On how thin submarine flows transported large volumes of sand for hundreds of kilometres across a flat basin plain without eroding the sea floor

Submarine gravity currents, especially long run‐out flows that reach the deep ocean, are exceptionally difficult to monitor in action, hence there is a need to reconstruct how these flows behave from their deposits. This study mapped five individual flow deposits (beds) across the Agadir Basin, offshore north‐west Africa. This is the only data set where bed shape, internal distribution of lithofacies, changes in grain size and sea floor gradient, bed volumes, flow thickness and depth of erosion into underlying hemipelagic mud are known for individual beds. Some flows were 30 to 120 m thick. However, flows with the highest fraction of sand were less than 5 to 14 m thick. Sand was most likely to be carried in the lower 5 to 7 m of these flows. Despite being relatively thin, one flow was capable of transporting very large volumes of sediment (ca 200 km³) for large distances across very flat sea floor. These observations show that these relatively thin flows could travel quickly enough on very low gradients (0·02° to 0·05°) to suspend sand several metres to tens of metres above the sea floor, and maintain those speeds for up to 250 km across the basin. Near uniform hemipelagic mud interval thickness between beds, and coccolith assemblages in the mud caps of beds, suggest that the flows did not erode significantly into the underlying sea floor mud. Simple calculations imply that some flows, especially in the proximal part of the basin, were powerful enough to have eroded hemipelagic mud if it was exposed to the flow. This suggests that the flows were depositional from the moment they arrived at a basin plain location, and that deposition shielded the underlying hemipelagic mud from erosion. Reproducing the field observations outlined in this exceptionally detailed field data set is a challenge for future experimental and numerical models.     

A multi‐dimensional analysis of pro‐glacial landscape change at Sólheimajökull,southern Iceland

Pro‐glacial landscapes are some of the most active on Earth. Previous studies of pro‐glacial landscape change have often been restricted to considering either sedimentological, geomorphological or topographic parameters in isolation and are often mono‐dimensional. This study utilized field surveys and digital elevation model (DEM) analyses to quantify planform, elevation and volumetric pro‐glacial landscape change at Sólheimajökull in southern Iceland for multiple time periods spanning from 1960 to 2010. As expected, the most intense geomorphological changes persistently occurred in the ice‐proximal area. During 1960 to 1996 the pro‐glacial river was relatively stable. However, after 2001 braiding intensity was higher, channel slope shallower and there was a shift from overall incision to aggradation. Attributing these pro‐glacial river channel changes to the 1999 jökulhlaup is ambiguous because it coincided with a switch from a period of glacier advance to that of glacier retreat. Furthermore, glacier retreat (of ~40 m yr^?1) coincided with ice‐marginal lake development and these two factors have both altered the pro‐glacial river channel head elevation. From 2001 to 2010 progressive increase in channel braiding and progressive downstream incision occurred; these together probably reflecting stream power due to increased glacier ablation and reduced sediment supply due to trapping of sediment by the developing ice‐marginal lake. Overall, this study highlights rapid spatiotemporal pro‐glacial landscape reactions to changes in glacial meltwater runoff regimes, glacier terminus position, sediment supply and episodic events such as jökuhlaups. Recognizing the interplay of these controlling factors on pro‐glacial landscapes will be important for understanding the geological record and for landscape stability assessments. Copyright © 2014 John Wiley & Sons, Ltd.     

Quaternary environmental changes in tropical Lake Towuti,Indonesia, inferred from end-member modelling of X-ray fluorescence core-scanning data

Continental and marine sediments are composed of a mixture from different sources and are influenced by a variety of environmental factors and transport processes prior to deposition. For analysis and interpretation, these sources and processes are often challenging to disentangle. We show that end-member modelling of X-ray fluorescence (XRF) core-scanning data helps to overcome these challenges by unmixing different environmental signals from high-resolution sediment geochemical records. We apply this approach to a 100 m long lacustrine succession from Lake Towuti, Indonesia, to separate the regional climate and tectonic history from local ecological and diagenetic processes. The resulting six end-members (EMs) are interpreted to represent changes in ecological (EM1), climatic (EMs 2–4), tectonic (EM 5) and geomorphic (EM6) processes determining changes in sediment composition. Because end-member analysis allows for the tracking of transient and overlapping processes, climatic changes can be followed throughout the 100 m-long succession, suggesting alternating wet and dry periods in Central Sulawesi over long (several 100 000 years) time scales. We show that end-member analysis on elemental data sets offers a detailed and objective means to disentangle depositional processes in sedimentary successions resulting from varying tectonic and environmental factors involved in sediment formation and deposition.     

