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

The stratigraphy of the Gault Formation (Middle & Upper Albian) in the BGS Arlesey Borehole, Bedfordshire M. A. Woods, I. P. Wilkinson and P. M. Hopson WOOD, M. A., WILKINSON, 1. P. & HOPSON, P. M. 1995. The stratigraphy of the Gault Formation (Middle & Upper Albian) in the BGS Ariesey Borehole, Bedfordshire. Proceedings of the Geologists' Association, 106,271-280. A cored borehole at Ariesey, 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. British Geological Survey, Keyworth, Nottingham NG12 SGG.

1. INTRODUCTION The British Geological Survey (BGS) Arlesey Borehole was drilled in June 1991 as part of a geotechnical study of the Gault. The borehole [TL 1887 3463], sited 40 m east of Arlesey Brickpit (Fig. I), provides the first complete section through the Gault of the area. The formation is better known to the northeast (Gallais & Morter, 1982) and southwest

(Owen, 1971, 1972; Eyers, 1991; Crux, 1991). Coring commenced at a depth of 3.16 m, and penetrated Chalk (c. 11.22 rn), Cambridge Greensand (c. 1.07 rn), Gault (c. 57.35 m) and Lower Greensand (c. 10.69 m), terminating in the last named at a final depth of 83.49 m. Previous studies of the Gault of the region between Leighton Buzzard and Cambridge, by Jukes-Browne (1875), Jukes-Browne & Hill (1900), Fearnsides (1904a;

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1904b), Woodward, Thompson & Mill (1909), Whitaker (1921), Bloom & Harper (1938), Spath (1943), Edmonds & Dinham (1965), Cole (1970), Hart (l973a, b) and Carter & Hart (1977), have relied mainly on scattered surface exposures (many no longer available) and old records of borings (mainly for water). The poorly documented lithologies in old borehole records often give only a tentative indication of the stratigraphy. The Lower and Upper Gault correspond with the Middle o

and Upper Albian substages respectively. The terms Lower and Upper Gault have no formal status in lithostratigraphical nomenclature, but are useful to retain for descriptive purposes. The Upper Gault is generally more calcareous and paler in colour than the Lower Gault, and a well marked erosive contact often separates them (Owen, 1971; Gallois & Morter, 1982). The standard Gault biozonation is based on ammonites (Owen, 1971, 1972, 1975, 1979, 1984), and this has been applied to the Arlesey

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THE STRATIGRAPHY OF T HE GA ULT FORMATION IN THE B G S ARLESEY BOREHOLE

Borehole succession (Fig. 2) which has thus been subdivided on the basis of combined litholog ical and macrofaunal characters. This has enabled the recognition of discrete units that can be correlated with the numbered divisions (Beds I to 19) established by Galloi s & Morter (1982) in the Gault of East Anglia (Fig. 2). The latter are used herein as a standard; for clarity and ease of reference, they are referred to in the following text and figures as 'G I ', 'G2 ' etc. The calcareous microfaunas (foraminifera and ostracods) are also described, and compared with the zonal scheme erected by Wilkinson & Morter (1981), and modified by Wilkinson (1990). In the following account, ' nodule ' is used as a descriptive term without genetic connotation.

