Facies and faunal assemblage changes in response to the Holocene transgression in the Lagoon of Mayotte (Comoro Archipelago, SW Indian Ocean)

This paper documents the facies change in response to the Holocene transgression within five sediment cores taken in the lagoon of Mayotte, which contain a Type-1 depositional sequence (lowstand, transgressive and highstand deposits underlain by an erosive sequence boundary). Quantitative compositional analysis and visual examination of the bioclasts were used to document the facies changes. The distribution of the skeletal and non-skeletal grains in the lagoon of Mayotte is clearly controlled by (1) the rate and amplitude of the Holocene sea-level rise, (2) the pre-Holocene basement topography and (3) the growth-potential of the barrier reef during sea-level rise, and the changes in bathymetry and continuity during this period. The sequence boundary consists of the glacial karst surface. The change-over from the glacial lowstand is marked by the occurrence of mangrove deposits. Terrigenous and/or mixed terrigenous-carbonate muds to sandy muds with a mollusc or mollusc-ostracod assemblage dominate the transgressive deposits. Mixed carbonate-siliciclastic or carbonate sand to gravel with a mollusc-foraminifer or mollusc-coral-foraminifer assemblage characterize the early highstand deposits on the inner lagoonal plains. The early highstand deposits in the outer lagoonal plains consist of carbonate muds with a mollusc-foraminifer assemblage. Late highstand deposits consist of terrigenous muds in the nearshore bays, mixed terrigenous-carbonate sandy muds to sands with a mollusc-foraminifer assemblage on the inner lagoonal plains and mixed muds with a mollusc-foraminifer assemblage on the outer deep lagoonal plains. The present development stage of the individual lagoons comprises semi-enclosed to open lagoons with fair or good water exchange with the open ocean

Compositional variations in calciturbidites and calcidebrites in response to sea-level fluctuations (Exuma Sound, Bahamas)

compositional variation of Pleistocene carbonate gravity deposits from the Exuma Sound Basin, Bahamas, was determined. Two types of gravity deposit were present in the cores of ODP Leg 101, Site 632A, i.e., calciturbidites and calcidebrites. In analogy with earlier studies, the compositional variations in the calciturbidites could be linked to different sources on the carbonate margin, i.e., platform interior, platform edge, and platform slope. Calciturbidites deposited during interglacial, sea-level highstands show a dominance of non-skeletal grains, largely derived from the platform interior, while calciturbidites of glacial, sea-level lowstands, show a dominance of skeletal platform-edge to platform-slope-derived grains. Thus, the calciturbidite composition can be used to reconstruct the position of absolute sea level. In addition, the mud content of the calciturbidites increased after Marine Isotope Stage 11. In contrast, the composition of the calcidebrites remained unaltered through time and showed a clear dominance of platform-edge-derived sediments during varying sea-level positions. The Bahamian carbonate platform is located in a tectonically stable passive-margin setting and the gravity-flow deposits were laid down in an environment exclusively controlled by eustatic sea-level fluctuations. This study shows that all types of gravity-induced carbonate deposits, calciturbidites, and calcidebrites, were deposited in response to global eustatic sea-level variations. The sediment composition could be linked directly to sediment input from specific facies realms along the carbonate platform margin. Hence, sediment composition analysis is a strong tool that may be used to discriminate between gravity-induced deposition triggered by eustatic sea-level changes and that related to tectonic events, when analyzing resedimentation processes in sedimentary basins.Geoscience & EngineeringCivil Engineering and Geoscience

Controlling Parameters on Facies Geometries of the Bahamas, an Isolated Carbonate Platform Environment

The Bahamas are among the most extensively studied carbonate regions in the world, and a number of phenomena typical of calcareous environments have been first observed in the Bahamas. Early geological research in the Bahamas was undertaken by Nelson (1853B) who surveyed their geography and topography. He noticed the “remarkable lowness of profile” and the dynamics of construction and destruction of the islands, outlined the biota and lithologies, described the formation of the carbonate rocks, and noticed the eolian origin of many Bahamian islands. Forty years later, the examination of modern carbonate environments rapidly progressed with the expedition of L. and A. Agassiz in 1893 (Agassiz 1894). Their explorations focused mainly on the fringing reefs of GE Great Bahama Bank. Research on abiotic carbonate components followed, by Vaughan (1914) who emphasized that carbonate constituents can originate from both skeletal secretion and chemical precipitation, and introduced the terms “organic” and “inorganic” limestones. Black (1933) first characterized the sedimentary facies on Great Bahama Bank and noted the significance of the widespread aragonitic mud. The sand-sized calcareous components of the Bahamas and their origin, including ooid sands, were described in detail in the classic papers by Illing (1954) and Newell et al. (1960)