ABSTRACT

Previous investigations (Duncan and others, 1970; Stokke and Carson, 1974) of Cascadia Basin clay mineralogy indicate a general trend in which montmorillonoids (smectites, 17 A) are enriched relative to illite (10 A) and chlorite (7 A) 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 (Russell, 1967). There exist, however, numerous local exceptions to these general patterns which are not readily explained by the previous works.

Fifteen samples from 5 cores in Northern Cascadia Basin and Juan de Fuca Abyssal Plain were fractionated into 6 size classes. Semi-quantitative determinations of clay mineralogy and "crystallinity" were made on each of the 90 subsamples. The results indicate a definite size-dependency in clay mineralogy and crystallinity which is consistent both areally and temporally. Discriminant-function analysis indicates that clay mineralogy and crystallinity of some individual size fractions are statistically distinct but that the stratigraphic units (Late Holocene, Early Holocene, and Late Pleistocene) are not statistically discernible. Textural variations within and between Holocene and Pleistocene parallel sediment clay-mineral changes. and mineralogy can be predicted on the basis of clay-min ral size-dependency and grain-size distribution. It is inferred that the stratigraphic textural-mineralogic variations result from selective transport and deposition and not from contrasting source areas or weathering conditions, as had previously been suggested (Duncan and others. 1970; Stokke and Carson. 1974).

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.