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

This paper describes large-scale hummock structures from an Oligocene-Mioce ne 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 amplitud e of about 50 m; they are characterized by a polygonal planform geometry. T wo successive depositional packages in this probably pelagic-dominated inte rval have this form of structural expression, but troughs of the younger se t are directly superposed on crests of the older set. They occur at a prese nt-day subsea-bottom depth of about 1000 m, and cover an area of similar to 6000 km(2). This extraordinary structural configuration is attributed to a protracted depositional and deformational history in which a density inver sion was established, hummock formation was initiated in response to differ ential loading above an irregular polygonal fault system, and then subseque nt collapse produced syndepositional troughs in the overlying units, These structures 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 den sity inversion deformation structure yet described from a clastic sedimenta ry succession.