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Manson Impact Structure



MANSON IMPACT

This is a response to William Wiesel's note in Digest 58 concerning
the Manson Impact Structure in Iowa (the largest known impact
structure in the continental U.S. with a diameter of 38 km).  Yes,
initial published radiometric ages from the Manson structure did
indeed indicate a K-T age, raising speculation about a possible role
in the K-T extinctions.  Intensive core drilling of the crater in
1991-92 resulted in a flurry of studies, which included new age dates
from melt-precipitated feldspars (Izett et al., 1993, Science, 262,
p.729-732) - 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").

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.  However, Russell (1993; Science, v. 24, p. 1211;
Geol. Assoc. Canada Spec. Pap. 39, p. 665-680) suggested that marine
vertebrate assemblages of the Western Interior Niobraran "age" "were
likely disrupted as a result of the Manson impact event", and "it
seems likely that local vertebrate faunas were badly distrubed by the
Manson impact and that taxa which were exterminated were replaced by
immigrants from beyond North America."  In particular, certain
lineages of mosasaurs, marine turtles, and fish vanished from the
Western Interior at a time that approximates the Manson impact event.

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.
The effects of the shock wave produced by the impact have been
tentatively scaled: 1) all combustible material within 200 km would
have been ignited, 2) all standing vegetation would have been
devastated to 600 km, 3) most terrestrial animals would have been
killed by the shock to 1000 km (as far as Montana), and 4) large
animals (e.g., dinosaurs) would have been knocked off their feet as
far as 1300 km.  Tsunami-like waves may have surged across the nearby
Western Interior seaway.  Considering the extent of the devastation,
the terrestrial biota apparently quickly recovered, undoubtedly
replaced by migrants from elsewhere on the continent.

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?).  It should also raise questions about
the reality of Raup's "Kill Curve," which supposedly correlates impact
size with percent global extinction.  At present, we simply don't know
the global or local biotic effects of large impacts - perhaps further
study of well-dated large impact events, like Manson, Manicouagan,
Popigai, Chicxulub, and others, can help constrain some of the
speculations.

I've had the pleasure of working closely with a number of colleagues
on the Manson Impact (and coeval ejecta in the Crow Creek), and
collectively, we've added much information to the still poorly known
field of large impact structures.  Manson is now one of the best known
large impact structures on earth.  For the most current information,
see our recent publication, "The Manson Impact Structure, Iowa;
Anatomy of an Impact Crater" (1996, edited by C.  Koeberl and
R.R. Anderson, Geological Society of America, Special Paper 302, 468
p.).

Brian Witzke
Research Geologist, Iowa DNR-Geological Survey
  and Adjunct Assoc. Prof., Dept. Geology, Univ. Iowa