Descriptive text to 1:500 000 sheet 12 Scoresby Sund. The regional geology of a segment of the Caledonian fold belt, adjacent Upper Palaeozoic - Mesozoic sediments of the Jameson Land basin, and Tertiary volcanics

The Scoresby Sund region was systematically mapped in the years 1968-1972 with the aim of producing sixteen 1:100 000 map sheets and obtaining a general description of the area. The 1:500 000 map compilation is based largely on this work and the descriptive text gives a summary of the geology.

The Scoresby Sund region includes the southernmost part of the N-S trending East Greenland Caledonian fold belt and adjacent to it a coast-parallel sedimentary basin with Upper Palaeozoic continental deposits and Upper Permian to Mesozoic shallow water and marine sediments . Tertiary plateau basalts dominate the southern part of the map sheet while Tertiary intrusive complexes occur in the north-east part of the region.

The crystalline rocks in the Caledonian fold belt have a composite origin , and it is possible to recognise Archaean , Early Proterozoic, Middle Proterozoic and Caledonian complexes. Four lithostructural zones can be distinguished: (1) tectonic window complexes along the margin of the Inland Ice interpreted as representing the Caledonian foreland beneath the Caledonian nappes; (2) a western gneiss and schist zone with a gneissic Archaean basement and a Proterozoic metasedimentary schist cover; (3) a migmatite and granite zone in the central part of the area with extensively migmatised rocks of presumed Proterozoic origin and Proterozoic and Caledonian plutonites mainly in the form of granitic intrusions; (4) a coast-parallel , horst-like region (Liverpool Land) bordering the North Atlantic Ocean comprising pre-Caledonian gneiss complexes and Caledonian migmatites and plutonic intrusives.

The oldest post-Caledonian deposits are terrestrial sediments of Devonian and Lower Permian age. These are succeeded in the Jameson Land basin by an Upper Permian to Early Cretaceous succession of shallow marine sediments with terrestrial intervals in the Triassic. The Upper Permian and Mesozoic sequence is approximately 3-4 km thick.

The Tertiary plateau basalts form a 2- 3 km thick sequence of tholeiitic rocks, erupted in two successive volcanic cycles during a period of approximately 3 m . y. at about the Paleocene-Eocene boundary. A Tertiary intrusive complex with alkaline rocks is found in the northern part of the map sheet area. Basic dykes and sills are widespread mainly in the post-Caledonian rocks.

Quaternary deposits occur abundantly over the whole region reflecting the former extensive ice cover. Marine shell-bearing silt and clay banks indicate that the sea at various stages reached more than 100 m above the present sea level.

Geophysical investigations include radiometric surveys over the eastern part of the region and extensive aeromagnetic and ship-borne shallow and deep seismic surveys over the post-Caledonian sedimentary areas on Jameson Land and offshore.

A molybdenum deposit with an estimated 120 x 1()6 tons of ore is associated with a Tertiary intrusive complex in the north of the map sheet area . The Upper Palaeozoic - Mesozoic Jameson Land sedimentary basin is a potential oil prospecting area , in which exploration began in 1985.