Impact Ejecta at the Mistastin Lake Impact Structure, Labrador, Canada

Introduction: One of the principal characteristics of impact events is the formation and emplacement of ejecta deposits. The compositional and physical characteristics of ejecta provide fundamental information about the shocked nature of target rocks and thus can provide one of the best and most accessible pieces of evidence for an impact origin of a structure [1] and provide a unique way to sample the sub-surface [2]. Impact ejecta is defined here as any target material, regardless of its physical state, that is transported beyond the rim of the transient cavity [2]. The original transient cavity reaches its maximum size during the excavation stage of crater formation, before rim collapse begins in the modification stage [3]. In complex craters, during the modification stage, rocks around the periphery of the bowl-shaped transient crater collapse downward and inward to form a series of terraces along the outer margin of the crater structure [1]. Most of the ejected material is deposited near the initial transient crater rim and can thus be found within the terraced region and beyond. Impact ejecta deposits, typically poorly preserved on Earth due to post-impact erosional processes, were recently identified in the rim region of the Mistastin Lake impact structure and are the focus of this study. Mistastin Lake Impact Structure: This impact structure, located in northern Labrador, Canada (55°53’N; 63°18’W), represents an intermediate-size, complex crater (28 km diameter) formed by a meteorite impact ~36 million years ago. The original crater has been differentially eroded; however, a subdued rim and distinct central uplift are still observed [4]. The inner portion of the Mistastin Lake impact structure is covered by the Mistastin Lake and the surrounding area is locally covered by soil/glacial deposits and vegetation. The topography directly surrounding the lake is slightly elevated in plateaus extending up to 5 km away from the shoreline. The crystalline target rocks of the Mistastin Lake region are dominated by granodiorite, mangerite, and anorthosite. Locally, allochthonous impactite units including impact melt and breccias are unevenly distributed around the lake and on Horseshoe Island (Fig. 1). Previous studies of the Mistastin Lake impactites have primarily focussed on the melt rocks (e.g., [4], [5]). This project builds on these studies and further evaluates the suite of impactite rocks in terms of their context within the crater structure and origin of their components. Fig. 1: Geology map (modified from [4], [5], and [6]).