- Email: [email protected]
What have we done to the chalk? A commentary
Proceedings of the Geologists’ Association 122 (2011) 229–231
Contents lists available at ScienceDirect
Proceedings of the Geologists’ Association journal homepage: www.elsevier.com/locate/pgeola
Context article
What have we done to the chalk? A commentary Haydon Bailey Network Stratigraphic Consulting Ltd., Cranborne Road, Harvest House, Potters Bar, Hertfordshire EN6 3JF, United Kingdom
To witness a complete issue of the Proceedings dedicated solely to two papers by Professor Rory Mortimore is worthy of note, although there are those who sometimes find his combined use of global eustatic sea-level change and regional tectonic events (e.g. Jarvis et al., 2006) a little difficult to accept. However, a little controversy helps science to move forward which is what we seek. The joint publication of these papers is rightly justifiable and they deserve their place in the established history of chalk research and the Geologists’ Association. It is immediately acknowledged here that this is not a fully comprehensive review, but serves only to put these papers into a more contextual setting. Just over a hundred years ago the Proceedings had just completed the publication of the final paper of a series of five written by Arthur Rowe on the Zones of the White Chalk (Rowe, 1900, 1901, 1903, 1904, 1908). These epic papers, together with the Survey work of Jukes-Browne and Hill (1900, 1903, 1904) became the ‘‘gold standards’’ for chalk stratigraphy for the next sixty years. As we are dealing with Mortimore papers we should not forget the part played by the GA and the numerous Proceedings publications on the Sussex chalk by Gaster (1924, 1928, 1929, 1939, 1941, 1951). Although more locally based, these were to play a major role in Mortimore’s interpretation of the Sussex chalk in his own research. This then brings us quickly into the late 1960s and 1970s when the demands of building the Channel Tunnel, including the 1960s thwarted attempt (Slater and Barnett, 1958; Varley and Warren, 1996) and the Thames Barrier (Carter and Hart, 1977) pointed to the need to revisit chalk stratigraphy after a fifty year hiatus and to apply techniques unavailable to Rowe and his contemporaries (e.g. SEM, micropalaeontology). The first hints of the changes to come were raised by Hancock (1975), again in the PGA, in his classic paper on ‘‘The Petrology of the Chalk’’ which introduced us to the effects of sea-level changes during the Cretaceous. The late 1970s were the time when Rory Mortimore appeared on the Late Cretaceous horizon as he completed his doctoral research at Brighton Polytechnic. He immediately endeared himself with the Geologists’ Association by leading a field trip to Sussex in 1977. I met Rory Mortimore on this trip and I am pleased to note that there is a grainy black and white photograph in the GA archive marking the event. His Ph.D., completed in 1979, started to raise a few eyebrows even with its title ‘‘The relationship of stratigraphy and tectonofacies to the physical properties of the
E-mail address: [email protected].
White Chalk of Sussex’’. Tectonics during the deposition of the chalk, just when the importance of global eustatic sea-level changes were becoming increasingly important (Haq et al., 1987)? Mortimore was swimming against a tide (or at least rising sea-levels), but his seven volume thesis is nothing if not thorough. Part of Mortimore’s work in Sussex was to define a lithostratigraphy for the chalk of the South Downs. Ironically, as is often the case, parallel research was being carried out on the chalk of the North Downs by Nick Robinson at Kingston Polytechnic. Precisely a quarter of a century ago, the Geologists’ Association regarded this situation as too good an opportunity to miss and they organised a meeting for the presentation of the North Downs versus the South Downs lithostratigraphies at Burlington House (Mortimore, 1986; Robinson, 1986). As with such events the initial outcome seemed more of a draw rather than a victory for either contender. Nevertheless, this meeting resulted in the formal publication of both the alternative lithostratigraphic schemes and their subsequent potential correlation (Gale et al., 1987; Mortimore, 1987) thereby bringing them to a much wider audience. This audience included the Geological Survey who had started to apply the Mortimore scheme in their mapping of the chalk in southern England (e.g. Bristow, 1990; Bristow et al., 1995). The Isle of Wight paper in the present issue is a further example of Mortimore’s approach to his geology in that it presents a series of basic, but thorough, lithological descriptions of numerous inland sections in the central part of the island. Many of these sections will probably be lost within a generation, as is the case with many similar sites across southern England as landfill and site neglect take effect; so their accurate recording is extremely important whilst they are still accessible. With the lithological variations and structural details described, Mortimore then goes on to use the information provided, together with the available geophysical and wireline log data to provide a much broader regional structural interpretation. As with all of Mortimore’s work, each and every project is carried out with a direct application in mind, in this case examining the hydrogeological behaviour of the chalk in a structurally complex area. It is a thorough, systematic piece of research which can be used at several different levels. The basic field logs in themselves are extremely useful, if sometimes a little mystifying (see Fig 15: 5.5 m up the section – ‘‘Possible 100 mm thick marl seam (gmz1) – an awkward fault!’’). The addition of the
1 gmz: Griotte marly zone. A regular Mortimore abbreviation, indicating a concentration of wispy horsetail like marl seams. Unfortunately, in this paper the abbreviation never receives a formal definition.
