Cretaceous to Paleocene depositional history of North-Pacific subduction zone: reconstruction from the Nemuro Group, eastern Hokkaido, northern Japan

The Campanian-Paleocene Nemuro Group comparing the oldest strata in the Kuril Arc, is distributed in the east of Hokkaido Island, northern Japan. Strata of the group in this region are sedimentologically classified into eight depositional facies, most of which are interpreted as sediment gravity flow deposits. These depositional facies comprise four facies associations. The distal and proximal basin plain facies associations are composed mainly of hemipelagic mudstones and sandstones that are interpreted as gravity flow deposits, and were deposited in a topographically flat environment inferred by good lateral continuity of lithofacies. The channel-levee complex and submarine slope facies associations, in contrast, are composed of hemipelagic mudstone, turbidites and debris flow deposits. These four facies associations stack in ascending order, and represent a regressive succession. Palaeocurrent data indicate that the deposits of the Nemuro Group were transported from the northwest. Hence, the group in the study area is concluded to record slope progradation away from the northern source area. Hiertherto, it has been known that the sea regressed from the Kuril Arc during the Eocene. This was attributed to ridge subduction beneath the Kuril Arc. My study has revealed that regression began as early as Maastrichtian. It may have been induced by volcanic activity in the Kuril Arc or by a significant eustatic fall in sea level

Three-dimensional Morphology of the Ichnofossil Phycosiphon incertum and Its Implication for Paleoslope Inclination

Details of the three-dimensional morphology of the ichnofossil Phycosiphon incertum collected from deposits on submarine slopes are reconstructed by processing a series of images obtained from polished sections of the samples. Samples were collected from the mudstone around a slump scar in the Paleocene Shiomi Formation, northern Japan, which is characterized by the occurrence of slump scars. The reconstructed morphology of Phycosiphon incertum is a meandering tube with a flattened ellipse cross section. The tubes are flattened in a plane oblique to the bedding surfaces and aligned along the same direction at both the interior and exterior of the slump scar. Flattening of the tubes was likely caused by sediment compaction, and the tube flattens toward the horizontal plane that is oblique to the bedding plane because of the paleoslope inclination. The difference between the bedding and flattening planes of the tubes of Phycosiphon incertum may imply paleoslope inclination. When the inclination of the bedding plane of the Shiomi Formation is corrected using the flattened surfaces, the bedding plane dips by 9° toward the southeast, which conforms to the paleocurrent direction of the turbidites. The morphology of Phycosiphon incertum can, therefore, be used as a paleoslope indicator

Inverse modeling of turbidity currents using an artificial neural network approach: verification for field application

Although in situ measurements in modern frequently occurring turbidity currents have been performed, the flow characteristics of turbidity currents that occur only once every 100 years and deposit turbidites over a large area have not yet been elucidated. In this study, we propose a method for estimating the paleo-hydraulic conditions of turbidity currents from ancient turbidites by using machine learning. In this method, we hypothesize that turbidity currents result from suspended sediment clouds that flow down a steep slope in a submarine canyon and into a gently sloping basin plain. Using inverse modeling, we reconstruct seven model input parameters including the initial flow depth, the sediment concentration, and the basin slope. A reasonable number (3500) of repetitions of numerical simulations using a one-dimensional layer-averaged model under various input parameters generates a dataset of the characteristic features of turbidites. This artificial dataset is then used for supervised training of a deep-learning neural network (NN) to produce an inverse model capable of estimating paleo-hydraulic conditions from data on the ancient turbidites. The performance of the inverse model is tested using independently generated datasets. Consequently, the NN successfully reconstructs the flow conditions of the test datasets. In addition, the proposed inverse model is quite robust to random errors in the input data. Judging from the results of subsampling tests, inversion of turbidity currents can be conducted if an individual turbidite can be correlated over 10 km at approximately 1 km intervals. These results suggest that the proposed method can sufficiently analyze field-scale turbidity currents

Visualization of the Internal Structure of the Massive Division in Experimental Sediment-Gravity-Flow Deposits by Mapping of Grain Fabric

