CHARACTERISTICS OF SANDSTONE DIKE INTRUSION IN THE EARLY PROTEROZOIC OF THE MARQUETTE TROUGH, MICHIGAN

On interstate 40 near Negaunee, Michigan, a roadcut through early Proterozoic Siamo Slate (Menominee Group) exposes a complex 10 cm thick sandstone (now a quartzite) dike that exhibits characteristics of being formed early in relation to post-deposition and compaction of the Siamo shales and sandstones but also synchronous with Penokean deformation and metamorphism.

The Siamo Slate outcrop strikes to the north-west and dips to the south and is mainly comprised of slate, metasiltstone, and a few quartzite beds. Several folds are observable. The sandstone dike resembles in grain size and appearance the sandstone (quartzite) beds. The dike is vertical and cuts across the dipping beds and is slightly displaced to the east along a large kink or fold in the bedding. The dike also seems to cut across the steeply dipping slaty cleavage. The dike contains quartz veins which formed along horizontal fractures.

Because the dike is vertical and the beds are dipping at a high angle it is assumed that the dike was emplaced after the beds were rotated into these high angles, and therefore syn-to post Penokean deformation. A large kink-fold originates at the intersection of a 10 cm thick sandstone bed and the sandstone dike, whereas most of the rock that the dike intrudes at this outcrop is slate and siltstone. This kink-fold also displaces the dike, supporting the claim that the deformation post-dates the dike. The dike is also fractured and extended vertically.

Sandstone dikes have been proposed to have formed in response to tectonic fluid pressures during closing of foreland basins, but it is not known how sandstone dikes could form in a greenschist facies metamorphic environment, which post-dates dewatering and lithification. Regardless, high fluid pressures are required, and may have been provided by the displacement of water from the Penokean Orogeny and closing of the Marquette trough. It may be that the dike intruded the surrounding material before metamorphism and slaty cleavage development was completed during rapid burial combined with high heat flow in the early Proterozoic.