TWO LATE EDIACARAN MASS EXTINCTIONS COMPARABLE WITH PERMIAN-TRIASSIC EVENTS

Two mass extinctions have long been recognized: (1) the Ediacaran Complex Acanthomorph-dominated Palynoflora (ECAP) was replaced within the upper Wonoka Formation of South Australia with a low diversity “Kotlin-Rovno” or Late Ediacaran Leiosphere-dominated Palynoflora (LELP), and (2) termination of most Ediacaran megafossils at the end of the Ediacaran between the Rawnsley Quartzite and Uratanna Formation of South Australia. The ECAP extinction of loosely fractal Avalon forms (including Fractifusus) was later succeeded by more compact Ediacara-White Sea forms (including Dickinsonia). The Ediacara-White Sea assemblage was not entirely extirpated, because cf. Swartpuntia survived into the Cambrian Uratanna Formation of South Australia, and comparable forms are known from Ordovician-Silurian (Rutgersella) and Devonian (Protonympha). Re-examination of rocks analyzed for stable isotopic analysis in the Flinders Ranges has shown that the so-called Shuram-Wonoka negative anomaly in δ¹³C and δ¹⁸O is a composite of pedogenic and marine carbonate spanning some 17 m. yrs in a local age model. Disregarding pedogenic carbonate, the synchronous marine organic-carbonate excursion is only -8 ‰ for δ¹³C organic and -6 ‰ for δ¹³C carbonate, and lasted less than a million years at about 564 ± 6 Ma using the same age model. This revised magnitude and duration is comparable with isotopic perturbations across the Permian-Triassic boundary, and the age model demonstrates that it coincided with the ECAP mass extinction. Permian-Triassic isotopic excursions have been attributed to greenhouse crises from CO₂ and CH₄ cracked from buried organic matter by flood basalts in Siberia. Similarly the revised Wonoka isotopic excursion and ECAP mass extinction were coincident with flood basalts of the Lac Matapédia dated at 565 ± 6 Ma by zircon U-Pb. A similar isotopic excursion has long been known at the Cambrian-Precambrian boundary (541 ± 0.6 Ma) in Yunnan (China), coincident with flood basalts of the Volyn Series of Ukraine (545.5 ± 4 Ma by K/Ar). The Antrim Plateau Volcanics of northwestern Australia are no longer believed to straddle the Cambrian-Precambrian boundary, but are Early Cambrian in age (508 ± 2 Ma from feldspar ⁴⁰Ar/³⁹Ar).