Rubielos de la Cérida impact basin: New impact breccia dikes in Paleozoic silicate rocks

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/dikeshttp://www.impact-structures.com/spainrubielos-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.

Image001Fig. 1. Location map.

Image003Fig. 2. Thick impact breccia dike cutting through Paleozoic siltstones at the new storage reservoir near Lechago (in the red circle of Fig. 1)

Image005Fig. 3. For comparison: impact breccia dike in Paleozoic siltstones; road to Santuario de la Virgen de Herrera (1 in Fig. 1). Azuara impact structure.

Image007Fig. 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.

Image009Fig. 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.

Image011Fig. 6. Segment of the wall in Fig. 5.

Image013Fig. 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.