The stratigraphy of the Middle Chalk (Upper Cretaceous) succession at Mundford, Norfolk, UK

Proceedings of the Geologists’ Association 127 (2016) 451–463

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The stratigraphy of the Middle Chalk (Upper Cretaceous) succession at Mundford, Norfolk, UK Ramues Gallois 92 Stoke Valley Rd., Exeter EX4 5ER, United Kingdom

A R T I C L E I N F O

A B S T R A C T

Article history: Received 28 August 2015 Received in revised form 20 March 2016 Accepted 21 March 2016

Four continuously cored boreholes and 82 observation shafts were drilled over an area of c. 45 km2 at Mundford, Norfolk in 1966–67 as part of a geological and geotechnical investigation of for a large (2.4 km diameter) proton accelerator and attendant experimental target areas that was to be commissioned by the Committe´ Europe´an pour Recherche Nucleaire (CERN). The site is a low-relief area underlain by Middle Chalk and the oldest part of the Upper Chalk in a region where the stratigraphy of the Chalk was poorly known prior to the investigation. The shafts enabled the unweathered succession to be examined in detail, both vertically and laterally, and proved numerous lithological marker beds, mostly flint bands and marl seams. Some were restricted to the site area; others remained unchanged over a much larger area. Penecontemporaneous erosion channels in the Middle Chalk that probably resulted from minor tectonic activity locally cut out some of the marker beds. The Mundford succession falls within the northern part of the Transitional Chalk Province. A few of the marker beds, notably the thicker marl seams, have been correlated with marker beds in the Southern and Northern Provinces. Some of the onsite names for marker beds were used to illustrate the geological succession and structure in the geotechnical report on the proposed site. As a result, the site subsequently became the de facto standard for the Middle Chalk stratigraphy of the region. A description of the full succession is included here for the first time. ß 2016 Published by Elsevier Ltd on behalf of The Geologists’ Association.

Keywords: Cretaceous Chalk Stratigraphy Norfolk Turonian Correlation Geophysical logging

1. Introduction In 1964 the CERN invited the fourteen participating member countries to propose sites for a 300 GeV proton synchrotron which would have been ten times larger than the next largest research facility of its type in the world. The principal requirements were that the site should be large enough to accommodate a 2.4 km diameter accelerator ring, a possible future second ring that would allow 600 GeV collisions, and beam lines up to 4.5 km long that would terminate in 250 m  1000 m experimental areas. The site should have a low topographical relief to minimise construction costs, should be uninhabited or sparsely populated, should be underlain by relatively uniform rocks whose geotechnical properties were well documented, and should be aseismic for all practical purposes. In 1965 the Institute of Geological Sciences (IGS) was asked by the Science Research Council to suggest possible sites for the facility. The site selected by IGS, and subsequently investigated and submitted to CERN as the UK proposal in competition with

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sites in 10 other countries, was an almost drift-free, largely afforested area of low relief (<20 m over an area of 5 km  9 km) on the Middle Chalk between Mundford and Brandon, Norfolk (Fig. 1). Reconnaissance surveys by C.R. Bristow, E.R. Shephard Thorn and R.G. Thurrell showed that the chosen site was underlain by lithologically uniform Chalk overlain in part by a veneer of drift deposits, mostly Cover Sand. Four continuously cored boreholes were drilled to confirm the full thickness and lithology of the Chalk beneath the centre of the site, to provide samples for analysis, and to allow seasonal variations in the level of the water-table to be monitored. The principal engineering requirement was that the site should be underlain by material whose geotechnical properties were such that there would be minimal differential settlements beneath the heavily loaded accelerator ring, the more lightly loaded beam tunnels, and the heavily loaded laboratory areas which included the radiation-shielded target areas (Ward et al., 1968). In geological terms this meant that the area must be drift free, or contain only thin patches of drift that could be relatively easily removed, and be underlain by a thick layer of relatively uniform rock of sufficient strength to meet the loading requirements.

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Fig. 1. The proposed 300 GeV proton accelerator site at Mundford, Norfolk showing the positions of the accelerator ring and beam lines, cored boreholes A–C, inspection shafts C1–C24 (1966) and F1–F54 (1967), and Plate Test Shafts T1–T4 (1967) (after Gallois in Anon, 1967, Figs. 3 and 5).

Additional criteria were that the site should have a very low seismic risk to avoid possible damage to the delicate magnetic and vacuum equipment, and that the ring and beam tunnels could be constructed above the highest predicted level of the water table. In this part of East Anglia, Cretaceous rocks rest with marked unconformity on a thick succession of folded Silurian and Devonian mudstones that have remained stable since the end of the Caledonian orogeny. The four cored boreholes all had poor core recoveries in highly fractured chalks with Rock Quality Designations (RQD) almost all <40% in the top 25 m, the layer of most interest to the investigation. The large area of the site combined with the stringent engineering requirements meant that conventional site-investigation methods based on cored boreholes would have been both unsuccessful and prohibitively costly. Dr W.H. Ward of the Building Research Establishment (BRE), the organisation responsible for the engineering feasibility study, therefore suggested that the stratigraphy and weathering profiles beneath the site could be recorded in situ by a geologist in 0.76 m diameter shafts. He did not propose that he or any of his engineers would enter the shafts. A total of 88 shafts (Fig. 2) ranging from 10 to 29 m deep were dug using a bucket-piling auger. This method of investigation proved to be cost effective. Shafts were dug to a depth of 14 m, including the time taken to insert casing to prevent the collapse of the surface layers, within one hour. Deeper shafts which involved the use of an extended kelly bar were dug to over 20 m depth in 2–3 h. The method also proved to be geologically effective for several reasons. First, the shafts had the stratigraphical advantage that they provided a circumferential section of c. 2.4 m for examination, and spoil material from a bedding plane area of about 0.45 m2. This enabled the lateral distribution of widely spaced flints and laterally impersistent marl wisps to be determined with confidence. Second, once a 0.1–0.2 m thick layer of puddled chalk had been plucked from the shaft wall using the modern equivalent of a Neolithic flint miner’s deer-horn pick, the