0016-7878/$ – see front matter ß 2011 The Geologists’ Association. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.pgeola.2011.01.001
230
H. Bailey / Proceedings of the Geologists’ Association 122 (2011) 229–231
seismic data takes us to a totally different interpretational level which is extremely informative. The recognition during the 1990s that we had a lithostratigraphy for the chalk which could be applied systematically across southern England, almost to an individual Bed level, was a major step forward, resulting in a number of key events in 1997: - Firstly, a meeting was held at the BGS offices in Keyworth bringing together chalk expertise from all directions to agree, if possible, on the formal adoption of the Mortimore scheme by both BGS and the wider chalk research community. - Secondly, the Proceedings published Bristow et al. (1997) on ‘‘Lithostratigraphy for mapping the Chalk of southern England’’, a paper which was to receive that year’s Geologists’ Association Richardson Award. - Thirdly, and most important to the majority of GA members, Mortimore published the GA Field Guide No. 57 ‘‘The Chalk of Sussex and Kent’’ which has provided so many people with their first detailed introduction to chalk stratigraphy and sedimentology in a much more accessible format. It is still a valuable asset in the backpack on any trip to the southeast corner of England. - Finally, in the 1997 volume of the Proceedings, Mortimore and his long term working colleague Bernard Pomerol also published ‘‘Upper Cretaceous tectonic phases and the end Cretaceous inversion in the Chalk of the Anglo-Paris basin’’. Thus Late Cretaceous tectonic phases were back on the agenda. Mortimore and Pomerol found that the tectonic phases they recognised in the Anglo-Paris basin had long been defined in Germany (Stille, 1924) and, in 1998, together with Christopher Wood and the late Gundolf Ernst they produced ‘‘Dating the phases of the Subhercynian tectonic epoch: Late Cretaceous tectonics and eustatics in the Cretaceous basins of northern Germany compared with the Anglo-Paris basin’’ (Mortimore et al., 1998). The implications of this work and the earlier Anglo-Paris paper were that chalk deposition was not simply controlled by a carbonate factory systematically generating algal fall out across a stable epicontinental shelf for thirty five million years as sea levels rose and fell. The preserved chalk succession represents a dynamic period where a plethora of plate scale events were taking place and the northern European shelf was a relatively stable back water area where the effects were represented more by facies change, often difficult to recognise in basinal chalk sequences. I initially became intrigued by the potential impact of Late Cretaceous tectonics about twenty five years ago whilst core logging through a continuous mass of Late Turonian radiolarian rich carbonates from an offshore Norwegian chalk field. This thick deep water ‘‘chalk’’ had been deposited during a period when all my previous training had taught me that there was global eustatic sea-level fall and I should be expecting to see section loss and condensation over the established positive structure I was helping to exploit for oil. I was seeing the opposite; I was logging through carbonates deposited in a rapidly deepening basin on the line of a known structurally positive lineament. Mortimore et al. (1998) and the late Ilsede inversion phase during the Late Turonian finally provided me with an explanation to the predicament I would found myself in. This dynamic Late Cretaceous model takes on a broader setting when we look at nearby areas of North Africa and the Middle East. What was happening in those areas during the Late Cretaceous had a direct impact on the European plate to the north. Mortimore’s ‘‘Revolution’’ paper here summarises briefly the work of Bosworth et al. (1999) and Guiraud and Bosworth (1997) and how the events they recognise (along the Syrian arc for example) relate to the tectonic phases recognised across northern Europe. Just to take one example, the so called ‘‘84 million year event’’ is widely recognised
