A.R. Farrant

Long-term Quaternary uplift rates inferred from limestone caves in Sarawak, Malaysia

ABSTRACTTherateoflong-term(2m.y.)base-levelloweringestimatedinanextensivesequenceoflimestonecavesinSarawak,Malaysia,fromuraniumseries,electronspinresonance,andpaleomagneticdatingis0.19 1 0.03/ 2 0.04m/ka.Thisratehasremainedconstantoveratleastthelast700ka,asshownbycomparisonofthenumberandspacingofwallnotchesformedduringphasesofinterstadialandinterglacialaggradationwithpeaksinthedeep-seaoxygenisotopecurve.Itisarguedthatbase-levelloweringoccursinresponsetoepi-rogenicupliftofthemoreresistantlimestonesduetoregionaldenudationofthesoftershales,andtoflexuralisostacyassociatedwithhighratesofoffshoresedimentation.INTRODUCTION Ratesofupliftareneededtoconstrainmodelsofreliefgeneration(PazzagliaandGardiner,1994)andtectonicprocesses(En-glandandMolnar,1990)overgeologictimescales.Mostestimatesofuplifthavecomefromtectonicallyactivecoasts,whereemer-gentmarineterracescanbedatedusingura-nium series and other techniques (e.g.,Bloometal.,1974),andstudiesofriverter-racesusingradiocarbondating(e.g.,Mer-rittsetal.,1994).However,althoughlong-termsequencesofbothcoastalandriverterraceshavebeenobtained(Pirazzolietal.,1993;PazzagliaandGardiner,1994),thechronologicalcontrolontheseisoftenpoor.Furthermore,fluvialterracesarenotubiq-uitous;theirdevelopmentdependsoncrit-icalinterrelationsbetweenupliftrate,basinarea,andtheeffectsofeustaticsealevel(Merrittsetal.,1994).Thusfewwell-con-strainedestimatesofratesofupliftoverthelongertimescaleareavailable.Herewepro-videestimatesofthelong-termrateofbase-levellowering(Merrittsetal.,1994)derivedfromuraniumseriesandpaleomagneticdat-ingofdepositsfromextensivecavesystemsin the Gunung Mulu National Park,Sarawak,Malaysia.

Karst geohazards in the UK: the use of digital data for hazard management

An essential prerequisite for any engineering or hydrogeological investigation of soluble rocks is the identification and description of their characteristic, observable and detectable dissolution features, such as stream sinks, springs, sinkholes and caves. The British Geological Survey (BGS) is creating a National Karst Database (NKD) that records such features across the country. The database currently covers much of the region underlain by 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, and separate from, the identification of specific karst features, the BGS has created a National Karst Geohazard geographical information system (GIS). This has been created to identify areas that may potentially contain karst geohazards. Initially, all the soluble rock units identified from the BGS 1:50 000 scale digital geological map are extracted. Each soluble unit has been given an objective score, interpreted, as based on factors including lithology, topography, geomorphological position and characteristic superficial cover deposits. This national zonation of these soluble rocks can then be used to identify areas where the occurrence potential for karstic features is significant, and where dissolution features might affect the stability of buildings and infrastructure, or where karstic groundwater flow might occur. Both datasets are seen as invaluable scientific tools that have already been widely used to support site investigation, groundwater investigations, planning, construction and the insurance underwriting businesses.

An analysis of Cotswold topography: insights into the landscape response to denudational isostasy

We have used the NEXTMap Britain digital terrain model (DTM) to determine the lithospheric response to erosional unloading and the contribution of tectonics, in the form of elastic plate flexure, to the Cotswold ‘scarp and vale’ landscape. The calculations take into account lithology variations and along-strike changes in escarpment retreat. We show that flexural rock uplift as a result of erosional unloading varies spatially and may contribute up to 50% of the relief in the Cotswold region. This is supported by morphometric analysis, of concavity and steepness, for 66 longitudinal river profiles that drain the scarp and dip slope of the escarpment. Viscoelastic plate models suggest that the uplift is initially rapid (up to 8 m ka−1) and essentially complete within 50 ka. These initial rates are compatible with an early post-Anglian incision rate inferred from the Thames terraces. The ‘staircase’ terrace pattern suggests, however, that there have been a number of denudational isostatic events, each associated with a climate cycle. Finally, the analysis reveals an inherited ‘proto-landscape’ that has a subdued relief when compared with the modern DTM. Such a relief is consistent with an early extension of the River Thames, through the Vale of Moreton, to the north of the present-day Cotswold Hills.