lithological and textural details of the Chalk were clearly visible in the unweathered surfaces. Third, the use of a miner’s lamp provided an even, well-lit surface that was independent of the weather or seasonal conditions. Fourth, the shafts were left open for over a year to enable them to be re-examined as more detailed lithostratigraphic data became available from the later shafts. Some of those that were dug at times when the water table was high were subsequently deepened and accessed using a rope ladder. The principal disadvantages were first, that the vibration of the auger bucket and the scraping action of the cutting blade meant that few of the larger fossils (ammonites, echinoids and inoceramid bivalves) survived. The smaller fossils, principally brachiopods, survived in large numbers at some levels. Secondly, a limited amount of time could be spent in the confined space of the shafts in atmospheres that were prone to a build up of carbon dioxide at times when atmospheric pressure was falling. This simple but effective method of examining strata in situ was banned as unsafe by the Factories Inspectorate when it was drawn to their attention shortly after completion of the site investigation. A detailed knowledge of the lateral and vertical variations in the stratigraphical succession was a key part of the site investigation given that the presence of any folds, faults or marked lithological variations would have meant that the engineering requirements with respect to differential subsidence could not be met. The preliminary geological results were described by the author (in Wilson, 1967) and the final geological and engineering requirements, tests and results were published in the UK submission to the CERN (Anon, 1967). The geological results have not previously been described except in an abbreviated form in the latter account, parts of which were included in a summary of the geotechnical results (Ward et al., 1968). The site lies in an area of low relief where the former Chalk escarpment was eroded by a west to east moving Anglian ice sheet that impinged on the escarpment at a high angle (Gallois, 1999). As

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Fig. 2. Geological sketch map of the site area (after Gallois in Anon, 1967, Fig. 2).

a result, the feature-forming Chalk Rock, which caps an escarpment up to 160 m above Ordnance Datum (OD) in the unglaciated parts of south east England, crops out at a topographical height at the CERN site (c. 30 m above OD) that is only slightly higher than that of the outcrops of the Lower and Middle Chalk. The site is relatively drift-free lying between the glacially infilled valley of the River Wissey to the north and a partially drift-filled dry-valley tributary of the River Little Ouse to south (Fig. 2). The older drift deposits comprise a complexly interbedded glacial succession of red-brown sandy till, grey chalk-rich till and fluvioglacial sand and gravel rich in chalk and flint clasts with rarer quartzite and other pebbles. The outcrops of these deposits are fringed by Head Deposits derived from them by solifluction, probably in the latest Pleistocene. Exposures in gravel pits in the dry valley adjacent to Grime’s Graves preserve possible collapse structures in the glacial deposits that suggest preservation in solution hollows in the Chalk. The highest part of the Chalk, a mostly 1–2 m thick Cryoturbation Bed, is a structureless chalk paste with and without chalk clasts, and with flints at various angles. It was present in every shaft and is presumed to be ubiquitous throughout the site. The base of the bed is commonly sharp suggesting minor lateral movement due to solifluction. It was referred to as Dead Lime by the 19th century flint knappers (Skertchly, 1879) who excavated thousands of shafts in the Brandon–Icklingham area adjacent to the CERN site. The youngest widespread drift deposit at the site, the Cover Sand, comprises up to 3 m of fine-grained sand with a few clasts. It has extensive outcrops adjacent to the eastern margin of Fenland where it is presumed to be of mixed aeolian and solifluction origin of late Pleistocene (Devensian) age (West, 2006). At the CERN site it is most conspicuous in those areas where it is less than 1 m thick

and concentrated in frost wedges which show up as prominent polygons and stripes in the unafforested areas.

2. Stratigraphy The deepest of the four cored boreholes (Mundford C) proved the full thickness of the Lower Chalk succession beneath the site, underlain by the Gault and Carstone Formations (Gallois and Morter, 1982) and the Sandringham Sands Group and Kimmeridge Clay Formation (Gallois, 1988). Following a recommendation by Penning and Jukes-Browne (1881) the Geological Survey of Great Britain separated the Chalk into Lower, Middle and Upper divisions which were bounded by the ‘‘hard rocky beds’’ of the Melbourn Rock and Chalk Rock. These give rise to pronounced topographical features that enabled the divisions to be mapped out over much of England. The divisions remained largely unchanged until the revision of the lithostratigraphy of the Chalk Group in the 1980s and 1990s and the recognition of up to 9 regional formations (Mortimore et al., 2001, Fig. 1.15). The CERN site lies on the Middle Chalk and the lowest part of the Upper Chalk, between the Melbourn Rock which crops out 2.5 km west of the site and the Top Rock which crops out on the site (Fig. 2). The succession proved in the shafts falls within the traditional Terebratulina lata to Plesiocorys (Sternotaxis) plana Zones of the Turonian and the lowest part of the Micraster cortestudinarium Zone of the early Coniacian. The site lies in the northern part of a region that is transitional between the Southern and Northern Chalk Provinces (Mortimore et al., 2001, Fig. 1.6) and has lithological and faunal affinities with both regions.