from the Middle East (Guiraud et al., 2005) to West Africa (Burke et al., 2003) where it resulted in coarse clastic hydrocarbon reservoir generation. This correlates directly with the subhercynian Wernigerode event recognised in Germany. There is a constant dynamic interplay between global eustatic sea-levels with regional variations being imposed by plate movements and inter-plate readjustments and, at a relatively local level, the sedimentary sequences we find ourselves logging, either along active structural lineaments or in the relatively stable areas in between. So perhaps this is the crux of Mortimores question ‘‘What have we done to the Chalk?’’ The chalk has been shifted from its comfort zone of being a uniform pelagic deposit across a widespread epicontinental sea and it has been placed into a totally different context. One ‘‘of major tectonic change on a global scale as well as one of exceptionally high sea levels globally. The supercontinents were breaking up, and Africa was beginning to under-ride and laterally shear along the southern edge of Europe’’ (Mortimore et al., 2001). The thirty five million year period between 100 ma and 65 ma begins to sound pretty dramatic and quite exciting. The high, yet fluctuating, sea-levels are still recognised and clearly represented in the sediments whilst at the same time there is the impact of an opening Atlantic, the development of the Icelandic hot spot, a shifting African plate and major ophiolite emplacement on the southern margins of the Arabian plate. All these were being translated through into Europe along well developed structural lineaments which then affect the sedimentary sequences preserved in the sections we log in the field (see Bless et al., 1986) or in offshore well sections (see Hampton et al., 2010 for an interesting example). This brings me back to the fundamental value of the papers presented in this issue of the Proceedings; they are founded initially on rigorous section logging and description, the fundamental training for every geologist. Once you have the basic information in your field logs, then you can start interpreting it. That is when the alternative views and interpretations come into play, but differences are fine, as with any reasoned scientific debate and argument, it is on the back of these disagreements that we make advances. Better to be part of an advancing dynamic system than a quiet backwater! I hope you agree and enjoy your chalk revolution. References Bless, M.J., Felder, P.J., Meesen, J.P.M.T., 1986. Late Cretaceous sea level rise and inversion: their influence on the depositional environment between Aachen and Antwerp. Annales de la Socie´te´ ge´ologique de Belgique 109, 333–355. Bosworth, W., Guiraud, R., Kesler, L.G., 1999. Late Cretaceous (84 Ma) compressive effects of the ‘‘Santonian event’’ and the origin of the Syrian arc. Geology 27, 633– 636. Bristow, C.R., 1990. Geology of the Country around Bury St. Edmunds. Memoir of the British Geological Survey (England and Wales), Sheet 189. HMSO, London, 99 pp. Bristow, C.R., Barton, C.M., Freshney, E.C., Wood, C.J., Evans, D.J., Cox, B.M., 1995. Geology of the Country around Shaftesbury. Memoir of the British Geological Survey (England and Wales), Sheet 313. HMSO, London, 182 pp. Bristow, C.R., Mortimore, R.N., Wood, C.J., 1997. Lithostratigraphy for mapping the chalk of southern England. Proceedings of the Geologists’ Association 108, 293– 315. Burke, K., Macgregor, D.S., Cameron, N.R., 2003. Africa’s petroleum systems: four tectonic ‘Aces’ in the past 600 million years. In: Arthur, T.L., MacGregor, D.S., Cameron, N.R. (Eds.), Petroleum Geology of Africa: New Themes and Developing Technologies, vol. 207Geological Society, London, pp. 21–60, special publication. Carter, D.J., Hart, M.B., 1977. Micropalaeontological investigations for the site of the Thames Barrier, London. Quarterly Journal of Engineering Geology, London 10, 321–338. Gale, A.S., Wood, C.J., Bromley, R.G., 1987. The lithostratigraphy and marker bed correlation of the white chalk (Late Cenomanian–Campanian) in Southern England. Mesozoic Research 1 (2), 107–118. Gaster, C.T.A., 1924. The chalk of the Worthing District of Sussex. Proceedings of the Geologists’ Association 35, 89–110. Gaster, C.T.A., 1928. Excursion to Newhaven and Brighton. Proceedings of the Geologists’ Association 39, 198–201.
