Rb-Sr Dating of Detrital Feldspar: A New Method to Study Till

• Published in: The Journal of geology 1981-01, Vol.89 (1), p.97-107
• Main Authors: Taylor, Karen S., Faure, Gunter
• Format: Article
• Language: English
• Subjects: Excavations; Feldspars; Geology; Ice sheets; Minerals; Moraines; Provenance; Ratios; Rocks; Sandstones
• ISSN: 0022-1376; 1537-5269
• DOI: 10.1086/628566

Late Wisconsin till in the midwestern U.S.A. contains feldspar concentrated in the sand-size fractions ($0.0 to 4.0\phi$). The feldspar originated almost exclusively from the igneous and metamorphic rocks of the Precambrian Shield of North America. Ice-flow directions support the hypothesis that feldspar in Late Wisconsin till of Ohio and Indiana is a mixture of grains derived from the Superior structural province (2.70 b.y.) by the Huron ice lobe and from the Grenville structural province (1.07 b.y.) by the Erie and Ontario lobes. Feldspar is well suited for dating by the Rb-Sr method, and such dates can therefore be used to study regional variations in its provenance. The K-feldspar/plagioclase ratios of feldspar and their Rb/Sr ratios increase with grainsize. Feldspar size fractions from two till samples form a linear array of data points on the Rb-Sr isochron diagram. The slope of this line is related to the proportion of mixing and is expressed as a provenance date of$1.70 \pm 0.52$b.y. Feldspar in the 2.0 to 3.0ø fractions of 24 samples of till from the Powell-Union City Moraine between Mansfield, Ohio, and the Wabach River, Indiana, have Rb-Sr provenance dates that increase from 1.2 b.y. (east) to 1.8 b.y. (west). The westerly increase in the provenance dates is probably due to an increase in the abundance of feldspar derived from the Superior Province. The close grouping of the provenance dates and the absence of a bimodal distribution of dates along the strike of the Powell-Union City Moraine indicate that thorough mixing of sediment occurred when the Erie lobe advanced over glacial deposits laid down earlier. Such extensive reworking implies that the older deposits were not frozen and may have been water-logged. The final advance of the Erie lobe was probably not caused by a deterioration of climate in Ohio and Indiana, but may have been a result of changes in the dynamics of the Laurentide ice sheet.