The ca 2.74 Ga Mopoke Member, Kylena Formation: a marine incursion into the northern Fortescue Group?

Abstract

The northern part of the Fortescue Group consists of interbedded flood basalts and sedimentary rocks that were deposited on the southern margin of the Pilbara Craton, Western Australia, during one or more periods of continental rifting between ca 2.78 and ca 2.63 Ga. Well-preserved sedimentary intervals within the group have yielded stable carbon and sulfur isotope data that have been used to infer changes in geobiological processes in the Neoarchean. However, the Fortescue Group is notable for being a predominantly subaerial succession, and it remains unclear whether data obtained from these intervals should be interpreted in the context of deposition in marine environments, possibly recording changes in the global ocean/atmosphere system, or in local and restricted lacustrine settings. Here, we describe the sedimentology, stratigraphy, stromatolites and stable carbon isotope geochemistry of the ca 2.74 Ga Mopoke Member, Kylena Formation, the oldest stromatolitic horizon in the Fortescue Group. This unit differs in terms of internal stratigraphic relationships, sedimentology, carbonate mineralogy and stable isotope geochemistry when compared with intervals of probable lacustrine origin in the overlying Tumbiana and Maddina formations. In contrast, we suggest that parts of the Mopoke Member may have been deposited under open marine conditions, or alternatively, in a lacustrine environment characterised by differing water chemistry and basement topography. Stromatolitic microfabrics of the Mopoke Member are dominated by spar, dolospar and vertically aligned calcitic crusts, rather than the micritic microfabrics described from other Fortescue Group stromatolites. Mud-draped ripples are common sedimentary features in the Mopoke Member, suggesting a tidal influence. Mopoke Member δ¹³C_carb values are generally slightly positive, but also include some significantly depleted values, which may relate to the reoxidation of ¹³C-depleted organic matter. δ¹³C_org values average –36.7‰, consistent with Neoarchean marine units reported from elsewhere, but significantly less ¹³C-depleted than values reported from overlying lacustrine intervals in the Fortescue Group. We conclude that some features of Fortescue Group datasets relevant to the field of geobiology may be facies dependent, and that more work focusing on the overall depositional environments of the Fortescue Group is needed in order to appropriately interpret geobiological data reported from that group.

Fortescue群的北部由互层洪流玄武岩和沉积岩石组成，在一个或多个2.78至2.63 Ga之间的大陆开裂期间沉积于西澳大利亚Pilbara克拉通南缘。该群内保存完好的沉积层段已经产生了稳定的碳和硫同位素数据，已用于推断太古代地球生物学过程的变化。然而，Fortescue群因主要是陆上地层序列而知名，对于从这些地层获得的资料是否应被解释为沉积于海洋环境，记录着全球海洋/大气系统的变化，还是沉积于局部并限制的湖泊环境，还没有论断。在这里，我们描述了约2.74Ga的Kylena组Mopoke段的沉积学、地层学、叠层石和稳定碳同位素地球化学特征，这是Fortescue群的最古老的叠层石岩层。与上覆Tumbiana组和Maddina组中的湖泊源岩层相比，本单元在内部地层关系、沉积学、矿物学碳酸盐和稳定同位素地球化学方面都有不同之处。与此相反，我们认为，Mopoke段可能沉积于开放海洋条件下，或者沉积在水化学和基底地形不同的湖泊环境中。Mopoke段的叠层石显微组构主要由亮晶、白云石亮晶和垂直排列的方解石结壳组成，而非从Fortescue群其它叠层石岩层中描述的微晶显微组构。Mopoke段中泥波是很常见的沉积特征，表明潮汐的影响。 Mopoke段d₁₃C_carb 值通常略微正值，但也包括一些显著减少值，这可能与13C减少的有机物再氧化有关。 D₁₃C_org 值平均为-36.7％，与从其它地方报道的新太古代海洋地层一致，但与上覆Fortescue群的湖泊地层相比13C耗尽比值显著减少。我们的结论是， Fortescue群的与地球生物学领域有关的资料的一些特征可能依赖于岩相，并且有必要有更多的工作侧重研究Fortescue群的整体沉积环境，以正确解释该组的地球生物学资料。

KEYWORDS:

• Neoarchean
• tidalites
• methanotrophy
• microbially induced sedimentary structures
• stromatolites
• lacustrine

ACKNOWLEDGEMENTS

We thank the Geological Survey of Western Australia for generous logistical support in the field and Tamsyn Garby, Yosuke Hoshino and Kee Wenyu for their contribution to fieldwork. We also acknowledge Anne Rich and Karen Privat at the UNSW Analytical Centre, Anita Andrew at Environmental Isotopes and the West Australian Biogeochemistry Centre for assistance with analytical work. DTF was supported by a Commonwealth Government of Australia Australian Postgraduate Award. MRW was supported by the University of New South Wales and a grant from the Australian Research Council. MVK acknowledges funding from the University of New South Wales and the Agouron Institute. RM is grateful to the University of New South Wales for a postdoctoral fellowship. We thank Aivo Lepland, Kath Grey and David Martin for their comments on the manuscript.