ERNSTSON CLAUDIN ESTRUCTURAS DE IMPACTO – CRÁTERES METEORÍTICOS

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).

Fig.1. Unconformable contact between the Jaulín breccia and the Jurassic limestones.

Fig. 2. Penetration of the breccia into the Jurassic host rock in the form of a dike.

Fig.3. Corrosion features in the contact zone between breccia and host rock. Neither aeolian nor karstification processes are responsible of the corrosion, which probably is related with decarbonization.

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).

Fig. 4. At first glance, the breccia reminds of a massive bone breccia.

Fig. 5. Close-up of the carbonate breccia clasts.

Fig. 6. Close-up of hollow and skeletal clasts embedded in the matrix. Note that the decomposition with possible relics of carbonate melt is confined to the interior of the cobbles reminding of the construction of a wasp’s nest.

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. 8. Evidence for the erosive strength of the breccia emplacement process: fragments of the limestone host rock have been carried away and embedded in the greenish matrix. Note the beginning «conglomeratization» (increasing roundness) of the clasts.

Fig. 9. Whitish rims of limestone fragments in contact with the breccia matrix: evidence of beginning decarbonization due to enhanced temperatures.

Fig. 10. Fragmented but coherent limestone clast as indicative of confining pressure during the emplacement.

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.

Fig. 12. Several sets of decorated planar deformation features (PDFs) in a quartz grain from the breccia matrix. Photomicrograph, crossed polarizers; the field is 220 µm 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

and/or

• 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).

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:

http://www.uni-wuerzburg.de/mineralogie/schuessler/Monforte-vein.pdf

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 ).

 2 3

 4 5

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.

 6 7

Images 6, 7: The Devils Wall breccia dike near Olalla.

 8 9

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).

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. 1. Location map for the Azuara – Rubielos
de la Cérida impact crater chain (frame in
Fig. 2) and suspected impact locations (A, B, C).

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.

Fig. 4. Part of the central uplift chain emerging from the Rubielos de la Cérida impact basin.

Fig. 5. The crater rim in the southern
part of the impact chain.

Fig. 6. Megabreccia and polished friction plane
in the southern part of the central-uplift chain.

Fig. 7. Impact breccia (suevite) exposed in
the southern part of the central-uplift chain.

Fig. 8. Probable impact ejecta near
Peñacerrada (location B in Fig. 1).