Treb.Mus. Geol. Barcelona, ll: 5-65(2002) The mid-Tertiary Azuara and Rubielosde la C6rida paired impact structures (Spain)*

Kord ERNSTSON**. FernandoCLAUDIN*i:*. Ulrich SCHUSSLER:r**:Fand Klaudia HRADIL*1.:k*

RESUMEN

ERNSTSON,K., CLAUDIN, F., SCHUSSLER,U. and HRADIL, K. Las estructuras de impacto doble del terciario medio de Azuara y Rubielosde la Cörida (Espana). El presentearticulo se centra en las estructurasde impacto de Azuara y Rubielos de la C6rida,que con diämetrosde aproximadamente35-40 Km fueron generadasen un objetivo puramentesedimentario. Ambas constituyenla estructuraterrestrc de impacto doble de mayor tamafloconocida hasta el momento.A panir de los datos estratigi4ficosy paleontoldgicos,su edadmr4s probable es Eocenosup. u Oligoceno. La cartografia geoldgica realizada ha pemitido localizar abundantesevidencias de impactoe inclusoefectos del impactoen depdsitosaut6ctonos distantes. La evidencia de impacto mäs importante para ambasestructuras viene dada por la presencia de un intensometamorfismo de choque,incluyendo fundido y vidrio diapldctico,msgos de deformacidn planar (PDFS), diferentes tipos de rocas de fundido de impacto (foma- das a paflir de fundido silicatado, de fundido carbonatado y fundido de carbonato- fosfato)y brechassueviticas. Particulas de vidrio carbonosoamorfo sitasen un com- ponentesölido de C-O puedenestar relacionadas con fullerenos,y haberseformado a partir del enfriamiento de un fundido procedente o bien de carbdn intercamente chocadoo bien de calizasintensamente chocadas. Pensamos cue el imDactotuvo una influencia considerableen el terciario medio de estaregiön del Sistima Ibörico, y sugedmosque aquellosmodelos en los que no se ha tenido en cuentaeste evento peculiary de ampliarepercusi6n necesitan una revisi6nconsiderable. Palabras claye: Estructura de impacto de Azuara, estructura de impacto de rubielosde la C6rida, CadenaIbdrica (Espafla),metamorfismo de choque,rocas de fundido de impacto,brechas de impacto,eyecta, Terciario.

'N Dr. FranciscoAnguita, AssociateProfessor (Planetary Geology), Universidad Complutensede Madrid (Spain), retereeof this work, sent us the following remarks: "In my opinion, the present paper means a quantic jump towards the confirmation of Azuara and Rubielos de la Cdrida structuresas a doublet impact crater Specifically, the analysesperformed at cen timeter to microscopic scales,reveal a body of evidence difficult to contradict. Any future researchon this areawill have to discussnecessarily the interpretationsnow offered by Emstson ?/ a/. Lacking this discussion. any hypothesis defending that Azuara and Rubielos structuresare the result ol "nonnal" sedimentaryand tectonic processeswill be utterly untenable." ** Fakultät für Geowissenschaftender Universität Würzburg, Pleicherwall l, D 9'70'70Würzburg, Germany.www-impact-structures,com *** IES Giola, Llinars del VallEs.Barcelona-08450, Spain- www.impact-strucnres.com **** Institut für Mineralogie der Universität Wüzburg, Am Hubland, D 97074 Würzburg, Germany. ABSTRACT We report on the Azuara impact structure and its Rubielos de la Cerida compa- nion cratei, which establish the largest terrestdal doublet imPact structue presently - known. Both structures have diameters of roughly 35 40 km and they have been formed in a purely sedimeltary target. From strati$aphic considerations anJ palaeontologicdating. an Upper'Eoceneor Oligoceneage.is very probable Geolägicalmapp-ing has eitablishää abundänt geologic impact evidencein the form Jmof;omictic änO"polymicticbreccias and brecciadikes, megabreccias,. dislocated megablocks,remartabie structuralfeatures, extensive impact ejectaand impact sig- natires even in distant autochthonousdeposits. The most striking.impactevidence for both süucturesis given by snong shoik rnetamorphism,including^melt and dia- oiecricslass, planar äeformätion fÄatures lPDFst. differentkinds of impactmelt iocLr (#orn färmer silicate melt. carbonatemelt, carbonate-phosphatemelt) and iuevite breccias.Glassy amorphous carbon particles in a solid C-O com^poundmay be related with fullerenesand may originaie from a quenchedmelt of extemely shockedcoal or from extremely shockedlimestones. It is assumedthat the impact had considerable influence on the Mid-Teniary regional geology of the Iberian System,and we suggestthat respectivegeologic models-which have.so far not con- sideredthis oeculiarand far-reachingevent. needconslderable revlslon' Key words: Azuara impact structure, Rubielos de la C6rida impact structure' Iberian chain (Spain)' shocl metamorphism; impact melt rocks, impact breccias' ej€cta, Tertiary.

INTRODUCTION

Emstsone/ al. ( 1985)published the hypothesisof theimpact origin for theAzua- ra structuresome 5b km.söuthof ZarJr;goza(41'10'N,0'55'W - Fig l) in 1985'The lzuara impact structurewas included in the Earth Impact Database,which was flrst "rr"-tf"aly researchersof the GeologicalSurvey of Canadain the sameyear' The CraterInveniory wasrecently updated and lists 163proven terestrial impact structures (http:':-brifitty,//www.unb.calpassc/Imp-alDalabage/ildgx.htmD' th" ""* f"-"rmation of the Azuara structureby impl9t-Jvaj -b3^sed mailly;n th;'occurrenceof shockmetamorphism- (French and Short, 1968; Stöffler' l972iand others)in polymictic(mixed) breccias. Later, the casefor an impactorigin *ur1tt"ttgth"n"d by geöphysicalmeasurements (Ernstson et al',1987;'Emstson and Fie6ae, tö9, ant tiy'*re aätectionof extendedimpact ejectadeposits (E- rnstson and Claudin,1990.1. Detailed mapping. extensive structural investiSations. photogeolo-g1 -a analyses(Bäle;1988; Fiebag,198& Gwosdek, 1988; König' 1988; f"ttog.uphi" 'iiaasmaier, Linnewebär,^1988; 1988; Hunolistein-Bunjevac,1989; Müller' 1989; Katschorek,'1990; Mayer, 1991;Müller and Ernstson,1990; Emstson and Fiebag' i99Z; g-.irott, 1994;ireld, pers comm.)provided a hostof impaclrelatedfindings and ;bservations.Despite the clear shockmetamorphism, the impact evidenceis questionedby someregional geologists (A]Ultell et a1.,1993;Aurell, !994i Cort's et ät.. ZOOZI,wlo stili claiman endogeneticorigin for mostof the impactfeatures' The idea that the impact event not only produced the Azuara structure but also the Rubielos de la C6rida structure, a twin crater of comparablesize about 50 km to the south (Fig.l), was frst suggestedby Emstsonet al. (1994) on the basisof the occunenceof snongly deformed Buntsandstein conglomerates peripheral to the Azuara struchtre (Emstson€, dl., 2001a).Since 1994,field work, petrographicand geochemicalanalyses have shown(llradll et a1.,200I:'Claudin et aI.,2001; Emstsoner al., 2001b,c) that this Rubielos de la C6rida structure bea$ all evidence of an impact strucnre. Stratigraphic considerations suggestRubielos de la Cdrida to be a companion to Azuara and thus we have establishedthat a large doublet impact s[ucture exists in east-centralSpain. Here, we report on the current stateof investigationsof this doublet impact struc- ture. Most of the data for the Azuara structurehave beenpublished previously or can be found in the many diploma thesesreferred to above.Therefore, this report widely summarizesthe comDarativeresults ofboth the Rubielos de la C6rida and the Azuara structures,with some emphasisfocused on new results for Rubielos de la C6rida and on the specialsituation of a paired impact.

Fig. L Location map for the Azuara and Rubielos de la Cdrida impact structures.CAL = Calomocha, CAM = Caminreal. CAR = Cadfiena.MUN = Muniesa. Fig 1. Mapa de situacidn de las estructüas de impacto de Azuara y Rubielos de la C6rida. CAL = Calamocha,CAM = Caminreal, CAR = Carifrena,MUN = Muniesa THE AZUARA IMPACT STRUCTURE

Topographic features and target rocks

Compared with other large impact structures (e.g., Manicouag:n, Ries' ClearwatärWest), the Azuara s-tructurädoes not show a particularly striking-mor- pholosv. Located at the margin betweenthe Alpidic fold belt of the Iberian Chains äna tnä feniary Ebro Basin. the structureis characterizedby Mesozoiclayers emer- ging from the Ebro Basin Tertiary sedimentsand forming a ring-like pattern(Fig'2)' thJ dia-eter of the ring is about 30 km, while the total diameterof the structureis estimatedbetween 35 and 40 km. The imDactaffected both the Palaeozoicand the Mesozoicof fte EastemIberian "rama is (the aragonesa")Chain. The Lower Tertiary at the ,time of the impact assumedto haväbeen thick (1000- 2000m or evenmore) and predominanfly uncon- solidated molassesediments overlaying the Palaeozoicand Mesozoic stable core (Emstsonand Fiebag, 1992). This sedimentarypile has a thicknessof rnorethan 10 i- (C-tr, 1983;Carls and Monninger, 1974) (Fig.3)' which showsthat despitethe laree diameter,the Azuam structurewas imprinted on a purely sedimentarytarget (see Gri-eveand Robertson, 1979; Grieve, 1982), if we neglecta few igneousintercalations of very small dimensions.The dominantly soft target and the advancederosion may be ths causeof the weak morphologicalsignature of the structure.

Impact-geologic features

Breccias

As hasbeen stated previously (Emstson et al., 1985;Emstson and Fiebag' 1992), polymictic (mixed) anä monomictic (monolithologic) breccias are very abundanl ihr6ughouttie Azuara structureand havebeen mapped in detail mostly on I : 10'000 and 1": 20.000scales (Bärle, 1988; Fiebag, 1988; Gwosdek, 1988; König' 1988; Linneweber, 1988; Waasmaier, 1988;Hunoltstein-Bunjevac, 1989; Müller. 1989; Katschorek1990; Mayer,1991; Feld, pers. comm.).

Fig. 2. Morphological signature of the Azuara structue taken from the digital mapof Spain,I : 250.000.The image has beenperformed by Manuel Cabedo. Fig. 2. Imagenmorfol6gica de la estruc- tura de Azuaraobtenida a partir del mapa digitalde Espafla,l: 250.000.La imagen fue elaboradapor Manuel Cabedo Basal breccia (suevitebreccia) "basal The original name breccia" (Emstson and Fiebag, 1992) refers to the fact that this peculiar breccia is regularly found at the baseof the post-impact(and post- Alpidic), tectonically undisturbedUpper Tertiary. The up to 20 m thick horizontally beddedbreccia layer is always unconformablyoverlying the folded rocks and at the contactsmore or less meshedwith them without any re-working or soil formation. Palaeozoic and Mesozoic rocks contribute to the sharp-edged(sometimes subrounded) clasts and minute splinters in an extremely hard and exceptionally cemented carbonate mahix (Pl. 1, Fig. 1) that often displaysdistinct flow texture.Clasts that arethemselves breccias(breccias-within-breccias) are abundant.Frequently, limestone and dolomite clastsshow a skeletaland vesiculartexture, sometimesto the extent that only the rim of a clast remains and the hollow may contain a residual white powder. Hightempe- rature decarbonization, the formation of carbonate melt and subsequentrecombination arethe probablecauses (Mayer, 1991;Emstson and Fiebag, 1992;for a comprehensive discussionof the basalbreccia and the relatedcarbonate melt seeKatschorek t19901).

Fig. 3. Stratigraphyof the target rocks in the Iberian chain. Data from Carls and Monninger (1974) and ITGE (1991). Fig. 3. Eshatigrafia de las rccas objetivo en la cadenaIb6rica. Datos extaidos de Carls y Monninger (1974)e ITGE (1991) 10

From X-ray fluorescenceand X-ray diffraction analysesand thin-sectioninspec- tion, Mayer (1991) infers an SiO2 amorphousphase (up to about l07o) contributing to the mätrix. This phaseis assumedto be finely dispersedg1ass. In a few cases,glass particlesin the basalbreccia can be observedby the optical microscope.Shock-meta- morphic effectsare frequently observedin mineralsfrom silicatefragments. Most sig- nificänt is the abundantoccunence of diaplectic quartz in the basal breccia (Mayer, 1991,p.215). The formation of diaplectic quartzrequires shock pressuresin excessof 15 GPa (= 150 kbar) (Bunch et aI., 1968). Planar deformation features (PDFs) are rare,but multiple setsof planarfractures (PFs, cleavage; see Fig. 139 in Fiebag' 1988) in quartz, miciotwinning in calcite (Metz'leret a1.,1988; and referencestherein) and strong kinkbanding in mica (Hörz, l97O) indicate moderate shock metamorphism. More evidence of shock metamorphism in the Azuara structure is given below. According to the cunent classification and nomenclature of impact rocks (IUGS Subcommission on the Systematics of Metamorphic Rocks, Study Group for Impactites), this polymictic basal breccia containing shocked clasts and melt, is termeda sueviteor suevitebreccia. In Fig.4, basal breccia exposuresso far mapped are plotted to show the general distribution, and the following locations are given by their UTM coordinates: 7 41 800;8 15 850,7 42 650:' 6 70 500.45'74'73 700:.6 82700,45 '73 75 100;8 '7o0,14 350, 6 636so. 45 200:6 62 350.4s 450,6 65 4576 500; 6 68200, 45 76 100:. 6 69600, 45 74 850;675 800,45 50600. In thevillage of Moyuela,the basal brec- cia has been used as building stonefor the constructionof a large retaining wall.

Breccia dikes (dike breccias)

Breccia dikes are prominent features in impact structures (Lambert, 1981; Wilshire eral.. 1972:Pohl et al.,1977; Dressler,1970, 1984;Bischoff and Oskierski, 1987, 1988; Schwarzmannet al., 1983; and others). Most researchersbelieve that they form in the excavationstage of impact cratering (Melosh, 1989)by the injection of material into the floor and the walls of the growing and changing excavation cavity. The Azuara structurepresents a large variety of impact breccia dikes (Fiebag, 1988; Ernstson and Fiebag, 1992). They have been mapped in and around the structurein nearly all stratigraphicalunits, and they have formed independentlyof the host rock both in silicate and carbonaterocks (Pl. l,Fig.2,3). They are up to 2 m wide and may be traced over a distanceof more than 200 m (Mayer, 1991). Whole systemsor networks of breccia dikes are developed(see, e.g., Fig. 17 in Emstson and Fiebag, 1992).An H-type (our definition) breccia-dike systemis often observedand charac- terized by a tying of vertical and horizontal dikes (Pl. 1, Fig. 2). The system is sug- gestedto have formed in a shofi-term sequenceof compressiveand tensile stress. The breccias of the dikes are monomictic and polymictic; two or more breccia generationsare frequent. Thereby,breccia dikes may run within or intersect another dike. Often, the brecciasdo not differ from the basalbreccia, and they show the same composition and texture discussedbefore. Shock metamorphismin the dike breccias is abundant and especially strong in the dikes that have formed in the Palaeozoic targetrocks (seebelow). 1l

Fig.5 showsthe locationswhere prominent breccia dikes and breccia-dikesys- temsare exposed. Apart from the dikesin Pl. 1, Fig. 2, 3, the UTM coordinatesare givenfor locationswhere breccia dikes can be readilystudied: 6 85 380,45 47 630t 6 88 070,45 52 5401683 500,45 75 9001'668 035,45 40 450.

