Significance of shallow core transects for reef models and sea‐level curves,Heron Reef,Great Barrier Reef

A sequence of shallow reef cores from Heron Reef, Great Barrier Reef, provides new insights into Holocene reef growth models. Isochron analysis of a leeward core transect suggests that the north‐western end of Heron Reef reached current sea‐level by ca 6·5 kyr bp and then prograded leeward at a rate of ca 19·6 m/kyr between 5·1 kyr and 4·1 kyr bp (pre‐1950) to the present reef margin. A single short core on the opposing margin of the reef is consistent with greater and more recent progradation there. Further to the east, one windward core reached modern sea‐level by ca 6·3 kyr bp , suggesting near ‘keep‐up’ behaviour at that location, but the opposing leeward margin behind the lagoon reached sea‐level much more recently. Hence, Heron Reef exhibited significantly different reef growth behaviour on different parts of the same margin. Mean reef accretion rates calculated from within 20 m of one another in the leeward core transect varied between ca 2·9 m and 4·7 m/kyr depending on relative position in the prograding wedge. These cores serve as a warning regarding the use of isolated cores to inform reef growth rates because apparent aggradation at any given location on a reef varies depending on its location relative to a prograding margin. Only transects of closely spaced cores can document reef behaviour adequately so as to inform reef growth models and sea‐level curves. The cores also emphasize potential problems in U‐series dates for corals within a shallow (ca 1·5 m) zone beneath the reef flat. Apparent age inversions restricted to that active diagenetic zone may reflect remobilization and concentration of Th in irregularly distributed microbialites or biofilms that were missed during sample vetting. Importantly, the Th‐containing contaminant causes ages to appear too old, rather than too young, as would be expected from younger cement.     

Autogenic avulsion,channelization and backfilling dynamics of debris‐flow fans

Alluvial fans develop their semi‐conical shape by quasi‐cyclic avulsions of their geomorphologically active sector from a fixed fan apex. On debris‐flow fans, these quasi‐cyclic avulsions are poorly understood, partly because physical scale experiments on the formation of fans have been limited largely to turbidite and fluvial fans and deltas. In this study, debris‐flow fans were experimentally created under constant extrinsic forcing, and autogenic sequences of backfilling, avulsion and channelization were observed. Backfilling, avulsion and channelization were gradual processes that required multiple successive debris‐flow events. Debris flows avulsed along preferential flow paths given by the balance between steepest descent and flow inertia. In the channelization phase, debris flows became progressively longer and narrower because momentum increasingly focused on the flow front as flow narrowed, resulting in longer run‐out and deeper channels. Backfilling commenced when debris flows reached their maximum possible length and channel depth, as defined by channel slope and debris‐flow volume and composition, after which they progressively shortened and widened until the entire channel was filled and avulsion was initiated. The terminus of deposition moved upstream because the frontal lobe deposits of previous debris flows created a low‐gradient zone forcing deposition. Consequently, the next debris flow was shorter which led to more in‐channel sedimentation, causing more overbank flow in the next debris flow and resulting in reduced momentum to the flow front and shorter runout. This topographic feedback is similar to the interaction between flow and mouth bars forcing backfilling and transitions from channelized to sheet flow in turbidite and fluvial fans and deltas. Debris‐flow avulsion cycles are governed by the same large‐scale topographic compensation that drives avulsion cycles on fluvial and turbidite fans, although the detailed processes are unique to debris‐flow fans. This novel result provides a basis for modelling of debris‐flow fans with applications in hazards and stratigraphy.     

Late‐glacial to Holocene depositional architecture of the Ombrone palaeovalley system (Southern Tuscany,Italy): Sea‐level,climate and local control in valley‐fill variability

The Ombrone palaeovalley was incised during the last glacial sea‐level fall and was infilled during the subsequent Late‐glacial to Holocene transgression. A detailed sedimentological and stratigraphic study of two cores along the palaeovalley axis led to reconstruction of the post‐Last Glacial Maximum valley‐fill history. Stratigraphic correlations show remarkable similarity in the Late‐glacial to early‐Holocene succession, but discrepancy in the Holocene portion of the valley fill. Above the palaeovalley floor, about 60 m below sea‐level, Late‐glacial sedimentation is recorded by an unusually thick alluvial succession dated back to ca 18 cal kyr bp . The Holocene onset was followed by the retrogradational shift from alluvial to coastal facies. In seaward core OM1, the transition from inner to outer estuarine environments marks the maximum deepening of the system. By comparison, in landward core OM2, the emplacement of estuarine conditions was interrupted by renewed continental sedimentation. Swamp to lacustrine facies, stratigraphically equivalent to the fully estuarine facies of core OM1, represent the proximal expression of the maximum flooding zone. This succession reflects location in a confined segment of the valley, just landward of the confluence with a tributary valley. It is likely that sudden sediment input from the tributary produced a topographic threshold, damming the main valley course and isolating its landward segment from the sea. The seaward portion of the Ombrone palaeovalley presents the typical estuarine backfilling succession of allogenically controlled incised valleys. In contrast, in the landward portion of the system, local dynamics completely overwhelmed the sea‐level signal, following marine ingression. This study highlights the complexity of palaeovalley systems, where local morphologies, changes in catchment areas, drainage systems and tributary valleys may produce facies patterns significantly different from the general stratigraphic organization depicted by traditional sequence‐stratigraphic models.     

