Age distribution of detrital zircon grains in sandstones and stream sediments from East Greenland north of 70°N. A contribution to the project: Provenance study of possible reservoir sandstone units in East and North-East Greenland

Sediment transport pathways fundamentally determines the three-dimensional geometry of clastic sedimentary bodies. One way to understand these pathways is to determine the source of the sedimentary detritus. Sedimentary provenance can be investigated in many ways and in this study absolute radiometric ages of detrital zircon grains will be used to: 1) fingerprint the potential source materials and 2) investigate Post-Devonian clastic sedimentary systems on-shore East Greenland to possibly determine their source areas. The sedimentary successions in East Greenland are direct analogues to potential petroleum reservoir rocks in the North Atlantic.

In this report U-Pb analyses from detrital zircon grains from 85 stream sediments and 194 sandstone samples will be presented and interpreted. The age distribution patterns will also be discussed within a palaeogeographic framework, focusing on possible pathways.

The analysed stream sediment samples are from 70°-79° N. They yield signals mainly from Archaean and Palaeoproterozoic basement gneisses and granites, Neo-proterozoic metasedimentary sequences and Caledonian and Devonian igneous rocks. This pattern may be primary, but may also be the result of reworking of older sediments with similar sources. Some signals are very influenced by the local geology, e.g. there is a strong dominance of Caledonian zircon grains in the southernmost part, there is a persistent influence of Devonian zircon grains in samples from Canning Land and there is a simple, almost bimodal distribution of ages in samples collected in the basement regions of the north. However, in large areas of East Greenland the exposed geology is dominated by Meso- to Neoproterozoic sedimentary successions (the Krummedal, Smallefjord and Eleonore Bay sequences) and they contribute with a wide range of Archaean and Proterozoic ages.

The analysed sandstones were deposited from the Neoproterozoic to the Palaeocene. The Neoproterozoic Eleonore Bay Supergroup samples are characterised by a small, but diverse Archaean population centered around 2800 Ma and Proterozoic grains with ages between 2000 and 900 Ma, with dominant peaks at 1650 and 1100 Ma. The Cambrian to Permian samples seem to have been sourced from the geological units present in East Greenland, mainly metasedimentary units and local Archaean and/or Proterozoic gneisses in the basement. A small, local Permian basin in the south of the area has a source area to the east, based on the abundance of Caledonian zircon grains. The Early to Middle Triassic samples generally have rather local sources and Liverpool Land acted as a contributor of detritus. The Jurassic Bajocian samples all display non-specific age patterns derived from mixed basement gneisses and metasediments. This is possibly in part due to the increase in basin size due to transgression and therefore a larger catchment. The Jurassic Bathonian samples are more distinctive with clearly separable source areas. The samples from the Late Jurassic constitute two groups: one with a source area to the south of 72° N and one with a source between 74° and 75° N, both groups were probably transported towards east. The Cretaceous samples generally have detritus both from basement gneisses and metasediments. It is possible to see a difference in provenance from samples derived from south and north of 73° N, based on their basement gneiss signal. In some Cretaceous samples there are Phanerozoic age populations that cannot be explained by local Greenland sources. The Palaeocene samples also have a mixed source from Proterozoic basement and metasediments.

In Peary Land, the sediments are sourced from older sediments and it is therefore difficult to pin-point source areas with any accuracy.