Polymictic (polymict, polygenetic, polylithologic) breccia
A polymictic breccia is a clastic sedimentary rock composed of angular clasts from different origin intermixed in a consolidated matrix.
According to a former nomenclature suggested by the IUGS Subcommision on the Systematics of Metamorphic Rocks, Study Group for Impactites, a polymict impact breccia is "a breccia with clastic matrix or crystalline matrix (derived from the crystallization of impact melt) containing lithic and mineral clasts of different degree of shock metamorphism excavated by an impact from different regions of the target rock section, transported, mixed, and deposited inside or around an impact crater or injected into the target rocks as dikes". Meanwhile (2007), a new provisional approach to an impactite classification has been performed: http://www.bgs.ac.uk/SCMR/docs/papers/paper_11.pdf
In accordance with common use to avoid genetically related names, the term "polymictic breccia" is applied here purely descriptive only. The term "polymictic impact breccia", a priori genetically related, may be used as a generic term for lithic breccias and suevite breccia .
Fig. 1. Polymictic breccia (Onaping Fm.), Sudbury (Canada) impact structure.
Fig. 2. A polymictic breccia from the Azuara impact structure (Spain); near Herrera de los Navarros. The reddish color of many limestone clasts is explained by enhanced temperatures.
Fig. 3. A polymictic breccia from the Azuara impact structure (Spain); near Fuendetodos. As in Fig. 2, the reddish color of many limestone clasts is explained by enhanced temperatures.
Fig. 4. A polymictic matrix-rich breccia from the Tüttensee crater, Chiemgau impact strewn field, Germany.
Breccia-within-breccia; breccia generations
In the extremely complex impact cratering process, breccias may incorporate earlier formed breccias, which leads to breccias-within-breccias and even to multiple breccia generations generally unknown from other geological processes.
Fig. 5. Breccia-within-breccia: polymictic breccia with flow texture and clasts being themselves brecciated (monomictic breccia). The distinct coherence of the monomictic breccia clasts proves high confining pressure during flow and emplacement of the polymictic breccia. Rim zone of the Rubielos de la Cérida impact basin, near Olalla.
Fig. 6. Polymictic breccia; Rubielos de la Cérida impact basin. Note the clast to the right of the coin which itself is a polymictic breccia (breccia-within-breccia).
Fig. 7. Polished slab of a polymictic limestone breccia, Azuara impact structure. The breccia tells a complex story of formation. At least, four breccia generations can be observed (see close-ups below).
Fig. 8. Close-up of the breccia in Fig. 7.
Fig. 9. Detail of Fig. 8. Note the breccia dikelet cutting through a clast. After the emplacement of the dikelet, the clast was again fragmented.