The BGS Lexicon of Named Rock Units — Result Details

Lower Mottled Beds (Reading Formation)

Computer Code: LMBED Preferred Map Code: notEntered
Status Code: Index Level
Age range: Thanetian Age (GT) — Ypresian Age (GY)
Lithological Description: Like the Lower Mottled Clay (LMCL), the Lower Mottled Beds typically consists of purple, red, green, blue-grey and brown mottled or multicoloured unbedded clays, some silty or sandy, and fine- to medium-grained sands. Typically, sands in the Reading Formation are brown or grey in colour, and generally not silty. Glauconite is absent in parts of the Lower Mottled Beds that have been derived from the Reading Formation. They are either not or only weakly bioturbated, but may be cross-bedded and can form steep-sided deposits up to 5m thick. Nodular calcrete, calcrete-cemented silt, or gravel- to cobble-sized calcareous nodules are widespread, which may coalesce to form a local limestone bed and which has been described in some borehole logs as ‘chalk’. The deposits are generally not shelly, but those that are contain a low diversity non-marine (brackish) fauna. In some areas, part or all of the Lower Mottled Beds are derived from the Upnor Formation. The composition varies accordingly although there is commonly also an element of alluviated clay derived from the overlying Reading Formation.
Definition of Lower Boundary: The base of the Lower Mottled Beds is placed at the lower extent of the multi-coloured mottling characteristic of the Reading Formation. In places this extends down into the Upnor Formation, so that the Lower Mottled Beds can cross the lithostratigraphic boundary at the base of the Lower Mottled Clay, and in some places they can occur in the absence of the Reading Formation, where the alteration is the result of weathering at the mid-Lambeth Group Hiatus.
Definition of Upper Boundary: The top of the Lower Mottled Beds is the same as the top of the Lower Mottled Clay. This is a generally clearly marked unconformity known as the mid-Lambeth Group Hiatus (Page and Skipper, 2000). Where the Woolwich Formation is present, the upper boundary of the Lower Mottled Beds is typically marked by an upwards change from vari-coloured mottled clays or, locally, brown or grey non-glauconitic sands, in places with calcrete, below the unconformity, to dark grey shelly clay or laminated clay and silt or fine- to coarse-grained sand, commonly with lignite fragments or, locally, beds, above the unconformity. Where the Woolwich Formation is absent, the nature of the boundary between the Lower Mottled Beds and the Upper Mottled Beds is not well known.
Thickness: none recorded or not applicable
Geographical Limits: The Lower Mottled Beds is probably present throughout the Reading Formation outcrop and subcrop. It extends eastwards beneath the Woolwich Formation, where the two units co-exist (Ellison, 1983; Hooker, 1991). Unlike the Lower Mottled Clay, the Lower Mottled Beds occur at the top of the Upnor Formation in some areas to the east of the extent of the Reading Formation.
Parent Unit: Reading Formation (RB)
Previous Name(s): none recorded or not applicable
Alternative Name(s): none recorded or not applicable
Stratotypes:
Type Area  The type area of the Lower Mottled Beds is central, south and south-east London. No type section has been designated. Ellison et al., 2004, fig. 20; Page and Skipper, 2000 
Reference(s):
Page, D P, and Skipper, J A E. 2000. Lithological characteristics of the Lambeth Group. Ground Engineering, Vol. 33, 38-43. 
Ellison, R A, Knox R W O'B, Jolley, D W and King, C, 1994. A revision of the lithostratigraphical classification of the early Palaeogene strata of the London Basin and East Anglia. Proceedings of the Geologists' Association, Vol.105, 187-197. 
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/. 
Ellison, R.A., 1983. Facies distribution in the Woolwich and Reading Beds of the London Basin, England. Proceedings of the Geologists' Association 94, 311-319. 
Skipper, J A. 1999. The stratigraphy of the Lambeth Group (Paleocene) of South East England. PhD thesis, Imperial College, London. 
1:50K maps on which the lithostratigraphical unit is found, and map code used:
none recorded or not applicable