Coral-rubble ridges as dynamic coastal features – short-term reworking and weathering processes

Abstract

Abstract.A coral-rubble ridge built by storm waves at Anegada (British Virgin Islands) underwent remarkable changes in shape and weathering in a 23-month period. The ridge is located along the island's north shore, in the lee of a fringing reef and a reef flat. This coarse-clast ridge showed two major changes between March 2013, when first examined, and February 2015, when revisited. First, a trench dug in 2013, and intentionally left open for further examination, was found almost completely infilled in 2015, and the ridge morphology was modified by slumping of clasts down the slope and by reworking attributable to minor storm waves. In size, composition and overall condition, most of the clasts that filled the trench resemble reworked clasts from the ridge itself; only a small portion had been newly brought ashore. Second, a dark gray patina formed on the whitish exteriors of the carbonate clasts that had been excavated in 2013. These biologically weathered, darkened clasts had become indistinguishable from clasts that had been at the ridge surface for a much longer time.The findings have two broader implications. First, coastal coarse-clast ridges respond not solely to major storms, but also to tropical storms or minor hurricanes. The modification and reworking of the ridge on Anegada most probably resulted from hurricane Gonzalo which was at category 1-2 as it passed about 60 km north of the island in October 2014. Second, staining of calcareous clasts by cyanobacteria in the supralittoral zone occurs within a few months. In this setting, the degree of darkening quickly saturates as a measure of exposure age.

Extreme waves in the British Virgin Islands during the last centuries before 1500 CE

“…Ridge-altering storms were therefore supposed greater than 2010 Hurricane Earl. However, the trench dug in 2013, and left open at that time, was found in 2015 to have been partly filled (Spiske, 2016). In addition, clasts at the trench-fill surface had already turned gray.…”

“…The coral rubble ridge on Anegada's north shore differs from the fields of scattered coral clasts in landscape position, internal structure, clast size, clast composition, and age. Here we summarize published descriptions (Spiske and Halley, 2014;Spiske, 2016), cite ridge clast ages obtained by Xu et al (2015), and briefly compare the ridge with storm deposits on rocky coasts elsewhere. Figs.…”

“…Geological effects of 2010 Hurricane Earl were examined at Anegada in February 2011, six months post-storm, with surveys of high-water lines and of predominantly fine-grained deposits (Atwater et al, 2014). The storm's effects were considered further in 2013 and 2015 with studies of a coral rubble ridge (Spiske and Halley, 2014;Spiske, 2016).…”

“…Downloaded from https://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/13/2/301/1001043/301.pdf by guest , 2009, 2016Manuscript drafts UtB 2007, 2009 Note: Yong Wei's participation was partly funded by the Joint Institute for the Study of the Atmosphere and Ocean (JISAO) under National Oceanic and Atmospheric Administration (NOAA) Cooperative Agreement NA10OAR430148. This report is JISAO contribution 2467 and NOAA Pacific Marine Environmental Laboratory contribution 4322.…”

Coastal erosion and sediment reworking caused by hurricane Irma – implications for storm impact on low‐lying tropical islands

“…The study takes advantage of available pre‐event field data collected on Anegada since 2011 (Atwater et al, 2012, 2014, 2017; Buckley et al, 2012; Cescon, 2014; Pilarczyk & Reinhardt, 2012; Reinhardt et al, 2012; Spiske, 2016; Spiske & Halley, 2014; Watt et al, 2012). The data includes, but is not limited to, global positioning system (GPS)‐surveys, light detection and ranging (LiDAR) images, rates of coastal erosion or accretion under normal conditions, and the documentation of deposits of historical tsunamis and storms.…”