Holocene Ooids of Aitutaki Atoll, Cook Islands, South Pacific
Ooids occur throughout the Archean, Proterozoic, and Phanerozoic rock record and are ubiquitous in many Holocene shallow marine carbonate provinces. Given their widespread spatial and temporal occurrence, a lingering question in carbonate sedimentology has remained - why are there no ooids in recent marine sediments of the Pacific Basin? The purpose of this study is to describe the occurrence and nature of Holocene oolitic sediments from Aitutaki Atoll, Cook Islands, South Pacific, the first documented occurrence of recent marine ooids from the Pacific Ocean.
On Aitutaki, ooids occur in water 2.1 - 6.3 m deep in shrimp- and worm-burrowed sediments within the platform interior lagoon; they have not been observed in sediments from shallower water on the hoa (inlets) or reef apron. They are most common just inboard of the windward reef apron and hoa, where at some locations, they constitute nearly 50% of the grains. They occur in low abundance (<2 %) in scattered samples from across the lagoon, however, and increase in abundance again near the leeward reef apron. Most ooids are between 200 - 300 μm in diameter, have between 1 and 20 regular, concentric to discontinuous laminae, and occur with peloids, composite grains and skeletal fragments. They appear dissimilar from Bahamian examples in that: 1) they are not found in shallow, high energy locations, but in deeper, lower-energy areas; and 2) they are generally smaller (< 300 μm versus > 350 μm for many Bahamian examples).
Discovery of ooids on Aitutaki resolves a lingering question - ooids are found in Holocene sediments of the Pacific Ocean. Yet, it raises even more: what are the conditions that favor formation and accumulation of ooids in these deeper, low-energy environments? Why are they absent in nearby high-energy, shallow environments? Are these conditions unique to Aitutaki, or do they occur elsewhere in the Pacific? Clearly, many questions remain in our understanding of Holocene ooids.
AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009