6Although the effects of sedimentation in caves have been recognised for many years, its role in 7 speleogenesis is frequently overlooked. Influxes of sediment into a cave system fundamentally alters 8 the way cave passages develop, either by alluviation in a vadose environment, forcing lateral 9 corrosion and the development of notches, or upwards dissolution in a phreatic environment through a 10 process known as paragenesis. Sediment influxes affect the hydrological functioning of a karst aquifer 11 by changing the way conduits behave and subsequently develop both in plan and long section. 12Here we give an overview of the mechanisms of cave sedimentation and describe how the process of 13 alluviation and paragenesis affect speleogenesis. A characteristic suite of meso-and micro-scale 14 dissolutional features can be used to recognise paragenetic development, which is reviewed here. In a 15 vadose environment these include alluvial notches, whilst in a phreatic environment, half tubes, 16 anastomoses and pendants, bedrock fins and paragenetic dissolution ramps result. Using these to 17 identify phases of sedimentation and paragenesis is crucial for reconstructing denudation chronologies 18 from cave deposits. We suggest that sedimentation and paragenesis is most likely to occur in certain 19 geomorphological situations, such as ice marginal and periglacial environments, beneath thick 20 residual soils and where rivers can transport fluvial sediment into a cave, either via stream sinks or 21 back-flooding. 22
The dispersal of human populations out of Africa into Arabia was most likely linked to episodes of climatic amelioration, when increased monsoon rainfall led to the activation of drainage systems, improved freshwater availability, and the development of regional vegetation. Here we present the first dated terrestrial record from southeast Arabia that provides evidence for increased rainfall and the expansion of vegetation during both glacial and interglacial periods. Findings from extensive alluvial fan deposits indicate that drainage system activation occurred during Marine Isotope Stage (MIS) 6
An essential prerequisite for any engineering or hydrogeological investigation of soluble rocks is the identification and description of dissolution features such as stream sinks, springs, sinkholes and caves. The British Geological Survey (BGS) is creating a National Karst Database that records such features across the country. The database currently covers much of the Carboniferous Limestone, the Chalk and particularly the Permo-Triassic gypsum and halite where rapid, active dissolution has caused significant subsidence and building damage. In addition to the identification of specific karst features, the BGS has created a National Karst Geohazard Geographical Information System (GIS). This has been guided by the National Karst Database and created by identifying all the soluble lithologies from the BGS 1:50 000 scale digital geological map and giving each a score, based on factors including lithology, topography, geomorphic position and superficial cover deposits. This national zonation of the soluble rocks can be used to identify areas where the potential for karstic features to occur is significant and where dissolution features might affect the stability of buildings and infrastructure, or where karstic groundwater flow might occur. Both datasets are invaluable scientific tools that have been widely used to support site investigation, groundwater investigations, planning, construction and the insurance businesses. Word Count 7530 No. References 34 No. Tables 7 No. Figures 8 Abbreviated title: Karst geohazards in the UK Karst features, developed over and within soluble rocks, are a well-known potential geohazard, and can cause significant engineering problems, such as subsidence and irregular rockhead. These can pose difficulties for planning and development and be very costly for the construction and insurance industries. There have been numerous examples of subsidence and infrastructure damage resulting from settlement and or collapse of karst features (Waltham et al., 2005); in extreme cases they can cause properties to collapse and put lives at risk. More commonly, karstic rocks can make ground conditions more difficult, increasing construction costs. Underground cavities can also act as pathways along which hazardous liquid and gaseous contaminants can travel, commonly some distance from their source, thus posing an environmental risk. Databases and maps of karst hazards are important for understanding the severity of the problem, and they constitute useful tools for hazard avoidance that have relevance to planning, engineering, development and the insurance industry. Developers, planners and local government can only operate effectively if they have advance warning about the hazards that might be present and have access to relevant geological information. In the UK, karst is most typically associated with the locally varied limestone successions of Early Carboniferous age, referred to informally as the Carboniferous Limestone, but karst features are also found in a host of other carbonate and evaporite...
Climatic changes in Arabia are of critical importance to our understanding of both monsoon variability and the dispersal of anatomically modern humans (AMH) out of Africa. The timing of dispersal is associated with the occurrence of pluvial periods during Marine Isotope Stage (MIS) 5 (ca. 130-74ka), after which, arid conditions between ca. 74 and 10.5 ka are thought to have restricted further migration and range expansion within the Arabian interior. Whilst a number of records indicate that this phase of aridity was punctuated by an increase in monsoon strength during MIS 3, uncertainties regarding the precision of terrestrial records and suitability of marine archives as records of precipitation, mean that the occurrence of this pluvial remains debated. Here we present evidence from a series of relict lake deposits within southeastern Arabia, which formed at the onset of MIS 3 (ca. 61-58 ka). At this time, the incursion of monsoon rainfall into the Arabian interior activated a network of channels associated with an alluvial fan system along the western flanks of the Hajar Mountains, leading to lake formation. Multiproxy evidence indicates that precipitation increases intermittently recharged fluvial systems within the region, leading to lake expansion in distal fan zones. Conversely, decreased precipitation led to reduced channel flow, lake contraction and a shift to saline conditions. These findings are in contrast to the many other palaeoclimatic records from Arabia, which suggest that during MIS 3, the latitudinal position of the monsoon was substantially further south and did not penetrate the peninsula. Additionally, the occurrence of increased rainfall at this time challenges the notion that the climate of Arabia following MIS 5 was too harsh to permit the further range expansion of indigenous communities.
Located at the crossroads between Africa and Eurasia, Arabia occupies a pivotal position for human migration and dispersal during the Late Pleistocene. Deducing the timing of humid and arid phases is critical to understanding when the Rub' al-Khali desert acted as a barrier to human movement and settlement. Recent geological mapping in the northern part of the Rub' al-Khali has enabled the Quaternary history of the region to be put into a regional stratigraphical framework. In addition to the active dunes, two significant palaeodune sequences have been identified. Dating of key sections has enabled a chronology of dune accretion and stabilisation to be determined. In addition, previously published optically stimulated luminescence (OSL) dates have been put in their proper stratigraphical context, from which a record of Late Pleistocene dune activity can be constructed. The results indicate the record of dune activity in the northern Rub' al-Khali is preservation limited and is synchronous with humid events driven by the incursion of the Indian Ocean monsoon.
Although the Chalk is only weakly karstified, tracer testing from stream sinks has demonstrated groundwater flow velocities comparable to those observed in highly karstic aquifers. Field survey of surface karst features in the catchments of the Pang and Lambourn rivers in southern England demonstrates the importance of overlying and adjacent Palaeogene strata in the development of karst features. Tracer techniques employed within the catchments enable further characterisation of the range and connectivity of solutional voids in this area of the Chalk, and allow assessment of the relative importance of different mechanisms of contaminant attenuation. Quantitative tracer test results suggest that groundwater flow may be through a complex combination of small conduits, typically 10 to 1000 mm in diameter, and more laterally extensive fissures with apertures of 1 to 50 mm. Evidence of connectivity between conduits and fissures suggest that in areas of the Chalk with rapid groundwater flow, fissures supplying abstraction boreholes may be connected to karst conduit networks with low potential for contaminant attenuation.
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