Chemostratigraphy of the Boolgeeda Iron Formation and Turee Creek Group, Hamersley Basin, Western Australia

Abstract

In the most general sense, the Great Oxygenation Event (GOE) refers to the transition from oxygen poor to oxygen rich conditions on ancient Earth (Holland, 2002; Holland, 2006). It has traditionally been defined as the sedimentary intervals in which the isotopic signature for mass independent fractionation of sulfur (MIF-S) disappears (Farquhar et al., 2000), which occurs above oxygen concentrations of 10-5 times present atmospheric levels (PAL; Pavlov and Kasting, 2002). In South Africa, this interval corresponds with the Pretoria Group in the Transvaal Basin; specifically, the lower Timeball Hill formation and upper Deutschland Formation (Bekker et al., 2001; Guo et al., 2009). The age for this event is constrained to approximately 2.32 Ga by Re/Os dating of pyrite within the Timeball Hill Formation (Bekker et al., 2001). However, Reinhard et al. (2013) suggested S isotopic signatures defining MIF-S are expected to be preserved for 10-100 million years after the GOE, as minerals hosting the MIF signature are cycled through terrestrial reservoirs and recycled during subduction and volcanic processes. The prospect of MIF-S recycling forces an investigation into the 100 Mya (or more) prior to 2.32 Ga, in hopes of identifying recycling signatures representative of the Reinhard et al. (2013) models, and highlighting proxies that record the initial rise in atmospheric O2 which eventually eliminated the MIF-S signal. Recently, Philippot et al. (2018) reported a MIF-S recycling signature in the Boolgeeda Iron Formation (IF) and overlying Turee Creek Group (TCG) of Western Australia. These sediments are particularly relevant to those in the Transvaal Basin as they represent time equivalent deposits (2.45-2.209 Ga; Trendall et al., 2004 and Martin et al., 1998, respectively). If the rise of atmospheric O2 was truly a global event, similar redox proxies should be prevalent in the sediments of both basins at similar stratigraphic intervals. The work presented in this thesis represents a search for O2 proxies other than MIF-S, that are prevalent within the Boolgeeda IF and TCG. This search was focused around major and trace element analyses, specifically Ce anomalies, Mn enriched intervals, and intervals portraying anomalous concentrations of redox sensitive trace elements. Results suggest the water column during Boolgeeda IF was persistently oxic in shallow environments, and anoxic and ferruginous in deep water sediments. Periodic weathering of continental and shallow marine sulfides may have also lead to temporary euxinic conditions in marine environments proximal to the continental margin. Thus, it would appear deposition of Boolgeeda and Turee Creek Group sediments occurred after the initial rise of atmospheric O2, but before levels reached appreciable levels to completely suppress MIF-S recycling signatures. The data presented in this thesis, combined with existing data from time equivalent basins in South Africa and North America, will lead to a better understanding of O2 dynamics during the early stages of the GOE and a more complete picture of the global rise of O2 on early Earth.