fold Belt, this interval is encompassed by the Ombombo Subgroup of the Otavi Group, as defined by Hoffman and Halverson (2008). This is approximately equivalent to the Zambian Katangan Copperbelt stratigraphy, which hosts world-class stratiform copper deposits (e.g. Cailteux et al., 2005, Miller, 2013, Rooney et al., 2015). Early work on this Namibian Neoproterozoic stratigraphy was undertaken by Söhnge (1957), Martin (1965), and Hedberg (1979), who established the first regional stratigraphic framework for the Otavi Group. Hoffmann and Prave (1996) developed a more complete framework which correlated the northern Otavi Fold Belt stratigraphy and identified two separate glacial intervals, representing a major advance in understanding. Hoffman and Halverson (2008) interpreted the basinal setting and basic stratigraphy of the Pre-Sturtian, Ombombo Subgroup in the southern Kaokoveld, and the area around the Kamanjab Inlier, northern Namibia. New age constrains in this succession have helped correlate this stratigraphy across northern Namibia (e.g. Halverson et al., 2005, McGee et al., 2012).

This study focuses on the stratigraphy and sedimentology of the early Neoproterozoic Ombombo Subgroup in the southern and northern Kaokoveld, Namibia. Building on the stratigraphic framework provided by Hoffmann and Prave (1996) and Hoffman and Halverson (2008), this work establishes the carbonate sedimentology of the Ombombo Subgroup with the aim of better understanding the paleoceanography and associated carbonate formation during the pre-Sturtian period. The significance of copper mineralization in the Ombombo Subgroup is also discussed, in relation to this stratigraphy and palaeoceanography.

Fieldwork was undertaken in the Kaokoveld, Namibia and included sample collection, geological mapping, and the measurement of stratigraphic sections and drill core logs. Unusual cemented carbonate breccias are prevalent in this succession and have been documented here in detail. Combined with detailed stratigraphic work, the carbonate sedimentology of the Ombombo Subgroup has revealed information about unusual seawater conditions during the Early Neoproterozoic. It appears that the build-up of marine anoxia was tied into the lead-up to the Neoproterozoic “Snowball Earth” in this ocean basin.

Section snippets

Early Neoproterozoic stratigraphy and basin formation

The well-exposed Otavi Fold Belt extends at least 750 km across Northern Namibia from the Otavi Mountainland in the east, to the Kaokoveld in the north-west (Hedberg, 1979, Hoffmann and Prave, 1996, Miller, 2008) (Fig. 1, Fig. 2). This fold belt represents the Northern Platform and Northern Margin Zone of the Kaoko and Damara belts (e.g. Hoffman and Halverson, 2008, Miller, 2008). The Otavi succession was deposited in Neoproterozoic sedimentary basins on the southern edge of the Congo Craton (

Results — stratigraphy and sedimentology

In this study, eleven outcrop stratigraphic sections and five drill cores were measured at several localities from north to south, on both the eastern and western margins of the Otavi Fold Belt (Kaoko Belt) in the Kaokoveld (Fig. 3, Fig. 4, Fig. 5). All outcrop stratigraphic sections were measured using a Jacob's staff (with Abney level). Around 100 large format thin sections were examined using plain light and cathodoluminescence microscopy. Cathodoluminescence microscopy was carried out on a

Beesvlakte Formation

The Beesvlakte Formation as recorded in stratigraphic sections in this study consists of a lower metre-scale dolomite unit, a middle, thick shale unit, and an upper carbonate unit. The lower Beesvlakte Formation records the initial marine transgression in the opening of this rift basin (Hoffman and Halverson, 2008). Therefore the lowermost dolomite unit may represent a condensed section developed on a transgressive surface during the early stages of marine incursion, similar to a Phanerozoic

Conclusions

This study documents the early Neoproterozoic stratigraphy and sedimentology of the Ombombo Subgroup, Namibia in an effort to understand marine conditions in the lead up to extreme Cryogenian glaciation. This stratigraphy builds on and adds to an earlier stratigraphic framework defined by Hoffmann and Prave (1996) and Hoffman and Halverson (2008). This work includes redefining the internal stratigraphy of the Beesvlakte Formation from the original type section; the recognition of

Acknowledgements

The authors would like to thank Teck Namibia Ltd., as well as the Namibian Geological Survey for their ongoing support during fieldwork. The authors acknowledge the Australian Research Council Discovery Project DP130102240 for financial assistance with this research. We are grateful to B. Jones, P. Pufahl, and an anonymous reviewer for their careful reviews and comments, which have improved the manuscript.

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