Relict sand wedges in soliflucted London Clay at Wimbledon, London, UK

Exposures on Wimbledon Hill, SW London, in the Palaeogene London Clay Formation, are described. The 3° slopes are mantled by clayey solifluction (head) deposits, comprising two superimposed sheets, each around 1.2 m thick. The lower sheet exhibits relict sand wedges forming polygons 1.0-1.5 m across, up to 50 mm wide and 1.25 m deep. These have no surface expression. The wedges are sand-filled and modify significantly the hydrogeology and properties of the solifluction mantle. There are no sand wedges in the upper solifluction sheet.The mechanics of emplacement of the lower solifluction sheet are explored and the high initial water content estimated. Subsequent desiccation and consolidation have reduced the thickness of this sheet by at least 35%, thus distorting contained features such as the wedges. An active layer about 0.4 m thick is inferred to have formed in the top of the lower solifluction sheet. No direct dating was undertaken but, by analogy with relevant dated sites in southern Britain and the Netherlands it is inferred that the lower solifluction sheet was emplaced during the first part of the Loch Lomond Stadial and frost-cracked during a subsequent, colder and more arid part. On this basis, following filling of the cracks by sand, emplacement of the upper solifluction sheet would have occurred in the later part of the Loch Lomond Stadial.     

A case history of a deep basement in London Clay

The case history of the deep excavation for the National Gallery extension in London is presented in this paper. Comparisons with data from other sites show that it is typical for that of similar projects in Central London. Class 1 predictions of the retaining wall behaviour (prior to construction) using the Model London Clay constitutive relationship considerably over-estimated wall and ground movements. Retaining wall and ground movements are also considerably over-predicted by analyses using a simple linear elastic/perfectly plastic soil model, despite optimistic parameters being assumed for the soils. Predictions made using the constitutive model BRICK are closer to the measured deflected shape, but are also higher than measured values. Parametric studies of the effect of various parameters suggest that a “best estimate” of the wall movements are still well in excess of those measured. It is concluded that these differences are due to three-dimensional effects and deficiencies in the model. The “beam-spring” computer software for retaining walls FREW gives similar results to the analyses using the simple model. Analyses of the same problem carried out by a different operator using another finite element code, but with the same constitutive model, yielded somewhat different results and highlight the need for careful interpretation of finite element analyses.     

Foraminiferids from the London Clay at Lower Swanwick and their palaeoecological interpretation

Sixteen foraminiferid assemblages have been studied from the London Clay succession at Lower Swanwick brickyard. These are divided into three faunules which are interpreted as indicating a shallow shelf regime with strong deltaic influences.     

A study of the water retention curve of lime-treated London Clay

This paper investigates the drying and wetting soil water retention curves (SWRCs) of statically compacted lime-stabilised London Clay specimens. A series of tests were performed using the contact filter paper method, pressure plate apparatus and a suction-controlled triaxial system incorporating the axis translation technique. These investigated the water retention of the soil under different boundary and stress-state conditions and simultaneously determined the volume change in the soil during drying and wetting. Factors relevant to the lime treatment of soils, such as curing period and method (air vs. water curing), were also considered. Finally, the hysteresis of the SWRC of the chemically treated soil (for which there appears to be a lack of information in the international literature) was investigated. The results showed that the treatment with lime increased the volumetric stability but reduced the water retention ability due to a more open structure enabled by the flocculation and chemical bonding effects. Curing period and method effect appears to be small. Hysteresis was noted to some degree in all instances.     

Holocene alluvial environments at Barking,Lower Thames Valley (London,UK)

Deposit modelling based on archived borehole logs supplemented by a small number of dedicated boreholes is used to reconstruct the main boundary surfaces and the thickness of the main sediment units within the succession of Holocene alluvial deposits underlying the floodplain in the Barking Reach of the Lower Thames Valley. The basis of the modelling exercise is discussed and the models are used to assess the significance of floodplain relief in determining patterns of sedimentation. This evidence is combined with the results of biostratigraphical and geochronological investigations to reconstruct the environmental conditions associated with each successive stage of floodplain aggradation. The two main factors affecting the history and spatial pattern of Holocene sedimentation are shown to be the regional behaviour of relative sea level and the pattern of relief on the surface of the sub-alluvial, Late Devensian Shepperton Gravel. As is generally the case in the Lower Thames Valley, three main stratigraphic units are recognised, the Lower Alluvium, a peat bed broadly equivalent to the Tilbury III peat of Devoy (1979) and an Upper Alluvium. There is no evidence to suggest that the floodplain was substantially re-shaped by erosion during the Holocene. Instead, the relief inherited from the Shepperton Gravel surface was gradually buried either by the accumulation of peat or by deposition of fine-grained sediment from suspension in standing or slow-moving water. The palaeoenvironmental record from Barking confirms important details of the Holocene record observed elsewhere in the Lower Thames Valley, including the presence of Taxus in the valley-floor fen carr woodland between about 5000 and 4000 cal BP, and the subsequent growth of Ulmus on the peat surface.     

