Age of the Manson Impact Crater

From: bwitzke@gsbth-po.igsb.uiowa.edu

This is a response to William Wiesel's note in [Dinosaur Mailing List] 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 disturbed 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 10²¹ [2.2E21] 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.).

From: Thomas_R_HOLTZ@umail.umd.edu (th81)

>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).

From: bwitzke@gsbth-po.igsb.uiowa.edu

I appreciated the remarks of Tom Holtz 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.

Still, it seems to precede a time of great dinosaur diversity. The age of Triceratops and Tyrannosaurus rex, the Maastrichtian, begins after the impact event. -- ed.