Early Cretaceous giant bivalves from seep-related limestone mounds, Wollaston Forland, Northeast Greenland

Anomalous mound-forming limestones. here termed the Kuhnpasset Beds, occur within Late Barremian (Early Cretaceous) mudstones on Wollaston Forland, Northeast Greenland. The normal mudstones contain a sparse Fauna of small nuculoids, arcoids and inoceramids: by contrast, the mounds contain an unusual faunal assembage, dominated by large bivalves. These include an abundant lucinid, Crytolucina kuhnpassetensis sp. nov., and, less commonly, Solemya, both known seep-associated genera. Locally, a large modiomorphid , Caspiconcha whitami gen. et sp. nov., is common and reaches > 300 mm in length and has a shell up to 28 mm thick. Also. the wood-boring bivalve Turnus is abundant in driftwood. Gastropods are rare. but the associated cephalopod fauna includes ammonites, belemnites, nautiloids and a remarkable large orthoconic phragmocone. The form of the mounds with calcite-cemented tube systems, associated laminated calcite crusts and void fills, together with the fauna, is analogous to those of methane-based cold-seep complexes. However, preliminary studies indicate that much of the original aragonitic shell is now replaced by silica. This precluded conclusive geochemical studies based on the shells themselves. it is believed that the mounds formed on the seafloor in a mid- to outer shelf situation at the end of a period of extensional rifting on the eastern Greenland passive Atlantic margin. The vents occur near the footwall crest of a tilted fault block. The underlying faults may have provided router or influenced direction of movement for nutrient migration. Source rocks were probably the Late Jurassic black shales from depths of < 600-1300 m. If methane was bring generated, it was probably forming by shallow-depth organic breakdown rather than by thermogenic processes, which require greater burial.