Azuara

Suevites in the Spanish impact structures have so far been found in two different outcrop situations:

a. as the so-called basal breccia in the Azuara and Rubielos de la Cérida paired impact craters

b. as large blocks quarried out from a megabreccia in the Rubielos de la Cérida structure

The name basal breccia refers to its stratigraphic position at the base of the undisturbed post-impact (and post-Alpidic) layers. The polymict and rather heterogeneous breccia is up to c. 20 m thick and is found in many parts of the Azuara and Rubielos de la Cérida structures unconformably overlying the Alpidicly folded and impact-deformed sediments of the Iberian System

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Basal breccia (suevite) unconformably overlying steeply dipping Jurassic limestones. Central uplift in the Rubielos de la Cérida structure.

Both Palaeozoic and Mesozoic rocks contribute to the matrix-supported breccia components which are angular to sharp-edged, but well-rounded components are also observed. Frequently, they show distinct reaction seams in contact with the matrix. Breccia generations (breccias-within-breccias) are abundant. The sharp-edged clasts, especially the limestone fragments suggest an immediate embedding after the brecciation and mixing of the Palaeozoic and Mesozoic components. Breccia specimens break into pieces across the clast boundaries due to the extremely hard matrix and an exceptional cementation.

The matrix is composed of calcite and (from X-ray fluorescence analysis) up to 10 % Si02. X-ray diffraction analysis and thin-section inspection with strong magnification, however, could only identify minute traces of quartz and silicates (e.g., mica). The major fraction of the Si02 content is thus expected to be in an amorphous phase finely dispersed within the breccia matrix. In rare cases, however, tiny glass fragments (see image below) can be observed in thin sections of the basal-breccia matrix, and we assume that the amorphous phase is glass, too. This would well fit a model by Kieffer & Simonds (1980) which explains the obvious absence of impact melt sheets in and around craters from sedimentary targets. The authors conclude that the large amount of shock-produced volatiles (from pore-water vaporization and limestone decarbonation) in sedimetary targets prevents melt-sheet formation and, instead, finely disperses the shock-melted material.

Regularly, components of the basal breccia are observed (see images below) to have undergone partial up to nearly complete decomposition which is obviously connected with flow texture of the basal breccia matrix. These zones were analysed in detail by Katschorek (1990), and she was able to establish the presence of former carbonate melt.

We further note that thin sections show shock-metamorphic effects in the form of PDFs and PFs in quartz as well as diaplectic quartz.

Thus, the basal breccia as a polymict breccia with glass components and mineral clasts displaying shock metamorphism, is a typical suevite or suevite breccia.