IMPACT BRECCIAS – A MANY-SPLENDORED THING

More than any other sedimentary deposit, extraterrestrial impact-generated breccias defy easy classification and do not fit into sedimentologic and stratigraphic nomenclature developed for non-impact units. Impact breccias are marked by their extreme and heterogeneous depositional environment, which evidences great variability in rock rheology, pressure, temperature, provenance, transport, textures, and depositional processes. These breccias reflect fundamentally such stages of the impact process itself as shock-wave effects near the impact, excavation flow, transient crater modification, ejecta formation, and, in oceanic impacts, collapse of adjacent margins. Ongoing challenges of impact breccia description and classification, mirroring the evolving nature of impact geology research, include:

(1) Textural classification of impact breccias and megabreccias. Most widely used textural classification schemes stress pebble-sized or smaller grains, and do not adequately describe the larger grain-size fractions found commonly within impact breccias. Alternative textural classification schemes are available, but largely impact geologists have not embraced these.

(2) Descriptive vs. interpretive terms. Traditionally, a distinct separation between descriptive and interpretive phases of sedimentary rock classification is stressed. However, many often-used impact breccia terms incorporate both aspects; this is necessary to describe accurately not only the breccia but also its genesis within the overall impact event. Such modifiers include ‘allochthonous (allogenic) polymict', ‘parautochthonous monomict', ‘suevitic', ‘fallback', ‘crater modification', ‘marine resurge', ‘tsunami runup', etc.

(3) Impact breccias and stratigraphic nomenclature. Impact deposits are not addressed specifically as formal units in NACSN and IUGS/ISSC treatises on stratigraphic nomenclature. Unlike many other sedimentary units, impact breccias are inherently both synchronous and isochronous in nature. Existing formal lithostratigraphic, allostratigraphic, and lithochronologic units can describe impact deposits; however, this terminology is currently not applied uniformly and future usage will determine whether the existing framework is adequate or if a new class of formal units is required.