| Computer Code: |
JOSA |
Preferred Map Code: |
notEntered |
| Status Code: |
Full |
| Age range: |
Ladinian Age (TD)
— Carnian Age (TC) |
| Lithological Description: |
The Joanne Sandstone Member is composed of interbedded pale grey and brown sandstones and grey and reddish brown siltstones and silty mudstones, with occasional stringers of cryptocrystalline and microcrystalline dolomite and argillaceous limestone. The sediments are very micaceous, such that in some sections average gamma-ray values are higher than those of the underlying Julius Mudstone Member (e.g. 30/12b-2). The sandstones are also feldspathic; they are argillaceous, mainly fine or very fine grained, and up to about 20 m thick. Many of the thickest beds form upward-fining units in which cross-bedded, medium to coarse grained sandstone grades up to current-ripple laminated silty sandstone; such units typically have sharp bases. However, the member also includes upward-coarsening units with gradational bases (e.g. 30/12b-2). |
| Definition of Lower Boundary: |
The base of the Joanne Sandstone Member is defined by a downward change from interbedded mudstones and sandstones to mudstones (Julius Mudstone Member). It is marked on wireline logs by a generally sharp downward increase in velocity, resistivity and density log values. In some sections, the boundary is marked by a downward decrease in gamma-ray values (e.g. 30/12b-2). In the Fulmar Field (Block 30/16), the Joanne sandstones onlap the flank of an Upper Permian salt diapir. |
| Definition of Upper Boundary: |
The top of the Joanne Sandstone Member is defined by a downward change from anhydritic mudstones (Jonathan Mudstone Member) to sandstones and mudstones. The lithological transition is generally gradational, but on wireline logs is taken at a relatively sharp downward change from wide to narrow density/neutron log separation. The Joanne sandstones are locally overlain by Jurassic strata. |
| Thickness: |
Proven thicknesses of the member vary between 70 m and 490 m (Goldsmith et al., 1995). |
| Geographical Limits: |
The Joanne Sandstone Member is patchily distributed in Triassic synclines of the south Central Graben. |
| Parent Unit: |
Skagerrak Formation (SKAG)
|
| Previous Name(s): |
none recorded or not applicable
|
| Alternative Name(s): |
none recorded or not applicable
|
| Stratotypes: |
| Type Section |
North Sea well 30/12b- 2: 4177-4393 m (13704-14413 ft) below KB (Cameron, 1993). |
| Reference(s): |
| Goldsmith, P J, Rich, B, and Standring, J. 1995. Triassic correlation and stratigraphy in the South Central Graben, United Kingdom North Sea. In: Boldy, S A R (ed.) Permian and Triassic rifting in NW Europe. Geological Society, London, Special Publication. |
| Hodgson, N A, Farnsworth, J, and Fraser, A J. 1992. Salt-related tectonics, sedimentation and hydrocarbon plays in the Central Graben, North Sea, UKCS. In: Hardman, R F P (ed.) Exploration Britain: Geological insights for the next decade. Geological Society, London, Special Publication No.67, 31-63. |
| 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. |
| 1:50K maps on which the lithostratigraphical unit is found, and map code used: |
| none recorded or not applicable |