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There is no natural exposure and few quarries in the Middle Chalk in or adjacent to the site. Jukes-Browne and Hill (1903) recorded only one exposure in the Middle Chalk in south Norfolk, a quarry in the lower part of the succession at Northwold [probably TL 7451 9742] which showed harder chalks overlain by softer chalks with flints. The highest part of the Middle Chalk succession adjacent to the CERN site includes the Neolithic flint mines at Grime’s Graves [TL 817 898] and, farther south in the Brandon to Icklingham area of Suffolk, excavations for flints for the manufacture of gun flints that were still in work at the times when they were recorded by Skertchly (1879) and Hewitt (1924). The latter (1924; 1935) also described over 50 sections in deeply weathered shallow excavations and a few quarries in the Middle Chalk and early Upper Chalk in south Norfolk and north Suffolk. Taken together, these sections probably exposed 5–10% of the Middle Chalk succession of north Suffolk/south Norfolk, mostly in the highest part of the succession. A few of the localities still contained exposures at the time of the site investigation. Notwithstanding the lack of exposures in much of the Middle Chalk succession, Hewitt (1924, 1935) was able to accumulate an extensive list of fossils through some of the more stratigraphically critical parts of the succession by repeatedly visiting the available exposures over a period of 15 years.

c. 2.5 km long and up to 100 m wide in which contours drawn on the Mount Ephraim Marl (Gallois in Anon, 1967, Fig. 5) showed it to be a locally fractured monocline (Fig. 4). A subsequent seismicreflection survey across the structure confirmed this to be the case (Grainger et al., 1973). Another area where it was thought that faulting might be present was on the SE side of the ring where several well-defined marker beds were absent from their expected position in one of the shafts (C6). Further investigation showed this to be due to a penecontemporaneously formed erosion channel (see below). Most of the succession exposed in the shafts comprises slightly marly chalks with common marl plexa and seams that is lithologically similar to the New Pit Chalk of the Sussex type area (Mortimore, 1983). The higher part of the succession at Mundford comprises similar marly chalks, but with a high concentration of flints culminating in the Brandon Flint Series of Skertchly (1879). The top c. 12 m of the Mundford succession consists of flint-rich marly chalks, gritty chalks, indurated chalks and mineralised hardgrounds which has no lithological equivalent in either the Southern or Northern Provinces, but is similar in part to the Chalk Rock succession of the Chiltern Hills (Bromley and Gale, 1982). Correlations between the successions proved in the shafts were primarily based on combinations of lithological characters, notably marl plexa and seams, and flint bands.

2.1. The Lower Chalk The Mundford C Borehole proved marls and nodular shelly chalks that can be matched with the Plenus Marl Member and Melbourn Rock Member respectively of the Holywell Nodular Chalk Formation of the Southern Province. The underlying Lower Chalk contains lithological and faunal marker beds, notably the Totternhoe Stone, Nettleton Stone and Nettleton Marl that can be matched those with in the central (Bristow, 1990) and northern parts of the Transitional Province (Gallois, 1994), and with the Ferriby Chalk Formation of the Northern Province (Wood and Smith, 1978). These similarities are reflected in the electrical resistivity and total-gamma-ray borehole signatures which highlight the more argillaceous (low resistivity/high gamma ray) and more indurated (high resistivity/low gamma ray) parts of the succession (Fig. 3). 2.2. Middle and Upper Chalk The generalised composite succession proved in the shafts is summarised in Fig. 4. The names of the more prominent marl seams and flint bands were created for informal use on site as an aide memoire. However, when some of these were used in the published site reports (Anon, 1967; Wilson, 1967) to illustrate the geological sections around the ring and along the beam lines they were adopted in later publications as the standard stratigraphy for the region (e.g. Bristow, 1990). The full list of names, including some that were added after the completion of the site investigation, is shown in Fig. 4. All the names are derived from localities on the Ordnance Survey 1–25,000 scale map or are based on a distinctive lithological character. The high concentration of shafts in the central (ring) area enabled much of the succession between the Devil’s Dyke Marl and the Double Patina Flints (Fig. 4) to be examined in 10 or more shafts. At the western and eastern extremities of the site, the oldest and youngest beds were examined in a minimum of 2 shafts (Fig. 5). The regional dip from west to east across the site, based on structure contours on the Mount Ephraim Marl, is <0.58 (Fig. 1). The oldest part of the succession was exposed in degraded chalk pits adjacent to the western edge of the site at Methwold and Northwold, the youngest beds in the more easterly shafts. The one exception to this simple structure is a NNE-SSW trending area

2.2.1. Marl wisps, plexa and seams Concentrations of clay minerals are common throughout the Chalk, and are especially common in the New Pit Chalk. They occur as thin (<1–2 mm thick) undulating discontinuous seams or groups of seams (wisps), concentrations of up to hundreds of anastomosing wisps (plexa), and thin (mostly 10–100 mm thick) finely interlaminated mudstones and muddy chalks (marl seams). Analyses of bulk samples of the Twin, Mount Ephraim and Grime’s Graves Marls showed them to be comprised of 71–81% CaCO3, 12– 18% montmorillonite (smectite sensu lato), 4–6% illite and 4–6% quartz (Sabine in Wilson, 1967, p. 208–211) with the clay minerals concentrated in laminae separated by laminae of purer (c. 96% CaCO3) chalks. Deconinck and Chamley (1995) identified three sources of smectite in Cenomanian and Turonian chalks in northern France. That derived from pedogenic weathering, overgrowths around detrital particles, and submarine weathering of volcanic glass derived from airborne ash falls at a time of enhanced subaerial volcanism related to accelerated sea-floor spreading in the North Atlantic. The CERN site shafts exposed a wide range of clay-mineral concentrations which included wisps that could not be relied on for correlation purposes within the site area, and over 50 plexa and seams all of which are probably laterally persistent within the site and many of which cover a much larger area than the site. Some of the marl seams were subsequently shown to be continuous over distances of tens to hundreds of kilometres. Many of the plexa and seams were observed to vary in thickness and character within the site and even within individual shafts with medium plexa passing into weak plexa and strong plexa passing into thin seams. In the higher parts of the shafts and in weathered quarry sections part of this variation is a secondary thickening effect caused by the breakdown of the rock fabric due to repeated freezing and thawing in a periglacial climate in the late Pleistocene. In the unweathered parts of the succession tectonic shearing along weak clay-mineral-rich beds can reduce the apparent thickness of plexa and seams or replace them with a bedding-plane joint (separation plane of authors) with or without a marl coating. In a total of 16 observations the thickness of the Mount Ephraim Marl varied from 48 mm to 150 mm due to secondary effects. The thicknesses shown in Fig. 4 are the average thicknesses based on unweathered, undisturbed sections.

