Weaubleau impact structure "round rocks" and possible Azuara/Rubielos de la Cérida analogues

7/28/2019 Weaubleau impact structure "round rocks" and possible Azuara/Rubielos de la Crida analogues

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The$Weaubleau$impact$structure$"round$rocks"$("Missouri$rock$balls",$"Weaubleau$eggs"):$possible$analogues$in$the$Spanish$Azuara/Rubielos$de$la$Crida$impact$

structures.$!

by!Kord!Ernstson!&!Ferran!Claudin!(July!2013)!

!$

Abstract.$D$The!"round!rocks"!of!the!WeaubleauBOsceola!impact!structure!have!phenomenological!counterparts!in!the!Spanish!Azuara/Rubielos!de!la!Crida!impact!

structures!where!they!occur!within!voluminous!heavily!brecciated!rock!units.!Related!

nodular!bodies!within!large!monomictic!movement!breccias!are!observed!also!in!the!Ries!

impact!structure.!A!process!similar!to!the!formation!of!monomictic!impact!breccias!with!

rounded!clasts!as!part!of!a!mortar!texture!is!suggested.!A!relation!to!the!WeaubleauB

Osceola!"round!rocks"!may!exist!but!not!necessarily.!!

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$1$Introduction$$!

The!Weaubleau!(http://en.wikipedia.org/wiki/Weaubleau_structure)!!(or!now!WeaubleauB

Osceola)!circular!feature!in!southwestern!Missouri!is!a!19!kmBdiameter!impact!structure!

that!formed!in!the!MidBCarboniferous!about!330!million!years!ago!(Evans!et!al.!2003).!

!

A!peculiar!feature!clearly!restricted!to!and!common!throughout!the!Weaubleau!structure!

are!the!"round!rocks"!called!also!"Missouri!rock!balls"!or!"Weaubleau!eggs"!(Figs.!1,!2).!

Originally!considered!to!be!of!glacial!origin!they!are!in!general!attributed!now!to!the!

impact!event.!The!idea!of!a!formation!as!megaBaccrecionary!lapilli!has!been!discarded!and!a!

diagenetic!formation!from!blasted!siltstone!clasts!intermixed!in!the!fallback!breccia!and!subsequent!silification!is!mostly!discussed.!Nonetheless,!the!process!of!formation!is!still!

poorly!understood.!Here,!we!present!evidence!of!roughly!similar!nodules!occurring!in!the!

Spanish!large!Azuara/Rubielos!de!la!Crida!impact!structures!where!different!from!the!

Weaubleau!"round!rocks"!they!can!be!observed!how!they!developed!in#situ.!

!

!

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!!Fig.$1.$Weaubleau$"round$rock".$The$typical$and$most$common$size$runs$from$golfDballs$to$grapefruit.$

Photo:$Harmil,$WIKIMEIA$COMMONS.$

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!!Fig.$2.$Nest$of$"Weaubleau$eggs"$in$a$paleokarst$pocket.$Photo:$Evans$et$al.$(2004).!!

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2$Observations:$the$in#situ#conglomeratization$in$the$Spanish$impact$structures$!

A!likewise!peculiar!nodule!formation!is!known!and!has!been!described!from!the!Spanish!

large!MidBTertiary!Azuara/Rubielos!de!la!Crida!impact!structures.!As!early!as!in!1973!W.!

Monninger!in!his!excellent!diploma!thesis!(University!of!Wrzburg,!Germany)!described!

"tectonic!nodules"!(Fig.!3)!intermixed!in!heavily!shattered!Cambrian!slates!(probably!

Valdemiedes!Fm.).!The!nodules!themselves!gave!evidence!of!also!strong!load!and!

deformation,!and!their!surfaces!were!frequently!smoothed!and!polished.!In!1973,!when!

meteorite!impact!and!impact!geology!were!anything!but!familiar!to!geologists,!Monninger!

concluded!that!the!nodules!could!have!formed!in!the!contact!zone!during!counter!

movements!of!tectonic!blocks!where!they!got!their!roundness!and!polish.!Later!the!

