The Fleetwood Dyke and the Tynwald fault zone, Block 113/27, East Irish Sea Basin

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Abstract
A series of Tertiary dolerite intrusions, the Fleetwood Dyke Group, is intruded into Triassic sediments of the northern East Irish Sea Basin (EISB). Well 113/27-1 penetrated a total of 375 m of dolerite within the Mercia Mudstone Group (MMG). Samples of dolerite have been dated (K–Ar) at between 61 and 65 Ma. Apatite fission track analysis on samples from the same well indicates that approximately 2000 m of uplift occurred in the early Tertiary, at about the same time as dyke intrusion. Using seismic and marine magnetic data, the Fleetwood Dyke Group has been mapped in the vicinity of Block 113/27 to reveal a complex set of dykes and sills. The presence of these intrusions within the MMG results in velocity pull-ups on the underlying Ormskirk Sandstone Formation (OSF), a potential exploration pitfall. The geometry of the igneous intrusion has been influenced by the structure of the Tynwald fault zone. This feature is interpreted to have formed in the Triassic as a result of listric faulting detached on Permian evaporites. This interpretation is supported by balanced section reconstruction.