60,000 years of recurrent volcaniclastic megabed deposition in the Marsili Basin, Tyrrhenian Sea

Megabeds, or ¨megaturbidites¨, are exceptionally large submarine deposits interpreted to originate from significant geohazard events. Megabeds result from rapid discharges of large volumes of sediments from continental margins to the deep ocean. Using high resolution 3.5 kHz subbottom profiler data, we discover four megabeds constituting ~75% of the deposits in the upper 60 meters (~ last 60 ky) of the western Marsili Basin, Tyrrhenian Sea. The megabeds are widespread and imaged as distinct acoustically transparent units with ponded geometries, 10 to 25 m thick, separated by parallel-bedded strata. Each megabed is thinner and volumetrically smaller than the proceeding one. Minimum volume estimates of 1.3, 9.4, 11.8, and 13.3 km³, respectively.  A synthetic seismogram and well tie at Site 650 of Ocean Drilling Program Leg 107 demonstrates that megabeds correspond to a thick unit of mud lying on top of a high amplitude facies of normally graded volcaniclastic silt or sand. Mud deposits are structureless, consistent with a turbidite origin, except for the transparent facies of Megabed 3, which corresponds to a highly deformed muddy debris flow. The detailed well-tie together with previous chronological constraints from volcaniclastics glass chemistry, suggest that Megabeds 3, and possibly 4 may be associated with the 39.8 ka Campanian Ignimbrite super-eruption of the Campi Flegrei caldera, Italy, described among the largest eruptions on Earth. Most likely source areas for the most recent megabeds are the volcanic provinces to the north or south, and the Stromboli canyon-channel system to the east. Surprisingly, the deposit geometries suggest the megabeds are not sourced locally from the Marsili seamount, which is the largest active volcano in Europe. Regardless of source locality, the megabeds entered the Marsili Basin rapidly as evidenced by significant basal erosion and fluid escape structures, some of which remain active. The newly discovered megabeds of the Marsili basin may indicate significant geohazard events for the circum-Tyrrhenian Sea coastlines.