Effect of groundwater and sea weathering cycles on the strength of chalk rock from unstable coastal cliffs of NW France

International audienceThe aim of this paper is to evaluate the role of groundwater and sea weathering on the strength of the chalk rocks exposed on the coastline of the English Channel in Normandy, NW France. We present a study of the rock strength variations of three representative chalk units (Lewes Chalk, Seaford Chalk and Newhaven Chalk) exposed at various locations on the coastal chalk cliffs. The combination of UCS tests and SEM observations have been used (1) on dry natural chalk samples, (2) on chalk samples at various moisture contents, (3) on dry chalk samples submitted to a 10-day cycle of alternating wetting and drying by distilled water and by sea water. Dry chalk samples show low UCS strength (3.46-4 MPa) indicative of very weak rocks. When chalk samples are submitted to progressive water wetting, they present a decrease of UCS strength and Young's modulus of 40% to 50%. This behaviour begins at low values of water content within the chalk, i.e., for a degree of water saturation ranging between 10% and 17%. When chalk samples are submitted to an artificial weathering cycle with distilled water, a decrease in strength is observed, whereas the Young's modulus increases. SEM observations indicate the occurrence of microcracks and particle aggregates in the sample. When chalk samples are submitted to an artificial weathering with sea water, the decrease of UCS strength and Young's modulus achieves a minimum. SEM observations indicate salt crystals within the chalk. On the coastal cliffs of NW France, weathering processes depend both on chalk lithology, which show a range of sensitivity to weathering and on the location of the chalk in the coastal area. Processes allied to the degree of weathering (e.g., salt crystallisation or fresh water disaggregation) differ in the chalk massif, on the cliff face and on the shore platform

Stonehenge: a unique Late Cretaceous phosphatic Chalk geology: implications for sea-level, climate and tectonics and impact on engineering and archaeology

Ground investigations for the A303 Stonehenge Tunnels revealed a unique and complex Chalk geology including the presence of the thickest (>20 m thick), and previously unknown phosphatic chalks in England, partly filling fault controlled erosional channels. The use of natural gamma-ray borehole logs to determine the presence and thickness of the phosphatic deposits is of particular value and combined with the lithostratigraphy, macrofossil and nannofossil biostratigraphy from cores has, for the first time, accurately constrained the Coniacian to Santonian age and the lenticular geometry of such deposits. Four phosphatic chalk events between 88.5–86.5 Ma are recognised associated with synsedimentary faulting. We suggest a causal link between tectonics, subsidence and channel-formation, phosphatisation events, pulses of oceanic upwelling on a frequency of about 0.5 million years to mantle-controlled plate tectonic episodes. The implications of this geology for construction of the A303 and the archaeology of the area are discussed

The Chalk of Suffolk

From its broad expanses of rolling downlands across southern England, the Late Cretaceous Chalk Group sweeps northwards across much of East Anglia, and on into Lincolnshire and Yorkshire. The Chalk is the major bedrock unit across Suffolk, and dips gently eastwards beneath much of the East Anglia region. The oldest strata are locally exposed along the western margin of the outcrop, for example near Mildenhall, and progressively younger horizons are introduced eastwards towards the coast. Consequently, the Chalk is thickest in the eastern part of the Suffolk region, reaching about 250 m in the Combs Borehole [TM 0427 5625] near Stowmarket; perhaps close to 300 m beneath Ipswich; and about 321 m in a borehole at Lowestoft [TM 5380 9260] (Moorlock et al., 2000). Northwards, boreholes in Norfolk have proved more than 400 m of Chalk at Trunch [TG 2933 3455] (468 m); Somerton [TG 4607 2120] (433 m); and West Somerton [TG 4736 1935] (423 m) (Arthurton et al., 1994), at least part of this increased thickness being attributable to the preservation of younger chalk in the upper parts of these successions. Just as in neighbouring Essex and Norfolk, much of the Suffolk Chalk is buried beneath a variable succession of post- Cretaceous, predominantly Quaternary deposits, but including Palaeogene and Neogene strata in the south-east of the county. For this reason, the region has not developed the typical downland landscape of southern England, and our geological understanding of the Chalk of Suffolk has to be assembled from rare natural exposures, chalk quarries and borehole data

The stratigraphy and sedimentation of the Turonian-Campanian in the Southern Province of England

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Coastal rock cliff erosion by collapse at Puys (France) : the role of impervious marl seams within chalk of NW Europe

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Transcultural Contacts and Literary Exchanges - Volume 8. 2018 (complete volume). Medieval Worlds|Transcultural Contacts and Literary Exchanges - Volume 8. 2018 medieval worlds Volume 8. 2018|

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