Role of Early Jurassic rift architecture in the dispersal of calciturbidites, as revealed by geological mapping in Central and Northern Apennines

Abstract

The role of sea-bottom topography in the dispersal of shallow water-derived calciturbidites across a submarine rift, as determined by the local extensional architecture, is under-investigated, namely with pelagic settings along ancient passive continental margins. A comparison with modern carbonate platform/basin analogues, or with siliciclastic systems, is not always feasible, as ancient carbonate systems were commonly home to anachronistic environments (e.g. the Western Tethyan Mesozoic). Geological mapping of the Jurassic rift basin exposed in the Apennines of Central Italy revealed a complex pattern of intrabasinal highs (pelagic carbonate platforms) and intervening basins, controlled by the high density, and oddly variable trends, of faults rooted in a shallow detachment layer corresponding to thick Triassic salt. A map pairing the occurrences of resedimented beds in pelagic successions with an updated palaeogeography becomes therefore the natural descriptor of the itineraries followed by sediment gravity flows. This qualitative method represents a companion, or even alternative, approach to the one strictly based on physical stratigraphy, and it greatly improves our knowledge of regional geology and rift-basin analysis. Our study focuses on: i) a reconstruction of the palaeotectonic architecture of the Umbria-Marche Basin in the Jurassic, and on ii) how this architecture produced a submarine topography which governed the dispersal of sediment, shed by the neighbouring Lazio-Abruzzo Carbonate Platform, for >40 million years. Geological mapping, coupled with the measurement and correlation of stratigraphic sections, shows that the marginal palaeoescarpments of pelagic carbonate platforms formed obstacles to the gravity flows as sediment load was discharged in confined basins at their toes and/or flows were deflected and forced along alternative itineraries. While turbidity currents were locally vigorous enough to climb the escarpments, leaving graded and laminated overbank deposits on the pelagic carbonate platform-tops, a “shelter” effect is evidenced by the resediment-free nature of those basins lying downflow, which were shielded by the highs. Our case study potentially represents the analogue of hydrocarbon fields both inland and in the offshore.