A method for mapping of grain fabric is proposed for analysis of the cryptic internal structure of massive sedimentary units. The method is applied to the analysis of an experimental debris-flow deposit, revealing a number of characteristic features of this type of mass sedimentation. The debris flow was simulated in the laboratory using a channel inclined 30° opening onto a 10° slope, and transverse thin sections were prepared from four longitudinal points in the depositional lobe. Back-scattered electron images of the sections obtained by scanning electron microscopy were processed and analyzed by mapping of grain fabric using an automated image-analysis procedure. Although the samples appear structureless by macroscopic observation, the grain-fabric map reveals a range of sedimentary features, including distinctive lineations from lower-upcurrent to upper-downcurrent in the most proximal section representing synsedimentary thrusts, a steepening-upward trend of grain imbrication angle in intermediate samples with very low-angle imbrication in the basal horizon, indicative of high-shear-rate flow, and complex imbrication features in the most distal samples. This analysis reveals that massive debris-flow deposits actually contain a range of distinctive structures which are characteristic of the mode of deposition and which are not identifiable by visible inspection or analysis of grain size or color. The proposed method is therefore of great utility for the investigation and characterization of massive deposits

Morphological Function of Trace Fossil Paleodictyon: an Approach from Fluid Simulation

This study examined the functional morphology of the trace fossil Paleodictyon in terms of computational fluid dynamics. The modern specimens show a unique morphology that is composed of a hexagonal mesh structure, vertical shafts opening to the seafloor, and a shield-like mound on the seafloor. The traces of the vertical shafts were also preserved in some fossil examples. To explain their characteristic morphology, a “passive ventilation” hypothesis has been proposed suggesting that their function was to ventilate their burrows with bottom currents, which supply both oxygenated water and food. However, this hypothesis has not yet been verified. This study conducted numerical experiments to understand the functions of the structures created by this ichnofossil by using a model of computational fluid dynamics with the 3D geometry of Paleodictyon and estimating the efficiency of the ventilation in burrows. As a result, it was observed that seawater flowed in the vertical shafts in the marginal area of the mound, and flowed out from the shafts located on the top of the mound, flowing through the mesh structure. This ventilation was observed only in the case that Paleodictyon had a shield-like mound. The ventilation rate rapidly increased as the bottom current velocity increased. In contrast, the rate also increased with the height of the shield-like mounds, whereas it once dropped after the minor peak at 4 mm in height, which corresponds to the value measured in the modern specimens. This coincidence may imply that the height of the mound observed in modern specimens resulted from the optimization in balancing between the efficiency of ventilation and physical stability against erosion. Full exchange of water in the mesh structure by ventilation took less than a few minutes at this mound height, which is presumably sufficient for the ability of Paleodictyon producers

Reconstruction of flow conditions from 2004 Indian Ocean tsunami deposits at the Phra Thong island using a deep neural network inverse model

The 2004 Indian Ocean tsunami caused significant economic losses and a large number of fatalities in the coastal areas. The estimation of tsunami flow conditions using inverse models has become a fundamental aspect of disaster mitigation and management. Here, a case study involving the Phra Thong island, which was affected by the 2004 Indian Ocean tsunami, in Thailand was conducted using inverse modeling that incorporates a deep neural network (DNN). The DNN inverse analysis reconstructed the values of flow conditions such as maximum inundation distance, flow velocity and maximum flow depth, as well as the sediment concentration of five grain-size classes using the thickness and grain-size distribution of the tsunami deposit from the post-tsunami survey around Phra Thong island. The quantification of uncertainty was also reported using the jackknife method. Using other previous models applied to areas in and around Phra Thong island, the predicted flow conditions were compared with the reported observed values and simulated results. The estimated depositional characteristics such as volume per unit area and grain-size distribution were in line with the measured values from the field survey. These qualitative and quantitative comparisons demonstrated that the DNN inverse model is a potential tool for estimating the physical characteristics of modern tsunamis

Understanding flow characteristics from tsunami deposits at Odaka, Joban Coast, using a deep neural network (DNN) inverse model

