Clustered, Stacked and Imbricated Large Coastal Rock Clasts on Ludao Island, Southeast Taiwan, and Their Application to Palaeotyphoon Intensity Assessment

Abstract

This work investigated the characteristics of a boulder field on the exposed south east coast of Ludao Island (Green Island) in southern Taiwan. Although the region regularly experiences seasonal Pacific typhoons, fieldwork on Ludao was prompted following the double-strike of Typhoon Tembin in August 2012, which followed an unusual looping track and was one of the strongest storms to affect the island in recent decades. In Wen Cuen Bay, large limestone and volcanic clasts (103–105 kg) occur both as isolated individuals and also grouped into distinct clusters across the gently-sloping emerged reef platform of Holocene age. Some individuals reach megaclast proportions. Observations revealed limited evidence for the production of new coastal boulders by Typhoon Tembin. However, clustering, stacking and notable imbrication of old large clasts provide evidence for multiple high-energy palaeoevents. Stacking and imbrication are significant depositional features, implying that (partial) lifting by wave transport was responsible. Boulders deposited by Typhoon Tembin suggest that storm produced minimum flow velocities of 3.2–5.1 m/s. This range of minimum flow velocity (MFV) values is lower than the 4.3–13.8 m/s range inferred from the pre-Tembin boulders, which indicates that older storm washovers must have been stronger, judging from their ability to stack and imbricate large clasts. One explanation for high upper values of palaeoevent MFVs is that localized funnelling of water flow through narrow relict channels (inherited spur-and-groove morphology, oriented perpendicular to the modern reef edge) concentrates onshore flow energy into powerful confined jets. Support for this hypothesis is the positioning and train-of-direction of the main imbricated boulder cluster at the landward head of one such feature. Geomorphic controls amplifying wave-driven flow velocities across the emerged Holocene reef mean that a palaeotyphoon origin is sufficient for explaining large clast stacking and imbrication, without the need to invoke a tsunami hypothesis.

The Safi boulders (South of Morocco): another evidence of the 1755 Lisbon tsunami?

“…In rocky shores, the detachment and landward displacement of mega-clasts is a known indicator of the occurrence of these extreme events. This type of deposit has been reported in multiple coastal areas around the world: Spain (Lario et al, 2023;Roig-Munar et al, 2019), Portugal (Costa et al, 2011;Oliveira et al, 2020), Morocco (Medina et al, 2011;Mhammdi et al, 2008;Sedrati et al, 2022), Japan (Namegaya et al, 2022;Watanabe et al, 2019), Chile (Abad et al, 2020;Carballeira et al, 2022), Scotland (Hall et al, 2010;Scheffers et al, 2010), Hawaii (Noormets et al, 2002;Richmond et al, 2011) and Taiwan (Ota et al, 2015;Terry et al, 2021). Unlike sandy deposits, boulders have a high probability to be preserved.…”

“…-Some boulder clusters are imbricated and form boulder trains. This arrangement characterizes both storm and tsunami boulder deposits, and is the evidence of a strong and unidirectional current (Terry et al, 2021). The orientation of imbrication is usually parallel to the direction of the incoming waves.…”

A Numerical Modeling Approach for Better Differentiation of Boulders Transported by a Tsunami, Storm, and Storm‐Induced Energetic Infragravity Waves

“…In this study, we conducted a one‐dimensional numerical simulation over the simplified topographies to investigate the general distribution characteristics of the tsunami and storm boulders (Figure 1), so this study has some limitations. First, we noted that local topography, such as slight depression and flow paths induced by three‐dimensional topography, influences the distribution of coastal boulders (Terry et al., 2021); however, these factors are not included in our calculations. Boulder sources are also site‐specific factors.…”

The Imprint of Recent Meteorological Events on Boulder Deposits along the Mediterranean Rocky Coasts

“…CBDs produced by highenergy waves usually have sedimentological evidence of landward transport; thus their origin mechanisms are readily identified. However, ascribing one of the two specific wave processes (tsunami or coastal storm) to CBD of unknown date of deposition is often quite difficult [5,22]. This problem does not exist for the purpose of this review as it deals with the activation of boulders during recent storms, while the occurrence of tsunamis can be excluded.…”

“…However, CBDs produced by high-energy waves have some peculiar characteristics (see Section 2). Thus a geomorphological survey is usually enough to recognize their origin with high confidence [3][4][5]. Boulder production and deposition by wave quarrying are strictly related to coastal erosion and flooding hazard.…”