Geomorphic and sedimentologic evidence of extreme wave events recorded by beachrocks: A case study from the island of St. Bartholomew (Lesser Antilles)

摘要

In spite of their preferential location inside tropical belts where extreme wave events occur, and in spite of their potential for rapidly lithifying coastal sediments and by doing so promoting their preservation, beachrocks have so far been poorly investigated for the record of tsunamis or storms. This contribution focuses on beachrock occurrences on the island of St. Bartholomew (French West Indies), which is located in a region at risk from such natural hazards. Geomorphic and sedimentologic criteria were used to characterize beachrocks from shores frequently exposed to extreme wave events. Besides known effects of waves on beachrocks, including exhumation and landward transport of broken slabs, this article reports less well-documented perturbations such as the immersion of beachrock layers and cementation of event deposits. During recent hurricanes, unlithified sands flooring beachrock slabs were washed away by highly turbulent flows, causing the collapse, fragmentation, and displacement of an 80 m long beachrock from the intertidal zone to the subtidal zone between -2 m and -3 m below mean sea level, with broad preservation of its characteristic orientation parallel to the shoreline. This example shows that, in regions subject to episodic extreme waves, submerged beachrocks might not be good indicators of past low stands of the sea because of the possibility they could have been dislocated from their primary intertidal setting. Using the magnitude of taphonomic imprint on reef-derived coarse clasts and their host sediments, rapid-burial event deposits and reworked event deposits cemented in beachrocks have been distinguished. The former represent high-energy wave sediments produced during a single short-lived event that could be a tsunami followed by permanent burial and rapid cementation; the latter contain coarse clasts produced during one or, more likely, multiple catastrophic events, and whose burial and cementation were delayed, thereby promoting their alteration and integration in time-averaged sediments, i.e., sediments produced during different time intervals but deposited together. This distinction is important for coastal-hazard risk assessments because establishing extreme event frequencies by dating event deposits must rely on single rather than composite events. The data collected during this investigation add to our understanding of the signature of extreme waves in beach systems undergoing early cementing processes, and by extension, their counterparts in the fossil record.