Imbricated trains of massive coastal rock clasts (103–104 kg) on Ludao Island, Taiwan: what they can and cannot tell us about palaeotyphoons

Abstract

<p>Ludao Island in south eastern Taiwan regularly experiences strong Pacific typhoons.  Fieldwork was undertaken to investigate the characteristics of a boulder field comprising massive limestone and volcanic clasts (10<sup>3</sup>–10<sup>4</sup> kg) on the exposed SE coast.  Old large clasts on the Holocene emerged platform provide evidence for multiple high-energy palaeowave events.  Of particular interest were clasts stacked and imbricated together to form distinct boulder trains.  Inferred minimum flow velocities of 4.3–13.8 m/s were needed for their deposition.  What can imbricated boulder trains tell us about the wave processes and geomorphic influences responsible?  One hypothesis here is that localized funnelling of water flow through narrow relict channels is able to concentrate onshore flow energy into powerful jets.  These channels represent inherited (fossil) spur-and-groove morphology, oriented perpendicular to the modern reef edge, now overdeepened by subaerial karstic solution.  Support for this idea is the location and train-of-direction of the main imbricated boulder cluster at the landward head of one such feature.  Geomorphic controls amplifying wave-breaking flow velocities across Ludao's coastal platform mean that a palaeotyphoon origin is sufficient to account for large rock clast stacking and imbrication, without recourse to a tsunami hypothesis.</p>