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Fig. 3. Total-gamma ray-and single-point resistivity logs of the Chalk Group in the Mundford C Borehole (in part after Murray, 1986). Geophysical logs reproduced with the permission of the British Geological Survey. ßNERC.

Some of the marl seams were recognised by the Brandon flint miners who used them as marker beds to identify the positions of the more desirable flint beds when digging trial pits (Skertchly, 1879). They recorded three seams that were laterally persistent over an area >50 km2 which they referred to, in descending order, as the First to Third Pipe Clays. All three were present in the CERN shafts (Fig. 3). In the flint workings and at the site the First Pipe Clay was the thickest (mostly 100 mm thick) and most laterally consistent seam. The Second Pipe Clay was a 25–50 mm thick seam (5–75 mm at the site), and the Third Pipe Clay varied from a wisp to a 25 mm thick seam (wisp to 12 mm at the site). The marl seams exposed in the shafts can be divided into two groups, an older group that includes the Methwold Marl to the Wellington Plantation Pair comprises 100–125 mm thick marl

plexa that include laterally variable marl seams that are locally 10–35 mm thick. The younger group, from the Mount Ephraim Marl to the First Pipe Clay, are the most prominent and thickest seams proved in the shafts where they comprise laterally persistent seams of laminated marl that are mostly 100 mm thick. They occur in the late Turonian to early Coniacian part of the succession and have possible long-distance correlatives in the volcanogenic marls of Wray and Wood (1998) (see below). The youngest marl seam recorded, the West Tofts Marl, was only seen in three shafts where it was 12–75 mm thick. The three youngest thick marls (Grime’s Graves, First Pipe Clay and West Tofts), are lithologically different from the older marls in that they are greyer, and are gritty as a result of included shell debris. Sponge spicules are locally common the West Tofts Marl and First Pipe

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Fig. 4. Generalised vertical section for the succession proved in the CERN shafts (after Gallois in Anon, 1967, Fig. 3).

Clay, and small rhynchonellid brachiopods and tiny spherical flints occur in the Grime’s Graves Marl. 2.2.2. Flints The Chalk contains a large variety of flint forms, many of which can be recognised as having precipitated in or around burrows, others of which have grown together or expanded into the sediment and have lost their distinctive shapes (Clayton, 1986). The most commonly recognisable forms in the shafts were vertical

to sub horizontal tubes, Thalassinoides and Zoophycos burrows, paramoudras, and a variety of tabular, pointed, knobbly and split forms that cannot be attributed to specific burrow types. The distribution of the flints at the CERN site can be divided into three broad groups on the basis of stratigraphy and type. The 17.2 m of chalk between the lowest observed flint band and the higher of the Wellington Plantation Marls contains 6 flint bands comprised of small and medium flints, three of which are widely spaced and three of which are tubular. Between the top of the

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Fig. 5. Geological cross sections around the accelerator circumference and along Beam Line 1 (after Gallois in Anon, Figs. 6 and 7).

Wellington Plantation Marls and the top of the Double Patina Flints (25.5 m) there are 7 flint bands composed of small to large flints, almost all of which occur in complex beds up to 1 m thick that contain a variety of forms including thalassinoid and tubular flints. Between there and the Top Rock, 34 lines of flint occur in a 23.2 m thick interval in which small to very large flints are mostly concentrated in bands up to 3 m thick. They include three levels with paramoudras, 3 with Zoophycos flints, and thin (2–5 mm thick) sheet flints, a form that was only recorded at one level in one shaft in the underlying succession. The most useful characteristics for correlating individual flint bands between exposures and in the shafts are their shapes, sizes, distribution, texture and patina.1 A wide variety of types of patina are exhibited by the flints, some of which are sufficiently distinctive and confined to a single flint band to enable them to be used as stratigraphical marker beds. Most of the patinas in the lower part of a succession, below the level of the Double Patina Flints, have a sharp boundary of grey to black flint with a grey patina less than 1 millimetre thick. The two exceptions to this are the Pink Patina and Chalky Patina Flints which have a uniform patina of partially silicified chalk up to 2 millimetres thick. The Double Patina Flints have an inner and an outer siliceous patina as the result of two stages of flint growth. Above the level of the Snake Wood Flints, almost all the larger flints have multiple patinas indicative of complex growth histories. Skertchly (1879) noted that when worked for gunflints by the flint knappers the outer layers of these flints commonly spalled away as thin flakes. The sizes, frequency and to a lesser extent the shapes of the flints in some of the flint bands at the Mundford site and in the flint-mining area are laterally variable, but others retain their characteristics throughout the area over distances of more 1 Patina is referred to by archaeologist as the weathered outer layer of flints found in drift deposits and as implements. As a result, cortex is now the preferred term used by geologists for the outer layer(s) of unweathered flints.

than 10 km. On the basis of discussions with the flint miners, Hewitt (1935) concluded that contrary to Skertchly’s (1879) account, not all of the named flints in the Brandon Flint Series were laterally continuous. The miners reported that the Floorstone, Wallstone and Toppings were the most continuous and the Upper Crusts the least continuous. The most distinctive of these are the 100 mm-thick tabular Floorstone, and the Wallstone with its accompanying ‘‘legs’’, tube flints up to 300 mm long. The Floorstone was recorded in both the shafts (F8 and F17) at the Mundford site that penetrated this stratigraphical level, in all the Brandon to Icklingham flint mines, and in a 1967 pipeline trench [TL 7066 6437] 10 km south of the Icklingham flint mines. The miners also noted that the Horns of Skertchly (1879, Fig. 2), tube flints 6 mm in diameter and up to 75 mm long, passed locally into ‘‘quite large nodules.’’ The latter are presumed here on the basis of their stratigraphical position to be the correlative of the Lynford Flints, large and very large burrowform flints and paramoudras. At the Mundford site representatives of the Upper Crusts were present in two of the five shafts that penetrated this stratigraphical level. They were also present, albeit widely spaced, in a nearby former gravel pit [TL 8274 9075] and as a single example in one of the Grime’s Graves shafts [TL 8168 8982]. In the lower and middle parts of the Mundford succession, all the bands of tube flints and the Double Patina Flints retain their characteristic shapes within the site area, but the spacing of the flints in some of the bands is laterally very variable. The Chalky Patina Flints, mostly small to medium sized, dark coffee brown translucent flints with few inclusions, were one of the few marker beds that was proved to die out across the site. Shaft F18, the most northerly of those in which it was recorded, contained 20 flints some of which were much larger than those recorded in the more southerly shafts. In the shafts in the central part of the ring there were only 4, and on the southern edge of the ring only one in F16. A search at the relevant stratigraphical level in Mounts Old Pit, Brandon [TL 7900 8698] covering c. 90 m of quarry face did not

