Palaeovalleys at the basal unconformity of the Palaeoproterozoic Karrat Group, West Greenland

© 2016 GEUS. Geological Survey of Denmark and Greenland Bulletin 35, 63–66. Open access: www.geus.dk/publications/bull In the Rinkian belt of West Greenland, reworked Archaean gneisses are overlain by supracrustal successions of the Palaeoproterozoic Karrat Group, defined by Henderson & Pulvertaft (1967) as comprising two formations: the Qeqertarssuaq Formation and the Nûkavsak Formation. The group was later extended to include the Mârmorilik Formation (Henderson & Pulvertaft 1987) originally considered to be of Archaean age (Henderson & Pulvertaft 1967) but later shown to be Palaeoproterozoic, resting with a depositional unconformity on Archaean gneiss (Garde 1978). Henderson & Pulvertaft (1987) suggested that the carbonatedominated Mârmorilik Formation in the south was laterally equivalent to the siliciclastic-dominated Qeqertarssuaq Formation in the north, the two subbasins being separated by a basement topographic high. The Karrat Group and the Archaean basement were metamorphosed and folded during the Rinkian orogeny (Henderson & Pulvertaft 1987; Grocott & Pulvertaft 1990). During shipand helicopter-supported fieldwork in 2015, the Kangerluarsuk–Maarmorilik area (Fig. 1) was visited to sample the Zn-Pb-mineralised horizons found by RTZ Mining and Exploration Ltd. (Coppard et al. 1992) along the basement-cover boundary and to obtain oblique photographs of the contact that was described as faults on the Mârmorilik geological map sheet (scale 1:100 000, Henderson & Pulvertaft 1987). The aim of this paper is to show preliminary new results on the geometry and type of unconformity between the Palaeoproterozoic Karrat Group and the underlying gneisses, and to describe a new stratigraphic unit representing here the lowermost sedimentary sequence above the unconformity. This unit is of particular relevance as it controls the distribution of the Zn-Pb mineralisation.