|
Sedimentology |
Edinburgh Oil & Gas plc, 10 Coates Crescent, Edinburgh EH3 7AL, Scotland
An early Viséan, anhydrite-rich, carbonate-clastic succession, of vertical thickness 3784 feet (no base) was found in 1990 at the Easton-1 well in the Solway Basin, onshore Cumbria. The succession is stratigraphically located in the Lower Border Group where only traces of evaporites had been found at outcrop. It is proposed to name the succession the ‘Easton Anhydrite Facies of the Lower Border Group’, abbreviated to ‘EA Beds’.
The EA Beds are divided into three units. Unit 1 at the top, about 400 feet thick, contains two sequences of upwardly thickening anhydrite beds within mixed lithologies. Unit 2, about 2100 feet thick, consists of nine cyclic sequences of limestones, shales and sandstones with numerous interbeds of anhydrite. Overall similarity of several of the sequences suggests similar origins. Unit 2 sequences are grouped into three megacycles, two of approximately 750 feet and one 590 feet thick. Deposition is interpreted to have taken place in arid or partly arid climatic conditions, within a shallow and rapidly, but uniformly subsiding basin with glacio-eustatically controlled rise and fall of sea level. Many of the Unit 2 anhydrite beds are interpreted to be of subaqueous origin. Unit 3, about 1500 feet thick, displays complex patterns of cyclicity with sedimentation apparently responding directly to solar insolation under orbitally induced control within Milankovitch periodicities of 100 000 and 400 000 years. Seismic interpretation indicates that the regional extent of the EA Beds is greater than 1000 km2.