2. LOWER GAULT The base of the Lower Gault (at n .80 m) is taken at an irregular (?eroded) junction, overlying a thin (c. 0.10 m), orange- stained, gritty and patchily glauconitic, carbonate unit at the top of the Lower Green sand. Petrographic analysis of the latter revealed a complexly laminated structure that is typical of an algal mat or stromatolite; concentrations of detrital quartz in ?desiccation cracks suggest periodic exposure in an intertidal env ironment. This depositional environment is similar to that inferred for the Shenle y Limestone (Eyers, 1992b) which underlies the Gault around Leighton Buzzard. The basal Lower Gault (71.1I- n .8 m) possibly equates with G I to G3. The lowest 0.48 m is a dark green, very sandy glauconitic mudstone with polished quartz granules (1-4 mm), and angular to subrounded glauconitic phosphatic nodule s. The poorly preserved macrofauna, includes the lon g-rangin g belemn ite Neohiboli tes, Birostrina concentrica (Parkinson), and T'Os trea' papyracea Sinizow. The last named is characteri stic of beds in the Hoplites (H.) dentatus Zone (G I to base G3 ) of the East Anglian succession, where it is associated with B. concentrica. It is locally abundant in Gl (Lyelliceras lyelli Subzone ?) (Gallois & Morter, 1982). The interval from 71.11 m to n .17 m is burrowed, and contains concentrations of gritty sediment. Dark to medium grey, fissile mudstone occurs between 69.8 m and 71.11 m, possibly representing ?G4 and G5. B. concentrica and Anomia sp. occur above 71.1 m and 70.28 m respectively; the former is abundant at 70.31 m. This bivalve association occurs in G4, and was regarded by Gallois & Morter (1982) as particularl y characterisic of G5 (Euhoplites loricatus Zone, Anahoplites intermedius Subzone). The base of G6 is possibl y rep resent ed bet ween 69.6 m and 69.8 rn, where sandy silty mudstone rich in B. conce ntrica (at 69.73 m) overli es mud stone with Chondrites. This association of litholog ies and fauna is typical of the base of G6 (E. loricatus Zone, Dimorphoplites niobe Subzon e) of East Anglian successions (Gallois & Morter, 1982).

273

Much of the remaining c. 1.2 m of Lower Gault was lost in drilling. The upper boundary of the Lower Gault (at 68.29 m) is an erosional surface at the base of a phosphatic nodule horizon.

3. UPPER GAULT The Upper Gault commences with a prominent , 0.06 m thick pyritic, phosphatic nodule horizon, overlain by a c. 0.8 m thick bed of silty glauconitic mudstone. The succession between the basal nodule bed at 68.29 m and a lithological change at 66.81 m is correlated with Gil (Mortoniceras (M.) infl atum Zone , D. cristatum Subzone). This unit contains the earliest occurrences (at 67.78 rn, 67.45 m and 67.42 m) of the bivalve Birostrina sulcata (Parkinson), which indicates the Upper Albian Dipoloceras cristatum and Hysteroceras orbignyi subzones (Owen, 1984). A specimen of B. concentrica ass ociated with abundant phosphatic nodules at 68.21 m may be derived from the Lower Gault. The interval from 66.29 m to 66.81 m includes poorly fossiliferous, very pale grey mud stones with Chondrites; these may equate with the typicall y very calcareous G 12 (H. orbignyi Subzone ). G 13 (H. orbignyi Subzone) occurs between 64.91 m and 66.29 m. Its base is defined by a sudden increase in the frequency of B. sulcata (typical of this unit; Gallois & Morter, 1982), and the top is marked by Chondrites-bearing mudstones at 64.91 m, a short distance above the highest B. sulcata specimen. The return of abundant B. concentrica at 64.84 m is indicative, in the Upper Albian , of the H. varicosum Subzon e (Owen, 1984), and characteri stic of the base of GI4 (Gallois & Morter, 1982). This is taken at a depth of 64.91 m, at a Chondrites-rich mudstone between the highest Birostrina sulcata spec imen and lowest B. concentrica. The B. concentrica-rich mudstones extend up to 64.32 m, and include a specimen of Euhoplites alphalautus Spath (at 64.55 m). In East Anglia, this ammonite is more typical of higher levels of the H. varicosum Subzone (Galloi s & Morter, 1982). Neohibolites occ urs regularly between 58.31 m and 63.47 m, and is not recorded again in the Gault until c. 15.64 m, near the base of the Cambridge Greensand. It is associated with a significant increase in the quant ity and diversity of the preserved macrofauna, and the frequen cy of this belemnite suggests a correlation with the higher part of G 14 (H. varicosum Subzone). The top of G 14, at 58.29 m, is the lithological change to mudstones rich in inoceramid shell debris , a short distance above the highest specimen of Neohibolites. G 15, between c. 52.15 m and 58.29 m, is represented by mudstones with inoceramid shells and locally abundant comminuted inoceramid shell debris. 'Inoceramus' /issa (Seeley) is present, with poorly preserved Mortoniceras and Callihoplites. Revised correlations of East Anglian Ga ult successio ns at this level, modifying the Gallois & Morter (1982) scheme, suggest that '/'. /issa does not range below the C. auritus Subzone (H. G. Owen, pers. comm., 1994), and that the base of the latter subzone is possibly coincident

