The mid-Proterozoic Ritscherflya Supergroup, Antarctica, is intruded by numerous mafic to intermediate sills and small layered bodies. Contact relations are well exposed where the Grunehogna and Kullen sills intruded argillaceous and arenaceous sedimentary rocks of the Schumacherfjellet and Hogfonna Formations of the Ahlmannryggen Group. A 3 m thick peripheral zone above the upper contact of the Grunehogna sill consists of reconstituted sedimentary rocks which have been fluidized and recrystallized due to the intrusion. Local melting and crystallization of the sediments, under hydrous conditions and pressures below 312 bars, have produced zones of granite and granophyre, which grade into arenites where primary sedimentary structures are preserved. The sedimentary rocks have been deformed into large disharmonic folds where the sill transgresses the Schumacherfjellet Formation. The contact relations suggest that emplacement of the mafic sills and sheets at Grunehogna occurred when the sediments were still water-saturated and only partially lithified.