Contrasted textural and taphonomic properties of high-energy wave deposits cemented in beachrocks (St. Bartholomew Island, French West Indies)

This study investigates the preservation potential of high-energy wave deposits by beachrocks located along shores contrastingly exposed to storms. In order to discriminate between clast populations originating in different formational and depositional conditions, namely fair-weather vs. extreme wave conditions, the applicability of quantitative textural and taphonomic analyses of sand- to boulder-sized sediments was evaluated using samples from beachrocks and non-cemented coarse-grained coastal deposits. The size, shape and roundness of coarse clasts were characterized by measuring axial dimensions (L, I and S), from which shape descriptors were derived. Taphonomic scores were established at macroscopic and microscopic (thin-section) scales by rating the degree of physical and biologically-mediated alteration of skeletal material. Combined with the examination of depositional morphologies and facies architectures, the procedure enables characterization of fair-weather and extreme wave-generated deposits, and provides criteria to distinguish between single-event deposits and reworked material resulting from the coalescence of sediments produced during multiple events. The distinction of single short-lived event deposits from those reflecting long-term reworking is crucial for correctly interpreting similar deposits in the geological record and evaluating the historical hazard along coastlines. Site-dependent controls such as the nature of available sediments, the geomorphology of coastal depositional settings, and the tectono-climatic context proved to be important for interpreting the origin of the high-energy wave deposits studied here. In the ongoing debate on the signatures of tsunamigenic and storm sediments in fringing coastal settings, the present study demonstrates that one promising research direction is the use of multiple sedimentological criteria to characterize event materials, including their morphology and taphonomy, combined with the precise characterization of site-specific controls. (C) 2011 Elsevier B.V. All rights reserved.