274

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'almost barren except for scattered Aucellina in the upper part' (Gallois & Morter, 1982). The absence of further records of inoceramid shell in either macro- or micro-faunal samples (see below ), appears to substantiate a position above G IS. The earliest record of Aucellina in the borehole is at 39.46 rn, above which it occurs with moderate frequency up to a 2 mm-thick pyritous mudstone at the top ofG l6 .

with the base of GIS. The lowest ' I.' Lissa at Arlesey is at 58.10 m. The top of G IS is the base of a hard. pale grey, phosphatic mudstone, which marks a change in fauna. G 16 is represented by sparsel y fossiliferous mudstones between 37.46 m and 52.15 m. This interval contains infrequent . poorly preserved ammonites and gastropods. and is compara ble with typical developments of G 16 (c. auritus Subzone-upper part), which is characteristically

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T HE S T RATIG RAPHY OF T I -IE G A U L T FOR MATION

Between 18.25 m and 37.46 rn, the succession is mainly shelly mudstone with local concentrations of phosphate and pyrite. The major increase in the freque ncy of Aucellina to the excl usion of virtually all other macrofossils, is characteris tic of G 17 (Stolic zka ia di spa r Zone , Mort oni cera s (M_) rostratum Subzone). A basal shelly phosphatic horizon contains abundant Au cellina spp., includi ng the 'gryph aeoides' and 'uerpmanni' morphotypes

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(S. dispar Zone, M. (M.) rostratum Subzone). The upward change to ornamented left valve umbones in Aucellina is characteristic of G18 (Morter & Wood, 1983), but the base of this unit can also be recognized by the downward change to Aucellina-rich mudstones at the top of G17 (Gallois & Morter, 1982). Only one left valve fragment of Aucellina is recorded, compared with some 24 occurrences and two population acmes in an equivalent thickness beneath. The umbonal region of this specimen is not preserved, but the valve surface bears distinct concentric flanges.

4. MICROPALAEONTOLOGY The distribution of biostratigraphically significant microfossil species are shown in Figs 3 (Ostracods) & 4 (Foraminifera). The ostracod and foraminifera biozonations follow Wilkinson (1990) and Carter & Hart (1977) respectively. In the Lower Gault, between 68.29 m and 72.8 m, the microfaunal assemblages consist of generally long-ranging foraminifera such as Gavelinella intermedia (Berthelin) and Hedbergella delrioensis (Carsey), although the presence of Arenobulimina macfadyeni Cushman tends to confirm a Mid-Albian age. A sample from the Upper Gault at 67-67.05 m contains evidence of the Cytherelloidea stricta Subzone of the Cythereis (Rehacythereis) luermannae s.slNeocythere (N.) ventrocostata ostracod Zone. The inception of Cytherelloidea stricta at this level is indicative of the D. cristatum ammonite Subzone (Wilkinson, 1990). The latter is endorsed by the earliest appearance of the foraminifer Citharinella pinnaeformis (Cushman) in the same sample, together with Arenobulimina macfadyeni and Epistomina spinulifera (Reuss); the latter species is very rare above the cristatum Subzone elsewhere in Britain, and benthonic foraminifera Zone 4a is indicated (Carter & Hart, 1977; Hart, Bailey, Crittenden, Fletcher, Price & Swiecicki, 1989). An assemblage from 61.00-61.05 m includes the ostracods Cythereis (C.) folkestonensis Kaye, Neocythere (P.) steghausi (Mertens) and N. (N.) vanveenae Mertens, with the foraminifera Vaginulina mediocarinata Ten Dam. These indicate the C. (C.) folkestonensis Subzone of the C. (R.) luermannae s.s.IN. (N.) ventrocostata ostracod Zone. The latter subzone equates with the 'mid' orbignyi to 'mid' varicosum ammonite subzones (Wilkinson & Morter, 1981). The apparent extinction of C. (R.) luermannae s.s. Triebel and N. (N.) ventrocostata Griindel at 57.00-57.05 m, suggests that this level is still within the H. varicosum Subzone. Samples from 55.00-55.05 m and 53.00-53.05 m lack age-diagnostic microfaunas, but contain abundant inoceramid shell debris. The disappearance of the foraminifer Citharinella pinnaeformis above 45.05 m indicates a position high in the C. auritus ammonite Subzone, as its last occurrence elsewhere in Britain is at the top of foraminiferal Zone 5 (Carter & Hart, 1977; Hart et al., 1989). The inception of the ostracod Mandocythere barringtonensis (Kaye), in a sample at 41.50-41.53 m, suggests an age probably no older