Alluvial fan records from southeast Arabia reveal multiple windows for human dispersal

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 (ca. 160–150 ka), MIS 5 (ca. 130–75 ka), and during early MIS 3 (ca. 55 ka). The development of active freshwater systems during these periods corresponds with monsoon intensity increases during insolation maxima, suggesting that humid periods in Arabia were not confined to eccentricity-paced deglaciations, and providing paleoenvironmental support for multiple windows of opportunity for dispersal out of Africa during the late Pleistocene.

Near-surface diagenesis of ophiolite-derived conglomerates of the Barzaman Formation, United Arab Emirates: a natural analogue for permanent CO2 sequestration via mineral carbonation of ultramafic rocks

Abstract Carbon capture and storage by mineralization is a potential method for storing anthropogenic CO2 emissions, and is based on the reaction between Mg silicate and CO2 to form Mg carbonate. The conglomerates of the Barzaman Formation exposed in the eastern United Arab Emirates represent an excellent natural analogue of this process. These conglomerates were deposited as a series of alluvial fans along the western margin of the Hajar Mountains, part of the Oman-UAE Ophiolite, and are composed largely of ultramafic and lesser-mafic clasts. The clasts and matrix have been extensively altered to dolomite during diagenetic processes. Analysis and interpretation of rock textures provide evidence for the various factors that influenced the diagenetic processes and shed light on the silicate–carbonate transformation process. All the reactions have taken place in the near-surface environment; the silicate–carbonate conversion reaction is exothermic and occurs spontaneously at near-ambient pressure and temperature, probably no greater than 50 °C. Estimates of the amount of CO2 stored in this way can be obtained from considerations of outcrop area, formation thickness and percentage of dolomite replacement, and show that c. 150 billion tonnes (equivalent to about 4 years of worldwide CO2 emissions at current rates) are stored.

Speleothem U-series constraints on scarp retreat rates and landscape evolution: an example from the Severn valley and Cotswold Hills gull-caves, UK

Modelling landscape evolution requires quantitative estimates of erosional processes. Dating erosional landscape features such as escarpments is usually difficult because of the lack of datable deposits. Some escarpments and valley margins are associated with the formation of mass-movement caves, sometimes known as ‘gull’ or ‘crevice’ caves, which are typically restricted to within 0.5 km of the valley margin or scarp edge. As in other caves, these mass-movement cavities may host speleothems. As gull-caves develop only after valley incision, uranium-series dating of speleothems within them can provide a minimum age for the timing of valley excavation and scarp formation. Here we present data from several gull-caves in the Cotswold Hills, which form the eastern flank of the Severn valley in southern England. U-series ages from these gull-caves yield estimates for both the minimum age of the Cotswold escarpment and the maximum scarp retreat rate. This is combined with data from geological modelling to propose a model for the evolution of the Severn valley and the Cotswold Hills. The data suggest that the location of the escarpment and regional topography is determined not by valley widening and scarp retreat, but by the in situ generation of relief by differential erosion.

Lithostratigraphy and regional correlation of the basal Chalk, Upper Greensand, Gault, and uppermost Folkestone formations (Mid-Cretaceous) from cored boreholes near Selborne, Hampshire

Three cored boreholes that penetrated the base of the Chalk through the Upper Greensand and Gault formations and into the top of the Folkestone Formation provide the fullest record of these beds in the Selborne district. They traverse the thickest Albian (Mid-Cretaceous) succession yet sampled in southern England and form a point of correlation link between the well documented stratal sequences of the North Downs with those of the South Downs. Their lithology is discussed and correlation with sequences around the Weald suggested. An outline biostratigraphical correlation is presented, which lends support to the hypothesis of synsedimentary tectonic activity throughout the Albian.

