Storm, rogue wave, or tsunami origin for megaclast deposits in western Ireland and North Island, New Zealand?

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...

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Bibliographic Details
Published in:Proceedings of the National Academy of Sciences - PNAS 2017-12, Vol.114 (50), p.E10639-E10647
Main Authors: Dewey, John F., Ryan, Paul D.
Format: Article
Language:English
Subjects:
Boulders
Comparative analysis
Deposits
Fluid mechanics
Hydrodynamic equations
Mathematical models
Miocene
Oceanography
Origins
Physical Sciences
PNAS Plus
Storms
Tsunamis
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Summary: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.
ISSN:0027-8424
1091-6490
DOI:10.1073/pnas.1713233114