Comparative Precambrian Stratigraphy and Structure Along the Mid-Continent Rift

Abstract

The Mid-Continent rift is a geophysically identified tectonic structure that has been traced from Kansas northeastward into the Lake Superior district. A related arm has been identified by gravity as extending from eastern Lake Superior southeastward into the lower peninsula of Michigan. This rift is of Precambrian (Keweenawan) age and began developing approximately 1.1 b.y.B.P. For most of its extent, this continental-class rift system is buried below Phanerozoic strata. Outcrops are found only in east-central Minnesota, northwestern Wisconsin, and the upper peninsula of Michigan. The most accepted tectonic model of the Mid-Continent rift is that of a central horst partly covered by clastic rocks, bounded by high-angle faulting, and flanked by basins filled with clastic rocks. This model was developed in conjunction with field studies in the southwestern Lake Superior area, and generally has been adopted for Minnesota, Iowa, Nebraska, and Kansas. A comparable model has yet to be presented for the related arm extending into southern Michigan. In the Lake Superior outcrop belt, the oldest recognized rifted rocks are layered basalts of middle Keweenawan age. Overlying these basalts is an upper Keweenawan thick sedimentary sequence composed of the Oronto (older) and Bayfield (younger) Groups. These sedimentary rocks are almost entirely clastic with texture ranging from conglomerate to shale (mudstone). Rock colors are principally reds and browns, with secondary shades of green and white. The Oronto Group is more immature mineralogically and texturally, and more complex structurally, than the Bayfield Group. The Oronto Group also is differentiated by one of its units--the Nonesuch Formation--which contains organic shales that bleed liquid hydrocarbons from subsurface fractures and vugs in a Michigan copper mine. Conditions of eposition for both groups are closely allied with an aquatic, continental environment. Specific paleogeography ranged from alluvial fan and plain, to deltaic, flood plain, fluvial, and lacustrine. Southwest and southeast of the Lake Superior outcrop belt, stratigraphic relationships are known principally from fewer than a dozen significant wells drilled into the Precambrian within the Mid-Continent rift trend. Analysis of the proposed upper Keweenawan red clastic sedimentary rock column within these wells suggests that the Lake Superior stratigraphy can be provisionally correlated throughout the Mid-Continent rift trend.