Depositional record of confined turbidites in syn-subduction intraslope basin: insight from the Tufiti di Tusa Formation (Southern Apennines, Italy)

A detailed lithostratigraphy and facies analysis of a type section of the Tufiti di Tusa Formation, including deepmarine clastic successions with syn-orogenic volcanic detritus and deposited in the late Eocene - early Miocene basin system at the front of the growing Maghrebian - Southern Apennines orogen, is discussed in the paper. Based on facies analysis and composition, the study section was subdivided into the following units, from bottom to top: Unit I, mostly formed by contained-reflected beds (including a bed similar to the Contessa megabed of the Marnoso-arenacea Formation in the Northern Apennines), with the ratio of sandstone intervals to mudstone intervals (S/M) of 0.6 and with mostly-calciclastic sandstone to siltstone fraction; Unit II, recording a moderate decrease in contained-reflected beds and a moderate increase in slurry beds, with S/M ratio of 0.9, and with mostly-siliciclastic sandstone to siltstone fraction; Unit III, recording a further decrease in contained-reflected beds and an evident increase in slurry beds and very-thick beds with a basal massive very coarse to coarsegrained sandstone, with S/M ratio of 2.5, and with mostly-volcaniclastic sandstone to siltstone fraction. In accordance with the depositional models for the infill of confined turbidite basins, Units I and II are here interpreted as representing a flow ponding depositional phase, while Unit III as iconic of a flow stripping depositional phase. The compositional variation from Unit I to Unit II records cutoff of calciclastic supply from underplate sources, possibly tied to tectonic uplift of the external basin margin; while that from Unit II to Unit III records sudden availability of volcaniclastic sediment possible due to burial of morphological high(s) between the internal volcanic arc (source of the volcaniclastic sediment) and the depositional basin, and/or establishment of tectonically-controlled conduits cutting the above high(s). This study may improve the knowledge not only of infilling evolution of confined turbidite basins, but also of the depositional setting of the late Paleogene Southern Apennines subduction margin in the Central Mediterranean.