The eugeosynclinal sequences

Abstract

The eugeosynclinal sequences of the Northern Apennines are characterized by the presence of ophiolites both as a primary stratigraphic substratum and as olistostromes and olistoliths interbedded in the sedimentary succession. A further characteristic is extensive turbidite sedimentation of Late Cretaceous-Eocene age, which preceded similar terrigenous deposition in the miogeosyncline. The eugeosynclinal sequences underwent intense horizontal displacements from a Tyrrhenian area, overriding the miogeosynclinal rocks. In spite of tectonic dismembering, several of the original stratigraphic successions have been reconstructed. The eugeosynclinal sequences have been grouped into: ( 1) Helminthoid Flysch sequences (Upper Triassic-Eocene); ( 2) Vara Supergroup (Jurassic?-Paleocene); ( 3) Calvana Supergroup (Upper Cretaceous-Eocene) and ( 4) Canetolo Complex (Paleocene-Miocene?). The Helminthoid Flysch sequences include most of the eugeosynclinal deposits, generally with remarkable thickness of calcareous and, subordinately, arenaceous turbidites of Late Cretaceous age. In this paper stratigraphic units of the Helminthoid Flysch and of the other major sequences are described, as well as their mutual tectonic relations. Schematically, the following upward geometric succession has been assumed, starting from the top of the miogeosynclinal rocks of the Tuscan and Umbrian sequences: the Canetolo Complex, the Calvana Supergroup, the Helminthoid Flysch sequences and the Vara Supergroup. Locally, this succession is present only in part, due to tectonic reductions. According to the structural positions and on the basis of various stratigraphic and sedimentological considerations, we have attempted to sketch the paleogeography of the eugeosyncline. The reconstruction is largely incomplete up to the Lower Cretaceous, whereas numerous paleogeographic data are available for the Upper Cretaceous and the Eocene. The areas of deposition of the Vara Supergroup and of those parts of the Helminthoid Flysch sequences with abundant Upper Cretaceous and Paleocene arenaceous turbidites, are believed to have been located along the internal (western) margin of the eugeosyncline. To the east of it, it is supposed, was located the basin of the Helminthoid Flysch sequences with calcareous turbidites. The external (eastern) margin of the eugeosyncline was occupied by the Calvana Supergroup and by the Canetolo Complex. The relations between the Alpine realms and the Apennines geosyncline are briefly discussed, especially in respect to the provenance of the clastics of the Apennines turbidites. The Corsica-Sardinia Massif is supposed to have been the main source area during the Cretaceous and Paleocene for the arenaceous clastics, whereas the coeval calcareous turbidites mainly derived from the South-Alpine zone and from the internal margin of the Briançonnais facies belt. A critical review of alternative paleogeographic reconstructions proposed by various authors, has also been attempted.