Abstract: 

Certain micrometer-scale fractures, or microtextures, on grain surfaces are the result of specific transport processes. Accordingly, these transport-induced microtextures are used to infer depositional setting in ancient deposits. Multiple transport histories complicate the microtextural record because modification of grain surfaces can reduce or eliminate the signal of earlier transport episodes. This study utilizes scanning electron microscopy to analyze surface microtextures to assess the role of fluvial overprint on glacially derived grains along ∼ 188 km of the proglacial Chitina River, SE Alaska. Results indicate that occurrence frequency of glacially induced microtextures (i.e., straight and curved grooves, deep troughs, and crescentic gouges) negatively correlates to distance downstream, yet these microtextures persist in every sample. Conversely, occurrence frequency of fluvially induced microtextures (i.e., v-shaped cracks and edge rounding) positively correlate to distance downstream. At least in the Chitina River watershed, the ratio of fluvially to glacially induced microtextures (F/G ratio) generally records physiographic variation, including increase in fluvial signal after a major tributary confluence, and decrease downstream from the intersection of valley glaciers. These results suggest that analysis of quartz grain microtextures in ancient fluvial deposits can be used to infer glacial influence when other diagnostic sedimentologic indicators of glaciation are lacking. Other potential uses for the proxy may exist, but need to be assessed after other modern proglacial systems are studied, including: 1) estimates of distance to paleo–ice margins using the respective downstream trends of glacially or fluvially induced microtextures, and 2) paleogeography of ancient glacio-fluvial systems through F/G ratio analysis.

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