VARIABLE THICKNESS OF THE IMPACT MELT SHEET AT MISTASTIN LAKE CRATER, NORTHERN LABRADOR

Mistastin Lake crater of northern Labrador is a 28-km wide near-circular structure formed by a meteorite impact 35-40 million years ago. The impact stratigraphy consists of underlying fractured and shock metamorphosed targets rocks, monomict to polymict impact breccias intermittently overlain by the impact melt sheet. The target rocks from which the impact melt was produced consist principally of anorthosite and mangerite (hypersthene monzonite). Mistastin may be the only terrestrial impact in which anorthositic rocks make up a majority of the target, and therefore it may be the best terrestrial analog for lunar impact melt formation.

The impact melt sheet crops out irregularly around the shore of the western two thirds of Mistastin Lake and at one location on the western shore of Horseshoe Island, the central uplift of the crater. The preserved thickness of the melt rock units varies significantly with location, and appears to be inversely related to the size and abundance of vesicles contained in the melt rock. Along the north side of the lake, at Steep Creek, the exposed melt sheet is about 6 m thick, though it may be much thicker as the base is not exposed; it has mm-scale spherical vesicles. At Cote Creek, further west, the basal contact of an 18 m thick melt sheet with abundant mm- to cm-scale spherical vesicles is exposed. A particularly distinctive butte on the west side of the lake, known as Discovery Hill, displays the thickest outcrop of the melt sheet, roughly 80 meters, with two tiers of columnar joints. This fraction of the melt contains few vesicles, with widths that never exceed 2 mm. A ridge just south of Discovery Hill consists of poorly vesiculated melt, 25 meter thick. Several dispersed melt outcrops are located all along the southern shore of the lake; these outcrops are never more than 1 meter thick and contain large elongated vesicles up to 25 cm in diameter. These rocks resemble pumice. Because the vesicle-rich melt rocks must have cooled more quickly than the more massive melt rocks, their predominance in thin melt unit outcrops at Mistastin suggests that the preserved melt sheet variations are primary features of the impact rather than the result of differential erosion by glaciers. Thicker parts of the Mistastin melt sheet such as at Discovery Hill may have formed by channelling of flow along preferred conduit pathways.