The Stratigraphy, Mineralogy, and Trace Element Concentrations of the Quaternary Sediments of the Northern Lake Superior Basin: Discussion
Mothersill and Fung (1972) declare in the introduction of their paper that one of the aims "of this investigation was to determine the Quaternary stratigraphy . . . of the northern part of the Lake Superior basin". Therefore, a reader expects not only new original data, but also consideration of generally accepted stratigraphic criteria including the code of the American Commission on Stratigraphic Nomenclature ( 1961 ), and correlation with up-todate stratigraphic schemes of other authors. Unfortunately, the authors are confusing rock stratigraphic with time stratigraphic terms, are defining a time transgressive contact as the boundary between two established time stratigraphic units, and are not using the latest references for correlations. To some extent, this has been done already before by the senior author (Mothersill 197 1 ) , and the present paper of Mothersill and Fung (1972) repeatedly refers to his equally disputable stratigraphic statements. In their Table 1, Mothersill and Fung (1972) call Farrand's (1969) names of sediments encountered in cores of Lake Superior as "Time Stratigraphic Units". They could be considered informal rock stratigraphic units, but never time stratigraphic. No reasons are given why, in the same table, the authors (and also Mothersill 1971) have chosen Hough's (1958) very seldom used classification of the Wisconsin Stage, when they already indicate in the table, that most of Hough's substages (five out of six) are missing in the area; the only one listed as present (Valders) is not a term introduced by J. L. Hough. Why not refer to any other more recently published and widely used classifications of the last glacial stage in the Great Lakes Region? No explanation is given by Mothersill and Fung ( 1972) and Mothersill (1971 ), why they prefer 11 000 yr B.P., based upon a few black carbon dates (Flint and Deevey 1951) for dating the Valders advance, rather than using, or at least mentioning the 18850 + 100 yr B.P. average date (Broecker and Farrand 1963) of the Two Creeks forest bed, which was flooded and the trees killed, during the Valders advance. The latter date is based upon a special investigation of new material and comparison with more than twenty other radiocarbon dates. One of the most confused concepts in Mothersill and Fung (1972) is the HolocenePleistocene boundary. They say that Mothersill ( 1971 ) , "defined the basal contact of the claysilt sequence with the underlying gray varves as the Holocene-Pleistocene boundary". They also state that this contact represents an unconformity. Comparison of their Tables 1 and 3 shows that chronologically this unconformity is partly in the Pleistocene, partly in the Holocene: in Table 3 the extrapolated dates of basal Holocene range from 5400 + to 13 000 2, while in the 'Time Scale' of Table 1 the date of 11 000 yr B.P. is included in Pleistocene. The Holocene-Pleistocene boundary is a timestratigraphic term and therefore it can not fluctuate over a wide span of time. It may be noted that the INQUA Commission on the Holocene announced at the VIII INQUA (International Union for Quaternary Research) Congress in Paris, 1969, that "For the reference boundary between the Pleistocene and Holocene with world-wide validity, the Commission proposes a date at about 10,000 B.P. measured in radiocarbon years" (Hageman 197 1 ) . Neither Mothersill ( 1971 ) nor Mothersill and Fung (1972) have proven that the contact between the clay-silt and underlying varves in