Impactites: some peculiarities in the Azuara structure
Fig. 1. Reddish breccia dike (hammer for scale) cutting through polymictic breccia. Azuara structure, between Plenas and Loscos villages. Obviously, the dike was emplaced after or during the deposition of the breccia. An intrusion from below is suggested. Detail in Fig. 2.
Fig. 2. Detail of the breccia dike in Fig. 1 showing pipes (arrows) within the dike which may be degassing pipes.
Fig. 3. Plastically strongly deformed Dogger/Malmian limestone slabs in a polymictic breccia dike near Muniesa. An excellent and detailed description of this prominent 300 m- length breccia dike is given in G. Mayer's Diploma thesis (1990).
Fig. 4. Breccia dikes penetrating a chert nodule in Liassic limestone. Lots of chert splinters have been dragged along from the shattered nodule. Coin diameter 23 mm. Ring wall near Belchite, Azuara impact structure.
Figs. 5, 6. Impact-induced flow processes in Muschelkalk limestones traced by chert splinters from crushed chert nodules. Fig. 5: Heavily crushed and disintegrated chert nodule in Muschelkalk limestone at Monforte de Moyuela, Azuara impact structure.
Fig. 6. Small white chert splinters forming continuous tracks which start at crushed nodules. Because of the extremely sharp-edged splinters, any kind of diagenetic processes can be excluded. A rose diagram of strike directions of the splinter tracks shows the strike maximum points to the center of the Azuara impact structure. More can be read and seen here: http://www.impact-structures.com/Archiv/wkarchiv11.html
Fig. 7. Azuara impact structure. This breccia is exposed near Almonacid de la Cuba at the NE rim of the structure and is so far unique with respect to composition and texture. Within a dense to porous and even foamy grayish carbonate matrix, components of snow-white color are embedded, which are in most cases extremely vesicular as shown in the photograph. The breccia is part of an extended deposit, which has in detail been investigated by Tanja Katschorek (diploma thesis). From field work and petrographic analyses, she concludes that the deposit has resulted from an expanded, turbulent, and dilute flow by inclusion of carbonate melt, similar to volcanic surges.
Fig. 8. Also unique with respect to composition and texture is a breccia deposit near Jaulín in the northern part of the Azuara structure. The sample in this image is dominated by a matrix composed of a green, probably Lower Tertiary sandstone and a pale Jurassic limestone both intimately interfingered. The breccia character of this peculiar rock becomes more evident in Fig. 9. More about the Jaulín outcrop and the interpretation of the breccia as an impactite can be read here: http://www.impact-structures.com/Archiv/wkarchiv18.html
Fig. 9. Another aspect of the Jaulín impact breccia suggesting some affinity to the Almonacid de la Cuba impactite in Fig. 7. Compared with the sample in Fig. 8, the intimate mixing of the green sandstone and the pale limestone material is even more pronounced. Jaulín samples courtesy of P. Bockstaller.
Fig. 10. Azuara impact structure: Polymict globular "breccia". This rock is a breccia in a broad sense only. The globular components turn out to be glomerations of smaller globular fragments, and the matrix itself is also composed of globules down to microscopic size. The texture of the rock shows intriguing features such as distinct strings (blue arrows) and whirls (white arrows). Therefore, soil formation (e.g., caliche) can be excluded. Instead, we suggest the rock to be an impactite and the globules to have grown by addition of finely dispersed material in an explosion cloud and, such, to form accretionary lapilli (Graup, 1981, in a paper about the suevite of the Ries impact structure).