Bevan M. French

THE GARDNOS IMPACT STRUCTURE, NORWAY : PETROLOGY AND GEOCHEMISTRY OF TARGET ROCKS AND IMPACTITES

Abstract The Gardnos structure, Norway is an approximately circular area of anomalously fractured and brecciated rock, about 4.5 km in diameter, emplaced in a metamorphic terrane composed chiefly of granitic gneisses with minor amphibolite and quartzite. The original recognition of Gardnos as a deeply eroded impact structure between 900 and 400 Ma old has been followed up by detailed petrographic and chemical studies of approximately thirty samples of target rocks and various types of shocked rocks (impactites). Deep erosion of the structure has erased the original rim, removed much of the crater-fill deposits, and exposed large areas at or near the original crater floor. However, a wide variety of distinctive impactites —fractured and blackened quartzites in the sub-crater basement rocks, lithic breccias, and melt-bearing breccias—are still preserved. These impactites show petrographic and chemical characteristics that confirm an impact origin: distinctive Planar Deformation Features (PDFs) in quartz and feldspar, incipient melting of feldspar clasts in the melt-bearing breccias, close matches between the chemical composition of the breccias and mixtures of the target lithologies, and the detection of an extraterrestrial component. A minor extraterrestrial component (≤0.15%) was detected in the melt-bearing breccias, based on significantly elevated Ir and Os contents and lower 187Os/188Os ratios compared to those in the target rocks. The Gardnos impactites are significantly enriched in C (5–10X) over the exposed target rocks. This may reflect the presence of a C-rich shale overlying the metamorphic basement at the time of impact; this idea is supported by δ13C values of −28.1 to −31.5%. measured in the impactites. Mixing calculations show that the chemical compositions of the impactites can be reproduced by mixtures of target rocks ranging from approximately 60–90 wt% granite gneiss, 0–30 wt% amphibolite, 0–12 wt% quartzite, and 3–19 wt% of a C-rich shale component. The deeply eroded state of the structure and the preliminary state of detailed geologic mapping make crater reconstruction difficult. One possible scenario involves the impact of a 300 m diameter stony meteorite that released 1019 J of energy and formed a transient cavity 3 km in diameter that evolved to a complex crater 5 km in diameter with a central uplift of about 350 m. The original crater was filled with at least 0.3 km3 of allochthonous melt-matrix breccias containing about 0.06 km3 of impact melt. The structure underwent low-grade (greenschist?) metamorphism in Caledonian time (about 400 Ma ago) and was subsequently eroded to its present appearance. The value of the Gardnos structure for further cratering studies lies in its easy access to large areas of the original crater floor zone, in the preservation of a possibly complete sequence of crater-fill breccias beneath a cap of elastic sediments, and in the unusual carbon enrichment of its impactites.

The Rock Elm meteorite impact structure, Wisconsin: Geology and shock-metamorphic effects in quartz

The Rock Elm structure in southwest Wisconsin is an anomalous circular area of highly deformed rocks, ∼6.5 km in diameter, located in a region of virtually horizontal undeformed sedimentary rocks. Shock-produced planar microstructures (PMs) have been identified in quartz grains in several lithologies associated with the structure: sandstones, quartzite pebbles, and breccia. Two distinct types of PMs are pres ent: P1 features, which appear identical to planar fractures (PFs or cleavage), and P2 features, which are interpreted as possible incipient planar deformation features (PDFs). The latter are uniquely produced by the shock waves associated with meteorite impact events. Both types of PMs are oriented parallel to specific crystallographic planes in the quartz, most commonly to c (0001), ξ{11\batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \(\overline{2}\) \end{document}2}, and r / z {10\batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \(\overline{1}\) \end{document}1}. The association of unusual, structurally deformed strata with distinct shock-produced microdeformation features in their quartz-bearing rocks establishes Rock Elm as a meteorite impact structure and supports the view that the presence of multiple parallel cleavages in quartz may be used independently as a criterion for meteorite impact. Preliminary paleontological studies indicate a minimum age of Middle Ordovician for the Rock Elm structure. A similar age estimate (450–400 Ma) is obtained independently by combining the results of studies of the general morphology of complex impact structures with estimated rates of sedimentation for the region. Such methods may be applicable to dating other old and deeply eroded impact structures formed in sedimentary target rocks.

Geochemistry of carbonaceous impactites from the Gardnos impact structure, Norway

Abstract The Gardnos impact structure in southern Norway is one of only two known impact structures (among ∼175) whose impactites contain significant amounts (typically 0.2–1.0 wt.%) of carbon, or 5 to 10 times the amount present in the target rocks; Sudbury, Canada is the other. This study extends a previous investigation of the geochemistry and petrology of Gardnos impactites (French et al., 1997) with additional sampling and a detailed investigation of the nature and possible origin of the carbonaceous material present. Two principal carbon components have been identified in Gardnos impactites: (1) impact-produced diamonds, 0.5 to 1 μm in size, with a cubic crystal structure, predominantly hexagonal morphologies with platey layers and an estimated concentration of Geochemical data suggests that there are no suitable target rocks that could provide a single source for the carbon in Gardnos impactites. However, Raman spectroscopy, stable isotope analysis and transmission electron microscopy of the impact diamonds and graphitic carbon suggests that there were at least two episodes of C emplacement in Gardnos impactites: an impact-related incorporation and shock transformation of graphitic material from target rocks followed by later mobilization of C, possibly during postimpact cooling or later regional metamorphism.

A TEM INVESTIGATION OF SHOCK METAMORPHISM IN QUARTZ FROM THE SUDBURY IMPACT STRUCTURE (CANADA)

Abstract The Sudbury basin in Canada is an elliptical feature (approx. 60 × 27 km in size) that is now widely believed to be part of a large (approx. 200 km diameter) meteorite impact structure which formed about 1.85 Ga ago and was subsequently deformed and metamorphosed. Despite prolonged debate over the origin and original size of the Sudbury structure, strong evidence for meteorite impact has been provided by the discovery in its rocks of a wide range of shock-metamorphic features, especially the optically observable traces (fluid inclusion arrays) of former Planar Deformation Features (PDFs) in quartz parallel to {10 1 3} planes. In this new examination of Sudbury samples, no preserved original PDFs (glassy lamellae) were observed optically in quartz grains from basement rock fragments in the Onaping Formation, a unit interpreted as a ‘fallback breccia’ deposited within the original crater. However, TEM revealed other thin lamellar features preserved in the quartz; these are identified as Brazil twin lamellae parallel to the basal plane (0001). These lamellae are 15–200 nm thick, show a typical spacing of 30 nm to several microns apart, and are occasionally decorated with fluid inclusions (≤ 0.5 μm in size), which probably formed during post-shock alteration and annealing. Numerous subgrain boundaries (SGBs) were also detected, many of them oriented roughly parallel to the basal plane (0001). The SGBs apparently formed during post-shock recrystallization, leading to the local disappearance of Brazil twin lamellae. Experimental evidence suggests that such basal Brazil twins are the unique product of high-pressure shock waves. The features in the Sudbury samples are identical to those observed in the Vredefort structure, South Africa, which is also widely accepted as an ancient impact structure about 2.0 Ga old. The recognition of basal Brazil twins in quartz at Sudbury provides additional evidence for meteorite impact origin and also emphasizes the high value of these durable shock features for identifying old and tectonically metamorphosed impact structures.