Dawn Wright - Thesis Defense (BS)

Using sieving to deconstruct the regolith: A first attempt Dawn Wright B.S. Candidate Advisor Dr. Gary Girty H.W. Nesbitt and colleagues published in 1996 and 1997 two papers that outlined a general model for the integrated role that regolith development in plutonic terranes, erosion, and sorting during riverine transportation play in producing quartzofeldspathic sediments. In this model, sorting of the eroded regolithic material produces a mud-rich component that is more weathered than a size-fractionated sandy component. In A-CN-K space, as a result of progressive weathering, the mud-rich and sandy components lie at opposite ends of rays extending from the A apex, the composition of kaolinite and gibbsite, downward to the plagioclase-K-feldspar join where they reflect as a result of the preferential incongruent alteration of plagioclase decreasing ratios of plagioclase to total feldspar. In order to test this model, I collected eleven corestone, four transition zone, and four saprock samples from a single site located within Japatul Valley, California. Each of the four saprock samples was divided into two parts. One part was labeled the bulk fraction, and the remaining part was sieved into the three following size fractions: 63 microns, 63 to 45 microns, and 63 micron fraction would be sorted into sand and pebble sized material, while the 63-45 micron fraction would represent coarse silt. The 63 micron fractions plot just to the right of and overlaps the cluster of bulk saprock samples. This relationship is consistent with the fact that the 63 micron fractions made up between 90 – 95% of the bulk fractions from which they were sieved. In contrast, the 63-45 and 45 micron fractions make up less than 5-10% of the bulk fractions from which they were derived, and are significantly poorer in K then are the bulk and the 63 micron fractions. This diminishment in K is likely due to the fact that most of the megacrystic K-feldspar was retained within the 63 micron fraction while plagioclase and its weathering products were retained within the 45-63 micron and 45 micron fractions. Significantly, the 45 micron fraction is more depleted in Ca and Na than is the 45-63 micron fraction, and both fractions plot about a.... more: http://sci.sdsu.edu/geology/theses/dawn-wright/