Asteroids - Extreme Events: A Physical Reconstruction and Risk Assessment - page 244

in a ferruginous stromatolite bed in the stratigraphic record within the Can-
ning Basin. Wallace et al. (1991)statethat several factors suggest that the Ir rich
layers here, and other sites with similar characteristics, are of terrestrial origin
because:

(1) no shock-metamorphosed minerals have been detected at the F--F

boundary;

(2) thegeochemistry of ferruginous crusts is invariably non-chondritic

(both platinum metals and other trace elements);

(3) downward-orientated ferruginous crusts in cavities have similar plat-

inum metal concentrations; this suggests a solution source (seawater)
rather than particulate ejecta products; and

(4) association between ferruginous stromatolitic horizons and subareal

exposure events suggest that changing sea levels (regression followed
by transgression) have indirectly controlled the crust formation.

As Cambrian, Devonian and Tertiary horizons investigated in the Canning
Basin show evidence of subaerial exposure in shallow-water lithologies, but not
in deep-water sequences, it is assumed that the regressive events were of moder-
ate scale. It is also possible that the ferruginous crusts are more directly related
to rapid sea-level rise than regression. Further research is needed, however, to
determine this. Together, these factors indicate that the Ir anomaly found at
the F--F boundary is related to microbial processes and sea-level change (most
likely rapid drowning). It is also possible that inorganic processes such as surface
adsorption of platinum metal complexes onto positively charged Fe-hydroxide
particles created the anomaly (Wallace et al. , 1991).
Nicoll and Playford (1993)also examined the Ir anomalies supposedly found
at the F--F boundary in the Canning Basin, Western Australia. They placed more
specific time constraints on the anomalies and were able to show that Ir anoma-
lies are located significantly both above and below the F--F boundary and the
major extinction event. They also found no direct evidence of a major extrater-
restrial impact.
The Hagenberg event, a mass extinction event found at or near the Devonian--
Carboniferous (D--C) boundary, has also been associated with an Ir anomaly.
Attrep and Orth (1993)examinedwhethertheIranomaly and the mass extinc-
tion at the D--C boundary was related to an extraterrestrial impact. They note
that even though this Ir anomaly is found within a bed of black shale, known
as the Hagenberg shale, globally, the stratigraphic position of this shale relative
to theD--C boundary varies between sites. In France, the black shale is located
about 20 cm under the D--C boundary whereas it lies immediately below the
boundary in China. The D--C boundary Ir anomaly is comparable in magnitude to