--> Understanding the Geographical Distribution of Historical and Modern Asphaltite Strandings Along the South Australian Coastline

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Understanding the Geographical Distribution of Historical and Modern Asphaltite Strandings Along the South Australian Coastline

Abstract

The coastal asphaltites of South Australia have source and age-specific biomarker signatures suggestive of an origin from a Cretaceous marine shale deposited under sulfidic conditions during an oceanic anoxic event (Edwards et al., 1998; Boreham et al., 2001). They are very likely to be of local origin in the offshore Ceduna or Otway basins, although the exact location of their parent petroleum system has yet to be established (Edwards et al., 1998, 1999; Totterdell et al., 2008; Hall et al., 2014). The process of stranding has been postulated to involve leakage of hydrocarbons on to the seafloor (possibly as asphaltic volcanoes, or via exposure of tar mats: Hall et al., 2014) followed by entrainment in the prevailing oceanic currents and eventual deposition on the shoreline by surface wave and wind forcing of seasonally variable strength and direction. The asphaltites typically have quasi-neutral buoyancy (4–18° API), implying that they were submerged or even bottom drifters. Hence, it is not surprising that their historical stranding pattern should differ markedly from that of the pelagic tar balls (waxy bitumens) also found along the coastline (McKirdy et al, 1994; Padley, 1995). An informed understanding of their journey from low-intensity seafloor seep(s) to the shoreline therefore requires a detailed knowledge of the stranding distribution and both the surface and deepwater currents which traverse the continental margin of South Australia, including zones of upwelling (Middleton and Bye, 2007; Middleton et al., 2007). Whilst historical standings of asphaltites have been reported from southeastern South Australia, western Victoria and southern Tasmania, there are significant lengths of the South Australian coastline which have not been surveyed, particularly along the Eyre Peninsula. Therefore, a more comprehensive understanding of the potential geographical origin of the asphaltites has not been possible. This paper outlines the preliminary findings of beach surveys recently conducted along 31 beaches of South Australia, from the Western Australian border to the Victorian border. We compare the spatial distribution and abundance of both asphaltite and waxy bitumen on different beaches with the results of previous surveys, and review our findings in the light of differences in seasonal oceanic conditions.