The Cretaceous seaway in the North Atlantic region – a molluscan viewpoint

Although narrow and occasionally restricted, the North Atlantic Ocean was a well established seaway during the Cretaceous Period. The northern part of this ocean was one of the gateways to the Boreal Arctic Ocean. In the North Atlantic, the Jurassic-Cretaceous transition was represented by sea-level lowstand. With rising sea level during the Cretaceous, shelf seas and the tropical zone reached their all-time greatest extents in Late Cretaceous, the subtropics extending to 40° latitude. The extensive chalk sea extended from the southern United States to Kazakhstan and northwards to the Viking Graben. Further northwards, siliciclastics characterised sedimentation up into the Arctic Ocean. Cretaceous sea and land temperatures from equator to the poles were higher and far more equable than today. On land or sea, polar ice caps did not exist; forested land and roaming reached well into the Arctic and Antarctic.

Cretaceous North Atlantic biotic changes are documented here from molluscan records, principally ammonites, belemnites and bivalves, particularly from eastern Greenland, British Isles and parts of northwestern Europe, but touching on other parts of Europe, Svalbard, the Sverdrup Basin and the Russian Arctic. The emergence of the Anglo-Brabant massif at the Jurassic-Cretaceous boundary, with development of Purbeck and Wealden facies, generated oyster-rich lagoons and fluvio-lacustrine systems with unionid bivalves. It created a physical barrier for the contemporary marine nekton which exhibit high provinciality. Boreal Berriasian (Ryazanian) craspeditine ammonite faunas of eastern England, the North Sea and eastern Greenland are isolated from Tethyan berriasellids of western Europe. Difficulties with Jurassic-Cretaceous boundary correlation between Boreal and Tethyan regions must be resolved in the less obstructed seaway of the Caspian-Russian Platform area. By Aptian time England had become a shallow seaway and by the Late Cretaceous high sea levels allowed near global distributions e.g. The heteromorph ammonite Diplomoceras cylindraceum. Whether North Atlantic molluscan diversity really is greatest in the earliest Cretaceous and decreases in the Late Cretaceous, as sea levels rise and the degree of endemism falls, will be examined. Also controls on the apparent lower diversity as one goes northward will be assessed. Early Cretaceous rifting defined narrow shelves to the northern North Atlantic seaway, which were maintained as fluctuating sea levels rose through the Cretaceous. Consequently deep water mudstones characterise much of the Late Cretaceous sediment preserved in this area in contrast to deposits of wide and shallow shelves further south in northwest Europe. The northern North Atlantic therefore may have a much depleted molluscan fauna. Final mention will be made of anomalous mollusca, independent of more general oxygenation, temperature and depth restrictions. A very limited number of methane seeps occur with associated limestones and chemosymbiotic molluscs, whose existence and global distribution have only recently become known and understood.