Sedimentary Facies of Bora Bora, Darwin's Type Barrier Reef (Society Islands, South Pacific): The Unexpected Occurrence of Non-Skeletal Grains

Abstract

Abstract “Barrier reefs, when encircling small islands, have been comparatively little noticed by voyagers; but they well deserve attention. In their structure they are little less marvelous than atolls, and they give a singular and most picturesque character to the scenery of the islands they surround.” (Darwin 1842, p. 2) Sediment sampling revealed six modern sedimentary facies in the barrier-reef system around the volcanic island of Bora Bora including mixed skeletal packstone, peloidal packstone, Halimeda-rich packstone to grainstone, coralgal grainstone, wackestone, and mudstone. Whereas sediments rich in corals and coralline algae are abundant on reefs and carbonate shoals, mud-rich sediments with mollusks and foraminifera occur in deep lagoons (15–40 m). Carbonate mud is composed largely of aragonite needles, probably derived from codiaceans like Halimeda, and coccoliths in moderate abundances. Despite the proximity to a volcanic island, the content of siliciclastics in the sediments is low (< 10% on average), which is a consequence of the absence of permanent water courses. In addition to skeletal grains, non-skeletal grains (cemented fecal pellets; aggregates) occur in high abundance (30–50%) in shallow lagoons (< 5 m) on the eastern side of Bora Bora. This observation opposes the widely-held concept that non-skeletal carbonate grains are abundant on platforms from the Atlantic Ocean but virtually missing in recent Indo-Pacific reefs and carbonate platforms. In Bora Bora, non-skeletal grains and aggregates apparently form in warm, shallow (< 5 m), moderately agitated areas with relatively low sedimentation rates. In addition, early cementation is probably favored by elevated alkalinity and pH in surface waters in this part of the south Pacific. In the same area, some 900 km to the southwest of the study area, ooids and peloids have recently been discovered in the shallow lagoon of Aitutaki, Cook Islands. More occurrences of abundant non-skeletal grains probably exist in the Indo-Pacific region; however, too few locations have been systematically studied. A comparison of the new Bora Bora data with other modern reefs and platforms shows that sediment occurrence and distribution exhibits basic patterns in that marginal areas are dominated by coarse-grained coralgal sediment whereas lagoons and platform interiors are characterized by mollusk, foram, Halimeda, and non-skeletal grains, and fines. Still, grain abundances may vary significantly among locations, and again suggest the need for more studies in order to fully understand the factors controlling occurrence and distribution of modern sediments.