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

James P Terry,A.Y Annie Lau,Yuei-An Liou,Kim Anh Nguyen,Adam D Switzer

doi:10.3389/feart.2021.792369

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.

Highlights

Boulder deposits attributable to high-energy wave (HEW) action have been found in a variety of coastal settings, such as submarine forereef slopes (Etienne, 2012), beaches (Lau et al, 2015), rocky shorelines (Jones and Hunter, 1992; Paris et al, 2011), coral reef platforms (Nott, 1997; Terry et al, 2013), elevated terraces (Sussmilch, 1912; Ota et al, 2015; Mottershead et al, 2017), and cliff tops (Williams and Hall, 2004; Hall et al, 2006; Roig-Munar et al, 2018)
A prominent characteristic of the boulder field on Dahu Reef is the presence of imbricated boulders among stacks of mixed lithology
Imbrication always begins with an obstacle at the landward end (Scheffers and Kinis, 2014)

Summary

Introduction

Wave-Transported BouldersDiscussions surrounding the origins of coastal boulder deposits have drawn upon field investigations worldwide (e.g., Scheffers, 2004; Mhammdi et al, 2008; Hansom and Hall, 2009; Scheffers et al, 2009; Goto et al, 2010a,b; Switzer and Burston, 2010; Engel and May 2012; Etienne and Terry, 2012; Kennedy et al, 2017; Erdmann et al, 2018). Boulder deposits attributable to high-energy wave (HEW) action have been found in a variety of coastal settings, such as submarine forereef slopes (Etienne, 2012), beaches (Lau et al, 2015), rocky shorelines (Jones and Hunter, 1992; Paris et al, 2011), coral reef platforms (Nott, 1997; Terry et al, 2013), elevated terraces (Sussmilch, 1912; Ota et al, 2015; Mottershead et al, 2017), and cliff tops (Williams and Hall, 2004; Hall et al, 2006; Roig-Munar et al, 2018). Assigning a specific wave process to boulder deposits of unknown origin is often difficult (Bryant and Nott, 2001; Noormets et al, 2002, 2004; Felton and Crook, 2003; Kennedy et al, 2007; Etienne and Paris, 2010; Switzer and Burston, 2010), and this continues to be a worthwhile focus of study

Methods

Results

Conclusion