| Computer Code: |
JOMU |
Preferred Map Code: |
notEntered |
| Status Code: |
Full |
| Age range: |
Carnian Age (TC)
— Norian Age (TN) |
| Lithological Description: |
The Jonathan Mudstone Member is composed largely of reddish brown and variegated silty mudstones, with sporadic laminae of well sorted, very fine sandstone and partings of white and pale brown, micritic limestone and cryptocrystalline dolomite. Anhydrite occurs as patches and occasional discrete laminae, particularly towards the base of the member. The mudstones are characterized by a monotonous sonic-log response and by a wide density/neutron separation. |
| Definition of Lower Boundary: |
The base of the Jonathan Mudstone Member is defined by a gradational transition from mudstones to sandstones with minor mudstones (Joanne Sandstone Member). Gamma-ray values often show little change across the boundary (e.g. 30/12b-3), but the lithological change is clearly marked by a sharp downward narrowing of the density/neutron log separation. |
| Definition of Upper Boundary: |
The top of the Jonathan Mudstone Member is defined by a downward change from sandstones with minor mudstones (Josephine Sandstone Member) to mudstones, and is typically marked by a sharp wireline-log break. Where the transition is more gradual, as in well 30/12b-3, the boundary is taken at the base of the lowest significant sandstone bed of the Josephine Sandstone Member. The Jonathan mudstones are locally overlain by Jurassic strata. |
| Thickness: |
The member is up to 250 m thick, with recorded thicknesses of between about 20 and 60 m for the Upper Jonathan Mudstone unit, between 35 and 120 m for the Middle Jonathan Mudstone unit, and up to 70 m for the Lower Jonathan Mudstone unit. |
| Geographical Limits: |
The Jonathan Mudstone Member is restricted to a few scattered outliers in the south Central Graben. These outliers occur in synclines that had been forming above zones of Permian salt withdrawal since the early Triassic (Hodgson et al., 1992). |
| 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- 3: 4188.5-4355.5 m (13742-14290 ft) below KB (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. |
| 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. |
| 1:50K maps on which the lithostratigraphical unit is found, and map code used: |
| none recorded or not applicable |
