BH250-167
Title
Subject
Description
Minor Mineral: glass
The contact between the North Shore Volcanic Group (NSVG) and the underlying sedimentary rocks (Nopeming Sandsonte) exposed near Eldes Corner, Minnesota, represents a geologically significant boundary associated with the development of the Midcontinent Rift System during the Mesoproterozoic era. In this area west of Duluth, the contact is a nonconformity, where younger volcanic rocks of the NSVG overlie older metamorphosed sedimentary rocks, primarily of the Paleoproterozoic Thomson Formation.
In some zones, thermal metamorphism and baked contacts are observed, where lava flows of the NSVG were emplaced directly over wet sediments or low-grade metasediments, resulting in localized contact metamorphism.
This contact is important for understanding:
The base of the North Shore Volcanic Group. The interaction between rift-related magmatism and older Proterozoic sediments. The early stages of Midcontinent Rift volcanism.
The Nopeming Sandstone is a quartzose sandstone, ranging from subarkosic to arkosic in composition. This is well observed in sample BH250-167. At its contact with overlying basalt flows, chilled margins and baked zones are present, indicating thermal alteration of the sandstone during lava emplacement. In some areas, individual sand grains appear to have been rolled or pushed by the advancing flow and may be cemented by volcanic glass derived from the chilled margins.
Overall, this outcrop marks a key stratigraphic and tectonic unconformity in the midcontinent rift.
Coverage
GPS Coordinates: 46.719913, -92.274972