Heterogeneity and Depositional Variability of Reef Sand Aprons: Integrated Field and Modeling of the Dynamics of Holocene Aranuka Atoll, Republic of Kiribati, Equatorial Pacific

Abstract

Depositional facies represent the net product of a complex set of processes that impact sediment supply and transport through geomorphic systems. Although the general facies motifs of many isolated platforms throughout the geologic record are well documented, the details of geomorphological and sedimentological patterns, and the physical oceanographical processes controlling sedimentological differentiation, are less well constrained. On isolated carbonate platforms, accumulation of reef-derived debris in platform-top reef sand aprons form expansive geomorphic elements, and can host prolific hydrocarbon reserves. To better understand the nature and scale of reef sand apron accumulations, this project integrates remote sensing, field, petrographical, and granulometrical observations of surficial Holocene sediments with physical oceanographical observations and modeling of Aranuka Atoll, Republic of Kiribati in the western equatorial Pacific. These results illustrate trends in hydrodynamics, geomorphology, and sedimentology from the platform margin to the platform interior. Current meter data and modeling illustrate how the tides (2.5 m spring tidal amplitude) modulate wave energy (open-ocean, annual average swell height of ~2 m; distal swell height can be larger) to produce dominant on-platform flow (speeds up to 90 cm/s) on the northern reef sand apron. These hydrodynamical influences are interpreted to have led to the development of the expansive northern reef sand apron (2000 m wide); the southeastern apron, with currents that reverse with the tides, includes a narrower sand apron. Concomitantly, the hydrodynamical patterns and platformward decrease in energy across the reef sand apron, coupled with changes in biota, are interpreted to control variability in sedimentary structures, bottom types and sediment attributes. Sediment near the margin on the reef sand apron contains well-sorted coral and red algal-rich coarse sand and gravel, transitioning to poorly sorted, foraminifera-rich, medium to coarse sand toward the lagoon. The lagoon includes even finer sediment. Collectively, the results of this study illustrate that selective winnowing,differentiation of sediment size, type, and sorting (e.g., depositional porosity and permeability), and nature and size of geomorphic elements, are linked ultimately to the hydrodynamical patterns across the platform. The results of this study provide a predictive conceptual model for the depositional variability and processes active on reef sand aprons, including some ancient reservoir analogs.