Giant hummocks in deep-water marine sediments: Evidence for largescale differential compaction and density inversion during early burial

This paper describes large-scale hummock structures from an Oligocene-Miocene succession in the Faeroe-Shetland Trough, off the northern coast of the United Kingdom. These hummocks have a wavelength of 1-2 km, and an amplitude of about 50 m; they are characterized by a polygonal planform geometry. Two successive depositional packages in this probably pelagic-dominated interval have this form of structural expression, but troughs of the younger set are directly superposed on crests of te older set. They occur at a present-day subsea-bottom depth of about 1000 m, and cover an area of -6000 km². This extraordinary structural configuration is attributed to a protracted depositional and deformational history in wich a density inversion was established, hummock formation was initiated in response to differential loading above an irregular polygonal fault system, and then subsequent collapse produced syndepositional troughs in the overlying units. These structure are similar in geometry to hummocks described from near-surface sediments in the Norway Basin by P. R. Vogt and are the largest type of density inversion deformation structure yet described from a clastic sedimentary succession.