Isolated carbonate platforms and atolls occur throughout the geologic record, from Archean to present. Although the roles of sea-level change, tectonics, and sediment supply on large-scale changes in stratigraphical architecture of these systems are relatively well understood, the details of the nature and controls on sedimentological variability between and within individual geomorphic elements on platforms and atolls are less well constrained. The purposes of this study are (1) to describe, compare, and contrast geomorphological and sedimentological characteristics of surficial Holocene sediments from two south Pacific atolls (Aitutaki and Maupiti) representing different physical and chemical oceanographic settings, and (2) to explore possible causes for sedimentological heterogeneity by contrasting the patterns with those present on other platforms and regions.
Both atolls include annular shelf-edge reef systems with a red algal rim and reef flat, flanked lagoonward by reef sand aprons up to 1,500 m wide. These aprons consist of moderately sorted coarse sand and gravel, with an abundance of Halimeda, coral, and red algal fragments; broadly, sediments become finer and better rounded away from margins. Water depths increase abruptly inboard of these reef sand aprons, into platform interiors with depths up to 25 m (Maupiti) and 9 m (Aitutaki). Although patch reefs dot these lagoons, sediment is commonly poorly sorted with bimodal and polymodal size distributions and includes a mix of whole and fragmented skeletal grains, peloids, and ooids; mud and fine sand are abundant (up to 80% on Maupiti, 85% on Aitutaki).
The occurrence of peloids, fine ooid sand, and oolitically laminated skeletal grains on Maupiti Atoll (as previously documented on Aitutaki) is consistent with interpretations of the impact of elevated regional ocean total alkalinity (TA) and pH levels in this part of the Pacific. On these wave-dominated atolls, open-ocean waves impinging on the platform margins generate currents that transport sediment from the reef, across the sand apron, and towards the platform interior. Unlike the tide-dominated platforms of the Bahamas, however, weak tides preclude strong daily agitation and circulation, so ooids do not grow large, and mud and fine sand transported into the atoll interiors is sequestered there. In the interiors, both sediment produced in situ and fine allochthonous sediment are more influenced by biological processes than by physical transport processes, so bioturbated muddy sand is most common. Collectively, the comparative analysis provides actualistic, process-based conceptual models for the nature and controls on variability of shelf-margin reef and reef apron systems and platform interiors, within and among atolls.