The alluvial architecture of a suspended sediment dominated meandering river: the Río Bermejo,Argentina

The alluvial architecture of fine‐grained (silt‐bed) meandering rivers remains poorly understood in comparison to the extensive study given to sand‐bed and gravel‐bed channels. This paucity of knowledge stems, in part, from the difficulty of studying such modern rivers and deriving analogue information from which to inform facies models for ancient sediments. This paper employs a new technique, the parametric echosounder, to quantify the subsurface structure of the Río Bermejo, Argentina, which is a predominantly silt‐bed river with a large suspended sediment load. These results show that the parametric echosounder can provide high‐resolution (decimetre) subsurface imaging from fine‐grained rivers that is equivalent to the more commonly used ground‐penetrating radar that has been shown to work well in coarser‐grained rivers. Analysis of the data reveals that the alluvial architecture of the Río Bermejo is characterized by large‐scale inclined heterolithic stratification generated by point‐bar evolution, and associated large‐scale scour surfaces that result from channel migration. The small‐scale and medium‐scale structure of the sedimentary architecture is generated by vertical accretion deposits, bed sets associated with small bars, dunes and climbing ripples and the cut and fill from small cross‐bar channels. This style of alluvial architecture is very different from other modern fine‐grained rivers reported in the literature that emphasize the presence of oblique accretion. The Río Bermejo differs from these other rivers because it is much more active, with very high rates of bank erosion and channel migration. Modern examples of this type of highly active fine‐grained river have been reported rarely in the literature, although ancient examples are more prevalent and show similarities with the alluvial architecture of the Río Bermejo, which thus represents a useful analogue for their identification and interpretation. Although the full spectrum of the sedimentology of fine‐grained rivers has yet to be revealed, meandering rivers dominated by lateral or oblique accretion probably represent end members of such channels, with the specific style of sedimentation being controlled by grain size and sediment load characteristics.     

Early Albian marine environments in Madagascar: An integrated approach based on oxygen,carbon and strontium isotopic data

New palaeotemperature reconstructions have been obtained on the basis of oxygen isotopic analysis of 178 aragonitic shell samples taken from specimens of three ammonoid orders (and some corresponding families): Phylloceratida (Phylloceratidae), Lytoceratida (Tetragonitidae) and Ammonitida (Oppeliidae, Desmoceratidae, Silesitidae, Cleoniceratidae and Douvilleiceratidae). Those obtained from aragonite shells, secreted in the lower epipelagic and in the middle mesopelagic zones during coolest season (winter), range from 15.4 to 16.8 °C, and from 11.8 to 12.0 °C, respectively. Presumed spring/autumn palaeotemperatures obtained from aragonite shells, secreted apparently in the upper and lower epipelagic, upper and middle mesopelagic zones, are somewhat higher. Presumed summer palaeotemperatures, calculated apparently for the upper and lower epipelagic, and upper mesopelagic zones range from 19.4 to 21.7 °C, from 17.7 to 19.4 °C, and from 14.4 to 16.1 °C, respectively. The predominant part of investigated ammonoids from Madagascar inhabited the epipelagic zone, but some phylloceratid, tetragonitid and silesitid ammonoids preferred deeper, cooler conditions (upper-middle mesopelagic zone). The study supports the hypothesis that Madagascar was located in middle latitudes within the tropical-subtropical climatic zone during the early Albian. Available carbon and strontium isotope data allow us to assume a more or less expressed carbon and strontium isotope stratification of the water column in this region in the early Albian. On the basis of the stable isotope data, following partly Lukeneder (2015), two large ethological groups can be recognised mainly in mid-aged and adult ammonoids. Some ammonoids (group 1) preferred apparently mesopelagic conditions, and to a lesser degree the epipelagic zone, being mainly cool-requiring animals. However, a significant part of the isotopically investigated ammonoids (group 2) preferred, on the contrary, only epipelagic conditions, being mainly thermophilic dwellers.     