than the auritus Subzone based on records of this species at Barrington. The earliest record of the ostracod zonal index species Cythereis (Rehacythereis) luermannae hannoverana Bertram & Kemper is at 41.00-41.05 m. Elsewhere in East Anglia, it ranges from the upper part of the varicosum ammonite Subzone to the top of the dispar Zone (Wilkinson & Morter, 1981; Wilkinson, 1990). In the present borehole, it has been recorded in only the upper part of this range, which might been be due to inadequate sampling and/or the general sparsity and sometimes poorly preserved nature of the microfaunas. The impoverished microfaunas from much of the remaining Upper Gault do not allow detailed biostratigraphical interpretation. The foraminifera Citharinella laffittei Marie, Globigerinelloides bentonensis (Morrow) and Arenobulimina sabulosa (Chapman) were recorded respectively at and above 36.90-36.95 m, 25.00-25.05 m and 23.00-23.05 m; all of these taxa become common within the lower part of the S. dispar ammonite Zone in Britain (Hart et al., 1989), and foraminiferal Zone 6 is inferred. The highest microfaunal sample (15.50-15.55 m; c. 0.05 m below the base of the Cambridge Greensand), contains the ostracod subzonal index, Planileberis scrobicularis (C. (R.) luermannae hannoverana Zone). The top of this Subzone is considered to be approximately equivalent to the upper boundary of the Mortoniceras (M.) rostra tum ammonite Subzone (lower S. dispar Zone). The record of Arenobulimina chapmani Cushman, in the same microfaunal sample indicates that the presence of foraminiferal Zone 6a is unlikely (Hart et al., 1989). There is no evidence for the presence of ostracod or foraminiferal zones equating with the M. (M.) perinflatum ammonite Subzone, although its presence at the nearby Arlesey Brickpit [TL 18793476] can apparently be inferred from nannofossil data (1. Eyers pers. comm., 1994). 5. COMPARISON OF FAUNAL DATA Both macro- and microfaunal data confirm that the erosion surface at 68.29 m is the Middle/Upper Albian boundary. Microfaunal evidence for the D. crista tum Subzone (G11), is below the pale mudstone samples that are inferred to represent G12 (lower orbignyi Subzone). The sample at 61-61.05 m (orbignyilvaricosum Subzone) is associated with a varicosum Subzone macrofauna, and is underlain (at 65.18-66.29 m) by abundant Birostrina sulcata that characterize G13, the top of which is coincident with the top of the H. orbignyi Subzone (Gallois & Morter, 1982). A sample at 57-57.05 m yielded probable microfaunal evidence of the varicosum Subzone, but the abundance of comminuted inoceramid shell in the associated mudstones might indicate reworking. These shell-rich mudstones are here assigned to the next youngest C. auritus Subzone (see above). Microfaunal evidence of the auritus Subzone at 41.5--41.53 m is supported by the sparse macrofauna. Macrofaunas are lacking from much of the thick interval attributed to G17, but the lowest S. dispar Zone micro-

