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Impact melt rocks formed during hypervelocity impact events are ideal for studying impact structures. Here, we describe impact melt rock samples collected proximal to the 31 km wide 58 Ma Hiawatha impact structure, northwest Greenland, which is completely covered by the Greenland Ice Sheet. The melt rocks contain diagnostic shock indicators (e.g., planar deformation features [PDF] in quartz and shocked zircon) and form three groups based on melt textures and chemistry: (i) hypocrystalline, (ii) glassy, and (iii) carbonate‐based melt rocks. The exposed foreland directly in front of the structure consists of metasedimentary successions and igneous plutons; however, the carbonate‐based impactites indicate a mixed target sequence with a significant carbonate‐rich component. Well‐preserved organic material in some melt rocks indicates that North Greenland at the time of impact was host to abundant organic material, likely a dense high‐latitude temperate forest. Geochemical signatures of platinum‐group elements in selected samples indicate an extraterrestrial component and support previous identification of a highly fractionated iron impactor in glaciofluvial sand. Our results illustrate the possibility to study impact structures hidden beneath a thick ice sheet based on transported samples and this opens a new avenue for identifying other potential impact craters in Greenland and Antarctica. [ABSTRACT FROM AUTHOR]

The article offers information about the Late Paleocene age for Greenland's Hiawatha impact structure. It mentions that Hiawatha structure, located beneath Hiawatha Glacier in northwestern Greenland, has been proposed as an impact structure that may have formed after the Pleistocene inception of the Greenland Ice Sheet.

Type material of Paliurus colombii Heer, Populus arctica Heer, Hakea arctica Heer, and Hedera macclurii Heer from the Atanikerdluk locality (Paleocene, Greenland) was restudied based on the original collection, stored in the National Museum of Ireland, Dublin. All these species were joined under the name Zizyphoides colombii (Heer) Seward et Conway, type species of the genus Zizyphoides Seward et Conway. A lectotype of Zizyphoides colombii is designated and the diagnosis of this species is emended. An emended diagnosis of the genus Zizyphoides is also presented. [ABSTRACT FROM AUTHOR]

The Paleocene volcanic rocks in the Nuussuaq Basin on Disko and Nuussuaq comprise the picritic Vaigat Formation (c. 62-61 Ma) and the overlying basaltic Maligât Formation (c. 60 Ma). The Maligât Formation is up to 2000 m thick on western Disko where the top of the formation is least eroded. The formation is divided into four members, the Rinks Dal, Nordfjord, Niaqussat and Sapernuvik members, which are formally defined here. On central and eastern Disko and Nuussuaq the Maligât Formation lavas are interbedded with fluvial and lacustrine sandstones and mudstones of the Atanikerluk Formation. The Rinks Dal Member is the lowest member and originally constituted around 61% by volume of the formation. It is divided into 12 informal units based on chemically recognisable oscillations in the fractionation state of the basalts. The oldest units are present on central and south Disko close to the Disko Gneiss Ridge. The younger lavas spread farther to the east, north and west, filled the Assoq Lake basin east of the ridge and gradually onlapped the shield of the earlier Vaigat Formation that rose to the north. Only the lavas of the upper Rinks Dal Member reached far into Nuussuaq. The lavas are generally not crustally contaminated and comprise evolved basalts with 4.4-9.2 wt% MgO and a few picrites. The most evolved basalts with 3.2-4.8 wt% TiO2 occur in the middle part of the member where they form the Akuarut unit. The Nordfjord Member originally constituted around 6% by volume of the formation. It is not subdivided because the lithological variability is local. The member is widespread but has its depocentre on north-western Disko where thicknesses reach 350 m and eruption sites, intermediate lavas and acid tuffs are present. Over most of the area the member consists of just a few lava flows with combined thicknesses of 30-100 m. The member has a very diverse lithology with rock types ranging from silicic basalt with 5.3-10.0 wt% MgO through magnesian basaltic andesite and andesite with 2.4-10.6 wt% MgO to dacite with 1.2-2.2 wt% MgO. Rhyolite with 0.2-1.2 wt% MgO and up to 77 wt% SiO2 occur in tuffs and conglomerate clasts. All rocks are crustally contaminated and some are native-iron-bearing. The Niaqussat Member originally constituted around 33% by volume of the formation. It is subdivided into three informal units. The member is widespread, but much of it has been removed by erosion. Lithologies in the lower unit range from silicic picrite with up to 15 wt% MgO to basalt with 6-12 wt% MgO and a few basaltic andesite flows. The middle and upper parts of the Niaqussat Member comprise more evolved basalts with respectively 6.1-7.2 wt% MgO and 4.9-6.4 wt% MgO. All rocks are crustally contaminated and a few lava flows are native-iron-bearing. The Sapernuvik Member comprises three uncontaminated basalt flows with 7.5-10.7 wt% MgO. It is only preserved in a small area on western Disko. Dyke systems with up to 80 km long dykes and subvolcanic intrusions associated with the Nordfjord and Niaqussat members occur on western and north-eastern Disko. The rocks are crustally contaminated and range from silicic basalt with 4-13 wt% MgO to magnesian andesite with 3-10 wt% MgO. They commonly form composite intrusions, some of which contain accumulations of native iron and sulfides. The contaminants are carbon- and sulfur-bearing sediments of the Nuussuaq Group. Major contamination mechanisms were mixing with partial melts from the sediment sidewall and xenoliths and selective exchange of some elements, including carbon and sulfur, between magma and sediment. Degrees of contamination vary from 2-5% in the basalts to 10-50% in the more silicic rocks. No rocks more evolved than basalt were produced by ordinary fractional crystallisation. [ABSTRACT FROM AUTHOR]

