6
Field Meeting on the Chalk at Pitstone, Central Chilterns 27 JWle 1970 Report by the Director: R. G. BLEZARD Revised text received 25 September 1971 ALTIIOUGH THE ASSOCIATION has held several Field Meet ings in this area it was considered of interest to review typical sections of the Chiltern Turonian-Cenomanian sequence in the light of the work of Jefferies (1962; 1963) and Kenn edy (1969). The object of the meeting was to demonstrate to members the fine sub- division of the Actinocamax plenus Subzone (lowest Turonian) in a classic area and to study localised aspects of the Lower and Middle Chalk succession, together with items of geomorphological interest on the central Chiitern escarpment. Basically the meeting was an examination of the route described by the Director in the early part of the first itinerary in the Geologists' Association Guide 30A (1967). The meeting was attended by forty members who met at Tring station. Transport was provided by Tunnel Cement Limited, whose Land-Rovers transported the majority of people to a quarry on the south side of the Upper Icknield Way (B488) near Brooks Statnalls Wood at the foot of Pitstone Hill. This quarry (SP 946147) shows some excellent sections of the A. plenus Subzone-I.labiatus Zonejunction, and the opportunity was taken to demonstrate the subdivision of the Plenus Marl (lowest Turonian) with respect to Jefferies' standard succession at Merstham, Surrey (Jefferies, 1962; 1963). A comparison is shown in Fig. 1. A section of the 'Plenus' succession in this quarry was given by Jukes-Browne & Hill (1903, p. 181, 463), with a sketch and a measured section. This particular section in the region of the old wood has since been destroyed by quarrying but good exposures are now to be seen farther west. The thickness of the bands is variable and is modified by numerous small faults. At a prominent point the succession was: Upper Marl (Jefferies, Beds 4 to 8) ... Intervening chalk band (Jefferies, Bed 3) Lower Marl (Jefferies, Beds 1 -2) em. 7·5 11'5 4·0 This succession of beds is generally thinner than the average section. Farther to the north-west only a single marl band (12 em. thick) was noted; faulting and erosion probably explain this. 349

Field meeting on the Chalk at Pitstone, central Chilterns: 27 June 1970

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Field Meeting on the Chalk atPitstone, Central Chilterns

27 JWle 1970

Report by the Director : R. G . BLEZARD

Revised text received 25 September 1971

ALTIIOUGH THE ASSOCIATION has held several Field Meet ings in this areait was considered of interest to review typical sections of the ChilternTuronian-Cenomanian sequence in the light of the work of Jefferies(1962; 1963) and Kennedy (1969).

The object of the meeting was to demonstrate to members the fine sub­division of the Actinocamax plenus Subzone (lowest Turonian) in a classicarea and to study localised aspects of the Lower and Middle Chalksuccession, together with items of geomorphological interest on the centralChiitern escarpment. Basically the meeting was an examination of theroute described by the Director in the early part of the first itinerary in theGeologists' Association Guide 30A (1967).

The meeting was attended by forty members who met at Tring station.Transport was provided by Tunnel Cement Limited, whose Land-Roverstransported the majority of people to a quarry on the south side of theUpper Icknield Way (B488) near Brooks Statnalls Wood at the foot ofPitstone Hill. This quarry (SP 946147) shows some excellent sections of theA. plenus Subzone-I.labiatus Zone junction, and the opportunity was takento demonstrate the subdivision of the Plenus Marl (lowest Turonian) withrespect to Jefferies' standard succession at Merstham, Surrey (Jefferies,1962; 1963). A comparison is shown in Fig. 1. A section of the 'Plenus'succession in this quarry was given by Jukes-Browne & Hill (1903, p. 181,463), with a sketch and a measured section . This particular section in theregion of the old wood has since been destroyed by quarrying but goodexposures are now to be seen farther west.

The thickness of the bands is variable and is modified by numerous smallfaults . At a prominent point the succession was:

Upper Marl (Jefferies, Beds 4 to 8) ...Intervening chalk band (Jefferies, Bed 3)Lower Marl (Jefferies, Beds 1-2)

em.7·5

11'54·0

This succession of beds is generally thinner than the average section.Farther to the north-west only a single marl band (12 em. thick) was noted;faulting and erosion probably explain this.

