INVESTIGATING THE CORRELATION BETWEEN WATERSHED SURFACE GEOLOGY AND DETRITAL ZIRCON AGE PATTERNS IN MODERN RIVERS

The purpose of this research is to discover whether there is a correlation between the surface area of exposed igneous rocks within a particular watershed and the distribution of age ranges measured using U-Pb dating of detrital zircons from modern river sediments taken from the watershed. In order to investigate this problem, we collected samples from five rivers that drain the Talkeetna and Chugach Mountains in southcentral Alaska. The most recent geologic map of Alaska (Wilson et al., 2015) was cropped for each watershed in ArcMap. These data were used to calculate the area of igneous, sedimentary, and metamorphic bedrock suites within each watershed. Where geochronologic age data are available for the igneous bedrock suites, that group was subdivided by age. For each detrital zircon sample, the U-Pb ages were classified using the same age categories as the igneous bedrock suites from the parent watershed. Preliminary comparisons between the percentage of area for each igneous bedrock age distribution within an individual watershed calculated from the geologic map to the percentage of each age distribution from the U-Pb data from our detrital zircon samples seem to indicate that there is a weak correlation between the two datasets. Potential explanations for this mismatch may be that 1) the watersheds have a large proportion of sedimentary and metamorphic bedrock relative to igneous bedrock such that a large number of zircons are being derived from the sedimentary and metamorphic units in each watershed, 2) there are both felsic and mafic igneous rocks in the watersheds, and because felsic units typically contain a greater relative abundance of zircons than mafic units, our U-Pb data could be biased toward the zircons derived from the felsic igneous units, or 3) there is only sparse geochronologic data from within our study area so our data could be exhibiting a large proportion of ages from undated or poorly dated units.