Inherited and Detrital 2:1 Type Phyllosilicates in Soils Developed from Mazama Ash1
- M. J. Dudas and
- M. E. Harward2
Previous studies of soils developed from Mazama ash have indicated the occurrence of a complex suite of 2:1 and 2:2 type phyllosilicates. Before interpreting the phyllosilicates as weathering products from the ash, it must be established that they have formed in situ. Origin of 2:1 and 2:2 type phyllosilicates in soils of the coarse Mazama pumice in central Oregon was traced to the pyroclastic lithic fragments in the deposit. Clay minerals separated from lithic fragments included chlorite, chloritic intergrades, mica, smectites, and possibly vermiculite. The inherited origin of these crystalline clays corrects previous suggestions of origin which involved weathering of ash.
For volcanic ash soils in forested areas located at distances farther than 100 km from the source at Crater Lake, 2:1 type clay minerals separated from ash horizons were shown to be relics from underlying paleosols. Crystalline clay suites in different ashy profiles exhibited greater similarity to their respective palesol clay fraction than to other profiles in the same ash deposit. Once incorporated into ashy horizons, palesol clay high in montmorillonite exhibited partial chloritization and samples high in mica exhibited some loss of interlayer K. Uprooting of trees is believed to be the major mechanism involved in mixing ash soils with palesols in forested areas where the Mazama deposit is relatively thin (< 100 cm).Please view the pdf by using the Full Text (PDF) link under 'View' to the left.
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