Reworking processes and deposits in coral reefs during (very) high-energy events: Example from a Pleistocene coral formation (125 ka), La Desirade Island, Lesser Antilles
Résumé
Abundant literature has documented the ecological impact of very high-energy (hurricane-type) events and high-energy (storm-type) events on modern coral reefs and sediment transport. In previous studies, sedimentary processes are often deduced from geophysical image analysis and scuba diving observations. Few field-based studies have been devoted to the impact of (very) high-energy disturbance on the structure and ecology of Holocene and Pleistocene coral reefs, and very few to the sedimentary processes acting around these reefs. This research examines the structure of the late Pleistocene (end of MIS5e stage) reefs of La Desirade Island, Guadeloupe Archipelago, Lesser Antilles, and focuses on the poorly known reworking processes and related coral-breccia sedimentation surrounding these reefs. Fieldwork was performed to document two coral-fragment rich breccia interbedded with coral patches. The breccia predominantly show the same petrography but differ in their internal structure, showing a ``massive breccia'' type and a ``stratified breccia'' type. This work infers that these breccia are the result of gravity-induced high-density mass flows triggered by high to very-high energy events such as storms and hurricanes. Damage and reworking vary according to the type of (very) high-energy event. Prior to transport, storms are thought to have broken the heads of branching and columnar coral colonies, and broken and overturn isolated lamellar and massive colonies. Hurricanes (particularly their eye walls) may have reworked much more sediment than storms, and may have also destabilised entire several metre-sized columnar colonies from the flank of coral patch reefs. We propose the name ``coral debrite'' for these coral breccia deposits caused by gravity-induced debris flows. The reworked deposits are interbedded between two coral reef levels, of which the main coral-species vertical succession should record the end of Marine Isotope Stage 5e sea-level rise. Thus, reworking is believed to occur during the transitional stage between a glacial and an interglacial period.