Role of Jurassic rift architecture in the dispersal of calciturbidites: New insights from the Central and Northern Apeninnes

Abstract

A geological mapping project was performed in three sectors of the Umbria-Marche Apennines (Central Italy), targeting the local Meso-Cenozoic succession: the Narni-Amelia, Sabini and Sibillini Mts. (Mt. Pennino Ridge). During our field-work we focused on pelagic carbonate platform (PCP)-basin systems analysis, with the aim of demonstrating how the Early Jurassic rift-related submarine topography forced the pathways of resedimented neritic material sourced by coeval active carbonate factories. Four stratigraphic sections for the northern sector of Sibillini Mts. and six for the Narni-Amelia and Sabina areas were measured.While calciturbidites are not unexpected in the Sabina region, due to the presence of the neighbouring Latium-Abruzzi carbonate platform (LACP), their presence in far removed basinal settings is less obvious. Resedimented shallow-water carbonates embedded in Middle and Upper Jurassic cherty pelagites (Calcari e Marne a Posidonia, Calcari Diasprigni and Maiolica Fms.) were discovered and mapped for the first time in the study sectors. The Middle-Upper Jurassic calciturbidites are graded and laminated grainstones-to-wackestones, locally rudstones, bearing loose shallow-water material. Coated grains (ooids, oncoids), peloids, aggregate grains, skeletal grains and benthic foraminifers are dominant, associated with typical pelagic elements (thin-shelled bivalves, radiolarians, crinoids). The neritic elements must have been sourced from productive carbonate platform(s), as intrabasinal highs had all drowned in the early Pliensbachian. The LACP is the prime suspect source-area for calciturbidites of both the Narni-Amelia-Sabina and Sibillini Mts. but shedding from carbonate platforms buried under the Adriatic Sea cannot be excluded for the Mt. Pennino area. The rugged palaeotectonic architecture inherited from the Early Jurassic rifting affected the dispersal pattern of calcarenites. The marginal palaeoescarpments of PCPs formed obstacles to the gravity flows as sediment load was discharged at their toe. The Sabina Plateau is an instructive case example: while certain W-directed flows were vigorous enough to climb its E-facing escarpment, leaving overbank deposits on its top, a "shelter" effect was by far dominant, as evidenced by the resediment-free nature of the basin lying west of it, which had to be shielded by the Plateau. To the north of the Sabina Plateau, the LACP-sourced flows were funnelled towards the Narni-Amelia area along a westward-branching arm of the Sabina Basin ("Terni corridor"). A comparable picture is observed in the northern Sibillini Mts., where calciturbidites were funnelled into a circa N-S-trending basin (Mt. Vermenone-Scurosa Valley basin) due to the presence of several PCPs, while "clean" onlap-successions characterize the northern flanks of the horst-blocks, suggesting a shield effect.