Permian rocks and faulting in Sandsvær at the western margin of the Oslo region.

Abstract

In the Sandvær area, a Permian quartz conglomerate and great thicknesses of both the B1-basalt and the rhomb porphyry RP2 are found above the Ringerike sandstone and the underlying Cambro-Silurian sediments. The younger Permian supracrustal units are engulfed by larvikite and ekerite. The larvikite is intersected by aplite, a granitic rock possibly formed by assimilation of Ringerike sandstone by the larvikite magma. The area is cut by a number of dip-slip faults, most of which are connected to the subsidence of the Oslo graben. The steep dip of the supracrustals towards the intrusives at the contact was caused by sinking of the supracrustals into the melt, while dip-slip faults parallel to the contact possibly were formed by a later pushing upwards of the half-consolidated magma.