Breccia dikes (dike breccias) are a prominent feature in impact structures, and they have told us a good many about the processes of impact cratering. Among the yet known roughly 200 terrestrial impact structures, Azuara and Rubielos de la Cérida are providing the probably by far best insight into these fascinating geological configurations concerning abundance, exposure accessibility, and diversity with regard to geometry, dimensions and material (also see breccia/dikes; http://www.impact-structures.com/spain; rubielos-breccia-dikes). Recent extensive construction works (storage reservoir, freeway, railroad) near Lechago, Calamocha (CAL; see Fig. 1, red circle) have yielded quite a few nice geological exposures once more documenting the almost everywhere existing impact signature thus underlining that construction work in the large impact region of the Azuara and Rubielos de la Cérida structures may be a hard undertaking – as was impressively shown when the new Autovía freeway line had to cut through the strongly destroyed Paleozoic of the Iberian Chain in the northern rim zone of the Azuara impact structure between Daroca and Cariñena (click here http://www.impact-structures.com/impact-spain/the-azuara-impact-structure/the-2005-autovia-mudejar-geological-exposures/ “The 2005 Autovía Mudéjar geological exposures”).

Below we show images of some newly exposed impact breccia dikes near Lechago exhibiting their characteristic properties and, for comparison, similar dikes and dike systems from the companion Azuara impact structure (location in Fig. 1).

Focusing here on breccia dikes in Paleozoic silicate rocks bears in mind the frequent claim of some Spanish geologists, especially from the Zaragoza university, that the impact breccia dikes are all karst phenomena. Their confusion of breccia dikes with karst features, however, meets some difficulties in the case of silicate siltstones. Also fault breccias can clearly be excluded since these dikes are filled with allochthonous material as is typical for impact-induced highly energetic injection processes.

Fig. 1. Location map.

Fig. 2. Thick impact breccia dike cutting through Paleozoic siltstones at the new storage reservoir near Lechago (in the red circle of Fig. 1)

Fig. 3. For comparison: impact breccia dike in Paleozoic siltstones; road to Santuario de la Virgen de Herrera (1 in Fig. 1). Azuara impact structure.

Fig. 4. For comparison: thick impact breccia dikes cutting roughly perpendicular through the bedding of the Paleozoic siltstones. Autovía Mudéjar in course of construction; near Cariñena (2 in Fig. 1). Azuara impact structure.

Fig. 5. Large system of impact breccia dikes in Paleozoic siltstones at the new storage reservoir near Lechago. Note that the dominant set of dikes is cutting sharply across the bedding (see Fig. 6). In the top: Unfolded post-impact Tertiary sediments.

Fig. 6. Segment of the wall in Fig. 5.

Fig. 7. For comparison: System of impact breccia dikes cutting sharply through Paleozoic silicate rocks. Autovía Mudéjar in course of construction; near Cariñena (2 in Fig. 2). Azuara impact structure.

Fig. 1. Amazingly similar: Regmaglypts on the surface of the Tabor meteorite and on a limestone clast from the Puerto Mínguez impact ejecta.

Among the various deformation features exhibited by the carbonate clasts in the Puerto Mínguez ejecta deposit (striae, nail prints, grooves, rotated fractures, irregular fractures with complex bifurcations, mirror polish, etc), regmaglypted clast surfaces are a prominent feature (Fig. 1). For the first time described by K. Ernstson (2004), they establish clear evidences of an aerial transport of quite a few clasts of the ejecta.

Source: Cascadia Meteorite Laboratory, Portland State University

Fig. 2. A Gibeon meteorite exhibiting distinct regmaglypts.

Regmaglypts (or thumbprints) are reliefs commonly reported for the surface of some meteorites (Figs. 1, 2). The depressions originate from dynamic air pressure [continue …] and from selective erosion by material melting (ablation) off the surface of the meteorite on its passage through the atmosphere. The relief may show polygonal, spherical, rounded or elliptical shape, and a pattern like fingers over wet clay is frequent.

We suggest the ablation features observed in the ejecta have originated from a similar process that is carbonate melting and ablation off the surface on the passage of the ejecta clasts through the heated impact explosion cloud.

Of course, the depressions on the Puerto Mínguez clasts in a way remind of lapiés (karren) features on surfaces of exposed limestones. Lapiés consist of shallow, straight grooves or runnels incised into the limestone by solution. Because of the striae and polish regularly coating the regmaglypted surfaces, and because the regmaglyptic clasts are embedded as individuals in the ejecta matrix (Fig. 3), an in situ formation as lapiés can clearly be excluded.

Fig. 3. The regmaglypted individual embedded in the Puerto Mínguez impact ejecta is incompatible with an in situ dissolution (“lapiés” formation).

Alternatively, the “lapiés” depressions existed already before the impact and survived rock fracturing, excavation, and emplacement of the ejecta. This can be excluded because many clasts are regmaglypted all around (Fig. 4), and the frequently observed sharp-edged ridges of the regmaglypts (Fig. 4) would not have survived the excavation and ejection process.

Fig. 4. Front and rear of a regmaglypted limestone clast from the Puerto Mínguez ejecta. The regmaglypts all around and the sharp ridges exclude a “lapiés” formation before excavation and ejection.

In a few cases, the ablation by obvious carbonate melting of the clasts has eaten deeply into the clast (Fig. 5) reminding of similar distinct ablation features of some meteorites (Fig. 6).

Fig. 5. A regmaglypted limestone clast from the Puerto Mínguez ejecta exhibits prominent ablation reaching deeply inside into the limestone clast. For comparison see the meteoritic ablation features in Fig. 6.

Fig. 6. The Derrick Peak, Antarctica, meteorite. Image courtesy of NASA.

An extended article on the Puerto Mínguez regmaglypted ejecta including many images can be read here.

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.