OR/14/008 An introduction to the Palaeogene Groups of southern England and some terminological conventions
|Aldiss, D T. 2014. The stratigraphical framework for the Palaeogene successions of the London Basin, UK. Nottingham, UK, British geological Survey. (OR/14/008)|
Palaeogene Groups in southern England
This stratigraphical framework uses four well-established major subdivisions for the London Basin, three of which extend into the Hampshire Basin: the Bracklesham Group, the Thames Group and the Lambeth Group. In the central and eastern part of the London Basin and in East Anglia, the Lambeth Group is underlain by the oldest subdivision, comprising onshore components of the Montrose Group (Table 2).
‘Lower London Tertiaries’
In some areas, notably the north-eastern part of the London Basin, it has proved difficult to subdivide the strata between the Chalk and the London Clay with confidence and at times they have been assigned to a broad category known as the ‘Lower London Tertiaries’ (Prestwich, 1852), corresponding to strata between the Chalk Group and the London Clay. This term survives on some published geological maps covering northern Essex. It should be taken to refer to ‘undifferentiated Harwich Formation, Lambeth Group and Thanet Formation’ and this composite term should now be preferred, if such an expression is required.
‘Pebbles’ and ‘gravels’
Historically, clasts found in British Palaeogene deposits that are larger than sand-size and smaller than cobbles have been described as ‘pebbles’. This use is reflected in much of the literature, including many BGS reports. The term ‘pebble’ was defined in the Udden- Wentworth scale as having a diameter between 2 mm and 64 mm. A modified form of the Wentworth scale used by BGS geologists also includes the term ‘granule’ for very fine gravel between 2 mm and 4 mm in diameter.
Some descriptions, however, prefer the term ‘gravel’, and this has been adopted by the geotechnical industry, which uses ‘gravel’ to describe clasts between 2 mm and 63 mm in diameter.
In practice, although many existing descriptions have used the term ‘pebble’ without reference to a particular definition, ‘pebbles’ and ‘gravel’ can normally be treated as interchangeable with no significant loss of accuracy. In this report, ‘pebble’ is used to honour long-established use (as in ‘Blackheath Pebble Beds’ and the like), and ‘gravel’ is used in formal descriptions of lithology. It should be borne in mind, however, that such descriptions are generalised and are not based on systematic grain size measurements.
Although both ‘pebble’ and ‘gravel’ are defined according to size and not to shape, vernacular use of ‘pebble’ carries a connotation of being rounded.
Most, if not all, of the Palaeogene formations described in this report include a component of gravel of very characteristic appearance, sometimes referred to colloquially as ‘Tertiary pebbles’ (that is, pebbles in — or derived from — Cenozoic, or Tertiary, deposits). These pebbles are typically composed almost exclusively of flint. The clasts are typically very well-rounded and subspherical in shape, commonly with a surface network of numerous short arcuate or crescentic percussion fractures known as ‘chatter marks’. These features arose in very high-energy coastal depositional environments (Gibbard, 1986; Plint, 1982) and essentially similar pebbles and cobbles can be found on many modern beaches in the region, for example on the Sussex coast. Where not bleached or stained by Quaternary weathering, they are generally black or dark grey in colour although some have a green-coloured glauconitic veneer, or have been stained red.
The pebbles are most abundant in some ‘pebble beds’ in the Upnor Formation and Harwich Formation but gravelly layers, or in some cases pebble strings or isolated clasts, occur at unit bases in most of the other formations. Isolated pebbles are recorded in some boreholes in the Thanet Formation in London.
In the following descriptions, references to gravel or ‘pebbles’ should be taken to refer to ‘Tertiary pebbles’, unless otherwise stated.
- GIBBARD, P L. 1986. Flint gravels in the Quaternary of southeast England. 141–149 in The Scientific Study of Flint and Chert. SIEVEKING, G D, and HAU, M B (editors). (Cambridge: Cambridge University Press.)
- PLINT, A G. 1982. Eocene sedimentation and tectonics in the Hampshire Basin. Journal of the Geological Society, London, Vol. 139, 249–254.