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Age of Manson impact
I appreciated the remarks of Tom Holz in Digest 72 concerning the
Manson impact, but I must correct the statement he made about its age
with respect to the Cretaceous NALMAs. He wrote that the age of the
Manson impact was "actually . . . mid-Campanian, between . . . Aquilan
and the highly diverse Campanian [Judithian] NALMAs." While I don't
mean to quibble, this is most assuredly incorrect.
The Aquilan-Judithian correlation for Manson was published by Russell
(1993), but the basis for this was not explained. Several lines of
evidence invalidate this proposal. First, Manson ejecta in the Crow
Creek Member (Pierre Shale) occurs within strata that are constrained
to the E. jenneyi or D. stevensoni marine ammonite zones (see Izett et
al., 93, and subsequent publications) [it could be as high as the
lower D. cheyennense zone]. Based on the definitions of Lillegraven
and Ostresh (1990, GSA Sp. Pap. 243), these ammonites occur high in
the Judithian NALMA, one zone short of their proposed boundary for the
Edmontonian [if D. chey., it is Edmontonian].
Second, the Crow Creek Member represents the basal deposit of the
Bearpaw marine cycle of the Western Interior (see Witzke et al., 96,
GSA SP 302). Marine shales of the Bearpaw cycle overlie the classic
Judith River sequence, suggesting a position near the top of the
Last, an age date of 74 Ma for the Manson impact is too young for the
Aquilan-Judithian boundary (which is about 79 Ma). These ages follow
radiometric ages-ammonite zonation scheme of Obradovitch (1993).
After Lillegraven and Ostresh, the top of the Aquilan is drawn above
the B. asperiformis ammonite zone (about 79 Ma).
As suggested by Russell (93), however, the position of the Crow Creek
Member (which approximates the age of the Manson impact) does indeed
correspond to the upper boundary of his "Niobraran" marine vertebrate
age. Is this mere coincidence? Russell thinks not.