THE STRATIGRAPHY OF THE GAULT FORMATION IN THE BGS ARLESEY BOREHOLE

faunal biomarker (Citharinella taffittei; Fig. 4) is almost coincident with the base of this Zone deduced from the abundance of Aucellina (Fig. 2). Hart (l973a, b) suggested, on micropalaeontological grounds, that the youngest Gault exposed at Arlesey and Barrington belonged to the Callihoplites auritus Subzone. Carter & Hart (1977) appear to sustain this view. However, the record of the foraminifer Gtobigerinelloides bentonensis throughout the top 3.0 m of Gault at both Arlesey and Barrington (Hart, 1973a) is indicative of a level at or above Bed XII (sensu Jukes-Browne & Hill, 1900) at Folkestone (Hart, 1993). A flood abundance of Globigerinelloides sp. occurs in the S. dispar Zone (Hart, 1993), which is the current macrofaunal interpretation of Bed XII and its

277

correlative G17 of East Anglian sequences (Gallais & Morter, 1982; Morter & Wood, 1983).

6. REGIONAL COMPARISON Previous estimates for the thickness of the Gault of the region are c. 62.2-65.2 m (Jukes-Browne, 1875) and c. 54.8 m (Jukes-Browne & Hill, 1900) and 61 m (Woodward et at., 1909). These figures compare favourably with that recorded in the Arlesey Borehole (57.35 m). Jukes-Browne & Hill (1900) considered the thickness to be entirely within the Lower Gault.

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278

M. A WOODS ET AL.

Lower Gault, with dentatus Zone ammonites, has been recognized at several localities in the region (Fig. 1), including a pit near Clophill [TL 091 388], Biggleswade (Newspring) Waterworks [TL 210 416] and Gamlingay [TL 237 530] (Jukes-Browne & Hill, 1900; Edmonds & Dinham, 1965). Spath (1923-1943) redetermined ammonites collected by Jukes-Browne & Hill (1900) from old 'coprolite' workings at Campton, near Shefford, and concluded that the phosphatic seam, which was not more than c. 7.6 m above the base of the formation, represented the local base of the Upper Gault (Edmonds & Dinham, 1965). Specimens of 'Inoceramus sulcatus' [= Birostrina sulcata (Parkinson)] are recorded from Clophill, Campton and Southill (Sowerby, 1821; Fitton, 1836). Edmonds & Dinham (1965) suggest that this species is likely to occur within c. 9 m of the base of the Gault, and probably not far above the Campton phosphate horizon. These values suggest that there might be local thickening of the Lower Gault, which at Arlesey is only 4.51 m thick. There is apparently no outcrop evidence for the presence of Lower Gault younger than the dentatus Zone. Upper Gault, with Hysteroceras orbignyi (Spath) and H. bucklandi (Spath), suggesting a level in the Callihoplites auritus Subzone, was worked at a brickyard near Meppershall (Fig. 1) (Bloom & Harper, 1938). The topmost c. 4.6 m of Aucellina-rich Gault recorded in a cementworks pit north of Barrington church [TL 393 510] (Edmonds & Dinham, 1965) has been assigned to G 17 in the S. dispar Zone (Gallois & Morter, 1982). Abundant Aucellina in the c. 9 m of Gault exposed north west of Shepreth station (Edmonds & Dinham, 1965) also suggest that Zone. In an adjacent borehole [TL 391 483] (Jukes-Browne & Hill, 1900, p. 289), Spath (1943, p. 749) suggested that the c. 1.5 m hard band logged at a depth of c. 45.7--47.2 m might correlate with that recorded by Feamsides (1904a) at Bamwell (i.e. Bamwell Hardband, the base of which marks the base of G15; Gallois & Morter, 1982). On this basis, a thickness of c. 31 m can be estimated for the combined C. auritus Subzone and S. dispar Zone hereabouts, compared to 42.65 m for the equivalent interval in the Arlesey Borehole. At Arlesey Brickpit [TL 1879 3476], c. 15 m of Gault are exposed (July, 1992) beneath the Cambridge Greensand (Hopson, Aldiss & Smith, in prep.). A fauna from c. 11 m below the Cambridge Greensand included Aucellina, and the ammonites Anahoplites (Lepthoplites) sp. and Callihoplites cf. leptus (Seeley), suggesting the Stoliczkaia dispar Zone (M. (M.) rostratum Subzone). Most of the East Anglian boreholes studied by Gallois & Morter (1982) contain less than 13 m of Upper Gault, of which G 17 is generally less than 1 m thick. Exceptions to this are Duxford [TL 469 457] and Little Chishill [TL 4528 3637] (Fig. I). Borehole records in BGS archives show that the Gault of the Little Chishill Borehole overlies Devonian limestones at a depth of c. 278 m (recorded as 377.4 m by Gallois & Morter, 1982). Gallois & Morter's (1982) depth of the Cambridge Greensand/Gault junction (214.9 m) corresponds with a positive (downhole) shift in the gamma log for this borehole, implying a total Gault thickness

