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

<p>Ludao Island in south eastern Taiwan regularly experiences strong Pacific typhoons.&#160; Fieldwork was undertaken to investigate the characteristics of a boulder field comprising massive limestone and volcanic clasts (10³&#8211;10⁴ kg) on the exposed SE coast. &#160;Old large clasts on the Holocene emerged platform provide evidence for multiple high-energy palaeowave events. &#160;Of particular interest were clasts stacked and imbricated together to form distinct boulder trains. &#160;Inferred minimum flow velocities of 4.3&#8211;13.8 m/s were needed for their deposition. &#160;What can imbricated boulder trains tell us about the wave processes and geomorphic influences responsible? &#160;One hypothesis here is that localized funnelling of water flow through narrow relict channels is able to concentrate onshore flow energy into powerful jets. &#160;These channels represent inherited (fossil) spur-and-groove morphology, oriented perpendicular to the modern reef edge, now overdeepened by subaerial karstic solution. &#160;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. &#160;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>