"tectonic!nodules"!fell!into!oblivion.!!

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!!Fig.$3.$"Tectonic$nodule"$from$the$Cambrian$outcrops$near$Olalla$(Spain).$Millimeter$scale.$Photo:$

Monninger$(1973).$

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These!nodules!and!their!curious!formation!attracted!attention!again!when!Azuara!and!

Rubielos!de!la!Crida!became!established!impact!structures.!In!the!thirty!years!of!their!

comprehensive!investigation!(Claudin!et!al.!2001,!2003;!Ernstson!&!Claudin!1990,!Ernstson!

&!Fiebag!1992,!Ernstson!et!al.!1985,!1987,!2001!a,!b,!2002,!2003)!we!were!frequently!surprised!to!find!outcrops!reflecting!what!we!have!been!calling!"in!situ#


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conglomeratization",!a!formation!of!spherical!segregation!within!the!autochthonous!

layering!even!in!competent!rocks!(Figs.!4!B!10).!And!in!this!context!we!also!rediscovered!W.!

Monninger's!"tectonic!balls"!within!the!heavily!shattered!slates!(Figs.!11,!12).!!

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In!the!following!we!show!and!explain!images!of!various!outcrops!where!typical!Spanish!

"round!rocks"!occur.!!

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!!Fig.$4.$"In#situ#conglomeratization":$formation$of$a$big$sphere$as$core$of$a$rotational$texture$within$

bedded$however$heavily$deformed$Jurassic$limestones.$Central$uplift$chain$of$the$Rubielos$de$la$

Crida$impact$basin;$road$to$Rubielos$de$la$Crida.$

$

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!!Fig.$5.$"In#situ#conglomeratization":$formation$of$nodules$within$bedded$however$heavily$deformed$

Jurassic$limestones.$Outer$ring$anticline,$Azuara$impact$structure,$near$Belchite.$!


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!!Fig.$6.$Outcrop$of$heavily$destroyed$Jurassic$limestones$having$become$a$distinct$grit$breccia.$Road$

between$Cella$and$Monterde$de$Albarracn$outside$the$western$rim$of$the$Rubielos$de$la$Crida$

impact$basin.$!

!!Fig.$7.$Closer$view$of$the$outcrop$in$Fig.$6$with$nodular$bodies$interspersing$the$grit$breccia.$

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Fig.$8.$CloseDup$of$one$of$the$nodular$bodies.$

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!!Fig.$9.$A$freshly$cracked$nodular$body$exhibiting$the$same$grit$brecciation.$

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!!Fig.$10.$Some$30$m$apart$from$the$outcrop$in$Fig.$6:$Obviously$an$early$phase$of$the$in#situ$

conglomeratization $of$the$bedded$Jurassic$limestones$that$came$to$standstill$before$the$final$formation$of$the$grit$breccia$with$a$few$individual$nodules$remaining$as$seen$in$Fig.$7.$

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!!Fig.$11.$Similar$situation$like$in$Figs.$6,$7:$in#situ$conglomeratization $within$heavily$shattered$

Cambrian$slates$(probably$Valdemiedes$Fm.)$D$the$"tectonic$nodules"$originally$described$by$Monninger$(1973).$

$

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!!Fig.$12.$Individual$nodules$within$the$Cambrian$heavily$shattered$slates.$

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3$Observations:$related$features$D$Spanish$impact$structures$and$Ries$impact$crater.$!

The!occurrence!of!larger!noduleBlike!segregated!bodies!within!voluminous!grit!breccias!is!a!

common!observation!not!only!in!the!large!Spanish!impact!structures!but!has!counterparts!

also!in!the!25!kmBdiameter!Ries!impact!structure!in!Germany!which!is!shown!and!

explained!in!Figs.!13!B!15.!!!

