Sedimentology and sequence stratigraphy of the middle Jurassic Tarbert Formation, Oseberg South area (northern North Sea)

This paper describes the development of a regressive-to-transgressive shoreline wedge within the Middle Jurassic Tarbert Formation in the Oseberg South area (northern North Sea), as interpreted from core and log data from more than hundred wells. The wedge is described in terms of four facies associations (FA1–FA4). The lower, regressive portion of the wedge (FA1–FA2) contains both coarsening upward wave/storm-dominated shoreline deposits as well as coal-bearing paralic deposits, and was deposited during ascending regression. The upper, transgressive portion of the wedge (FA3–FA4) is characterised by wave-dominated estuarine deposits, exhibiting an upward change from inner to central to outer estuarine deposits. In contrast to some earlier studies, it is argued that this part was deposited during accretionary transgression. The present study documents an estuarine system that developed without any preceding fall of relative sea level and valley incision. It is argued that differential fault-induced subsidence created a broad gentle sag wherein one or several estuarine systems developed as the depositional system became transgressive. The subtle fault-induced subsidence is related to the tectonic evolution in the North Sea Basin.     

Susceptibility evaluation and mapping of China’s landslides based on multi-source data

Landslides are occurring more frequently in China under the conditions of extreme rainfall and changing climate, according to News reports. Landslide hazard assessment remains an international focus on disaster prevention and mitigation, and it is an important step for compiling and quantitatively characterizing landslide damages. This paper collected and analyzed the historical landslide events data of the past 60 years in China. Validated by the frequencies and distributions of landslides, nine key factors (lithology, convexity, slope gradient, slope aspect, elevation, soil property, vegetation coverage, flow, and fracture) are selected to construct landslide susceptibility (LS) empirical models by back-propagation artificial neural network method. By integrating landslide empirical models with surface multi-source geospatial and remote sensing data, this paper further performs a large-scale LS assessment throughout China. The resulting landslide hazard assessment map of China clearly illustrates the hot spots of the high landslide potential areas, mostly concentrated in the southwest. The study implements a complete framework of multi-source data collecting, processing, modeling, and synthesizing that fulfills the assessment of LS and provides a theoretical basis and practical guide for predicting and mitigating landslide disasters potentially throughout China.

     

Sediment chemistry and diatom stratigraphy of two high arctic isolation lakes,Truelove Lowland,Devon Island,N.W.T., Canada

Three sediment cores from two lakes, Fish Lake and Phalarope Lake, in Truelove Lowland, Devon Island, N.W.T. were analyzed for diatoms and chemical composition. Multivariate statistical techniques using a range of chemical variables successfully isolated three sediment groupings in the cores. Allochthonous and autochthonous chemical components in the sediments have been used to reconstruct paleoenvironmental conditions. The two lakes began approximately 10600 years ago as shallow marine lagoons that were isolated from the sea as a result of glacio-isostatic rebound. Based on the presence of distinctive diatom assemblages, the three stratigraphic zones are identified as a basal marine zone, an intermediate and transitional brackish/marine zone and an upper freshwater zone. Following isolation from the sea, the lakes were flushed with freshwater produced by snow and ice melt. In Fish Lake, the period of transition from marine to freshwater, which began approximately 7000 years ago, lasted approximately 800 years. In Phalarope Lake, which was isolated from the sea approximately 5000 years ago, flushing by fresh water was completed only within the last 300 years. Fe, Cr, and Mo in the sediments are associated with the isolation phase when lake sedimentation is sensitive to the presence of brackish water and erosion within the lake catchments. In particular, the precipitation of Mo as MoS₂ reflects the presence of hypolimnetic anoxia associated with lake isolation. During the early post-isolation phase the response of lake biota to an influx of nutrients is reflected in an increase in biological silica and organic carbon in the lake sediments. On the other hand, the generally low organic content of the sediments indicates that sedimentation in these lakes has been largely determined by variations in non-biogenic factors through time. During the mid Holocene the progressive stabilization of surface materials within the lake catchments is marked by decreasing Cr, As and Na in the sediments. At the same time, an increase in allochthonous Mn and Fe is attributed to progressive soil development. During the last 2500 years the catchments have experienced decreased erosion resulting in a decrease in both allochthonous clastic input and lake productivity.     

Comparison of Turbulent Flows and Suspended Sediment Particle Motions Simulated around a Submerged Breakwater Using RANS and LES

Beach erosion is a serious problem that can be aggravated by human-made structures, and the modeling of breaking waves near the coast and around coastal st     

Abyssal currents driven by a local wind forcing through deep mixed layer: implication to the east sea

A simple analytical model is considered in an attempt to demonstrate a formation mechanism of the abyssal current in the East Sea. In this model, the abyssal currents are driven by wind through an outcrop region and flow along closed geostrophic contours. A rough estimate of the abyssal currents, arrived at by applying this model to the region of deep mixing in the East Sea, gives currents comparable to those observed, although there is an uncertainty in the surface area of the outcrop region. It seems that the spin-up of deep water by wind forcing through the region of deep winter mixing is, at least partly, an important contribution to the formation of the abyssal currents in the East Sea.     

