Control of clay-mineral stratigraphy by selective transport in late Pleistocene-Holocene sediments of northern Cascadia Basin-Juan de Fuca abyssal plain; implications for studies of clay-mineral provenance

Previous investigations indicate a trend in which montmorillonoids (smectites, 17 Aa) are enriched relative to illite (10 Aa) and chlorite (7 Aa) in Holocene lutites, whereas illite and chlorite dominate in late Pleistocene deposits. The apparent crystallinity of all clay-mineral groups declines from Pleistocene to Holocene; however, there are local exceptions to these patterns. Semi-quantitative determinations of clay mineralogy and "crystallinity" were made on each of the samples. The results indicate a definite size-dependency in clay mineralogy and crystallinity. Discriminant-function analysis shows clay mineralogy and crystallinity of some individual size fractions are statistically distinct but that the stratigraphic units are not statistically discernible. Textural variations parallel sediment clay-mineral changes, and mineralogy can be predicted on the basis of clay-mineral size-dependency and grain-size distribution. The stratigraphic textural-mineralogic variations result from selective transport and deposition and not from contrasting source areas or weathering conditions. Future investigations of clay-mineral provenance, dispersal, or correlation, in any area, should be based upon data which has been corrected for the effects of clay-mineral size-dependency.--Modified journal abstract.