Saturday, August 23, 2014
No mountain range, no active slipping fault,
And yet this plain had lava seas erupt.
We call them Kalkarindji flood basalt.
It’s hard to know just when these rocks were formed.
The weathering and rock type complicates
Radiometric dates of dykes that swarmed
When seas contained the first protochordates.
For ten long years they searched the outback rocks
For grains unhurt since fossils first were formed.
In hopes the nucleii-related clocks
Survived half billion years, still undeformed.
510 MA, a date of some distinction.
Flood basalts can lead to mass extinction.
F. Jourdan, K. Hodges, B. Sell, U. Schaltegger, M.T.D.Wingate, L.Z. Evins, U. Söderlund, P.W. Haines, D. Phillips and T. Blenkinsop. (2014) High-precision dating of the Kalkarindji large igneous province,
synchrony with the Early–Middle Cambrian (Stage 4–5) extinction. Geology 42
The voluminous Kalkarindji flood basalts erupted in
during the Cambrian and covered >2 × 106 km2. New U-Pb
and 40Ar/39Ar age data from intrusive rocks and lava
flows yielded statistically indistinguishable ages at ca. 511 Ma, suggesting a
relatively brief emplacement for this province. A zircon age of 510.7 ± 0.6 Ma
shows that this province is temporally indistinguishable at the
few-hundred-thousand-year level from the Early–Middle Cambrian (Stage 4–5)
boundary age of 510 ± 1 Ma, which marks the first severe extinction of the
Phanerozoic and an extended marine anoxia period. Sulfur concentration
measurements ranging from less than 50 to 1900 μg/g, and fractal analysis of extensive explosive volcanic breccias, suggest that blasts and phreatomagmatic explosions have contributed to injection of large amounts of sulfur into the stratosphere. In addition, magma intrusions in oil, gas, and sulfate deposits may have generated significant emission of CH4 and SO2
which, along with volcanic gases, would have combined to cause an oscillation
of the climate and led to the Cambrian extinction.