The Tertiary development of the Porcupine and Rockall basins is compared in terms of stratigraphy and sedimentation. Four main unconformities are correlated between the basins and these are interpreted as being of Paleocene (C40), latest Eocene-Early Oligocene (C30), latest Early Miocene (C20) and Early Pliocene (C10) age. Seismic stratigraphic analysis of both basins suggests a greater similarity in post-Eocene deposition than in the Paleocene to Eocene stratigraphy. During Early Tertiary time a regressive succession, punctuated by minor transgressions, marks a major interruption in the general post-rift thermal subsidence pattern of the region. This regression, possibly triggered by lithospheric thermal effects and/or ridge-push stresses, resulted in deltaic and submarine fan deposition in the Porcupine Basin, with submarine channel trends indicating that sediment was sourced mainly from the Porcupine High to the north and west. Sand deposition in the Porcupine Basin occurred principally during Mid- to Late Eocene times. In contrast, Early Eocene sand input is postulated in the Rockall Basin, whereas deposition during the Mid- to Late Eocene times was more mud-prone. In Oligocene and Mio-Pliocene times, sediment build-ups, interpreted as contourites, developed towards the margins of both basins, with sedimentation principally influenced by oceanographic circulation patterns at this time. During Neogene to Recent times limited marginal sediment influx occurred in the Porcupine Basin whereas sediment input continued locally, during Neogene time, in the Hebrides region of the Rockall Basin.
The margins of the deep-water sedimentary basins west of Ireland contain a number of large clusters (provinces) of spectacular carbonate mounds and build-ups. These basins have a complex development history involving the interplay of rift tectonics, thermal subsidence, igneous activity and oceanographic variations. The Porcupine and Rockall basins both rest upon thin continental crust, the consequence of major rift episodes in Permo-Triassic, Late Jurassic and Early Cretaceous times. Phases of volcanism occurred in the Early Cretaceous and especially in the Early Cenozoic. Fluid flow within the basins is likely to have been controlled by the overall basin geometry and by the distribution and linkage of permeable strata with fault systems, stratal surfaces and unconformities. A number of regional unconformities, controlled by both basin tectonic and regional oceanographic effects, can be mapped and correlated throughout the Porcupine and Rockall basins. The youngest of these unconformities (C10: Early Pliocene) can be traced throughout much of the NW European Atlantic margin. It forms the horizon on which virtually all the carbonate mounds in the basins develop, suggesting a geologically instantaneous mound nucleation and growth event. Although the control on their development is uncertain, the mound clusters show a spatial association with lithified strata, buried contourite and deltaic deposits, slope failure features and with the basin margins. Analysis of these relationships points to a combination of geological and oceanographic processes controlling mound initiation and growth.
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