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Re: Manson Impact Structure



Brian Witzke wrote:

>This is a response to William Wiesel's note in Digest 58 concerning
>the Manson Impact Structure in Iowa [...] an age of about 74 Ma now
>seems secure, placing the age of the impact during the late
>Campanian (near the boundary of the Judithian-Edmontonian vertebrate
>"ages").

Actually, the date (which is correlated with some interesting beds within
the Western Interior marine sequence, see Izett et al.) is at the
mid-Campanian, between the poorly-known Aquilian and the highly diverse
Campanian NALMAs.

>This date does not apparently correspond to any major terrestrial
>extinction event (local or global), and there is little evidence among
>the many lineages of marine invertebrates for noteworthy extinctions
>at that time.

True.  In fact, it immediately precedes the peak diversity of North American
and Asian dinosaurs, but this is more likely attributable to the poor early
Campanian rock record than to anything else.

>The Manson event was undoubtedly a major catastrophe: the impact
>released about 2.2 x 10 (21 power) Joules of energy (!) and launched
>about 1000 cubic kilometers of ejecta (15% into the troposphere,
>shrouding the earth).  Manson ejecta is now clearly identified within
>the Pierre Shale (Crow Creek Member) of South Dakota and Nebraska.

That's what I love about the Izett et al. paper: uses good old fashioned
stratigraphy as well as radiometrics and all that jazz!

>Considering the extent of the devastation, the terrestrial biota
>apparently quickly recovered, undoubtedly replaced by migrants from
>elsewhere on the continent.

Or Asia, Russell's theory.

>There is probably a lesson here concerning dinosaur (and K-T)
>extinctions - if Manson wasn't large enough to do them in (and if an
>impact was involved in the extinction process at all), it must have
>taken one whale of an exceptional impact event to compromise the
>global biota (like Chicxulub?).

Unless Officer et al. are correct, and Chicxulub is mid-Campanian as well,
in which case NO impact known can be big enough to cause a mass extinction.
(I don't buy this, but if they do turn out to be correct, impact theory is
on pretty shaky grounds).

Thomas R. Holtz, Jr.
Vertebrate Paleontologist     Webpage: http://www.geol.umd.edu
Dept. of Geology              Email:th81@umail.umd.edu
University of Maryland        Phone:301-405-4084
College Park, MD  20742       Fax:  301-314-9661

"There are some who call me...  Tim."
-- Tim the Enchanter, "Monty Python and Quest for the Holy Grail"

"Tim?!?  They called me TIM?!?!"
-- me, on seeing the credits to "The Ultimate Guide to T. rex"  :-)