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hereabouts of c. 63.1 m. Correlation of the Arlesey, Duxford and Little Chishill boreholes is shown in Fig. 5. The laterally variable nature of the basal Cambridge Greensand erosion (Morter & Wood, 1983) means that the total thickness of the preserved Gault provides no guide to depositional patterns in the Gault, and enhances the value of Gallois & Morter's (1982) bed subdivision employed herein. These authors also noted the rapid expansion of parts of the Upper Gault (especially G14--G16) southwards through Cambridgeshire. Thicknesses of Lower Gault at Duxford and Arlesey (4.73 m and 4.51 m respectively) are comparable, but core loss at the latter makes detailed comparisons difficult. However G1-G5 appear relatively condensed at Duxford (Fig. 5), which implies either condensation of units in the higher (not recovered) interval of Lower Gault at Arlesey, or else a less completely developed sequence. Arlesey has a slightly thicker, and more complete Upper Gault sequence compared to Duxford, where the absence of G18 might reflect locally enhanced basal Cambridge Greensand erosion. Although the Little Chishill Borehole appears to have the thickest Upper Gault sequence, lengthy uncored intervals limit detailed comparison with either Arlesey or Duxford.

7. DEPOSITIONAL CONTROLS The Gault of East Anglia thins onto the London Platform east of its Cambridge outcrop (e.g. Four Ashes, Fig. 5) and

THE STRATIGRAPHY OF THE GAULT FORMATION IN THE BGS ARLESEY BOREHOLE

northwards onto a Carstone (Lower Greensand) shoal on which Red Chalk accumulated (Gallois & Morter, 1982). Compared to other boreholes described by Gallois & Morter (1982), Duxford, Little Chishill and Arlesey are a distinct southern group of localities with much thicker Gault sequences. Proximity of these thick sequences to the inferred margin of the London Platform (Gallois & Morter, 1982, Fig. 1), might suggest a tectonic influence. There is geophysical evidence for a major WNW-ESEtrending basement fault which downthrows to the south of Bury St Edmunds (Pattison, Berridge, Allsop & Wilkinson, 1993). The Devonian limestones beneath the Gault at Little Chishill are part of a thick Devonian sequence occupying a basin in the Luton-Cambridge area which is locally separated from a similar, more extensive Devonian basin to the south by an upwarp ('massif') of Silurian sediments around Ware (Allsop, 1985; Allsop & Smith, 1988); the northern margin ofthis massif may be fault-bounded. To the northwest, a NE-SW-trending lineament ('Cambridge Line'), coincident with the northwestern edge of the Luton-Cambridge Devonian basin (Woodcock, 1991), is interpreted as a fault responsible for the northwestward uplift and truncation of the Silurian subcrop in East Anglia (Woodcock & Pharaoh, 1993). This lineament, with the ?fault bounding the 'Ware Massif' to the south, thus defines a trough in the Palaeozoic basement of the region. These structures may have influenced deposition in the Early Cretaceous. Eyers (1991) has inferred localized block fault tectonics as an influence on the sedimentation of the Lower Greensand of Bedfordshire, and suggested that this might

