JUST HOW BIG IS COUGAR POINT TUFF XIII?

Large-volume ignimbrites are well-known from the youngest part of the Yellowstone hotspot track in the Yellowstone and Heise eruptive centres but their existence in the older record of volcanism is less well-constrained. Recent work along the southern margin of the Snake River Plain (SRP) has managed to correlate the ignimbrite sheet of Cougar Point Tuff XIII (CPT XIII) >75 km eastwards from where it was first recognised in the Bruneau Jarbidge region to the Cassia Mountains. Correlation of units in the SRP has been achieved by using the existence of multiple compositional populations of both pigeonite and augite within an ignimbrite sheet in concert with high precision ⁴⁰Ar/³⁹Ar geochronology, O isotopes, magnetic polarity and whole rock compositions. The upshot of this correlation is that a volume of approximately 1,000 km³ was estimated for CPT XIII. Given that the source for CPT XIII is not exposed and inferred to lie within the Snake River Plain, it might be expected that some of the outflow should be exposed to the north of the plain. Our work in the East Bennett Mountains, north of the plain has shown that the Tuff of Fir Grove (previously thought to be somewhat older than CPT XIII) overlaps in age with CPT XIII via ⁴⁰Ar/³⁹Ar geochronology derived in two laboratories. In addition, the two ignimbrites share identical mineral compositions in terms of augite, plagioclase and ilmenite plus show similarity in whole rock compositions. The possibility of correlation is further supported by the two units containing identical isotopic compositions in terms of δ¹⁸O and Sr, Nd and Pb ratios. Should CPT XIII and the Tuff of Fir Grove represent the products of a single eruption, then the volume of the resultant eruption truly would be ‘super’.