Pliocene mass failure deposits mistaken as submarine tsunami backwash sediments - An example from Hornitos, northern Chile

Spiske, M., Bahlburg, H., Weiss, R.

Abstract

In this study we question the former interpretation of a shallow marine backwash tsunami origin of a conspicuousPliocene coarse clastic unit at Hornitos, northern Chile, and instead argue for a debris flow origin for this unit.We exclude a relation to a tsunami in general and to the Eltanin impact in particular. The observed deposit atHornitos was not generated either directly (impact-triggered tsunami) or indirectly (submarine mass flowcaused by seismic shaking) by an impact. Re-calculation of the alleged impact tsunami including considerationof the Van Dorn effect shows that an impact in the Southern Ocean did not cause a significant tsunami at Hornitos.Impact-related seismic shaking was not able to trigger slides several thousands of kilometers away because theEltanin event was a deep sea-impact that did not create a crater. Additionally, the biostratigraphic age of5.1-2.8 Ma of the associated La Portada Formation is not concurrent with the newly established age of 2.511 ±0.07 Ma for the Eltanin impact.Instead, we argue for an origin of the conspicuous unit at Hornitos as a debris flow deposit caused by an earthquakein the Andean subduction zone in northern Chile. Our re-interpretation considers the local synsedimentarytectonic background, a comparison to recent submarine tsunami sediments, and recent examples of masswasting deposits along the Chilean margin. The increased uplift during the Pliocene caused oversteepening ofthe coastal scarp and entailed a contemporaneous higher frequency of seismic events that triggered slope failuresand cliff collapses. The coarse clastic unit at Hornitos represents an extraordinary, potentially tsunami-generatingmass wasting event that is intercalated with mass wasting deposits on a smaller scale.