Lithology and palynology of Neogene sediments on the narrow edge of the Kitakami Massif (basement rocks), northeast Japan: Significant change for depositional environments as a result of plate tectonics

Abstract A controversial stratigraphic section, the Taneichi Formation, is exposed along the Pacific Coast of northeastern Honshu, the main island of the Japanese Archipelago. Although most sediments of the formation have long been dated as late Cretaceous, the northern section of it has been assigned to (i) the Upper Cretaceous; (ii) the Paleogene; or (iii) the Neogene. In the present report, we present the data of palynological and sedimentological studies, showing that the northern section should be assigned to the Neogene. A more important point in the present study is that we invoke some basic principles of fluvial sedimentology to resolve this stratigraphic subject. The lignite layers full of Paleogene–Miocene dinoflagellate cysts and pollen assemblages drape over the boulder‐sized (>40 cm in diameter) clasts in the northern section. However, the layers totally consist of aggregates of small lignite chips, indicating that the lignites are allochthonous materials. The mega‐clasts with derived microfossils in the lignites are thought to have been deposited as Neogene fluvial (flood) sediments in the newly formed Japanese Archipelago. Prior to the Miocene, the northern Honshu was part of the Eurasian Plate, thus the boulder‐sized clasts cannot be envisaged as long river flood deposits along the continental Paleogene Pacific Coast. Instead, the mega‐clasts with the draping lignites were probably derived from nearby Miocene highlands in the newly born island arc.     

龙门山国家地质公园飞来峰成因的新证据

近来发现四川彭州（原称彭县）、什邡一带的飞来峰是第四纪冰川作用形成的。笔者等在塘坝子、葛仙山等飞来峰的底界及周边，多次发现飞来峰是压在早更新世的冰水堆积层之上。葛仙山北寨门、丁家湾，什邡大垭口等剖面的地质现象，充分说明塘坝子、葛仙山等飞来峰是第四纪中更新世时“飞”来，压盖上去的。同时可见飞来峰与第四系之间是“冷接触”，找不到确切的断层证据。笔者等还用现有的区域地质调查资料及新的地质成果，编制了1：10万更新世时的彭县至汶川的地质剖面图，并设想了冰雪层掩盖情况。那时岷江尚未剥蚀下切，按现在标高计的5000m处，即在光光山、太子城之上约1000m的“天”上。当时应在该处出露的石炭系-二叠系石灰岩，就是塘坝子、葛仙山飞来峰的根部。塘坝子、葛仙山就是从那里被冰川作用剪切、从母体分离的石灰岩，滑动、随冰川固体流，滑移到彭县来的。笔者还从理论上阐述了塘坝子、尖峰顶、天台山等如此巨大的山体，能被冰川作用拔起、挖掉、推走的原因和根据。从而解释了飞来峰“飞”行的过程。顺理成章，合乎自然，揭开了飞来峰之迷。     

Chevron Ridges and Runup Deposits in the Bahamas from Storms Late in Oxygen-Isotope Substage 5e

Landward-pointing V-shaped sand ridges several kilometers long are common along the windward margin of the Bahama Islands. Their axes share a northeast–southwest trend. Internally, the ridges contain low-angle oolitic beds with few erosional truncations. Commonly interbedded are tabular, fenestrae-rich beds such as those formed by the sheet flow of water over dry sand. Defined here as “chevron ridges,” these landforms appear to have originated in the rapid remobilization of bank margin ooid bodies by the action of long-period waves from a northeasterly source. Deposits along adjacent coastlines also preserve evidence of the impact of large waves. Reworked eolian sand bodies preserve beach fenestrae and hydraulic scour traces up to +40 m on older ridges. On cliffed coasts, 1000-ton boulders have been thrown well inland, recording the impact of large waves. Amino acid ratios confirm a correlation of the ridges across the archipelago, while stratigraphy, spacing, and cross-cutting relationships indicate emplacement as sea level fell rapidly from the substage 5e maximum at or above +6 m.     

