Mass extinction of peat-forming plants and the effect on fluvial styles across the Permian-Triassic boundary, northern Bowen Basin, Australia

摘要

The most spectacular extinction event in Earth's history occurred across the Permian-Triassic boundary. In the northern Bowen Basin, a major coal-bearing sedimentary basin in eastern Australia, a long-lived (c. 9 Myr), cold climate, peat mire ecosystem collapsed at the Permian-Triassic boundary when the vast majority (c. 95%) of peatproducing plants became extinct. The environmental change marked by the Permian-Triassic boundary is expressed as an abrupt and sharp change in sedimentary regime at the contact between the Rangal Coal Measures and the Sagittarius Sandstone. The stratigraphic record shows no diminution in the thickness, lateral extent or spatial distribution of coal seams prior to the boundary event. The abrupt ecological shift at the Permian-Triassic boundary was coincident with and interrelated to a change in landscape attributes and fluvial style. The boundary shift is considered to reflect a short-period radical atmospheric change accompanied by an abrupt change in plant ecosystems. However, palynological data indicate that it was preceded by a more gradual gross taxonomic progression in the floral succession. The boundary shift is unlikely to reflect change in the tectonic setting of the northern Bowen Basin because the detrital character of clastic sediment supply shows no provenance change within the boundary sequence. The Late Permian fluvial style is characterised by large-scale (up to 1 km wide), sandstone-dominated, low sinuosity, trunk river channel deposits. The trunk river channels were flanked by extensive levee/composite crevasse-splay systems. Channel tracts were relatively stationary in position over enduring periods, and developed stacked sediment accumulations up to 30 m thick. The constrained character of the Late Permian trunk river systems was most likely due to progressive compaction of thick tracts of peat substrate, and the stabilising effect of vegetation adjacent to the channel complex. The well-developed crevasse splays, coupled with the low sinuosity style of the fluvial channels, might suggest a perennial fluvial system, characterised by short discharge periods, as common in high-latitude settings. The fluvial architecture of the Sagittarius Sandstone, the basal formation of the Lower Triassic Rewan Group, is characterised by sheet-like elements, suggestive of broad, shallow channels in a deforested braid-plain setting. The channel deposits are considered to represent highly mobile sandy systems, dominated by a flashy runoff regime. The mass extinction of plants in the northern Bowen Basin at the Permian-Triassic boundary thus had a significant impact on the Early Triassic landscape and fluvial architecture.