The BGS Lexicon of Named Rock Units — Result Details
Sudbury Formation
| Computer Code: | SBRY | Preferred Map Code: | notEntered |
|---|---|---|---|
| Status Code: | Full | ||
| Age range: | Early Pleistocene (QPE) — Mid Pleistocene (QPM) | ||
| Lithological Description: | Formation encompasses fluvial, lacustrine and organic deposits of the pre-diversionary River Thames. Most of the surviving deposits are fluvial gravels, with sedimentary structures indicating deposition by a braided river. Lacustrine silts and clays and organic peats are uncommon. The formation represents the first significant input of far-travelled materials into the Thames river sediments, and is characterised by quartz and quartzite from the Triassic, Carboniferous and Devonian rocks of the West Midlands, Welsh Borderland and possibly southwestern Pennines, and felsic volcanic rocks from northern Wales. The presence of mega-erratics and glacially fractured sand grains indicate glacial erosion in the headwater regions of the river. The fluvial gravels occupy terrace levels and the members are defined on the basis of altitude and pebble clast content. The members comprise bodies of cross-bedded and massive, moderately sorted sand and gravel. The aggradations are generally entrenched into bedrock with a difference in surface elevation of at least 10 m. | ||
| Definition of Lower Boundary: | Unconformable on bedrock: Cretaceous, Palaeogene and Crag Group. Differentiation from the Crag Group can be difficult where the latter is reworked, and the distinction must be made on the basis of sedimentological structures: the Crag Group is marine, the Kesgrave Catchment Subgroup is fluvial. | ||
| Definition of Upper Boundary: | Commonly overlain by Mid Pleistocene glacigenic deposits. Upper boundary may be difficult to determine where overlain by glaciofluvial sand and gravel, but the presence of more angular clasts, chalk, and poorer sorting in the latter is usually helpful. | ||
| Thickness: | About 18 m. Individual terrace aggradations typically 5-10 m thick. | ||
| Geographical Limits: | Formation is restricted to the Thames Valley, Essex and Suffolk. It does not extend into Norfolk (Hamblin and Moorlock, 1995). | ||
| Parent Unit: | Kesgrave Catchment Subgroup (KGCA) | ||
| Previous Name(s): | Kesgrave Sands And Gravels [Obsolete Name And Code: See KGCA]
(KGSG)
Essex White Ballast [Obsolete Name and Code: Use SBRY, KES, CCHR, KGCA] (-3284) Kesgrave Formation [Obsolete Name and Code: Use KES, CCHR, KGCA, SBRY] (-1621) |
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| Alternative Name(s): | Stoke Row Member Satwell Member |
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| Stratotypes: | |||
| Type Area | The course of the pre-diversionary River Thames from Stebbing, Essex to Claydon, Suffolk. Whiteman and Rose, 1992. | ||
| Reference(s): | |||
| Whiteman, C A and Rose, J. 1992. Thames river sediments of the British Early and Middle Pleistocene. Quarterly Science Reviews, Vol.11, 363-375. | |||
| Hamblin, R J O, and Moorlock, B S P. 1995. The Kesgrave and Bytham Sands and Gravels of eastern Suffolk. Quaternary Newsletter, No.77, 17-31. | |||
| McMillan, A A, Hamblin, R J O, and Merritt, J W. 2011. A lithostratigraphical framework for onshore Quaternary and Neogene (Tertiary) superficial deposits of Great Britain and the Isle of Man. British Geological Survey Research Report, RR/10/03. 343pp. | |||
| 1:50K maps on which the lithostratigraphical unit is found, and map code used: | |||
| E206 E207 E208 E221 E222 E223 E239 E240 E255 E256 E257 | |||