|Road constructors are the friends of the impact geologists. Without their work, most of the highlighting impact outcrops in the Spanish Azuara and Rubielos de la Cérida impact structures would not exist, and many of the rocks typifying impact would not have been discovered. In the last decade, kilometers and kilometers of new geological exposures have been prepared, and we mention the road cuts between Luco de Jiloca and Lechago, at the Puerto Mínguez, between Navarrete and Barrachina, between Fuendetodos and Azuara, between Lécera and Muniesa, between Fuendetodos and Jaulín, and many more. Not only the road cuts but also the many new quarries mostly exploited for road construction material have supplied new geologic outcrops of high geologic importance, as for example the quarries between Belchite and Puebla de Albortón, the many temporary quarries between Navarrete and Barrachina, the large quarries of Corbalán, San Blas, Villafranca del Campo, near Muel, and so on.
The road cut at the crater rim and view down into the Rubielos de la Cérida impact basin.
Only recently, the new road cutting into the Rubielos de la Cérida impact basin rim in the ascent between Alfambra/Escorihuela and El Pobo/Cedrillas has prepared a breath-taking extraordinary continuous geologic exposure of currently about 2 km length. The exposure does not show only the unimaginably disastrous forces of the impact excavation and modification of the Permotriassic/Buntsandstein and Muschelkalk rocks leaving a gigantic megabreccia, but also reveals large-scale rock deformations hitherto obviously unknown to geologists. We want to call them stop-and-go deformations.
The impact stop-and-go deformation is characterized by a multiple rapid sequence of erosion, sedimentation, folding, faulting and flow in a limited rock unit. This extremely peculiar process is not explicable by “normal“ geological forces and is understood only by the complex impact excavation and modification movements with permanently and in a short time strongly varying stress fields probably supported by the action of water and shock-produced volatiles.
Simple model of stop-and go deformation.
More stop-and-go deformations: megabreccia near Barrachina.
About 30 km north of the center of the Azuara impact structure (Spain) near the village of Jaulín (0°59.3′ W; 41°27.2′ N), a peculiar breccia is exposed. The breccia, not mapped geologically thus far, is intercalated between fossil-rich Jurassic limestones and brownish Miocene(?) gypsum marls. The breccia is unconformably overlying the Mesozoic rocks (Fig. 1) and may penetrate the limestones (breccia dikes, Fig. 2) as well as corrode them (Fig. 3).
On cursory inspection, the greenish rock looks like a massive bone breccia (Fig. 4). On closer examination, the «bones» prove to be limestone clasts having become hollow or more or less completely skeletal (Figs.5, 6).
Beginning (Fig. 7) and complete fragmentation of the clasts is observed. Beside these decomposed clasts, fragments of the limestone host rock are intermixed in the breccia (Fig. 8). They frequently show distinct whitish rims (Fig. 9) which we interpret as the result of beginning decarbonization from enhanced temperatures. Occasionally, the fragmented clasts remain coherent giving evidence of confining pressure upon emplacement (Fig. 10). For the present, it is not clear whether the hollow and skeletal clasts originate also from the local limestones, but from the discussion below we have to assume they are allochthonous.
Fig. 7. Breccia sample exhibiting a decomposed, fragmented clast and distinct flow texture of the matrix.
Fig. 9. Whitish rims of limestone fragments in contact with the breccia matrix: evidence of beginning decarbonization due to enhanced temperatures.
The clasts are immersed in a greenish matrix (Fig. 8) partly exhibiting flow texture as indicated by lined-up small elongated clasts (Fig. 7). In thin section (Fig. 11), the matrix shows to be fine-grained carbonate streaked with irregular bands of poorly rounded quartz grains in a slightly different matrix. Quite a few quartz grains exhibit planar deformation features (PDFs) as in proof of shock metamorphism (Fig. 12).
Fig. 11. Breccia matrix composed of dark carbonate material streaked with irregular bands of poorly rounded quartz grains in a slightly different matrix. Photomicrograph, plane light; the field is 10 mm wide.
Breccia formation. – The contacts between breccia and underlying autochthonous rocks as well as the peculiar characteristics of the clasts absolutely exclude a karstification process. A «normal» sedimentation and any diagenetic processes are not consistent with the observations either. From the stratigraphic position at the base of the unfolded Upper Tertiary and from the evidence of enhanced temperatures and of shock metamorphism we conclude the breccia to be an impactite related with the formation of the Azuara structure in the giant multiple impact event that, beside the Azuara structure, formed also the large Rubielos de la Cérida impact basin and crater chain (see http://www.impact-structures.com/spain/). We explain the breccia to be impact ejecta excavated from a region in the Azuara structure where shock intensities were enough to decarbonize and melt limestone cobbles and boulders and to produce PDFs in quartz grains contributing to the breccia matrix.
