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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 
     Judithian.
     
     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.