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Home , Gault, Greensand, Leighton Buzzard, Woburn Sands Formation

Journal of the Geological Society, , Vol. 153, 1996, pp. 334-336, 2 figs. Printed in Northern Ireland

Discussion on facies analysis in the Lower using ground-penetrating radar

Journal, Vol. 152, 1995, pp. 591-598.

J. Eyers writes: In his recent paper Bristow (1995) presents field relations with the and underlying are an excellent account of the use of the ground-penetrating incorrect on his figure (see text below). This unit isof radar in deducing large-scale sedimentary structures in tardefurcata Zone age. The lithology is that of a sands.However, thereare several inaccuracies and ferruginous claystone with typically a 5050 ratio of ragged misleading statements in the paper and these are dealt with clasts to ferruginous matrix. It is thereforemore in the order they appear in the text. correctly termed an ironstone. (3) ‘Carstones’ at the top of the Silver Sands (base Silty Beds). These are channel with a very localised Tectonic influence on sedimentation. The statement that the distribution. The lithology is that of quartz grains and minor tectonic influence proposed by Eyers (1991) is ‘refuted by other clasts held together with a silica or ferruginous the most recent Geological Survey Memoir(Shephard- cement.This is more correctly termedsandstone or Thorn et al. 1994)’ is unfair. Theauthor is not going to ferruginous channel sanhtone. discuss the influences on sedimentationnor the sandunits (4) ‘Carstone reefs’, e.g. of Shephard-Thorn et al. (1986, that are the subject of the Eyers paper. The proposal of a 1994), which are erosional remnants of the channels of (3) tectonic influence on sedimentation patterns within the above. Lower Greensand is thus dismissed as requiring no further (S) A ‘carstone’ at the top of the Brown Sands. Some, investigation, even though the author does not propose an and probably all, of these beds are channel sandstones. The alternative model or discuss the issue. Tectonic influence has lithology is similar to (3) with commonly a better developed also been proposed by Ruffell & Wignall (1990). Bristow’s oxide cement-ferruginous channel . paper describesa small and unrepresentative unit of the (6) Ferruginous horizons andiron pan throughoutthe majority of the Lower Greensand, the ‘Red Sands’, a unit Brown Sands. These horizons are not in channel form and previously intepreted as channelized deposits (C. R. Bristow can be laterally extensive. They are commonly a few to a 1963; Shephard-Thorn et al. 1994; Eyers 1992a,1995a, and few tens of centimetres in thickness, with a ferruginous references within). Readers interested in this possible tec- matrix containing occasional other clasts, usually quartz. tonic aspect may judge the evidence for themselves in Eyers They are always preceded by a clay drape atthe base. (1992a,1995a). Correct lithological term: iron pan. For the above reasons, and for the reason that the term ShenleyLimestone. Contraryto the statement that the Carstone applies to a distinct lithological unit of the Lower Shenley Limestone is not exposed at the present day, this Greensand Group, it is not appropriate to use the term for horizon hasbeen exposed at various quarries during the any of the sediments in the Leighton Buzzard area. period of my research, 1988-1995, including the best sec- It was of greatinterest to seethe use of ground- tions at Reach Laneand Munday’s Hill, bothnorth of penetrating radar with the Red Sands and, given the right Leighton Buzzard (Eyers 19926,19956). Today only the conditionsand suitable reflectors, this has proved a very Munday’s Hill section is still exposed, although this is cur- valuable technique and produced good results. However, I rently being excavated at a rapid rate (from August 1995). am very worried to see sweeping generalizations such as that used throughout the conclusions and previous text (e.g. the caption for fig. 10) thatthe results are forthe ‘Lower Carstone. The use of the term ‘Carstone’ (p. 591 and fig. 1) Greensand’. The sandunits within the Lower Greensand is not clear and stratigraphically confusing. Thereare six (Red, Silver, and Brown Sands and the Carstone) are highly occurrences of ferruginous deposits in the Leighton Buzzard distinctive. The bedformgeometry and sedimentary areaand unfortunately all have beenreferred to as ‘car- structures are quite different for each subdivision. The stone’. These are listed below, in order, from the top of the Red Sands have a notably different structureand very sequence downwards and illustrated in Fig. 1. localised distribution to any of the other units. Distinction (1) A ‘carstone’ which cuts down through the Silty Beds should have been made throughout that the conclusions are and topmost Silver Sands. This is a coarse sandstone with a valid only for theRed Sands subdivision of the Lower silica or ironoxide cement and is in channelform. (This Greensand. sandstone is probably contemporaneous with the topmost Silty Beds, but precise evidence is lacking.) Based on Field relations. Figure 1 of Bristow’s paper is an unusual lithology it is therefore more correctly termed a ferruginous interpretation of the field relations between the Red Sands, channel sandstone. Silty Beds, Shenley Limestoneand ‘Carstone’ and is one (2) A ‘carstone’ at the top of the Silty Beds and within that is not borne out by field evidence. Field evidence which the lenses of ShenleyLimestone occur. It is not in suggests an alternative interpretation of the relationbet- channel form and is cut by sandstone (1) above. Bristow’s ween these units (Fig. 2). fig. 1 would most closely approximate with this one, but the 23 August 1995 334