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!!Fig.$13.$Part$of$the$Barrachina$megabreccia,$Rubielos$de$la$Crida$impact$basin:$a$completely$

shattered$and$in$part$pulverized$quartzitic$rock$(aroca$quartzitic$sandstone?)$that$has$preserved$

some$ghost$bedding.$Right$in$the$middle$of$the$voluminous$grit$breccia$a$few$segregated$quartzitic$

blocks$of$roughly$spherical$shape$have$survived$the$intense$destruction$strongly$reminding$of$the$situation$shown$in$Figs.$6$D$8.$etail$in$the$photo$to$the$left.$

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!!Fig.$14.$Very$similar$situation$in$the$Ries$impact$structure.$The$Iggenhausen$quarry$is$located$in$a$big$

dislocated$megablock$of$Malmian$limestone$far$outside$the$crater$structure.$The$very$competent$

limestone$has$been$shattered$to$such$a$degree$that$the$material$can$simply$be$exploited$like$in$a$

gravel$pit.$The$roundish$megaclast$with$some$preserved$bedding$appears$to$be$surrounded$by$a$halo$

of$a$slightly$rotational$texture.$

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!

!!

Fig.$15.$Once$more$the$Rubielos$de$la$Crida$impact$basin:$eastern$crater$rim,$road$between$Escorihuela$and$El$Pobo.$Voluminous$megabreccias$and$grit$brecciation$of$the$Muschelkalk$

limestones/dolostones$are$hosting$rounded$blocks$embedded$in$a$kind$of$rotational$texture$of$the$

grit$breccia.$

!

4$iscussion$!

To!begin!with!a!clear!statement,!we!are!still!unable!to!give!an!explanation!for!the!formation!

of!the!Weaubleau!"round!rocks".!We!aim!at!showing!that!similar!features!occur!at!another!

large!impact!site,!and!thus!similar!formation!processes!could!possibly!be!indicated.!To!

better!understand!the!Weaubleau!situation!where!the!round!rocks!are!weathering!from!a!

fallback!breccia!it!may!help!to!compare!it!with!the!Spanish!in#situ!occurrences!demonstrating!how!"round!rocks"!develop!more!or!less!autochthonously!and!

simultaneously!with!the!formation!of!and!within!voluminous!grit!breccias.!Voluminous!grit!

breccias!or!monomictic#movement#breccias!(Reiff!1978)!are!a!typical!impact!formation!and!

may!be!diagnostic!if,!e.g.,!giant!landslides!to!form!these!deformations!can!be!excluded.!

FractureBmechanically!these!heavy!deformations!through!and!through!of!extended!

volumes!can!be!understood!only!if!extremely!high!confining!pressures!are!acting!on!the!

whole!complex.!In!impact!cratering!these!forces!do!exist!considering!the!wellBknown!

processes!of!crater!excavation!and!modification.!

!

Obviously,!confining!pressure!alone!does!not!lead!to!a!loosening!up!of!competent!rock!

units!ending!in!the!existence!of!selective!spherical!bodies!within!in!a!fineBgrained!gritty!matrix.!In!addition!radical!movements!are!required,!and!on!a!smaller!scale!monomictic!


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$$Fig.$18.$etail$of$the$mortar$texture$of$the$breccia$shown$in$Fig.$17$with$individual$clasts$having$in$

part$become$distinctly$rounded.$Polished$section.$

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!!Fig.$19.$Suevite$breccia$from$the$Rubielos$de$la$Crida$impact$structure,$Spain$with$distinct$flow$

texture$and$a$larger$limestone$clast$having$disintegrated$into$a$monomictic$breccia$with$largely$wellDfitting$components.$CloseDup$in$Fig.$20.$Polished$section.$

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!!

Fig.$20.$etail$of$Fig.$19:$Probably$the$most$instructive$example$of$the$in#situ$formation$of$spheroidal$

clasts$probably$supported$by$material$flow$(see$Fig.$19)$under$high$pressure.$

!!