Introduction: The ~31-km-wide Hiawatha structure, located beneath Hiawatha Glacier in northwestern Greenland, has been proposed as an impact structure that may have formed after the Pleistocene inception of the Greenland Ice Sheet [1]. However, available age constraints for the Hiawatha structure have only been tentative. The bedrock under the structure is likely a continuation of the highly metamorphosed 1.95-1.75 Ga [2] bedrock exposed immediately adjacent to Hiawatha Glacier, providing a maximum constraint on the structure's age [1]. Further age estimates have been based on indirect constraints, such as estimates of erosion rates and the structure's relationship to the Greenland Ice Sheet [1]. Here we dated the structure using detrital materials sampled immediately downstream thereof. We report 40Ar/39Ar analyses of impact-related glaciofluvial sand collected in 2016 and U-Pb analyses of shocked zircon from pebble-sized glaciofluvial clasts of impact melt rock collected in 2019. Results: Of the 50 grains that underwent 40Ar/39Ar step-heating analysis, 29 developed 'U'- or 'saddle'-shaped age spectra, with 23 giving minimum apparent ages in the Paleocene. Two mini-plateaus gave Late Paleocene ages of 58.5 ± 0.3 Ma and 60.2 ± 0.5 Ma, respectively. The 62 U-Pb analysis performed on 43 zircon grains using secondary ion mass spectrometry (SIMS) defined a discordant array in concordia space, spreading between an upper intercept of ~1915 Ma and a Late Paleocene lower intercept. The ~1915 Ma age for the majority of unshocked zircon grains from the impact melt rocks agrees with known geological events and zircon ages from the bedrock adjacent to Hiawatha Glacier [2], supporting a local origin for the samples. Of the 45 analyses performed on shocked zircon grains, the youngest nine data points are statistically indistinguishable, giving a robust concordia age and a weighted mean 206Pb/238U age of 57.99 ± 0.54 Ma (MSWD = 1.18; probability = 0.30; n = 9). We consider this to be the most robust estimate for the age of the impact structure [3]. Discussion: Impact melt rocks collected less than 10 km downstream from the edge of the Hiawatha structure contain unshocked zircon with ages matching that of the bedrock adjacent to Hiawatha Glacier. Shocked zircon in these samples gives a ~58 Ma U-Pb age, which agrees with the younger of two 40Ar/39Ar mini-plateau ages from the sand sample. The samples were thus derived from a Late Paleocene impact structure somewhere upstream but sufficiently proximal that the target geology is indistinguishable geochronologically from that exposed where the samples were recovered. Given existing geomorphic evidence for an eroded complex impact structure beneath Hiawatha Glacier, we suggest this is the source of the samples and thus that the Hiawatha structure is a relatively large impact structure that formed in the Late Paleocene. The new age indicates that the Hiawatha impact structure significantly predates Pleistocene glaciation and is unrelated to either the Paleocene-Eocene Thermal Maximum or flood basalt volcanism in east Greenland. The impact was contemporaneous with the Paleocene Carbon Isotope Maximum, although the impact's exact paleoenvironmental and climatic significance await further investigation. [ABSTRACT FROM AUTHOR]

The article presents a geological assessment of the East Greenland margin. It notes that the margin runs for almost 3,000 kilometers (km) from Kap Farvel to Nordostrundingen. It discusses the geology of the margin south of Shannon which holds sub-basalt and post-Paleocene sediments. Assessment of the margin north of Shannon shows that the shelf area is not affected by Tertiary flood basalts, offering a fair to good seismic imaging of Paleocene and older successions.