The Gardnos structure : the impactites, sedimentary deposits and post-impact history
Appears in the following Collection
- Naturhistorisk museum 
AbstractThe Gardnos structure in Hallingdal, Norway is an eroded impact crater, presently consisting of impactites and crater infill sediments exposed within a roughly circular area of about five km diameter. Investigations in the early 1990s confirmed its impact origin, however a number of issues regarding the crater formation and post-impact history were still unresolved. The time of impact was poorly constrained to between 900 and 400 Ma, the geological setting uncertain and the source of organic carbon within the impactites unknown. These obviously inter-related questions touch different fields within geology (involving radiometric dating, sedimentology and organic chemistry) thus a multi-disciplinary approach was required. Another premise at Gardnos is the availability of outcrops. Erosion has erased some of the original crater shape; however, it has also exposed impact-related lithologies from breccias and fractured basement deep underneath the crater floor, to the allochthonous melt-bearing breccias and post-impact sediments. Whereas previous works were based on samples from a few central localities, this study aims to describe the entire structure and the impact-related lithologies based on detailed mapping and much broader sampling. The stratigraphic and lateral variations within each lithology are key information to understand the crater formation and post-impact process. During this study the structure has been mapped in detail with special emphasis on the crater suevite and sedimentary infill lithologies. Tentative reconstruction of the fresh crater indicates a central peak in the centre, surrounded by a relatively flat plain and crater walls, making the original crater diameter likely about six kilometers. Melt-bearing breccias (dominated by suevite) directly overlies the original crater floor which consists of shattered basement rocks constituting the (par)autochthonous Gardnos Breccia. The suevite contains a mix of shocked and unshocked material, melt and lithic fragments of various sizes. Orientations of planar deformation features (PDFs) in quartz grains indicate maximum shock pressures above 20 GPa. The distribution of lithic fragments originating from large parts of the crater demonstrates the extreme forces responsible for the formation of this unit. Melt generally occurs as individual fragments, amounting from zero to 40 vol% of the bulk rock. Minor amounts of a clast-rich impact melt rock occur, where melt constitutes the matrix (up to 85 vol% of bulk rock). Volume calculations of the original melt content within the crater are in fairly good agreement with previous estimates and with general models for melt volumes in craters of this size. The melt fragments within the suevite appear with a variety of shapes, textures and chemical composition, depending on the original target rock composition and degree of melting. This indicates that most melting during impact took place locally before mixing with clastic impact debris, and there never existed a large homogeneous melt sheet during crater formation. Most chemical variations within the suevite unit can be explained by incorporation of mafic rocks into a dominant mix of granitic, gneissic and quartzitic target rocks. The variations in lithic clast content in the suevite indicate mixing of material from large part of the crater. Melt fragments often appear stretched in one direction, in accordance with deposition by flow. Other melt fragments are apparently rotated and deformed, and we speculate if the mark the boundaries between successive pulses of suevite flows. In the Branden core from the central parts of the Gardnos crater a one meter thick fine-grained layer occurs between the suevite and the main sequence of post-impact sediments. This layer is lacking shocked mineral grains, lapilli and other evidence for deposition from the impact plume, and thus should probably be assigned to the post-impact succession. This indicates a brief (?) period of relative quiet conditions in the crater between deposition of the suevite and the coarse post-impact sediments. The main sequence of post-impact sediments filling in the crater depression comprises a wide range of siliciclastics: sedimentary breccias, coarse conglomerates, conglomeratic sandstones, sandstones, and interbedded fine sandstones, siltstones, and shales reflecting the shifting depositional environment. The impact probably happened in a shallow marine environment and rock avalanches and debris flows probably initiated as the crater rim was broken at its weakest parts and water entered the crater depression shortly after impact. The overlying conglomeratic and sandy sequences show significant local thickness variation, consistent with coalescing fan-shaped deposits along the lower crater wall, as well as on the crater floor. Sand-enriched density flows dominated in the water-filled crater. Above finegrained sandstones, siltstones and shales were deposited, representing the re-establishment of quiet conditions, maybe comparable to the pre-impact depositional conditions. Zircon and titanite grains have been dated by U-Pb isotope dilution - thermoionization mass spectrometry (ID-TIMS). Some zircon grains appear almost unaffected by later events, retaining close to original (>1000 Ma) ages. Concordant ages of 995-999 Ma for titanite represent late Sveconorwegian metamorphism, and concordant titanite and zircon ~380 Ma ages likely recorded the Caledonian orogeny. A large group of zircon grains have UPb ages reflecting the influence of the Caledonian orogeny and recent Pb-loss. A minor group of zircon grains yielding data with relatively high discordance for moderate U contents, including a grain with proven granular, probably impact-related, texture. Most likely the zircon subject to impact-induced deformation suffered contemporary extensive lead-loss or complete resetting. This group of zircon grains fits a discordia line with an upper intercept of 546 ± 5 Ma, suggested to be the approximate time of impact. Rocks within the Gardnos impact structure have elevated concentrations of organic carbon relative to rocks outside the structure. The carbon content and stable C-isotope values in the different impact-related lithologies (impactites and post-impact sediments) have been studied in order to establish the origin of carbon and its mobilization. The carbon probably was derived from carbon-bearing sediments overlying the crystalline basement at the time of impact. Though the carbon entered the structure during impact, the akkumulation of carbon in the overlying coarse-grained post-impact sediments indicate that re-distribution of carbon during post-impact cooling may have been significant. Later mobilization by Caledonian metamorphism probably had local limited effects.
List of papers
Paper I: Kalleson, E., Dypvik, H., and Naterstad, J. (2008) Post-impact sediments in the Gardnos impact structure, Norway, in Evans, K., Horton Jr., J.W., King Jr. D.T., and Morrow, J.R. (eds.) The Sedimentary Record of Meterorite Impacts, Geological Society of America Special Paper 437, 19-41.
Paper II: Kalleson, E., Corfu, F. and Dypvik, H. (in press) U-Pb systematics of zircon and titanite from the Gardnos impact structure, Norway: evidence for impact at 546 Ma? Geochimica et Geochimica Acta 73, 3077-3092.
Paper III: Kalleson, E., Dypvik, H., and Gilmour, I. (in prep.) Distribution of carbon in different impact-related lithologies from the Gardnos structure. 37 p.
Paper IV: Kalleson, E., Dypvik, H., and Nilsen, O. (in prep.) Melt-bearing impact breccias within the Gardnos structure. 51 p.