The BGS Lexicon of Named Rock Units — Result Details

Orwell Member

Computer Code: ORW Preferred Map Code: notEntered
Status Code: Full
Age range: Ypresian Age (GY) — Ypresian Age (GY)
Lithological Description: The Orwell Member commences in glauconitic fine-grained sands (Unit A) with well-rounded flint gravel and faunal debris at the base, conformably overlain by bioturbated silty sands and sandy clayey silts with shell fragments and discontinuous laminae of dark grey-black claystone (Unit B). The upper part of the member (Unit C) is characterised by tuffaceous sandy silt with laminae of fine-grained sand. The tephra layers are discontinuous. Microscopic tuffaceous material is disseminated through Units B and C (Jolley, 1996; King, 1981; Knox and Ellison, 1979). King (in prep.) describes more varied lithofacies from this part of the sequence.
Definition of Lower Boundary: The basal pebble bed of the Orwell Member rests disconformably on sands of the Reading Formation or on the Ormesby Clay Member, on an undulating erosion surface.
Definition of Upper Boundary: The sandy silts and fine-grained sands of the upper part of the Orwell Member give way at a sharply defined disconformity to clayey silts and silty clays at the base of the Wrabness Member.
Thickness: Unit A is typically up to about 1m thick; Unit B up to about 1.75m; Unit C up to about 2.5m (Jolley, 1996). King (in prep.) suggests that channel-filling sands and gravels up to 10m thick in the Harwich area can be correlated with his Ipswich Member.
Geographical Limits: The Orwell Member occurs in eastern Norfolk, Suffolk and north Essex. It onlaps to the south-west, then passes laterally into glauconitic silty sands of the Swanscombe Member. To the south, Unit C of the Orwell Member passes laterally into the Oldhaven Member of the north Kent coast (King, 1981; Jolley, 1996).
Parent Unit: Harwich Formation (HWH)
Previous Name(s): none recorded or not applicable
Alternative Name(s): none recorded or not applicable
Stratotypes:
Partial Type Section  Low cliffs on the north bank of the River Orwell at Bridge Wood, just south of Ipswich [TM 1824 4064] to [TM 1870 4010] expose the middle and upper part of the unit (Jolley, 1996). 
Reference Section  Shotley Gate Borehole (TM23SW19)9.93 to 23.55m depth (Knox and Ellison, 1979). 
Reference Section  Cliff and foreshore exposures at Ferry Cliff on east bank of River Deben just east of Woodbridge, Suffolk [TM 2774 4854] to [TM 2800 4867](George and Vincent, 1976; King, 1981). 
Reference(s):
Aldiss, D.T., 2012. The stratigraphical framework for the Palaeogene successions of the London Basin, UK. British Geological Survey Open Report OR/12/004. Available from http://nora.nerc.ac.uk/17121/. 
Boswell, P G H. 1915. The stratigraphy and petrology of the Lower Eocene deposits of the north-eastern part of the London Basin. Quarterly Journal of the Geological Society, Vol. 71, 536-591. 
Boswell, P G H. 1927. The geology of the country around Ipswich. Memoir of the Geological Survey of England and Wales, Sheet 207 (England and Wales). 
Curry, D. 1958. Part 3a XII Palaeogene. Lexique Stratigraphique International. Whittard, W F, and Simpson, S (editors). Vol.1 Europe (Paris: Centre Nationale de la Research Scientifique.) 
George, W, and Vincent, S. 1976. Some river exposures of London Clay in Suffolk and Essex. Tertiary Research, Vol. 1, 25-28. 
King, C. in prep. A revised correlation of Palaeogene and Neogene deposits in the British Isles. Geological Society of London Special Report. 
King, C, 1981. The stratigraphy of the London Clay and associated deposits. Tertiary Research Special Paper No.6. (Backhuys: Rotterdam). 
Jolley, D.W., 1996. The earliest Eocene sediments of eastern England: an ultra-high resolution palynological correlation, In: Knox, R.W.O., Corfield, R.M., Dunay, R.E. (Eds.), Correlation of the Early Palaeogene in Northwest Europe. Geological Society of London Special Publication 101, pp. 219-254. 
Knox, R.W.O., Ellison, R.A., 1979. A Lower Eocene ash sequence in SE England. Journal of the Geological Society 136, 251-253. 
1:50K maps on which the lithostratigraphical unit is found, and map code used:
none recorded or not applicable