Physical and Chemical Oceanographic Influences on Sedimentology and Geomorphology of Equatorial Carbonates: Aranuka Atoll, Gilbert Island Chain, Republic of Kiribati, Pacific Ocean

Isolated platforms are an iconic carbonate geomorphic construct. Although general concentric reef-reef sand apron-lagoon facies pattern is common and well-documented in many atolls, the details of sedimentologic patterns and processes controlling sedimentologic differentiation are less well constrained. To develop more rigorous predictive conceptual models for the facies character and dynamics of platform-top reef sand apron and lagoon accumulations, this study describes field, petrographic, and observations of surface and shallow subsurface Holocene sediments from Aranuka Atoll, in the equatorial Republic of Kiribati.

The triangle-shaped atoll covers ~10^7 km2. One island stretches between the NE and SE apices, and another occurs near the western apex. The atoll is rimmed by an annular reef (with only one pass), and between islands, expansive shallow reef sand aprons extend from the reef in excess of 2 km onto the platform. The northern, intertidal apron is rocky, reefal, and rubbly within a few 100 m of the shelf edge, and passes onto the platform into sand, with a concomitant on-platform change in dominant grain types, from coral-red algal-dominated to foraminifera-dominated. This apron includes several elongate, margin-normal, low-amplitude (<1 m) barforms in the lee of earlier Holocene islands. These barforms are coarser than the deposits of flanking aprons, and include superimposed gravel and coarse sand subaqueous dunes 20-50 cm high with crests oblique to the margin. This apron abruptly dips into the lagoon. The southern apron includes elongate, margin-normal coral ridges up to 1 km long that stand ~30-60 cm higher than intervening sand- and gravel-filled troughs. This reef-and-ridge morphology passes gradationally into a shallow subtidal sand apron and the lagoon with ubiquitous patch reefs.

Collectively, and by comparison with Bahamian platforms and other atolls in the Pacific, these patterns are interpreted to reflect the importance of on-platform physical transport influenced by waves and tides, and the elevated nutrient levels of open-ocean surrounding this equatorial atoll. These controls, and the sedimentologic and geomorphic products, may serve as conceptual models for variability in subsurface reservoir analogs.

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California