The miogeosynclinal sequences

Abstract

The Tuscan and Umbrian sequences represent miogeosynclinal deposition in the Northern Apennines. They are composed of five major rock groups, which are here described in detail and interpreted. The sequences start with a terrigenous basal section, Upper Carboniferous to Carnian, composed of clastic rocks with shallow-marine, lagoonal and continental facies, and of acid volcanics. This group probably represents the Hercynian postgeosynclinal stage. A carbonate-evaporitic section follows, Noric to Rhaetic, overlain by Hettangian neritic limestones. These facies are of wide extent; a geosynclinal basin in the Northern Apennines had probably not yet differentiated. A progressive deepening of the sea began in the Sinemurian; the carbonate-siliceous section (Lower Jurassic-Lower Cretaceous) is made of frequently siliceous micritic limestones and marlstones, and contains bedded cherts, but locally there are biostromal and oolitic limestones. In parts of the geosyncline, submarine dissolution on top of tectonic rises caused the development of paraconformities. The pre-flysch section includes pelitic and marly formations, with some bedded chert, and graded limestones (prototurbidites) in the Tuscan sequence (Lower Cretaceous to Lower Oligocene); limestones and marlstones in the Umbrian sequence (Upper Cretaceous to Middle Miocene). The pre-flysch stage indicates a further deepening of the sea and a considerable slowing of the rate of deposition, but the bottom physiography was in parts irregular and paraconformities were formed. The topmost flysch section consists of thick turbidite formations, migrating in age from west to east, Early-Middle Oligocene to Early Miocene in the Tuscan sequence, Early to Late Miocene in the Umbrian sequence. Each flysch unit starts abruptly and is typically arenaceous in the lower part (ortotubidites), but it becomes progressively marly-argillaceous in the upper part (kataturbidites).