Coastal boulders as evidence for high-energy waves on the Iranian coast of Makran

Abstract

Coastal boulder deposits attesting to large waves are found along the rocky coast of Makran (Iran) from Chabahar to Lipar. Boulders are either scattered on the rocky coastal platform or accumulated in imbricated clusters. The boulders are mostly rectangular and composed of biogenic calcarenite deriving from the present coastal platform. Significant morphological features observed on the boulders include supratidal karstic pools, sharp broken edges and fractures. Some boulders contain boreholes and shells of marine bivalves, suggesting detachment and transportation from the subtidal zone. The dimensions, elevation and distance from the coastline of 58 representative boulders are documented to estimate their volume, weight and inland displacement. The boulders, weighing up to 18 t, are found up to 6 m above present mean sea level and up to 40 m from the present shoreline. We applied hydrological models to estimate and compare the wave height and inundation distance required to transport the boulders inland. Our results demonstrate that no known or probable storm event on the Makran coast is capable of detaching and transporting the boulders. In contrast, tsunami wave height of 4 m is enough to detach all the boulders from the rocky coast and transport them inland. We conclude that a tsunamigenic origin for boulder deposits is most plausible. Our results imply that the western part of the Makran coast has archived evidence of paleotsunami events, probably generated by large earthquakes at the Makran subduction zone.