Shape analysis of mud clasts in turbidites and hybrid event beds: application of elliptic Fourier analysis with PCA to sedimentary rocks in the Pleistocene Otadai Formation, Japan

Spatial variation in mode of occurrences of mud clasts in several sediment-gravity flow deposits was investigated by bed-by-bed correlation and shape analysis of the mud clasts, aiming to shed light on entrainment processes of muddy sediment. Previous studies have suggested that behavior of turbidity currents varies due to addition of muddy sediment into flows even in a distal area (Marr et al., 2001; Haughton et al., 2003; Waltham, 2004; Talling et al., 2004; Amy and Talling 2006; Ito, 2008). Thus, estimation of the origin and transport mechanism of muddy sediment is important in terms of understanding the behavior of sediment-gravity flow. In this research, bed-by-bed correlation of turbidites was conducted in the Pleistocene Otadai Formation, Boso Peninsula, Japan. Photographs of the mud clasts were taken at each outcrop, and the elliptic Fourier analysis was conducted on the binarized images of mud clasts. Principal component analysis was then applied to the resultant Fourier descriptors to assess variations in shape of mud clasts.

As a result, it was suggested that turbidites and hybrid event beds (co-genic debrite-turbidite deposits) can be discriminated in terms of size and shape of mud clasts. In this study, 4 turbidites and 2 hybrid event beds were correlated over 6 outcrops, and the mud clasts were distributed in a region about 5-10 km in length along the direction of paleocurrent. Shape analysis indicated that the clasts in turbidites show relatively small and rounded, and did not show a significant downstream change in shape. In contrast, the clasts in hybrid event deposits show a great variety not only in size but also in shape. Roundness of mud clasts in hybrid event beds varies downstream largely. The rounded shape of mud clasts in turbidites imply a strong abrasion process after entrainment of mud clasts. On the other hand, the various types of clasts in hybrid event beds suggest that they have different origins and transport processes from those in turbidites . In conclusion, we succeeded to visualize the diversity of mud clasts in sediment-gravity flow deposits, which suggests that the entrainment processes of muddy sediment can be largely different between turbidity currents and debris flows.