Seismic architecture and sediment distribution within the Holocene barrier reef–lagoon complex of Mayotte (Comoro archipelago, SW Indian Ocean)

Twenty gravity cores and a large set of high-resolution seismic profiles from various lagoonal settings were studied to determine the Holocene sediment distribution and sequence architecture within the Mayotte barrier reef–lagoon complex. The Holocene seismic sequence comprises a type 1 sequence with lowstand, transgressive and highstand systems tracts. The lowstand systems tract consists of a paleosoil horizon formed during subaerial exposure. The transgressive systems tract is composed of four depositional systems: (1) inner transgressive layer, (2) proximal and distal incised valley fills, (3) mid-lagoonal layer and (4) keep-up or catch-up fringing and barrier reef sequence. The highstand systems tract comprises three depositional systems: (1) of a proximal terrigenous wedge, (2) mid-lagoonal and distal carbonate sands or muds and (3) reefal carbonates. Our studies show that the nature of the Holocene sequence is controlled by the rate and amplitude of sea-level rise and environmental changes, which are expressed by changes in clastic sediment supply and carbonate production. The pre-Holocene topography and water dynamics steer the vertical and spatial sediment thickness distribution of the Holocene. Additional important parameters are the proximity to a source area (carbonate or terrigenous) and the width of the depositional area. Climate dynamics are also of great importance while they determine carbonate production and terrigenous runoff. Sedimentation rates in the subtidal settings always lacked behind sea-level rates. Thus, a steep relief was created keeping most lagoonal parts within the deep subtidal realm in which sediment production was not efficient enough to fill up accommodation space. In addition, wave and/or current energy might prevent the fill up of the lagoon. This ultimately resulted in a typical empty bucket morphology. Only a high amplitude sea-level fall would allow the subtidal lagoon to build up to base-level. Unfilled accommodation space, therefore, must be a very common feature in the geologic record.