Mineral Composition, Sr/Sr Ratios, and Concentrations of Strontium and Rubidium in Late Wisconsin Till of Ohio

Samples of Late Wisconsin till from the Powell-Union City Moraine of Ohio have bimodal grainsize distributions and concentrations of carbonate minerals, illite, and kaolinite-chlorite that are primarily the result of grinding of clasts derived from local bedrock. Quartz and feldspar have unimodal grainsize distributions that suggest their derivation from distant sources on the Precambrian Shield of Canada. The carbonate content of the silt and clay fractions (<62.5jum) of 13 till samples collected along the Powell-Union City Moraine increases from less than 10% in the east to about 35% in western Ohio. The change in carbonate content is compatible with bedrock lithology and a southwesterly iceflow direction. The silt and clay fractions (non-carbonate) contain Rb= 154.7=*= 13.5 ppm, Sr= 104.8 ±11.0 ppm, Rb/Sr = 1.49±0.21, Sr/Sr = 0.7345±0.0041. The average Sr/Sr ratio of these till samples is similar to results obtained elsewhere for strontium of the continental crust and supports previous suggestions that the average Sr/Sr ratio of the continental crust is 0.730±0.010. The ratio of radiogenic Sr/Rb increases from about 0.0060 in the east to 0.0075 in western Ohio. The low magnitude of this ratio is due to the predominance of locally derived sediment of Palezoic age in the silt and clay fractions of till. The westerly increase is probably the result of several factors, including: 1) Increasing age of the Precambrian component (feldspar); 2) Increasing abundance of the Precambrian component; 3) Increasing age of the local bedrock from Mississippian in the east to Silurian in western Ohio. OHIO J. SCI. 79(4): 149, 1979 "Till is the only sediment stemming directly and solely from glacial ice . . . and . . . it holds the secrets to the action of ice at the bottom of a glacier." These eloquent words of Goldthwait (1971, p. 3) suggest the importance of till as a source of information about the geologic and climatic history of the Earth during the Pleistocene epoch. The methods used to study till and its fabric were reviewed recently by Raukas et al (1978) and include determinations of the abundances of selected light or heavy minerals. Studies by other investigators have contributed significantly to our understanding of ice flow patterns and of the relationships between grainsize, Manuscript received 12 September 1978 and in revised form 11 January 1979 (#78-56). Publication of Institute of Polar Studies, Misc. Series No. 124. abundance of minerals, and distance of transport from their sources. Chemical composition of till in Ohio has received very little attention. Wilding et al (1971) reported concentrations of Ti, Zr, Fe, Ca and K and reviewed the sparse literature relevant to the midwestern area of the United States. The concentrations of both major and trace elements in till may be useful in the study of iceflow directions and bedrock compositions. For example, the clay fraction of till has been shown to be a representative sample of the bedrock over which the ice moved (Goldschmidt 1954) and the chemical composition of this fraction may be used to estimate the chemical composition of the bedrock or to distinguish among tills derived from different sources. The elements Rb and Sr are of particular interest because Rb is radio-