The Geochemistry of Some Recent Argillaceous Sediments

Abstract

The nature of sedimentation processes, the mineralogy and the geochemistry of the argillaceous sediments from the Solway Firth mud flat and from Dowdeswell Reservoir Residuum have been investigated. The results of the investigation have enabled the diagenesis of the sediments to be studied with particular reference to the role of iron. The mud flat sediments have a mean size between 3.98 and 4.87 Ø and are moderately to poorly sorted. The residuum sediments are finer (mean size 5.26 to 6.78 6 Ø) and are very poorly sorted. In each case a diminishing transport energy is the prime factor in the sedimentation, but on the mud flat the physico-chemical process of flocculation is also involved. The mineralogy of the two sediments, identified by X-ray diffractometry, is similar, The sediments from the Solway Firth are dominated by quartz, chamosite and illite whereas, at Dowdeswell, kaolinite, calcite and quartz are the major constituents. The successful impregnetion of the soft muds allowed thin, and polished, sections to be studied and these proved invaluable in the study of mineral diagenesis. The evidence for diagenetic chamosite, glauconite, iron sulphides, oxides and hydroxides is clearly visible in such sections. The geochemistry has shown the association of trace elements (Cu, Pb, Zn etc.), aluminium and iron with clays. The major concentrations of trace elements in the Solway Firth occur in the upstream region rather than in the mud flat sediments. The environmental conditions are assessed in terms of the stability fields of the various minerals present. The diagenesis of the iron minerals is discussed and attention is drawn to certain physical and chemical aspects of the sedimentation in the two contrasting environments.