Fukada Geological Institute

We examined 47 specimens of ammonoid and inoceramid bivalves from the Cretaceous Taneichi Formation in Iwate Prefecture, northeastern Japan and assigned them to seven species of ammonoids and two species of inoceramids. Of these, five species of ammonoids, Protexanites ( Anatexanites ) fukazawai (Yabe and Shimizu, 1925); Eubostrychoceras valdelaxum Aiba, Yamato, Kurihara, and Karasawa, 2017; Hyphantoceras transitorium Matsumoto, 1977; Polyptychoceras yubarense (Shimizu, 1935); and Po . obatai (Matsumoto, 1977); and one species of inoceramid, Platyceramus japonicus (Nagao and Matumoto, 1940) are reported for the first time from this Formation. The inoceramid Pl . japonicus and a few biostratigraphically informative ammonoids indicate that the middle Member of the Taneichi Formation was deposited in Santonian–early Campanian (Late Cretaceous). Ammonoid species found in the Taneichi Formation were generally similar to those of the Santonian–lower Campanian in other regions of the northwestern Pacific. Heteromorph taxa were the most abundant ammonoids, accounting for approximately 70 % of the total. Additionally, it is notable that “Leiostraca” taxa, such as Tetragonitoidea and Desmoceratoidea, were not included in the examined specimens. These patterns of occurrence might suggest the palaeoecology of ammonoids, but determining whether they reflect the true distribution requires verification of taphonomic processes, such as post-mortem drifting. • A taxonomic study of 47 specimens of ammonoid and inoceramid bivalves from Cretaceous shallow marine deposits (middle to lower shoreface) of the Taneichi Formation, in northeastern Japan was conducted. • Five species of ammonoids: Protexanites ( Anatexanites ) fukazawai ( Yabe and Shimizu, 1925 ); Eubostrychoceras valdelaxum Aiba, Yamato, Kurihara and Karasawa (2017) ; Hyphantoceras transitorium Matsumoto (1977) ; Polyptychoceras yubarense ( Shimizu, 1935 ); and Po. obatai ( Matsumoto, 1977 ); and one species of inoceramid, Platyceramus japonicus ( Nagao and Matumoto, 1940 ) are newly recorded from the Taneichi Formation. • The depositional age of the Taneichi Formation was re-estimated to range from the Santonian to the lower Campanian, based on the occurrence of a stage diagnostic inoceramid and a few biostratigraphically informative ammonoid species described. • Focusing on the faunal composition of ammonoid, heteromorph taxa occurred abundantly, while “Leiostraca” taxa were not included in the examined specimens. These patterns might reflect the palaeoecology of ammonoids.

IntroductionCarbonate concretions form at shallow burial depths (Raiswell, 1971), with most bicarbonate ions derived from the anaerobic oxidation of organic matter (Claypool and Kaplan, 1974). Specifically, spherical carbonate concretions are proposed to grow concentrically through the diffusion-driven outward migration of the carbonate supersaturation front (Yoshida et al., 2018). Consequently, the center-to-edge isotopic and geochemical profiles of spherical carbonate concretions provide valuable records of pore-water evolution. Zoned spherical concretions, which exhibit distinct zonation, can potentially preserve traces of multiple diagenetic processes. This presents a unique opportunity to reconstruct the temporal and spatial evolution of the pore-water environment at higher resolution. This study presents a detailed analysis of zoned spherical concretions from the Miocene marine sediments of the Atsuta Formation, Japan, to elucidate their formation processes.Results and DiscussionWell-preserved zoned spherical concretions from the Atsuta Formation are subdivided into two distinct zones: a spherical nucleus (Inner-Concretion) and an outer crust (Outer-Concretion) (Fig. 1). Its spherical shape and the presence of fossilized mud shrimp claws at the center suggest a concentric growth mode. Both the Inner- and Outer-Concretion are inferred to have formed rapidly at shallow burial depths, based on the following evidence:Fossilized fecal pellets and mud shrimp claws were found in both the Inner- and Outer-Concretions, exhibiting no signs of compaction. The carbonate content was 83.0 ± 0.5 wt% in the Inner-Concretion and 79.4 ± 0.1 wt% in the Outer-Concretion, indicating precipitation within porous sediments prior to significant compaction. The average stable oxygen isotope ratios (δ18O) were +0.63 ± 0.77‰ (n = 76) in the Inner-Concretion and +0.92 ± 0.48‰ (n = 44) in the Outer-Concretion, suggesting the absence of isotopic fractionation effects related to burial depth. Detailed measurements of stable carbon isotope ratios (δ13C) (n = 129) revealed characteristic trends in each zone, with abrupt changes at their boundaries. The δ13C values in the Inner-Concretion steadily increased from -15‰ near the center to +10‰ toward the edge, reflecting isotopic fractionation associated with enhanced methanogenesis. In contrast, δ13C in the Outer-Concretion was approximately -15‰ and relatively constant. The presence of pyrite throughout the concretion suggests that at least the Outer-Concretion formed within the sulfate reduction zone.These findings collectively indicate a shift in dominant organic matter degradation processes from methanogenesis to sulfate reduction, a trend that deviates from the classical geochemical sequence of sedimentary environments (Berner, 1981). A plausible hypothesis is that methanogenesis occurred entirely within the sulfate reduction zone, potentially as a localized microenvironment around organic matter during the formation of the Inner-Concretion.

