Sedimentology and palaeogeographic significance of certain red limestones and associated beds in the Lias of the Alpine region

Abstract

Synopsis A stratigraphical and petrological description is given of a selected number of red limestones and associated grey limestones and red and grey-green marls, in several classic localities in the Lias of the Alpine region. These include the Ammonitico Rosso of southern Switzerland and northern Italy and the Adnet Limestone and Fleckenmergel, etc. of the Salzburg Province of Austria. There are a number of characteristics of these rocks which contrast with those of equivalent age in north-west Europe. Detrital quartz, organic carbon, pyrite, kaolinite, chamosite and siderite are all rare or absent, whereas they are common in the latter region. Chert nodules are, in contrast quite common in the Alpine beds but rare further north. Red limestones and marls are stratigraphically more condensed and more nodular than their grey equivalents; in cases of extreme condensation manganese oxide-coated nodules and corrosion surfaces occur. Stratigraphical evidence favours deposition in quite shallow water. The fauna of the Mediterranean region is the more diverse, but individual abundance, for beds of a given rate of deposition, is greater in north-west Europe, most notably in the case of bivalves. These facies differences can best be accounted for by interpreting north-west Europe as an area of coastal and inshore waters, whose salinity was lowered slightly by the influx of fresh water from rivers, which brought large quantities of terrigenous detritals, organic matter and iron to the sea. The Mediterranean region was in contrast an area of open but quite shallow sea, of normal marine salinity but deficient in particulate organic matter and hence benthonic fauna. The relevance of this interpretation to other Mesozoic formations is briefly discussed.