Discontinuities in carbonate successions: Identification, interpretation and classification of some Italian examples

The growing use of unconformities as bounding surfaces for new types of stratigraphic units seems not to be matched by a corresponding effort on process-oriented researches on unconformities themselves. This paper aims at giving a contribution to classification and understanding of discontinuities in carbonate successions from the outcrop perspective, based on some Italian examples. A number of integrated criteria (geometry, sedimentology, diagenesis, biostratigraphy) is proposed to recognise stratigraphic breaks. Old terms are discussed and sometimes redefined, while some new terms are introduced. The term discontinuity surface (DS) is proposed as a general one to indicate any stratigraphic interface where an interruption of sedimentation can be proved. A DS can be characterized as a firm ground or a rock ground depending on its coherence at the moment of the renewal of sedimentation. Polygenic and simple omission surfaces are separated within firm grounds if a recognizable gap is either present or not; a third kind of firm ground, revealed only by biostratigraphy, has been called hidden discontinuity surface. Two categories of rock grounds are also distinguished: hard grounds and inherited rock grounds if, respectively, generation of the DS and/or deposition of the overlying sediments took place in the same environment of the underlying sediments or not. A genetic interpretation for each type of discontinuity is proposed. The largest variety of DS's occurs in pelagic sediments. Some discontinuities (hard grounds and simple omission surfaces) are attributed to increased bottom-current activity during sea-level falls; others (polygenic omission surfaces and hidden DS's) are interpreted as submarine slide scars either with or without the overprint due to exposure on the seafloor. The DS's separating different facies (e.g. platform and pelagic ones) are interpreted to be due to tectonics. The relevance of the recognized DS's to sequence stratigraphy is briefly discussed.