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[HCG21-11] Characteristics of two types of inversely graded bands of spaced stratification identified by microtextural analysis
Keywords:spaced stratification, traction carpet, CNN, image analysis
Spaced stratification is a sedimentary structure composed of multiple inversely graded bands (Hiscott & Middleton, 1980), also known as traction carpet deposits. Spaced stratification has been observed in coarse-grained deposits formed by high-density gravity flows, such as turbidity currents, subaerial pyroclastic flows, and hyperconcentrated flows. It has been considered an indication of high-density gravity flows (Lowe, 1982; Hiscott, 1994; Sohn, 1997). This sedimentary structure provides keys to understanding the flow dynamics of submarine and subaerial gravity flows. However, its formation process is still in debate, and several formation hypotheses have been proposed without sufficient observations or flume experiments. The characterization of spaced stratification is needed to validate these hypotheses and conclude the discussion. Therefore, this study analyzed the characteristics of spaced stratification, focusing on its microtexture using Convolutional Neural Network (CNN) image analysis techniques.
This study investigated the characteristics of spaced stratification observed in the Bandodani Formation and Nada Formation of the Upper Cretaceous Izumi Group, distributed in southwestern Japan. Sandstone samples exhibiting spaced stratification were cut parallel to the paleocurrent, and vertical cross-sectional images were obtained using a desktop scanner. We observed the size and fabric of sand grains by applying a CNN model to the high-resolution scanned images of the sandstone cross-sections. This methodology allowed observing cross-sectional images larger than 10 cm2 at a resolution of 5 μm per pixel.
Multivariate analysis using the characteristics of grains measured from cross-sectional images of sandstone samples revealed that the inversely graded bands constituting spaced stratification can be classified into two different types. Type A bands exhibit bimodal imbrication angles, are composed of coarse grains (average of 0.6 φ), and show local variations in grain size and imbrication angles within the band. In contrast, Type B bands exhibit unimodal imbrication angles, are composed of finer grains (average of 1.2 φ), and do not show local variations in grain size or imbrication angles within the band. These two types of inversely graded bands suggest that spaced stratification may include multiple sedimentary structures formed by different processes.
Although the detailed formative mechanism of the imbrication observed in the two types of bands is still unclear, the differences between the two types may reflect variations in high-density flow characteristics. The imbrication angles observed in the two types of bands were higher (>40°) compared to those in deposits formed by bedforms or low-density turbidity current deposits. Such high imbrication angles have been reported from experimental deposits formed by high-density gravity flows where grains frequently collide with each other (Taira, 1989).
The field survey also suggests that two types of bands constituting spaced stratification may have formed in different depositional environments. Sandstone beds containing Type A bands are observed in the spilled-out deposits from channels, suggesting a deposition by supercritical flows. In contrast, sandstone beds containing Type B bands are observed in the distributary channel-fill deposits in the lobes, suggesting deposition from subcritical flows. Therefore, even though spaced stratification appears to be the same structure to the naked eye, it may have formed under different environments and flow characteristics.
This study investigated the characteristics of spaced stratification observed in the Bandodani Formation and Nada Formation of the Upper Cretaceous Izumi Group, distributed in southwestern Japan. Sandstone samples exhibiting spaced stratification were cut parallel to the paleocurrent, and vertical cross-sectional images were obtained using a desktop scanner. We observed the size and fabric of sand grains by applying a CNN model to the high-resolution scanned images of the sandstone cross-sections. This methodology allowed observing cross-sectional images larger than 10 cm2 at a resolution of 5 μm per pixel.
Multivariate analysis using the characteristics of grains measured from cross-sectional images of sandstone samples revealed that the inversely graded bands constituting spaced stratification can be classified into two different types. Type A bands exhibit bimodal imbrication angles, are composed of coarse grains (average of 0.6 φ), and show local variations in grain size and imbrication angles within the band. In contrast, Type B bands exhibit unimodal imbrication angles, are composed of finer grains (average of 1.2 φ), and do not show local variations in grain size or imbrication angles within the band. These two types of inversely graded bands suggest that spaced stratification may include multiple sedimentary structures formed by different processes.
Although the detailed formative mechanism of the imbrication observed in the two types of bands is still unclear, the differences between the two types may reflect variations in high-density flow characteristics. The imbrication angles observed in the two types of bands were higher (>40°) compared to those in deposits formed by bedforms or low-density turbidity current deposits. Such high imbrication angles have been reported from experimental deposits formed by high-density gravity flows where grains frequently collide with each other (Taira, 1989).
The field survey also suggests that two types of bands constituting spaced stratification may have formed in different depositional environments. Sandstone beds containing Type A bands are observed in the spilled-out deposits from channels, suggesting a deposition by supercritical flows. In contrast, sandstone beds containing Type B bands are observed in the distributary channel-fill deposits in the lobes, suggesting deposition from subcritical flows. Therefore, even though spaced stratification appears to be the same structure to the naked eye, it may have formed under different environments and flow characteristics.