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.
We should be aware that it may be difficult to distinguish between monomictic impact breccias and monomictic tectonic breccias and that the use of the term "monomict impact breccia" may in some instances be problematic. Therefore, 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).
A new provisional approach (2007) to an impact breccia nomenclature by the IUGS Subcommission on the Systematics of Metamorphic Rocks can be read here: http://www.bgs.ac.uk/SCMR/docs/papers/paper_11.pdf
Fig. 1. Monomictic movement breccia; dislocated megablock, Malmian limestone; Ries impact structure, Iggenhausen quarry. Typical grit brecciation.
Fig. 2. Iggenhausen quarry, close-up.
Fig. 3. Extensive monomictic movement breccia in Paleozoic quartzites; quarry near Lagueruela, rim zone of the Azuara (Spain) impact structure.
Fig. 4. Close-up of the monomictic movement breccia in Fig. 3. Over large volumes, the quartzite rock is ground to finest fraction and in part pulverized which has required enormous compressive forces.
Fig. 5. Extensive monomictic movement breccia in the rim zone of the Azuara impact structure; quarry in Jurassic limestones, north of Belchite.
Fig. 6. Close-up of the monomictic breccia in Fig. 5.
Fig. 7. Monomictic movement breccia, dislocated megablock; Azuara impact structure, near Cucalón.
Fig. 8. Monomictic movement breccia, dislocated megablock; Azuara impact structure, near Cucalón. Grit brecciation in close-up.
Fig. 9. 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.
Fig. 10. Typical mortar texture in a monomictic breccia. Azuara impact structure, near Herrera de los Navarros.
Fig. 11. Monomictic movement breccia displaying distinct mortar texture; Siljan (Sweden) impact structure.
Fig. 12. Monomictic movement breccia, Lake Hummeln (Sweden) probable impact structure.
Fig. 13. Strongly brecciated chert nodule taken from Malmian limestone. Ries impact structure, crater rim near Holheim.
Fig. 14. Heavily brecciated and polished scour surface. Contact between Muschelkalk limestone (photo) and Paleozoic shales. Rim zone of the Rubielos de la Cérida impact basin, near Olalla.
Sometimes, some problems may arise with regard to a proper classification of impact breccias. Fig. 14 shows a breccia from the rim zone of the Rubielos de la Cérida (Spain) impact basin. The clasts unquestionably have originated from the same Muschelkalk dolomite basically constituting a monomictic breccia with distinct mortar texture in a clastic brownish-grayish Muschelkalk matrix. However a second-generation carbonate matrix of reddish color has intruded into the first-generation breccia, and a third dike-like breccia of whitish color can be observed to have penetrated the reddish matrix (Fig. 15) - all in all a three-generation bereccia/breccia dike rock. Obviously, this rock does not match any of the well-established impact breccias and classifications. Obviously, too, a formation in a "normal" geologic process provides nearly insoluble problems.
Fig. 15. Complex three-generation breccia texture in a rock from the rim zone of the Rubielos de la Cérida impact basin near Olalla (see text).
Fig. 16. Close-up of the breccia in Fig. 15.