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produce any flints. This bed is therefore presumed to be absent south of Shaft F16, becoming steadily more prominent when traced northwards. This was confirmed by geological mapping which showed that although the flints are mostly small, their stratigraphical position could be correlated with a well-marked topographical feature that was traced from the northern edge of the site to the vicinity of F16 where it rapidly died out (Fig. 2). Notwithstanding the low relief, the presence of up to 1.5 m of Cover Sand over much of the site, and the ubiquitous presence of the Cryoturbation Bed, it was possible to map topographical features that could be correlated with some of the flint bands proved in the shafts. The most prominent of these is the Floorstone which is marked by a feature break on either side of the valley on the north side of Grime’s Graves, and which can be traced across the full width of the site. Similar, but more subdued features were correlated with other flint bands (Fig. 2) and with the Chalk Rock and the Top Rock. In the absence of any surface evidence related to the underlying chalk succession, these features would not have been interpretable without the evidence from the shafts. The method by which the Neolithic flint miners discovered the presence of the exceptionally high quality Floorstone at Grime’s Graves and nearby areas has never been satisfactorily explained. The outcrop is everywhere concealed by glacial deposits and/or Cover Sand, and there are no stream sections as the area is a freedraining, gently undulating plateau. Neither are there any known exposures of drift deposits with large flints that could have prompted them to dig speculative shafts in the area. When traced northwards to north west Norfolk into the region where the former Chalk escarpment was less eroded by the Anglian ice sheet, the crest of the escarpment taken to mark the base of the Upper Chalk is at c. +70 m OD. The Chalk Rock and Top Rock have not been reported in field brash in that area, but the presence of abundant large burrowform and tabular flints suggests that the feature break is within the Brandon Flint Series. The presence of tabular flints in the soil brash close below the escarpment crest in the Hillington area cannot be assumed to be Floorstone given that the Wallstone and Toppings were almost continuous tabular beds in some of the Mundford shafts. To the south of the Mundford site where the crest of the escarpment is at c. +80 m OD in the Newmarket area, the succession adjacent to the Middle-Upper Chalk boundary was exposed in a water-pipeline trench [TL 7066 6437] that crossed the escarpment between Moulton and Gazely. The 2 m-deep trench exposed the succession between the Twin Marls and the Wallstone, and showed that the feature break was at the level of the Floorstone. 2.2.3. Mundford succession 2.2.3.1. Plenus Marl to Methwold Marl. The oldest bed proved in the shafts was estimated from a comparison of the lithologies with the geophysical logs to be c. 1 m above the Methwold Marl. At the time of the site investigation the marl was exposed in a deeply weathered chalk pit at Methwold [TL 7343 9469]. This showed 4.6 m of shelly chalks with inoceramid shell debris and whole Mytiloides. labiatus (Schlotheim) interbedded with softer more marly chalks, overlain by a group of separation planes with marl streaks, an Orbirhynchia-rich bed and a 24 mm thick marl seam, overlain by 2.1 m of marly chalk. The lithological boundary between the Holywell Nodular Chalk and the New Pit Chalk is transitional here and lies between the marl and the base of the section. The boundary between the Mytiloides spp. and T. lata Zones is at about the level of the marl seam (C.J. Wood, pers comm.) 2.2.3.2. Methwold Marl to Pilgrim’s Walk Marl. Rhythmic interbeds of smooth textured marly chalk and gritty, shelly chalk with whole and fragmentary Mytiloides common in the lower part of the

succession, becoming rare in the upper part. Orbirhynchia abundant at a few levels. The seven named marls are laterally variable plexa up to 100 mm thick with discontinuous marl seams 6–24 mm thick, mostly at their bases. Flintless except for four closely spaced beds of mostly small rounded, burrowform and tube flints between the Feltwell Lodge and Denton Lodge Marls, and a band of widely spaced burrowform flints close above the highest shelly bed. 2.2.3.3. Pilgrim’s Walk Marl to Twin Marls. Smooth textured slightly marly chalks with relatively widely spaced marl plexa. Flints mostly concentrated in 5 bands containing up to 6 beds. These include small and medium burrowform and tube flints including partially silicified tubes up to 0.9 m long. The medium and large Crescent Wood and Rougham Corner Flints mark an upward change from small and medium flints to medium and large flints. The Mount Ephraim Marl marks the incoming of laterally persistent marl seams that are mostly c. 100 mm thick. 2.2.3.4. Twin Marls to Chalk Rock. Smooth textured slightly marly chalks with interbeds of indurated mineralised chalk (chalkstones of authors) becoming common above the Upper Crusts Flints. The oldest of these, immediately below the First Pipe Clay, was correlated by Hewitt (1924) with the ‘‘spurious Chalk-rock’’ of Whitaker et al. (1891). This part of the succession at Mundford is flint rich with closely spaced large and very large, unusually pure flints at several levels. It includes the Brandon Flint Series of Skertchly (1879) who restricted the name to the succession between the Rough and Smooth Blacks and the Toppings as these were the only beds recorded to have been worked by the flint miners. He noted that the Rough and Smooth Blacks were only used when found in trial excavations that were dug for the Floorstone. This probably explains why the distinctive grey, sponge-spicule-rich Grime’s Graves Marl was not recorded. High purity flints of the quality needed for making implements and gun flints first occur in the Double Patina Flints at the Mundford site, and continue up the succession to at least the Lynford Flints. The Double Patina Flints are too variable in size and shape to be worth excavating for flint knapping, but the quality and size of the overlying Snake Wood Flints are such that they would presumably have been used if they had been discovered. There is no direct evidence that the Lynford Flints were worked. However, Hewitt (1935) noted that a ‘‘saleable’’ flint (as a flint-knapping stone) that had been encountered in some of the excavations in the Brandon area had been referred to as the Toppings, but was not recognised as this bed by the flint miners. It was thought to have come from a slightly higher level (Hewitt, 1935, p. 26). In the flint-mining area, Hewitt (1935) defined the base of the P. (S.) plana Zone as the incoming of Micraster corbovis Forbes of the ‘‘Holaster plana Zone type’’. In the almost total absence of fossiliferous exposures he placed the boundary for practical purposes at a ‘‘bed of hard chalk’’ immediately under the First Pipe Clay (the Spurious Chalk Rock in Fig. 4). 2.2.3.5. Chalk Rock to Top Rock. The succession between the Chalk Rock and Top Rock comprises interbedded marly chalks and chalkstones in the lowest part and grey, slightly gritty chalks and interbedded chalkstones in the middle and upper parts. It comprises a highly condensed succession in which the Chalk Rock and Top Rock represent major sedimentary breaks and the intervening chalkstones represent non sequences of unknown duration. The Chalk Rock is a highly bioturbated, glauconitised and calcitised hardground which Hewitt (1924) referred to as a ‘‘green rock bed’’. At the CERN site, a chalkstone c. 1.5 m above the Chalk Rock contains the high-diversity Reussianum fauna in its typical preservation (see below). The bioturbated erosion surface at the

