Size and shape of carbonate grains: A comparison of measurement techniques

Grain size and shape significantly influence sediment transport and deposition, so accurate characterization is essential for understanding depositional dynamics. Biogenic carbonate grains, such as skeletal fragments, exhibit irregular morphologies that challenge traditional, single-diameter-based size estimates. To better characterize these non-spherical grains, two- or three-dimensional analyses are required. Micro-computed tomography (micro-CT) offers high-resolution, three-dimensional models of individual grains, enabling accurate quantification of volume, projection area, and shape. However, the high cost and limited accessibility of micro-CT in standard sedimentology labs hinder its widespread use. This study compares caliper measurements, sieve analysis, static image analysis (SIA), and dynamic image analysis (DIA) against a micro-CT reference dataset for sand-sized skeletal carbonate grains. We assess the accuracy of each method in capturing key grain properties relevant for hydrodynamic and geomechanical modeling, including volume, maximum projection area, nominal diameter, and Corey shape factor. We also evaluate the potential of predicting a grain’s third dimension based on its maximum projection dimension, as measured through SIA of loose grains and thin sections. A regression model comparing SIA to known micro-CT values yields 72–78% accuracy. Results show that methods relying on axial dimensions systematically overestimate volume and maximum projection area. DIA tends to oversimplify grains as spherical, while SIA on thin sections may underestimate grain size due to slicing orientation effects. This study highlights the advantages and limitations of various measurement techniques and underscores the importance of selecting appropriate grain measurement techniques in sedimentological research.