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[HCG22-10] Grain fabric analysis of the spaced stratification in the Upper Cretaceous Izumi Group, Nada Formation
Keywords:Spaced stratification, Traction carpet, Izumi Group, CNN
The spaced stratification, also known as the traction carpet deposit, occurs in coarse-grained sandstones deposited from sediment gravity flows. The spaced stratification is composed of repetitive inverse or normal grading layers. This structure has been reported from deposits formed by various sediment gravity flows, such as turbidity currents, hyperconcentrated flows, and pyroclastic flows. Several studies have suggested that the spaced stratification was deposited from high-concentration sediment-laden flows. The vertical variation of grain size and grain fabric of the spaced stratification has rarely been measured in high resolution, although grain size and fabric are thought to be important in reconstructing the state of the flow that formed the spaced stratification.
In this study, the CNN (Convolutional Neural Network) was used to observe the microtextures of the spaced stratification in high resolution. We applied the method to the coarse sandstone samples showing spaced stratification that were collected from the Nada Formation in the southeastern part of Awaji Island, Hyogo Prefecture. The Nada Formation, which comprises a main part of the Upper Cretaceous Izumi Group, is characterized by sand-dominated alternating beds of sandstone, conglomerate, and mudstone. The sandstone samples were cut and polished, and then a desktop document scanner took the high-resolution images of their polished cross-sections parallel to the paleocurrent directions. The pre-trained CNN models were applied to the obtained images of the vertical rock sections to classify grains and matrices of sandstones. We then measured spatial variations in grain size and fabric at the mm scale.
As a result of grain size and fabric analyses, we found several different types of normal- or inverse-graded layers in the spaced stratification. Grain size and fabric analysis indicated that some poorly sorted layers have high angle (>30°) or random grain imbrications, while the other layers are well sorted and have low angle (<20°) imbrications. Also, layers with low angle imbrications are usually on layers with high angle or random imbrications. Thus, this result suggests that several different flow can form the spaced stratification. In future work, it is necessary to analyze the grain fabric of deposits in different depositional environments to distinguish the diversity and formation process of the spaced stratification.
In this study, the CNN (Convolutional Neural Network) was used to observe the microtextures of the spaced stratification in high resolution. We applied the method to the coarse sandstone samples showing spaced stratification that were collected from the Nada Formation in the southeastern part of Awaji Island, Hyogo Prefecture. The Nada Formation, which comprises a main part of the Upper Cretaceous Izumi Group, is characterized by sand-dominated alternating beds of sandstone, conglomerate, and mudstone. The sandstone samples were cut and polished, and then a desktop document scanner took the high-resolution images of their polished cross-sections parallel to the paleocurrent directions. The pre-trained CNN models were applied to the obtained images of the vertical rock sections to classify grains and matrices of sandstones. We then measured spatial variations in grain size and fabric at the mm scale.
As a result of grain size and fabric analyses, we found several different types of normal- or inverse-graded layers in the spaced stratification. Grain size and fabric analysis indicated that some poorly sorted layers have high angle (>30°) or random grain imbrications, while the other layers are well sorted and have low angle (<20°) imbrications. Also, layers with low angle imbrications are usually on layers with high angle or random imbrications. Thus, this result suggests that several different flow can form the spaced stratification. In future work, it is necessary to analyze the grain fabric of deposits in different depositional environments to distinguish the diversity and formation process of the spaced stratification.