La Formation de Chibougamau, d'âge Paléoprotérozoïque, se situe aux alentours de la ville de Chibougamau (Québec, Canada). Cette formation clastique se compose de conglomérats, de grès, de diamictites et de laminites granoclassées. Les études antérieures ont conclu que cette formation est d'origine glaciaire à paraglaciaire. Cette hypothèse de genèse s'appuie notamment sur la présence de laminites granoclassées encaissant des dropstones qui sont associées à des diamictites, roches pouvant être, entre autres, formées par l'action des glaciers. Cependant, des preuves indubitables d'une origine glaciaire sont absentes : - aucun claste à surface striée et facettée n'est recensé; - le substratum archéen ne montre aucune abrasion due au passage d'un glacier; et - aucune séquence d'argilite varvée n'est présente. Cette absence d'évidences de glaciations et la découverte récente d'indices de métamorphisme de choc au sein de la Formation de Chibougamau, ont contraint à entreprendre des recherches portant sur la genèse de la formation. Les études pétrographiques ont mis en évidence de nombreux indices de métamorphisme de choc indubitablement associés à un impact météoritique. Ces indices sont : - des figures de déformations planaires affectant des minéraux de quartz et de feldspaths ; - des échardes de verre de haute pression non altérées et du verre diaplectique altéré au sein d'une matrice fragmentaire ; - de la maskélynite ; - des amygales indicatrices d'une fonte ; - des textures en mosaïque ; et - des rnicrobrèches comportant dans certains cas une matrice de verre dévitrifié. Ces nouvelles découvertes suggèrent une réinterprétation de la genèse de la Formation de Chibougamau par des processus d'impact météoritique. L'hypothèse d'un éjecta d'impact est favorisée par les observations pétrographiques, mais ne pourrait être généralisée à l'ensemble de la formation, puisque les conglomérats et certaines diamictites, porteurs d'indices de métamorphisme de choc, encaissent une grande quantité de clastes arrondis non impactés. Ces unités suggèrent ainsi un remaniement de l'éjecta source. Cette hypothèse semble soutenue par les relations stratigraphiques de la formation. En effet, à la base se trouve un conglomérat non affecté par le métamorphisme de choc qui est recouvert par une unité diarnictique, interprétée comme l'éjecta d'impact, le tout surmonté par un conglomérat similaire à l'unité basale mais porteur de quelques indices de métamorphisme de choc dérivant de l'érosion de l'éjecta sous-jacent. Ces relations stratigraphiques suggèrent donc un impact pénécontemporain aux dépôts de grande séquence conglomératique alors que la nature du dépôt d'éjecta suspecté suggère un environnement supra à intertidal. La présence de clastes de carbonate oolithique et de carbonate, dérivant probablement des roches du Groupe de Mistassini, favorise un âge maximum à 2,2 Ga. De plus, les figures de déformations n'affectant que des clastes de roches quartzofeldspathiques et la présence de fragments de carbonate à cœur de verre diaplectique favorisent un lieu d'impact en domaine continental, probablement dans un bassin intracratonique peu profond. L'absence de sphérules de verre au sein de la formation indique le caractère proximal de l'éjecta et favorise donc un lieu d'impact sur ou proche de la Province du Supérieur. ______________________________________________________________________________ MOTS-CLÉS DE L’AUTEUR : géologie, éjecta, impactite, métamorphisme de choc, Chibougamau, Abitibi.

