Investigating the subsurface structure of the main crater of the proposed Sirente meteorite crater field (Central Italy): new clues from reflection seismics

The Sirente crater field (Central Italy) consists of a main, ≈130 m wide, droplet-shaped in plan view depression with a prominently elevated rim, and 30 smaller depressions in its close vicinity. It was discovered in 2002 and proposed to be of meteorite impact origin based on its geological and geomorphological similarities to known crater fields. It is developed in non-lithified lake sediments (i.e. carbonate mud). Given the age of formation in the 3rd to 5th centuries A.D., the inferred catastrophic origin was, in the media, soon related to the celestial sign ("Chi Rho") said to have been seen by Emperor Constantine in 312 A.D. and suggested to have changed the course of both Roman and Christian history. However, the meteoritic origin is not yet confirmed and has been contested. This work presents new geophysical data on the subsurface structure of the main crater, which provides further clues around the controversy of its origin. Two roughly transversal seismic reflection profiles across the main crater reveal a deep (53 m on average), rootless, bowl-shaped structure with a poorly developed, deep-seated central mound, as well as different seismic facies representing a crater infill of disturbed material resembling the breccia lens in craters formed in rock. This survey also allowed the recognition of possible compaction-fissures and upturned strata below the rim, similar to what is known from explosion craters in porous, unconsolidated targets. Some of these structures had already been noted in published resistivity surveys (ERT). The structural features noted in this geophysical survey are consistent with the impact hypothesis and do not support other proposed mechanisms of formation such as karst or mud volcano.