Hydraulic fracturing is a crucial technology for resource development, such as shale gas, and its optimization is necessary to enhance development efficiency. However, evaluating fracture shapes involves technical uncertainties. Japan Organization for Metals and Energy Security (JOGMEC) and Kyoto University have conducted laboratory-scale hydraulic fracturing experiments using coagulable fluorescent resin as the injection fluid (resin fracturing test) to visualize hydraulic fractures and investigate their relationship with acoustic emissions (AEs) generated during fracturing. Since lab-scale experiments can only examine the phenomena near the injection hole owing to size limitations, we designed an experiment to apply the visualization method to the outcrop scale. This paper presents the results from an in situ, outcrop-scale hydraulic fracturing experiment conducted at the Kamioka Mine, Gifu Prefecture, Japan, from 2022 to 2023, with goals similar to those of the laboratory experiments. A resin fracturing borehole (RF1) with a diameter of 76 mm was core-drilled to a depth of approximately 10 m for the resin fracturing tests. AEs were observed in five boreholes drilled around RF1 at the same depth. Resin fracturing tests were performed at two different depths, with breakdown confirmed at both. A core of a larger diameter (205 mm) was recovered by coaxial overcoring around RF1, and resin-filled fractures were observed under black light on the core surfaces. After the resin fracturing experiment, two analyses were performed using the acquired core and AE data to predict the fracture extension and the mechanism of AE occurrence. We compared the distribution of AE events and visualized fractures in the core. Additionally, we compared the stress direction estimated from failure mechanism analysis using AE data with the maximum stress direction estimated from hydraulic fracturing. Our analysis provided several insights into fracture extension. The distribution of AE hypocenters was consistent with the direction of the hydraulic fractures confirmed by coring after the resin fracturing test. The failure mechanisms are classified based on the polarity of the first P-wave motion of the AE waveform. However, the actual scale of oil fields is significantly larger than that considered in this study. Discussing visible fractures created by hydraulic fracturing is deemed meaningful. We expect that the results of this study will provide valuable information for the precise estimation of hydraulic fractures.

ABSTRACT The ammonites in spherical carbonate concretions often preserve their original three-dimensional (3-D) shell shapes and detailed fragile structures. However, the formation process of spherical ammonite concretions is not fully understood. Herein, the ammonite concretions identified in the Cretaceous (Campanian) Osoushinai Formation, Yezo Group, Japan, are examined to understand their formation process during soft-tissue decomposition after burial in marine sediments. In the Osoushinai Formation, almost all observed ammonites in concretions preserve their 3-D form without phragmocone deformation. The calcite filling in the remaining body chamber of ammonites (BC1) shows that shells were buried with soft tissues. These occurrences, negative δ13C values, and the near-zero δ18O values of BC1 as well as the concretions indicate that both BC1 and concretions rapidly formed from dissolved inorganic carbon derived from organic matter, including the soft tissue of dead organisms, in the shallow part of the sediments. The increasing Fe concentration in BC1 shows that BC1 formed in the iron-reduction (FeR) zone, where organic matter was decomposed owing to the activity of iron-reducing microorganisms. The similarity of the elemental and isotopic compositions of BC1 and concretions show that they concurrently formed in the FeR zone. In the Osoushinai Formation, an abundant influx of Fe(III) and intense bioturbation during the deposition of the formation promoted organic decomposition in the FeR zone, causing rapid formation of BC1 and concretions. Such rapidly formed calcite fillings and concretions protected fossils from deformation and dissolution during diagenesis to preserve their 3-D form. Overall, the findings of this study provide new insight into the relation between sedimentary environments and the fossil preservation process via rapid concretion formation.