349

350 R. G. BLEZARD

The party climbed a small distance up Pitstone Hill to inspect theMelbourn Rock and the labiatus Zone chalk above the Plenus Marl.The age of the topmost bed of the 'sub-globosus-t ype ' chalk below the marlvaries considerably in the Ch ilterns, and in th is area some 20 ft. of the top'sub-globosus-ty pe' chalk seems to have been removed, a point to be con­sidered when studying the con stitution of the microbenthos in this section(Jefferies, 1962, fig. 4).

Dr. M. B. Hart kindly expla ined his interpretation of the Cenomanian­Turonian boundary which differs slightly from that of Dr. R. P. S. Jefferies.

M

CM l b

K l a

L e g en d

H.R. Melb ou r n r o ckC Chal k

CM Chalky ..arlM Ma rl

S .G . Subglobosus-typ ec hal k* Erosio n surface

Cn Cen omanianT Turonian

••.~..•......•..,.:o·.~"'.• 'M

0..>· .~..:<=>

5 -lr4 T T·· ····ca···· -_.. Ca

C 3

Merstham Stand a r dPlenus Ma r lSuccession

Pitsto n ePlenus MarlSuccession

0 876

5 .---.......*~ 4

"....,"ec 3

.~

2

".... l b~ 1.0u

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Fig. I . Comparison of Plenus Marl succession at Pitstone with the standa rd successionat Merstham, Surrey (type locality after R. P. S. Jefferies)

FIELD MEETING IN CENTRAL CHILTERNS 351

On the evidence of the microfossils (and in particular planktonic foramini­ferida) Dr. Hart considers that there is a major faunal change at the top ofJefferies Bed 3 and that this faunal change can be traced over the whole ofEurope and North America. The Plenus Marls are designated under themicrofaunal zonation scheme as Zone 14, with Jefferies' bed numbersbeing used for finer division. Bed 3 contains a fauna characterised by thespecies Rotalipora cushmani (Morrow), which is used as an Upper Ceno­manian indicator all over the western hemisphere. This species appears atthe boundary between the Turrilites costatus and Turrilites acutus assem­blage zones of Kennedy (1969). The fauna from Bed 4 and above ischaracterised by advanced species of the genus Praeglobotruncana Ber­mudez, almost identical with the association recorded from the FretevouChalk of the Cher Valley (Type Turonian).

The Director corrected a statement made in Regional Guide 30A (page4) on the extent to which the A. plenus can be found in the Lower Marl;this was confirmed by locating the subzone fossil indicator.

A deep excavation in the Lower Chalk was examined at Bulbourne(934139). The Totternhoe Stone was detected as a thin phosphatic pebblelayer, one centimetre thick, in an upper section of the quarry. It is at thislevel that there is a most remarkable microfaunal change that has alreadybeen described by Carter & Hart (in discussion of Kennedy, 1969) andHart (1971). The Lower Cenomanian and the lower levels of the MiddleCenomanian contain microfaunas that are dominated by benthonicforaminiferida and only rarely are planktonic forms found. Above theTotternhoe Stone there is a rich association of planktonic species charac­terised by Rotalipora cushmani (Morrow) and Praeglobotruncana stephani(Gandolphi). This faunal change has been shown to constitute the onlyevidence-in the south-east of England-of a major non-sequence in theLower Chalk succession. This break becomes more visibly manifest in thesouth-west of England, where this level is marked by the appearance of the'Chalk Basement Bed' of other workers.

The vertical cutting below this level was some 18 m. deep and showed agood face of the Chalk Marl which was devoid of flint. The soft nature ofthe marl was judged by the ease of excavating in this section using a mam­moth German-made LMG type 150 bucket-wheel excavator (see Plate 18),which can remove this chalk at a rate of 1000 tons per hour. The operatingof the excavator was demonstrated to members. The excavating wheels hada cutting circle of 4'5 metres diameter, carrying eight buckets and eightintermediate cutters spaced alternately, and the optimum operating heightof the face for this machine was 10'5 metres. This technique of excavatingcould not be used for sediments containing a flint band content in excessof 3 per cent and thus excavating in normal Upper Chalk would not berecommended. A deep boring made in 1855 and reported by Jukes-Browne