H. Bailey / Proceedings of the Geologists’ Association 122 (2011) 229–231 Gaster, C.T.A., 1929. Chalk zones in the neighbourhood of Shoreham, Brighton and Newhaven, Sussex. Proceedings of the Geologists’ Association 39, 328–340. Gaster, C.T.A., 1939. The stratigraphy of the chalk of Sussex. Part II. Eastern area— Seaford to Cuckmere Valley and Eastbourne, with zonal map. Proceedings of the Geologists’ Association 50, 510–526. Gaster, C.T.A., 1941. The chalk zones of Offaster pillula and Actinocamax quadrata. Proceedings of the Geologists’ Association 52, 210–215. Gaster, C.T.A., 1951. The stratigraphy of the chalk of Sussex. Part IV. East Central area—between the valley of the Adur and Seaford. Proceedings of the Geologists’ Association 62, 31–64. Guiraud, R., Bosworth, W., 1997. Senonian basin inversion and rejuvenation of rifting in Africa and Arabia; synthesis and implications to plate-scale tectonics. Tectonophysics 282, 39–82. Guiraud, R., Bosworth, W., Thierry, J., Delplanque, A., 2005. Phanerozoic geological evolution of Northern and Central Africa: an overview. Journal of African Earth Sciences 43, 83–143. Hampton, M.J., Bailey, H.W., Jones, A.D., 2010. A Holostratigraphic approach to the chalk of the North Srea Eldfisk Field, Norway. In: Vining, B., Pickering, S. (Eds.), Petroleum Geology: From Mature Basins to New Frontiers. Proceedings of the 7th Petroleum Geology Conference. Geological Society, London. Hancock, J.M., 1975. The petrology of the chalk. Proceedings of the Geologists’ Association 86, 499–535. Haq, B.U., Hardenbol, J., Vail, P.R., 1987. Chronology of fluctuating sea levels since the Triassic (250 million years ago to present). Science 235, 1156–1167. Jarvis, I., Gales, A.S., Jenkyns, H.C., Pearce, M.A., 2006. Secular variation in Late Cretaceous carbon isotopes: a new d13C carbonate reference curve for the Cenomanian–Campanian (99.6–70.6 Ma). Geological Magazine 143 (5), 561–608. Jukes-Browne, A.J., Hill, W., 1900. The Cretaceous Rocks of Britain, volume 1: The Gault and the Upper Greensand of England. Memoir of the Geological Survey of the United Kingdom. HMSO, London, 499 pp. Jukes-Browne, A.J., Hill, W., 1903. The Cretaceous Rocks of Britain, volume 2: The Lower and Middle Chalk of England. Memoir of the Geological Survey of the United Kingdom. HMSO, London, 568 pp. Jukes-Browne, A.J., Hill, W., 1904. The Cretaceous Rocks of Britain, volume 3: The Upper Chalk of England. Memoir of the Geological Survey of the United Kingdom. HMSO, London, 566 pp.
231
Mortimore, R.N., 1986. Stratigraphy of the upper Cretaceous White Chalk of Sussex. Proceedings of the Geologists’ Association 97, 97–139. Mortimore, R.N., 1987. Upper Cretaceous White Chalk in the North and South Downs, England: a correlation. Proceedings of the Geologists’ Association 98, 77–86. Mortimore, R.N., 1997. The Chalk of Sussex and Kent. Geologists’ Association Field Guide No. 57. Geologists’ Association, London, 193 pp. Mortimore, R.N., Pomerol, B., 1997. Upper Cretaceous tectonic phases and the end Cretaceous inversion of the Anglo-Paris Basin. Proceedings of the Geologists’ Association 108, 231–255. Mortimore, R.N., Wood, C.J., Pomerol, B., Ernst, G., 1998. Dating the phases of the Subhercynian tectonic epoch: Late Cretaceous tectonics and eustatics in the Cretaceous basins of northern Germany compared with the Anglo-Paris Basin. Zentralblatt fu¨r Geologie und Pala¨ontologie, Teil 1 1996 (11/12), 1349– 1401. Mortimore, R.N., Wood, C.J., Gallois, R.W., 2001. British Upper Cretaceous Stratigraphy. Geological Conservation Review Series, vol. 23. Joint Conservation Committee, Peterborough, 558 pp. Robinson, N.D., 1986. Lithostratigraphy of the Chalk Group of the North Downs, southeast England. Proceedings of the Geologists’ Association 97, 141–219. Rowe, A.W., 1900. The zones of the White Chalk of the English Coast. I Kent and Sussex. Proceedings of the Geologists’ Association 16, 289–368. Rowe, A.W., 1901. The zones of the White Chalk of the English Coast. II Dorset. Proceedings of the Geologists’ Association 17, 1–76. Rowe, A.W., 1903. The zones of the White Chalk of the English Coast. III Devon. Proceedings of the Geologists’ Association 16, 289–368. Rowe, A.W., 1904. The zones of the White Chalk of the English Coast. IV Yorkshire. Proceedings of the Geologists’ Association 18, 193–296. Rowe, A.W., 1908. The zones of the White Chalk of the English Coast. V Isle of Wight. Proceedings of the Geologists’ Association 20, 209–352. Slater, H., Barnett, C., 1958. The Channel Tunnel. Allan Wingate, London, 213 pp. Stille, H., 1924. Grundfragen der verglieichenden Tektonik. Borntraeger, Berlin, 443 pp. Varley, P.M., Warren, C.D., 1996. History of the geological investigations for the Channel Tunnel. In: Harris, C.S., Hart, M.B., Varley, P.M., Warren, C.D. (Eds.), Engineering Geology of the Channel Tunnel. Thomas Telford, London, pp. 5–18.
