143-3 The Evaluation of Three-Dimensional Shapes of Sand Grains Using Microfocus X-Ray Computed Tomography: Implications for the Discrimination Between Fluvial and Foreshore Environments

Abstract:

The shape of clastic grains has been considered as an essential indicator of the depositional environments and hydrodynamic experience that grains have undergone. Therefore, many studies have been pursued to quantify the shape of clastic grains.

For example, Suzuki et al. (2015) applied elliptic Fourier principal component analysis to two-dimensional sand-grain outlines and showed that grains from glacial, fluvial, foreshore, and aeolian environments could be quantitatively discriminated.

However, this scheme is based on arbitrary two-dimensional projections of grains, which cannot fully capture their three-dimensional shape. Microfocus X-ray computed tomography enables three-dimensional characterization of modern grains and may permit nondestructive analysis of ancient grains within sandstones. This study investigates whether three-dimensional grain shape can discriminate between modern fluvial and foreshore environments.

Approximately 30 sand grains of 0.5–2.0 mm grain size were analyzed at each of twelve fluvial sites along the Shinano River, central Japan, and seven foreshore sites scattered in Japan. Three-dimensional surface models of sand grains were acquired using microfocus X-ray computed tomography (ComscanX Mini90-1616, Comscantecno Co., Ltd., Yokohama, Japan). We conducted visual inspections of sand grains by stereo microscope and confirmed that all 3D images used in this study were accurate. Volume-equivalent diameter, elongation ratio, flatness ratio, sphericity, and solidity (concavity) were calculated for each grain.

Standardized Principal Component Analysis was performed on the individual-grain values to produce the composite indices of macroscopic grain shape parameters. Principal Component 1 (PC1) showed positive correlations with sphericity and solidity. Therefore, PC1 represented a composite shape index, with positive scores indicating more spherical and less concave grains and negative scores indicating less spherical and more concave grains.

PC2 was mainly associated with volume-equivalent diameter as a positive load and elongation ratio as a negative load. Therefore, PC2 captured grain-size variation. Six of the seven foreshore sites had positive PC1 scores, whereas nine of the twelve fluvial sites had negative scores, indicating that foreshore grains were generally more spherical and less concave than fluvial grains. Little separation was observed along PC2, indicating that the environmental separation was associated primarily with three-dimensional grain shape rather than grain size.

This preliminary study indicates that 3D analysis of sand grains are promising and may enable discrimination of depositional environments by further studies.

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