Slice depth of 2-knots

We introduce the notion of slice depth of a 2-knot K, which is the minimal integer n such that K is n-slice. We give an upper bound for the slice depth of the n-twist spin of a classical knot which belongs to several specific classes, namely, certain types of 2-bridge knots, pretzel knots, ribbon knots of 1-fusion, and knots with given unknotting number.