Regional and global changes around the Triassic–Jurassic boundary reflected in the late Norian–Hettangian history of the Apennine basins
Abstract Palaeogeographic restorations for the Late Triassic and Early Jurassic of the west-Mediterranean area show that significant facies changes occurred in this period. The study of three basinal successions in the Northern, Central and Southern Apennines indicates that changes in ecological conditions occurred at the same time in different palaeogeographic and geodynamic domains. The early Mesozoic evolution of the Apennines is controlled by the opening of two oceans: the Ionian/East-Mediterranean Tethys to the south-east and the Alpine Ocean to the north-west. The Lagonegro basin, in the Southern Apennines, was the northern continental margin of the Ionian Tethys from the Triassic, while the La Spezia basin, in the Northern Apennines, became part of the south-eastern continental margin of the Alpine Ocean in the Middle-Late Jurassic. The third studied basin (Mt. Camicia basin, in the Gran Sasso, Central Apennines) was an intra-platform basin from the early Norian up to the late Hettangian–early Sinemurian. During the Norian–Hettangian period these basins were not directly connected to each other, being separated by wide carbonate platforms, and show different evolutionary trends. Stratigraphic studies in these successions place the Triassic–Jurassic boundary in 6–8 m thick intervals with anoxic facies in the three basins. Relevant facies changes are also recorded in the late Norian: in the Northern Apennines the sedimentation of sulphate evaporites ended, being replaced by the Rhaetavicula contorta facies; whilst in the Southern Apennines (Lagonegro basin) the deposition of radiolarites began. The end of evaporite accumulation can be explained by a climate change from arid to humid conditions, and it is also probably linked to an extensional phase preceding the Jurassic rifting of the Alpine Ocean, and may therefore have a regional significance. Nevertheless, it corresponds to the spread of the R. contorta facies into other parts of southern Europe, from France to Germany and Austria. The beginning of radiolaritic sedimentation in the Lagonegro basin could be related to the rise of atmospheric CO2 content, linked to volcanic activity in the Ionian ridge. These facies changes reflect changes of environmental conditions, that caused the decline of some fossil associations favoring the development of opportunistic forms, e.g. Triasina hantkeni. The end-Triassic extinction, coupled with an anoxic episode in all the examined basins, immediately preceded a new environmental change, documented by the recovery of the carbonate platforms with oolitic facies and different associations. The evolution of all these basins suggests that geodynamics were probably responsible for regional events and that, at certain moments, development reflected global effects as a response to new connections between formerly different domains. The Central Atlantic Magmatic Province (CAMP) volcanism, considered as one of the possible causes for the end Triassic mass extinction, would have had even greater importance if coupled with the volcanism linked to the rifting of the Ionian Tethys.