Rates of volcanic deposition, facies changes and movements in a dynamic basin: the Nuussuaq Basin West Greenland, around the C27n-C26r transition

The initial stages of Palaeogene volcanism in the Nuussuaq Basin in West Greenland were characterized by eruption of basaltic and picritic magmas through sediments of Cretaceous to early Paleocene age into a marine or low-lying coastal environment. Recent magnetostratigraphic work has recognized the C27n-C26r transition (estimated duration less than l0 ka and here assumed to be 5ka) as a c. 170 m thick zone within a succession of thin picritic lava flows. Multi-model photogrammetry combined with chemical and lithological analysis of the volcanic rocks has allowed detailed 3D analysis of the facies variation within this narrow time window. Subaerial lavas flowed eastwards over a more than 40 km wide front. On northern Disko they covered an existing lava plateau and buried a subaerial landscape of dipping Cretaceous sandstones, while on Nuussuaq they flowed into an up to 700 m deep marine embayment and formed prograding hyaloclastite fans passing into fine-clastic mass flows. With a progradation rate of 0.5-1 ma ^-1 the palaeogeography of the basin changed considerably during the short time interval. In addition to substantial basin subsidence, the volcanic facies changes have also preserved a record of synvolcanic differential movement of extensional fault blocks.The following parameters are estimated for the volcanism within the Nuussuaq Basin during the C27n-C26r transition: Production rate c. 0.042km ³ a ^-1, productivity c. 1.2 x l0 ^-3 km ³ a ^-1 km ^-1 (rift), volcanic aggradation c. 33mka ^-1, subsidence c. 25mka ^-1. If the volcanism evolved continuously at this high aggradation rate, all of the Vaigat Formation could have erupted in 70 ka. However, the complex geological record indicates a much longer total duration, and the volcanism must have had an intermittent character.