A reappraisal of the stratigraphy and depositional development of the Upper Greensand (Late Albian) of the Devizes district, southern England

Three members are recognized within the Upper Greensand Formation of the Devizes district on the basis of outcrop, newly acquired cored borehole and petrographical data. These are, in ascending stratigraphical order, Cann Sand Member, Potterne Sandstone Member and Easterton Sandstone Member. Compared to the imprecise historical subdivisions, the members provide a much clearer indication of lithological variation through the Upper Greensand and this, in turn, provides clues to its depositional development. The biostratigraphy of each member was determined using macrofossils and microfossils. The new biostratigraphical data clarify the relationship of the Potterne Rock to the traditionally named ‘Ragstone’, which caps the Shaftesbury Sandstone in the Shaftesbury district, and suggest that the correlation of the Potterne Rock and ‘Ragstone’ is less straightforward than suggested previously. There are some distinct contrasts with the stratigraphy of the Upper Greensand southwest of Devizes (Shaftesbury and Wincanton districts). Whilst tectonic influences have been demonstrated to affect coeval strata in parts of the eastern Weald, these may not be the dominant control on the Devizes succession, which seems to be influenced more strongly by its palaeogeographical setting with respect to sediment source areas, and the effect this had on the volume and timing of sediment infill. Palaeogeography may also be indirectly responsible for the absence of cherts in the Upper Greensand of the Devizes area, in contrast to their conspicuous development in the Upper Greensand of southwest England and the Weald.

Invited comment on Wray & Gale's 'The palaeoenvironment and stratigraphy of the Late Cretaceous chalks'

Despite the context of this reply we would wish to echo the tributes to Jake Hancock expressed elsewhere in this volume. His contribution in papers over many years (e.g. 1961, 1972, 1975, 1991, 2000) is considerable and, in many cases, still relevant. We would also like to thank Wray and Gale for demonstrating the breadth of research into the understanding of the Chalk and its environment of deposition. Our major concern is with the inaccuracy, incorrect assumptions and misrepresentations implied and stated by Wray & Gale (2006) in their discussion of the modern Chalk lithostratigraphical framework applied to British Geological Survey (BGS) maps and demonstrated in many published documents. This reply gives the BGS an opportunity to restate in brief the lithostratigraphical framework for the Chalk Group of England and to deal with specific points raised by Wray and Gale. Our reply is prompted in part by the implications in Wray & Gale that the geological mapping of the Chalk Group in England is carried out in a haphazard and uncontrolled manner without scientific rigour and is of ‘little value’. This implication is refuted absolutely. In fact, it is a test of the robustness of the framework that a large number of field geologists can apply the scheme across southern England and provide such a powerful predictive tool for the practical benefit of the nation. A further verification of the framework’s pertinence, over and above its application to pure scientific endeavours, is its widespread use in applied geological studies. For example, the framework provides the foundation for investigations into the structure of the Chalk, its engineering characteristics and hazards and, perhaps most importantly of all, provides the key and new impetus to studies, including predictive modelling, of the hydrogeology of the UK’s largest and most important aquifer. As stated by Wray & Gale, the development of the regional Chalk lithostratigraphies in the late 1970s to the mid-1980s (Wood & Smith 1978; Mortimore 1983, 1986; Jarvis & Woodroof 1984; Robinson 1986) made it apparent that the traditional tripartite scheme embodied by Jukes-Browne & Hill (1903, 1904) did not delineate fully the lithological variation in the Chalk. This inadequacy led to the development of a more detailed and broadly applicable framework by BGS. The modern lithostratigraphical framework builds upon the expertise of a large number of field geologists. It was presented in a model (Bristow et al. 1997) that was modified at a workshop of the UK’s Chalk experts in 1999. It belongs to all those who contributed and signed up to it, and is the ‘agreed lithostratigraphical framework’ (Rawson et al. 2001). The outline framework with minor additions at member level is given in Figure 1. The full framework report for the Chalk Group of the UK, approved by the Geological Society of London’s Stratigraphy Commission (GSLSC), appears in Hopson (2005) and is available (in pdf format) as a free download from the BGS website (http:// www.bgs.ac.uk) as a joint BGS/GSLSC publication.

Reply to discussion on ’An analysis of Cotswold topography: insights into the landscape response to denudational isostasy'Journal, Vol. 165, 85–103

We thank Scourse and Preece for their interest in our paper. We agree that we should have included a reference to Gale et al . (1999), who used stratigraphic data to quantify the uplift history of the northern limb of the Sandown pericline, Isle of Wight. We did, however, include a reference to Green et al . (2001), who used an apatite fission-track analysis of outcrops of Palaeozoic basement rocks to determine the uplift history of the English Midlands. Together, these studies suggest a mean long-term uplift rate of c . 0.10 m ka−1 for the …