Editing Weaubleau structure

{{Infobox terrestrial impact site
| name            = Weaubleau structure
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| photo           = Weaubleau Structure shaded relief.jpg
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| photo_caption   = The {{convert|12|mi|adj=on}} circular Weaubleau structure is discernible in the drainage patterns of this shaded-relief image.
| map             = United States#Missouri
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| confidence      = Probable
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| age             = 335 and 340 million years
| exposed         = 
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[[Image:Weaubleau Structure outline v2.svg|thumb|This diagram illustrates the large-scale structures interpreted in the shaded-relief image.]]

The '''Weaubleau structure''' is a probable [[meteorite]] [[Impact event|impact]] site in western [[Missouri]] near the towns of [[Gerster, Missouri|Gerster]], [[Iconium, Missouri|Iconium]], [[Osceola, Missouri|Osceola]], and [[Vista, Missouri|Vista]]. It is believed to have been caused by a {{convert|1200|ft|adj=on}} [[meteoroid]] between 335 and 340 million years ago<ref>Miller, J.F., Evans, K.R., Rovey, C.W., II, Ausich, W.L., Bolyard, S.E., Davis, G.H., Ethington, R.L., Sandberg, C.A., Thompson, T.L., and Waters, J.A., Mixed-age echinoderms, conodonts, and other fossils used to date a meteorite impact, and implications for missing strata in the type Osagean (Mississippian) in Missouri, USA. Echinoderm Paleobiology, 2008, 53p.</ref> during the middle [[Mississippian age|Mississippian Period]] (Latest Osagean to Earliest [[Meramecian]]).
It is listed by the [[Impact Field Studies Group]] as a "probable" impact structure.<ref>{{cite web
  | last = Rajmon
  | first = David
  | title = Impact database 2009.1
  | date = 2009-07-01
  | url = http://impacts.rajmon.cz
  | accessdate = 2009-08-25 }}
</ref>

The structure consists of an area of severe structural deformation and extensive [[breccia]]tion that was poorly understood and had been thought to be the result of either [[Thrust fault|thrusting]] over a [[Dome (geology)|dome]]<ref>Beveridge, T.R., 1949, The Geology of the Weaubleau quadrangle, Missouri [Ph.D. thesis]: Iowa City, State University of Iowa</ref> or a [[cryptoexplosion|cryptoexplosive]] event.<ref>Snyder, F.G., Gerdemann, P.E., Hendricks, H.E., Williams, J.H., Wallace, G., and Martin, J.A., 1965, Cryptoexplosive structures in Missouri: Guidebook, 1965 Annual Meeting of the Geological Society of America: Missouri Geological Survey and Water Resources, Report of Investigations No. 30, 73 p.</ref>  A {{convert|12|mi|adj=on}} circular structure was discovered by [[geologist]] Kevin R. Evans through examination of [[Digital elevation model|digital elevation data]].<ref name=Evans>Evans, Kevin R.; Mickus, Kevin L.; Rovey, Charles W. III; & Davis, George H. (2003). Field Trip I: The Weaubleau Structure: Evidence of a Mississippian Meteorite Impact in Southwestern Missouri. ''Association of Missouri Geologists Field Trip Guidebook, No. 26, 50th Annual Meeting'', pp.&nbsp;1-30. Missouri Department of Natural Resources. [http://courses.missouristate.edu/KevinEvans/RI-75(2003AMGguidebook).pdf PDF]</ref>  The structure was originally called the Weaubleau-Osceola structure after Weaubleau Creek and [[Osceola, Missouri|Osceola]]. It is now known as the Weaubleau structure.

