Impactites of the Mistastin Lake impact structure: Insights into impact ejecta emplacement

Abstract

The Mistastin Lake impact structure is an intermediate-size (~28 km apparent crater diameter), complex crater formed ~36 Myr. The original crater has been differentially eroded; however, a subdued terraced rim and distinct central uplift are still observed and impactites are well exposed in three dimensions. The inner portion of the structure is covered by Mistastin Lake and the surrounding area is locally covered by soil/glacial deposits and vegetation. The crystalline target rocks of the Mistastin Lake region are dominated by anorthosite, granodiorite, and quartz monzonite. Previous studies of the Mistastin Lake impactites have primarily focussed on the impact melt rocks. This study further evaluates the entire suite of impactite rocks in terms of their location within the crater structure and emplacement mechanisms. Locally, allochthonous impactite units including impact melt and various types of breccias are distributed around the lake in the terraced rim and are interpreted as proximal ejecta deposits. A multistage model for the origin and emplacement of impact melt rocks and the formation of impact ejecta is proposed for the Mistastin Lake impact structure based on a synthesis of the field and petrographic observations. This model involves the generation of a continuous ballistic ejecta blanket during the excavation stage, followed by the emplacement of melt-rich, ground-hugging flows during the terminal stages of crater excavation and the modification stage of crater formation. Impact melt-bearing breccias—also termed “suevite” at other sites—are present in several distinct settings within the Mistastin Lake structure and likely have more than one formation mechanism.