Fabric and origin of multiple diamictons within the pre-Illinoian Kennedy Drift east of Waterton-Glacier International Peace Park, Alberta, Canada, and Montana, USA

Abstract

Pebble fabrics and sedimentological properties indicate a glacigenic origin for multiple, superposed diamictons within the (pre-Illinoian) Kennedy Drift east of Waterton-Glacier International Peace Park, Alberta, Canada, and Montana, USA. Determining the origin of these diamictons is particularly important in light of their antiquity. Eight diamictons on Mokowan Butte, Saint Mary Ridge, and Two Medicine Ridge have reversed magnetic polarity and are correlated with the Matuyama Chron (2.6–0.78 Ma). On Mokowan Butte, two diamictons with normal polarity underlying two reversed diamictons are correlated with the Gauss Chron (3.6–2.6 Ma) or possibly the Reunion or Olduvai events (2.23–2.20 and 1.93–1.76 Ma, respectively). Fabrics were taken from 17 units within the Kennedy Drift on five interfluves. On the basis of comparison of properties of these diamictons with those of modern glacial deposits, eight, including the uppermost one or two units on each interfluve, are probably undeformed lodgment tills. These are typically massive, matrix-supported bouldery sandy loam with strong unimodal fabric orientations, relatively high eigenvalue (S 1 /S 3 ) ratios, and striations on resistant clasts. Some include boulder pavements, bullet-shaped boulders, and weak fissility. Eight other units have weaker fabrics typical of either deformed lodgment tills or glacigenic sediment flows. Because these also include bullet-shaped boulders, stone pavements, and striated clasts, most are probably deformed lodgment tills. A colluvial or mudflow origin for the material is ruled out on the basis of the lateral and vertical extent of the diamictons (at least 300 m wide and 5–10 m thick), wide range of clast lithologies and sizes, and distance from the mountain front (2–10 km), as well as pebble fabrics and sedimentological properties.