PRELIMINARY ASSESSMENT: FLUVIAL OVERPRINTING OF GLACIALLY-INDUCED MICROTEXTURES ON QUARTZ GRAINS DERIVED FROM THE CHITINA GLACIER, ALASKA

Scanning Electron Microscopy (SEM) of quartz sand grain surfaces can be useful for interpreting sedimentary transport mechanisms, especially in eolian, fluvial, and glacial depositional environments. Few studies have applied SEM microtextural analysis to systematic sampling to assess the degree of overprinting under multiple transport histories. Transport-induced microtextures can be grouped based on inferred fracture process into: (1) high-stress fractures created through sustained grain-to-grain contact, (2) percussion fractures created through grain-to-grain contact during saltation or traction flow, and (3) polygenetic fractures occurring under varied transport processes. This study attempts to ascertain the degree of fluvial overprinting imparted onto glacially-derived grains utilizing the ratio of sustained high stress (glacially-induced) microtextures to percussion (fluvially-induced) microtextures from the proglacial Chitina River in SE Alaska. Samples were progressively taken approximately every 5km downstream from the glacial terminus to a distance of 190 km.

Data shows that percussion microtextures present on grain surfaces generally increased (from ~1-22%) progressively downstream from the glacial front. Percussion microtextures show a modest positive correlation (R² = 0.40) versus distance downstream. Percentage of high-stress microtextures present ranges from ~17-36%. However, a correlation between high-stress microtextures and distance downstream is not present (R² = 0.005) and likely reflects recycling of older moraines and/or tributary input. The increase in percussion textures observed downstream appears to be at the expense of polygenetic fracture observed on grain surfaces. Polygenetic fracturing negatively correlates (R² = 0.52) with distance downstream. This data indicates that percussion fracturing increases downstream from the glacial front, but that saltation and traction appear to overprint cursory polygenetic fractures rather than robustly imprinted high-stress gouged and grooved microtextures. Future data analysis is addressing a method to consistently count v-shaped percussion scars within a consistent grain surface area. This should provide quantitative data for assessing downstream grain surface overprinting.