Stratigraphical interpretation of Chalk Group macrofossils from the Aylesbury district (Sheet 238): Spring/Autumn 2012 fieldwork Geology & Landscape England Programme Open Report OR/12/067 BRITISH GEOLOGICAL SURVEY GEOLOGY & LANDSCAPE ENGLAND PROGRAMME INTERNAL REPORT OR/12/067 Stratigraphical interpretation of Chalk Group macrofossils from the Aylesbury district (Sheet The National Grid and other Ordnance Survey data © Crown 238): Spring/Autumn 2012 Copyright and database rights 2012. Ordnance Survey Licence No. 100021290. fieldwork Keywords Chalk Group, Macrofossils, Cenomanian, Turonian, M A Woods Coniacian, Lithostratigraphy, Biostratigraphy, Chronostratigraphy. Map Sheet 238, 1:50 000 scale, Aylesbury Bibliographical reference WOODS, M A. 2012. Stratigraphical interpretation of Chalk Group macrofossils from the Aylesbury district (Sheet 238): Spring/Autumn 2012 fieldwork. British Geological Survey Internal Report, OR/12/067. 20pp. Copyright in materials derived from the British Geological Survey’s work is owned by the Natural Environment Research Council (NERC) and/or the authority that commissioned the work. You may not copy or adapt this publication without first obtaining permission. Contact the BGS Intellectual Property Rights Section, British Geological Survey, Keyworth, e-mail [email protected]. You may quote extracts of a reasonable length without prior permission, provided a full acknowledgement is given of the source of the extract. Maps and diagrams in this book use topography based on Ordnance Survey mapping. © NERC 2012. 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Material was collected by MAW, A R Farrant and P M Hopson during fieldwork in the Spring and Autumn of 2012. This work is in connection with revision of the digital mapping and lithostratigraphical classification of the Chalk Group on Sheet 238. Contents Foreword.........................................................................................................................................i Contents...........................................................................................................................................i Summary........................................................................................................................................ii 1 Introduction............................................................................................................................ 1 2 Stratigraphy............................................................................................................................ 1 3 Discussion.............................................................................................................................. 19 Appendix 1 – author citations for fossil species.................................................................. 20 References .................................................................................................................................... 21 FIGURES Figure 1. Map showing distribution of sites discussed in this report. Figure 2. The stratigraphy of the Chalk Group referred to in this report Figure 3. The chalk succession seen at [SP 90164 11342] ((12) of report). (a), (c), (d): contact of Plenus Marls and Melbourn Rock members of the Holywell Nodular Chalk Formation (arrowed). Note high angle fault lined with flint in (d) showing small offset of Plenus Marls/Melbourn Rock. Red note book is 0.19 m high. (b): view of the complete section (about 5m high). Figure 4. The chalk succession at [SP 84678 05598] ((26) of report) Figure 5. The chalk succession at [SP 90939 03524] ((30) of report) i OR/12/067; Draft 0.1 Last modified: 2013/02/12 18:34 Summary Geological details and macrofossil collections from 53 localities within the Chalk Group of the Aylesbury district are described and stratigraphically interpretated. These data are used to support new digital mapping and lithostratigraphical subdivision of the Chalk Group on Sheet 238. ii OR/12/067; Draft 0.1 Last modified: 2013/02/12 18:34 1 Introduction New collections of macrofossils and geological details of 53 localities in the Chalk Group of the Aylesbury district (Sheet 238) are described below. Many of the localities are old, largely degraded chalk pits or cuttings, with relatively poor exposure of bedrock. Nevertheless, these localities provide valuable insights into lithological variation within the Chalk Group of the district. Combining these lithological observations with biostratigraphical dating of the macrofaunas, permits correlation with the standard lithostratigraphical framework established for the Chalk Group of southern England (e.g. Mortimore et al., 2001). These interpretations are used to support new digital field mapping and lithostratigraphical classification of the Chalk Group of the Aylesbury district, following the field methodology described by Aldiss et al. (2012). The distribution of the localities described in this report is summarised on Figure 1, and Figure 2 shows the stratigraphy referred to in this report. Author citations for fossil species are given in Appendix 1. 2 Stratigraphy The 53 localities in the Chalk Group of the Aylesbury district that form the basis of this work are described sequentially below. (1) Exposure on east side of bridleway adjacent to Grove Wood, c. 350 m NNW of Hastoe House, Hastoe, Hertfordshire. 1:10 000 SP90NW NGR: SP 91606 09755 Specimen nos: WMD 16035 – 16045 The specimens were collected from poorly fossiliferous, grainy-textured chalk; mostly rather soft but with more indurated patches. The sparse fossil material comprises fragmentary remains of the inoceramid bivalve Cremnoceramus, including C. crassus?, and a cidarid echinoid fragment. Interpretation: Cremnoceramus crassus suggests assignment to the M. cortestudinarium Zone, in which Cremnoceramus is characteristic. In conjunction with the lithology, assignment to the higher part of the Lewes Nodular Chalk might be inferred. Conclusion: White Chalk Subgroup, upper Lewes Nodular Chalk Formation; Coniacian, M. cortestudinarium Zone. (2) Exposure on West side of bridleway. c. 420 m NNW of Hastoe House, Hertfordshire. 1:10 000 SP90NW NGR: SP 91600 09801 Specimen nos: WMD 16046 – 16048 The specimens were collected from a similar Chalk to that seen at (1) (above), although somewhat more indurated in character. The fauna includes the inoceramid bivalve Cremnoceramus, including C. denselamellatus. Interpretation: Cremnoceramus denselamellatus suggests assignment to the M. cortestudinarium Zone, in which Cremnoceramus is characteristic. In conjunction with the lithology, assignment to the higher part of the Lewes Nodular Chalk might be inferred. 1 OR/12/067; Draft 0.1 Last modified: 2013/02/12 18:34 Conclusion: White Chalk Subgroup, upper Lewes Nodular Chalk Formation; Coniacian, M. cortestudinarium Zone. (3) Exposure on west side of bridleway, 380 m SSE of West Leith Farm, Tring, Hertfordshire. 1:10 000 SP90NW NGR: SP 91643 09967 Specimen nos: WMD 16049 – 16055; 16056 – 16057 This locality exposes the Chalk Rock, at the base of the Lewes Nodular Chalk Formation. There is a rapid downward transition, within 1 m, from the intensely hard chalk of the Chalk Rock, into firm, smooth-textured chalk resembling New Pit Chalk. This relationship is somewhat unexpected, as previous studies (Bromley & Gale, 1982) suggest that