Heavy mineral analysis in late Wisconsinan tills of southeastern Manitoba

Abstract

The heavy mineral assemblages in the fine and very fine sand fraction (2-4 0) of the four upper late Wisconsinan tills in southeastern Manitoba were catalogued and evaluated. Twenty-six heavy mineral species were identified under the microscope. Typically in the samples of all till units there is 34-49 % hornblende, 8-15 % epidote, 4-14 % pyroxene, 7-14 % magnetite, and 1-9 % garnet; apatite, tourmaline, biotite, pyrite, and hematite normally make up 1-5 % of the heavy minera1 suite in all samples. Till units identified previously on the basis of texture and clast lithology couid not be distinguished consistantly or reliably using heavy mineral suites, single species, or ratios of selected species. The Paleozoic bedrock of central Manitoba contributed little to the heavy mineral suites of the tills. In spite of local variations, the Precambrian source rocks display an overall (average) similarity in mineralogy. The result is a striking uniformity in the heavy mineral content of the four upper tills in southeastern Manitoba. Sandy tills of the Labrador lobe, which were deposited by ice moving from northeast to southwest, are weakly distinguished by a higher total heavy mineral content from tills deposited by the northwest to southeast advancing Keewatin lobe. This higher percentage is explained by the relatively close proximity of the Precambrian source area of the Labrador tills. The correlations of tills in Manitoba with those in North Dakota and Minnesota made by previous workers mainly on the basis of texture, clast lithology, and stratigraphic position, cannot be substantiated by heavy mineral studies. The tills in Manitoba contain significantly more epidote and less hornblende and garnet than their correlatives to the south, and large differences in other major mineral species occur between the units.