Easton Anhydrite Member
Easton Anhydrite Member, Carboniferous, Northern England Province
Easton Anhydrite Member is part of the Lyne Formation.
Originally named the ‘Easton Anhydrite Facies of the Lower Border Group’ by Ward (1997) the unit is here formally renamed the Easton Anhydrite Member. It is derived from the Easton 1 borehole, which was drilled near the hamlet of Easton 16 km north of Carlisle, and includes the type section.
Grey or white anhydrite interbedded with limestones, shales, siltstones and sandstones.
The sedimentary rocks are predominantly of shallow water origin. They were deposited in a rapidly subsiding basin, with restricted marine circulation, in which sedimentation and rapid burial is inferred to have kept pace with subsidence. Most of the anhydrite is likely to have had a subaqueous rather than a sabkha origin, the purer and thicker beds probably precipitating as gypsum in regionally extensive salinas.
The partial type section is the Easton 1 borehole (BGS Registration Number NY47SW/15) (NY.44124.71694) from 1054 to 2260 m depth (the base of member not being proved). An upper unit, 126 m thick, includes successions of generally upward-thinning beds of anhydrite, interbedded with limestone, some dolostone, sandstone and shale. A middle unit, 639 m thick, includes nine large cyclical sequences of limestones, shales, sandstones and anhydrite. A lower unit, at least 440 m thick, includes composite limestone-anhydrite beds, 2.4–6.1 m thick, separated by shales or claystones that thicken downwards. Sandstones in this unit are numerous but thin.
Lower and upper boundaries
The base of the member is not seen, but seismic interpretation indicates the presence of ‘several thousands of feet’ of conformable strata below the succession in some parts of the Solway Basin (see Ward, 1997, p. 283).
The top of the member is taken where numerous beds of anhydrite interbedded with limestones, shales, siltstones and sandstones give way to strata with variously algal, oolitic, peloidal, shelly, crinoidal and serpulid limestones, which may represent the Main Algal Member of the Lyne Formation. However, this has not been positively identified.
More than 1153 m (base not seen).
Distribution and regional correlation
Solway Basin, proved in the Easton 1 borehole (see above). The member is not exposed at the surface (breccias present suggest it was removed by solution), but seismic reflectors suggest that the top of the member extends at depth from west Cumbria eastwards to the flanks of the Bewcastle Anticline and beyond (see Ward, 1997, fig. 11). A total area well in excess of 1000 km2 is inferred.
Early Visean, determined by limited palynology (Ward, 1997).
- Ward, J. 1997. Early Dinantian evaporites of the Easton-1 well, Solway Basin, onshore Cumbria, England. 277–296 in Petroleum geology of the Irish Sea and adjacent areas. Meadows, N S, Trueblood, S P, Hardman, M, and Cowan, G (editors).Geological Society of London Special Publication, No. 124.