Paired 10Be sampling of polished bedrock and erratic boulders to improve dating of glacial landforms: an example from the Western Alps

Cosmogenic nuclide dating of glacial landforms may lead to ambiguous results for ice retreat histories. The persistence of significant cosmogenic concentrations inherited from previous exposure may increase the apparent exposure ages for polished bedrocks affected by limited erosion under ice and for erratic boulders transported by glaciers and previously exposed in high-altitude rock walls. In contrast, transient burying by moraines, sediments and snow decreases the apparent exposure age. We propose a new sampling strategy, applied to four sites distributed in the Arc and Arve valleys in the Western Alps, to better constrain the factors that can bias exposure ages associated with glacial processes. We used the terrestrial cosmogenic nuclide ¹⁰Be (TCN) to estimate the exposure time from paired sampling of depth profiles in polished bedrock and on overlying erratic boulders. For a given sampling site, the exposure ages for both the polished bedrock and boulder are expected to be the same. However, in six cases out of seven, boulders had significantly higher ¹⁰Be surface concentrations than those of the associated polished surfaces. In present and past glacial processes, the ¹⁰Be distribution with depth for boulders and bedrocks implies the presence of an inheritance concentration of ¹⁰Be. Our study suggests that ¹⁰Be concentrations in erratic boulders and in polished bedrocks provide maximum and minimum exposure ages of the glacial retreat, respectively. © 2019 The Authors. Earth Surface Processes and Landforms published by John Wiley & Sons Ltd     

论雷暴风化作用

在位于冀东地区海拔１８４０多米的都山顶峰，笔者首次发现了雷暴风化作用，并目睹了雷暴风化作用的进行。在都山顶部，发现了雷暴风化砾石堆，分析了雷暴风化作用的物理依据，估算出一次雷暴作用所释放能量的０．０７％－０．００７％就可使１ｍ＾３的花岗岩断裂。阐述了雷暴风化作用的时空分布规律，指出雷暴风化作用主要发生在低纬度近山地地区。气候比较温暖且降水量较多的地史时期是雷暴风化作用较发育阶段。对于中国东部中低山     

Physical and Human Factors Influencing Potential Fish Habitat Distribution along a Mountain River, France

Potential fish habitat along the Drôme River, France, is a function of the distribution of large woody debris, boulders, undercut banks, gravel substrate, and pools. The distribution of these features is, in turn, a function of channel geomorphology, watershed and riparian forest characteristics. We conducted field work and analysed aerial photographs for 190 elementary segments of 500 m length along the Drôme River's 95 km course from the Alps westward to its confluence with the Rhône River near Loriol. The Drôme River does not follow the classic pattern of a monotone downstream decrease in gradient and change in channel characteristics. Although channel gradient, braided index and channel incision all decrease downstream, stream power is independent of longitudinal distance. These variables are largely controlled by geomorphic, human or hydrologic factors at the reach scale. Potential fish habitat richness decreases downstream, but individual habitat variables affecting habitat richness do not necessarily decrease downstream, many being controlled by local factors rather than by position along the continuum. Large woody debris is more abundant in braided reaches located directly downstream of confluences with main tributaries or downstream input sites. Boulders are most abundant downstream of failed bank protection works or in gorges. To improve fish habitat in the Drôme River, we recommend taking a long-term and large-scale perspective. Because structures placed in this unstable channel are likely to be washed downstream, we propose to emulate natural river dynamics and to permit large woody debris to enter the channel in unstable reaches via bank erosion, and that this debris not be removed (as is routinely done now) but permitted to migrate downstream through the system, creating fish habitat en route.     

Prehistorical frequency of high‐energy marine inundation events driven by typhoons in the Bay of Bangkok (Thailand), interpreted from coastal carbonate boulders

Rapidly rising populations of low‐lying megacities in Asia mean that understanding the potential risk of coastal flooding by storm surge is of paramount concern. The city of Bangkok and the wider Chao Phraya River delta at the head of the Gulf of Thailand is a region topographically vulnerable to coastal flooding, but without the record of a high‐energy marine inundation (HEMI) event in historical time owing to the atypical path that a typhoon must take to be able to produce such an event. This work builds upon previous findings that identified coastal depositional evidence for HEMI events in the form of coastal carbonate boulders (CCBs) located on Ko Larn Island in the eastern Bay of Bangkok. The HEMI events were most likely driven by typhoons and the CCBs are therefore interpreted as typhoon deposits. Through uranium/thorium dating, it is revealed that from ad 1400 to ad 1600 the Bay of Bangkok possibly experienced a phase of relatively heightened storm impact. During this period, the frequency of typhoon‐driven HEMI events was approximately four events in 200 years. Waves generated onshore minimum flow velocities (MFVs) in excess of 5 m/s. Such exceptional MFVs are unlikely to be produced during the annual northeast monsoon, but are consistent with typhoon‐impacted coastlines elsewhere in the tropical Asia–Pacific region where similar CCB evidence exists. Since ad 1600, the Bay of Bangkok has enjoyed a relatively quiescent phase, recording less frequent HEMI events and of lower magnitude. However, the re‐occurrence of a typhoon‐driven HEMI event on the scale of the prehistorical events that emplaced carbonate boulders at elevation on Ko Larn Island would threaten low‐lying coasts in the Bay of Bangkok, including the Chao Phraya delta, with potentially damaging inundation. Copyright © 2015 John Wiley & Sons, Ltd.     

