Holocene Facies Patterns from a Tide-Dominated Isolated Carbonate Platform: Nonouti Atoll, Republic of Kiribati, Central Pacific
Facies patterns and sedimentology of isolated carbonate platforms are strongly influenced by waves and tides. As a step towards better understanding these influences, this study documents the depositional patterns of Nonouti Atoll (Republic of Kiribati), a tide-dominated platform.
Nonouti is a large (21 x 43 km), oblong, SE-NW oriented atoll less than a degree south of the equator. As such, it is not directly influenced by tropical storms, and dominant winds and oceanic currents are from the east. Tides of Nonouti exceed 2 m amplitude. Nonouti includes several distinct facies types. On the NE-facing margin, the forereef passes to a 50-100 m wide rocky-red algal-coral reef margin with surge channels. This margin is flanked by a broad (300-800 m) intertidal rocky reef flat, and then rubbly to sandy islands less than 1 km wide. The platformward side of the islands is rimmed by a broad intertidal sand flat up to 1.5 km wide that gradually slopes into the deeper platform interior. The southern margin of Nonouti is characterized by a well-developed coral-red algal barrier reef system (>1 km wide) and sandy reef apron that gradually slopes down to the platform interior. The western margin of Nonouti is unrimmed, and includes a deeper rocky to sandy bottom with only very patchy reef growth. The platform interior reaches depths of up to 25 m, although some circular to elongate patch reefs and banks in the interior are < 1 m deep; banks exceed 5 km in length and 500 m in breadth. Sediments of the interior are dominantly sandy skeletal debris.
The results of this study illustrate facies patterns that contrast with other Holocene tidally influenced platforms such as those of the Bahamas and Caicos, and with wave-dominated platforms of other parts of the Pacific Ocean. These differences are likely reflected in ancient tide- and wave-dominated platforms as well.
AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009