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A bifurcated fiber-optic light guide was used to measure the colors of clays in a groundwater stream over the Upper Ordovician Queenston Formation of the Niagara Escarpment (Ontario, Canada). One branch of the light guide illuminated the samples, and the other branch gathered reflected light for spectrophotometry. Measuring Munsell rock color chips, and using the weighted-ordinate method to calculate chromaticity coordinates from reflectance spectra, the dominant chemically reduced clay was greenish-gray (Munsell 5G 6/1, x=0.348, y=0.352, Y%=62.3) while the dominant oxidized clay was grayish-red (Munsell 5R 4/2, x=0.388, y=0.338, Y%=23.8). The electrical impedance of clays was correlated with x (r=-0.92 for capacitance and r=0.96 for resistance, P<0.001 at 10 kHz). The peak wavelengths for correlations were around 500 to 540 nm. The matrix between the corallites of a Devonian fossil coral above the collection site (Eridophyllum seriale) was within the statistical range of oxidized grayish-red Ordovician shale. There was no evidence that low pH in the groundwater stream had caused the variation in clay color, thus, leaving conditions when the source shale was deposited as the most likely cause of color variation.

Colored Clays in a Groundwater Stream, Correlating Fiber-optic Reflectance with Electrical Impedance