Holocene Carbonate Sedimentation in Aitutaki Lagoon, Cook Islands, South Pacific

Bruce M. Richmond, James R. Hein

Aitutaki, an almost-atoll in the Southern Cook Islands, is characterized by shallow enclosed lagoon. Sediment distribution within the lagoon can be broadly placed into three sedimentary-bathymetric provinces. (1) A low-relief reef rim (< 2 m deep), including sand flats and washover fans, is comprised mostly of clean sand and gravel. (2) The majority of the lagoon floor, which lies between 3 and 6 m water depth, is dominated by sand and silt; coral-algal patch reef are common with densities greater than 500 reefs/km². Sediment commonly is coarser grained near the patch reefs. (3) Enclosed and elongate-sinuous topographic lows (basins) up to 10 m deep are marked by coral-algal reef growth along their margins. These features are typically narrow, less than 100 m wide, and are U-shaped in cross section and infilled by carbonate and terrigenous muds.

High-resolution continuous seismic profiling and limited drilling indicate that differences in thickness of Holocene sediment result from primary irregularities in the pre-Holocene basement surface.

Aitutaki was formed by late Miocene volcanism, with a post-edifice building mid-Pleistocene (0.77 Ma) volcanic episode. Two islets within the lagoon are also of volcanic origin, and sinuous coral ridges which extend for several kilometers probably developed on Quaternary lava flows. The coral ridges and meandering enclosed basins appear to be unique to Aitutaki in the Cook Islands.

AAPG Search and Discovery Article #91038©1987 AAPG Annual Convention, Los Angeles, California, June 7-10, 1987.