The BGS Lexicon of Named Rock Units — Result Details

Emerald Formation

Computer Code: EMLD Preferred Map Code: notEntered
Status Code: Full, pending ratification
Age range: Bathonian Age (JN) — Oxfordian Age (JO)
Lithological Description: The formation comprises very fine- to medium-grained, shelly, bioturbated sandstone and subordinate siltstone, often with a thin basal conglomerate. In many sections, the lower part of the formation is coarser and cleaner than the upper part, but in some sections log signatures indicate upward coarsening.
Definition of Lower Boundary: The Emerald formation may overlie, lower Jurassic, Upper Palaeozoic, Lower Palaeozoic and Precambrian strata. In some sections (e.g. 2/10-8) the base can be placed between the basal conglomerate Lower Jurassic sandstones and this is shown on geophysical logs by a distinct downward increase in velocity. In other sections there is a down-section change from clean brownish sandstone (Emerald Formation) to red or varicoloured sandstones, siltstones or limestones of Devonian age, accompanied by a downward increase in gamma values and velocity. Where the formation rests on Lower Palaeozoic or Precambrian rocks, there is a downward change from clean sandstone to gneiss and a downward increase in gamma values and decrease in velocity.
Definition of Upper Boundary: The upper boundary is characterised by a downward change from grey mudstones and siltstones (Heather Formation) to sandstones (Emerald Formation). The boundary is marked by a down-section decrease in gamma values and increase in velocity.
Thickness: 10-30 m (average c. 15 m).
Geographical Limits: 'Transitional Shelf', North Sea quadrants 2 and 3.
Parent Unit: Humber Group (HMBG)
Previous Name(s): Emerald Sandstone (-5100)
Alternative Name(s): none recorded or not applicable
Stratotypes:
Type Section  North Sea well 2/10-8 at 2706.5-2734 m depth below KB. Wheatley et al. (1987); Richards et al. (1993). 
Reference Section  North Sea well 2/15-1 at 1705-1725 m depth. Richards et al. (1993). 
Reference Section  North Sea well 3/11b-3 at 1672-1686.5 m depth. Richards et al. (1993). 
Reference(s):
Richards, P C, Lott, G K, Johnson, H, Knox, R W O'B. and Riding, J B. 1993. 3. Jurassic of the Central and Northern North Sea. In: Knox, R W O'B and Cordey, W G. Lithostratigraphic nomenclature of the UK North Sea. [British Geological Survey, Nottingham] 
Stewart , D.M. & Faulkener, J.G. 1991. The Emerald Field, Blocks 2/10a, 2/15a, 3/11b, UK North Sea. In: Abbotts, I.L. (ed.) United Kingdom oil and gas fields 25 years commemorative volume. Memoir of the Geological Society, London, 14, 111-116. 
Wheatley, T.J., Biggins, D., Buckingham, J. & Holloway, N.H. 1987. The geology and exploration of the Transitional Shelf area, an area to the west of the Viking Graben. In: Brooks, J. & Glennie, K.W. (eds) Petroleum geology of North West Europe, 979-989. [Graham Trotman, London] 
1:50K maps on which the lithostratigraphical unit is found, and map code used:
none recorded or not applicable