Monomictic breccia - monomictic impact breccia  - monomictic movement breccia

Basically, a monomictic breccia results from rock deformation by shearing and granulation (cataclasis) in the process of tectonism or - more general - dislocation metamorphism. If the dislocation metamorphism is impact-related, the produced cataclasite may be termed a monomict impact breccia.

According to the IUGS Subcommision on the Systematics of Metamorphic Rocks, Study Group for Impactites, a monomict impact breccia is a "cataclasite produced by impact and generally displaying weak or no shock metamorphism; occurs in the (par)autochthonous floor of an impact crater or as clast (up to the size of blocks and megablocks) within allochthonous impact breccias".

We should be aware that it may be difficult to distinguish between monomict impact breccias and monomict tectonic breccias and that the use of the term "monomict impact breccia" may in some instances be problematic. Therefore, and owing to the rather restricted IUGS definition "... occurs in the ... floor of an impact crater or as clast ... within allochthonous impact breccias", the term "monomictic movement breccia" may be more appropriate.

This term has been introduced by Reiff (1978) by discussing monomictic breccia complexes in the Steinheim, Ries, Sierra Madera, Flynn Creek, Decaturville and Wells Creek impact structures. In the Wells Creek impact structure, these breccias were called crackle breccias and homogeneous rubble breccias (Wilson and Stearns, 1968) and later considered variants of the same phenomenon (Reiff 1978). Related textures are grit brecciation and mortar texture (Hüttner, 1969). In each case, a drastic brecciation of whole rock complexes with resulting grain sizes down to sand and silt fraction and frequently preserved fitting of the fragments is observed. This peculiar brecciation requires intense movement under very high confining pressure. In the impact structures referred to above, these movement breccias occur in excavated/ejected megablocks, in the crater floor, in inner rings and central uplifts. Reiff (1978) points to the fact that the typical texture of monomictic movement breccias is also observed in breccias from giant rock falls (for example the 1,500 m Flims, Switzerland, rock fall) and may in rare instances occur along tectonic fault zones. The occurrence of monomictic movement breccias in environments lacking tectonic fault zones and gradients sufficient for mass rock falls are strong clues to impact cratering events (Reiff 1978).

Monomictic movement breccia; dislocated megablock, Malmian limestone; Ries impact structure, Iggenhausen quarry. Typical grit brecciation.

Iggenhausen quarry, close-up.

Extensive monomictic movement breccia in the rim zone of the Azuara impact structure; quarry in Jurassic limestones, north of Belchite.

Close-up.

Monomictic movement breccia, dislocated megablock; Azuara impact structure, near Cucalón.

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Monomictic movement breccia, dislocated megablock; Azuara impact structure, near Cucalón.

Grit brecciation in close-up.

Extensive monomictic movement breccia, rim zone of the Rubielos de la Cérida impact basin, road between Escorihuela and El Pobo/Corbalán. Grit brecciation and mortar texture.

Typical mortar texture in a monomictic breccia. Azuara impact structure, near Herrera de los Navarros.

Strongly brecciated chert nodule taken from Malmian limestone. Ries impact structure, crater rim near Holheim.

Heavily brecciated and polished scour surface. Contact between Muschelkalk limestone (photo) and Palaeozoic shales. Rim zone of the Rubielos de la Cérida impact basin, near Olalla.