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top of this bed marks a sudden upward change from marly chalks similar to those of the New Pit Chalk of the Southern Province to grey gritty chalks. Hewitt (1924) recorded the Reussianum fauna in situ in an old pit at Thetford Warren [Loc 6: TQ 841 841] and in an iron-stained nodular bed at Santon Downham [Loc. 7: TQ 816 875], and as loose blocks in a quarry near Lynford Cottages [Loc. 27: TQ 8216 9125]. His Santon Downham succession is similar to that at the CERN site in which two nodular beds are separated by 0.6 m of white chalk. The lower bed was sparsely fossiliferous at Santon Downham: the upper bed contained an abundant Reussianum fauna and was overlain by grey gritty chalk. The Top Rock was only encountered in two of the CERN shafts, in both cases in the weathering zone where it comprised 0.5– 0.6 m of dense, cream coloured, crystallised limestone capped by an irregular burrowed surface with green-stained nodules and pebbles. Hewitt (1924) described a 0.3 m thick partially phosphatised, hard crystalline limestone in a former quarry at Croxton [Loc. 14: TL 8848 8457], 10 km SE of the CERN site, which was rich in sponges and which he correlated with the Top Rock of other areas. He took the erosion surface at the top of this bed to mark the base of the Micraster coranguinum Zone, and noted that this bed caps a marked topographical feature in that area, as it does at the CERN site. 2.2.3.6. Erosional channels. An unusual feature of the Mundford site that had not previously been recorded at that time was the presence of erosional channels in the middle part of what is now the New Pit Chalk. These were recorded at two principal stratigraphical levels. Erosion surfaces in shafts C20, C19, C6 and C18 n the SE part of the ring were interpreted as the floor of a channel that cuts out the Double Patina Flints and Twin Marls, marker beds that maintain their stratigraphical position and spacing throughout the remainder of the site (Fig. 6). The erosion surfaces are marked by highly bioturbated, glauconitised and calcitised hardgrounds which, in the central part of the channel, are lithologically indistinguishable from the ‘‘green rock bed’’ that caps the Chalk Rock at the site. The presumed channel is partially infilled with harder, gritty (shelldebris-rich) chalk that is lithologically distinguishable from the more marl-rich chalks that separate the Double Patina Flints and Twin Marls elsewhere on the site. On the western side of the ring (shafts F41–F43), similar erosion surfaces locally cut out the Mount Ephraim Marl and the Chalky Patina Flints. A similar channel-like feature in the New Pit Chalk at Lewes, Sussex in which the infill material differs from that of the local standard succession has been described by Mortimore (1976). If the suggested correlation between the Mundford succession and the Sussex type area is correct (see below) the channel exposed at Southerham Works Quarry [TL 424 093] is older than the Mundford examples. Evans and Hopson (2000) described seismically imaged channels in the Chalk in Hampshire which they suggested had resulted from penecontemporaneous fault movements. These are younger than and on a much larger scale, several kilometres across and over 100 m deep, than the Mundford or Lewes examples. However, the latter are also likely to have formed in response to penecontemporaneous movements on reactivated faults. The more westerly of the Mundford channels is within the fractured Mount Ephraim fold, a structure that is underlain by a fault that was almost certainly repeatedly reactivated during Cimmerian earth movements. 3. Correlations with other areas In the absence of natural or man-made exposures in much of the Middle Chalk in East Anglia, the most useful method of correlation between boreholes has been geophysical logs. Gray

