The significance of sand aprons in Holocene atolls and carbonate platforms

Abstract

Sand aprons are located in the back-reef area of atolls and carbonate platforms and form transport pathways of reef-derived sediment into adjacent lagoons and platform interiors. Though there are studies focusing on sand apron sediment dynamics, the knowledge of Holocene sand apron evolution is limited, because hardly any subsurface data are available. In the current model, antecedent topography, i.e., the elevation of the underlying Pleistocene karst surface and platform area are major factors controlling Holocene sand apron development. Because data on marginal reef thickness in Holocene atolls and carbonate platforms are limited, maximum lagoon depth was also used as proxy for the depth to the underlying Pleistocene surface. Sand apron proportions of 122 atolls and carbonate platforms from the Atlantic, Indian and Pacific Oceans were quantified and correlated to maximum lagoon depth, total platform area and marginal reef thickness, when available. Analyses show significant correlations between sand apron proportions and both maximum lagoon depth (r = −0.420; p = 0.000) and total platform area (r = −0.226; p = 0.012). There is no statistically significant correlation (r = −0.364; p = 0.165) between sand apron proportions and depth to Pleistocene surface at platform margins. It is assumed that the lack in correlation can be explained by the limited data set of 16 atolls. Principal component analyses might allow the separation of Atlantic (Caribbean) from Indo-Pacific atolls, which probably arise from spatial and regional variations in sea-level history, i.e., in the Atlantic (transgressive) and Indo-Pacific region (transgressive–regressive), but more subsurface data are necessary to confirm the interpretation. In general, Holocene sand apron formation started when marginal reefs approached relative sea level some 6–3 kyrs BP. Because accommodation space became limited, depositional systems of sand aprons prograded toward the lagoon. Estimated sedimentation rate data of atoll lagoons show that the accumulation of sand aprons is up to three times higher than lagoonal background sedimentation and therefore suited to fill lagoonal accommodation space and create “filled buckets” in small and intermediately sized platforms (up to hundreds of km2 size), even during times of high-amplitude sea-level fluctuations of an icehouse world. Although we postulate an application of the antecedent topography model to Holocene sand apron development, further factors controlling reef development such as hydrodynamics and carbonate production still influence sand apron development and pattern. This study shows the need for further work and knowledge of the subsurface of carbonate platforms and reef sand aprons, which is still limited. Collectively, hydrodynamic (waves, tides and currents), biologic (carbonate production and distribution of organisms) as well as geomorphological (present-day and antecedent topography) factors should be combined, when generating new models for sand apron development to better understand sand apron history in the geologic past.