The facies distribution of carbonate sediments on Porcupine bank, northeast Atlantic

Porcupine Bank (51–54°N, 12–15°W), approximately 10,000 km² in area and 150–500 m deep, is effectively removed from present-day terrigenous sedimentation by the broad Irish Shelf to the east, and deep water to the north, west and south. Water temperatures range from 10° to 14.5°C. Carbonate sediments presently accumulate on the bank, mixing with relict quartz sands and lithic pebbles, cobbles and boulders of glacial origin. Traction current structures are absent but biogenic hollows, pits, mounds and trails are prevalent. The lithofacies have a crudely concentric zonation with pelagic foraminiferal oozes in the deepest waters ( > 500 m), passing to foraminiferal (pelagic and benthic) glauconitic quartz sands over the bulk of the bank (500-200 m) where scattered patches of Lophelia pertusa coral branches are found, which in turn pass into a narrow zone associated with the lithic boulder field at the bank crest (< 200 m) which has coarse sands composed of quartz and skeletal fragments of benthic organisms-molluscs, echinoids, bryozoans and serpulids. This facies pattern is closely similar to that on Rockall Bank, 500 km to the north, and supports the construction of a depth-related facies model for modern open-sea temperate carbonate sediments: > 500 m, pelagic foraminiferans; 500-200 m, benthic foraminiferans plus Lophelia coral patches; 200-100 m, bivalve-echinoderm; 100-50 m, bryozoan-bivalve-gastropod-serpulid; <50 m, calcareous red algae-bivalve-gastropod-barnacle.