<p>Granite is distributed all over the world and one of the rock types that are very susceptible to various kinds of mass movements including rockfall, rock slide, debris slide and debris avalanche. For example in Japan, Hiroshima rainstorm disasters in 1999, 2014, and 2018, and southern Miyagi rainstorm disaster induced by typhoon 19 in 2019. This is because its special characteristics of formative processes and weathering behavior. The primary structures of granite have long been believed as orthogonal cooling joints since the pioneer work of Cloos (1921, 1922), but we found that a granite body has columnar joints near its roof using UAV and SfM. Whether granite has columnar joints or not leads to different mass movement types. Rock columns separated by columnar joints form high unstable rock towers or tors, which are susceptible to rockfalls. When rock columns are weathered under the ground, they form boulders surrounded by saprolite; when they are eroded to form hills they frequently fail during rainstorms and transform to debris avalanche or debris flow with high destructive potential because of large mass of boulders. Granite without columnar joints is not suitable for spheroidal weathering but is sheeted by unloading; sheeting forms dip slopes, on which rock slides occur. Some granite is micro-sheeted by unloading and micro-sheeted granite is weathered to form a loose soil layer beneath slope surfaces. Such soil layers are very prone to heavy rainfalls and frequently slide, transforming debris avalanches and debris flows.</p><p>Primary structures of granite and following weathering schemes thus define landslide behavior in granite areas.</p>

Abstract Well‐developed tors and associated boulder fields are present in exposed granitoid pluton, but their geomorphological origins have not been fully addressed. Although the tor‐like features are commonly attributed to the orthogonal joint system along which weathering proceeds and the weathering materials are removed, the majority of previous studies lack three‐dimensional (3‐D) morphological observations. One of the well‐known granitoid rock towers in Japan, namely, the obelisk at Mt. Jizogadake in the Japanese Southern Alps, was investigated using unmanned air vehicles (UAVs). A 3‐D observation of the granitoid obelisk found that it is shaped by columnar joints with an undulation along a long axis and irregularly shaped cross‐sections. Because of these features and intersection angles between the rock columns and exposure surfaces, the obelisk exhibits a peculiar morphology appearing as a heap of rock blocks when observed from the ground. The columnar jointing might have occurred in a marginal zone of the granitoid pluton due to faster cooling rates. The 3‐D observations also confirmed the columnar joints formation was predated by high‐angle, planar joints. The columnar joints divide the rock mass between those planar joints. This may imply that the high‐angle joints would have formed probably during cooling under the presence of tectonic and thermal stresses.

Abstract Rock pillars of granite form a special type of characteristic topography in granite, but their origin is not well understood, partly because their 3D morphology has never been well characterized. Rock pillars were investigated at Mt. Mizugaki, which is underlain by Neogene granite that intruded into the Cretaceous accretionary complex in central Japan. Three rock pillars and their surrounding areas were investigated in detail using unmanned aerial vehicle (UAV) images and structure from motion (SfM). The rock pillars were up to 70 m high and were shaped by columnar joints, which has hardly been reported before in granitic rocks. Columnar joints in granite are much larger than columnar joints in volcanic rocks. The columnar joints curved vertically, and some of them showed dipyramid shapes. The large size and curved morphology of the columnar joints may be the result of the slow cooling of the granite, compared to that of volcanic rocks. Thus, the columnar joints at Mt. Mizugaki made large, high rock pillars, which are not common in volcanic rocks.

Spherical carbonate concretions are commonly observed in marine clayey sedimentary strata and often contain well preserved fossils. Previous studies revealed that the spherical concretions are formed by the very rapid reaction with decomposed organic matter from inside and Ca2+ ion of seawater. However, the detailed mass transport process during concretion formation has not been completely understood. Here two different size of spherical concretions, cm size of tusk-shell concretions and metre size of Moeraki boulders, are re-examined to understand the diffusion oriented formation process. Field observations, and detailed mineralogical (XRD) and geochemical analyses (SXAM, XRF, δ13C) revealed diffusive transport of HCO3− from decaying organic matter and Ca2+ from surrounding pore-water of marine origin led to solid carbonate precipitation reactions that progressed from the margin of a concretion. Based on the compositional gradients across the concretions, a diffusion based diagram has been applied to estimate the growth rates of the different size of spherical concretions. The process and rate estimation indicate that even gigantic spherical concretions can form quite rapidly in the muddy matrices under a diffusion-controlled transport regime.

Abstract The aim of this study is to select an appropriate method for CH4 adsorption in organic nanopores for shale‐gas development. Molecular dynamics (MD) and grand canonical Monte Carlo (GCMC) simulations were performed. Three comparison studies were included: (i) comparison of the adsorption behavior in kerogen nanopores using different schemes for dispersion correction, (ii) comparison of the adsorption behavior in graphite nanopores using MD and GCMC simulations, and (iii) comparison of the adsorption behavior in kerogen and graphite nanopores using MD simulations. The result was reliable when using a particle‐mesh Ewald scheme or a cut‐off ≥1.5 nm without dispersion correction. The simulation results were essentially identical for the MD and GCMC simulations. The free‐gas CH4 density inside the nanopores started to deviate from the bulk density at ~2 nm for the graphite model and at ~7–10 nm for the kerogen model, whereas the total CH4 density deviates from the bulk density at ~20 nm.