Holocene ooids of Aitutaki Atoll, Cook Islands, South Pacific

Abstract

Although oolitic sands are widespread throughout the Phanerozoic carbonate rock record, and they are abundant in some modern shallow-marine carbonate systems, recent ooids have not been recognized from any locality in the Pacific Basin. On Aitutaki Atoll (Cook Islands, South Pacific), oolitic sands occur in water depths of 2.1–6.3 m within the lagoon and, less abundantly, on shallower reef aprons. Most ooids are between 150 and 300 μm in diameter, have between 1 and 25 concentric to irregular laminae, and occur with peloids and skeletal grains. Cortical laminae include Mg-calcite crystals with radial, tangential, and random orientations. The occurrence and distribution of oolitic sands on Aitutaki are facilitated by a convergence of hydrodynamic and chemical factors. Wave-driven currents, combined either with flow separation over a sharp increase in depth at the reef apron–lagoon interface or with oceanward flow in the lee of islands, serve to transport ooids without flushing them from the system. Here, pH, alkalinity, and carbonate supersaturation are at some of the highest levels in the tropical Pacific. The restricted occurrence of Holocene oolitic sands in areas of the Pacific and Atlantic Basins with elevated pH and total alkalinity reflects the importance of carbonate saturation thresholds in limiting the spatial distribution of these sands in modern shallow-marine settings. These recent examples are consistent with interpretations of the importance of these variables in determining the distribution, abundance, and cortical mineralogy of oolitic sands throughout the Phanerozoic stratigraphic record of carbonate accumulation.