Unique marine Permian–Triassic boundary section from Western Australia

Abstract

A unique marine Permian–Triassic boundary section containing rich oil source rocks has been continuously cored in a petroleum borehole from the Perth Basin of Western Australia. Such sequences, which provide a biostratigraphic and environmental record at the time of the largest extinction event of the past 500 million years, are globally rare, and this is the first to be documented in Australia. Throughout geological history there have been periods of global marine anoxia that commonly resulted in the widespread deposition of petroleum source rocks, most notably in the mid‐Cretaceous and Late Jurassic. An apparent paradox is that, previously, source rocks have not been recognised in association with the Permian–Triassic boundary, despite widespread marine anoxia at this time. The Perth Basin source rocks contain abundant and unusual biomarkers, apparently related to the highly specialised and limited biota that flourished in the aftermath of the end‐Permian extinction event. Local conditions may have favoured source‐rock development, either due to higher productivity resulting from coastal upwelling or through enhanced preservation under strongly anoxic conditions.

Occurrence and significance of phytanyl arenes across the Permian-Triassic boundary interval

“…The local PTB in the Hovea-3 core was identified at a depth of 1980.95 m by Thomas et al (2004) but, due to a likely hiatus, the horizon also corresponds to the level at which the Permian fauna and spores/pollen disappear (Grice et al, 2005b). Phytanyl benzene and phytanyl toluene were present in samples from 1980.92 m and above, corresponding to the Lower Triassic upper Sapropelic Interval of the Hovea Member, but not in any Permian sample.…”

“…The mid-southern palaeolatitude (Fig. 1) Hovea Member was deposited in a marine shelf environment, with a lower 'Inertinitic Interval' of bioturbated sandstones and mudstones, and an upper 'Sapropelic Interval' of laminated mudstones, including oilprone source rocks (Thomas et al, 2004). The abrupt facies change at the boundary between these two intervals (at a depth of 1980.95 m) implies the presence of a hiatus, due to nondeposition during the transgression.…”

“…The abrupt facies change at the boundary between these two intervals (at a depth of 1980.95 m) implies the presence of a hiatus, due to nondeposition during the transgression. A major palynological change occurs at this surface, which Thomas et al (2004) used to define the local PTB in the absence of conodonts. In more complete sections elsewhere, similar palynological changes record the extinction event, which occurred prior to the PTB (e.g.…”

“…Abundance of phytanyl benzene (pbenz) and phytanyl toluene (ptolu) relative to phenanthrene (phen) and C 33 n-ACH relative to n-C 34 (from Grice et al, 2005b). Stratigraphic log of the Hovea-3 core, Perth Basin, Western Australia is redrawn from Thomas et al (2004). Permian/Triassic boundary (PTB) as defined by Thomas et al (2004).…”

“…The core was drilled in the northern Perth Basin through the source rocks of the organic-rich Hovea Member (Thomas et al, 2004). The mid-southern palaeolatitude (Fig.…”