A critical examination of evidence used to re-interpret the Hornitos mega-breccia as a mass-flow deposit caused by cliff failure.

Abstract

The mega-breccia at Hornitos, northern Chile, was recently re-interpreted as a mass flow deposit caused by cliff failure, without any link to a tsunami backwash or the Eltanin meteorite impact at 2.5 Ma. While agreeing with the latter in the light of new microbiological data, it is here argued that mass flow can also be caused by tsunami backwash events and would be difficult to distinguish from those caused by gravity alone, especially as the Hornitos outcrop is of limited extent. However, a mechanism for downward dyke injection can be postulated for tsunami-related mass flows, but would not be applicable to normal debris flows generated by cliff collapse. The new age range of the Portada Formation coincides with similar deposits at Carrizalillo, Ranquil, Caldera and possibly Caleta Verde, suggesting that one or more mega-tsunamis might have struck the Chilean coastline during the Messinian.