Glacial comminution of quartz sand grains and the production of loessic silt: A simulation study

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Traditional views have attributed the formation of loess-sized quartz silt particles to glacial processes. Glacial grinding was considered to be the only mechanism capable of producing loessic silt in the quantities necessary for the formation of loess deposits. This idea has been strengthened by the apparent close association between present or former glacierised regions and major global loess deposits. However, some doubts exist as to whether or not glacial grinding is capable of producing sufficient quantities of quartz silt. Because of the difficulties entailed in the observations and direct measurement of subglacial abrasion and crushing, this paper describes and discusses the results of a simulation study designed to examine abrasion and size reduction of quartz grains under “grinding” conditions analogous to those experienced in a subglacial environment. The findings from these experimental runs suggest that, although particle breakage and comminution does occur, little silt-sized material is actually produced. The amount of silt produced ranges from approximately 9% of the original sample in one experimental run to less than 1% for the other seven runs. It is suggested that the results obtained may show considerably less abrasion and loessic silt production than may be expected to occur within a natural subglacial environment because: (1) subglacial grains experience much greater overburden pressures for much longer periods of time; (2) the use of sand-sized grains may represent a stable component resistant to further size reduction through crushing or abrasion; and (3) the use of a monomineralic sample of relatively sound quartz grains. Although care must be taken when relating results obtained from these experimental runs to silt production within a glacial environment, findings from this work do question whether large amounts of quartz silts are produced within subglacial environments. The importance of glacial processes in the formation of loess deposits may lie in the reworking of pre-existing sediments by ice rather than the actual production of vast quantities of silt. This in turn suggests that care should be taken in the interpretation of the provenance of particles comprising loess deposits.