Hollow and skeletal limestone boulders, cobbles and pebbles are not uncommon in the impact region, and they have been found in e.g. the basal suevite breccia (see suevite) and the impactite from Almonacid de la Cuba (see peculiarities). The process of the decomposition obviously confined to the interior of the clasts is not completely understood thus far, but we suggest two possibilities that must not exclude one another. The clasts belonged to Lower Tertiary conglomerates in the target, and they experienced
- a shock concentration in the cobbles’ interior by shock-wave reverberation and focusing effects
- a shock heating of the cobbles and a rapid external cooling upon ejection, only allowing the interior to be decarbonized and melted.
On excavation and ejection, the shocked limestone clasts were mixed with the greenish matrix material possibly originating also from the Lower Tertiary. The emplacement was a process of ballistic erosion and sedimentation (Oberbeck 1975) under high pressure (penetration into the Jurassic host rock that was partially fragmented and incorporated into the breccia) and still enhanced temperatures (partial decarbonization of the host rock).
With regard to its stratigraphical position, the Jaulín impactite must be seen as a special variety of the Azuara/Rubielos de la Cérida basal breccia, and, due to the observed shock effects and the relics of probable carbonate melt, as a special type of a suevite breccia (see IUGS classification, the_suevite_page).
This peculiar polymict breccia uniformly exposed at the base of the unfolded Upper Tertiary over a distance of at least 120 km is a strong clue to the Mid-Tertiary multiple impact in northern Spain. A lot has already been said and written about this breccia (Ernstson & Fiebag, 1992; Ernstson & Claudin 2002, Ernstson et al. 2003; Claudin & Ernstson 2003; http://www.impact-structures.com/impact-rocks-impactites/the-suevite-page/suevites-from-the-azuara-and-rubielos-de-la-cerida-spain-impact-structures/, http://www.impact-structures.com/impact-spain/the-rubielos-de-la-cerida-impact-basin/basal-suevite-breccia-in-the-rubielos-de-la-cerida-impact-basin/), and that is why we confine ourselves to show some new varieties exposed about 2 km northeast of Olalla in the rim zone between the Azuara impact structure and the Rubielos de la Cérida impact basin. We point to the dominating Paleozoic and Triassic components, to the flow texture, to halos around many clasts, to the remarkable fitting of fragmented clasts, to breccias-within-breccias, and we think that some of the breccias are simply beautiful.
Here, we want to mention that regional geologists from the Zaragoza university and the Madrid Center of Astrobiology (Ángel Luis Cortés, Marcos Aurell, Enrique Diaz-Martínez, and others), which basically deny an impact in that region, consider this typical breccia a lacustrine sediment or even a conglomerate (Aurell et al. 1993; Cortés, Nov. 6, 2002, Heraldo de Aragón).
Outcrop of Muschelkalk dolomite crosscut by a dark vein of impact melt rock in the vicinity of Monforte de Moyuela.
Black vein under the microscope: light matrix of carbonate minerals (Cc), black particles and gas vesicles (gv). Long side of the figure is about 1 mm.
The full article can be read here:
key words: Azuara impact structure, Rubielos de la Cérida impact structure, carbonate melt, breccia dike, manganese, quench crystallization.
Accretionary lapilli is a term originally solely related with volcanism. Accretionary lapilli are pellets that form by the accretion of fine ash around condensing water droplets or solid particles, particularly in steam-rich eruptive columns. Commonly, they exhibit a concentric internal structure, and, once formed, they can be transported and deposited by pyroclastic fall, surge, or flow processes (Allaby & Allaby, 1999; A Dictionary of Earth Sciences). Armoured lapilli is the term that is especially used in the case the ash has accumulated around a small rock fragment. The armoured-lapilli variety is frequently found in deposits of basaltic base surges.
Image 1: Accretionary lapillus (diameter 0.5 mm) from the basal
suevite breccia in the Azuara impact structure (Mayer 1990).
Photomicrograph, xx nicols.
