Permian-Triassic Boundary Outcrop
The Permian–Triassic (P-T) boundary outcrop near Wollongong, Australia, particularly at Austinmer, a coastal suburb in the Illawarra region, is a significant geological site for studying the most severe mass extinction event in Earth’s history, dated to approximately 251.9 million years ago.
This boundary, marking the transition from the Permian to the Triassic period, is associated with the collapse of up to 96% of marine species and 70% of terrestrial vertebrates, likely driven by massive volcanic activity in the Siberian Traps, global warming, ocean anoxia, and widespread wildfires.
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| Permian-Triassic Boundary Outcrop.This place is located at Austinmer, a coastal suburb between Sydney and Wollongong. |
The Permian–Triassic (P–Tr) extinction event, colloquially known as the Great Dying, the End Permian or the Great Permian Extinction. occurred about 252 Ma (million years) ago, forming the boundary between the Permian and Triassic geologic periods, as well as the Paleozoic and Mesozoic eras.
It is the Earth's most severe known extinction event, with up to 96% of all marine species and 70% of terrestrial vertebrate species becoming extinct. It is the only known mass extinction of insects. Some 57% of all families and 83% of all genera became extinct.
Because so much biodiversity was lost, the recovery of life on Earth took significantly longer than after any other extinction event, possibly up to 10 million years.
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| Permian-Triassic boundary outcrop near Wollongong, Australia |
The Bulli Seam, the uppermost coal seam of the Illawarra Coal Measures, is a key marker at or just below the P-T boundary in this region. At Austinmer, the boundary is identified within or above the coal-bearing Permian strata, transitioning to the Triassic Narrabeen Group (e.g., shales and siltstones). The contact is often marked by a sharp change in lithology, from coal and carbonaceous mudstones to reddish or grayish clastic sediments, reflecting a shift from lush, swampy environments to more arid, post-extinction conditions.
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| PT boundary section at Bells Point: Tongarra Coal Measures capped by deltaic Austinmer Sandstone, Sydney Basin. Photo by: Dietmar Down Under. |
Evidence of the P-T Boundary:
Stratigraphy: The Bulli Seam at Austinmer and nearby sites (e.g., Clifton, Coalcliff) represents the late Permian coal-forming environment. The P-T boundary is typically located at the top of the coal or in the overlying thin shale/siltstone layers, where coal deposition ceases.
Biostratigraphy: Fossil floras show a dramatic turnover. Permian glossopterid-dominated plant assemblages (e.g., Glossopteris leaves and pollen) disappear, replaced by sparse Triassic floras (e.g., Dicroidium). Palynological studies from Austinmer and nearby sections confirm this abrupt change.
Geochemical Signatures: Carbon isotope excursions (negative shifts in δ¹³C) are detected in the coal and overlying sediments, consistent with global P-T boundary signatures linked to volcanic CO₂ emissions and ecosystem collapse.
Trace Fossils: Large burrows at the top of the Bulli Seam suggest environmental stress, possibly due to desiccation or wildfire activity, which are implicated in the extinction event.
Environmental Indicators: The Austinmer outcrop records evidence of late Permian environmental deterioration, including increased aridity, wildfire activity (charcoal layers), and loss of peat-forming ecosystems. These changes align with global patterns of climate disruption during the P-T extinction.
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