Impactbreccia near Almonacid de la Cuba

A peculiarimpact breccia is exposednear Alrnonacid de la Cubaat theNE rim of the structure(Fig. 4; UTM coordinates6 84 300,45 73 900)and is sofar uniquewith respectto compositionand texture (P1. 1, Fig. 4). Within a denseto porousand even foamy greyishcarbonate matrix, componentsof snow-whitecolour are embedded, which are in many casesextremely vesicular. The brecciais part of an extensive

" 10km t$l 'ü

tr aELCH|TE

ffir llllilTllllillz7Zj =)4 o5 16 07

Fig. 4. Location map for impact brecciasand breccia dikes in lhe Azuara structure.Geological sketch map simplified and modified from IGME (1970, 1981) and ITGE (1991). 1 = palaeozoic,2 = Mesozoic, 3=PelardaFm.ejecta,4=Cenozoic;5=basal(suevite)breccia,6=brecciadikes,?=impactbreccia near Almonacid de la Cuba.

Fig. 4. Mapa de situaciönde los afloramientosde brechasde impacto y diques de brechaen la esffuctura de Azuara. El mapa geol6gico ha sido obtenido por simplificaci6n y modificaci6n de los mapas del IGME (1970, l98l) e ITGE (1991). I = Paleozoico,2 = Mesozoico, 3 = eyectade la Fm. pelarda,4 = Cenozoico,5 = brechabasal (suevita),6 = diquesde brechas,T = brechade impacto cercanaaAlmonacid de la Cuba l2 deoosit which has been investigatedin detail and describedby Katschorek (1990)' Fröm field work and petrographii analyses,she concludes that the deposithas resulted from an expanded,turbulent, and dilute flow by inclusion of carbonatemelt, similar to volcanic surges.

Megabreccia

In the imoact literature, extensivebreccias consisting of very large components are generally- termed megabreccias. Megabreccias have been reported for- other imna-ctstmciures such as Wells Creek, Steinheimbasin, Gosses81uff, SierraMadera, Wetumpka,and others. In ihe Azuara impact structure,the megabrecciain the narrow sense(megabrec- cias in the broader sänseare discussedwith the Rubielos de la C6rida structure)is exposednear Belchite/Almonacidde la Cuba and Herrerade los Navarrosin the outer (1988) Katschorek ring 619."the-deposits 5). According to the detailed descriptigl blr Fiebag and (t9b0), co-veran area of roughly : f-t, their thicknessmay exceed 80 -, und th" larlest componentshave a size up to about 20 m (P1. l, Fig. 5). Although

.tir urrrl|4Xfi 10 km lo 'rtr 0

ffiffir ffilllllllllzZZs t 14 -5 o6 07

Fig.5. Location map for the megabreccia (5), monomictic movement breccias (6)' dislocated mJgablocks(7), and the gravity profile in Fig. 6 (8) Geology sameas in Fig. 4 Fig. 5. Mapa de localizaci6n de la megabrecha(5), brechasmonomicticas de movimiento (6)' megablo- qu"esdislocados (?), y el perfil de grave

Monomicl i c movcment breccia s "monomict The term movementbreccia" has been introducedby Reiff (1978) by discussing monomictic breccia complexes in the Steinheim, Ries, Sierra Madera, Flynn Creek, Decaturville and Wells Creek impact structures.In the Wells Creek impact structure, these breccias were called crackle breccias and homogeneous ru6ble breccias(Wilson and Stearns,1968) and later consideredvariants of the same phenomenon(Reiff 1978). Related textures are grit brecciation and mortar texture (Hüttner, 1969). In each case, a drastic brecciation of whole rock complexes with resulting grain sizes down to sand and silt fraction and frequently preserved fitting of the fragmentsis observed.This peculiar brecciationrequires intense movement under very high confining pressure.In the impact structuresreferred to above,these move- ment breccias occur in excavated/ejectedmegablocks, in the crater floor, in inner rings and central uplifts. Reiff (1978) points to the fact that the typical texture of monomictic movementbreccias is also observedin brecciasfrom giant rock falls (for example the 1,500 m Flims, Switzerland,rock fall) and may in rare instancesoccur along tectonic fault zones.The occurrenceof monomictic movementbreccias in envi- ronmentslacking tectonic fault zonesand gradientssufficient for massrock falls are strong clues to impact crateringevents (Reiff 1978). In the Azuara structure, monomictic movement breccias are very abundant. Although also found in Palaeozoicsilicate rocks, they are concentratedin Triassic, Jurassic and Cretaceous limestones. An example of a typically brecciated Muschelkalk limestone is shown in PI.1, Fig. 6. More outcrop locationsof monomic- tic movement brecciasare plotted in Fig. 5. lmpressive breccia exposures can be visited a few kilometres outside the northem ring near Belchite (UTM coordinates 6 87 000, 45 83 000; UTM coordinates 6 83 000, 45 83 000). Occasionally, large isoiated blocks of Jurassic limestones emerge from the post-impact Upper Tertiary Ebro basin sediments.Quarrying in these blocks has enabled insight into the enormous impact deformations experiencedby very large rock volumes. The limestones are drastically destroyed through and through to form a more or less continuous breccia displaying abundant grit brecciation and mortar texture. The uninter- rupted brecciation over a distance of at least 300 m and the absenceof topographic relief show that these monomictic movement breccias formed in the impact cratering process. 14

J 0.0 -2.5 t J -5.0

z t -10.0 LIJ l f -15.0 d)

DISTANCE,KM

Fig. 6. Gravity profile (Bouguerresidual anomalies)across the Azuara structure.Location for the pro- file in Fig.5. Fig. 6. Perfil gravitatorio (anomaliesresiduals de Bouguer) a trav6s de la estructurade Azuam La situaciöndel perfil puedeverse en la Fig. 5.

, Dislocated (allochthonous) megabl.ocks

Dislocated megablocks are characteristic of impact structures and are understood to be related to the different stages of impact cratering (excavation, ejection and modifi- cationof the transientcrater; see Melosh [1989]).Thorough mapping within the Azuara structure(Bärle, 1988; Fiebag, 1988; Gwosdek, 1988; König, 1988; Linneweber, 1988; Waasmaier,1988; Hunoitstein-Bunjevac,1989; Mül]er, 1989; Katschorek,1990; Mayer, 1991)has revealeda large numberof dislocatedmegablocks. Gravity gliding can in most casesbe excludedas an explanation,and the rootlesslayering of many megablockshas been shown by geophysicalmeasurements (Bärle, 1988, König, 1988). A locationmap for allochthonousmegablocks is given in Fig. 5. UTM coordinatesof typicallocations are:6 67 350,4547 79O,670140,4547 790;,670390,45 4'7 520:6 77 660.45 45 700l.6 88 850,45 60 000,6 69 760,45 45 470.

Structuralfeatures

Structuralfeatures in the Azuarastructure may haveoriginated from both Alpidic tectonicsand the impact cratering process.Investigations of the completestructural inventory including folds, faults,jointing, tectonic(horizontal) stylolites, photo- lineations,satellite imagery drainagepattern have beenperformed in detail and are discussedby Bärle (1988),Fiebag (1988),Gwosdek (1988), König (1988), Linneweber(1988), Waasmeier (1988), Hunoltstein-Bunjevac (1989), Müller (1989), Katschorek(1990), Mayer (1991), and Feld (pers. comm.). In mostcases it is not pos- sible to decidewhether a distinct structuralsituation is relatedwith tectonics,impact or evenboth, as both processesmay lead to the samefeatures (e.g., folds, faults, 15 joints).From impact cratering considerations (Grieve et aL,1981;Melosh 1989; and many others),divergent and convergentmass transportis expectedto dominate during the stagesof excavation and modification, implying radial and tangential wideningand shorteningwith respectto the cratercentre. In agreementwith this, theseelements are frequently documentedin the structuralstyle of the Azuara struc- ture (Fiebag,1988, König, 1988,Katschorek, 1990, Emstson and Fiebag, 1992) and may be relatedwith impactrather than with tectonics.In a few cases,peculiar struc- tural featuresoccur, which are simply incompatiblewith tectonic processes (Gwosdek,1988; Katschorek, 1990; Müller andErnstson, 1990; Ernstson, 1994).

Geophysicaldata

From a gravitysurvey comprising 350 gavity stations,a Bougueranomaly map of the Azuarastructure has been plotted (Emstson and Fiebag,1992). For selected profiles, Bouguerresidual anomalies were computedwith the aid of provisional regional-fie1ddata (IGCE 1976,and data kindly suppliedby A. Casas,Barcelona). In Fig. 6, a northto southrunning profile (seeFig. 5) is shownto displayan overallne- gativeanomaly of about10 mgalamplitude. A negativegravity anomaly conforms to mostother impact structures for whichgravity surveys exist. Relatively positive local anomaliessuperimposing the main anomaly may be relatedwith an innerring buried beneaththe youngpost-impact sediments of the Ebro basin(Ernstson and Fiebag, 1992).Simple model calculations and a gravity-derivedmass deficiency are compat- ible with resultsfor other large impactstructures (Pohl et al-, 1978;Emstson and Fiebag,1992). At eachgravity station, the total intensityof the Earth'smagnetic field wasmea- sured.The field contoursshow the central areaof the Azuara structureto be oracti- cally void of anomalies.Surprisingly, short-wavelength anomalies in the periphery have considerableamplitudes exceeding 100 nT (Ernstsonand Fiebag, 1992). Susceptibilitymeasurements of a wide varietyof rocks (Palaeozoic,Mesozoic and Cenozoicsediments, igneous rocks) from theAzuara structure and its environs(com- pilationin Hammannand Ernstson, 1987) show that highestvalues 1k-r* = 0.0016 S.L) are attainedin samplesfrom the basal(suevite) breccia. Althoufli-iystematic rock-magneticstudies are outstanding, a strongremnant magnetization of somebasal brecciasamples is observed,which is possiblya thermalremnant magnetization. Impactbreccias showing stronger magnetization are known from manyother impact structures.Shock pressure and post-shock temperatures are important factors that may leadto enhancedmagnetization of impactbreccias (Pohl 1994).

Shockmetamorphism

The find of strongshock-metamorphic effects in polymicticdike brecciasinter- calatedin Palaeozoicrocks near Nogueras and Santa Cruz de Nogueras(Emstson u/ al., 1985)established the impact origin for theAzuara structure. Here, we summarize newobservations and investigations related to shockmetamorphism in Azuararocks. In orderof decreasingshock intensity, we discussmelt glass,-draplecticcrystals and l6 diaplectic glass,PDFs, and moderateshock effects like PFs in quartz, kinkbands_in miCa, and microtwinning in calcite. A discussionof moderateshatter coning in the Azuara structure is added. A general map for the occurrence of shock metamolphism is given in Fig. 7. UTM coordinatesfor definitive exposuresare addedbelow.

Melt glass

Melt glass in shockedbreccias from the Azuara structurewas first reported and describedin detail by Fiebag (1988). In the polymictic dike breccias exposednear Noguerasand SantaCruz de Nogueras(UTM coordinates,e.g., 6 61 240,45 53 400), yellowish to reddish glass is observedto form clastswith vesicles,schlieren and mi- neral splinters (Pl. 2, Fig.1). Partly recrystallized glass occurs as aggregatesin the matrix (Pl. 2, Fig. 2) or may coat mineral fragments. Because of the occurrence together with strongly shocked quartz (see below), the glass itself was probably formed by shock. This assumed,parts of the breccia must have experiencedshock peak pressuresexceeding 500 kbar (50 GPa) (ErrgelhaÄt et al., 1969).Melt glasshas 6eeniound also in brecciadikes exposedin the Mesozoic rim zone (Mayer, 1990)and in the basalbreccia (Mayer, 1990).

ffir flllllTillllllllrV-Vz f )t ffis@o {z

Fig. 7. Location map fbr strongly shockedrocks (5), moderately shockedrocks (6), and shattercones (7). Geology same as in Fig. 4. Fig. 7. Mapa de situacidn de las rocas intensamenteafectadas por choque (5), rocas moderadamente chocadas(6), y conos astittados(7). La geologia es la misma que la de la Fig. 4 17

Diaplectic crystals and diaplectic glass

Patly (diaplectic crystals) and completely optical isotropic quartz (diaplectic glass) is abundant il breccias frorn the Azuara structure. Diaplectic glass was irst reported by Emstson (1994) to occur in the polymictic dike brecciasnear Noguerasand SantaCruz de Nogueras,which also contain melt glass and PDFs @. 2, Fig. 2). Diaplectic quartz crys- tals are abundant[also] in breccia dikes exposedin the Mesozoic rim zone and in the basal heccia Mayer, 1990; and Pl. 2,Fig.3). The formation of the amorphousquartz phaseby shock without melting requires pressuresof more than 15 GPa (150 kbar) for partial isohopization and 25 - 4O GPa (250 - 400 kbar) for complete transformarionto diaplectic glass(Engelhardt et al.. 1969:Bunch et al.. 1968:Sröffler and Homerumn! 1972; and otherst.