Since similar processes are related with the turbulent explosion plume raising above the expanding excavation cavity in an impact cratering event, it is not surprising that accretionary lapilli have been found also in impact deposits. Graup (1981) describes accretionary lapilli to occur in the suevite fall-back breccia of the Ries impact structure. They are also reported for ejecta deposits of the K/T Chicxulub impact structure in Mexico (http://www.lpi.usra.edu/meetings/metsoc2000/pdf/5124.pdf,
http://www.lpi.usra.edu/meetings/largeimpacts2003/pdf/4113.pdf) and Belize (http://www.icdp-online.de/news/workshops/abstracts/EGS03/EAE03-J-06925.pdf).
Concentrations of lapilli formed lapillistone that occurs as discontinuous, reworked clasts within the megabreccia related with the Late Devonian Alamo impact (http://gsa.confex.com/gsa/2002AM/finalprogram/abstract_42158.htm).
For the Azuara impact structure, accretionary lapilli was first reported by Mayer (1990). Image 1 shows a typical lapillus from the matrix of the basal suevite breccia near Muniesa. The interior is composed of very badly sorted material (calcite, quartz, ore). The rim zone is formed by concentric layers of finer material (mostly micritic calcite). Similar accretionary lapilli are observed also in the matrix of Azuara breccia dikes.
Accretionary lapilli also contribute to the matrix material of the basal suevite breccia and breccia dikes in the Rubielos de la Cérida elongated impact basin (as part of the Azuara/Rubielos de la Cérida impact crater chain, see Highlights ).
Images 2 – 5: Accretionary lapilli from the basal suevite breccia in the Rubielos de la Cérida impact basin. Photomicrographs, xx nicols; the fields are 3.5, 5, 6.5 and 3 mm wide.
Images 2 – 5 show accretionary lapilli from the basal suevite breccia near Corbatón in the Rubielos de la Cérida impact basin. The lapilli are basically carbonate with some accessory silicate material (e.g., quartz fragments, Image 3) and regularly exhibit the typical «onion skin» internal structure. The angular core of the lapillus in Image 5 probably is a fragment of a former lapillus thus reflecting some kind of reworking in the lapilli formation and deposition process.
Near the village of Olalla in the northern rim zone of the Rubielos de la Cérida impact basin, a prominent breccia dike is exposed (Images 6, 7). In the early seventies, in the course of his geologic mapping activities, W. Monninger called this dike «Teufelsmauer» (Devils Wall) because of its peculiar structure and composition. At that time, impact processes and impact rocks were not well known among geologists.
Images 6, 7: The Devils Wall breccia dike near Olalla.
Images 8, 9: The matrix of the Devils Wall breccia dike composed of accretionary lapilli. The whitish crusts of some breccia fragments are suggested to result from high-temperature decarbonization of limestone. Polished sections, the fields are 13 and 15 mm wide.
In polished sections (Images 8, 9), the matrix of the carbonate dike breccia for the most part proves to be completely composed of accretionary lapilli (lapillistone). Limestone fragments from the disintegrated host rock of the dike abundantly make up the core of larger lapilli. In doing so, the fragments my be broken but still remain coherent within the lapillus, as can be seen in Image 10. This shows that the accretion process continued upon injection of the vapor plume material into the host rock.
Image 10: Fragments of the breccia dike host rock make up the core of larger accretionary lapilli.
Regional geologists from Spain (M. Aurell, E. Díaz-Martínez, A. L. Cortés, and others) vehemently refusing the impact origin of Azuara and Rubielos de la Cérida, suggest the basal suevite breccia and breccia dikes to be diagenetic, pedogenetic or karstification features. For these geologists, the accretionary lapilli is calcrete (caliche).
|Spallation is a well-known process in fracture mechanics as well as in impact cratering and has been investigated theoretically and experimentally by many researchers. Unfortunately, it is less well known that spallation can also be observed in nature as an actually existing geologic phenomenon in and around impact structures. The present WEEKLY IMAGE shows already known spallation features in conglomerates around the Azuara/Rubielos de la Cérida impact structures (Spain), now recognized to form a 120 km long impact crater chain (see
– KLICK here; and Ernstson, K., Schüssler, U., Claudin, F. & Ernstson, T. (2003): An Impact Crater Chain in Northern Spain. – Meteorite, 9/3, 35-39 – KLICK here), and prominent spallation fractures only recently observed in ejecta (Pelarda Fm. ejecta) from this crater chain.Spallation takes place when a compressive shock pulse impinges on a free surface or boundary of material with reduced impedance (= the product of density and sound velocity) where it is reflected as a rarefaction pulse. The reflected tensile stresses lead to detachment of a spall or series of spalls.Prominent spallation effects have been reported for shocked Buntsandstein conglomerates exposed around the Azuara/Rubielos de la Cérida impact structures. Details about these geologic spallation features have been described in Ernstson, K., Rampino, M.R., and Hiltl, M. (2001): Cratered cobbles in Triassic Buntsandstein conglomerates in northeastern Spain: An indicator of shock deformation in the vicinity of large impacts. Geology, 29, 11-14., and can be found here.