The!preceding!figures!suggest!that!the!formation!of!larger!spheroidal!bodies!and!even!ballB

like!clasts!as!observed!in!outcrops!of!very!strong!and!voluminous!monomictic!destruction!

has!much!in!common!with!deformations!on!a!much!smaller!scale!to!produce!monomictic!

breccias!with!distinct!mortar!texture.!In!particular!Figs.!19,!20!demonstrate!how!ballBlike!

clasts!can!develop!yet!in!contact!with!and!even!inside!the!rock!main!mass!if!the!

decomposition!acts!under!highBpressure!flow.!In!the!case!of!the!largerBscale!spheroidal!

body!formation!we!suggest!that!in!principle!the!same!process!takes!place,!and!the!"round!

rocks"!are!a!few!remaining!nodular!"bricks"!that!for!whatever!reason!have!survived!

complete!brecciation!within!the!gritty!"mortar"!in!the!course!of!the!highly!energetic!impact!

cratering!process.!!

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5$Conclusions$!

The!phenomenon!of!the!"round!rocks"!is!not!restricted!to!the!WeaubleauBOsceola!impact!

structure!but!the!formation!of!spheroidal!clasts!and!larger!nodular!bodies!is!observed!also!

in!the!Spanish!large!impact!structures!of!Azuara!and!Rubielos!de!la!Crida!and!in!the!Ries!

impact!crater.!For!the!latter!occurrences!a!formation!in!the!course!of!an!intense!macroB

scale!brecciation!of!larger!rock!complexes!in!the!impact!cratering!process!requiring!

movements!under!high!confining!pressures!is!suggested.!The!"round!rocks"!are!remnants!in!a!voluminous!monomictic!breccia!with!a!macroBmortar!texture.!FractureBmechanically!


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the!rounding!may!be!an!effect!roughly!similar!to!the!rounding!of!cobbles!in!a!stream!to!

form!in!the!end!true!conglomerates.!The!basic!difference!is!the!very!short!duration!of!the!

impact!in#situ#conglomeratization!enabled!by!the!high!confining!pressure!during!the!

movement.!In!any!case,!the!"round!rock"!formation!has!nothing!to!do!with!the!wellBknown!

spheroidal!chemical!weathering!that!creates!rounded!boulders!also!in#situ.!

!As!for!the!WeaubleauBOsceola!"round!rocks"!our!explanation!may!apply!but!not!

necessarily,!and!we!may!point!to!convergence!that!is!quite!different!processes!may!lead!to!

the!same!outcome.!For!the!time!being,!our!contribution!here!is!a!proposal!to!consider!

whether!the!"Weaubleau!eggs"!originally!formed!also!in!an!impact!brecciation!process!

before!they!were!displaced!and!silicified.!!

!

!

References$!

!

Claudin,!K.,!Ernstson,!K.,!Rampino,!M.R.,!and!Anguita,!F.!(2001).!Striae,!polish,!imprints,!

rotated!fractures,!and!related!features!in!the!Puerto!Mnguez!impact!ejecta!(NE!Spain).!

Abstracts,!6th!ESF!IMPACT!workshop,!Impact!Markers!in!the!Stratigraphic!record,!pp.!15B

16.!

!

Claudin,!F.!and!Ernstson,!K.!(2003).!Geologia!planetaria!y!Geologia!regional:!el!debate!sobre!

un!impacto!mltiple!en!aragn.!Enseanza!de!las!ciencias!de!la!Tierra,!vol!11,!n!3,!pp!202B

212.!

!

Ernstson,!K.,!Hamman,!W.,!Fiebag,!J.,!and!Graup,!G.!(1985).!Evidence!of!an!impact!origin!for!

the!Azuara!structure!(Spain).!!Earth!Planet.!Sci.!Let.,!74,!361B370.!!

Ernstson,!K.,!Feld,!H.,!and!Fiebag,!J.!(1987).!Impact!hypothesis!for!the!Azuara!structure!