Because the site was covered by later [[Pennsylvanian (geology)|Pennsylvanian Period]] sediments, and only partially exposed to erosion relatively recently, its structure is well preserved, and its age can be determined with fair accuracy. It is one of a series of known or suspected impact sites along the [[38th parallel north|38th parallel]] in the states of [[Illinois]], [[Missouri]], and [[Kansas]]. These [[38th parallel structures]] are thought to possibly be the result of a serial impact, similar to that of [[comet Shoemaker-Levy 9]] on [[Jupiter]], an extremely unlikely event on [[Earth]]. The argument for a serial strike would be greatly strengthened if the ages of the other 38th parallel structures could be constrained to the same period as the Weaubleau structure.<ref name="Dulin Elmore serial">{{cite journal | url=https://books.google.com/books?id=mLswN4OgQYEC&q=Weaubleau%20structure%2038th%20parallel&pg=PA63 | title=The Sedimentary Record of Meteorite Impacts | volume=437 | journal=Geological Society of America Special Paper | editor=Evans, Kevin R. | year=2008 | page=63 | author=Dulin, Shannon | author2=Elmore, R.D.| isbn=9780813724379 }}</ref>

The Weaubleau structure is one of the fifty largest known [[impact structure]]s on earth and the fourth largest in the United States. The three larger ones in the US either have been glaciated and buried ([[Manson crater]]), are under water ([[Chesapeake Bay impact crater|Chesapeake Bay crater]]), or have been subjected to [[orogeny]] ([[Beaverhead impact structure]]). Therefore, the Weaubleau structure is the largest exposed [[Tectonics|untectonized]] impact structure in the US.<ref name=Evans/>

==Round rocks==
[[Image:Missouri round rock.jpg|thumb|200px|One of the "round rocks" found in [[Osceola, Missouri]]]]
{{see also|Weaubleau egg}}

Long thought to be a glacial remnant, these [[Conglomerate (geology)|conglomerate]] rocks are found in the area of the Weaubleau structure. They are nearly perfectly round, and are referred to locally simply as "geodes", "round rocks", "Missouri rock balls", or "Weaubleau eggs".<ref name=Evans/>

It has been suggested that the round rocks are [[chert]] [[concretion]]s or nodules. The round rocks may have formed when the impact threw [[shale]] [[clastic rock|clast]]s of the [[Northview Formation]] away from the center of the impact structure and subsequent silica-saturated waters precipitated silica around the shale clasts. The formation of the round rocks is poorly understood and requires further research.<ref name=Evans/>

==See also==
*[[38th parallel structures]]
*[[List of possible impact structures on Earth]]

==References==
{{reflist}}

==Further reading==
*Evans, K. R., Rovey, C. W., II, Mickus, K. L., Miller, J. F., Plymate, T., and Thomson, K. C. 2003.  Weaubleau-Osceola structure, Missouri: deformation, event stratification, and shock metamorphism of a mid-Carboniferous impact site. ''Third International Conference on Large Meteorite Impacts, Nördlingen, Germany, August 5–7, 2003''. Extended abstract from conference proceedings available online.  [http://www.lpi.usra.edu/meetings/largeimpacts2003/pdf/4111.pdf PDF]
* Evans, Kevin R.; Mickus, Kevin L.; Fagerlin, Stanley; Luczaj, John; Mantei, Erwin; Miller, James F.; Moeglin, Thomas; Pavlowsky, Robert T.; & Thomson, Kenneth C. 2004. ''Oblique Impact at Weaubleau-Osceola Structure, Missouri.'' Abstract online [http://gsa.confex.com/gsa/2004AM/finalprogram/abstract_76553.htm]
* Space Today Online. [http://www.spacetoday.org/SolSys/Earth/Meteorites/MeteoritesExplained.html The Planet Earth Today: Meteorite vs. Meteor, Meteoroid and Micrometeoroid].  Retrieved November 27, 2004.

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{{Impact cratering on Earth}}

[[Category:Carboniferous impact craters]]
[[Category:Landforms of St. Clair County, Missouri]]
[[Category:Geology of Missouri]]
[[Category:Impact craters of the United States]]
[[Category:Possible impact craters on Earth]]
[[Category:Landforms of Missouri]]