P.M. Hopson

The development and seismic expression of synsedimentary features within the Chalk of southern England

Seismic reflection data acquired onshore to the north of Bournemouth, southern England, image clearly a series of prominent concave-up (troughs or depressions) and concave-down (mounded) structures within the White Chalk Subgroup. The Chalk lithostratigraphy in the area of study is established from borehole geophysical log correlations. These correlations then provide calibration of the seismic reflection data, indicating that the structures are concentrated at levels between the Lewes Nodular Chalk Formation and Tarrant Chalk Member, and towards the base of the Portsdown Chalk Formation. Onlap, downlap and truncation of reflectors are observed, with the most dramatic features forming a stacked series directly overlying faulting of Jurassic strata. Similar stratal geometries have been described from comparable levels in the Chalk of the North Sea, and in outcrop in Britain and France. To many they represent channels formed by submarine erosion and redeposition of the Chalk during relative sea-level falls, linked to tectonics. We also interpret a number of apparently incisive structures and slumped infill sequences described here as being of primary syndepositional origin. The slumping and the close association of these features with underlying faults, and proximity to nearby major crustal faults, may be indicative of submarine erosion of the Chalk caused by sea-level changes linked to syndepositional fault movements. However, evidence of toplap–downlap pairs is also suggestive of more aggradational mounding or laterally migrating surfaces or facies belts. The presence of any such structures implies rapid lateral variations in the Chalk that would have implications not only for aquifer management and future storage schemes but also during the processing of seismic reflection data and general studies and correlations of the Chalk across southern Britain.

The stratigraphy of the Gault Formation (Middle & Upper Albian) in the BGS Arlesey Borehole, Bedfordshire

A cored borehole at Arlesey, Bedfordshire proved, in descending sequence, the Lower Chalk, Cambridge Greensand, Gault, and Lower Greensand. The macro- and microfaunas, together with lithology, are used to subdivide the c . 57 m thick Gault succession. Comparison with the Upper Gault of the Duxford and Little Chishill boreholes indicates a possible tectonic influence on sediment accumulation in the Arlesey area.

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.