Planar deformation.features (PDFs)

Despite the clear PDFs documentationby photomicrographsand universal-stage measurementsof crystallographicorientations in Emstsonet al. (1985), doubts about shock metamorphism in the Azuara structure were expressedby Langenhorst and Deutsch (1995). In their paper, however, all referencesto the shock metamorphism describedin Ernstson et al. (1985) and in Emstson and Fiebag (1992t are missing. Therefore, two additional independent analyses of PDFs in shocked quartz from Azuara samples (P1. 2, Fig. 4) were performed in the Departamento de petrologia y Geoquimica,Universidad Complutense,Madrid, (by E. GuerreroSerrano) and at the Geological Survey of Canada(by A. Theniault). Universal-stagemeasurements in the Madrid institution were performedon quartz of sandstoneclasts from the polymictic dike breccia exposed near Santa Cruz de Nogueras (see above) and of quartzite clasrsfrom the Pelarda Fm. ejecta (see4.1). jhock The prominent and prevailing o: I l0T3 ] and r I 1012] planessuggest pressures exceeding10 GPa (= 100 kbar) (Stöffler and Langenhorst1994). A. Therriault inves- tigated quartz of sandstoneand quaftzite clastsalso from the polymictic dike breccia and also from the Pelarda Fm. ejecta. She analyzed the crystallographic orientation of PDFs and other parameterssuch as their density, sharyrness,spacing, and spreading over the grain. Up to five setsof PDFs per grain were observed.The spacingis 1 pm or less,the spreadingover the grain in most casesI00Vo, and the PDF density high. Practically all sets are decorated.All shockedgrains have reduced birefringence of 0.004 - 0.008. A frequencydiagram for the PDF crystallographicorientation is shown in Fig. 8. Comparedwith the frequency diagram in Ernstson et aL (1985, Fig. 7), the generalcorrespondence is obvious. Quaftz grains displaying PDFs in the manner as describedhere, are generallyconsidered (StöffIer and Langenhorst1994,Gneve et al. 1996) to be strongly shocked,thus excluding any tectonic deformation.

Moderate shock effects

While according to cunent knowledge, diaplectic crystals, diaplectic glass and PDFs can form only by strong impact shock, there are mineral defirrmationswhich have been reported from both impact shock metamorphisrn and endogenetic processes. 18

Shock deformationof moderateintensity createsplanar fractures(PFs, cleavage)in quarti, whichUelong to the regularshock inventory of.impact structures' Cleavage is cimmonty unknowi to occurin quafiz. In rate cases,however, planar fracturesmay be producedtectonically in zonei of extremeregional metamorphism Vcry strong L"ä"i" a"rorrnutlonsmay also causekink banding in mica, which also is a typical low-grade- - shock effect. e-ottrlFs ln quartzand strongkink bandingin mica are abundantin rocks from the Az,rarast ocnre'6ig.7). They oicur in polyrnicticallochthonous breccias' but alsoin autochthonousroiksl Waasmaie.(19881 analysed 9 thir sectionsfrom Cretaceous ünliu, ,*arton"s exposedwest of Blesaand found that an averageof 7O9o of the rnicas rtto* tiot Uu"aing.ln all thin sections,she observed PFs in quartz'Regularly, 2-- 3 sets occur,and in a fei cases,up to six setsof differentorientation per grain wereobserved tPl. 2, Fie.5). As the Cretaceoussediments have experienced Alpidic tectonicsonly a1d no i"nio"ä tn.*orphism ar all, a formationof ttrekinkbands and the PFsother than by rtt*f "- u" "*O"däd. The assumedshock pressures range between roughly |,qnd tQ 1969)afi ci" iso -a 100kbar) for the formationof PFsin quartz@ngelhardt et al'' -" t"ot" tft- I GPa(i0 kbar) for the formationof kink bandsin mica (Hön' I97O)'

AZUARAIMPACT STRUCTURE - PDF OHIENTATION IN OUAFTZ POLYMICTDIKE BRECCIA AND PELARDA FM. EJECTA

20 18 z 16 14 o 12 p lI 10 I 6 '1011) ,.^i I l,vl {1121} t5161}

iA 30 45 60 75 ono Anglebetween c-axis and polesto PDF (PDFS)in Fig. 8. Frcquencydiagram of crystallographicorientation ot planar^oeformation features The dia- orärtz from'Azuara mixed-breccianear SantaCruz de Noguerasand from PelardaFm ejecta gram has beencompiled from data put at our disposalby A Therriault' planar Fig. 8. Diagramade frecuenciasde la orientacidnc stalogräficade los rasgosde deformacidn y de los tPöF.t p.Ä"nt", en cuarzosprocedentes de las brechascercanas a SantaCruz de Nogueras a partirde los "v".," i" f" g* pa^taa lesuucturade impacto de Azuan).El diagamaha sidorealizado datossuministlados por A. TherTiault 19

Carbonaterocks are known to be lesssusceptible to shock-metamorphiceffects. As a characteristicshock effect, microtwinning in calciteis considered(Me%ler et al., 1938;and references therein). Multiple sets(up to six per gain) of microtwinningin calcitefrom Azuararocks are common(Pl. Z, Fig. 6y and have beenreportedby Fiebag(1988), Hunoltstein-Bunjevac (1989) and Mayer (1990).

Shattercones

_ It is,gexerallyaccepted that shattercones form by shock waves(Roddy and Davis,,l977, Saggyet al.,2002)in a pressurerange between ab out 2 and,25Gpa (20 and 250 kbar) (Grieve,1987). Because of thick posGimpactsediments in the inner partofthe Azuarastmcture, the finding of shatteiconesii resrictedto the sediments of the rim zone.In this zone,the shockwave was alreadyweakened to the lower pressurelirnit of shatter-coneformation. Consequently, it ii not surprisingthat shat- ter cones are rarely found and that the fracture cones often display only diffuse markings.Shatter cones in Jurassiclimestones have been found ät threeiocations (descriptionand photographsin Mayer (1990),Katschorek (1990), Müller (1989), howeverin eachcase they were sampled from rock debrisin thering zone.The UTM coordinatesof the threeplaces are 6 84 440,45 49 550;6 g4 70O:4555 150;6 83 100,4575 850.

THE RUBIELOS DE LA CfRIDA IMPACT STRUCTURE

Topographic features and target rocks

The Rubielos de la Cdrida structure,about 50 km south-southwestof the A_zuara_structure (Fig. 1), is defined by an approximatelycircular uplift of Mesozoicrocks (in the middle of Fig. I ), sumoundödby a seÄi-circularto ellipti_ cal.depression^ofQuaternary and poslimpact Neogenedeposits (Fig.9; simplified andmodified from the geologicalmaps 1 : 200.000;ITGE,-1991; IGME, 198:6).As will be discussedlater (seeImpact melt rocks,puerto Minguez ejecta. Age of the impact.event),the Neogeneage of someunits may be disputed.The diameterof the uplift is roughly 15 km and the west-eastdiameter of the depressionamounts to^some40_ km- The geologicalsketch map (Fig. 9) and a simplified section(Fig. 10) show that the existenceof the centraluplift is structurallycontrolled. In th--e weslernand northernpart of lhe struclure.its rim is madeup of Mesozoicand Palaeozoicrocks. belonging to the W€sternIberian Chain (for the stratigraphyof the target see Fig.3). A number of hills of Mesozoic rocks emereefroh- the Qu_a1e1aryin the western part of the depression.In the south a-ndeast, the Rubielosde la c6rida structureis lesswell däfined.In the literatureon the resional geology(ITGE, 1991; IGME, 1986;and references therein), no specialaccoirnt is given of the Rubielosde la Cdridastructure. 20

Geology de la In contrastwith the Azuara structure,a systematicmapping of the Rubielos e Ce.iar-iÄ"t*" on 1 : 10.000 and 1 : 20.ö00 scales is so far lacking' gql"-4 scales(IGME' ;;;i"il;ir;" uy itre geotogicatmaps on I : 50 000 and 1 : 200 000 iöä,-\öis,"islo, issiul rsa5t,1e8ic, 1es6; IrGE 1ee1),that do not address the i.pu"r-i"tui"a features.The new datapresented here result from our discontinuous ten yearperiod' !"älogi" unap"ttographic field work overan approximate

Central uPlift cor- The oldestlayers exposed in the centreof the uplift ar-e Muschelkalkage -of - 1000m ."roondinsto u ,t atigraphicaluplift (Gieve et al',19811of theorder of 500 on the pie-impacLläyeringl. The youngestlayers are upper Cretaceous iä;;;ffi g. ;i;;J;;"lt periitreruttyin LhäwesirFigs. l0). Frommorphometrical considera-

%n It NSNz ffio ls ffifllllllllllllllla 77Vz

7)t l-

10

10km

simplified and modified 9. Geological sketch map for the Rubielos de la Cdrida impact structure' Fig. Tertiary'2 = Upper i.J. Ircg tisst) -o tcME (1986).I = Palaeozoicum,Mesozoicum and Lower = 5 = Keuper'6 = i;;;;d a;;i;;.y, 3 = iehrda Fm. ejecta;central uplift:- 4 tr4uschelkalk' Rhaetiänandüassic,7=Dogger,8=Malmian,g=Cretaceous:10=drainagepattem' y modificado Fis. 9. Mapa geoldgrcode la estructurade impacto de Rubielos de la Cerida' simplificado paleozoico, = Terciario " irii. J"iir?eiis91) e IGME (1986). 1 = Mesozoico y Terciado inferior,2 4 = Muschelkalk' 5 = ;";;.;; ö;;;-;o, 3 = Ey""tu de la Fm Pelarda; levantamiento central: g = l0 = sistemade drenaje' äJö"r. ,i =-t"ir""* vliari"o' r = nogg"t,8 = Malmiense, Cretäcico; 2l

tions (Grieveet al., l98l; Melosh, 1989),one would expecta much larser strati- graphicaluplift. The divergence could possibly be relared-withthe enormöus thick- nessof the softmolasse top targetlayers (perhaps more than 2 km; seeabove for the datafor the Azuarastructure). Besides the soft argillaceousKeuper layers and a few Keuperand Cretaceous sandstones, carbonate rocks, mainly limestonei, dominate. The mostsignificant structural feature in the centraluplift is the enonnouscom- pressivesignature. Strong deformation up to continuousmegabrecciation is evident nearlyeverywhere and can best be observedin variousroad cuts (Pl. 3, Fig. 1).In the southemcontinuation of theuplift at Bueflavillage, deformation in the form of criss- crosslayering is especiallyimpressive (P1. 3, Fig. 2). Quarryingnear the roadjunc- tion CN 211 - Rubielosde la Cdridavillage (UTM coordinates6 45 000,45 20 800) reveals.,roluminous strong deformation (grit brecciation,mortar texture) typical of monomicticmovement breccias (see the respective descriptions for theAzuara struc- ture).About 1 km northof Rubielosde la C6ridavillage (UTM coordinates640870, !t!7780), quarryinghas exposed a largeverrical fault plane(superfault; cf. Spray, 1997)with some300 m? of continuousminor polish.In thecontaci of theptane, äO;ä- centcarbonate rocks have changed to whitishporous masses possibly as the resultof decarbonizationorland melting and as such representing a variety of pseudotachylite. On a mesoscopicscale, we observea peculiarmechanical behaviour of limesiones exposedover large areasin the uplift. On hitting them gently with a hammertwo or tkee times, they disintegrateinto innumerablesmall fragmentsand splinters as if stresswas frozen within therocl.

Crater depression

Going outwards from the uplift, the remarkabledeformation continues.In the Mesozoic hills emergingfrom the Quaternary a number of quarriesenable insight into drasticand voluminous brecciation of thelimestones. In theextended quarrv near Villafrancadel Campo(UTM coordinates639200; 4505200),large complexäs are intenselycrushed and groundto display grit brecciationand mortar texturetypical of

RUBTELoSDE LA cERtDA ! roo. CENTRALUPLIFI

IMTflIIIIMNNIZ 't2 456789

Fig l0 Generalizedgeological w-E section acrcssthe Rubielos de la cdrida structure.stratisraDhical unitssame as in Fig.9.

Fig. 10. Secci6ngeolögica geneml, de direcci6n E-W, a trav6sde la estuctura de Rubielos de la C6rida. Las unidadesestatigräficas son las mismas que las de la Fig. 9. 22 - monomicticmovement breccias. A compressivestrength of perhaps150 200 MPa (= 1.5- 2 kbar)assumed for thesemassive and dense Muschelkalk limestones means ifr"V-utt ft*6 operiencedpressures cleady exceeding these values, not only locai- ly but through and through.

Craterrims At the rim of the depression,the rocks continueto be seento be heavily defotmed' howeverwith varying intensity.Outcrops resuldng frorn road constructionfrequently Jo* u *gut *"'iatiän of the'mostlycarbonate roiks. Very often within the megableg- "iÄ,---'-we"emphasirJ r*g" 6n"r., uppear to havebeen rotated in situ,a featurealso obser.red in the uplift. ihat suchheavy and voluminousdeformation is restrictedto the Rubielosdö la C6ridast,ucture (and the Azuara structure) and is unknownfrom other res.ionsin lhe lberianchain overprinted by Alpidictectonics'

sameas in Fig Fig. 11. Location map for impact featuresin the Rubielos de la Cdrida structure Geology = g."pp = peuraa frn. ejecta,PM = Itrerto Minguez ejecta' b = basal (suevite)breccia' d breccia dikes' = M = impact melt rocki, Su = suevite,su = suevite-like impact breccia, s shock metamorphism' de Rubielos Fig. 11. Mapa de situaci6n de los rasgosde impacto plesentesen la estructurade impacto = Eyecta d;la C6rida'.La geologia es la misma que la d; la Fig. 9 PF = Eyectade la Fm Pelarda' PM = de impacto' ä" i""tt ui.g"är, t ] brecha basal (suevita)' d = diques de brecha,M rocas de fundido Su = Suevita, iu = suevita similar a brecha de impacto' s = meta:morfismode choque 23

Brecciasand brecciadikes

..flart from tne widely distributedmegabreccias already mentioned in the central uplift, andthe voluminousmonomictic movement brecciai in the rim zone,breccias andbreccia dikes, as described for theAzuara stnrcture, also occo. in theRubielos de la cdrida structure.The basal(suevite) breccia is againfound in irffnediatecontact with foldedMesozoic rocks and,therefore, at the baseof the post-impact(or post- Alpidic),Upper '720,Tertiary. UTM coordinaresfor exposures(Fig. l i) are6'6 S 6iO,45 02 988;6 59 44 98 970;6 35 870,45 08 490. Comparedwith the Azuarastructure, breccia dikes seemto be less abundant. which.however. is assumedto be relatedto theresricted mapping activities only. Typicaldikes are shown in Pl. 3, Figs.3, 4. While the brecciadil.e in pl. 3. Fis. 3 is part_of an extendeddike systemcutting through competent Muschelkalk limeitones, thedike in Pl.3, Fig.4 cutting-throughsteeplydipping Jurassic limestones actually i; not a brecciadike. The whitefilling of the dike ii moie or lesshomogeneously -any com_ posedof highly porous,par{y vesicularcarbonate material without clasis.The tharp cut and the sharp-edgedfragments of the limestone,without any dissolution features-exclude any karstification.Instead, the white materialis suggestedto be relicsofcrystallized carbonate melt injectedinto thecrater wall duringÄä excavation stageof impactcratering. The locations for brecciadikes in theRubie.-los de la C6rida structureare plotted on Fig. 11.urM coordinatesfor a locationof brecciadikes cut- ting throughPalaeozoic rocks near Olalla are6 53 240,45 37 l7O.

