Calciclastics in the Rosso Ammonitico Fm from the Umbria-Sabina Apennine (Central Italy): new evidence for Toarcian synsedimentary tectonics - 3rd International Congress on Stratigraphy - Strati 2019

Toarcian calciclastic deposits made of pelagic carbonate elements embedded in other pelagic carbonates were identified in two key-sectors of the Umbria-Sabina Apennine: the Reatini and Narnesi-Amerini Mts. The studied outcrops pertain to basinal successions of the Umbria-Marche-Sabina type and, in particular, to the Rosso Ammonitico Fm of three different Pelagic Carbonate Platform-Basin systems. The pelagic successions of these areas onlap Early Jurassic rift-related structural highs. These clastic bodies are mass-transport deposits (MTDs) which partially replace the typical Toarcian reddish marls and shales at different stratigraphic levels. The clasts, ranging from blocks to megablocks, are dominated by facies typical of the Corniola Fm (Pliensbachian p.p.), coupled with Rosso Ammonitico Fm intraclasts and, sporadically, Calcare Massiccio Fm (Hettangian-lower-Pliensbachian) extraclasts. The internal architecture (source and accumulation areas) of the MTDs and their potential emplacement processes were identified. Three distinct lithofacies characterise the clastic bodies, each one corresponding to a different emplacement process or to a different portion of the flow. The occurrence of lithified megablocks (>20 m across) of Corniola Fm suggests the exhumation of buried portion of the unit. Synsedimentary extensional tectonics more than differential compaction is the most likely triggering mechanism. Stratigraphic-sedimentological analysis and geological mapping of the study areas reveal the key role played by Pelagic Carbonate Platforms in the genesis of these MTDs. The early Toarcian reactivation of the Hettangian palaeofaults bounding the structural highs is inferred. These faults, characterised by moderate offsets, crosscut the onlap wedges of the hangingwall successions, exhuming the older and lithified portion of Corniola Fm. The planes of the no more active faults were then eroded producing palaeoescarpments. Their backstepping, coupled with seismic shocks, produced the accumulation of the study clastic bodies.