PROVENANCE OF GLACIOMARINE STRATA OF THE YAKATAGA FORMATION, SOUTHERN ALASKA, USING ELEMENTAL GEOCHEMISTRY AND MINERALOGY OF THE SILT-SIZE FRACTION

Fine-grain strata are abundant, especially in glacial environments where diamicts are found in both terrestrial and marine settings. Evaluating the utility of diamicts as records of glacial dynamics depends on analyzing particle composition, from boulders to clays. Whereas sand-size and coarser grains can be studied optically or with detrital geochronology, there are fewer studies that investigate provenance of the fine fraction of diamicts. We examine provenance of the fine fraction of the glaciomarine Neogene Yakataga Formation that is uplifted and exposed onshore in the foothills of the St. Elias/Chugach range of southern Alaska. Accumulation of the Yakataga began at ~6 Ma and continues to the present day. The Yakataga is the thickest and possibly most complete Neogene section on earth. The provenance of the coarse fraction within the Yakataga Fm, previously determined from detrital zircon spectra (Perry et al., 2009), reflect sources in the inboard Chugach/Prince William terranes along with older Cenozoic marine strata exposed in nearby thrust sheets. The Yakataga, however, is primarily composed of fine-grain diamictite, so detrital zircon analyses may provide an incomplete provenance story. Here we analyze the silt fraction of the Yakataga for mineralogy and elemental geochemistry to determine provenance. Samples were collected from both the exposed Yakataga strata as well as modern sediment sourced from glaciers in the region to characterize the changes of provenance through time. Within this muddy strata, we focus on the silt fraction because it may contain lithic fragments and products of mechanical weathering that retain source geochemistry better than clays. To separate out the silt fraction we use a technique of gentle disaggregation through repeated water saturation under vacuum followed by freezing and thawing. XRD analyses reveal three dominant mineral phases present in most of the silt samples: quartz, albite, and chlorite. The presence of chlorite indicates a metamorphic source rock that is possibly the Chugach metamorphic complex. Additional elemental analyses are compared with published results from the possible source regions to provide an additional provenance tool. Provenance of the fine fraction and coarse fraction are compared to investigate differences in sediment transport.