Pebble Fabric in an Ice-Rafted Diamicton

Abstract

Pebble fabric studies on ice-rafted diamictons have been limited to general observations, with authors noting preferences toward vertical, random, or horizontal orientations. To clarify such observations, pebble fabric data were collected from a fossiliferous diamicton of late Pleistocene age located on Whidbey Island, Washington. The ice-rafted origin of this unit is supported by several independent characteristics including in situ macrofauna and microfauna, conformity with subaqueous lithofacies containing dropstones, lower bulk densities and higher void ratios than associated tills, soft sediment deformation structures suggestive of iceberg dumping, textural gradations, and facies relationships. Analysis using the eigenvalue method indicates that ice-rafted fabrics are nearly random with little consistency of vector orientations between sites and without any relationship to the probable direction of glacial flow. The weak fabric is mainly the product of settling through the water column and impact with, or penetration of, the bed. Samples that possess a weak preferred long axis orientation with a low angle of dip, including those from laminated muds, can best be explained by the intermittent effects of bottom currents, a resistant substrate at the time of deposition and post-depositional flowage. Comparisons of pebble fabrics from basal tills, recent sediment flow deposits, and basal, debris-laden ice of an active glacier demonstrate that the ice-rafted fabrics are distinct from those of basal ice and till but are quite similar to those of sediment flow diamictons. Ice-rafted diamictons appear, however, to contain a greater number of elongate stones with long axis plunge angles exceeding 45° than other glacigenic diamictons.