In the northern part of Swedish Caledonides, north of Lake Torneträsk is a 7 km long exposure of a breccia layer. The layer thins westwards and eastwards from the central part where it is up to 27 m thick. It is called the Vakkejokk Breccia after the type section. The breccia has been described in literature since about a century, but its origin is enigmatic. The breccia layer is since the summer of 2012 investigated by three geologists specialized in impact craters, Paleozoic sediments, and the Caledonian orogeny. They put forward evidence for the breccia being formed by a hypervelocity impact during the Lower Cambrian at approximately 520 Ma (Ormö et al. 2017). At that time the target area was a shallow epicontinental sea that surrounded the mainly peneplanized continent Baltica. An impact into the sea is known to generate tsunami waves as well as resurge deposits when the water brings ejected and rip-up material back into the crater. Ormö et al. (2017) suggest the top part of the Vakkejokk Breccia to include such resurge deposits. The depositional marine environment is also known to rapidly protect an impact crater from further erosion. It is possible that only the topographic rim of the Vakkejokk crater was eroded during the millions of years it may have taken before the crater was covered by younger sediments. About 100 m.y. after the formation, it was completely covered by overthrust nappes during the Caledonian orogeny, when Baltica and Laurentia collided. The crater itself is not exposed today, merely parts of what is thought to be the ejecta layer and resurge deposits. This Bachelor of Science project aimed to investigate the putative resurge deposits to learn more about the process of formation and the provenance in the target of the clasts in the deposits. This was carried out by three short drillcores through the resurge deposit part of the Vakkejokk Breccia layer. The place to drill the boreholes was chosen at an outcrop which is proximal to the putative hidden crater. The retrieved drillcores were cut longitudinally, then polished and photographed in high resolution. Each core was then analyzed in an image analysis software with respect to clast granulometry and lithology. To the results are presented as graphs showing clast size, size sorting, clast shape, of the relative amounts of different lithologies and the matrix content. The results are discussed with respect to well-documented analogue marine-target craters

The recent discovery of the 31-km-wide Hiawatha impact crater has raised unanswered questions about its age, impactor and highly unusual organic carbon component. Previous research suggests a fractionated iron meteorite impactor, a probable maximum 3–2.4 Ma impact age and a possible Younger Dryas impact age. The first objective in this study has been to investigate a possible link between the Cape York meteorites and the Hiawatha impact crater by comparing the chromium isotopic signature in chromite from a Cape York meteorite with the chromium isotopic signature in potential chromite from the Hiawatha impactor. The second objective has been to investigate a possible Hiawatha signature in the Younger Dryas deposits from Baffin Bay. The third objective has been to study the organic carbon component in impactites derived from the Hiawatha impact crater. Heavy mineral grains were separated from glaciofluvial sediment which contains Hiawatha impactite grains. Not a single chromite grain was found and the possible link to the Cape York meteorites could not be tested. The petrographic examination of Younger Dryas marine deposits resulted in absence of impact-related Hiawatha grains. A petrological investigation revealed that organic carbon was likely found in five of six variably shocked impactites derived from the Hiawatha impact crater. The character of the organic carbon varies between the samples and also within individual samples. Vitrinite reflectance measurements of the organic carbon in two impactites yielded low reflectance values compared to charcoalification experiments of wood. Organic particles with different reflectance in the same sample suggest that the particles had different impact histories prior to settling and becoming a rock. Diagnostic conifer cellular texture was found in at least one of the samples. The character of the organic particles in the impactites supports the suggestion in a previous study that the sources of the Hiawatha organic carbon component are unmetamorphosed surficial deposits containing dead conifer tree trunks and fine-grained layered clay and organic matter.  In this study it is concluded that the apparent absence of chromite in the examined glaciofluvial sediment sample corroborates the significance of previous research which suggests that the Hiawatha impactor was an iron meteorite. The apparent absence of impact related grains in the Younger Dryas deposits suggests that although a Younger Dryas age for the Hiawatha impact crater is less likely now, the possibility remains open. The organic carbon with diagnostic conifer cellular texture in the Hiawatha impactites corroborates the conclusion in a previous study that the Hiawatha impact-related organic carbon component stems from local, thermally degraded conifer trees with a probable age of ca. 3–2.4 Ma. It is also concluded that the relatively low reflectance values of the organic carbon in the Hiawatha impactites seem to be related to the short duration of the high-temperature excursion during the hypervelocity impact event.