Stereological analysis of ice flow-induced preferred orientation of small clasts in Tertiary tillite matrix of Mt. Feather

The Sirius Group tillite at Mt Feather, McMurdo Dry Valleys, Antarctica, is a deposit of considerable importance to tl1e early glacial history of the Antarctic Ice Sheet. We investigate the power of stereological analysis for a11 objective (determination of the preferred direction of small clasts (< 5 mm) in two cores and, hence, in two core samples drawn at different depths below the tillite surface. Orthogonal sets of vertical thin sections were prepared on this core material. Images measuring 34x22 mm were prepared of 14 subareas, jointly representing most of the area, of at vertical thin section. From each of these sub-areas, a field of constant 340x340 pp-size was analysed. This involved at systematic line scanning operation, in which the number of intersections between particle traces and the line system was determined. To reduce structural scatter, the resulting 14 roses of the number-of-intersections were averaged per vertical section. These roses reveal the signal coming from the preferred orientation of the clasts to be weak, but above the sensitivity level of the approach. However, this signal is camouflaged by the effect of digitization on the intersection counts, preventing under the present conditions to assess a reliable estimate for the tilt angle of the clasts.