Beachrocks of the last low sea level, substrate of the barrier reef system along the outer Guiana shelf

Abstract

The Great Amazon Reef System is a living biogenic mesophotic reef ecosystem that has been recently described along the shelf break of Brazil. An exploration mission was carried out in 2019 along the outer edge of the French Guiana Shelf. A side-scan sonar survey was conducted to locate reef outcrops and allowed twelve in situ 80 to 120 m depth dives and sampling of the reef rocks and peripheral sands. The majority of the hard-rocks are composed of biological concretions. However several fragments revealed the inside presence of sandstone clasts. These clasts, more or less enveloped by biogenic coatings, probably represent destroyed clasts of underlying or neighboring beachrock banks. Their dominant cement is micritic (high-magnesian calcite); the intergranular or extragranular porous field was later filled with low-magnesian sparry calcite. The sand or gravel that accumulated near the barrier mainly consist of the blunt debris of coastal fauna and flora associated with different carbonate or ferruginous neoformed ooids. At 104 m depth, ooids extracted from Dive 11 samples dated from the start of MIS2 (27,370 cal yr BP) and attest to the presence of a significant coastal accumulation. At this same site, cementing did not take place until about 3,500 years later (23,990 cal yr BP). The cement of a nearby beachrock indicates a much more recent age (16,170 cal yr BP). Lastly, the age of 4,100 year BP measured on the barnacles attached to the top of the reef attests to the late Holocene reef's biological activity.