Contents lists available at ScienceDirect
Proceedings of the Geologists’ Association journal homepage: www.elsevier.com/locate/pgeola
Context article
What have we done to the chalk? A commentary Haydon Bailey Network Stratigraphic Consulting Ltd., Cranborne Road, Harvest House, Potters Bar, Hertfordshire EN6 3JF, United Kingdom
To witness a complete issue of the Proceedings dedicated solely to two papers by Professor Rory Mortimore is worthy of note, although there are those who sometimes find his combined use of global eustatic sea-level change and regional tectonic events (e.g. Jarvis et al., 2006) a little difficult to accept. However, a little controversy helps science to move forward which is what we seek. The joint publication of these papers is rightly justifiable and they deserve their place in the established history of chalk research and the Geologists’ Association. It is immediately acknowledged here that this is not a fully comprehensive review, but serves only to put these papers into a more contextual setting. Just over a hundred years ago the Proceedings had just completed the publication of the final paper of a series of five written by Arthur Rowe on the Zones of the White Chalk (Rowe, 1900, 1901, 1903, 1904, 1908). These epic papers, together with the Survey work of Jukes-Browne and Hill (1900, 1903, 1904) became the ‘‘gold standards’’ for chalk stratigraphy for the next sixty years. As we are dealing with Mortimore papers we should not forget the part played by the GA and the numerous Proceedings publications on the Sussex chalk by Gaster (1924, 1928, 1929, 1939, 1941, 1951). Although more locally based, these were to play a major role in Mortimore’s interpretation of the Sussex chalk in his own research. This then brings us quickly into the late 1960s and 1970s when the demands of building the Channel Tunnel, including the 1960s thwarted attempt (Slater and Barnett, 1958; Varley and Warren, 1996) and the Thames Barrier (Carter and Hart, 1977) pointed to the need to revisit chalk stratigraphy after a fifty year hiatus and to apply techniques unavailable to Rowe and his contemporaries (e.g. SEM, micropalaeontology). The first hints of the changes to come were raised by Hancock (1975), again in the PGA, in his classic paper on ‘‘The Petrology of the Chalk’’ which introduced us to the effects of sea-level changes during the Cretaceous. The late 1970s were the time when Rory Mortimore appeared on the Late Cretaceous horizon as he completed his doctoral research at Brighton Polytechnic. He immediately endeared himself with the Geologists’ Association by leading a field trip to Sussex in 1977. I met Rory Mortimore on this trip and I am pleased to note that there is a grainy black and white photograph in the GA archive marking the event. His Ph.D., completed in 1979, started to raise a few eyebrows even with its title ‘‘The relationship of stratigraphy and tectonofacies to the physical properties of the
E-mail address: [email protected].
White Chalk of Sussex’’. Tectonics during the deposition of the chalk, just when the importance of global eustatic sea-level changes were becoming increasingly important (Haq et al., 1987)? Mortimore was swimming against a tide (or at least rising sea-levels), but his seven volume thesis is nothing if not thorough. Part of Mortimore’s work in Sussex was to define a lithostratigraphy for the chalk of the South Downs. Ironically, as is often the case, parallel research was being carried out on the chalk of the North Downs by Nick Robinson at Kingston Polytechnic. Precisely a quarter of a century ago, the Geologists’ Association regarded this situation as too good an opportunity to miss and they organised a meeting for the presentation of the North Downs versus the South Downs lithostratigraphies at Burlington House (Mortimore, 1986; Robinson, 1986). As with such events the initial outcome seemed more of a draw rather than a victory for either contender. Nevertheless, this meeting resulted in the formal publication of both the alternative lithostratigraphic schemes and their subsequent potential correlation (Gale et al., 1987; Mortimore, 1987) thereby bringing them to a much wider audience. This audience included the Geological Survey who had started to apply the Mortimore scheme in their mapping of the chalk in southern England (e.g. Bristow, 1990; Bristow et al., 1995). The Isle of Wight paper in the present issue is a further example of Mortimore’s approach to his geology in that it presents a series of basic, but thorough, lithological descriptions of numerous inland sections in the central part of the island. Many of these sections will probably be lost within a generation, as is the case with many similar sites across southern England as landfill and site neglect take effect; so their accurate recording is extremely important whilst they are still accessible. With the lithological variations and structural details described, Mortimore then goes on to use the information provided, together with the available geophysical and wireline log data to provide a much broader regional structural interpretation. As with all of Mortimore’s work, each and every project is carried out with a direct application in mind, in this case examining the hydrogeological behaviour of the chalk in a structurally complex area. It is a thorough, systematic piece of research which can be used at several different levels. The basic field logs in themselves are extremely useful, if sometimes a little mystifying (see Fig 15: 5.5 m up the section – ‘‘Possible 100 mm thick marl seam (gmz1) – an awkward fault!’’). The addition of the
1 gmz: Griotte marly zone. A regular Mortimore abbreviation, indicating a concentration of wispy horsetail like marl seams. Unfortunately, in this paper the abbreviation never receives a formal definition.
