Mass extinction of peat-forming plants and the effect on fluvial styles across the Permian–Triassic boundary, northern Bowen Basin, Australia
Introduction
The Permian–Triassic boundary event represents the most extensive mass extinction in the geological record (e.g. Erwin, 1990, Hallam and Wignall, 1997, Lucas and Yin, 1998, Hansen et al., 2000, Jin et al., 2000), marking the extinction of 80–97% of all species (Retallack, 1995). The Permian–Triassic boundary is well constrained in the northern sector of the Bowen Basin of eastern Australia (Fig. 1) by palynology (Michaelsen et al., 1999), organic carbon isotopes (Hansen et al., 1999, Hansen et al., 2000) and stratigraphic and sedimentological records (Michaelsen et al., 2000). The boundary is essentially conformable but marks a major environmental shift which is reflected in the basinal stratigraphy by separation of the coal-bearing Blackwater Group from the coal-barren Rewan Group (Fig. 2).
The end-Permian mass extinction event marked the collapse of marine and terrestrial ecosystems on a world-wide scale (e.g. Erwin, 1993, Erwin, 1994). Significantly, the boundary event extinguished c. 95% of peat-forming plants in Australia (Retallack, 1995). In the northern Bowen Basin a long-lived (c. 9 Myr) peat mire ecosystem was abruptly terminated (Michaelsen et al., 1999). The mass extinction is represented in the stratigraphic record by a global fungal event (e.g. Eshet et al., 1995, Visscher et al., 1996), recording excessive dieback of arboraceous vegetation, an event documented in the Bowen Basin by Foster (1982). The mass extinction of peat-forming plants was followed by a global Early Triassic coal gap, which lasted some 6 million years (Retallack et al., 1996, Retallack, 1999).
The aim of this contribution is to examine how the mass extinction of peat-forming plants at the Permian–Triassic boundary affected the fluvial architecture across the boundary succession in the northern sector of the Bowen Basin. Fluvial architecture is thought to be essentially controlled by three factors: accommodation space, sediment supply and hydrodynamics (e.g. Puigdefabregas, 1993). However, this study highlights a fourth element, vegetation, as an architectural control of considerable importance affecting channel morphology and behaviour through bank stability (e.g. Smith, 1976, Stanistreet et al., 1993, Miall, 1996, Ward et al., 2000).
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Strato-tectonic context of the Permian–Triassic succession
The northern Bowen Basin forms the northernmost extension of the Early Permian–Middle Triassic Bowen–Gunnedah–Sydney Superbasin, a major feature in the crustal fabric of eastern Australia, extending for >2000 km north–south. The basin has a complex, polyphase history with an early extensional, back arc phase followed by an episode of thermal recovery with a subsequent retro-arc foreland stage of evolution (Murray, 1983). The northern sector of the Bowen Basin has a maximum stratal thickness of
Methods
This study has drawn on a new database compiled in the context of regional exploration work targeting coal seam gas in the early 1990s by Mitsubishi Gas Chemical Resources Australia. This database consists of 611 line-km of seismic profiles, calibrated with deep, well-distributed drillholes (Fig. 1). The present study is based on detailed logging of 88 drillholes, totalling almost 9.5 km in composite length, 11 of which penetrate the Permian–Triassic boundary. Event signatures, lithofacies
Sandstone petrology across the Permian–Triassic boundary
Sandstone petrology analyses were conducted on 38 samples from the Rangal Coal Measures and 28 samples from the Sagittarius Sandstone (Fig. 3). In the Rangal Coal Measures the highest proportion of framework grains are lithics (62%), followed by quartz (29%) and feldspar (9%), with the source being an undissected to transitional magmatic arc flanking the basin to the east (Michaelsen and Henderson, 2000a). Within the Sagittarius Sandstone a significant enrichment of quartz and decrease in
Organic carbon isotope data across the Permian–Triassic boundary
A new comprehensive record of organic carbon isotope across the Permian–Triassic boundary sequence at the Newlands coal mine (Fig. 1) has recently been disclosed by Hansen et al., 1999, Hansen et al., 2000. The organic carbon isotope record, derived from 55 samples, shows a prominent negative excursion 1.5 m beneath the lithostratigraphic boundary between the Rangal Coal Measures and the Sagittarius Sandstone (Fig. 4). This excursion is also reflected in data from the southern sector of the
Palynological record across the Permian–Triassic boundary
A palynological investigation of the Permian–Triassic boundary sequence exposed in the Newlands coal mine based on a limited number of samples found a gradational floral change with the proportion of ‘Triassic’ taxa gradually increasing through the boundary succession (Michaelsen et al., 1999). Samples from the Rangal Coal Measures were found to be quantitatively rich in species such as Triplexisporites playfordii and Falcisporites australis, which become persistent and characteristic
Late Permian fluvial style
Facies and stacking patterns of the Rangal Coal Measures have been investigated in drillcores from extensively drilled exploration areas and in spaced regional drillcores (Fig. 1, Fig. 5). Highwall exposures of the Rangal Coal Measures in three coal mines in the northern Bowen Basin, provided an excellent opportunity to calibrate drillcore data with outcrop information (Fig. 6A). This integrated approach has identified six recurring fluvio-lacustrine depositional systems: (1)
Early Triassic fluvial style
The Sagittarius Sandstone is dominated by green, well-sorted, fine- to coarse-grained sandstone, with subordinate conglomerate, siltstone and mudstone. The basal part of the Sagittarius Sandstone is characterised by abundant rip-up clasts consisting of elongate siltstone and organic debris, ranging up to 25 cm in size (Fig. 5, Fig. 6). The Sagittarius Sandstone is characterised by a rarity of thick beds, they are typically 0.3–1 m. The formation is characterised by a significantly higher
Discussion
The Permian–Triassic boundary in the northern sector of the Bowen Basin appears conformable but marks a dramatic environmental shift reflected in the basinal stratigraphy by separation of the coal-bearing Blackwater Group from the coal-barren Rewan Group. The boundary is well constrained by a prominent negative organic carbon isotope anomaly (Fig. 4), palynology and stratigraphic data.
The sedimentary architectural record shows a significant change in fluvial style across the Permian–Triassic
Summary
The Permian–Triassic boundary in the northern Bowen Basin is marked by ecological disaster. A long-lived (c. 9 Myr) peat mire ecosystem collapsed when c. 95% of all peat-producing plants became extinct. The stratigraphic record shows no diminution in the thickness, lateral extent or spatial distribution of coal seams prior to the boundary event. However, the palynological record indicates a gradational floral change with the proportion of ‘Triassic’ taxa gradually increasing prior to the
Acknowledgments
This work is part of an ongoing sedimentological and stratigraphic research programme in the northern Bowen Basin. I would like to thank the German Creek, Moura, Newlands and Burton coal mines for access to highwalls, company data, and generous help with logistics. In particular, thanks to mine geologists Bernadette Williams, Sarum Peou and Ray Slater. Special thanks to Hans Jørgen Hansen and Steen O. Mikkelsen, University of Copenhagen, for the use of isotope data, cheerful company at Newlands
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