Modeling discontinuous rock mass based on smoothed finite element method

This paper aims at developing a method for modeling rock mass with preexisting multiple discontinuities within the framework of the smoothed finite element method (SFEM). The discontinuity is simulated by an interface element with zero thickness, the stiffness matrix of which are derived explicitly based on the SFEM. An elastic damage constitutive relation with residual strength is introduced in order to describe the nonlinear mechanical behavior of the discontinuities. The computation codes of the present method were developed. The present method has been verified to be a sound approach for modeling discontinuous rock mass, inheriting the advantages of the SFEM.     

Mesh-free analysis applied in reinforced soil slopes

One of the most applicable geotechnical structures whose analysis is carried out through iterative procedures is the reinforced soil slope. In this regard, the most successful method for the reinforced slope analysis through numerical methods is the finite element method whose updating mesh may result in some difficulties. In this study, the Natural Element Method (NEM), which is a mesh-free method, in conjunction with conventional limit equilibrium is implemented to find the slip surface in the reinforced slopes. Results demonstrate the convergence and preciseness of the present method in comparison with the other numerical methods and conventional limit equilibrium method.     

An efficient quasi-Newton approximation-based SORM to estimate the reliability of geotechnical problems

The first order reliability method (FORM) is efficient, but it has limited accuracy; the second order reliability method (SORM) provides greater accuracy, but with additional computational effort. In this study, a new method which integrates two quasi-Newton approximation algorithms is proposed to efficiently estimate the second order reliability of geotechnical problems with reasonable accuracy. In particular, the Hasofer–Lind–Rackwitz–Fiessler–Broyden–Fletcher–Goldfarb–Shanno (HLRF–BFGS) algorithm is applied to identify the design point on the limit state function (LSF), and consequently to compute the first order reliability index; whereas the Symmetric Rank-one (SR1) algorithm is nested within the HLRF–BFGS algorithm to compute good approximations, yet with a reduced computational effort, of the Hessian matrix required to compute second order reliabilities. Three typical geotechnical problems are employed to demonstrate the ability of the suggested procedure, and advantages of the proposed approach with respect to conventional alternatives are discussed. Results show that the proposed method is able to achieve the accuracy of conventional SORM, but with a reduced computational cost that is equal to the computational cost of HLRF–BFGS-based FORM.     

DEM simulations of methane hydrate exploitation by thermal recovery and depressurization methods

Methane hydrate (MH, also called fiery ice) exists in forms of pore filling, cementing and load-bearing skeleton in the methane hydrate bearing sediment (MHBS) and affects its mechanical behavior greatly. To study the changes of macro-scale and micro-scale mechanical behaviors of MHBS during exploitation by thermal recovery and depressurization methods, a novel 2D thermo-hydro-mechanical bonded contact model was proposed and implemented into a platform of distinct element method (DEM), PFC2D. MHBS samples were first biaxially compressed to different deviator stress levels to model different in-situ stress conditions. With the deviator stress maintained at constant, the temperature was then raised to simulate the thermal recovery process or the pore water pressure (i.e. confining pressure for MH bond) was decreased to simulate the depressurization process. DEM simulation results showed that: during exploitation, the axial strain increased with the increase of temperature (in the thermal recovery method) or decrease of pore water pressure (in the depressurization method); sample collapsed during MH dissociation if the deviator stress applied was larger than the compression strength of a pure host sand sample; sample experienced volume contraction but its void ratio was slightly larger than the pure host sand sample at the same axial strain throughout the test. By comparison with the laboratory test results, the new model was validated to be capable of reproducing the exploitation process by thermal recovery and depressurization methods. In addition, some micro-scale parameters, such as contact distribution, bond distribution, and averaged pure rotation rate, were also analyzed to investigate their relationships with the macroscopic responses.     

The behavior of a multilayered porous thermo-elastic medium with anisotropic thermal diffusivity and permeability

With the aid of integral transform techniques, this paper presents an extended precise integration solution for thermal consolidation problems of a multilayered porous thermo-elastic medium with anisotropic thermal diffusivity and permeability due to a heat source. From the fundamental governing equations, ordinary differential equations are derived by employing Laplace–Hankel transforms. By applying the extended precise integration method, equations in the transformed domain can be solved, and the actual solutions are further obtained by adopting a numerical inverse transformation. The accuracy and feasibility of the proposed theory is demonstrated by contrastive analysis with existing studies. Finally, several examples are carried out to investigate the influence of heat source’s type, axial distance, burial depth of heat source, ratio of thermo-permeability, permeability anisotropy, thermal diffusivity anisotropy and stratification on the thermal consolidation process.