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Abstract

AAPG Bulletin, V. 88, No. 10 (October 2004), P. 1433-1460.

Copyright copy2004. The American Association of Petroleum Geologists. All rights reserved.

Transgressive changes from tidal estuarine to marine embayment depositional systems: The Lower Cretaceous Woburn Sands of southern England and comparison with Holocene analogs

Shuji Yoshida,1 Howard D. Johnson,2 Kenneth Pye,3 Richard J. Dixon4

1Department of Geological Sciences, University of Texas at Austin, 1 University Station C1100, Austin, Texas 78712-0254; [email protected]
2Department of Earth Science and Engineering, Imperial College, Prince Consort Road, London SW7 2BP, United Kingdom; [email protected]
3Department of Geology, Royal Holloway, University of London, Egham, Surrey TW20 0EX, United Kingdom; [email protected]
4BP Trinidad and Tobago, 5-5a, Queens Park West, Port of Spain, Republic of Trinidad and Tobago; [email protected]

AUTHORS

Shuji Yoshida has a B.Eng. degree (mining engineering) from Kyushu University, Japan, an M.Sc. degree (petroleum geology) from Imperial College, United Kingdom, and a Ph.D. (basin analysis) from Toronto University, Canada. He held industry and postdoctoral positions in reservoir characterization (Imperial College), coastal geomorphology (Royal Holloway, University of London), and hydrocarbon exploration (University of Wyoming). His main interest is shallow-marine and nonmarine sedimentology.

Howard Johnson holds the Shell Chair of Petroleum Geology at Imperial College. His interests include clastic sedimentology, sequence stratigraphy, reservoir characterization, and basin studies. He spent 15 years with Shell working in research, exploration, and development geology and engineering. He holds a B.Sc. degree in geology from the University of Liverpool and a D.Phil. in sedimentology from the University of Oxford.

Ken Pye has B.A. and M.A. degrees from Oxford University and a Ph.D. and an Sc.D. from Cambridge University. He has been professor of environmental geology at Royal Holloway since 1999. He was Natural Environment Research Council (NERC) and then Royal Society Research Fellow at Cambridge University, then lecturer, reader, and professor at Reading University. His publications include 11 books and more than 180 papers.

Richard Dixon joined BP in 1989 with a Ph.D. in marine geology and volcaniclastic sedimentology. He spent several years working on deep-water sediments and published several papers in this field, notably on postdepositional modification and sandstone intrusion. Since 1996, he has worked almost exclusively on paralic sequences, including examples from the North Sea, Alaska, Algeria, east Siberia, Sakhalin, and Trinidad.

ACKNOWLEDGMENTS

The outcrop mapping of the Woburn Sands was carried out as a part of the Forum for Reservoir Characterization and Reservoir Engineering project conducted at Imperial College in 1997–1999 and sponsored by five oil companies (BP, Statoil, Conoco, Norsk Hydro, and Fortum) and coordinated by the Norwegian Petroleum Directorate. It was enhanced in 1999–2001 at Royal Holloway by the comparative studies of modern, Quaternary, and ancient tidal systems worldwide as a part of the Estuarine Morphology and Processes Holistic Assessment System project funded by the Ministry of Agriculture, Food, and Fishery (United Kingdom), the Environment Agency (United Kingdom), and English Nature. We have benefited from stimulating discussions with Robert Dalrymple, Daphne van der Wal, Roland Goldring, Graham Evans, Dag Nummedal, Sanjeev Gupta, Jonathan Wonham, Bernadette Tessier, Randi Martinsen, Rob Gawthorpe, and Paul Heller. Ron Steel and Andrew Miall are thanked for internal reviews of the manuscript. Finally, we thank the AAPG editor John Lorenz and reviewers Lesli Wood, Ron Kreisa, and one anonymous reviewer for useful suggestions.

ABSTRACT

The Lower Cretaceous Woburn Sands (Lower Greensand Group) in southern England constitutes one of the most intensively studied tidal sandstone outcrops for sedimentological and reservoir analog studies. Most recent workers have interpreted the whole 30–60-m (100–200-ft)-thick succession around Leighton Buzzard as representing an ancient tide-dominated estuary. However, unequivocal estuary characteristics are limited to the lowermost part (about 15–20 m [50–66 ft]). We suggest that a significant portion of the Woburn Sands, and most of the middle part, was formed in a tide-dominated marine embayment. Hence, the vertical facies change from the lower to middle part of the Woburn Sands is interpreted as a change from (1) a narrow estuary to (2) a broad marine embayment. The Wash embayment in eastern England is a striking modern analog; it receives most of its sediments and waters from marine sources and is largely filled with nondiluted seawater. Moreover, the Holocene transgressive history of The Wash is remarkably similar to the transgressive evolution of the Woburn Sands.

Early estuarine sequence models predict landward translation of facies zones along the valley thalweg during transgression, but eventual facies translation in the strike direction has not been fully documented or discussed. An embayment facies that is not commonly confined in the early incised valley can occur vertically between the estuarine valley fill and marine shelf deposits and is probably underrepresented in current models. However, this segment of the transgression, comprising along-strike bay expansion and the development of a broad marine embayment, may be more important than previously thought.

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