Impact melts

Impactmelts were found at four locationsin theRubielos de la Cdridastructure. We emphasizethat these melts do not reflectthe completemelt-rock spectrum in the Rubielosde la Cdridastructure. As alreadysuggested for theAzuara structure, abun- dantrelics of formercarbonate melt arepropoied to alsooccur here in its companion crater.A carbonatemelt cannot be chilledto form glass,but rapidly crystallizes-to car- bonateagain. Thergfore, the origin from a melt canonly indlrecily6e suggestedby theoccurrence of skeletal,dendritic crystallites, vesicular texture and relatäd featurei (seethe discussionon Azuaracarbönate melts by Katschorek[1990]). The melts describedbelow are characterized by amorphousglass phases.

Thehost rocks

_ The_m€lfbearing rocks are exposednorth of the centraluplift in the valley of the Pancrudobrook roughly_ 10 km eastof thetown of CalamochalMin Fig.t 1).Between the junction of the road to Cutanda./Olallaand the Barrachinavillagl, tire impact_ relatedrocks areexposed as a rnegabrecciaover roughlyhve kilomJtres.A further outcropis locatedalong the steepbanks of thepancrudo brook. On both sidesof the roadbetween the junction and Barrachina,temporary outcrops exist or haveexisted and may in the futurebe enlargedfor gravelexplöitatiön. The Äegabrecciais deposited and faulted. in conlact with bedded sediments,which are, however, strongly folded ,.megabreccia" and the overall är;-te; refers to both the size of the components distinguish for fl""k";;;, *tti8n -uy reach up to 50 m. Within the megabreccia, we the '_present: a massive diamictic material which is not consolidatedand has characteristics described for the Pelarda Fm. ejecta (Emstson and Claudin' ,"* ,i-if- to those ( iödO,ärä ."" +l ianotor" the ejecta deposits ol'the Puerlo Minguez. C.fulilet aL, plastrcallyo:l:"tl:o 2001; also see4.2). As in thesedeposits. we observestrongly uccaslonally' comDonentswhich point to highconlining pressures upon detormatlon' process ^'-i'"."Ut"""i" is intercalateä,somedmöi giving evidence of an injection (Pl.'^ .' 3, Fig.s). -'T1fi diamictic materiai incorporates,intermixes and interstratifieswith multi- (Pl 6)' The "ofouJlypto. marls,claystones, li-".ton"s andpure gypsum 3' Fig' *"iti""i"üäa materialshows a complexinterlayering tith the diamicticmaterial. i'il' ;;p;;ii" Äay overlie the diamictites, theriby iunnelling them and forming even aooohvleswithin them. Frequenlly. bodies of üe multicolouredmaterial are may iä"iJ nou,ing*ithin thediamictii material' ln otherplaces-' the marly material in the ""ä"ifl" tft" d"iamicticunit. In this case'we may observea fluidal megatexture marly äiamictite obviously correspondingto a fluidai megatextJrrein the underlying the mul- il;üry ;ft";, th; impacimelt röcks are embeddddwithin this zone.Both i.ncom- ,i"orooilä "o-rj"sition of themegabreccia and its strangetexture have much Pohl mon with the bunte brecciaejeäta of the Ries impaci crater(Hüttner' 1969; et- ' al., 1977;Hörz, 1982). showinga megatexture -'rüo"gty ut"cciateähmestone megablocks tPl ^3. Fig T) .hi"h ;;;;ät;";;b1es grit brecciaidescribed for part of the Ries craterejecta (Hüttner,1969).

Silicatemelt

Within the outcropwall at the northeastemroadside (UTM coordinates6 55 100' gO0),melt rocki occuras soft,porous, fine-grained' whitish blocks of variable 45 29 j sizein a rangeof decimetresup lo I - metres'intermixed in thepolymictic t:-ry?fi: rn cia describeäabove (Pl. 4, Figs. 1, 2). Two of theseblocks- have been lnvestlgated and il"il Th;;;Ä *ainly consiit of-a milky white gllss which forms tiny spheroids Uoaies.Thäir diameters are röughly 0'5 mm (P1'4, Fig A second'sub- tens-strapea -3) glass "ä*i"iä* ptt^iei, tranrluc.ntgreyish aid öccursinterstitial within the white pr. romake up more than 9ovo "äät.'i!,?aä]."r*i i" +. Fig.3i Thegtass is estimared "hump"' äi,r'.'i".t. Th'isis typicallyshlown by a distinctamorph-ous glass occumng minor in x-ravpowder ditfiäcrograms (Fig. l2). Somerelics of plagioclaseand' to a reflec- ;;;;;"iil;; unJtl"i*itttln tüeglass masses are indlcated by respecÜve tionoeaks..Grainso|quanz.twinned-plagioclaseandoccasionalmicaarealso|ound planar i" irti" ,Ä.,i""t "f the'glassmatrix. In ti. cattt. the quanzfragments show ä"io.rnutio" features(iDFs) and, more frequently.multiple sets of planar fractures F"rä.p- grains show isotropizationin the form of-mul4Ple setsoJ isotropic ipirl. have twin*ns taniettä andisotropic spois (diaplectic crystals' Pl' 2'Fie'7)' andthey 25 sometimesbecome almost completely isotropic (diaplectic glass), indicating shock peak pressures of the order of 30 GPa (300 kbar) (Engelhardt et al., 1969). Four bulk samplesof the melt rocks were analyzedby RFA Philips PW1480, and separatedparticles of the white and the greyish glasswere measuredusing a CAME- CA SX50 electronmicroprobe with wavelengthdispersive spectrometers at operating conditionsof 15 kV acceleratingvoltage, 15 nA beamcuraent and a defocussedbeam size. The results are given in Table 1. Contentsof Mn, Cr, Sc, Co, Ni, Mo and S are below the detection limit of the respectiveinstrument. The poor totals of the micro- probe analysesare most probably due to the presenceof HzO which enteredthe glass by alteration. If corrected by LOl-determination, the totals are close to 100 70 as shown for the bulk analyses. The compositionsof the milky white glass spheroidsand the interstitial greyish glassparticles do not differ significantly. The sameholds true if microprobe analyses and RFA bulk analysesare compared.Major oxides are SiOz between53 and59 wt.Vo and Al2O3around 20 wt.%o.Thecontent of MgO in the glassparticies (arotndT wt.Vo) is somewhathigher than in the bulk analyses(4.8 - 6.1 wt.%o).Differences between the microprobe and the bulk analyses may be ascribed to secondary pore fillinss or the mineral content.

100 29 (des.)

Fig. 12. X-ray powder diffractogram of the silicate melt rock shown in Pl. 4, Fig. 3. Beside the sharp diffraction peaks belonging to the feldspar phase (f), strongly broadened mica p€aks (m) can be observed.The broadeningreflects the low crystallinity of this phase.In the 2Q-range between 20' and 30", a typical glass "hump" is displayed,with peaksof feldspar and mica phasessuperimposed. Fig. 12. Difractograma de rayos X de la roca de fundido silicatado mostradaen la Pl.4, Fig. 3. al lado de los picos de difraccidn abruptospertenecientes a la fase feldespätica(f), puedenobservarse amplios picos correspondientesa las micas (m). Esta amplitud refleja la baja cristalinidadde estafase. En el inter- valo 2Q comprendido entre 20" y 30', puede obseNarseun "abultarniento" tipico del vidrio, con picos superimpuestosde fasesde felde.patoy mica. 26

Suevite

Silicate melt was also found in a polymictic breccia quarried out as large blocks in a temporary outcrop at the northeastem roadside in the Pancrudo valley (UTM- coordinates 6'55 4OO,-4529 600). Apart from the melt, the breccia is composedof matrix-supported subrounded to angula.r limestone, sandstone and quartzite- clasts having vdrious grain sizes.The dominantly carbonatematrix is pepperedwith -frag- mente:d, predominantly quartz and calcite grains, and shows flow texture with the larger cla^stspreferentiält adjustedto the flow (P1.4,Fig.4). In somecases, clasts are thäselves Lrecciated (breccia-within-breccia)' In thin sections, shock metamor- phism in the form of quartz grains with multiple setsof PDFs and diaplectic quanz is äbser.red.The melt oicors in elliptic whitish clasts with diametersbetween several millimetres and 2-3 cm. The soft alasts are extremely fine-grained and can easily be carved by a small spatulato get material for analysis.From x-ray powder diffraction analysis,these clastsare not pure melt, but consist of a mixture of amorphousmate- rial (typical glasshump) togetherwith dolomite, calcite and minor amountsof quartz, m.,rcävit" und gypsum. This mixture is also reflectedby the chemical bulk composi- rion (bulk-5 anilßis in Tabl" 1). High contentsof MgO and CaO and the distinct LOI values reflect dominating carbonates,mainly dolomite and subordinatecalcite. The bulk-5 composition of tfie clasts closely matches a mixture of 507o pure dolomite, 107opure calcite and 25Voof the white glass (mean composition) in Table l. A dom- inant-amount of carbonates and a subordinate portion of melt is also estimated from the x-ray diffractogram. According to the current classification and nomenclature of impact iocks GUÖS Subcoffnission on the Systematicsof Metamorphic Rocks, Study Group for Impactites),this polymictic impact brecciacomposed of shockedand melt clastsis termed a sueviteor suevitebreccia.

Suevite-like breccia

Along the road betweenBarrachina and Torre Los Negros,exposed limestones are strongly deformed and frequently show allochthonous material intercalated in the form of mägäblocks and dikes cbmposed of multicoloured marls and shales and Palaeozoic Pelard--aFm. facies (Ernstsonänd Claudin, 1990; also see4.1). At UTM 6 57 900' 45 28 300, black shales have been injected into the bedded limestones exposing a pecu- liar macroscopicbreccia zone (P1.4, Fig. 5). On a smaller scale, fragmentedblack shales are intensively mixed with a white material to form a fine-grained breccia ln thin section, flow texture can be observed within the dark brownish to black, extremely fine-grained matrix. This matrix contains clasts of quartz and subordinate feldspq,-and some'iimesvery fine-grained aggegates of light minerals. From x-ray powder diffrac- analysis, the rock consists of quartz, kaolinite and illite, carbonate and an amor- tion "glass phous phase,the latter defined and documentedby a typical hump" (Fig' 13) backscättered electron images of electron microprobe investigations show an extremely fine-grained mixture of medium greyish and darker greyish components on a scale of few Äicrons. The analysis of the medium greyish parts reproducibly yields illite mix-ed with other clay mineials and a composition of about 53 wt.Vo SiOz, 25Vo AlzOz' 27

5VoFeO,3Vo KzO and,2.5Vo each, CaO andMgO. The darker greyishparts arerich in SiOz and contain variable, but subordinateAl2O3 content. Most probably, this fine- grainedmixture reflects strongly alteratedrelics ofthe amorphous(glass) phase which in an uncorrodedstate could not be detectedby microprobe analysis.By corrosion, a glass may be transformed to clay minerals passing through an intermediate gel stage with typically enhancedSi-contents (Rösch e/ d/., 1997).No clear shock-metamorphic features have so far been identified in mineral clasts of this glass-bearing breccia. Therefore, we chose the term suevite-like breccia for this peculiar rock.

Carbonate -p hos p hate meIt

A very special kind of former melt was found within the megabreccia of the north- easternroadside outcrops at UTM coordinates6 51 800, 45 31 400. The whitish melt rocks (Pl. 4, Fig. 6) are composed of irregular spheroids up to 4 mm in size, which are embeddedwithin an extremely fine-grained matrix. Under the microscope, the spheroids

ca

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A'o

10 2e @ee.)

Fig. 13. X-ray powder diffractogram of the suevite-like breocia. Next to the sharp diffraction peaks of low quartz (q) and calcite (c), more or less broadenedpeaks of mica phases(kaolinite, illite, montmoril- lonite) are observed,In comparisonto the silicate melt rock, a lessprcnounced glass "hump" is displayed due to the supedmposed strong and sharp quartz and calcite peaks. The enlarged part of the diffractogram "hump". with a logadthmic scale for the intensity clearly reveals the Fig. 13. Difractograma de rayos X correspondientea la brecha tipo suevita. Prdximos a los abruptos picos de difraccidn del cuarzo (q) y de la calcita (c), puedenobservarse picos mäs o menos amplios de fasesde micas (kaolinita, illita, montmorillonita). En comparaci6ncon la rcca de fundido de impacto, "abultamiento" puede observarseun correspondientea viddo menos pronunciadodebido a la fuerte y abrupta superimposici6nde picos de cuarzo y calcita. La parte aumentadadel difractograma con una escalalogaritmica para la intensidad,revela de manen clara el abultamiento. 28 turn out to be globular to amoeba-like calcite particles. They-are coarse-grainedin their centres and di;phy decreasing grain size towards the rims. Regularly, a perpendicular grain orientation towards the ims is observed. The contact with the matrix is extremely fine-srained tPl. 4, Fig. 7). The isotropic glass matrix in part is intensively pervaded bv tiiv. elonsated, sorietimes flaser-li1ie microcrystals, often orientated tangentially to ttie lÄ of tnä calcite panicles rPl. 4, Fig. 7). The whole rock compositionyields 52 7 wt.Vo CaO.8.3 wt.7o PzOs and 1.5 wt.VoBaO (RFA, bulk in Table 2) From micro- Drobeinvestigations, the carbonateof the particles is pure calcite. The glassy matrix inainly consiits of CaO and PzOs(Table 2), with minor contentsof F (l'0-2'5 wt Vo)' (0'3-0'6 wt%)' The b 1l.t'-Z.t wt.7o,if calculatedas SO:), C1(0.5-0.8 wt-Vo) and NaO light componentswithin the Ca-P- 'slass,poor torals ofthe analysespoint to high amountsof presumablyHzö which may have enteredthe glassduring corrosion.The exis- i.nce öf consideiableamounts of C or COz,however, must also be taken into account' --content Locally, a strongenrichment ofBa and S at the expenseof the CaO and PzOs is observed, wliich is lowered to the range of trace elements or below the detection limit, whereasAl2O3 is presentin minor concentrationsof about 1 wt'qo' In part, the 6u-p-g1ass is recrysialtiieO to form apatite, as verified by x-ray powder diffraction

3

-a d

h d 'th. b

20 \deg.)

of the carbonate-phosphatemelt. Peaksof calcite (c) and baryte (b) Fig. 14. X-my powder diffractogram "hump" "hump" unä airtinctty U-aOened peaks of hydroxylapatite (h) are supedmposed o-na glass The dis- shows in the enlargedpart of the diffractogram with a logarithmic scale for the intensity, but is l€ss tinct due to a subordinatecontent of phosphateglass in the melt. Fig. 14. Difractograma de rayos X corespondiente al fundido de cartonato-fosfato Pueden aprecia$e (h) supelponen piäos de calcita G), barita (b) y caracteristicospicos amplios de hidroxiapatito que se ..abulramiento" "abultamiento" parte iobre un con;spondiente a vidrio. El puede observarseen la aumentada del difractograma con escala logaritmica para la intensidad, pero es menos ostensible debido al contenido subordinado de vidrio fosfatado presente en el fundido analysis. The diffraction peaks of this apatite, however, are broadened compared to those of a well crystallizedone (not shown here), indicating its very low crystallinity (Fig. 14). The existenceof baryte has also been proved by x-ray diffraction analysis. This baryte may occur as a very fine-grained phase within the Ba- and S-enriched loca- tions in the Ca-P-matrix, detected by microprobe analysis.