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Hill and appear to thicken towards the South where the Red Sands crop out. Theerosional truncation of the Silty Beds is well exposed at the southern end of Mundays Hill Quarry but this is not shown on Eyers diagram. Furthermore, Eyers (199%) describes erosion of the silty beds which she links to a‘Carstone Conglomerate’ in Chamberlains BarnQuarry which containsreworked clasts of the Shenley Limestone and the silty beds. I agree with Eyers’ (19956) interpretation and suggest that this ‘Carstone Conglomerate’ would tend to support my interpretation of the stratigraphy. Eyers disputes the evidence for the Red Sands cutting the Silty Beds when this relationship is clearly shown in Shephard Thorn et al. (1994), their summary log for Mundays Hill (fig. 14). The fact that the Silty Beds are not found above the Red Sands provides additional circumstantial evidence to support my interpretation that the Red Sands post date and erode into the Silty Beds following the stratigraphy of Shephard-Thorn Fig. 1. Figure to illustrate the stratigraphic horizons with previously cited ‘carstone’ horizons. Descriptions of 1 to 6 appear in the text. et al. (1994). Carstone. In order to avoid the confusion over the use of the term ‘Carstone’ as suggested by Eyers, I used the name C.S. Bristow replies: In her discussion of my recent paper on in a lithostratigraphic context. I did not make any reference the Lower GreensandEyers raises severalpoints onthe to other iron-carbonate cemented conglomerate or concre- stratigraphic nature of the Red Sands, Silty Beds, Shenley tionary horizons within the Woburn Sands as ‘Carstone’. In Limestone and Carstone in the Leighton Buzzard area. This thissense the ‘Carstone’ is alithostratigraphic unit, not a subject was not central to my paper which concerns the use ‘lithological unit’, and may be correlated with the Carstone of ground-penetrating radar in investigating the Lower of . Greensand. However, I welcome the opportunity to respond to her comments because they raise interesting concerns for Shenley Limestone. In my paper I stated that the Shenley others working in this area. Limestone is not exposed at the present day. Eyers believes I do not agree with Eyers’ generalized stratigraphy (Fig. that the Shenley Limestone is exposed at the southern end 1) for the following reasons. Eyers’ diagram shows the Silty of Mundays Hill Quarry. However, there is a difference in Beds andIronstone with ShenleyLimestone pinching out interpretation of this section. My interpretation of this out- laterally towards the Red Sands with what appears to be an crop is based partly on evidence brought to my attention by offlap relationshipprior to being onlapped by . The Jonathan Wonham. The ‘lenses of Shenley Limestone’ re- pinching out of theseunits is notcompatible with field ported by Eyers from this locality are reworked clasts as evidence which indicates at least oneand possibly two stated in my paper. The limestone clasts are found in very phases of erosion between the Silty Beds and the Gault, as coarse grained iron rich sand above an unconformity surface shown in Shepard-Thorn et al. (1994). Eyers (19956) shows which can be traced in the field cutting down through the that the Silty Beds are thickest at Reach Lane and Mundays silty beds and passing into the base of the iron cemented channel sandstone which erodes into the top of the Silver Sands. I identify both iron rich sands above the unconfor- mity as the ‘Carstone’. There is direct field evidence for the unconformity between the ‘Carstone’ and the Silver Sands as shown in my figure one.This relationship is shown in Eyers’ Carstone 1, which may be equivalent to the ‘Carstone Conglomerate.’ of Eyers’ (19956) whichis not included in Eyers’ list of Carstones.

