Applicability of automatic image analysis in quartz-grain shape discrimination for sedimentary setting reconstruction

Abstract

The quartz-grain shape is an important sedimentological attribute typically used to reconstruct depositional settings and sediment transport conditions and predict the duration of grain transport. A common assumption is that the extent and duration of eolian and fluvial transport are reflected in the increased sphericity and roundness of sand grains, and this study aims to verify this relationship. Automatic particle-shape image analysis was performed on quartz grains (0.8–1.0 mm) from three sedimentary environments: weathered with angular fresh grains (GNU), eolian with well-rounded grains (GRM), and fluvial with well-rounded grains (GEL). This enabled us to obtain four associated shape-description parameters: highly sensitive circularity (circularity HS), convexity, solidity, and aspect ratio. Subsequent principal component analysis and discriminant analysis of the parameters indicated that circularity HS (associated with the degree of roundness) best distinguished the shape of GNU, GRM, and GEL grains. However, the aspect ratio was more useful for analyzing the degree of grain sphericity. GRM and GEL grains, those with a high degree of roundness, had either low or high sphericity, which should be considered a characteristic of long-term reworking by eolian and fluvial processes. The presence of grains with low sphericity and high degree of roundness, such as those found in environments where transport prevails, should be considered a shape inheritance. Using both sphericity and roundness parameters facilitates an accurate interpretation of transport environments in terms of transport duration and sediment maturity. However, neglecting grain sphericity when analyzing these sorts of sedimentary settings may result in the misinterpretation of their true sedimentary settings.