| 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.
Marine: inner to mid shelf.
Early Eocene (early Ypresian). |
| 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 1 m thick; Unit B up to about 1.75 m; Unit C up to about 2.5 m (Jolley, 1996). King (in prep.) suggests that channel-filling sands and gravels up to 10 m 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): |
Hales Clay Member [Obsolete Name and Code: Use UPR]
(HAC)
|
| 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 |
BGS Shotley Gate Borehole (TM23SW 19) 9.93 to 23.55 m 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): |
| George, W, and Vincent, S. 1976. Some river exposures of London Clay in Suffolk and Essex. Tertiary Research, Vol. 1, 25-28. |
| Aldiss, D T. 2014. The stratigraphical framework for the Palaeogene successions of the London Basin, UK. British Geological Survey Open Report OR/14/008. 95 pp. |
| 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 |
