ARENA

In this study, Landsat 7 ETM+ data sets were used to study the geomorphology and mineralogy of different dune types in the eastern part of Abu Dhabi in United Arab Emirates. The extraction and mapping of the dominant mineral phase of the investigated sand dunes were carried out using image processing integrated with field techniques. Band ratios 6/4 and 5/7 were very useful for mineralogical distinction of (i) high mafic content areas (ratio 5/7), and (ii) carbonate and quartz content (ratio 6/4). The results of the present study show that these dunes are classified into several main classes based on composition. The first class is low dunes composed of a mixture of quartz and carbonate, the second class is carbonate-rich, the third quartz-rich and the fourth mafic mineral-rich. Morphologically, the dunes in the investigated area belong to two main types. The first is linear and is trending NE–SW. This type shows transitional change in mineralogy from carbonate-rich components to iron oxide-rich components. This mineral gradient was observed on Landsat images as spectral variations and color tones, and was confirmed from ground truth data. The second type is star dunes and appears in images as radially symmetrical to mound shape that characterize multidirectional wind systems. The results of this study show that multispectral data can be used to differentiate between different dune types and their associated mineralogy, and to reveal information on the dynamic processes shaping dunes, such as prevailing wind directions.

Most submarine canyons are erosive conduits cut deeply into the world’s continental shelves through which sediment is transported from areas of high coastal sediment supply onto large submarine fans. However, many submarine canyons in areas of low sediment supply do not have associated submarine fans and show significantly different morphologies and depositional processes from those of ‘classic’ canyons. Using three-dimensional seismic reflection and core data, this study contrasts these two types of submarine canyons and proposes a bipartite classification scheme.The continental margin of Equatorial Guinea, West Africa during the late Cretaceous was dominated by a classic, erosional, sand-rich, submarine canyon system. This system was abandoned during the Paleogene, but the relict topography was re-activated in the Miocene during tectonic uplift. A subsequent decrease in sediment supply resulted in a drastic transformation in canyon morphology and activity, initiating the ‘Benito’ canyon system. This non-typical canyon system is aggradational rather than erosional, does not indent the shelf edge and has no downslope sediment apron. Smooth, draping seismic reflections indicate that hemipelagic deposition is the chief depositional process aggrading the canyons. Intra-canyon lateral accretion deposits indicate that canyon concavity is maintained by thick (>150 m), dilute, turbidity currents. There is little evidence for erosion, mass-wasting, or sand-rich deposition in the Benito canyon system. When a canyon loses flow access, usually due to piracy, it is abandoned and eventually filled. During canyon abandonment, fluid escape causes the successive formation of ‘cross-canyon ridges’ and pockmark trains along buried canyon axes.Based on comparison of canyons in the study area, we recognize two main types of submarine canyons: ‘Type I’ canyons indent the shelf edge and are linked to areas of high coarse-grained sediment supply, generating erosive canyon morphologies, sand-rich fill, and large downslope submarine fans/aprons. ‘Type II’ canyons do not indent the shelf edge and exhibit smooth, highly aggradational morphologies, mud-rich fill, and a lack of downslope fans/aprons. Type I canyons are dominated by erosive, sandy turbidity currents and mass-wasting, whereas hemipelagic deposition and dilute, sluggish turbidity currents are the main depositional processes sculpting Type II canyons. This morphology-based classification scheme can be used to help predict depositional processes, grain size distributions, and petroleum prospectivity of any submarine canyon.

The undrained response of cohesive soils is of paramount importance in geomechanics and it has been modelled extensively for the last 50 years. In comparison, drained behaviour of clays has received only modest attention. Drained and undrained behaviour is significantly affected by past consolidation stress history. This paper evaluates the capabilities of the MIT-S1 effective stress model, described in a companion paper, for predicting the anisotropic stress-strain-strength behaviour of clays. The paper illustrates the selection of model parameters for Lower Cromer Till, using data from standard types of laboratory tests. Comparison of model simulations with measured response for Lower Cromer Till and Boston Blue Clay illustrate model capabilities. The work focuses initially on comparisons of model predictions with measurements from undrained triaxial and plane strain tests on initially K0-consolidated specimens. Comparisons with measured data from undrained shear tests performed in different modes of shearing for LCT and BBC show that the model: (a) gives excellent predictions of maximum shear stress conditions and accurately describes the non-linear shear stress-strain behaviour; (b) accurately describes the anisotropic shear stress-strain-strength conditions for different consolidation stress histories; and (c) gives realistic description of mobilized friction angles, especially at large OCR's. The paper then focuses on the effects of consolidation stress history for isotropically consolidated specimens of resedimented Lower Cromer Till and Boston Blue Clay. Finally, the paper compares model predictions for drained shear tests on K0 and isotropically consolidated specimens with overconsolidation ratios, OCR10, used to evaluate particular aspects of the critical state framework of soil behaviour. Overall, the model gives excellent predictions of the effect of initial anisotropy and overconsolidation stress history on the shear stress-strain and volumetric behaviour of clays.