Amorphous carbon

In the Barrachina megabreccia described above, blocks of a fine-grained micro- breccia are locally intercalated.The microbreccia consistsof loosely cementedcar- bonate and, subordinate, quartz particles. Small black clasts measuring between 0.5 and 2 mm are widespread and striking. To separatethese particles ftom the sediment, the carbonatewas dissolved in HCI and then the particles were removed from the remainins mineralfracdon by hand-picking.Two kindsbf blackcomponenrs can be distinguishei The frst kind is ordinary charcoal which under the microscope shows the typical charcoal struchre. Becauseof the weakness,the charcoal can easiiy be destroyed with a preparation tool, and actually, it is not of further interest for the present paper. The sec- ond kind of black paniclesis very hard.occurs in very inegular fbrms and hasa surface gleaming like glass (Pl. 4, Fig. 8). Qualitative micrbprobe element scans show the par- ticles to be composed of carbon and oxygen as the only major components. Additional elements are Ca in varying concentrations up to 2.7 wt.Vo and S varying between 0.2 and 0.8 wt.7o. X-ray powder diffracrion analysis of the particles resulted in diffrac- togams without any reflections, but showing a typical amorphous glass "hump".

Origin of the melt rocks

Silicate melt

The blocks of silicate glass are assumedto originate ftom melted shales.This is conoboratedby the chemical compositionofthese melt rocks. The analysesshown in Table 1 comparequite well to a composition of shales,especially with respectto the high Al-contents and the low contentsof Ca, Fe and the alkaline elements.The dis- crimination log(SiO2/Al2O3)vs. log(Fe2O3/K2O)(Henon, 1988), not shown here, also suggeststhat the origin of the silicate melt rocks is shale,mainly basedon the low SiO2/Al2O3-ratiowhich is typical for pelitic rocks (Wimmenauea 1984). Using the pure SiO2-Al2O3system (compilation in Levin er al. t19641),a maximum melting temperatureof 1750'C can be deducedfor the silicate glassrocks, which in any case is lowereddue to the contenlof alkalineand earthalkaline elements. Very similar shock-meltedshales are reportedfor the Haughton impact crater on Devon Island in Canadato form a highly porous network of non-transparentsilicate glass with some very fine-grainedparticles of quartz, sheetsilicates and calcite (Metzler et al., 1988). Using data of Kieffer et al. (1976) and Stöffler (1984), a minimum shock pressureof 30 GPa (300 kbar) is required for the onset of melting in sandstonesand shales,as is the casewith the Haughton impact melts (Metzler et aI., 1988). The Rubielos de la C6rida silicate glass rocks are clearly not of volcanic origin' due to the occufrence of strongly shocked clasts in the melt. If these melt rocks wefe to represent a deformed ash layer, the rocks should contain pyroclastic fragments and' ..intermediate" with resoect to an SiO2-concentration, mafic relic minerals or andesitic rock fragments. Such is clea y not the case. Moreover, the chemical composition should bie similar to that of andesitesor basaltic andesites.Those rocks, however, eenerally have distinctly lower contents of AlzO: and much higher contents of FeO, öaO anä (Na2O+K2O) than the investigatedsilicate melt rocks (a comparisonwas canied oui with all analysesof volcanic rocks given in Wilson [1989]). Furthemore, the melting temperature estimated for the investigated rocks does not really match lhe temperatur:esin^an andesite volcanic system. Apart from impact events, the only other geoiogic possibility to produce melt rocks (pseudotachylites)is by frictional heating äurinf ex'tremedynamic metamorphism in a thrust zone. Such a process is assumedto occur'also in large impact evend during the excavation and modification stages-of impact cratering.'The Äarrachina silicate melt cannot be assumed to have formed by friätional meltiig of the target rocks, as the coexistence of glass and highly shocked minerals clearly ipeaks in favour of a shock-produced melt. . . Pelitic rocks, i.e. claystonesand shales,as the deducedorigin of the silicate melt, to the are abundant in the stratigraphic-ouicrops sequence of the target. only a few kilometres norlh of the Barrachina near Olalla, they contribute, for example to -the Cambrian Valdemiedesand Hu6rmeda Fms. (ITGE 1991; Monninger 1973)' They also occur in the BuntsandsteinFm., in the upper Malmian PurbeckFacies and in the Eocene/Oligoceneactually exposedin the Rubielos de 1aC6rid4 structure'

Sueviteand suevite-likebreccta

The melt clasts of the suevite are composed of a mixture of dolomite' calcite and slass. which is more or less similar to the silicate glass of the larger melt bodies' öarbonate, which has crystallized as calcite and dolomite, may derive from a carbo- nate melt. A fine-grained carbonate-rich suevite breccia with formeriy melted carbonate material häs been reported for the Chicxulub impact structure (Heuschkel et at., 1998; Joneset a1.,2000) and may servefor compa.rison.From its macroscoplc aDpearance,the suevite-like breccia is quite different from the suevite. Like the sue- uiä, ho*"u"., it mainly consists of a very fine-grained mixture of carbonateand silicate glass,the latter i; part strongly conoded Both brecciasare composedof clasts which, äpart from the melt, representa mixture of pre-impact Palaeozoic,Mesozoic and ?Cenozoiclithostratigraphical units.

Carbonate-pho sphat e meIt

The carbonate-phosphate rocks from Barrachina in the Rubielos de la C6rida stmcture do not looi< like typical melt rocks, and at first glance, the only hints of a for- mer melt are the glassyreliis within the phosphatematrix. The only comparablerock described up to nöw häs been reported for the original Ries suevite, which is the melt- 31

bearing impact breccia of the Nördlinger Ries crater (Engelhardtet al.,1969). Within this suevite, inegular, amoebalike carbonate particles aie embedded within a matrix of silicate glass. The carbonateparticles show chilled margins in contact with the silicate matrix, and along this rim, calcite crystalsgrew elongätedperpendicular to the contact.This texture was interpretedas the result of a quench cryitallization (Graup, 1999),and the author was able to show that theseparts of the suevitewere formed bv the imrniscibility of impact-producedcarbonate and silicate melts. In general, the carbonate-phosphate melt rocks from Barrachina resernble the suevite from the Nördlinger Ries crater. Both rocks show these amoebalike carbonate particles which result from quench crystallization, with chilled mareins and calcite orientatedperpendicular to the margin. Additionally, in both rocks, carbonateglobules and carbonatepalticles with curvilinearrims towardsthe matrix occur Like resoective parts of the Ries suevite,the melt rocks from Barrachinaare assumedto result from an immiscibility of two kinds of impact-induced melts, different from the Ries suevite, however, of a carbonate and a phosphate melt. A themal decomposition of carbonates. which-commencesat a shockpressure of about45 Gpa (450 kbar), is also reportedfor the sedimentarytarget rocks ofthe Haughtonimpact (Metzler et al.,19gg; Kieffer and Simonds, 1980). At Haughton,carbonate globules and inegular blebs within silicate glass, similar to those from the Ries impact, are described and again interpreted as a result of carbonate*ilicateliquid immiscibility (Osinsky and Spray,2001a;b). The carbonateof the carbonate-phosphatemelt may be ässumedto be derived from the Mesozoic, especially the Jurassicand Upper Cretaceouslimestones, which are very abundantin the shatigraphicsequence ofihe centraluplift ofthe Rubielos de la C6rida structure(ITGE, 1991). Limesronesalso occur in the EoceneOTGE. i991) and largely contribute to the conglomeratesof the Lower Teniary which made up a large part.of the pre-impact target area (Ernstson and Fiebag, lg92). The phosphäte of the carbonate-phosphate melt may be derived also from ihe Jurassic "boundary rocts in ttre target. There are two oolithes" (Geyer et al., 1974) which mark the Lower/Middle and Middle/upper Jurassicboundaries in the whole Iberian Cordillera. From these boundaries, phosphoritic components in the fossiliferous oolites are described(Geyer et al., 1974). Phosphatemay also originate from coprolites in the sedimentarysequence of the target. A prominent Oligocene coprolite layer has been reported, e.g., exposed near the town of Calatayud some 80 km northwest of the Rubielos de la Cdrida structure(Hammann, pers. comm.). Ba, as locally observedin the carbonate-phosphate melt, is common in the target area, occurring as ba.rite in fissures,dikes and as irregular massesin Palaeozoicrocks (ITGE, 1991).

Amorphous carbon

Carbon in elemental form, sometimesas diamonds, sometimesas C-rich shale fragments,has repeatedlybeen describedfrom impact structures(Kobavashj et at-- 1997:Bunch et al-. 19971.A possiblesource of elementalcarbon is carbonate.esoe- cially for target areas showing a thick and carbonate-rich sedimentary cov-er. Hypervelocity impact experiments verified the production of highly diiordered graphitefrom dolomite or limestonetargets (Bunch et al., 1997)-ForTaiural impacts, a transformation of CO to CO2+C in the cooling atmosphericimpact plume is assumed(Heymann and Dressler, 1997). Miura et al. (1999a, b) propose-that amor- pt oo. .*Uoti occurringin impact structurescan form from vaporizedlimestone iu,e;i ,".ü in multiple"impact;with reductionstate. Ca-contenfs within the carbon ui"" int.rp."t"d to bi remäinsfrom the limestonetarget rocl{s.Thick limestone ,"qo.n""i in th" Rubielosde la Cdridatarget area thus may easily havebeen a source ioiit," "urUonpu.ticles in theBarrachina megabreccia. This is underscoredby theCa- "Lnt"rrt.up to).2 wtTodetected in theseparticles. As a further.possibilityand taking into u""ouitt ttt" glassyappearance and the inegular shapesof-the particles, the amor- "frour "-U.n mäv bä quinchedcarbon melt from extremelyshocked coal of the bi"tu."oot Utrillaslignite depositsin the target.The meltingtemp€rature of car.bon i, iougfrfy3500"C wrfich is eiceededat highestshock levels' The lignitelayers have teetr 3t Änsia".aUleeconomic value in Späinand are deposited roughly 20 km-west oi the Rubieiosde la C6ridacentral upüft (ITGE, 1991).In the srructureitself, the stratisraohicallvadiacent layers of Albian to Senonianage are exposedin the central ""iiä rücg, issil, -a äoal_bearingutrillas layerscould have been deposited immediatelyat the imPactPoint The ro1äof remarliablJcontentsof oxygen detectedin the carbonpalticles is still unclear.Compounds of carbonand oxygen do not occur-insolid state' We propose the nossibilitv thät the carbonmay occur äJfu ereneswhich are able to trap gaseswithin iheir.agäs.Fullerenes have been reponed in relationwith-the^Sudbury impact struc- tor" -i tt.,"permian-Triassic boundary (Becker er a1.,1996. 2001). More investiga- tronsare necessary.

ShockmetamorPhism

Apartfromtheshock-metamorphiceffectsalreadydescfibedforthemeltrocksand tfr" roLrtit", moderateshock effecis are regularly observedin samplesfrom silicate i."rc, Ä"rirv Cretaceoussandstones frorn the central uplift and the rim zone' They atsooccu. Itisandstonesof the small Buntsandsteinhill emergingfrom the Quatemary in tfr" a"pr"rtio" southwestof the centraluplift, and the P-ermotriassicrocks exposed in t6" ruitounaingsof Visiedo, southeastofthe centraluplift (erroneouslymapped- as IriJman ttrere,sJe ITGE t19911,IGME [1979]).As in theAzuam sedimentary rocks' rnoä"iut" shoci

Shatter cones

Although competent rocks are widespread in the Rubielos de la C6rida structure and_areespecially well exposedin the central uplift, only very few shattercones of moderatequality have beenfound so far This is believedto be'dueto limited outcroD conditions (freshly exposedrocks are rare) and to the lack of a detailed mapping. Also, there may have been influencing factors which preventedshatter cone fb-rmä- tion. We point to the assumedvery thick unconsolidärcdmolasse sedimentsof the target and a possible buffering effect. Moreover, we refer to the above-mentioned peculiar behaviour of the limestonesin the central uplift, that is, breakins into small lragments and splinrersupon hitting them gently with a hammer.The i'rocessthat evidently froze stresswithin the limestonesis unknown so far. but mav have been incompatible with shatterconins.

ÄZUARA AND RUBIELOS DE LA CERIDA IMPACT EJECTA

Two impact ejecta deposits have so fa.rbeen investigated in more detail: the pelarda Fm. ejecta (Emstson and Claudin, 1990) and the prrerto Minguez impact ejecta (Claudin et al.,200l). Both may be tracedback to an early observationof an;.enigmaticdeposit', at Puerto Minguez (Moissenet et al., 1972). This at that time small outcröo of quartzite Palaeonic compon€nts was also addressedby Carls and Monnin ger\197 4 by comparing it with the Pelarda Formation. The reported similarities betweän the ,,enis_ matic"PuenoMfnguez deposir and the Pelarda Fm. initiateda new thoroughinvestigä- tion which related the Pelarda Fm. sediments with the Azuara imDact structure (Emstson and Claudin, 1990). A completely new insight into the deposii ar the puefio Mfnguez has been provided by road-construction work for the new stretch of the CN 211 between Caminreal and Montalbi{n. From field work in the enlarged outcrops it is suggestedthat the deposits are related with the Rubielos de la C6rida impact strücture.

