Sand remobilisation and intrusion in the Upper Jurassic Hareelv Formation of East Greenland

Abstract

An extensive Late Jurassic intrusive sandstone complex is exposed in Jameson land, East Greenland. The sandstones and the host mudstones form the Upper Oxfordian – Volgian Hareelv Formation. The formation covers an area of 55×70 km, is 200–400 m thick and consists of black basinal mudstones and highly irregular sandstone bodies, dykes and sills. Failure of shelf-margin sandbodies resulted in downslope sediment gravity flows and deposition of massive sands on the slope, at the base-of-slope and in the basin. The sands flowed in steep-sided gullies formed by retrogressive slumping of slope muds or loaded directly into the muds. Sandbodies deposited within the gullies have steep commonly stepped margins while those deposited at the downslope termination of gullies have a sheet-like geometry. All sandbodies underwent some degree of fluidization and liquefaction subsequent to burial and sand was intruded into the surrounding black mudstones. Remobilisation and intrusion took place over a long time interval ranging from almost syndepositional to relatively deep burial and primary sediment structures were lost in most cases. Sandstone dykes and sills are ubiquitous and were emplaced by all combinations of stoping and dilation. The intrusive sandbodies range in dimensions from centimetres to many hundreds of metres. The degree of post-burial remobilisation ranges from rather small-scale modifications to wholesale fluidization, liquefaction and out-of-place intrusion of the sand over tens to hundreds of metres. The Hareelv Formation was deposited during the most important Mesozoic rift event in East Greenland and the pervasive remobilisation of all sandbodies in the formation is interpreted as caused mainly by cyclic loading by seismic shocks. Additional important factors were slope shear stress, build up of pore pressure due to loading, slumping, upwards movement of pore waters expelled from the compacting muds and possibly also of biogenic and thermogenic gas. The Hareelv Formation is an excellent field analogue for deeply buried hydrocarbon reservoirs, which have been modified by remobilisation and injection of the sands.