| Computer Code: |
COYS |
Preferred Map Code: |
notEntered |
| Status Code: |
Full |
| Age range: |
Late Paleocene (GPL)
— Late Paleocene (GPL) |
| Lithological Description: |
The Colsay Sandstone Member 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 bed of basaltic lava 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 Colsay Sandstone Member 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 more proximal sections 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 most sections the top of the Colsay Sandstone Member is defined by a sharp downward change from mudstone to sandstone. The overlying mudstone is distinguished by being thicker and less sandy than those occurring within the overlying and underlying sandstone units. |
| Thickness: |
The member reaches a maximum thickness of c. 260 m in well 206/1- 2. |
| Geographical Limits: |
UK north west margin. The Colsay Sandstone Member is present in central basinal areas. In the southern part of the basin it appears to be absent through non-deposition or erosion (see Ebdon et al., 1995). The absence of the Apectodinium spp. acme biomarker in many other marginal sections may also indicate the absence of Colsay equivalents; alternatively it may reflect facies control. |
| Parent Unit: |
Flett Formation (FLET)
|
| Previous Name(s): |
none recorded or not applicable
|
| Alternative Name(s): |
none recorded or not applicable
|
| Stratotypes: |
| Type Section |
North Sea well 206/02- 1A: 2454 – 3053 m (8051 – 10017 ft) (Knox et al., 1997). |
| Reference Section |
North Sea well 214/28- 1: 2525 – 2889.5 m (8284 – 9480 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.) |
| 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. |
| Ebdon, C, Granger, P J, Johnson, H D, and Evans, A M. 1995. Early Tertiary evolution and sequence stratigraphy of the Faeroe-Shetland Basin: implications for hydrocarbon prospectivity. In: Scrutton, R A, Stoker, M S, Shimmield, G B, and Tudhope, A W (eds.) The tectonics, sedimentation and palaeoceanography of the North Atlantic region. Geological Society, London, Special Publication 90, 51-69. |
| 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 |
