Fluvial overprinting of glacially induced microtextures on quartz grains derived from the Chitina Glacier, Alaska



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Microtextures revealed through scanning electron microscopy (SEM) of quartz 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 with systematic sampling to assess the degree of surface modification when grains experience multiple episodes of transport style. Transport-induced microtextures can be grouped based on the process of fracturing into: (1) sustained high shear stress fractures created through grain-to-grain stylus contact (i.e. troughs, grooves and gouges), (2) percussion fractures created through grain-to-grain impacts during saltation or traction flow, and (3) polygenetic fractures that reflect fracturing under a wide variety of transport processes. This study attempts to ascertain the degree of fluvial modification of glacially derived grains within the Chitina River of SE Alaska. In total, 41 samples were collected in ~5km increments over the entire 188 km length of the river beginning at the Chitina Glacier terminus for a total analysis of 1,000 quartz grains.

Data shows that the number of grains exhibiting percussion microtextures generally increased (from ~1-31%) progressively downstream from the initially sampled glacial till (1%-percussion, 30% sustained high stress features). Percussion microtextures demonstrate a fairly strong positive correlation (R2= 0.6535) versus distance downstream. In contrast, percentage of grains exhibiting sustained high shear stress microtextures ranges from ~7-36% and negatively correlates (R2 = 0.6248) with distance downstream. The reciprocal relationship suggests that grains exhibiting percussion microtextures increase with downstream transport at the expense of grains exhibiting sustained high shear stress microtextures. This trend is supported since percussion textures positively correlate, sustained high stress textures negatively correlate, and polygenetic microtextures display no downstream transport correlation (R2=0.02).

In this study, sustained high stress microtextures persisted for at least 188 km downstream indicating that the presence of such microtextures in an ancient coarse grained fluvial system can be used to differentiate from proglacial and non-glacial fluvial systems. Additionally, minor increases in sustained high stress features can be corroborated with glacially fed tributaries, including the Barnard, Hawkins, and Kennicott Glaciers.

Finally, the ratio of percussion to sustained high stress (P/S) occurrence frequency was used to correlate surface feature changes relative to river morphology. Changes in the P/S ratio coincide with confluencing glaciers and the Tana and Nizina Rivers. P/S ratio decreases when proximal to glacial environments and increases with further transport distance from glacial headwaters or tributaries. The P/S ratio relationship was used to make conclusions concerning river morphology, linear regression application to downstream distance, and changes in microtexture occurrence frequency during downstream fluvial transport. Thus, the results of this study support the merits of using this method as proxy in ancient systems to determine a glacial influence on depositional systems and potentially as a proxy to predict extent of glacial advances.



Quartz, Microtextures, Glacier