0016-7878/$ – see front matter ß 2011 The Geologists’ Association. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.pgeola.2011.01.001
230
H. Bailey / Proceedings of the Geologists’ Association 122 (2011) 229–231
seismic data takes us to a totally different interpretational level which is extremely informative. The recognition during the 1990s that we had a lithostratigraphy for the chalk which could be applied systematically across southern England, almost to an individual Bed level, was a major step forward, resulting in a number of key events in 1997: - Firstly, a meeting was held at the BGS offices in Keyworth bringing together chalk expertise from all directions to agree, if possible, on the formal adoption of the Mortimore scheme by both BGS and the wider chalk research community. - Secondly, the Proceedings published Bristow et al. (1997) on ‘‘Lithostratigraphy for mapping the Chalk of southern England’’, a paper which was to receive that year’s Geologists’ Association Richardson Award. - Thirdly, and most important to the majority of GA members, Mortimore published the GA Field Guide No. 57 ‘‘The Chalk of Sussex and Kent’’ which has provided so many people with their first detailed introduction to chalk stratigraphy and sedimentology in a much more accessible format. It is still a valuable asset in the backpack on any trip to the southeast corner of England. - Finally, in the 1997 volume of the Proceedings, Mortimore and his long term working colleague Bernard Pomerol also published ‘‘Upper Cretaceous tectonic phases and the end Cretaceous inversion in the Chalk of the Anglo-Paris basin’’. Thus Late Cretaceous tectonic phases were back on the agenda. Mortimore and Pomerol found that the tectonic phases they recognised in the Anglo-Paris basin had long been defined in Germany (Stille, 1924) and, in 1998, together with Christopher Wood and the late Gundolf Ernst they produced ‘‘Dating the phases of the Subhercynian tectonic epoch: Late Cretaceous tectonics and eustatics in the Cretaceous basins of northern Germany compared with the Anglo-Paris basin’’ (Mortimore et al., 1998). The implications of this work and the earlier Anglo-Paris paper were that chalk deposition was not simply controlled by a carbonate factory systematically generating algal fall out across a stable epicontinental shelf for thirty five million years as sea levels rose and fell. The preserved chalk succession represents a dynamic period where a plethora of plate scale events were taking place and the northern European shelf was a relatively stable back water area where the effects were represented more by facies change, often difficult to recognise in basinal chalk sequences. I initially became intrigued by the potential impact of Late Cretaceous tectonics about twenty five years ago whilst core logging through a continuous mass of Late Turonian radiolarian rich carbonates from an offshore Norwegian chalk field. This thick deep water ‘‘chalk’’ had been deposited during a period when all my previous training had taught me that there was global eustatic sea-level fall and I should be expecting to see section loss and condensation over the established positive structure I was helping to exploit for oil. I was seeing the opposite; I was logging through carbonates deposited in a rapidly deepening basin on the line of a known structurally positive lineament. Mortimore et al. (1998) and the late Ilsede inversion phase during the Late Turonian finally provided me with an explanation to the predicament I would found myself in. This dynamic Late Cretaceous model takes on a broader setting when we look at nearby areas of North Africa and the Middle East. What was happening in those areas during the Late Cretaceous had a direct impact on the European plate to the north. Mortimore’s ‘‘Revolution’’ paper here summarises briefly the work of Bosworth et al. (1999) and Guiraud and Bosworth (1997) and how the events they recognise (along the Syrian arc for example) relate to the tectonic phases recognised across northern Europe. Just to take one example, the so called ‘‘84 million year event’’ is widely recognised