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(1958) used the distinctive signatures produced by electrical logs that picked out the contrast between low-resistivity marls and the high-resistivity cemented chalks to make correlations between boreholes in the Lower and Middle Chalk in the London Basin. The same method was used by Murray (1986) to make correlations in Cenomanian and Turonian chalk successions covering the London Basin, East Anglia and Lincolnshire-Yorkshire regions, and by Mortimore and Pomerol (1987) in combination with outcrop studies to correlate all the major sections in the Turonian and Campanian chalks in southern England and northern France. Some of the East Anglia correlations made on the basis of simplified copies of resistivity logs that show only the more prominent resistivity lows between the Melbourn Rock and the Top Rock appear convincing, but need to be treated with caution. For example, Murray (1986, Fig. 7) correlated the Mount Ephraim Marl and several of the higher marl seams at the CERN site with marl seams throughout East Anglia and the Northern Province even though this part of the succession was not geophysically logged in the Mundford cored boreholes. The correlations are less obvious when detailed geophysical signatures are correlated with the full successions of marl seams and plexa, and when the possibility of tectonic shearing along the marl-rich beds is taken into account. Comparison of a single-point resistivity log of the East Harling Pilot Borehole [TM 011 860], c. 20 km east of Mundford in which the positions of the Plenus Marl and Chalk Rock were recognised from rock cuttings, with the Mundford succession showed that the spacings between the Plenus, Methwold, Mount Ephraim and Twin Marls were closely similar. This allowed tentative correlations to be made between the resistivity signature and many of the marl plexa (Fig. 7). However, not all the prominent resistivity lows are represented by marl seams at the CERN site and not all the marl seams recorded in the shafts are represented by prominent resistivity lows. For example, a marked low at 108 m depth in the borehole does not correlate with a marl seam at the site. If the overall correlation is correct, then this it likely to be the result of a lateral variation in the succession. The weak representation of the Grime’s Graves and West Tofts Marls in the resistivity log might be due to lateral variations or to local shearing along the marl seams. Correlation of the CERN site Turonian succession with representative examples of the successions exposed at the nearest Geological Conservation Review (GCR) sites in the Transitional and Northern Provinces, at Kensworth [TL 015 197], Bedfordshire, c. 100 km SW and at Welton Wold Quarry [SE 970 277], East Yorkshire c. 160 km NNW, and a representative Southern Province site are summarised in Fig. 8. In the general absence of palaeontological marker beds in the Middle Chalk in East Anglia, combined with the condensed nature of the regional successions at some stratigraphical levels, the most reliable method of interprovince correlation described to date is that based on the correlation of a small number of volcanogenic marl seams. Wray and Wood (1998) and Wray (1999) identified five Turonian marl seams in the Northern Province that can be correlated with marl seams in the Southern Province and northern Germany on the basis of rare-earth-element (REE) ratios. This was used in combination with the identification of palaeontological marker events to propose a tephrostratigraphical framework for the Turonian in Germany (Wiese et al., 2004). The correlations between the Northern England and German Provinces were confirmed by the carbon-isotope signatures of the two successions (Wray and Gale, 2006). The identification of the five volcanogenic marl seams in the Transitional Province is less certain. At Kensworth Quarry [TL 017 197], Cambridgeshire the Latimer Marl of Bromley and Gale (1982) has been correlated with the Southerham 1 Marl of the Southern Province on the basis of clay-mineral geochemistry (Wray and Gale, 1993). There is palaeontological support for this in that the

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Fig. 6. Correlations between the anomalous successions proved in the shafts on the SE side of the ring and their interpretation as a penecontemporaneous erosion channel.

Latimer Marl contains the large foraminifera Labyrinthidoma southerhamense Hart (Mortimore and Wood, 1986) which reaches its acme in the Southerham 1 Marl in Sussex (Hart, 1993). The presumed correlative of the Mount Ephraim Marl in the Stowlangtoft Borehole [TL 9475 6882] Suffolk, c. 30 km SE of the Mundford site also contains L. southerhamense (Bristow,

1990). It is not possible at the present time to correlate any of the 30+ marl seams and plexa below the Mount Ephraim Marl at the CERN site with individual seams/plexa in the Northern and Southern provinces or even those in other successions in the Transitional Province. The lateral variability in thickness of the thin seams within the relatively small area of the site

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Fig. 7. Comparison of the single-point resistivity log of the East Harling Pilot Borehole with the succession of marls and plexa recorded in the Mundford shafts.

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suggests that some may not be laterally persistent over distances of more than a few kilometres. The correlation of the Barton 1 and Glynde 1 Marls with the Pilgrim’s Walk Marl based on a simplified summary of the Mundford succession (Ward et al., 1968, Fig. 3) and simplified copies of resistivity logs (Murray, 1986; Woods and Chacksfield, 2012) is speculative. In the higher part of the Mundford succession the Twin, Grime’s Graves and West Tofts Marls have been correlated with volcanogenic marls in the Southern and Northern Provinces partly on the assumptions that the Mount Ephraim-Southerham 1 – Melton Ross correlation is correct and that the volcanogenic marls are laterally persistent, and partly on their positions with respect to lithological and faunal events that have been recognised in all three provinces. Mortimore and Wood (1986) correlated the Brandon Flint Series of Skertchly (1879) with a ‘‘belt of maximum flint development’’ in the youngest part of the Turonian in the onshore outcrop of the Chalk which they called the High Turonian Flint Maximum (HTFM). This is a stratigraphically loosely defined event that includes a wide range of flint types, sizes and concentrations spread over a stratigraphical range that varies from area to area depending on where the lower and upper boundaries are taken. At the Mundford site, the highest concentration of flints ranges from the Emily’s Wood Flints to the Santon Downham Flints, a total of 27 bands of flint in c. 22.5 m of succession. It includes all the flint types and sizes recorded in the remainder of the Turonian succession plus paramoudras, sheet flints and the very large flints of the Brandon Flint Series. The distinguishing character of the Brandon Flints is their unusual purity which gives rise to their largely unblemished, very dark translucent grey colour and a conchoidal fracture which allows them to be split and shaped in a predictable manner. The purity of the flints won at Grime’s Graves and in the Brandon flint workings can be matched with those at few other localities in the U.K. Prominent among these are those worked in Neolithic flint mines at Beer Head, Devon [SY 227 879], and Cissbury [TQ 137 979] and Harrow Hill [TQ 082 100] in Sussex. The first of these is at a similar stratigraphical level to the flints at Grime’s Graves (Mortimore et al., 2001), the latter are in a higher (Offaster pilula Zone) part of the Chalk (Mortimore, 1986). The Brandon Flint Series has been correlated with a concentration of tabular flints in the lowest part of the Burnham Chalk in the Northern Province on the basis of resistivity-log correlations (Mortimore and Wood, 1986). Although at a similar stratigraphical level close above the base of the P. (S.) plana Zone they are morphologically and lithologically markedly different from the Brandon Flints. A second concentration of flints higher in the Late Turonian in the Northern Province, 15 tabular flints up to 30 cm thick in 7–13 m of succession, has been named the Vale House Flint Member (VHFM) by Hildreth (2013). Mortimore and Wood (1986) noted that the HTFM falls below the Reussianum fauna in all three provinces. The presence of the fauna within the VHFM (Hildreth, 2013) suggests that these flints are at a similar stratigraphical level to some of the flint bands between the Chalk Rock and Top Rock at the CERN site. Much of the upper part of the Turonian succession in the south Norfolk area is lithologically different from that of the Lewes Nodular Chalk of Southern Province, from the Transitional Province successions of the Chiltern Hills, and from the more evenly bedded, pervasively indurated chalks of the Welton and Burnham Chalk Formations in the Northern Province. At the CERN site and in the adjacent quarries, the beds between the Holywell Nodular Chalk and a little above the Chalk Rock are in what were described as ‘‘typical lata Zone lithologies’’ at the time of the investigation; New Pit Chalk lithologies in the modern classification. At Kensworth (Shephard-Thorn et al., 1994; Mortimore et al., 2001) the Chalk Rock is underlain by c. 16 m

