Comparative Tertiary stratigraphic evolution of the Porcupine and Rockall basins

Abstract

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.

Modern carbonate mounds: porcupine drilling

“…In general, the Paleocene succession is more mud dominated, whereas the main coarse clastic input occurred in the middle Eocene to earliest late Eocene (McDonnell and Shannon, 2001). The Paleocene-Eocene is subdivided into five sequences (PT1-PT5) (Fig.…”

“…The Albian PK2 sequence is affected by a minor rifting pulse of early actual North Atlantic seafloor spreading that started in the late Aptian (Shannon, 1991;Moore and Shannon, 1995). A local uplift of the basin margins produced a regressive period with an outer shelf deltaic sedimentation environment with locally sand rich deltas and associated beach complexes (McDonnell and Shannon, 2001). The onset of the Late Cretaceous (PK3) was characterized by a further relative sea level rise, featuring offshore sandstone bars, followed by a northward thinning and onlapping outer shelf to slope sequence of carbonates (chalk).…”

“…A major basinwide unconformity developed in the latest Eocene to early Oligocene. This unconformity C30 marks the onset of deepwater sedimentation and establishment of regional contour-hugging bottom currents (McDonnell and Shannon, 2001). …”

“…In sequence PT8, a slope-parallel sediment drift is described in the southeast part of the basin. The C20 reflector, which forms the lower boundary of PT8, is related to a relative sea level fall and is noticeably erosive in the south part of the basin (McDonnell and Shannon, 2001). This intensified scouring may be related to intensified bottom current activity elsewhere in the North Atlantic.…”

Seabed geomorphology of the Porcupine Bank, West of Ireland

“…The bank was first surveyed in 1981 on MV Challenger expedition 11/81 using seabed samples, underwater video and shallow seismic profiling (Scoffin & Bowes, 1988), and multiple surveys have been carried out since (Cullen, 2003;Dorschel, Wheeler, Monteys, & Verbruggen, 2010;McDonnell & Shannon, 2001;Naylor & Shannon, 2009). Scoffin and Bowes (1988) subdivided the bank into two broad regions, with a clear depth zonation: (1) an area centred on and surrounding the Porcupine Ridge, dominated by up to boulder-sized clasts in a matrix of coarse sand-sized fragments of benthic organisms, with some quartz sand; and (2) an area between 200 and 500 m water depth dominated by foraminifera-rich (benthic and pelagic) glauconitic quartz sand.…”