from the Middle East (Guiraud et al., 2005) to West Africa (Burke et al., 2003) where it resulted in coarse clastic hydrocarbon reservoir generation. This correlates directly with the subhercynian Wernigerode event recognised in Germany. There is a constant dynamic interplay between global eustatic sea-levels with regional variations being imposed by plate movements and inter-plate readjustments and, at a relatively local level, the sedimentary sequences we find ourselves logging, either along active structural lineaments or in the relatively stable areas in between. So perhaps this is the crux of Mortimores question ‘‘What have we done to the Chalk?’’ The chalk has been shifted from its comfort zone of being a uniform pelagic deposit across a widespread epicontinental sea and it has been placed into a totally different context. One ‘‘of major tectonic change on a global scale as well as one of exceptionally high sea levels globally. The supercontinents were breaking up, and Africa was beginning to under-ride and laterally shear along the southern edge of Europe’’ (Mortimore et al., 2001). The thirty five million year period between 100 ma and 65 ma begins to sound pretty dramatic and quite exciting. The high, yet fluctuating, sea-levels are still recognised and clearly represented in the sediments whilst at the same time there is the impact of an opening Atlantic, the development of the Icelandic hot spot, a shifting African plate and major ophiolite emplacement on the southern margins of the Arabian plate. All these were being translated through into Europe along well developed structural lineaments which then affect the sedimentary sequences preserved in the sections we log in the field (see Bless et al., 1986) or in offshore well sections (see Hampton et al., 2010 for an interesting example). This brings me back to the fundamental value of the papers presented in this issue of the Proceedings; they are founded initially on rigorous section logging and description, the fundamental training for every geologist. Once you have the basic information in your field logs, then you can start interpreting it. That is when the alternative views and interpretations come into play, but differences are fine, as with any reasoned scientific debate and argument, it is on the back of these disagreements that we make advances. Better to be part of an advancing dynamic system than a quiet backwater! I hope you agree and enjoy your chalk revolution. References Bless, M.J., Felder, P.J., Meesen, J.P.M.T., 1986. Late Cretaceous sea level rise and inversion: their influence on the depositional environment between Aachen and Antwerp. Annales de la Socie´te´ ge´ologique de Belgique 109, 333–355. Bosworth, W., Guiraud, R., Kesler, L.G., 1999. Late Cretaceous (84 Ma) compressive effects of the ‘‘Santonian event’’ and the origin of the Syrian arc. Geology 27, 633– 636. Bristow, C.R., 1990. Geology of the Country around Bury St. Edmunds. Memoir of the British Geological Survey (England and Wales), Sheet 189. HMSO, London, 99 pp. Bristow, C.R., Barton, C.M., Freshney, E.C., Wood, C.J., Evans, D.J., Cox, B.M., 1995. Geology of the Country around Shaftesbury. Memoir of the British Geological Survey (England and Wales), Sheet 313. HMSO, London, 182 pp. Bristow, C.R., Mortimore, R.N., Wood, C.J., 1997. Lithostratigraphy for mapping the chalk of southern England. Proceedings of the Geologists’ Association 108, 293– 315. Burke, K., Macgregor, D.S., Cameron, N.R., 2003. Africa’s petroleum systems: four tectonic ‘Aces’ in the past 600 million years. In: Arthur, T.L., MacGregor, D.S., Cameron, N.R. (Eds.), Petroleum Geology of Africa: New Themes and Developing Technologies, vol. 207Geological Society, London, pp. 21–60, special publication. Carter, D.J., Hart, M.B., 1977. Micropalaeontological investigations for the site of the Thames Barrier, London. Quarterly Journal of Engineering Geology, London 10, 321–338. Gale, A.S., Wood, C.J., Bromley, R.G., 1987. The lithostratigraphy and marker bed correlation of the white chalk (Late Cenomanian–Campanian) in Southern England. Mesozoic Research 1 (2), 107–118. Gaster, C.T.A., 1924. The chalk of the Worthing District of Sussex. Proceedings of the Geologists’ Association 35, 89–110. Gaster, C.T.A., 1928. Excursion to Newhaven and Brighton. Proceedings of the Geologists’ Association 39, 198–201.
