ALBERT

Description: The Baie du Lévrier (some 800 km2, 〈 20 m deep) lies at about 21° N. The climate is arid. Hydrographie conditions are controlled by (1) Trade winds, which largely determine wave base, (2) tidal currents, and (3) upwelling on the shelf nearby. Sediments range from carbonate-poor silty muds to carbonate-rich sands. Below wa ve base sediments characteristically have 〈 30% sand, sand medians 〈 125 μm and 55-80% silt, whereas wave-washed sediments have 〉 70% sand with medians 〉 400 μm. Other dynamic factors, notably tidal currents, may blur the distinction between these two populations. Sources for the terrigenous components are (1) modern aeolian dunes whose sands enter the sea to form a prograding coastal sediment wedge; (2) wind dust deposited as silty muds below wave base; (3) sub-Recent relict material on the seafloor; ( 4) cliff and bank erosion (local); (5) sea bed-load, significant in current-washed areas. Non-terrigenous components include pellets, ooids and skeletal debris. Pellets, incompletely indurated and mainly faecal, form up to 2/3 of the sand fraction in muds below wave base. Relict pellets are rare. Superficial ooids occur in trace quantities in some beach and nearshore samples. Skeletal debris accounts for most of the carbonate sand and also contributes significant quantities of biogenic opal and phosphatic material. Relict and mineralized (goethite and Fe-silicates) skeletal grains are locally important. Like the modern skeletals they are of foramol type. In the modern skeletal sand molluscan material is ubiquitous ( 〈 20% to 〉 90% of skeletal sand) but, as little can be identified further, molluscs as a whole have little interpretative value. Other skeletals form two groups: ( 1) typical of the banks and flanks of the bay, includes barnacles, worm tubes, calcareous red algae, bryozoans and alcyonarian spicules (the two lattcr tend to occur close to rather than on the banks and to lie below wave base rather than in the wave zone as the others do); (2) found in "basinal" sediments below wave base, includes calcarcous, opalinc and phosphatic forms: foraminifers (benthonic and planktonic), echinoderms, ostracodes, sponge spicules, diatoms (planktonic and benthonic) and fish remains. From the relationships between the bank and "basinal" groups five skeletal grain assemblages are recognized and shown to be related to sedimentary facies (ranging from carbonate-poor, silty mud facies with foraminifers, sponge spicules, diatoms and fish remains, to carbonate-rich sand facies characterized by bank group skeletals, particularly barnacles). Within some of the assemblages there are marked and systematic lateral shifts in relative importance of skeletal components. All these features can be interpreted in terms of source and transportability of the grains, some (e.g. barnacle) being moved principally as bed load, while others are carried in suspension. Laterally graded suspensions are proposed to explain the distribution of the components of the "basinal" group and also, near banks (or flanks), of alcyonarian spicules and bryozoans. Proximity to oceanic influences is expressed particularly in fine-grained "basinal" sediments by relative increase of echinoderms, ostracodes and molluscs (particularly Pinna needles) as well as of planktonic foraminifers and diatoms relative to benthonic ones. Significant regional environmental features are expressed in the sediments as follows: Aridity limits terrigenous sediments to a coastal wedge of dune-derived sands, aeolian dust in silty muds below wave base, and products of local cliff erosion. By precluding freshwater "dilution" it allows evaporation to increase salinity and a few ooids form. Combination of offshore upwelling and the Canary Current accounts for the presence of the foramol skeletal association in the tropics, and for the unusually high percentage of biogenic opal (up to 〉 12% of total sample), (cool-water) planktonic foraminifers (up to 〉 25% of total foraminifers), fish debris, and perhaps also of ostracodes. The general water temperature and salinity regime is such that the Baie du Lévrier lies in a critical position on or near the boundaries between major provinces of carbonate sedimentation both for skeletal (foramol, but chloralgal known 150 km to south) and non-skeletal components (normally pellet, but oolithaggregate locally reached).