The geology of Sarqaqdalen, Nûgssuaq, West Greenland, with special reference to the Creataceous boundary fault system

Sarqaqdalen is a wide valley which runs northwards from the coast of Vaigat into the Nugssuaq peninsula. The valley lies along a major geological boundary - that between the Cretaceous-Tertiary sedimentary basin to the west and the Precambrian basement to the east. This boundary is a fault. Fluvial, coal-bearing Cretaceous sediments on the west side of the fault are largely the deposits of a braided river system, passing north into a swampy plain. The Cretaceous beds dip at 10°-16° towards the fault, but Tertiary strata, which overlie the Cretaceous with angular unconformity, are virtually horizontal. This implies that the main movement on the fault was pre-middle Danian (the age of the earliest Tertiary sediments). The actual fault contact between Cretaceous and Precambrian is only exposed at one locality to the east of outer end of Sarqaqdalen, where there is a relay fault striking ca. 110°. Neither here nor anywhere else is there evidence in the Cretaceous strata of sediments having been fed in from the east or north-east or of the proximity during sedimentation of a steep fault scarp on this side of the valley. On the contrary, palaeocurrent directions arealmost all towards the north and north-west, parallel to the boundary fault system.

It is suggested that either 1) the north-south Cretaceous river valley in the Sarqaqdalen area was localised by subsidence along the boundary fault system, but sedimentation kept pace with the fault movements so that no marked relief developed along the line of the fault, or 2) the valley developed in a sag which in this area did not develop into a fault-bounded basin until the end of the Cretaceous. (To the north, however, on the islands Upernivik Ø and Qeqertarssuaq, there is evidence of syn-sedimentary faulting having begun as early as during the Albian-Cenomanian).

A striking feature of inner Sarqaqdalen is the major dolerite sheet which coats the steep gneiss slopes on the eastern side of the inner part of the valley. The gneiss in contact with this sheet is bleached and brecciated, and there is no doubt that, as previous workers have suggested, this sheet was intruded along the fault plane between the Precambrian and Cretaceous. The contact between the sheet and the Cretaceous sediments to the west is not however exposed.