Articles | Volume 38
Adv. Geosci., 38, 55–61, 2016
Adv. Geosci., 38, 55–61, 2016

  29 Feb 2016

29 Feb 2016

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

Michaela Spiske1,2 Michaela Spiske
  • 1Universität Trier, Geozentrum, Behringstr. 21, 54296 Trier, Germany
  • 2Westfälische Wilhelms-Universität, Institut für Geologie und Paläontologie, Corrensstr. 24, 48149 Münster, Germany

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.

Short summary
A coastal ridge on Anegada was surveyed in 2013 and 2015 to document short-term changes in morphology and weathering. Minor storms changed the ridge morphology and added new clasts to the lower parts. Observations imply that the ridge was initially emplaced during a more severe event; minor storms are only able to modify preexisting ridges. Second, bioweathering is a rapid process in tropical settings, as proven by the fact that white coral clasts in 2013 already depicted a grey patina in 2015.