A syn-rift to post-rift transition sequence in the Main Porcupine Basin, offshore western Ireland

The evolution of the Main Porcupine Basin can be divided into pre-rift, syn-rift and post-rift stages. The syn-rift/post-rift transition represents a gradual change from extensive fault activity through localized faulting and passive infill to sporadic faulting during post-rift times. This Transition Sequence is present in localized areas (sub-basins). It infills the remnant syn-rift topography after which the post-rift stage begins. Sub-basins developed during the syn-rift stage have a general N-S to NE-SW orientation and range in size from approximately 30 km × 20 km to approximately 12 km × 8 km. Their shapes range from symmetrical fault-bounded depocentres to half-graben. The sub-basins have several internal unconformities which merge into a single composite erosion surface at the margins. Areas in which the Transition Sequence is developed usually have an element of fault control, especially during the early stages. Clastic fans are identified prograding from fault scarps. Later passive onlap during a rising sea-level progressively infills the subbasins. The age of the Transition Sequence is problematical, but borehole data suggests that it spans uppermost Kimmeridgian/Tithonian to end-Berriasian times. The sub-basins appear to be broadly coincident with small Permo-Triassic basins. This may point to a deep-seated fault control on the location of the Transition Sequence sub-basins. Comparable syn- to post-rift Transition Sequences are also present in the Orphan and Jeanne d’Arc Basins, offshore Eastern Canada, and possibly in other North Atlantic rift basins also.