Faithe Lovelace - Thesis Defense (BS)

First study of corestone-saprock development within NE-block San Jacinto fault, Anza, California: Implications for the genesis of clastic sediment derived from granodiorite – quartz monzodiorite Faithe Lovelace B.S. Candidate Department of Geological Sciences San Diego State University Advisor Dr. Gary Girty ABSTRACT Weathering, erosion, and sorting during transportation in the fluvial regime play important integrated roles in the development of quartzofeldspathic sediment. For example, H. W. Nesbitt and colleagues showed that sorting of eroded regolithic material produces a mud-rich component that is more weathered than sand derived from the same source. In order to test this idea, I collected 5 corestone samples and 5 saprock samples from a single site located east of the Clark segment of the San Jacinto fault, SE of Anza, California. Each of the five saprock samples was split into two parts. One part was used as the bulk sample, while the other part was sieved into the following size fractions: 63 microns, 63-45 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 at the lower end of this trend while the finer grained fractions plot about the trend but nearer the p(A)-p(CN) join. Weathering intensity factors for the two 45-63 micron fractions are 0.86 and 0.95 while weathering intensity factors for the three 45 micron fractions are 0.85, 0.97, and 0.95. These results contrast markedly with those provided above for the bulk saprock samples, and clearly indicate that the sieved finer grained fractions captured a part of the weathering trend that is masked by the bulk saprock samples. In short, the results of this study document for the first time, a trend derived from the weathering of granodiorite to quartz monzodiorite that is controlled primarily by the loss of K from biotite. This trend has not been previously recognized but given that the composition of the corestone studied during this investigation is relatively common in eroded continental margin magmatic arcs on a global basis it may be more widespread than currently recognized. Clearly more work is warranted.