Progressive downstream overprinting of glacially induced quartz microtextures during fluvial saltation, Salmon river, British Columbia, and Alaska

Scanning electron microscopy (SEM) has been used to infer sedimentary transport mechanisms through careful analysis of quartz grain microtextures since the 1960s. In previous studies, it has been found that certain microtextures are indicative of unique transport processes. This study utilizes a tripartite microtextural suite to categorize transport-induced microtextures as follows: 1) sustained high shear stress microtextures which result from the stylus effect of grain-to-grain contact in a highly viscous medium such as ice or viscous debris flow, 2) percussion microtextures created through grain-to-grain collision during saltation of bedload sediment in a low viscous medium such as water or air, and 3) polygenetic microtextures created through multiple environmental processes. This paper investigates the progressive overprinting of glacially induced microtextures during fluvial saltation as a function of transport distance in the Salmon River, British Columbia and Alaska. Furthermore, a comparison to a previous study done on the Chitina River, Alaska is made to ascertain the value of the technique among different river systems. \nIn evaluating the Salmon River, 10 samples were taken every 2 to 4 km, as terrain allowed, over 26 km. Quartz grains (n = 824) were analyzed via SEM. Two grain-size populations were analyzed (250-850 µm and 850-2000 µm) to assess grain size variations of microtextures. Microtextures from each grain-size population were divided by total grains observed. The subsequent microtextural ratio for the 250-850µm grain-size population was divided by the 850-2000µm grain-size population. The resulting numbers for each of the 14 microtextures observed were plotted on a logarithmic scale from 0.1 to 10. Numbers closer to 10 indicate a bias towards the smaller grain-size population. Numbers closer to 0.10 indicate a bias towards the larger grain-size population. Complete unity is indicated by a divisional result of one. Through these spider plot analyses, this study found there to be little or no bias of microtexture abundance based on grain size. \nIn the Salmon River, sustained high shear stress microtextures display a strong negative correlation (R2 = 0.93) and decrease in occurrence frequency progressively as a function of distance downstream. Percussion microtextures display a strong positive correlation (R2 = 0.81) and increase in occurrence frequency with distance downstream. Polygenetic microtextures displayed a strong positive correlation (R2 = 0.86) and increased in occurrence frequency with distance downstream. Sustained high shear stress microtextures survived 26 km of fluvial transportation without additional glacial sediment input. In the Chitina River, sustained high shear stress microtextures survived 188 km of transport, but downstream glacial input likely freshened the glacial microtexture input. Due to additional glacial input in the Chitina River downstream, only the first 17 km of the river is used as a comparison to the Salmon River. \nLastly, the ratio of fluvial to glacial (F/G) microtextures in the Salmon River steadily increases over the distance of the river. This indicates that as quartz grains saltate further downstream, fluvial microtextures increase in comparison to glacial microtextures. The ever-increasing trend of the Salmon River is similar to that of the first 17 km of the Chitina River. Similar trends within the Chitina and Salmon Rivers indicate that the F/G microtextural ratio may prove useful in predicting ancient glacial fronts in two hydrologically different river systems.

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