DOES CLAST SIZE INFLUENCE FABRIC STRENGTH

Clast-fabric strength is commonly used as a quantitative tool to deter mine the depositional history of subglacial tills. This paper emphasiz es the variance in fabric strength relative to clast size. Data on 46 clast-fabric analyses were sampled from two basal till localities in s outheast Denmark and another eleven analyses from a basal till localit y beyond a glacier in south Iceland. It is demonstrated that clasts lo nger than 2-3 cm show a stronger preference for parallel orientation t han do shorter clasts, This implies that two operators sampling fabric data a few meters apart within the same till unit, but selecting clas ts longer or shorter than 20 mm, may classify the till as two differen t types of basal till because of differences in clast size alone. Addi tionally, this study demonstrates considerable variations in fabric st rength over short lateral distances within the same basal till bed. It is concluded that caution should be exercised in using eigenvalues to infer depositional processes and strain histories in subglacial envir onments; sedimentological criteria other than clast fabric must also b e considered before drawing final conclusions on subglacial deposition .