Sediments of the almost-atoll Aitutaki, Cook Islands, South Pacific

Abstract

Modern sedimentary facies in the shallow platform interior of the almost-atoll of Aitutaki, Cook Islands, is significant, because abundant non-skeletal grains have been discovered in this Pacific location and also in some adjacent carbonate platforms. Abundant non-skeletal carbonate grain occurrences have been classically placed in the western Atlantic and the Arabian/Persian Gulf, but not in the Indo-Pacific realm. In order of decreasing abundance, non-skeletal grains in Aitutaki include peloids, aggregate grains, and ooids. Peloids are largely cemented fecal pellets, and their occurrence is controlled by the occurrence of mollusk and crustacean producers in the interior platform. Ooids, which are characterized by thin and irregular cortices, and aggregate grains appear to be more common at the platform margin in environments of somewhat elevated depositional energy. In general, high alkalinity in this part of the south Pacific seems to be another precondition of non-skeletal grain formation. The fine grain-size fraction reaches an abundance of 70% in the platform interior (average 41.5%) and is largely of skeletal origin. Based on the quantitative analysis of texture and composition of samples, two groups of sediment types may be distinguished including coralgal grainstones and packstones that occur at the platform margin as well as peloidal and mixed skeletal-peloidal wackestones and packstones in the platform interior. These two types may be further differentiated into eight sedimentary facies including (1) peloidal wackestone, (2) peloidal packstone, (3) mud-rich skeletal-peloidal packstone, (4) skeletal-peloidal packstone, (5) skeletal-peloidal grainstone, (6) coralgal packstone, (7) coralgal grainstone, and (8) coralgal-aggregate grainstone. Sediment texture is correlated with water depth, which is considered a good proxy for depositional energy. Considerable bathymetric overlap exists, however, between sedimentary facies. Also, facies distribution in the platform interior appears to be fragmentary rather than being characterized by laterally extensive units. These observations have to be considered when using sedimentary facies distribution in modern reefs and carbonate platforms as models for ancient systems in the fossil record.