The BGS Lexicon of Named Rock Units — Result Details

Flett Formation

Computer Code: FLET Preferred Map Code: notEntered
Status Code: Full
Age range: Late Paleocene (GPL) — Early Eocene (GEE)
Lithological Description: The Flett Formation is dominated by mudstones and sandstones mostly of progradational shallow marine facies, but ranging from paralic 'delta-top' facies in proximal sections to 'prodelta' facies in distal sections. A series of basaltic lavas is present in well 205/9-1. Paralic 'delta-top' facies are represented by claystones, mudstones, siltstones and sandstones of highly variable facies, with thin lignites. The more distal 'progradational' successions display a more limited range of lithologies, which are dominated by grey laminated or bioturbated mudstones and siltstones interbedded with fine to coarse grained sandstones. The beds are mostly organized into upward-coarsening cycles, clearly identified on wireline-log character. The 'prodelta' successions consist largely of mudstones, which are relatively extensive in the upper part of the Flett Formation (Hildasay equivalent).
Definition of Lower Boundary: In more distal sections, the base of the Flett Formation is defined by a downward change from lithologically varied facies, represented by erratic log signatures, to uniform mudstone facies (Lamba Formation), represented by consistent wireline-log signatures. A small gamma-ray peak commonly occurs at the base of the Flett Formation (e.g. 214/28-1). In some proximal sections (e.g. 204/23-1; 208/19-1), the basal beds of the Flett Formation are generally more sandy, and the uppermost sediments of the Lamba Formation are more variable in lithology, rendering the boundary more difficult to define on lithology alone. In all sections where the top of the Lamba Formation is in mudstone facies, the boundary is marked by a marked increase in the abundance of agglutinated foraminifera, particularly the distinctive Bathysiphon/Rhabdammina type. In proximal shelf areas, some sections also show a downward change from grey or variegated mudstones to more consistent pale grey-green mudstones. No such change in mudstone colour is displayed in more distal sections, however.
Definition of Upper Boundary: In mudstone-dominated sections the top of the Flett Formation is marked by a downward change from mudstone with abundant tuffs (Balder Formation) to darker grey mudstones with few tuffs (Flett unit 3). It is marked by a downward increase in gamma values, although the gamma-ray profile is commonly stepped and the precise boundary therefore somewhat arbitrary. The boundary usually falls at or slightly below a marked downward decrease in velocity. In some marginal successions the boundary is less clear-cut, with sandy tuffaceous mudstone or sandstone of the Balder Formation resting directly on silty or sandy facies of the Flett Formation. In such sections the boundary is somewhat arbitrary, although the tuffaceous character of the Balder mudstones, associated in many sections with a spiky sonic log signature, is usually still identifiable. In such sections a low-velocity unit commonly occurs at the top of the Flett Formation, reflecting the presence of lignitic facies, including true lignites in some sections (e.g. 206/11-1).
Thickness: The formation reaches a maximum thickness of c. 860 m in 206/2-1A, thinning to c. 80 m in 208/17-1, the most distal of the pro-delta sections.
Geographical Limits: UK north west margin. The Flett Formation occurs in most Moray Group sections, but is absent in the extreme south and in some central marginal sections. In the north of the region, the formation passes laterally into a high-gamma mudstone assigned to the Sele Formation (unit 3) (e.g. 209/4-1A; 210/13-1).
Parent Unit: Moray Group (MORA)
Previous Name(s): none recorded or not applicable
Alternative Name(s): none recorded or not applicable
Stratotypes:
Type Section  North Sea well 205/09- 1: 2360 – 3014 m (7743 – 9888 ft) (Knox et al., 1997). 
Reference Section  North Sea well 204/19– 1: 1564 – 1820 m (5132 – 5972 ft) (Knox et al., 1997). 
Reference Section  North Sea well 214/28– 1: 2407.5 – 2896 m (7898 – 9502 ft) (Knox et al., 1997). 
Reference Section  North Sea well 208/21– 1: 1805 – 2000 m (5922 - 6562 ft) (Knox et al., 1997). 
Reference(s):
Waters, C N, Gillespie, M R, Smith, K, Auton, C A, Floyd, J D, Leslie, A G, Millward, D, Mitchell, W I, McMillan, A A, Stone, P, Barron, A J M, Dean, M T, Hopson, P M, Krabbendam, M, Browne, M A E, Stephenson, D, Akhurst, M C, and Barnes, R P. 2007. Stratigraphical Chart of the United Kingdom: Northern Britain. (British Geological Survey.) 
Knox, R W O'B and Holloway, S. 1992. 1. Paleogene of the Central and Northern North Sea. In: Knox, R W O'B and Cordey, W G (eds.) Lithostratigraphic nomenclature of the UK North Sea. British Geological Survey, Nottingham. 
Shanmugam, G, Bloch, R B, Mitchell, S M, Beamish, G W J, Hodgkinson, R J, Damuth, J E, Straume, T, Syversten, S E, and Shields, K E. 1995. Basin-floor fans in the North Sea: sequence stratigraphic models vs. sedimentary facies. AAPG Bulletin 79, 477-512. 
Knox, R W O'B, Holloway, S, Kirby, G A and Baily, H E. 1997. Stratigraphic nomenclature of the UK North West Margin. 2. Early Paleogene lithostratigraphy and sequence stratigraphy. British Geological Survey, Nottingham. 
1:50K maps on which the lithostratigraphical unit is found, and map code used:
none recorded or not applicable