Citation

Dewey JF, Ryan PD. Proc. Natl. Acad. Sci. U. S. A. 2017; 114(50): E10639-E10647.

Affiliation

School of Natural Science, Earth and Ocean Sciences, National University of Ireland Galway, Galway H91 TK33, Ireland jfdeweyrocks@gmail.com.

Copyright

(Copyright © 2017, National Academy of Sciences)

DOI

10.1073/pnas.1713233114

PMID

29180422

Abstract

The origins of boulderite deposits are investigated with reference to the present-day foreshore of Annagh Head, NW Ireland, and the Lower Miocene Matheson Formation, New Zealand, to resolve disputes on their origin and to contrast and compare the deposits of tsunamis and storms. Field data indicate that the Matheson Formation, which contains boulders in excess of 140 tonnes, was produced by a 12- to 13-m-high tsunami with a period in the order of 1 h. The origin of the boulders at Annagh Head, which exceed 50 tonnes, is disputed. We combine oceanographic, historical, and field data to argue that this is a cliff-top storm deposit (CTSD). A numerical model for CTSDs is developed which indicates that boulder shape in addition to density and dimensions should be taken into account when applying hydrodynamic equations to such deposits. The model also predicts that the NE Atlantic storms are capable of producing boulderites that, when size alone is considered, cannot be distinguished from tsunamites. We review the characteristics that identify the origins of these two deposits.

Language: en

Keywords

megaclast; storm; tsunami