Plate 128

Brecciated structure


A breccia can be viewed both as a deposit and as a kind of structure, or fabric; in the latter respect, it indicates an extreme deformation affecting rigid, brittle materials. A breccia is often a puzzling geological object as it can be originated by several processes, not only sedimentary ones. A volcanic or a tectonic breccia, for example, can be produced in at least twenty different ways, according to experts. I shall here enumerate the more common cases of sedimentary  and pyroclastic  breccias, and their causes. They are: 1) mass flows (fresh clasts of whatever origin are not rounded in matrix-supported transport); 2) fragmentation caused by sliding; 3) fall of single particles from rock cliffs (talus debris); 4) ballistic fall of pyroclastic products; 5) in place production of intraformational (mud) clasts by desiccation, expanding crystals or mechanical action of organisms; 6) collapse of solution (karst) cavities; 7) fluid injection (water, gas-charged mud) from below; 8) substrate liquefaction beneath cohesive or brittle beds, and, last but not least, 9) earthquake shaking (which again poses the problem of using the term seismite!).

As a consequence, a brecciated structure can represent many things and cannot be a reliable indicator  of any in particular. In most cases, the interpretation of a breccia needs the help of independent evidence and cannot be given if the geological context is ignored or unknown.

This specific example shows elements of bituminous marl embedded in a carbonate matrix at the base of an evaporitic formation (Gessoso-solfifera, northern Apennines). Several processes, among those listed above, could have contributed to its origin, acting alone or in combination; namely, 1, 2, 5, 6, 7, 8, 9.


Sedimentographica