A new bourgueticrinid (Crinoidea) from the Castle Hayne Formation (Eocene) of southeastern North Carolina,USA

Cenozoic fossil stalked crinoids are poorly known. Based on a large, new collection of disarticulated columnals and cups, a new gracile bourgueticrinid, Democrinus simmsi species nov., is described from the Eocene Castle Hayne Formation as exposed at the Martin Marietta Quarry, New Hanover County, southeastern North Carolina (USA). The smooth, conical cup of D. simmsi commonly is widest at the oral surface, moderately flared orally in small (juvenile?) specimens, but tends to be subcylindrical in larger examples. The basals are at least 1.5 times the height of radials. Some columnals of mesistele and dististele have distinctly warty latera. This is the first nominal bourgueticrinid from the Paleogene of North America, despite their moderate diversity locally in the Paleogene of Eurasia. The small size of cups and disarticulated columnals of gracile bourgueticrinids are almost certainly a factor in our poor knowledge of their fossil record.     

Hydrochemical trends,palaeorecharge and groundwater ages in the fissured Chalk aquifer of the London and Berkshire Basins,UK

The hydrochemical, radiochemical, stable isotope, ¹⁴C and dissolved noble gas composition of groundwaters has been determined along two profiles across the confined, fissured Chalk aquifer of the London Basin of southern England, and for selected sites in the adjacent Berkshire Basin. During downgradient flow in the London Basin aquifer, the groundwater chemistry is modified by water–rock interactions: congruent and incongruent reaction of the carbonate lithology resulting in enhanced Mg/Ca and Sr/Ca ratios and ¹³C contents with increased residence times; redox and ion exchange reactions; and towards the centre of the Basin, mixing with a residual saline connate water stored in the Chalk matrix. There is evidence from anomalous water chemistries for a component of vertical leakage from overlying Tertiary beds into the confined aquifer as a result of historical dewatering of the aquifer. Dissolved noble gas contents indicate the climate was up to 4.5°C cooler than at present during recharge of the waters now found in the centres of both Basins; stable isotope (²H and ¹⁸O) depletions correspond to this recharge temperature change. For evolved waters having δ¹³C > −8‰ PDB a negative linear correlation is demonstrated between derived recharge temperatures and δ¹³C values, which is interpreted as mixing between relatively warm, light isotopic, fracture-borne waters and cooler stored waters of the matrix having a ¹³C signature more or less equilibrated with the Chalk. From geochemical (¹⁴C, ⁴He) age estimates, the abstracted water is interpreted as being either of wholly Holocene/post-Devensian glacial origin, or an admixture of Holocene and Late Pleistocene pre-glacial (cold stage interstadial) recharge. Devensian pleniglacial stage waters of the Last Glacial Maximum are not represented.     

Magnetostratigraphic and cyclostratigraphic calibration of a prospective Palaeocene/Eocene stratotype at Zumaia (Basque Basin, northern Spain)

ABSTRACT Detailed magnetostratigraphic work at the Palaeocene/Eocene boundary interval in the Zumaia section has led to a precise delineation of the position of polarity Chron C25n from −35.0 m (or −35.7 m) to −25.2 m below the base of the Carbon Isotopic Excursion that marks the Palaeocene/Eocene boundary. This chronostratigraphic constraint suggests that the lithological carbonate–marl couplets making up most of the studied section are the expression of orbital precession cycles, and confirms the expanded and continuous nature of the section, with mean sediment accumulation rates of ∼ 2.5 cm kyr⁻¹. The new data render Zumaia a unique land-based marine section with available magnetostratigraphic data coupled to cyclostratigraphy. The revised chronostratigraphic framework presented supports previous cyclostratigraphic calibration of the latest Palaeocene geomagnetic timescale; it should facilitate better assessment of the different chemobiostratigraphic events that span this critical interval as a consequence of past climate and oceanographic change. Consequently, the Zumaia section is proposed as a candidate Global Stratotype Section and Point (GSSP) for the Palaeocene/Eocene boundary.