A subaqueous rootless cone field at Niuluut, Disko, Paleocene of West Greenland

During the formation of the lavas of the Maligât Formation (60 Ma) on Disko in West Greenland, lava flows erupted from subaerial vents were emplaced into an extensive and deep lake, resulting in the formation of hyaloclastite breccias, subaqueous lava flows, and invasive flows. In the central part of the lake, unusual mounds of volcaniclastic material mingled with sediment were formed on the lake floor. Two well-exposed mounds are about 25 m high and 100-200 m wide, and each is composed of three normally graded units. The lowest units are coarse deposits with pillows, pillow fragments and large shale fragments, fining upwards through lapilli tuff into tuffaceous layers rich in small shale clasts. The upper units in the mounds are tuffaceous with sediment matrix and grade into dark shale horizons. Each mound has a central, sediment-filled 'chimney'. The mounds are overlain by 2-8 m shale followed by a c. 50 m thick subaqueous lava flow that disturbed the soft sediments and filled local depressions between the mounds. The mounds are interpreted as subaqueous rootless cones that formed by hydroclastic explosions where a subaqueous lava flow was locally brecciated throughout, thereby providing outlets for the pressurized and heated water in the underlying mud. The three normally graded units in the mounds may correspond to successive flow pulses in the central liquid part of the flow. The distribution of breccia deposits and lava pillars suggests the presence of a rootless cone field around 1.5 km in diameter. The structure of the mounds contrasts to the structure of subaerial rootless cones which have inversely graded units and upwards decreasing amounts of sediment. This is explained by higher water/magma mass ratios in the subaqueous than in the subaerial environment, which were situated on either side of an explosive energy maximum. In both environments the water/magma mass ratio decreased during formation of the cones, but the explosivity of the subaqueous cones moved towards the maximum whereas the explosivity of the subaerial cones moved away from the maximum.