Morphology and Grain Composition of Sand-Size till Pellets from ANDRILL Core AND1B, Ross Sea, Antarctica

A 1,285 m-long sediment core (AND-1B) was drilled from beneath the McMurdo Ice Shelf sector of the Ross Ice Shelf as part of an Antarctic geological drilling program (ANDRILL). This core contains the glacial- interglacial record over 14 million years of Antarctic geologic history.

Analysis of diamictite samples throughout the core has shown that up to 80% of the sand-size fraction are aggregate grains dominated by a clay matrix, with inclusions of mineral grains or lithic fragments. The occurrence of these aggregate grains is associated with massive diamictites that are interpreted as being deposited subglacially by the Antarctic Ice Sheet. These aggregates are not present in stratified diamictite facies, which are interpreted as being deposited by iceberg rafting. Following the terminology used by sedimentologists in the Ross Sea, we refer to these aggregate grains as till pellets.

Grains from the 1.4–1.0 millimeter size fraction were mounted in epoxy and thin sectioned. Till pellets were then photographed and their long and short axes were measured under crossed polars and plain light. Grain roundness was estimated based on the Wadell Roundness chart. Analysis has shown that the till pellets are primarily subrounded, with an average elongation of 0.8. Constituent grains within till pellets have a variable composition that includes quartz and feldspar grains, volcanics, and sedimentary lithic fragments. Grain arrangement within the till pellets vary, including: coarse silt-size grains distributed evenly and randomly throughout a clay matrix, a dominant sand-size grain (up to 1mm) surrounded by clay, and a grain-rich core surrounded by a clast-free clay rim. Investigation of till pellets within Ross Sea diamictites may to lead to an understanding of their formation and therefore, the depositional processes of the diamictite during glacial periods.