Geometry of obstacle marks at instream boulders—integration of laboratory investigations and field observations

Obstacle marks are instream bedforms, typically composed of an upstream frontal scour hole and a downstream sediment accumulation in the vicinity of an obstacle. Local scouring at infrastructure (e.g. bridge piers) is a well-studied phenomenon in hydraulic engineering, while less attention is given to the time-dependent evolution of frontal scour holes at instream boulders and their geometric relations (depth to width, and length ratio). Furthermore, a comparison between laboratory studies and field observations is rare. Therefore, the morphodynamic importance of such scour features to fluvial sediment transport and morphological change is largely unknown. In this study, obstacle marks at boulder-like obstructions were physically modelled in 30 unscaled process-focused flume experiments (runtime per experiment ≥ 5760 min) at a range of flows (subcritical, clear-water conditions, emergent and submerged water levels) and boundary conditions designed to represent the field setting (i.e. obstacle tilting, and limited thickness of the alluvial layer). Additionally, geometries of scour holes at 90 in-situ boulders (diameter ≥ 1 m) located in a 50-km segment of the Colorado River in Marble Canyon (AZ) were measured from a 1 m-resolution digital elevation model. Flume experiments reveal similar evolution of local scouring, irrespective of hydraulic conditions, controlled by the scour incision, whereas the thickness of the alluvial layer and obstacle tilting into the evolving frontal scour hole limit incision. Three temporal evolution phases—(1) rapid incision, (2) decreasing incision, and (3) scour widening—are identified based on statistical analysis of spatiotemporal bed elevation time series. A quantitative model is presented that mechanistically predicts enlargement in local scour length and width based on (1) scour depth, (2) the inclination of scour slopes, and (3) the planform area of the frontal scour hole bottom. The comparison of field observations and laboratory results demonstrates scale invariance of geometry, which implies similitude of processes and form rather than equifinality.     

Putting a spin on palaeotsunami deposits

Palaeotsunami research is a relatively young discipline and while considerable progress has been made in identifying the evidence of past events there are still difficulties on occasion in differentiating between palaeotsunami and palaeostorm deposits. This has tended to focus debate on the similarities and differences between deposits laid down by the two differing processes at the expense of alternative hypotheses. Although coastal research in New Zealand a decade ago drew attention to high elevation coarse sediment deposits laid down by waterspouts, it was largely ignored by the research community. Re‐analysis of a possible palaeotsunami deposit on a nearshore island in New Zealand, however, highlights the likelihood that this deposit, and perhaps many similar coastal ones, may have been laid down by waterspouts. In essence, the palaeotsunami–palaeostorm debate can no longer ignore waterspouts as an alternative hypothesis, but only time will tell how significant a contribution waterspouts have made to catastrophic coastal sedimentation. Copyright © 2016 John Wiley & Sons, Ltd.     

Statistics of Boulder Encounters during Shaft Excavation

Summary. We study the problem of boulder encounter during shaft excavation and the bias that results from directly using statistics from borehole samples without correction for probe diameter. We specifically focus on two quantities, the rate of boulder encounters λ₁ and the total length of obstructed shaft drilling L ₁. Assuming that boulders have spherical shape or cylindrical shape of a particular type, we evaluate how λ₁ and the mean and variance of L ₁ depend on the shaft diameter, the distribution of the boulder diameter, and the minimum boulder diameter that constitutes an obstruction to drilling. The statistics on L ₁ assume that the boulder centers are distributed according to a homogeneous Poisson process in space. Finally, we briefly discuss the problem of inferring the spatial density of boulders and their diameter distribution from borehole data. Both λ₁ and L ₁ increase significantly with increasing shaft diameter; hence the uncorrected statistics from borehole samples severely under-predict these two quantities during large-shaft drilling. Moreover, for typical parameter values, the mean value of L ₁ may be a significant fraction of the total shaft length.