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

Wasserman, Hannah N.; Rankey, Eugene C.; Uriam, Tion

Isolated carbonate platforms record complex depositional patterns and processes and can host prolific hydrocarbon resources. Although reef-reef sand apron-lagoon facies patterns have been well documented on many atolls, the details of sedimentologic patterns and the physical oceanographic processes affecting the sedimentologic variability within and among these geomorphic elements is less well understood. To better understand the facies character and lateral variability in platform-top reef sand apron accumulations, this project integrates remote sensing, field, petrographical, and granulometrical observations of surficial Holocene sediments and physical oceanographic observations and modeling of Aranuka Atoll, Republic of Kiribati.

Aranuka Atoll is influenced by dominant easterly trade winds and an open-ocean annual average swell height of ~2 m. Because of the equatorial location, Aranuka is not influenced directly by tropical storms; therefore, day-to-day processes dominate. Waves are also strongly modulated by semi-diurnal tides; spring tidal amplitude is > 2.5 m. Strong, flood-dominant currents exceed 1.1 m/s in some channels on the reef sand apron, and control circulation and sediment transport. The atoll covers ~72km2 and contains a range of geomorphic elements including forereef, reef crest, reef sand apron, lagoon (including patch reefs), and islands. The largest geomorphic constructs of the atoll are the northern and southern intertidal to subtidal reef sand aprons which extend > 2km from the platform margin toward the lagoon, and consist of reef-derived sediment. These sand aprons are not simple homoclinal planes dipping into the lagoon, however; subtle (> 2 m deep) subtidal to intertidal channels, some of which contain margin-normal barforms on their flanks, focus and direct flow across the sand apron. Physical sedimentary structures such as ripples, dunes, and low amplitude (< 1 m) barforms also occur across these sand aprons. Sediment near the margin on the reef sand apron is well-sorted coral and red algal-rich coarse sand and gravel (> 1 mm), transitioning to poorly sorted foraminifera-rich medium-coarse sand (~0.5 mm) toward the lagoon. Collectively, the results of this study illustrate that selective winnowing and differentiation of sediment size, type, and sorting (e.g., depositional phi/k), and bedform geometries along transport pathways are linked closely to the hydrodynamic patterns across the platform.

AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013