Geological mapping reveals the role of Early Jurassic rift architecture in the dispersal of calciturbidites: new insights from the Central and Northern Apennines
A geological mapping project was performed in three sectors of the Umbria-Marche Apennines (Central Italy): the Narni-Amelia, Sabini, and Sibillini (Mt Pennino) Mountain Ridges. One of the outcomes of this project is the improvement of our database of the occurrence of levels or stratigraphic intervals with resedimented shallow-water carbonate material in the Jurassic part of the succession. The Umbria-Marche region was characterised by an exceedingly complex pattern of intrabasinal highs (pelagic carbonate platforms) and intervening basins in the Jurassic. Due to this, a map pairing the occurrences of such resedimented beds with an updated palaeogeography becomes the natural descriptor of how the rift-related submarine topography (an extreme instance of it) could influence the dispersal of materials sourced by coeval active carbonate factories. Resedimented shallow-water carbonates were discovered and mapped for the first time in the study areas, embedded in uppermost Lower to Upper Jurassic cherty pelagites.While calciturbidites of this age are not unexpected in the Sabina and Narni-Amelia sectors, lying in the neighborhood of the productive Lazio-Abruzzo Carbonate Platform, their occurrence in settings far removed from any obvious shallow-water source is puzzling, since intrabasinal highs in the Umbria-Marche Apennines had all drowned by the early Pliensbachian. The Lazio-Abruzzo Carbonate Platform is the most likely source-area for ooidal/peloidal/bioclastic calciturbidites of both the Narni-Amelia and the Sabina sectors (~50 km away from the present-day margins of the carbonate platform), while occurrences in the northern part of the Sibillini Mts (~100 km away from those margins) might require alternative sources. Geological mapping shows that the marginal palaeoescarpments of pelagic carbonate platforms formed obstacles to the gravity flows as sediment load was discharged at their toes. While turbidity flows were locally vigorous enough to climb the escarpments, leaving overbank deposits on the pelagic carbonate platform-tops, a “shelter” effect is evidenced by the resediment-free nature of those basins lying downflow, which had to be shielded by pelagic carbonate platforms. Attached in this data are the raw field photographs of the analysed outcrops, clear of the line-drawing layer.