The 2011 Tohoku-oki tsunami inundated the Joban coastal area in the Odaka region of the city of Minamisoma, up to 2818 m from the shoreline. In this study, the flow characteristics of the tsunami were reconstructed from deposits using the DNN (deep neural network) inverse model, suggesting that the tsunami inundation occurred in the Froude supercritical condition. The DNN inverse model effectively estimated the tsunami flow parameters in the Odaka region, including the maximum inundation distance, flow velocity, maximum flow depth, and sediment concentration. Despite having a few topographical anthropogenic undulations that caused the inundation height to fluctuate greatly, the reconstructed maximum flow depth and flow velocity were reasonable and close to the values reported in the field observations. The reconstructed data around the Odaka region were characterized by an extremely high velocity (12.1 m s⁻¹). This study suggests that the large fluctuation in flow depths on the Joban Coast compared with the stable flow depths in the Sendai Plain can be explained by the inundation in the supercritical flow condition

High-resolution upper Maastrichtian carbon isotope stratigraphy of terrestrial organic matter from northern Japan

High-resolution stable carbon isotope stratigraphy of terrestrial organic matter was established for the upper Maastrichtian Senpohshi Formation of the Nemuro Group in eastern Hokkaido, northern Japan. The Senpohshi Formation, approximately 1, 300 m thick, is dominated by hemipelagic mudstone deposited along an active margin in the North Pacific region. Microscopic observations of extracted kerogen samples from the formation revealed the presence of sedimentary organic matter (SOM), predominantly phytoclasts and a minor amount of non-fluorescent amorphous organic matter, indicating material of a terrestrial higher plant origin. The atomic hydrogen/carbon ratios of the kerogen samples indicated a coalification rank at the anthracite stage or below. Therefore, the stable carbon isotope values of the bulk SOM obtained for the Senpohshi Formation represent the unmodified, original values of terrestrial organic matter. The stable carbon isotope profile reconstructed for the formation provides the first high-resolution terrestrial record of the Mid-Maastrichtian Event (MME), which is comparable to high-resolution marine carbon isotope data from other sections. The carbon isotopic signatures defined by the marine records are recognized in the terrestrial data from the formation, especially in middle to upper part of the event. However, the terrestrial record showed a discrepancy from the marine data in the lower part of the MME, suggesting local variation of the hinterland environment in the North Pacific region. This study provides new insight into environmental changes during the late Maastrichtian by establishing a detailed carbon isotope record of terrestrial materials

Collisional bending of the western Paleo-Kuril Arc deduced from paleomagnetic analysis and U–Pb age determination

The Paleo‐Kuril Arc in the eastern Hokkaido region of Japan, the westernmost part of the Kuril Arc in the northwestern Pacific region, shows a tectonic bent structure. This has been interpreted, using paleomagnetic data, to be the result of block rotations in the Paleo‐Kuril Arc. To understand the timing and origin of this tectonic bent structure in the Paleo‐Kuril arc‐trench system, paleomagnetic surveys and U–Pb radiometric dating were conducted in the Paleogene Urahoro Group, which is distributed in the Shiranuka‐hill region, eastern Hokkaido. The U–Pb radiometric dating indicated that the Urahoro Group was deposited at approximately 39 Ma. Paleomagnetic analysis of the Urahoro Group suggested that the Shiranuka‐hill region experienced a 28° clockwise rotation with respect to East Asia. The degree of clockwise rotation implied from the Urahoro Group is smaller than that of the underlying Lower Eocene Nemuro Group (62°) but larger than that of the overlying Onbetsu Group (−9°). It is thus suggested that the Shiranuka‐hill region experienced a clockwise rotation of approximately 34° between the deposition of the Nemuro and Urahoro Groups (50–39 Ma), and a 38° clockwise rotation between the deposition of the Urahoro and Onbetsu Groups (39–34 Ma). The origin of the curved tectonic belt of the Paleo‐Kuril Arc was previously explained by the opening of the Kuril Basin after 34 Ma. The age constraint for the rotational motion of the Shiranuka‐hill region in this study contradicts this hypothesis. Consequently, it is suggested that the process of arc–arc collision induced the bent structure of the western Paleo‐Kuril Arc