Thrust geometries in unconsolidated Quaternary sediments and evolution of the Eupchon Fault, southeast Korea

Abstract The Korean peninsula is widely regarded as being located at the relatively stable eastern margin of the Asian continent. However, more than 10 Quaternary faults have recently been discovered in and reported from the southeastern part of the Korean Peninsula. One of these, the Eupchon Fault, was discovered during the construction of a primary school, and it is located close to a nuclear power plant. To understand the nature and characteristics of the Quaternary Eupchon Fault, we carried out two trench surveys near the discovery site. The fault system includes one main reverse fault (N20°E/40°SE) with approximately 4 m displacement, and a series of branch faults, cutting unconsolidated Quaternary sediments. Structures in the fault system include synthetic and antithetic faults, hanging‐wall anticlines, drag folds, back thrusts, pop‐up structures, flat‐ramp geometries and duplexes, which are very similar to those seen in thrust systems in consolidated rocks. In the upper part of the fault system, several tip damage zones are observed, indicating that the fault system propagates upward and terminates in the upper part of the section. Pebbles along the main fault plane show a preferred orientation of long axes, indicating the fault trace. The unconformity surface between the Quaternary deposits and the underlying Tertiary andesites or Cretaceous sedimentary rocks is displaced by this fault with a reverse movement sense. The stratigraphic relationship shows normal slip sense at the lower part of the section, indicating that the fault had a normal slip movement and was reversely reactivated during the Quaternary. The inferred length of the Quaternary thrust fault, based on the relationship between fault length and displacement, is 200–2000 m. The current maximum horizontal compressive stress direction in this area is generally east‐northeast–west‐southwest, which would be expected to produce oblique slip on the Eupchon Fault, with reverse and right‐lateral strike‐slip components.     

A multi‐azimuth seismic refraction study for a horizontal transverse isotropic medium: physical modelling results

To investigate the characteristics of the anisotropic stratum, a multi‐azimuth seismic refraction technique is proposed in this study since the travel time anomaly of the refraction wave induced by this anisotropic stratum will be large for a far offset receiver. To simplify the problem, a two‐layer (isotropy–horizontal transverse isotropy) model is considered. A new travel time equation of the refracted P‐wave propagation in this two‐layer model is derived, which is the function of the phase and group velocities of the horizontal transverse isotropic stratum. In addition, the measured refraction wave velocity in the physical model experiment is the group velocity. The isotropic intercept time equation of a refraction wave can be directly used to estimate the thickness of the top (isotropic) layer of the two‐layer model because the contrast between the phase and group velocities of the horizontal transverse isotropic medium is seldom greater than 10% in the Earth. If the contrast between the phase and group velocities of an anisotropic medium is small, the approximated travel time equation of a refraction wave is obtained. This equation is only dependent on the group velocity of the horizontal transverse isotropic stratum. The elastic constants A₁₁, A₁₃, and A₃₃ and the Thomsen anisotropic parameter ε of the horizontal transverse isotropic stratum can be estimated using this multi‐azimuth seismic refraction technique. Furthermore, under a condition of weak anisotropy, the Thomsen anisotropic parameter δ of the horizontal transverse isotropic stratum can be estimated by this technique as well.     

Bedload transport processes in a braided gravel-bed river model

A 1:50 scale hydraulic model was designed, based on Froude number similarity and using hydrological and sediment data from a small braided gravel-bed river (the North Branch of the Ashburton River, Canterbury, New Zealand). Eighteen experiments were conducted; seven using steady flows, and eleven using unsteady flows. The experiments were carried out in a 20 m × 3 m tilting flume equipped with a continuous sediment feed and an automated data acquisition and control system. In all experiments water at 30°C was used to reduce viscosity-related scale effects. Analyses of the experimental data revealed that bedload transport rates in braided channels are highly variable, with relative variability being inversely related to mean bedload transport rate. Variability was also found to be cyclic with short-term variations being caused by the migration of bedforms. Bedload transport was found to be more efficient under steady flow than under unsteady flow, and it was postulated that this is caused by a tendency for channel form to evolve towards a condition which maximizes bedload transport for the occurring flow. Average bedload transport rate was found to vary with channel form, although insufficient measurements were made to define a relationship.     

Rates of surface processes on slopes,slope retreat and denudation

Results taken from 270 publications on rates are summarized, and collated with those from 149 publications reviewed previously (Young, 1969, 1974). The data are classified by major climatic zone, normal or steep relief, and consolidated or unconsolidated rocks. Representative rates and their ranges are given for soil creep, solifluction, surface wash, solution (chemical denudation), rock weathering, slope retreat, cliff (free face) retreat, marine cliff retreat, and denudation, the last being compared with representative rates of uplift. Solifluction is of the order of 10 times faster than soil creep, but both cause only very slow ground loss. Solution is an important cause of ground loss for siliceous rocks, on which it may be half as rapid as on limestones. Total denudation, brought about mainly by surface wash, reaches a maximum in the semi-arid and probably also the tropical savanna zones. Acceleration of natural erosion rates by human activities ranges from 2–3 times with moderately intense land use to about 10 times with intensive land use (and considerably higher still where there is recognized accelerated soil erosion). Where there is active uplift, typical rates are of the order of 10 times faster than denudation, although in some high, steep mountain ranges these may approach equality.