Contours of saturated and unsaturated overhanging slopes at limiting equilibrium condition

Overhangs are frequently observed in riverbanks, coastal headlands, and rock formations. The geometry of an overhang is an input into most slope stability analyses and is often idealised or back-calculated from empirical data. This study investigates the geometries of overhang slopes, which exist while in limiting conditions satisfying static equilibrium with soil strength governed by the Mohr-Coulomb failure criterion. The overhanging contour is formulated as the unknown in a boundary value problem and solved for using the slip line theory. Analyses consider nonhomogeneous soils, where cohesion and, if unsaturated, the contribution of suction to the effective stress vary linearly with depth. The solutions are presented in general dimensionless charts. Applications of the charts are illustrated via examples. It has been observed that soil with varying amounts of friction and cohesion could develop overhanging arches of different sizes and shapes. This study shows that the curvature of an overhang becomes more pronounced for small values of φ′ . It is also demonstrated that changing the contribution of suction to the effective stress has a direct impact on the size of an unsaturated soil overhang. Overhanging slope shapes observed in reality may be different from the idealised subset studied in this paper since real slopes are not necessarily at impending failure. The real shapes may be influenced by various physical processes such as weathering, stress variations caused by cycles of wetting-drying. Even so, the results presented in the paper indicated how key soil properties influence slope shapes, albeit it while in limiting conditions.     

Linking interannual river flow river variability across New Zealand to the Southern Annular Mode, 1979–2011

River flow constitutes an important element of the terrestrial branch of the hydrological cycle, yet knowledge regarding the extent to which its variability, at a range of timescales, is linked to a number of modes of atmospheric circulation is meagre. This is especially so in the Southern Hemisphere where strong candidates, such as El Niño Southern Oscillation and the Southern Annular Mode (SAM), for influencing climate and thus river flow variability can be found. This paper presents the results of an analysis of the impact of the SAM on winter and summer river flow variability across New Zealand, purposefully controlling for the influence of El Niño Southern Oscillation and the tendency for the SAM to adopt a positive phase over the last 10–20 years. Study results, based on identifying hydrological regions and applying circulation‐to‐environment and environment‐to‐circulation approaches commonly used in synoptic climatology, reveal a seasonal asymmetry of the response of river flow variability to the SAM; winter flows demonstrate a higher degree of statistical association with the SAM compared to summer flows. Further, because of the complex orography of New Zealand and its general disposition normal to zonal flows of moisture bearing winds, there are intraseasonal spatial variations in river flow SAM associations with clear rain shadow effects playing out in resultant river flow volumes. The complexity of SAM river flow associations found in this study warns against using indices of large scale modes of atmospheric circulation as blunt tools for hydroclimatological prediction at scales beyond hydroclimatological regions or areas with internal hydrological consistency.     

The Quassaic Group,a Medial to Late Ordovician arenite sequence in the Marlboro Mountains Outlier,mid-Hudson Valley,New York,U.S.A

