Transport of Indicator Clasts by Ice Sheets and the Transport Half-Distance: A Contribution to Prospecting for ORE Deposits

Abstract

The abundance (A) of granitic and metamorphic clasts in Wisconsinan till in the Dummer Moraine at the edge of the Precambrian Shield of Ontario decreases exponentially but then remains nearly constant at about 4% for more than 800 km down-ice in southwestern Ontario, Ohio, and Indiana. The exponential decrease is attributed to the transport and deposition in the basal ice of the Laurentide ice sheet after it crossed the contact between the granitic rocks of the Precambrian Shield and the overlying Paleozoic carbonate rocks. The transition from exponential to linear distribution was caused by a change in transport mechanism from basal to englacial transport of clasts that were moved above the base of the ice sheet and thus escaped attrition and deposition in basal till. The transport half-distance of a boulder train can be estimated from a plot of In A vs. transport distance within the exponential transport regime. The half-distance can then be used to determine the transition distance, which indicates how...