BRECCIAS AND MELT ROCKS IN TERRESTRIAL IMPACT STRUCTURES: AN OVERVIEW OF OCCURRENCE, TYPES, COMPOSITION, FORMATION, AND USE

In contrast to the situation on (nearly) atmosphereless bodies in the solar system, where impact craters remain recognizable by their morphology for a very long time, on Earth the active geo-, hydro-, and atmosphere cause craters to be eroded, obscured, or distorted fairly rapidly. Thus on Earth remote sensing and morphological observations may yield important initial data regarding the recognition of a potential impact structure, but cannot provide confirming evidence. Geophysical methods are also useful in identifying candidate sites for further studies, especially for subsurface features. In complex craters, the central uplift usually consists of dense basement rocks and frequently contains severely shocked material. This uplift is often more resistant to erosion than the rest of the crater, and, thus, in old eroded structures, may be the only remnant of the crater that can still be identified. However, only the petrographic and geochemical study of actual rocks from the potential impact structure can provide the evidence for confirmation of an impact origin. Good materials for the recognition of impact evidence are various types of breccia and melt rocks. These rocks often carry unambiguous evidence for the impact origin of a structure in the form of shocked mineral and lithic clasts or of contamination from the extraterrestrial projectile. Impact craters can be filled with and surrounded by various breccia types, which include fragmental impact breccia (or lithic breccia), a “monomict or polymict impact breccia with clastic matrix containing shocked and unshocked mineral and lithic clasts”. Impact melt breccia has been defined as an “impact melt rock containing lithic and mineral clasts displaying variable degrees of shock metamorphism in a crystalline, semihyaline or hyaline matrix (crystalline or glassy impact melt breccias)” Suevite is a polymict breccia with cogenetic impact melt particles. The distribution of the rock types is a function of their formation and the order in which they formed. Distal ejecta con comprise shocked rocks and minerals as well as impact glass or tektites. The study of impact breccias yields important information on the formation and age of a structure.