Correlating river terrace remnants using an Equotip hardness tester: An example from the Miٌo River, northwestern Iberian Peninsula

This paper describes a new method to distinguish river terrace levels based on clast hardness and degree of weathering measured with an Equotip hardness tester. The technique was applied to a series of terraces on the Miٌo River in the northwestern Iberian Peninsula, where the lack of suitable material, high degree of weathering, and intense iron cementation precluded routine dating. Detailed mapping demonstrated that terrace sediments occupy a range of altitudes that make assignment to a specific terrace, and/or correlation between levels, difficult. Statistical analysis of the Equotip hardness data from quartzite clasts using k-means clustering allowed four probable terrace levels to be identified; a series of t-tests generally supported these groupings. A fifth, lowermost terrace level, was not included in the analysis because of limited exposure above a reservoir. Clast hardness and degree of terrace weathering were generally consistent with progressive river downcutting. The occurrence of faulted sediments, however, suggested that terrace elevations were modified locally by post-depositional tectonic movements, which may explain why probable younger terraces in some sectors of the Miٌo River are at higher elevations than older terraces in adjacent sectors. The Equotip tester helped to resolve stratigraphic uncertainties and to assign deposits to specific terrace levels and was found to be a useful tool to distinguish and correlate river terraces.