The Pelarda Fm. ejecta

The PelardaFm. covers an area of approximately 12 x 2.5 k:n2 (Fig.9), has in many casesa thicknessof more than 200 m and, between 1,100and 1.450m altitude- forrns the top of a mountain chain which belongs to the highest ones in the region (Carls and Monninger, 1974). The Pelarda Frn. ünconformibly overlies the Foifria Lower Tertiary (Palaeogene)which is composedof alternationsof conglomeratesand multicoloured marls and is overlain by the Olalla Tertiary materials (Adrover et al., 1982).In the environmentsof Fonfria, the limestone cobbles of theseconslomerates are heavily deformedand show intensestriations, deep imprints and polishlThe stria_ tions display more or less homogeneousSW - NE strike.ln general,a lower, middle andupper zone ofthe PelardaFm. can be distinguished(Ernsison and Claudin. 1990.1. The contactsbetween the zonesare gradual and not very distinct. In all three zonei, stratificationis completely lacking except for few conglömeraticintercalations in the middle zone,which are more or less laminatedbut do not displaytabular bodies No structurescan be Gtfr". tÄui "ot large)nor do they form channelingstructur€s itr"-"a indicatingälfferent pe.iods of depositionle'g , thin sandstonebeds from ir,-rt*tun"oot Ut"ufr, ,on", oftt"*orked material) Likewise, a clear grain size distri- üutlon""-r'urrti''""zonesofthePelardaFm''cobblesandbouldersareobservedtodis-"unnot t" observed,and the textureis alwaysmatrix-supported (Pl. 5' Fig. 1). onu äiiün"i tmutions(Pl. 5, Fig. 2). A statisticalanalysis of the strikefor morethan - äö ;;i;;:;6 t;"eals a clearäaximum in the SW NE direction(Emstson and CiuoAi",fqSOl which is the directionof line betweenthe centesof both the Azuara ur,änoti"tor ä" ta C6ridastructures. Sets of inegular,frequently open, fractures with complexbifurcations and rotated displacementslrotated fractures) occ]rr in the c*- ;i;;*ffi;;id;it, which indicatemore strongplastic deformation (Pl' 5' Fig' 3)' S;ca,rseifirmeaiate disintegration would occur' any transport of suchdeformed clasts can-- be excluded,which proves in situ defomation' I" th. sectionsof quartzitecobbles and boulders(both Bämbolaquartzite and Armoricanquaftzite), quartz grains regularly show strong mechanical deformation' We-otreiue'Oistl"ct'iräcturing,strong-unduiatory extinction, deformation lamellae' (mul- Äur,ior" ,"o of planardeformätion feätures (PDFs; Pl' 5' Fig' a) andcleavage tiof. i"it of pfuti. fractures,PFs). Kink bandsin mica arefrequent' The PDFshave üä"n unufvzjaon the universalstage (Fig. 8) and showcrystallographic orientations typicai' ' oiimpact shock. Fromtheie data, Ernstson and claudin (1gg0)conclude that the Pelarda Fm. con- stitutestüe remnanti of an ejectadeposit originally extendedaround the Azuara and Rubielosde la C6ridaimpact srruciures. Thä unuiual thicknessis explainedby the inie.action-*Tdi;p;;t;gin of ttte materialhore or lesssynchronously excavated from both structures' of the PelardaFm' is not generallyaccepted' Previous models andmodels'opposä to an impactorigin considJr-a-Quaternary mud flow depositof it",t"* ix,äti indinezet il., 19ggtp6rez,lgSgiFeneiro.etal, 1991.,Avell et al., iöS:; Äo."if, 1994;Cortes and Martinez, 1999) and a Tertiaryfluvial conglomerate With regardto theobser.- iC*fi ""a fra"t"i nger,797 4; J. Smit,2000, written comm')' "raia" iuti*, unO.ut".ia"lso far presented,the interpretation ofthe PclardaFm' asa iuo" d"porit o. u n"vial conglomerateimplies basic problems The almostcompletely äi*i"ä tü",iiii"tlon ana tfrematrix-suppoded teiture suggesttransport- by.plastic and ä'i;ilil*"ifü; iather than fluvial'transport(see Lowe, 1979; colombo ir,r-ä", iöszi. rh" observationsalso discard models of fluvial meandersedimentation littl^i.Igii, tl8t; lriage, 1975,1978: Allen, 1963,1964, .196s,1970; McGowe,n ;;ä;;" lti0iänd m"od"t oibraided streamsedimentation (Miall, 1977'1978l' and Marzo' il;;;r ;i Sopena,1983; Tunbridge, 1981; Friend, ^1978;. Castelltort lDS6j. sh""t fläod depositsas descibedby Frien{ (1983)do not occureither' The p"irä" f-. depositis locatedat the highe;taltitude of the region.Therefore, a flu- tectonicsexplicitly in uiJ oi t"oO flow depositionrequires eiceptional Quatemary 'tafla" tfrir ."nion, ot sedimäntationagainst gravity. Likewise, the suggested deposit ;;;i;ü;;; d"""loped at the b'aseof-a gradientbut not olr a topographicbigh' The observeddeformatibn, macroscopic and microscopic (PDFs) cannot be explainedby iioäaL tectonlcs",und un origin from syn-tectonicsedimentation clearly must be "r"ioO"a. ftlo."ouer,given a Q-uaternaryäge of the PelardaFm and consideringits io.uüo" ut u topoglupi'ichigh, the overlying thick sedimentsrequired for the confining 35

pressureto producethe observedstriations and plastic deformationshave never been present.We explicitly also excludefaults tfrom Quatemarytectonics!) to have caused the deformations,because faults have never been observed in this zone. Also, the strike consistency of the striae throughout the huge voiume of the deposit is incom- patible with faulting. A formation of the striae by the action of glaciärs can also be basically excluded.Otherwise, we had to establishverv specid cönditionsdurine the in Quaternary exactlythis localzone. Moreover. and äs älreadymenrioned. the [nn- cipal orientation of the striae points to the centers of both the Azuara and Rubielos de la C6rida impact shucture. Large amounts of the clasts display plastic deformation (seeErnstson & Claudin, 1990) in such a mannerthat even ä verv short fluvial trans- port would not have been possible without complete disintegiadon of the clasts. Therefore, the field data suggesttranspofi by non-Newtonian flow, contrastingwith normal fluvial deposition.On the other hand, suchplastic deformationhas nevei been reported for debris flows and mud flows (McGowen and Groat, 1971; Rust, 1979; Säez,1985; Boothroyd and Nummedal,1978; Miall, 1981;Heward, i987; Bluck, 1987;'.Cabreraet a/., 1985; Gloppen and Steel, 1981).Moreover, ir is very difficult to explain the rotated fractures in these clasts by normal tectonic defomation. when comparedto synorogenicconsolidated molasse deposits.

The Puerto Minguez ejecta

The geologicalmap ( 1 : 50.000 scale;sheet 492, Segurade los Baflos) and its text explanation (IGM.E, 1917) report the deposits at the puerto Minguez as monogenetic conglomeratesof Stampienage, composedof Mesozoic limestone componeniswith large boulders_.inc1uded.Conglomerates of Santonian age are reponed to uncon- formably overlie these deposits.Later, the deposits werä includeä in the sheet of Daroca (map and texr explanationsat the I : 2ö0.000 scale; ITGE t19911)as Upper Oligocene to Upper Miocene materials and their orisin--a attributed to fluvial nroieis- es. The Pueno Mfnguezmaterials are describedas successionof conelomerates consisting exclusively of sub-roundedlimestone boulders (dimension up to 50 cmt and showing pueno a .grain-supporledtexture". In a recentlypublished paper. the Minguez deposits are classedwith the sedimentsof the intra-mounüin Montalbän basin (Casaset a1.,2000). The new stretch of the road CN 211 betweenkilometric milestones 111 and 117 now gives easyaccess to the Puerto Minguez outcrop whose aerial extent is at least5 x 0.3 km .squared.The up to several decametersthick depositsconsist basically of polygenetic,weakly consolidatedrudites. They show a rnoitly matrix-supportediex- ture and are poorly sorted.They include conglomeraticpatches, occasionafly layered, as well as brecciatedareas with sharpand sometimeserosive contacts.On the whole, the deposit can be termed a diamictite (Pl. 5, Fig. 5). paleozoic materials are generally more cofirmon in the eastem part. Big clasts of prior depositional sequenies,stiil intact,are found in the deposit,which as a whole presentspoor sorting and is massive or weakly layered.Isolated Mesozoic limestoneboulders in the paleozoic-dominated .,islands,, clast zonesare abundant(Pl. 5, Fig. 6), and frequently, like floating within the matrix, there are localisedpatches of conglomeraies.Towards öosa, and near the 36

Portalrubiojunction, Palaeozoic materials become more and morescarce, until they äi*oo"- aitogether.Here, the conglomeraticpatches consist of large (up to meter- ln sizeäjsub-ang"ular to sub-roundedMesozoic iimestone blocks and an increment "iurt-roppo.tä texture.The patchesare found dispersed in a sandyto clayeymatrix' io i""t,i.ä. areessentially märix-supported Side by sidewith lheseconglomerates' 6i...iu ,on., -. .*por.ä (pl. 6. Fig. l) consisringmosrly of polygeneticPalaeozoic boulders.They showvarious dimensions, they arematrix-supported and layenngrs "o-of""fn laäking.Some of thesebreccia zones are composedof fragmentedand i;i;iä;i""-k;, ;fii.h huu.largely conserved their original lavering Frequently' the ir"""iu ,on". areintercalated i-n tfie conglomeratic,poorly layereddeposit' In other partsof the outcrop,the contactsbetweeti breccias ant conglomeratesare sometimes iham- -ön" and sometimes erosive. of the most intriguing observations.which can be made at the Puerto Mineuezdeposits. is the generallystrong deformalion ol thecomponenLs' even it^Äu?nir,.u'ur" uncemenr;dand ömbeddedwithin a soft matrix. The most abun- ä;;;ä;;6f deformationis the striation of the surfaces(Pl 6' Fig' 2) without ".""riiä", "ta irrespecdveof their size,all pebbles,cobbles and bouldersof car- ü"r"i. iiir,"r.gv arä striatedmore or less all around.The striaehave obviously originatedfroÄ- countermovements of compolenlsand matrix in an environment whärethe confining pressurewas considerable.Frequently, the grainsthat made the striationscan bie'foundin small pits at the end of the striaepath. with the ä;;;;" of the particle sizeof the matrix, the striatedsurfaces become more and proba- moreelossv, and finally they may show distinct mirror polish {Pl' 6' Fig 3)' Uiuari" ,o'tft. contaciwithclayey and silty matrix The Palaeozoiccomponenls shäwstriations as well, althoughless developed than thoseof the limestonecom- pon"nit.ptoUuUfy because of tfiedifference in lithologyVery often' imprints have grooves iormed li'kedeepgrooves and indentation hollows (Pl' 6, Fig' 4) The deep seemto be "uoj"J by scrapingout by matrix pebbles,and the indentationhollows r".- to n" causedby the penletrationof an adjacentcomponent under conditions oi-oui" pfustlc defoimation.Sometimes, the penetrationmarks resembleshapes rnuä" tV " knife over soft butter.When severalof thesepenetration marks come i"i" ""i ""i, a facetedsculpture of the clast develops'A-very unusualtype of is a ä-"}o.-ution'tf,utis very frequentlyobserved in the PüertoMinguez material, ,tr-f int"tnuf torsion of thä components(PI 6,.Fig' 5.1which has alreadybeen descr"ibedfor the pelardaFm. ejecä lEmstsonand Claudin, 1990). The prominent torsionrelates to macroscopicailyuntouched hinges and rotatedfractures' cuttlng ifriorgrr *r" whole cobble,withoit howeverbreaking it to pieces'More irregular ä""i"?t .itft complexbiiurcations and prominentdisplacements are alsotypical oJdeformed cobbles. The cobblesremain together despite squashing, a fact thatrte i"t"ipi"i "t "t expressionof a shongconfitting pressure'which permitssimulta- n"oui U.ittf" andplastic behaviour ofthe cobblästo the appliedstress Frequently' to slices(Pl 6' Fig' i;;;;;;;p;";nts at first sight-brokenlook like a loaf of .br-eadcut Oi.ftorv"u"., ihe clastsare not to piecesat all' In many cases,widely open ü""t,*"" ur" fitt"d by splintersof the fräcturedcomponent and matrix material' ur; ,o äp."."", a t

From the analysis of the lithologic characteristics, surface features and intense deformation of the components and the matrix of the Puerto Minguez deposits, we conclude that the origin of thesefeatures is connectedwith simultaneousplastic and brittle deformation, acting rapidly, and under high contact pressure between the matrix and larger components.The rotated ftactures are regarded as typical of dynamic, rapid deformation under high confining pressure. They are not compatible with an origin from fluvial deposition, quasi-statictectonics, debris flows, alluvial fans, the action of glaciers,or, generally,with syntectonic sedimentation(Casas et al.,2OOO). In summary, the Puerto Minguez deposits are diamictites which have been depositedand deformed within a shofi time span and under high confining pressure. Such a setting is well known from meteoriteimpact ejecta.Identical depositionalfea- tures and defomations (striae, polish, rotated fractures,bread-cut-to-slices features, all kinds of imprints, and others) have been described in detail for the Ries crater ejecta (Chao, 1976, 1977'1and for the Belize eiecta of the Chicxulub impact structure (Marshalfer al.. 1998:Ocampoeral..1997:Rampinoeral..1996.1997a. |997b).For the origin of the Belize brittle and plastic deformations,the authors suggesthigh- velocity flow, violent collisions and shock effects as well as partially melting during excavation,transport and ballistic emplacement.

Related deposits

Deposits showing similar features as studied at the Puerto Minguez have been observedat many other places (about 20; between the villages of Almonacid de la Cuba/Belchitein the north, Ventasde Muniesa in the east,Alfambra and Mesquita de Jarque in the south and southeastand Blancas and Daroca in the west), within the Tertiary intra-mountain basins of the region under discussion.Compared with the Puerto Minguez, they are in general of smaller size and do not display all the features describedabove. Results of a more detailedinvestieation with referenceto their oosi- tion in the Teniarystratigraphy will be presentedii a fonhcomingpaper.

DISTANT IMPACT SIGNATURE

Originally, the observation of strongly deformed quartzite cobbles in Buntsandstein conglomerates drew attention to the Rubielos de la C6rida structure (Ernstsonet al., 1994). Well known to geologists,all pebbles of the Bunrsandstein basai conglomerates surrounding the Azuara and Rubielos de la C6rida structures up to a distance of roughly 100 km, show distinct pock-marks and a conspicuous cratering on their surfaces. These featues are obviously related to intense sub-parallel fracturing of the quadzite pebbles,both on a macroscopic and a microscopic scale (Emstson et al., 2001c). Comrnonly, the pock-marks and the miniature crateis are explained by pressuredissolution from overburden and./ortectonic stress (IGME 1986, Cortds et al.,2OO2b,Stel et a1.,2002), but Ernstsonet al. (2O0Ib, c; 2002) have shown that there is no evidence for this explanation. Sections and thin sections throush the 38 crateredcobbles reveal pervasiveinternal fracturing, concaveand subparallelspall fractures, and zones mark"d by quartz grains with planar deformation features (PDFs),6ut they fail to display'theprecipitation products of the allegeddissolved materiäl.Compärison with risults of impactexperiments on afiificial conglomerates ,unn".t. thutth" tnong deformationis relatedto shock-wavepropagation though the co"nälomerates.Accorlingly. the featuresare explainedto have occurredin the Azrära/Rubielosde la C6rldaimpact event by intemalaccelerations and multiple col- lisionsof the cobblesproducing the pock-markindentations and spallation.fractures discussionshow 1i.n.tron et al.,200ti). ThesJobservations, experiments -and the that a distinctmacroscopic impact signature may be found in autochthonousrocks' evenal largedistances from the impact sites.

