The BGS Lexicon of Named Rock Units — Result Details

Argyll Carbonate Member

Computer Code: ARCA Preferred Map Code: notEntered
Status Code: Full
Age range: Guadalupian Epoch (PUG) — Lopingian Epoch (PUL)
Lithological Description: The Argyll Carbonate Member is generally described from cuttings as consisting of light grey and pale brown dolomites and minor dolomitic limestones. The carbonates are massive or microcrystalline, commonly sucrosic, and locally porous and vuggy. Irregular gamma-ray and sonic log signatures are typical of many basin-margin sections, indicating that the dolomites are interbedded with more argillaceous facies (Deegan and Scull, 1977). In some basin-margin wells, high-gamma sections are partly related to the presence of mudstone beds, and partly to the presence of organic-rich dolomites. In more central parts of the basin (broadly defined by the distribution of the Shearwater Salt Formation), many sections display a unit of relatively low velocity argillaceous dolomites overlain by a unit of cleaner, anhydritic dolomites, generating a distinctive stepped profile on the sonic and resistivity logs (e.g. 21/11-1; 28/5-1). The basal dolomites are anhydritic at the northeastern flank of the Mid North Sea High (e.g. 30/29a-1).
Definition of Lower Boundary: The base of the Argyll Carbonate Member is defined by a downward change from dolomites to the high-gamma mudstones of the Kupferschiefer Formation. It is generally marked by a sharp downward increase in gammaray values and decrease in velocity (e.g. 14/4-1), but is gradational in areas of thick Kupferschiefer mudstone development (e.g. 20/2-2). Where the Kupferschiefer Formation is absent, the Argyll Carbonate Member rests unconformably on Lower Permian or pre-Permian clastic sediments (e.g. 27/10-1).
Definition of Upper Boundary: Where the Iris Anhydrite Member is present, the top of the Argyll Carbonate Member is defined by a downward transition from dominant anhydrite to dominant carbonate. The transition is commonly gradational, but a highly radioactive bed of mudstone or sapropelic dolomite occurs close below the boundary in many wells (e.g. 27/10-1). In some basin-margin sections, the Iris Anhydrite Member is absent, and the Argyll Carbonate Member is overlain, possibly disconformably, by dolomite of the Innes Carbonate Member. In such sections, the boundary may not be easily distinguished, although most of these sections display a downward change from high-gamma, argillaceous dolomites to cleaner dolomites (e.g. 20/2-2). A superficially similar succession occurs in many basinal sections, where the Argyll Carbonate Member extends upwards to include equivalents of the Iris Anhydrite Member. Here, however, the boundary with the Innes Carbonate Member is clearly defined by a downward transition from argillaceous to clean dolomites, and is generally marked by a sharp downward decrease in gamma-ray values, and increase in velocity and resistivity (e.g. 21/11-1).
Thickness: Its thickness varies regionally between 1 m and 30 m, but up to 60 m of Z1 carbonates have been encountered in the Ettrick area (Blocks 20/2 and 20/3), and on the Long Forties High.
Geographical Limits: The Argyll Carbonate Member extends across most parts of the Central North Sea and South Viking Graben, but is not differentiated north of 60°N, where few wells penetrate the Halibut Carbonate Formation. It is absent from basin-fringing areas of contemporary clastic deposition and from areas of deep Mesozoic erosion.
Parent Unit: Halibut Carbonate Formation (HACA)
Previous Name(s): none recorded or not applicable
Alternative Name(s): none recorded or not applicable
Stratotypes:
Type Section  North Sea well 30/24- 33: 3078-3100 m (10098-10170 ft) below KB (Cameron, 1993). 
Reference Section  North Sea well 20/02- 2: 3631-3689.5 m (11912-12105 ft) (Cameron, 1993). 
Reference Section  North Sea well 27/10- 1: 1440.5-1493 m (4726-4898 ft) (Cameron, 1993). 
Reference(s):
Cameron, T D J. 1993. 4. Triassic, Permian and pre-Permian 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. 
Bifani, R. 1985. A Zechstein depositional model for the Argyll field. In: Taylor, J C M et al. (eds.) The role of evaporites in hydrocarbon exploration. JAPEC Course Notes No.39,11-118. 
Deegan, C E and Scull, B J. 1977. A standard lithostratigraphic nomenclature for the Central and Northern North Sea. Report of the Institute of Geological Sciences, 77/25; NPD Bulletin No.1. 
Trewin, N H, and Bramwell, M G. 1991. The Auk Field, Block 30/16, UK North Sea. In: Abbotts, I L (ed.) United Kingdom oil and gas fields 25 years commemorative volume, Geological Society, London, Memoir No. 14, 227-236. 
1:50K maps on which the lithostratigraphical unit is found, and map code used:
none recorded or not applicable