Nature and distribution of the sand fraction components in the Cadiz Bay Bottoms (SW-Spain)

Zusammenfassung

The content and distribution of the terrigenous and biogenic component in the sand fraction of Cádiz bay sediments were analysed. Their relation and variation with the grain size fraction were determined. The components of terrigenous origin represent an average content of 70%. The most important mineral is the quartz with 62%, and trends to accumulate in the finer sizes fraction, while the coarsest sub-fractions, are composed mainly by bioclastics components. The glauconite is the authigenic components more represented in the sand fraction, and appear in dark green colour. In spite of their siliciclastic character, the sediments show an average content of carbonates of the 23%, their origin is fundamentally biogenic. The mollusks are the majority components; its content diminishes in the finest subfractions where they are replaced by the quartz. The foraminifera display generally a good state of conservation, the test is of calcite and sometimes they appear fillers of glauconite. The presence and the nature of these components were found to be related to the petrographic nature of source area and to the condition of erosion affecting to rocky shoals and plioquaternary coastal cliffs present in the study area. Other control factor is related to the hydrodynamic behaviour of each component during the transport and deposit processes.