PROC. GEOL. ASS., YOLo 83, PART 3, 1972 24

352 R.G.BLEZARD

& Hill (1903, 177) proved 34 m. (111 ft.) of Chalk Marl. In the lower levelsof the quarry specimens of the sponge Exanthesis labrous (T. Smith) werefound, which indicated a position in the Mantelliceras mantelli zone of theLower Cenomanian. Samples collected for microfaunal analysis on theexcursion have shown that from this level up to the Totternhoe Stone thesuccession is precisely the same as that recorded at Dover. The onlydifference is the thickness of Dr. Hart's zone 11 (i) immediately below theTotternhoe Stone. This variation would seem to be due to the removal ofthe layers of Chalk Marl immediately below the Mid-Cenomanian non­sequence. It has been shown over the greater part of southern Englandthat variations in the thickness of the Cenomanian can in part be accountedfor by the removal of material at this level.

The party retraced its steps in an easterly direction along the UpperIcknield Way past Folly Farm. As the meeting took place in the NationalConservation Year it was fitting to remark that an excavated area of 31acres north of Folly Farm had been filled in, levelled and returned tocultivation. A large disused quarry (referred to in G.A. Guide 30A aslocation Ia, grid reference 944145) exposed a coombe rock based on a'subglobosus type' chalk rubble and showing effects of ice disturbance.Holaster subglobosus is not suitable as a zonal index in this area andAcanthoceras rhotomagense was not detected. The occasional occurrence ofStohamites simplex was noted where the Turrilites costatus subzone wasinferred. In moving northward into the Chalk Marl the decreasing contentof calcium carbonate became more obvious with the grey colour of thechalk (CaCOa about 55 per cent), and fossils were more evident. ManySchloenbachia species were found, S. subvarians and S. intermedia; forzoning purposes Mantelliceras gr. dixoni is diagnostic, but Schloenbachiavarians is an unsatisfactory index fossil.

At the conclusion of the meeting the President, Mr. S. C. A. Holmes,passed a vote of thanks to the Director who, in turn, thanked members fortheir attendance and acknowledged the kindness of Tunnel CementLimited in allowing access to the exposures. Mr. Blezard was grateful forthe comments made by Dr. M. B. Hart during the meeting and later dis­cussions.

REFERENCESKIRKALDY, J. F. & R. G. BLEZARD. 1967. Itinerary I, The London Region. Geologists'

Association Guide 30A, 4-6.BROWN, E. H. 1964. Field Meeting in the Chiltems, near Tring. (21 July 1963). Proc.

Geol. Ass., 75, 341-4.EVANS, P. & K. P. OAKLEY. 1952. Field Meeting in the Central Chilterns (7 May 1949),

Proc. Geol. Ass., 63, 59-62.JEFFERIES, R. P. S. 1962. The Palaeoecology of the Actinocomax plenus Subzone (Lowest

Turonian) in the Anglo-Paris Basin. Palaeontology, 4, 609-47.---. 1963. The Stratigraphy of the Actinocamax plenus Subzone (Turonian) in

the Anglo-Paris Basin. Proc. Geol. Ass., 74, 1-33.

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Large-scale excavating in th e Lower Cha lk at Bulbon e, near Pitstone, using a LMG-1 50 bucket-wh eel excavato r

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FIELD MEETING IN CENTRAL CHILTERNS 353

JUKES-BROWNE, A. J. & W. HILL, 1903. The Cretaceous Rocks of Britain. Vol. 2, Mem.geol SUTV. U.K.

KENNEDY, W. J. 1969. The Correlation of the Lower Chalk of South-East England,Proc. Geol. Ass., 80, 459-560.

HART, M. B. 1970. The Distribution ofthe Foraminiferida in the Albian and Cenomanianof South-West England. Unpublished Ph.D. Thesis, University of London.

----. 1971. Micropalaeontological Evidence for Mid-Cenomanian Flexuring inSouth-West England, Proc. Ussher Soc. (in press).

R. G. Blezard114 Purfleet RoadAveley, South OckendonEssex