(Spain)!strengthened.!Meteoritics,!22,!373.!

!

Ernstson,!K.!&!Claudin,!F.(1990).!Pelarda!Formation!(Eastern!Iberian!Cains,!NE!Spain):!

Ejecta!of!the!Azuara!impact!structure.!!N.Jb.Geol.Palont.Mh.,!1990,!581B599.!

!

Ernstson,!K.!&!Fiebag,!J.(1992).!The!Azuara!impact!structure!(Spain):!new!insights!from!

geophysical!and!geological!investigations.!!Int.!J.!Earth!Sci.,!81/2,!403B427.!

!

Ernstson,!K.,!Rampino,!M.R.,!and!Hiltl,!M.!(2001).!Cratered!cobbles!in!Triassic!

untsandstein!conglomerates!in!NE!Spain:!Shock!deformation!of!inBsitu!deposits!in!the!

vicinity!of!large!impacts.!Geology,!v.!29,!no.1,!11B14.!

!

Ernstson,!K.,!Claudin,!F.,!Schssler,!U.,!Anguita,!F,!and!Ernstson,!T.!(2001)!Impact!melt!

rocks,!shock!metamorphism,!and!structural!features!in!the!Rubielos!de!la!Crida!structure,!

Spain:!evidence!of!a!companion!to!the!Azuara!impact!structure.!Abstracts,!6th!ESF!IMPACT!

workshop,!Impact!Markers!in!the!Stratigraphic!record,!pp.!23B24.!

!

Ernstson,!K.,!Claudin,!F.,!Schssler,!U.,!Hradil,!K.!(2002).!The!midBTertiary!Azuara!and!

Rubielos!de!la!Crida!paired!imapct!structures!(Spain).!Treb.!Mus.!Geol.!arcelona,!11,!5B65.!


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!

Ernstson,!K.,!Schssler,!U.,!Claudin,!F.,!Ernstson,!T.!(2003).!An!Impact!Crater!Chain!in!

Northern!Spain.!!Meteorite,!9,!35B39.!

!

Evans,!K.R.,!Rovey!II,!C.W.,!Mickus,!K.L.,!Miller,!J.F.,!Plymate,!T.G.,!and!Thomson,!K.C.!(2003):!!

WeaubleauBOsceola!structure,!Missouri:!Deformation,!event!stratification,!and!shock!metamorphism!of!a!MidBCarboniferous!impact!site.!B!Third!International!Conference!on!

Large!Meteorite!Impacts,!Nrdlingen,!Germany,!August!5B7,!2003.!URL:!

http://www.lpi.usra.edu/meetings/largeimpacts2003/pdf/4111.pdf!

!

Evans,!K.R.,!Mickus,!K.L.,!Rovey!II,!C.W.,!and!Davis,!G.H.!(2004):!Field!Trip!I:!The!

WeaubleauBOsceola!Structure:!Evidence!of!a!Mississippian!Meteorite!impact!site!in!

Southwestern!Missouri.!B!Association!of!Missouri!Geologists!Field!Trip!Guidebook,!50th!

Annual!Meeting,!Springfield,!Missouri,!September!26B27,!2003,!Missouri!Department!of!

Natural!resources,!Geological!Survey!and!Resource!Assessment!Division,!p.!1B30.!!

URL:!http://missourigeologists.org/Meeting2003/RIB75(2003AMGguidebook).pdf!

!

Monninger!W.!(1973).!Erluterungen!zur!geologischen!Kartierung!im!Gebiet!um!Olalla!

(Prov.!Teruel),!NEBSpanien.!Diplomarbeit!Universitt!Wrzburg,!140!p.!

!

Reiff,!W.!(1978).!Monomict!movement!breccias;!an!indicator!of!meteorite!impact.!

Meteoritics,!13:!605B609.!

!

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Weaubleau impact structure "round rocks" and possible Azuara/Rubielos de la Cérida analogues