Study of the sedimentary transference between the Bay of Cadiz and the adjacent continental shelf: Model of transport by means of the use of clay minerals as natural tracers (Spanish text, Spain)

The recent sediments of the Cadiz bay and adjacent continental shelf have been studied in order to know the sedimentary exchange processes between the adjacent emerged lands, the coastal zones and the continental shelf. The characteristics of the recent sedimentary infilling, facies distribution, mineralogical associations, source area and degree of textural maturity and reworking have been established.; The distribution of depocenters and sedimentary processes is influenced by morfoestructural control. The Sediments have siliciclastic character. The quartz is the major component in sandy areas and philosilicates in muddy sectors. The main carbonate mineral is the calcite and has a biogenis origin. From granulometric point of view, three sectors are distinguished, the sandy external sector, the inner other with predominance of argillaceous muds and salt marsh and emerged tidal flat zones constituted by sandy muds sediments.; Four sedimentary facies have been differentiated: (a) bioclastic quartziferous silty-sand, (b) bioclastic quartziferous sandy-silt, (c) bioclastic quartziferous phillosilicated silty-clay and (d) quartziferous bioclastic sandy gravel. The sedimentary dynamics and facies distribution are mainly controlled by the hydrodynamic system, rate of fluvial supply and sea-level changes.; In the heavy fraction two mineralogical associations are differentiated: (a) zircon > hornblende > andalusite and (b) garnet > topaz > tourmaline > rutile. Both of them coincide of those found in different sedimentary units present in the neighbouring continental areas. The exoscopic analysis of heavy minerals and quartz grains indicates a high reworking grade, policyclic character of the sediments and diversification of source areas.; The most abundant clay minerals are Illite in recent sediments and Smectite in suspension matter. The observed mineralogical associations are: illite >> chlorite > kaolinite > smectite > illite-smectite in the Cadiz bay and salt marshes and emerged tidal flat zones, and illite >>> smectite > illite-smectite > kaolinite > chlorite in the adjacent continental shelf. These are similar to mineralogical associations of adjacent continental areas, with differences observed in secondary minerals.; The transport dynamics of fine sediments in Cadiz bay and the continental shelf has been determined by using clay minerals as natural tracers. Two flow paths have been established: Out flows proceeding from Cadiz bay and littoral zones and In flows coming from external marines zones. These flows are controlled by tidal currents. Surge current generated during dominant winds are also important.