A Late Paleocene age for Greenland’s Hiawatha impact structure

Abstract

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. To date the structure, we conducted 40 Ar/ 39 Ar analyses on glaciofluvial sand and U-Pb analyses on zircon separated from glaciofluvial pebbles of impact melt rock, all sampled immediately downstream of Hiawatha Glacier. Unshocked zircon in the impact melt rocks dates to ~1915 million years (Ma), consistent with felsic intrusions found in local bedrock. The 40 Ar/ 39 Ar data indicate Late Paleocene resetting and shocked zircon dates to 57.99 ± 0.54 Ma, which we interpret as the impact age. Consequently, the Hiawatha impact structure far predates Pleistocene glaciation and is unrelated to either the Paleocene-Eocene Thermal Maximum or flood basalt volcanism in east Greenland. However, it was contemporaneous with the Paleocene Carbon Isotope Maximum, although the impact’s exact paleoenvironmental and climatic significance awaits further investigation.

Impact melt rocks from the Late Paleocene Hiawatha impact structure, northwest Greenland

“…Observations presented here provide the first comprehensive investigation of glaciofluvial impact melt rock samples collected from the ice margin in Inglefield Land (Figure 1a). With the only likely origin of the samples being from the Hiawatha impact structure, this work adds to previous studies of impact-related material collected in Inglefield Land and strongly supports the identification of a hypervelocity impact event (Garde et al, 2020(Garde et al, , 2022Gustafsson, 2020;Kenny et al, 2022;Kjaer et al, 2018). All the studied samples are melt rocks and contain a variety of shocked, unshocked, and melted materials mixed during their formation.…”

“…We follow the recommendation of Osinski et al (2022): "samples in which shock metamorphic criteria are determined need not be in situ, as long as there is a clear topographic crater form and evidence (e.g., glacial flow directions) to connect the two". Clear evidence connecting sampled material to the Hiawatha structure under the ice has already been demonstrated (Figure 1a) (Kenny et al, 2022;Kjaer et al, 2018), and here, we therefore refer to the crater as the Hiawatha impact structure herein. FIGURE 1.…”

“…Dominant mineral clasts in the silicate melt rocks, summarized as quartz + plagioclase + Kfeldspar + biotite AE sillimanite AE garnet (Table 1), mirror the metamorphic mineral assemblage of the Etah Group and Etah meta-igneous complex (Dawes, 2004;Nutman et al, 2008), as well as the predominant mineral species in the melt grain-rich glaciofluvial sand exhumed from the structure (Garde et al, 2022;Kjaer et al, 2018). Furthermore, most U-Pb ages of apparently unshocked zircon from HW19-01 and HW19-05 (Kenny et al, 2022) correspond to known zircon ages found in Inglefield Land, proximal to the Hiawatha impact structure (Nutman et al, 2008).…”

“…These samples are proximal to glaciofluvial sand samples collected in 2016 (Garde et al, 2020(Garde et al, , 2022Gustafsson, 2020;Kjaer et al, 2018). Samples HW19-01 and HW19-05, first described by Kenny et al (2022), were both collected here. Sample localities B and C are located on topographic highs along the western flank of the crater rim and glacial margin (Figure 1a).…”

“…Given the ∼ 58 Ma age of the impact, it can be ruled out that Hiawatha melt rocks were sampled from an ejecta blanket preserved until today under the ice (or that any of the ejecta blanket remains in the foreland). Previous studies have shown that impact-related material was transported subglacially from the Hiawatha impact structure through the main subglacial drainage channel to the adjacent floodplain (Figure 1a; Garde et al, 2022;Kenny et al, 2022;Kjaer et al, 2018).…”