Tectonics. My comments on tectonics are ‘Eyers (1991) has KEY suggested that there was a tectonic control on sedimentation Shenley Lst. whichis refuted by the most recent Geological Survey Ironstone Memoir (Shephard-Thorn et al. 1994)’. I stand by this state- Silty Beds Red Sands ment which recognizes the work of Eyers and the survey. It Silver Sands makes it clear that Eyers has suggested a tectonic control Brown Sands and it also reports the findings of the most recent geological v1u)vIvI survey memoir which contradicts her interpretation.

Fig. 2. Replacement for fig. 1 of Bristow: generalized stratigraphy RedSands. TheRed Sands are an informal unit of the of the Leighton Buzzard area. Points of notable difference to which is in turn part of the Lower Bristow’s figure are: (1) No ‘Carstone’ over Red Sands: (2) Shenley Greensand Group. In my descriptions and interpretations I Limestone forms discrete lenses, not a continuous bed; (3) evidence have made some generalisation in using thethe terms for the Red Sands cutting Silty Beds is disputable, Silty Beds could Woburn Sands and Lower Greensand. However, I believe be the younger deposit. but no reliable field evidence is available. that I made it clear that my paper was only based on a study

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of the Red Sands e.g. ‘In this study the ground-penetrating penetratingradar. Journal of the GeologicalSociety, London, 152, radar surveys have been restricted to the Red Sands.. .’ (p. 587-598. EYERS,J. 1991. The influence of tectonics on early sedimentation 591). in ,. Journal GeologicalSociety, London, 148, With reference tothe specific figure caption whichis 405-414. criticized (fig. lO), I would like to point out that the - 1992a. Lithostratigraphy of the Lower Greensand and Gault of the accompanying text (p.597) states ‘A reconstruction of Bedfordshire Province, England. PhD thesis, Open University. -19926. Sedimentology and palaeoenvironment of the Shenley Limestone bedforms and sedimentarystructures in theRed Sands is (Albian,Lower Cretaceous): an unusual shallow-water carbonate. shown in fig. 10’. The Red Sands are part of the Woburn Proceedings of the Geologists’ Association, 103, 293-302. Sands Formation and while I do not explicitly exclude the - 1995a. Correlation of the Lower Greensand Group (Woburn Sands and Silver or Brown Sands in my paper I do not make any claims Carstone) of the Bedfordshire Province, England. Cretaceous Research, 16,385-413. to have investigated them. -19956. The Silty Beds: a tidal flat sequence at the junction of the Lower Greensandand Gault (Albian, Lower Cretaceous) of Bedfordshire, England. Proceedings of the Geologists’ Association, 106, 107-118. 4 October 1995 RUFFELL,A. & WIGNALL,P.1990. Depositionaltrends in theUpper -Lower Cretaceous of the northern margin of the Wessex Basin. Proceedings of the Geologists’ Association, 101,279-288. SHEPHARD-THORN,R., E. HARRIS, P. M,, HIGHLEY,D. E. AND THORNTON,M. References H. 1986. An outline study of the Lower Greensand of parts of south-east England. Reportfor the Department of theEnvironment, British BRISTOW,C. R. 1963. The stratigraphy and structure of the Upper Jurassic and Geological Survey, Keyworth, Nottingham. Lower Cretaceous rocks the in area between -, MOORLOCK,B. S. P,, Cox, B. M,, ALLSOP,J. M. & WOOD,C. J. 1994. () and LeightonBuzzard (Bedfordshire). PhD thesis, Geology of the country around Leighton Buzzard. Memoir of the University of London. Geological Survey,Sheet 220, 127 pp. (HMSOfor British Geological BRIBIOW, CS. Facies analysis in TheLower Greensand using ground- Survey London).

J. EYERS,Science Faculty, Open University, Walton Hall, MK7 6AA, UK. C. S. BRISTOW,Department of Geology, Birkbeck College, Malet Street, London, WClE 7HX. UK.

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