279

also have had an effect on at least the Lower Gault ammonite subzonal thicknesses recorded in the Munday's Hill area at Leighton Buzzard. Data from recent geological mapping of the Thame district by the BGS suggest significant northwestward expansion of (at least) the lower S. dispar Zone (Mortoniceras (M.) rostra tum Subzone), coupled with rapid lateral changes in facies; these effects appear to be related to tectonic structures with a history of pre-Cretaceous activity that may also have influenced sedimentation in the Late Albian (Horton, Sumbler, Cox & Ambrose, in press). 8. CONCLUSIONS Correlation of the numbered beds with the standard Middle and Upper Albian ammonite zones (Gallais & Morter, 1982; Morter & Wood, 1983), is confirmed by the microfaunas and the sporadic ammonite records. The speculative evidence for a tectonic role in the deposition of the Gault requires further investigation. ACKNOWLEDGEMENTS

We thank Dr B. M. Cox, Dr E. R. Shephard-Thorn, Prof. M. B. Hart and Dr J. Eyers for the valuable comments on the manuscript; Dr G. K. Lott for petrographic descriptions; M. G. Sumbler for discussion on the Gault of the Thame disrict and Dr H. G. Owen for helpful correspondence on the correlation of Upper Gault successions. This paper is published by permission of the Director, British Geological Survey (NERC).

REFERENCES ALLSOP, J. M. 1985. Geophysical investigations into the extent of the Devonian rocks beneath East Ang1ia. Proceedings of the Geologists' Association, 96, 371-379. - - & SMITH, N. J. P. 1988. The deep geology of Essex. Proceedings of the Geologists' Association, 99, 249-260. BLOOM, E. F. D. & HARPER, J. C. 1938. Field meeting in the Hitchin district. Proceedings of the Geologists' Association, 49, 415--419. CARTER, D. J. & HART, M. B. 1977. Aspects of mid-Cretaceous stratigraphical micropalaeontology. Bulletin of the British Museum of Natural History [Geology], 29, 1-135. COLE, M. J. 1970. Records of Wells in the Area of New Series One-Inch (Geological) Hitchin (221) Sheet. Water Supply Papers of the Institute of Geological Sciences [Well Catalogue Series]. CRUX, J. A. 1991. Albian calcareous nannofossils from the Gault Clay of Munday's Hill (Bedfordshire, England). Journal of Micropalaeontology, 10, 203-222. EDMONDS, E. A. & DINHAM, C. H. 1965. Geology of the Country around Huntingdon and Biggleswade. Memoir of the British Geological Survey, Sheets 187 & 204 (England & Wales). EYERS, J. 1991. The influence of tectonics on early Cretaceous sedimentation in Bedfordshire, England. Journal of the Geological Society, London, 148, 405--414.

- - 1992a. 'Lithostratigraphy of the Lower Greensand and Gault (Lower Cretaceous) of the Bedfordshire province, England.' PhD thesis, Open University. - - 1992b. Sedimentology and palaeoenvironment of the Shenley Limestone (Albian, Lower Cretaceous): an unusual shallowwater carbonate. Proceedings of the Geologists' Association, 103,293-302. FEARNSIDES, W. G. 1904a. On the Occurrence of a limestone with Upper Gault Fossils at Barnwell, near Cambridge. Quarterly Journal of the Geological Society of London, 60, 360-363. - - 1904b. The Geology of Cambridgeshire. In (Marr, J. E. & Shipley, A. E.; eds) Handbook to the Natural History of Cambridgeshire. Cambridge University Press, Cambridge, 9-50. FIlTON, W. H. 1836. Observations on the strata between the Chalk and the Oxford Oolites in the south-east of England. Transactions of the Geological Society, Series 2, 4,103-388. GALLOIS, R. W. & MORTER, A. A. 1982. The stratigraphy of the Gault of East Anglia. Proceedings of the Geologists' Association, 93, 351-368. HART, M. B. 1973a. Foraminiferal Evidence for the age of the Cambridge Greensand. Proceedings of the Geologists' Association, 84, 65-82. - - 1973b. A correlation of the macrofaunal and microfaunal zonations of the Gault Clay in southeast England. In (Casey, R.

