Jan Audun Rasmussen

Conodont geothermometry and tectonic overburden in the northernmost East Greenland Caledonides

Kronprins Christian Land lies at the northernmost limit of the East Greenland Caledonides, and may be divided into four main tracts: foreland, parautochthon, thin-skinned thrust sheets containing Neoproterozoic sediments, and thick-skinned thrust sheets of crystalline basement. The eastern part of the foreland and the parautochthon are composed of Ordovician–Silurian shelf carbonates overlain by Llandovery–lower Wenlock turbidites deposited at the southern margin of the Franklinian Basin. Deformation of the parautochthon occurred beneath a major detachment, the Vandredalen thrust, with Neoproterozoic sediments of the Rivieradal Group lying in the hanging wall. The Rivieradal Group was deposited in an E-facing half-graben and was displaced westwards across its rift shoulders during Caledonian thrusting. Extensive sampling for conodonts has been carried out within the Lower Palaeozoic rocks of the foreland and parautochthon in order to increase the precision of structural interpretations and to provide data on maximum burial temperature and, in turn, the thickness of overburden. In contrast to earlier studies, conodont colour alteration indices (CAI) show a gradual and continuous increase from CAI 3 in the eastern part of the foreland to CAI 5+ in the easternmost parts of the parautochthon. The isopleths are not disrupted or truncated by thrusting, as previously suspected, indicating that the heating is not attributable to pre-thrusting stratigraphic overburden. Furthermore, considerations of the regional geology indicate that there was no significant accumulation of sedimentary overburden in post-Caledonian time; the predominant component is thus considered to be loading by thrust sheets. Modelling of the overburden thickness suggests that, prior to erosion, it increased from 3.9 km (CAI 3) in the easternmost foreland to a maximum of 12.5 km beneath the Vandredalen thrust sheet in the easternmost part of the area, providing constraints for restoring cross-sections across the orogen.

The Neoproterozoic Hekla Sund Basin, eastern North Greenland: a pre-Iapetan extensional sequence thrust across its rift shoulders during the Caledonian orogeny

The Vandredalen thrust sheet forms the allochthonous Caledonian front in Kronprins Christian Land, eastern North Greenland. It transported Neoproterozoic shallow marine, arkosic sediments of the Rivieradal Group and shelf sediments of the Hagen Fjord Group across parautochthonous Neoproterozoic–Ordovician sediments on the foreland. The thrust sheet roots to the east along the Brede Spærregletscher–Hekla Sund lineament and has a demonstrable westwards displacement of c. 40 km. A large part of that displacement (c. 22 km) is achieved on a single structure, the Vandredalen thrust, but this is only one of a linked system of thrusts which involve parautochthonous thin-skinned thrust sheets to the west and thicker skinned thrust sheets now exposed in the east. Conodont geothermometry suggests that the thick-skinned thrust sheets formerly extended westwards above the Vandredalen thrust sheet at higher structural levels. Restoration of the displacement on the thin-skinned thrusts reveals the Rivieradal Group to be a Neoproterozoic syn-rift sequence deposited in an eastwards-facing extensional half-graben (Hekla Sund Basin) which originally lay immediately to the east of the Brede Spærregletscher–Hekla Sund lineament. The transgressive Hagen Fjord Group represents a period of post-rift thermal re-equilibration in latest Riphean time and the youngest units in the group overstep the original rift shoulder. The Hekla Sund Basin represents another example of the rift-sag episodes which are an important feature of the pre-Iapetan evolution of the Laurentian margin successions and comparisons are made with coeval basins in Scotland and Svalbard.

Palaeokarst systems in the Neoproterozoic of eastern North Greenland in relation to extensional tectonics on the Laurentian margin

Palaeokarst, in the form of large, uncollapsed cave systems, is described from the Proterozoic of Kronprins Christian Land, eastern North Greenland. The endokarst, of entirely meteoric origin, is developed in dolostones of the Fyns Sø Formation (Hagen Fjord Group, Riphean). At one locality, Hjørnegletscher, shallow, sub-horizontal phreatic conduits are present below an unconformity surface and are infilled by the overlying Ediacaran Kap Holbæk Formation. In Sæfaxi Elv, the unconformity is overlain by the Wandel Valley Formation, an Early Ordovician carbonate sequence that is widely transgressive over northeastern Greenland. Vertical vadose fissures extend down towards the phreas, but the cave systems are again filled by Kap Holbæk Formation sediments. At Hjørnegletscher, channels up to 40 m wide incise the phreatic system, pointing to relative base-level lowering before, or during, deposition of the Kap Holbaek Formation. Recognition of a depositional hiatus between the Fyns S0 and Kap Holbaek formations, in what was previously thought to be a continuous Vendian Hagen Fjord sequence, has implications for regional correlation and tectonics. The unconformity could represent most of Vendian time, accounting for the absence, in this area, of glaciogenic sedimentary rocks in the Hagen Fjord Group. This permits correlation of the Fyns SøFormation with other end-Riphean transgressive carbonate sequences developed in East Greenland, Svalbard and perhaps Scotland, that represent the culmination of a major pre-Iapetan rift-sag cycle. Secondly, recognition of the scale of the sub-Wandel Valley unconformity points to regional uplift and tilting of northeastern Greenland in mid-Cambrian to earliest Ordovician time. This must represent a phase of renewed extension of the Iapetus passive margin that is unique to this corner of Laurentia, not terrane collision as previously suggested.

First Permo-Carboniferous conodonts from North Greenland

Upper Palaeozoic conodonts are described for the first time from the North Greenland Wandel Sea Basin. In eastern Peary Land, the Moscovian species Idiognathodus incurvus and the Kasimovian—Gzhelian I. magnificus occur in the Upper Carboniferous Foldedal Formation, while an assemblage from the lower part of the succeeding Kim Fjelde Formation suggests deposition in the Upper Artinskian Neostreptognathodus pequopensis—N . clarki Zone. These datings confirm the existence in the northern part of the Wandel Sea Basin of the pronounced early Permian hiatus previously recognized in Holm Land and Amdrup Land in the southern part of the basin. The single conodont specimen found at Prinsesse Ingeborg Halvo further corrobates the local absence of this regional hiatus in the central part of the Wandel Sea Basin.

Microfossil biostratigraphy of the Palaeogene succession in the Davis Strait, offshore West Greenland

Abstract A new microfossil based biostratigraphy of the Paleocene and Lower Eocene sediments of the Hellefisk-1, Ikermiut-1, Kangâmiut-1, Nukik-1, and Nukik-2 boreholes offshore West Greenland has been established. In general, the five boreholes contain fairly well-preserved, diverse microfossil faunas and floras consisting mainly of foraminifera, radiolaria, ostracods and diatoms. The studied interval was subdivided into three foraminiferid biostratigraphic intervals, in ascending stratigraphic order, the Stensioeina beccariiformis , Praeglobobulimina ovata and Pseudohastigerina wilcoxensis intervals and five biostratigraphic intervals based on additional microfossil groups (the Thalassiosiropsis wittiana , Fenestrella antiqua–Coscinodiscus morsianus , Ostracod, Aulacodiscus hirtus and Cenodiscus–Cenosphaera intervals). The intervals are more easily recognised in the two basinal boreholes, Ikermiut-1 and Kangâmiut-1, than in the three more nearshore boreholes, Nukik-1, Nukik-2 and Hellefisk-1, due to a higher microfossil diversity and abundance.