Images D, E. Concave spallation fracture surfaces in quartzite boulders from the Pelarda Fm. ejecta.
|The Images A and B show typical shock-produced spallation features in these Buntsandstein quartzite cobbles: subparallel open spallation fractures (Image A) and a concave fracture surface forming a crater after the detachment of a lens-shaped spall (Image B). This concave spall fracture near a spherically shaped reflection surface is predicted by theory (and hardly explained by any other geologic process) and can be produced experimentally as shown in Image C.|
|The shock experiments were performed at the Fraunhofer Institute for High-Speed Dynamics (Ernst-Mach-Institut) in Freiburg, Germany. A single-stage powder gun was used to accelerate steel projectiles. As targets, we used two quartz spheres (rock crystal) in contact, embedded in a synthetic epoxy matrix. The shots were performed with impact velocities in the range of 25 to 115 m/s, corresponding to initial impact pressures between 0.55 and 2.5 GPa (5.5 and 25 kbar). The recovered samples were cut in half (see Image C, shot 3), thin sections were made, and the results of our observations were presented in Ernstson, Rampino, and Hiltl (see above). Here, the recovered sample of shot 3 at lowest impact velocity is shown (Image C) displaying a clear spallation fracture in the right-hand sphere otherwise untouched.|
|In the Ernstson/Rampino/Hiltl paper published by Geology (see above), the importance of such shock-deformed autochthonous conglomerates for an easy recognition of regional impact signature has been pointed out.|
|Here, we report on recent observations of prominent spallation fractures in quartzite boulders from the Azuara/Rubielos de la Cérida impact ejecta (Pelarda Fm.). The Image D shows a typically deformed boulder that displays a concave spall fracture surface being a mirror image of the convex surface (sketched as white broken line in Image D) of the detached (and now missing) large spall. A similar concave spallation fracture can be seen in Image E.|
|The quartzite boulders (mostly Cambrian Bámbola quartzite and Ordovician Armorican quartzite) contributed to the upper part (dominating molasse sediments) of the purely sedimentary target and, upon impact, experienced moderate to strong shock before excavation and ejection. The shock is documented by abundant multiple sets of PDFs in quartzite boulders (see, e.g., the sound PDF analysis made by Dr. Ann Therriault, in: Ernstson, K., Claudin, F., Schüssler, U. & Hradil, K. (2002): The mid-Tertiary Azuara and Rubielos de la Cérida paired impact structures (Spain). – Treb. Mus. Geol. Barcelona, 11, 5-65 – KLICK here, and on shockeffects ). We assume that the prominent spall fractures in the large quartzite boulders have originated also from the initial shock event, although a formation by collision of quartzite boulders during excavation and ejection must also be taken into consideration.|
This is the title of an article only recently published in
METEORITE, The International Quarterly of Meteorites and Meteorite Science
For the readers of METEORITE (and others), the black-and-whites of the article are shown here as original color prints.
Fig. 2. The topography of the Azuara/Rubielos
de la Cérida crater chain (from the digital map
of Spain, 1 : 250,000; provided by Manuel Cabedo).
Fig. 3. Photomicrograph of strongly shocked
quartz from the Rubielos de la Cérida basin.
|Near the village of Monforte de Moyuela at the SSW rim of the Azuara structure, chert nodules in Muschelkalk limestones show very peculiar deformations. The nodules are heavily shattered, fragmented down to millimeter-sized sharp-edged splinters and partly even pulverized (Image A). From these shattered nodules, flow lines of miniature chert splinters run into the limestone as shown in Image B and, for reasons of clarification, in the sketch of Image C. The area interspersed with the flow lines is of the order of some 100 square meters. A statistical analysis of the strike of the flow lines shows a distinct maximum pointing to the center of the impact structure (rose diagram in Image D).|
A diagenetic origin of this peculiar texture can clearly be excluded, and we suggest a short-term shock-induced fragmentation and fluidization of the Muschelkalk limestones and the embedded chert nodules. A relation may be seen to the acoustic fluidization proposed as possible mechanism to enable impact crater collapse in the modification stage (H. J. Melosh (1989): Impact Cratering. A Geologic Process).