H. Bailey / Proceedings of the Geologists’ Association 122 (2011) 229–231 Gaster, C.T.A., 1929. Chalk zones in the neighbourhood of Shoreham, Brighton and Newhaven, Sussex. Proceedings of the Geologists’ Association 39, 328–340. Gaster, C.T.A., 1939. The stratigraphy of the chalk of Sussex. Part II. Eastern area— Seaford to Cuckmere Valley and Eastbourne, with zonal map. Proceedings of the Geologists’ Association 50, 510–526. Gaster, C.T.A., 1941. The chalk zones of Offaster pillula and Actinocamax quadrata. Proceedings of the Geologists’ Association 52, 210–215. Gaster, C.T.A., 1951. The stratigraphy of the chalk of Sussex. Part IV. East Central area—between the valley of the Adur and Seaford. Proceedings of the Geologists’ Association 62, 31–64. Guiraud, R., Bosworth, W., 1997. Senonian basin inversion and rejuvenation of rifting in Africa and Arabia; synthesis and implications to plate-scale tectonics. Tectonophysics 282, 39–82. Guiraud, R., Bosworth, W., Thierry, J., Delplanque, A., 2005. Phanerozoic geological evolution of Northern and Central Africa: an overview. Journal of African Earth Sciences 43, 83–143. Hampton, M.J., Bailey, H.W., Jones, A.D., 2010. A Holostratigraphic approach to the chalk of the North Srea Eldfisk Field, Norway. In: Vining, B., Pickering, S. (Eds.), Petroleum Geology: From Mature Basins to New Frontiers. Proceedings of the 7th Petroleum Geology Conference. Geological Society, London. Hancock, J.M., 1975. The petrology of the chalk. Proceedings of the Geologists’ Association 86, 499–535. Haq, B.U., Hardenbol, J., Vail, P.R., 1987. Chronology of fluctuating sea levels since the Triassic (250 million years ago to present). Science 235, 1156–1167. Jarvis, I., Gales, A.S., Jenkyns, H.C., Pearce, M.A., 2006. Secular variation in Late Cretaceous carbon isotopes: a new d13C carbonate reference curve for the Cenomanian–Campanian (99.6–70.6 Ma). Geological Magazine 143 (5), 561–608. Jukes-Browne, A.J., Hill, W., 1900. The Cretaceous Rocks of Britain, volume 1: The Gault and the Upper Greensand of England. Memoir of the Geological Survey of the United Kingdom. HMSO, London, 499 pp. Jukes-Browne, A.J., Hill, W., 1903. The Cretaceous Rocks of Britain, volume 2: The Lower and Middle Chalk of England. Memoir of the Geological Survey of the United Kingdom. HMSO, London, 568 pp. Jukes-Browne, A.J., Hill, W., 1904. The Cretaceous Rocks of Britain, volume 3: The Upper Chalk of England. Memoir of the Geological Survey of the United Kingdom. HMSO, London, 566 pp.
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Mortimore, R.N., 1986. Stratigraphy of the upper Cretaceous White Chalk of Sussex. Proceedings of the Geologists’ Association 97, 97–139. Mortimore, R.N., 1987. Upper Cretaceous White Chalk in the North and South Downs, England: a correlation. Proceedings of the Geologists’ Association 98, 77–86. Mortimore, R.N., 1997. The Chalk of Sussex and Kent. Geologists’ Association Field Guide No. 57. Geologists’ Association, London, 193 pp. Mortimore, R.N., Pomerol, B., 1997. Upper Cretaceous tectonic phases and the end Cretaceous inversion of the Anglo-Paris Basin. Proceedings of the Geologists’ Association 108, 231–255. Mortimore, R.N., Wood, C.J., Pomerol, B., Ernst, G., 1998. Dating the phases of the Subhercynian tectonic epoch: Late Cretaceous tectonics and eustatics in the Cretaceous basins of northern Germany compared with the Anglo-Paris Basin. Zentralblatt fu¨r Geologie und Pala¨ontologie, Teil 1 1996 (11/12), 1349– 1401. Mortimore, R.N., Wood, C.J., Gallois, R.W., 2001. British Upper Cretaceous Stratigraphy. Geological Conservation Review Series, vol. 23. Joint Conservation Committee, Peterborough, 558 pp. Robinson, N.D., 1986. Lithostratigraphy of the Chalk Group of the North Downs, southeast England. Proceedings of the Geologists’ Association 97, 141–219. Rowe, A.W., 1900. The zones of the White Chalk of the English Coast. I Kent and Sussex. Proceedings of the Geologists’ Association 16, 289–368. Rowe, A.W., 1901. The zones of the White Chalk of the English Coast. II Dorset. Proceedings of the Geologists’ Association 17, 1–76. Rowe, A.W., 1903. The zones of the White Chalk of the English Coast. III Devon. Proceedings of the Geologists’ Association 16, 289–368. Rowe, A.W., 1904. The zones of the White Chalk of the English Coast. IV Yorkshire. Proceedings of the Geologists’ Association 18, 193–296. Rowe, A.W., 1908. The zones of the White Chalk of the English Coast. V Isle of Wight. Proceedings of the Geologists’ Association 20, 209–352. Slater, H., Barnett, C., 1958. The Channel Tunnel. Allan Wingate, London, 213 pp. Stille, H., 1924. Grundfragen der verglieichenden Tektonik. Borntraeger, Berlin, 443 pp. Varley, P.M., Warren, C.D., 1996. History of the geological investigations for the Channel Tunnel. In: Harris, C.S., Hart, M.B., Varley, P.M., Warren, C.D. (Eds.), Engineering Geology of the Channel Tunnel. Thomas Telford, London, pp. 5–18.