The Quassaic Group occurs as an outlier in the north-south trending Marlboro Syncline which extends 40 km between Kingston and Newburgh in southeastern New York. The group is comprised primarily of 3050 m of late Medial to medial Late Ordovician marine arenites. It is subdivided (oldest to youngest) into five formations: Creek Locks, Rifton, Shaupeneak. Slab Sides, and Chodikee. On the western limb of the syncline the group overlies a minimum of 610 m of the early late Medial Ordovician Bushkill Shale and the bottom three formations grade north and south to Bushkill Shale. Abundance of graded but occasionally conglomeratic beds, common tabular cross-lamination, much less common trough cross-lamination, comparative lack of channelling, general lack of shales, and scarcity of fossils indicate that the average depositional environment of the formations was near the base of the slope of a sedimentary apron or delta which lay to the southeast. The environment, however, probably extended from a basin plain, in the marginal Bushkill Shale, through the lower and middle slope and, rarely, to the upper slope. The occasional molasse-like deposits in the Shaupeneak Formation may indicate local transport of molasse to an upper slope position. The Quassaic Group thus records an initial, local extension of arenites of the lower slope of an apron or delta onto a downwarping basin plain in the Bushkill Trough. This was followed by progressive enlargement and extension of the delta slope with attendant shallowing, culminated by the appearance of molasse-like sediments. Subsequent deposition involved greater slope extension followed by recession with gradual deepening. These activities were prelude to the Hudson Valley Phase of the Taconian Orogeny which downfolded the Quassaic Group into the Marlboro Syncline during latest Ordovician times.     

Mapping the archaeological soil archive of sand and gravel mineral reserves in Britain

Primary sand and gravel deposits in Britain play an important role in preserving our cultural heritage and are also a valuable aggregate resource. While an understanding of the extrinsic properties of the soil archive (such as pH, redox, groundwater) can provide a firstorder assessment of the potential risk to any archaeologically sensitive deposits, we have very poor definition of spatial variations in the extrinsic properties of soil that influence archaeological preservation at a regional and national scale. Developments in digital geological mapping, remote sensing, and geochemical survey data undertaken by the British Geological Survey (BGS) have, however, significantly extended capabilities in this respect and can potentially be used to provide a primary assessment of the sensitivity of the present soil archive and the potential risk from changes to the soil process on cultural material in areas earmarked for aggregate extraction. Two of the major factors affecting archaeological preservation—soil acidification and groundwater—can be mapped or predicted at scales of better than 1:50,000 across increasingly large parts of the country using a combination of regional hydrogeological, geophysical, and geochemical data. Additional data from site investigations may further refine preservation potential as a function of changes in redox potential and acidity. These data, maps and models can be used to (1) better establish a baseline for archaeological preservation at a regional and national scale and (2) improve our understanding of how the physical and chemical properties of the near surface environment can be managed to sustainably preserve archaeological materials in areas impacted by sand and gravel extraction. © 2009 Wiley Periodicals, Inc.     

Geophysical Surveys in the Jebel Hamrat Fidan,Jordan

The Jebel (Jebel is mountain in Arabic) Hamrat Fidan marks the “gateway” to the Feinan district of southern Jordan—one of the largest sources of copper during the prehistoric and Early Bronze Ages in the eastern Mediterranean. Preliminary excavations and surveys at sites along the Wadi Fidan have revealed a long history of settled occupation extending from the Pre‐Pottery Neolithic (ca. 6,500 B.C.) to early medieval times. Because of this long history of occupation, and the fact that this area was a regional center for the production of copper, the study of this area is important for understanding early metallurgy, craft specialization, and social evolution. During the summer of 1997, geophysical investigations at a series of Neolithic and Bronze Age sites identified specific areas within Wadi Fidan for future intensive excavations. Three geophysical techniques (electromagnetic induction, ground‐penetrating radar, and magnetometry) were used to help locate buried architectural and industrial features remaining from early mining and metallurgical operations, including copper ore bodies or voids. Geophysics was not used at the actual mining sites because of scheduling constraints; however, geophysics did delineate buried stone walls at three distinct Wadi Fidan sites. Magnetometry and ground penetrating radar provided little useful information. Buried stone walls were apparently “masked” by numerous magnetic stones on the ground surface making magnetometry useless. Reflections from known strata demonstrated that radar penetrated the ground adequately; however, known shallowly buried walls were not recognizable. Electromagnetic induction produced maps of linear and rectilinear features that suggested spatial distribution of widespread buried stone walls suitable for future excavation. A significant and unexpected finding was that electromagnetic induction proved capable of delineating buried stone walls. © 2000 John Wiley & Sons, Inc.