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Peculiarities


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Breccias and Breccia dikes: peculiarities

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Reddish breccia dike (hammer for scale) cutting through polymict megabreccia. Azuara structure, between Plenas and Loscos villages. 


Detail showing ? degassing pipes (arrows) within the dike.

Plastically strongly deformed Dogger/Malmian limestone slabs in a polymict 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).

Breccia dikes penetrating a chert nodule in Liassic limestone. Arrows mark chert splinters which have been dragged along from the shattered nodule. Coin diameter 23 mm. Ring-wall near Belchite, Azuara impact structure.

Impact-induced flow processes in Muschelkalk limestones traced by chert splinters from crushed chert nodules. Top: Heavily crushed and disintegrated chert nodule in Muschelkalk limestone. Bottom: 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 that the strike maximum points to the center of the Azuara impact structure. - Monforte de Moyuela.

The megabreccia

The megabreccia of the Azuara structure is a structural peculiarity which may be important for understanding some impact-cratering aspects. The breecia is exposed in the region of the Mesozoic outer ring where it concentrates in the north-east and north-west. It occurs as a nearly stratiform layer of up to 80 m thickness. The components have resulted predominantly from in-situ brecciation of Carniolas (Keuper/Liassic transitional layer) and/or Liassic limestones; however, rocks of stratigraphically adjacent layers are frequently intermixed. Occasionally, blocks of stratigraphically much younger (e.g. Eocene) sediments contribute to the breccia. The clast size ranges from a fraction of a millimeter to the order of ten meters. Mortar texture and breccia generations (breccias-within-breccias) are abundant. In thin section cataclastic flow texture of the matrix can be observed. Breccia dikes cutting across the megabreccia are mainly of the polymict type. The megabreccia also causes a strange landscape which is characterized by a hummocky vegetationless surface and isolated large breccia megablocks emerging from the ground.

The principal macroscopic characteristics of the megabreccia can best be studied 5 km south of Belchite along the old railway-cutting. For more detailed information, the reader is referred to the work of FIEBAG (1988) and KATSCHOREK (1990).

The megabreccia should not and, in fact, cannot be confused with the well-known Carniolas rauhwacke brecciation, although there are geologists who are evidently unable to make a distinction between these totally different breccias (see e.g., M. Aurell et al. 1993, M. Aurell 1994). This suggests that they have never studied the megabreccia in the field.

Azuara impact structure: Typical aspect of the megabreccia (3 km southwest of Belchite; photograph by courtesy of Tanja Katschorek).

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 greyish 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.

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. Therefore, the name glomerolite is proposed for this special rock type. 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).

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