THE AGE OF THE IMPACT EVENT

No radiometricabsolute age is so far availablefor the Azuara and Rubielosde la c6rida impacts.The advancedcorrosion of the glassfrom the impactmelt rocks is expected' io prevetttany reliable dating A stratigiaphicage may be addressedconsidering the youngestsediments affected bv the impait,'and thä oldestundisturbed post-impact layers. A roughestimate is given bv the stratieraphicposition of the PelardaFm. ejecta at the boundarf betweenthe t-owerTedär änd tie UpperTertiary (Caris and Monninger, 1974; also see Fig' 3)' According to ihis old andiimple stratigraphicsubdivision,-the Lower Tertiary expe'- encedthe"complete Alpidic täctonic movements,and the Upper Tertiary is the-post- tectonictime, rrihenthebasins and valley systemsformed with theh sedimentaryfilling. Evidently,a comparablesubdivision may applyto an impactevent in thisregion - Althäugh thä palaeontologicdating of Tertiaryunits in-tbe Iberian chain has madeprogäss, the stratigraphicdating still offersmany problems.Explicitly, Perez et al..ig{, statethat th;oulcrops in the zoneare limited andthat the rapid changes of the faciesprevent the use of lithological guide bedsf-or correlationpurposes' Accordingly,ihe exactstratigraphic age of the impact will remainunresolved for the Dresent. irrom the sediments(units 55 - 57, in ITGE [1991])exposed near Fonfria and Allueva and underlying-1unitthe PelardaFm. ejecta,a lower limit is Upper,Eoceneor earliest Oligocene 571. An upper limit is given by palaeontologicdata' Foraminiferäand ostracodsin post-impact,Upper Tertiary gastropodrnarls, about3 km northof Monevain the Azüarasüücture, point to a Lower Mioceneage (Doeb1, in Gros, lg74). A datingof the gastropodsthemselves (Geyer, in Gwosdek,1988) nrovidesan Upper Rupälianor Chattial (Oligocene)age with a high degreeof ärobabilitv.A positionät thebase of theAquitanian, however, cannot be excluded'A iurther upper-timitdating is given by gastropods(Potaminidae) in Upper Tertiary sandyliriästones near Ve-ntas-deMunieia in the Azuara structure.These gastropo-ds iiu"d b"t ""n the UpperEocene and the earliestMiocene (Geyer, in Mayer,1990), which doesnot corieipondwith the Middle Mioceneage for the re!p99tive,ylit "Areniscasen bancos,-conglomeradosno cementados y arcillas" in IGME (1981)' The Middle Mioceneage il not palaeontologicallyproven. Similar problemswith 39 "Arcillas Mioceneages are found also in the Rubielosde 1aCdrida structure. Unit 64 rojas,arenas y conglomerados"exposed south of Navarrete,is dated(ITGE, 1991) to be lateLower Miocene or Middle Miocene.Within this unit however,we observe strong structural deformationswith a pronouncedhorizontal component(large fault planeswith prominentslickensides, excluding atectonic collapse structures by karsti- fication). This implies eitherremarkable tectonics in the post-tectonicUpper Tertiary, a wrong stratigraphicclassification, or an origin from the impactcratering process, which,on the otherhand, is questionedby Corteset al. (2O02),Aurell et al. (1993), Aurell (1994),and others. Disregardingthese incompatibilities, we concludefrom the lower andupper time limits givenabove, that the impact event very probablyoccurred in theUpper Eocene or Oligocene.

CONCLUSIONS

The proximity of the Rubielos de la Cdrida structureto the Azuara structure,the conspicuouslocation of the largeejecta complex (Pelarda Fm.), exactlyin between them,the stratigraphyof the involved sedimentaryunits, the uniform stratigraphicage of thepost-impact sediments and, especially, the widespreadpolymictic impact brec- cia at their base, suggesta Mid-Tertiary paired impact in the Upper Eocene or Oligocene,corresponding to the hitherto establishedage of the Azuara structure.On acceptanceof this impactscenario, some basic consequences are inevitable. This is becausean impactof this dimensioncannot be regardedas a geologicallyisolated eventat an isolatedplace (see also Emstson [1994]). An impactofthis dimensionhas a strongsignature, which showsup with the developmentof small-and large-scale structuralfeatures not necessarilydifferent from orogenictectonics. Such a large impactalso means a suddendecisive and significantbreak in the erosionand sedi- mentationhistory of the regionunder discussion. It impliesthe productionof thick andextensive ejecta deposits (as we haveseen in the abovediscussion), the blocking and changingof the drainagesystem and subsequent,longJasting erosion and reworkingprocesses in an areaof the orderof severalthousands square kilometres. Thus,the existenceof a very largedoublet impact shucture in the regionof the intra-mountainbasins of the IberianChain is a remarkablesetting in the Lower to Mid-Tertiaryregional geology. As the projectilesimpacted a target,which was alreadyfolded in the Alpidic orogenyand which included Tertiary basins filled with "impact" thick molassesediments, a distinctionbetween "normal" and geologymay be problematic.This involvesboth the distinctionbetween Alpidic tectonicsand imDactstructural features on the one handand the distinctionbetween normal basin sedimentsand impact-affectedsediments, on the otherhand. While mountainbuil- ding and impacts may lead to very similar structural features,there are observable structuresand characteristicswhich enabledeposits to be clearly establishedas impact-relatedand not to be confusedwith fluvial, alluvial-fanor debris-flowsedi- mentation.These observations can exemplarily be madewith thePelarda Fm. andthe Puefio Minguez depositsand include the peculiarlayering and significanthigh- pressure/short-termdeformations typical of impactejecta. The impacts,their structuralfeatures and the impact-relateddeposits. within and outsidethe äraters, have so far not beenconsidered in the many modelsof intra-moun- tain syn-tectonicsedimentation, developed in the pasl for the.region underdiscussion (öÄÄ etaL,2OOOiPardo eta1.,1984;P6rez' 1989;P6rez etal.' 1985,1990' 1991) We tlerefore conclude that these models give a rather limited picture of the Tertiary iegional geologyin that area,and we suggestthat thesespecific models need revision'

ACKNOWLEDGEMENTS

The authors K.E. and F.C. want to give very heartfelt thanks for the help of a p".ron undttit t"um whichis ratherfar from the world of geology'We havehad long äna iometimeshard field campaigns,where a good meal'a comfortablebed, and a üi"nOtv utrnotptt"re.well beyönd-normalcommercial standards. are paramount So tnir *ä.t owe'smuch to th; Hostal Legido (at Daroca)staff' Gracias,Pepe and Ramön,Miguel Ängel, Alejandro,-preparationMaria, Josefa,and the rest of this nice group' ir,_tr'to rl spatne"toi the of the rhin sectionsand to K.-p. Kelber for r"u"*t pttotoeiuphsshown itt this pape.'but also to T. Ernstsonwho madesome i-o"""Ät disiov'eriesin the field.-ti. Mtiller-Sigmund,Institute of Mineralogy, Fö;i;;t und Geochemistrl, University of Freiburg, carried out the qualitative -i".oo.",iU" "f.."nt scanson the carbonparticles. E. Guerreroand A' Therriault sup- oil"a äüo"aunta"ta from PDFanalyses oi Azuarasamples. Reading of themanuscript t"i C"*aiu,"rulted in consideräbleimprovements. Thank you very muchfor this.

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F r-) ü; -;v-) s^q J ,'oooloo-ä{äs"s"hqsm-.riFr'| 6 9 ö ci q o + g I -.E=_ o. 3ä533*HpsSü:333ä 8i: E; . Ei ses o\ c.] .: \ I q :Z biF ir t.) c.l F3:l:; Hg!ä:IElsBö a lii Y EE<+; r >. .= ts >o-< gSgBgEEäE?gSEBsE trx F s 6 fJ

wt.Vo 1 zJ+5 6 mean I bulk PzOs 22,13 2r,26 24,47 27,52 32.61 i2.42 26.74| 8.2s Al2O3 0,00 0,00 0,00 0,00 0,00 0,00 0,00| 0,00 CaO 35,83 35,91 3'7,46 42,93 48,76 sr,62 42,10| s2,6s BaO 0,00 0,00 0,00 0,00 0,00 0,00 0,00I r,47 Na2O 0,35 032 0,46 0,57 0.s3 0,5s 0,46| 0,23 SO: |,67 1,15 1,77 2,1.2 1.47 r,37 1,s9| 0,92 F 1,57 1,56 t,02 2,26 2,24 2,39 1,84| n.d. ct 0,62 0,71 0,50 0,79 0,49 0,55 0,61 | n.d. LOI 34,3r Total 62,18 60,98 65,68 76,19 86,10 88.91 73.34| 97.83

Table 2. Electronmicroprobe analyses of the glassyphosphate matrix (dark arcäsin Pl. 4, Fig. 7), mean compositionof this matrix, and x-ray fluorescencemeasured bulk compositionof the carbonate-phos- phatemelt

Tabla2. Anaksis mediantemicrosonda electrdnica de la mariz vihea fosfatada(6rcas de color oscuroen la lämina 4, Fig. 7.), mosftandola principal composicidnde esta mahiz, y fluorescenciade rayos X dondese indica la composici6nglobal del fundido de cabonato-fosfato. Plate I Brecciasfrom the Azuara impact structure' Azuam' Lämina I Brechasprocedentes de la eshucturade impacto de

coordinates6 70 500' 45 74 700 The Fig.l. Basal (suevite)breccia. El Portillo variety Location: UTM field is 17 cm wide. coordenadasUTM 6 70 500' 45 74700' iin. f."ei""hu t".uf Auevita). variedad el portillo Locatizaci6n: La-anchurade camPoes de 17 cm' limestones Near Rudilla; UTM coordi- Fig.2. H-type breccia dike cutting through strongly brecciated nates 666660, 4540625 brechadas cerca de Rudilla; coor- än.ä.-ni*" ä" tt""rtas del tipo Hque ataviesa calizas fuertemente ae"nadasÜtu 6666e0. 4540625 Palaeozoicrocks near Santa Cruz de Fis.3. Schocked polymictic breccia from a dike cutting through cemenlationafter lhe brecciation ffi"".;. N;i; ih"'fitting of the component'prouin! immediate w'de Loätion: UTM coordinarer661240. 4553400 The lleld is 2J cm sito cercade SantaCruz de Nogueras' que ei*i. g;"lhu pori-i.tica chocadaque proviene de.un-dique, de la misma que prueba ulra ^ii-i"* ."""t'ä"i paleozoico. observese el errcajede los componentes coordenarlasUTM 661240'4553400 i"."4i"i"""-"**iOn despu6sde la brechificacid; Localizaci6n: La anchura del campo es de 23 cm (see The field is l6.cm wrde' Fig.4. Impact breccia from Almonacid de la Cuba tex) (ver el texto) La anchurade campo es Fie. 4. Brecha de impacto procedenrcde Almonacid de la Cubä ae tO cm. near Belchite' UTM coordimtes 689450' Fis.s. Megaclasts from the megabreccia;old railway cut +s?s-lso.äot. tl'turrhe lefr bloc{ is completel}convened inlo a Sritbreccia d" tt"n ""ttuno u Belchite; coorde- pin.-i. rJ"eu"ru.to, sitos en la megabräa; cärte de la antigua'iiu a la izquierda esta completa- ;::d; Ui"N7ä;;;"0, üäsäS6. pr"a" apreiiarse como el bloque situado mente transfomado en una brecha arenosa'

LnM coordinates667050' 4547030' Fig.6. Monomictic movement breccia; near Monfofte de Moyuela' The field is 20 cm wide. Monforte de Moyuela; coordenadasUTM Fie. 6. Brecha de movimtento monomictica situadacerca de 6ö'7050.4547030.La archura de campo es de 20 cm' 55 to

Plate 2 Shock metamorphismin rocks ftom the Azuara (Fig l-6) and Rubielos de la C6rida (Fig 7, 8) impact stuctures' Lämina 2. Metamofismo de choque presenteen las rocas de las estructuras de impacto de Azuara (Fig. l-6) y Rubielos de la C6dda (Fig 7' 8)' *ä:ffi:'"1xäJ{iffii:l:ii":"#fä?il""ü*#'::'.1'"'J3'Ji:t';9i"Jiü!i:'Tllii'3fäi.T'ffi?'i"'.:: l'*i:ilt"'*:**äl;"",t'älf*d,xx"f"?,'"?"#iiinä""'ä:T;ääif';!TJ,Tä""?'ä'J$iäi::":3; anchurade campo es de 9 mm. Fig.2.Diaprectic todiaprectic srass;.photomicrogratl".iä:1i.J?l;_*1t"T,t'*?Tfj:flt"i'*"-'"d i*t'ut"'m*:lir-"l.litt*:i*tmt.'n section;otto be co.r","o 'r,nä*'J,Jff;t'flli""g#;l:

;'**vik'r:"$'s"*T::1;i;ä#f;:siif*:T:ffi?:Ä1"'Täi:':3?säJiSi",nrrlceerjrhartainmerso en el seno ä;.;riä;";ü;i;;t" i-niio.fiuao "n "oalzo diapld;tico..Etfragmenro se

*äj*l'l*-"*r:*:"ii:f;'i:":ä1ilää'"1:J;',Tf;:'i:':'"ii,r'ä:ilil3,i"""31--JöTJ';i:;'äpm. ä,"i"äI"t" lF"""aäleicano a Sa'ta Cru, de Noguerai.La anchurade campoes de 600 *äi;,li*Y;i9$ss1"r3!fläditi"t*''!"'",f;?"',Hi#ä""T'ffi1'I'.*".::',1;J;:if'siUffi'*'yJ' I'f';:]äst+:"tg: i*tä::r"t:*1;:""'.itr'fl'os'äsli..J!.,tJ1fl,ffi'3; iöÜ:i,ffif#';:'""'1; realizadaa nicoles cruzados;la anchurade campo es de 195 Um' iiä;lyJlllil""?i'j,iJfilffi:,::'"^1i31"",i":i:'i:"i'T35:l:äFH3il'S:'JJ.:*I:$'1?y#J'l;'*;i: crossednicols; the field is 140 Um wide