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& Rawson, P. E; eds) The Boreal Lower Cretaceous. Geological Journal Special Issue, 5, 267-288. - - 1993. Cretaceous foraminiferal events. In (Hailwood, E. A. & Kidd, R. B.; eds) High Resolution Stratigraphy, Geological Society, London, Special Publication, 70, 227-240. - , BAILEY, H. w., CRITIENDEN, S., FLETCHER, B. N., PRICE, R. J. & SWIECICKI, A. 1989. Cretaceous. In (Jenkins, D. G. & Murray, J. w.; eds) Stratigraphical Atlas of Fossil Foraminifera (2nd edn). Ellis Horwood, Chichester, 273-371. HOPSON, P. M., ALDISS, D. T. & SMITH, A. In prep. Geology of the country around Hitchin. Memoir of the British Geological Survey, Sheet 221 (England & Wales). HORTON, A., SUMBLER, M. G., COX, B. M. & AMBROSE, K. In press. The Geology of the country around Thame. Memoir of the British Geological Survey, Sheet 237 (England & Wales). JUKES-BROWNE, A. J. 1875. On the relations of the Cambridge Gault and Greensand. Quarterly Journal of the Geological Society ofLondon, 31, 256-315. - - & HILL, W. 1900. The Cretaceous Rocks of Britain, 1, The Gault and Upper Greensand of England. Memoir of the Geological Survey of the United Kingdom. MORTER, A. A. & WOOD, C. J. 1983. The biostratigraphy of Upper Albian-Lower Cenomanian Aucellina in Europe. Zitteliana, 10, 515-529. OWEN, H. G. 1971. Middle Albian Stratigraphy in the Anglo-Paris Basin. Bulletin of the British Museum of Natural History [Geology], Suppl. 8. - - 1972. The Gault and its Junction with the Woburn Sands in the Leighton Buzzard Area, Bedfordshire and Buckinghamshire. Proceedings of the Geologists' Association, 83, 287-311. - - 1975. The stratigraphy of the Gault and Upper Greensand of the Weald. Proceedings of the Geologists' Association, 86, 475-498.

- - 1979. Ammonite Zonal Stratigraphy in the Albian of North Germany and its setting in the Hoplitinid Faunal Province. In (Wiedmann, J.; ed.) Aspekte der Kreide Europas. LU.G.S. Series A, 6, 563-588, Stuttgart. - - 1984. The Albian Stage: European province chronology and ammonite zonation. Cretaceous Research, 5, 329-344. PATTISON, J., BERRIDGE, N. G., ALLSOP, J. M. & WILKINSON, L P. 1993. Geology of the country around Sudbury (Suffolk). Memoir of the British Geological Survey, Sheet 206 (England & Wales). SOWERBY, J. 1821. Mineral Conchology of Great Britain. London. _ SPATH, L. E 1943. A monograph of the Ammonoidea ofthe Gault. Palaeontographical Society [Monograph], 721-787. WHITAKER, W. 1921. The water supply of Buckinghamshire and of Hertfordshire from underground sources. Memoir of the Geological Survey. WILKINSON, L P. 1990. The biostratigraphical application of Ostracoda in the Albian of eastern England. Courier Forschungsinstitut Senckenberg, 123, 239-258. - - & MORTER, A. A. 1981. The biostratigraphical zonation of the East Anglian Gault by Ostracoda. In (Neale, J. W. & Brasier, M. D.; eds) Microfossils from Recent and fossil shelf seas. Ellis Horwood, Chichester, 163-176. WOODCOCK, N. H. 1991. The Welsh, Anglian and Belgian Caledonides compared. Annales de la Societe Geologique de Belgique, 114,5-17. - - & PHARAOH, T. C. 1993. Silurian facies beneath East Anglia. Geological Magazine, 130,681-690. WOODWARD, H. B., THOMPSON, B. & MILL, H. R. 1909. The water supply of Bedfordshire and Northamptonshire from underground sources. Memoir of the Geological Survey (England & Wales.

Received 12 September 1994; revised typescript accepted 12 January 1995