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Fig. 8. Correlation f the CERN site succession with representative successions in the northern, Transitional and Southern Chalk Provinces. See text for details.

of Lewes Nodular Chalk that is the presumed correlative of >40 m of marly chalks at the CERN site that includes the Brandon Flint Series (Fig. 8). 4. Summary and conclusions A low-relief area of 5 km  9 km on the Middle Chalk adjacent to Mundford, Norfolk was investigated by means of geological mapping, cored boreholes and eighty eight 0.76 m diameter shafts

as a possible site for a 2.4 km diameter proton accelerator and attendant target areas for the Committe´ Europe´an pour Recherche Nucleaire. The site lies in the northern part of the Transitional Chalk Province of Mortimore et al. (2001) and is important for correlation between Turonian successions in the Southern and Northern provinces. In the absence of laterally persistent faunal marker beds apart from the L. southerhamense acme and the Reussianum fauna, the most reliable inter-province method of correlation in the Turonian succession to date has been the

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recognition of volcanogenic marl seams. Correlation of the CERN site succession with that of the Chiltern Hills has shown it to be expanded and more complete. It is presumed therefore to have been deposited on a steadily subsiding shelf on the north side of the Anglo-Brabant stable massif (London Platform of authors). The most condensed part of the Turonian succession, the beds between the Chalk Rock and the Top Rock, is the most laterally variable in thickness with c. 4 m at Kensworth (Shephard-Thorn et al., 1994), c. 8 m at the CERN site, c. 14 m in the East Harling Borehole (Fig. 7) and c. 22 m in the Stowlangtoft Borehole (Bristow, 1990). In the event, none of the 10 competing international sites was chosen for the new proton accelerator. It was built adjacent to the existing facilities at the CERN headquarters at Meyrin near Geneva, close to the site of the present-day Hadron Collider. Acknowledgements The author is especially indebted to the late Chris Wood, who descended many of the shafts, for stratigraphical and palaeontological advice; to the F. Smith and Son (Grimsby), Ltd. drillers who enabled the shafts to be accessed safely during the investigation and to Adrian Morter and Paul Hildreth who did the same after the drillers had left. The geological research formed part of a multidisciplinary site investigation commissioned by the Science Research Council with on-site management by J.B. Burland of the Building Research Station. Paul Hildreth and Chris Jeans are thanked for their reviews which led to numerous improvements to the first draft. References Anon, 1967. CERN 300 GeV Proton Accelerator Project: Report on Ground Conditions at the British Site. Science Research Council, London. Bristow, C.R., 1990. The Geology of the Country Around Bury St Edmunds. Memoirs of the Geological Survey, Sheet 189 (England and Wales) HMSO, London. Bromley, R.G., Gale, A.S., 1982. The lithostratigraphy of the English Chalk Rock. Cretaceous Research 3, 273–306. Clayton, C.J., 1986. The chemical environment of flint formation in Upper Cretaceous chalks. In: Sieveking, G. de G., Hart, M.B. (Eds.), The Scientific Study of Flint and Chert. Cambridge University Press, pp. 43–54. Deconinck, J.F., Chamley, H., 1995. Diversity of smectite origins in late Cretaceous sediments: example of chalks from Northern France. Clay Minerals 30, 365–379. Evans, D.J., Hopson, P.M., 2000. The seismic expression of synsedimentary channel features within the Chalk of southern England. Proceedings of the Geologists’ Association 111, 219–230. Hart, M.B., 1993. Labyrinthidoma Adams, Knight & Hodgkinson; an unusually large foraminiferal genus from the Chalk facies (Upper Cretaceous) of southern England and northern France. In: Kaminski, M.A., Geroch, S., Gasinski, M.A. (Eds.), Proceedings of the Fourth International Workshop on Agglutinated Foraminifera, Krakow, Poland, September 12–19, 1993, 3. Grzybowski Foundation Special Publication, pp. 123–130. Hewitt, H.D., 1924. Notes on some Chalk Sections in the District around Thetford, Norfolk. Proceedings of the Geologists’ Association 35, 220–244. Hewitt, H.D., 1935. Further notes on the Chalk of the Thetford District, Norfolk. Proceedings of the Geologists’ Association 46, 18–37. Gallois, R.W., 1988. Geology of the Country Around Ely. Memoir of the British Geological Survey, Sheet 173 (England and Wales) HMSO, London. Gallois, R.W., 1994. Geology of the Country Around King’s Lynn and The Wash. Memoir of the British Geological Survey, Sheet 145 and Part 129 (England and Wales) HMSO, London. Gallois, R.W., 1999. Pre-glacial drainage of Fenland. Proceedings of the Geologists’ Association 110, 257–258.

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