-nicoles *j;y:ä:ii:i:[*:"x"gr'"lgg""l;::ru3J.1]:lT;'J":Ji::.ä".:1'ffij""$"1".:'T::xjJi:j;de campo es de 140 pm ä'i'öÄ-ä;-.-;."d;;. Microfotagrafia realizadaa cruzados;la anchura i€J;tü1135"::j'ij"'li"31j'ä:ii::"'"!'Fil:i:älä,i],ü iäi'f;Ji::*::l"J'xY"H"'ä;llff:'äff$ nicols; the field is 80 Pm wide iy;ty"'*ni:li*:if'i::im,',,1:ll;i;;il''""':ä:'fifi:::I:::l'"'äffi:h3iä1'##:"""J:ä3 iF. Mi"ioiotog.uäoobtenida a niioles cruzados;la anchurade campoes de 80 Fm' fM coordinale' Fip.b.MicrolwinninP and Unkband in catcile:polymictic dile breccic\. Munie'r' U ää"jjso'+r+iojor-'")errqe',phoromicr:'"iffi:,Lll';"ilf,"J:;ji"rlf;"$,:l,ll"äJul1$"ä"nuni",u, k;f;#j:',"ü+',fi"räiäl'Ä.iäIrir** lee.).Micrororosrari a reori"aü inlcoles cruzarros; ra anchurade campo es de 150 Um. iitR'H:ä*:$ltlf,:il"'Jä"#1i.::i$i",i3ß'i;Jt""ä:"Tl:l'1,'"1''i.:l3.1":ä'?'""ä?:l3i;??i:ii is I mm wide. ::';:l*:lm:'*";'f:ä*3$i",$%'""1':'rT"T?il:'::""i""äJ""3'?'Jiii.TäJ'i$::i33'::ff8:lix: lÄ"räi"t"*"i'" i""tiiada a nicolescruzados;ia anchura de campoes de I mm' x:1""'"i#:j1$3'iit'*:f?lj*"ä'Jg:li:lf;'#":i"Tj;*j}:ltti':ls,i:'lJ's#::"xl*tfffl:ä11T;; unintenso macrado mecänico.v fl?jt"i:l.il"o*'i::'Xläl:g-töääi11,ra äSlffi#:li?:"$: i:'u"::',n.*,**ll,l";:a*"*1"'""*lS*t?'iä::"$::::'gtb#ifälii,i1o"" nico,escruzados; ra anchurade campo es de 900 Pm. 57 58

Plate 3. Rubielos de la Cerida impact structure:Breccias and breccia dikes Ldmina 3. Estructurade impacto de Rubielos de la Cdrida: brechasy diques de brechas'

criss- Fig.l, Fig.2. MegabrecciationofJurassic limestonesin the southemcentral uplift Note the chaotic "ghost" (l) UTM coordi- "t3rr'f"v?.itg C]l and some layering having sur"rivedthe intensebrecciation nabs 643310, 4510560 (l); 646100,4508000 (Bueäa village) (2) Fis.l.. Fis.2. Megdbrechificaciön en calizas del Juräsico ubicadas en la parle sur del levantamlento que ha ""ir.oi. pi"a" upä.ial"se la esrrarificaci6nca6rica entrelazada (2) y alguna estratificaci6nrelicta (1); poaiao .otr"uiil. a la intensa brechiacidn (1). CoordenadasUTM 643310, 4510560 646100' 4508000 (pueblo de Buena) (2) UTM Fig.3. Polymictic breccia dike cutting through competent Muschelkalk limestones Near Olalla' coordinates657750, 4537800 de Fig. 3. Dique de brechaspolimicticas que atraviesacalizas competentesdel Muschelkalk. Cerca Ol-alla;cooidenadas UTM 657750,4537800. 4510900 Fig.4. Dike cutting thrcugh Jurassiclimestones near Ojos Negros; UTM coordinates626100' Thi white material is assumedto be crystallized carbonatemelt Negros; coordenadasUTM 626100' fig.+. biqo"-El que atraviesacalizas dei Juräsico cercanasa Ojos 4510900. material blanquecinocoresponde a fundido carbonatadocristalizado- megaclasts Fis.s. Mesabreccia near Barrachina (uTM coordinates655 100,45 29 900) showing thlee into the "iäff"."ti, lithology in contact A microbreccia (whitish colbur) seemsto have been injected middle, diamictic clast. pueden aprc- ri!.!. ru"guU.""rtu ""rcana a Banachina (coordenadasUTM 655100,-4529900)donde se parece ciä 3 meluclastos en contacto de diferente litologia. Una microbrecha (de color blanquecrno) haber sido inyectadaen la parte media del clasto diamictico

Fig.6.MulticolouredbrecciaaspartofthemegabrcccianealBarrachina:.anintensemixtureofdiamictlc' quarry mästly Palaeozoic material and ?Lower Tertiary red and green marls (darker clasts) Temporary (now filled up) at the bank of the Pancrudobrook. intensade iig.O. nr""hu ;ulti"olor que forma pafie de la megabrechacercana a Barachina: una mezcla (i?) ma-terialesdiamicticos, errsu -ayoria del Paleozoiio y de margasrojo verdosasdel Terciario inferior (clastosmas oscuros).Cantera temporal (ahorarellenada) situada en la orilla del arroyo de Pancrudo' a marly Fig.7. MegabreccianearBarrachina: grit-brecciated limestone megaclast paftly interspersedwith preservedghost- io"runay äut it. A few larger clastJhave survived the heavy brecciation, and some layering is observed.UTM coordinates6 52 310' 45 30 950. (hasta tamaäo Fig.7. i'4egabrechacercana a Barrachina: megaclastocalizo intensamentebrechificado de gran tamaflo arJnol p.äiu!m"nt" enhemezclado con una matriz arenoso-margosa Unos pocos clastos i* loLt"uiuiao a la intensa brechificaci6n. De igual modo, puede apreciarsealguna estratificaciön relicta. CoordenadasUTM 6 52 310,45 30 950. 59 60

Plate 4. Melt rocks in the Rubielos de la C6rida structure Lämina 4. Rocas fundidas en la estructurade impacto de Rubielos de la C6rida'

Fig.1, Fig.2: Megabreccianear Barrachina:Typically intercalatedsilicate melt rocks (the whitish ribbons and clasts). Fig.l, Fig. 2: Megabrechacercana a Barrachina: Rocas de fundido silicatado tipicamente intercaladas (los clastosy cintas blanquecinos) o/o Fig.3. The silicate melt rock under the microscope.The rock is composedof estimatedmore than 90 gliss forming tiny spheroidsand lens shapedbodies. The field is about 8 mm wide' Fig.3. ,lrp""to Ai d.oca de fundido silicatadobajo el microscopio. La rcca estacompuesta por mäs de un'907o de vidrio bajo la forma de delgadasesfdrulas y cuelpos con forma de lentej6n. La anchurade campo es de 8 mm.

Fig.4. Sawed surface of a suevite sample from a large block quaried out from the Barachina mega- breccia. Fig.4. Superficie cortada de una muestrade suevitaprocedente de un gran bloque exhaido de la mega- brecha de Barrachina.

Fig.5. Suevite-like dike breccia intercalatedin strongly brccciatedlimestones Fit.5. Dique de brecha del tipo suevitico intercaladoen calizas intensamentebrcchadas

Fig.6. Clast of cafbonare-phosphatemelt rock (white) in the Bafrächina megabreccia. coin diameter 23 nm Fi;.6. Clasto de roca de funaido carbonatado-fosfatado(color blanquecino)presente en la megabrccha de Barrachina.El diämetuode la monedaes de 23 mm.

Fig.7. Photomicrograph(crossed polarizers) of amoebae-likecalcite bodies within a matrix of phosphate gl;ss (dark) from ihe;last in Fig.6 Note that the size ofthe individual calcite crystals increasestowards ihe centersof the bodies. Also note that the peripheral calcite obviously has grown perpendicularto the rim becauseof the orientation. In part, especially along the borders to the calcite bodies, the phosphate glasshas recrystallizedto folm apatite(elongated, sometimes flaserlike minerals tangentially orientated to the calcite bodies).The field is 6 rnm high Fig.7. Microfotografia (a nicoles cruzados)de cuerposde calcita ameboidalessitos dentro de una matriz d;idrio fosfatad; (color oscuro), obtenida a partir del clasto de la Fig 6. Puede observarse que el tamafro de los cdstalesindividuales de calcita aumentahacia el centlo de los cuerpos.Tambien puede apreciarse que la calcita de Ia pedferia ha crecido obviamente de modo perpendicularal borde a causade la orien- äciön. En parte, y especialmente a lo largo de los bordes de los cuerpos de calcita, el vidrio fosfatado ha recristalizaäopaia formar apatito (en mineralesde formas elongadas,a vecescon fomas filamentosas, orientadostangencial^ent" ."rpecto u 1o, "uelpos de calcita) La anchurade campo es de 6 mm

Fig.S. MicroslagJike paticles are common in blocks of a finegrained microhreccia within the Ba-.achina megibreccia. They consist of amorphouscarbon with subordinateamounts of Ca and S, but also contain remarkableamounts of oxygen. Scalebar is I mm. Fig.8. Particulasde morfologia similar a la microescoriason comunesen bloquesde üna miclobrecha de grino fino sita dentro de la megabrechade Barrachina.Estas pafticulas consistenen catb6n amorfo con iantida

't. ,.-

., .l

.r f ü'-; -

i&. 7 .

trin R OL

Plate5. PelardaFm and Pueflo\4rnguel impacl ejecle Lämina 5. Eyectasde la Fm. Pelarday de Puerto Minguez'

Fiq.l. TyDical aspectof the rniddle unit of the PelardaFm. showing matrix-supportedtexture por la eiE.i. e,'.'p".,ouil"o oe lä unidad media de la Fm. Pelardaen el que exhibe una textura soportada matriz.

coordinates Fig.z. Strongly deformed quartzite clast from the Pelarda Fm ejecta; near Olalla, UTM ts"s;oO. +s:liso. Despite ihe remarkabledisplacements' the clast remains coherent and is not broken Lntopreces. cerca de Fig.d. Clasto de cuarcita intensamentedeformado prccedentede los yecta de la Fn Pelarda: perma- Olälla. coordenadasUTM 655700,4537250. A pesarde los evidentesdesplazamientos, el clasto nece unido y no se halla roto en fragmentos.

Fig.3. Multiple setsof striaeon a quartzite cobble The field is 2 5 cm wide' cm Rii.:. l,ltttiptes coniuntosde estri;s sobre un canto de cuarcita-La anchuradel campo es de 2'5

quarlz: sh9:\e9 quartzite clast from the Fig.4. SEM image of two sets of crossing PDFS in llTboY p.iarau frn. ejeia. Note the spacingof th; individual PDFS,which is distinctly less than I pm in many cases, cuar Fig.4. lmagen SEM de dos conjuntosde PDFSque se intersectanen un cuarzo: clasto chocadode "äBu-uä* p.o""dente del eyectade la Fm- Pelarda.Observese el espaciadode las PDFs individuales' que es tipicamente inferior a 1 pm en algunoscasos

Fig.s. Typical aspectof the Puerto Minguez ejecta.The outcrop heighLis about 12 m' piä.S. aspecto tipico ael eyectade Puerto Minguez La altun del afloramiento es de 12 m'

dominating Fig.6. Puerto Minguez impact ejecta: Mesozoic limestone cobbles and boulders in Paiaeozoicmatrix material The outcrop height is about 6 m' una Fis. 6. Evecta de impacto de Pueno Manguez:cantos y bloques de calizas del Mesozoico dento de mitriz dominante clemateriales del Paleozoico.La altura del afloramiento es de 6 m' 63 64

Plate 6. Puerto Minguez impact ejectaand shock-deformedBuntsandstein cobbles' Ldmina 6. Eyecta de impacto de Puerto Minguez y cantosdeformados por imPacto del Buntsandstein.

Fig.1. Breccia zone in the Puerto Minguez diamictite Hammer length is 40 cm. Fig.l. Zona de brechaen la diamictita de Puerto Minguez. La longitud del martillo es de 40 cm'

Fig.2. Stdated surfaceof a limestoneboulder. Centimeterscale. Fig. 2. Superficie estdadade un bloque calizo. Escala centimdtrica.

Fig.3. Fine stdation of a limestoneboulder merging into minor polish. Centimeter scale' Fi;.3. Estriaci6n fina en un bloque calizo que se halla en una zona de pulido en espejo. Escala centimdtrica.

Fig.4. Penetation marks in strongly deformed limestone cobbles from the Puerto Minguez ejecta Centimeterscale. Fig.4. Marcas de penetracidn presentesen cantos calizos intensamente deformados procedentes del eyecta de PuenoMinguez. Escala cenlimdtrica.

Fig.s. Limestone cobbles from the Puerto Minguez ejecta showing intemal rotations with macroscopi- cally untouchedhinges (arrows) and rotated fractures.Note the two axes of rotation in the cobble down to the right. Centimeter scale. Fig.5. Cintos calizos procedentesdel eyecta de Puerto Minguez que exhiben rotacionesintemas con chämehs macrosc6picas no deformadas (flechas) y fracturas rotacionales. Aprdciense los dos ejes de rotaci6n en el canto hacia la parte inferior derecha. Escala centimetrica "Bread Fig.6. cut to slices" type of deformationin limestonecobble from the PuertoMinguezejecta' Note rharthe cobbleis not brokeninto pieces "rodajas Fig.6. Deformacidn del tipo de pan" presente en un canto calizo procedente del eyecta de Puerto Minguez. Puedeob.ervarse como el canlo no se encuenfraroto en fragmentos

Fig.7. Tlpically shock-deformedquartzite cobble from autochthonousBuntsandstein basal conglomera- 17 cm. The pock- t".; d"poiitr-have b"t*een Rueda de la Siena and Molina de Arag6n. Length of cobble marks originated from fracturing upon collision with neighbouring cobbles and lack any dissolu- tion features. Fig.?. Canto de cuarcita, tipicamente deformado por choque, prccedentede los conglomeradosautöc- toios del Buntsandsteinbaial; dep6sito localizado entre Rueda de la Siena y Molina de Aragdn La longitud del canto es de l7 cm. Los hoyos se han originado a partfude la fracturaci6npor el choque con los cantosvecinos, no presentandoningün rasgo de disolucidn.

Fig.8. Cratered quartzite cobble from autochthonous Buntsandstein basal conglomelatesi deposits between Rueda de la Siena and Molina de Arag6n. Maximum size of cobble 6 cm. The crater proves brittle ftacturing and has probably fomed by spallation ftom shock collision. Fig.8. Canto de cuarcita ion crätercs procedente de los conglomerados autdctonos del Bultsandstein ba;ali depdsito localizado enhe Rueda de la Sierra y Molina de Arag6n El tamaflo mäximo del canto es de 6 cm.-El cräter es una prueba de fracturacidn frägil y se ha formado probablemente por espalaci6n a partir de la colisi6n. 65