Peperites From the Limagne Trench (Auvergne, French Massif Central): A Distinctive Facies of Phreatomagmatic Pyroclastics. History of A Semantic Drift

The term “peperite” was initially coined in the Limagne Oligocene Trench for a rock made of altered grains of basaltic lava within an apparently sedimentary cement, designated as “marlycalcareous”. As early as the 19th century, two generic theories were opposed: some saw in peperites the result of a sub-lacustrine volcanism, with granulation and mixing of hot magma in contact with freshly deposited argilo-calcareous mud, prior to diagenesis; while others saw dikes and intrusive breccias, much posterior to the enclosing sedimentary sequence. As the first interpretation has prevailed for sometime and moreover was expressed in English, this was the meaning referred to in English literature; “peperites” were understood as being hyaloclastics with a sedimentary matrix. As a matter of fact, all the studies which have been conducted for the last 25 years have firmly established that Limagne peperites were products of sub-aerial phreato-magmatic eruptions, of an average Miocene age, whose carbonated matrix results ftom the pulverization of former sedimentary rocks. This does not match in any the international definition of peperites. We propose giving up this definition, and keeping the word to mean simply a facies in the polylithologic pyroclastic range.