Sediment distribution and facies of modern tropical reefs and carbonate platforms revisited: A global perspective

Surface sediment samples (n = 886) collected on tropical reefs and carbonate platforms in the western Atlantic, the central and eastern Indian Ocean, the south and west Pacific Ocean, and the Persian Gulf have been revisited and examined in their entirety. The samples stem from twelve depositional environments, i.e., fore reef, reef margin, sand apron, shallow lagoon, deep lagoon, restricted lagoon, patch reef, island, nearshore; inner ramp, mid ramp, and outer ramp. Grain composition, texture, mineralogy, and geochemistry have been analyzed in a quantitative and statistical manner. Fragments of coral skeletons, calcareous algae (red coralline algae, Halimeda), mollusk shells, and non-skeletal grains (predominantly peloids) are the most abundant constituent carbonate grains. Coralgal, grain-supported textures are common at the margins whereas mud- and grain-supported textures with mollusks, Halimeda, and non-skeletal grains are ubiquitous in rimmed platform interiors. Non-rimmed platforms are characterized by non-skeletal grains on the inner ramp, while mid and outer ramps are rich in mollusk shells. Multivariate statistics of composition and texture data have been used to discern fourteen facies including (1) quartz-rich marl, (2) quartz sand, (3) foraminiferal grainstone to packstone, (4) mollusk-foraminiferal grainstone to packstone, (5) mollusk grainstone to packstone, (6) non-skeletal grainstone to packstone, (7) Halimeda grainstone to packstone, (8) coral-rich grainstone, (9) quartz-rich biogenic grainstone, (10) mollusk packstone, (11) coral packstone, (12) non-skeletal packstone to wackestone, (13) wackestone, and (14) mudstone. There is a statistically significant correlation between the amount of fines (‘mud’) and water depth, i.e., depositional energy, however, individual facies exhibit large depth ranges, thereby underlining previous studies that have challenged intuitive and long-standing concepts of sediment distribution. An exception to this scheme is the ramp system in the northern Persian-Arabian Gulf where facies boundaries approximately follow bathymetric contours. Aragonite (83.2 ± 0.6 % relative abundance ± standard error of mean) and high-magnesium calcite (11.1 ± 0.6 %) are the most common carbonate phases reflecting skeletal mineralogy of producers. The abundance of low-magnesium calcite is lowest (5.2 ± 0.1 %). Stable isotopes of oxygen (δ¹⁸O: −3 ‰ to +3 ‰) and carbon (δ¹³C: −2 ‰ to +6 ‰) of bulk samples exhibit wide ranges and appear to be controlled principally by region rather than by depositional environment within reefs and carbonate platforms. Western Atlantic and south Pacific platform samples show the highest δ¹³C, as a consequence of the occurrence of abundant non-skeletal grains, which are largely lacking in the open Indian Ocean samples. Samples from the Persian Gulf and Shark Bay reach the highest δ¹⁸O due to elevated salinity in surface waters. High amounts of Halimeda platelets in bottom sediments appear to be more abundant in the western Atlantic (platform interiors with restricted circulation) as compared to Indo-Pacific platform sediments. There is evidence that non-skeletal grains are found preferentially where carbonate supersaturation is high and skeletal production and sedimentation rates are low; these preconditions are fulfilled in the western Atlantic, the Persian-Arabian Gulf, Shark Bay, and in certain areas of the south Pacific. The origin of mud is still controversially discussed, but from a modern global perspective it appears that biogenic (detrital) origins predominate over abiogenic origins.