Abstract

Rockall Bank (56 degrees N to 58 degrees N and 13 degrees W to 16 degrees W) is approximately 20,000 km 2 in area and 100 m to 300 m deep, and is effectively removed from present day terrigenous sedimentation by surrounding deep water. Water temperature ranges from 8 degrees C to 12 degrees C. Carbonate sediments presently accumulate on the bank burying Tertiary and older rock outcrops and Pleistocene boulders of glacial origin. Superficial samples were collected by grabbing and dredging and the environments of carbonate production and deposition were observed by underwater television. Component analyses of the sediments indicate a roughly concentric zonation of lithofacies with coarse bryozoan and serpulid remains dominant in the shallow central portion less than 120 m deep. The inner zone is surrounded by a zone rich in bivalve and echinoderm fragments and benthic foraminifera, which covers most of the bank from depths of about 120 m to 220 m. The peripheral zone below 220 m is dominated by pelagic foraminifera. In water 200 m to 400 m deep, 10 m to 20 m wide patches of I m high living colonies of the branched coral Lophelia pertusa abound and form an annular zone around the bank margin. The bulk of the carbonates are calcitic with some aragonitic remains present in the shallow (molluscan) and deep (coral) zones. Microbial borings are rare in sediments from agitated waters less than 100 m deep but very abundant in sediments (particularly mollusc, echinoderm and coral) from calmer deep waters. The bioerosion of coral framework is probably more rapid than the rate of burial of the upright colonies which will lead to their preservation as irregular bands of broken branches. Below 200 m many pelagic tests are filled with diagenetic glauconite and rarely silica, though conspicuous evidence of inorganic carbonate dissolution or precipitation is absent. The carbonate deposits are not very thick but if accumulation continues to keep pace with the subsidence of the bank, aphotic temperate limestones will ultimately be formed, having a central zone of coarse fragments of benthic organisms rimmed by pelagic deposits containing bands rich in corals.

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