Keweenawan Rift System

Summary
 The Keweenawan Rift System is an Early Proterozoic, tectonic event that is very popular for geologic studies.  Nearly 1.109 billion years from the present, a basaltic magma upwelling began to rift the Lake Superior Craton.  This magmatic upwelling or hotspot is very similar to what is found at Yellowstone National Park.  The hot spot created a dome that covered all of the Lake Superior region.  A linear belt of subsidence  was produced along the rift valley and in turn 25 million years of volcanism followed.  Many basalt flows erupted from the central rift axis, similar to the expansion of the mid-ocean ridge in the Atlantic Ocean.  Early basaltic flows were found to be pillowed and displayed evidence for the rift being once underwater. Subsequent flood basalts cover lower pillow basalts  and ranged in thickness up to 3 miles thick.  Maximum total thickness of the basalt flows ranged from 2 to12 miles in thickness, while the rift itself extended over 100 miles on either side.  It is also noteworthy that these early flows exhibit a paleomagnetic signature indicating a magnetic  reversal of the Earth’s poles.

 Rifting continued and eventually caused the thick basaltic units to sink.  The majority of the subsidence occurred after the volcanics stopped (~1084ma.).  Sedimentation and erosion quickly followed and began to fill the rift valley.  After deposition of the Oronto Group, a thick sedimentary package, the region underwent a closure of the once extensional rift valley.  The failed continental break-up was signified by compressional forces that caused normal fault blocks in the rift valley to reverse and uplift.  Large horsts of volcanic rock were thrusted on top of the sedimentary rocks (Oronto Group) that were formally deposited on top of the volcanics.  These features were indicators of the compressional forces subjected to the Lake Superior region and marked the end of the once extensional Keweenawan Rift System.
 

Interstate Park  (St. Croix Falls, WI., Taylor Falls, MN.)
  •  Amygdules produced by filling of basaltic vesicules with secondary minerals.  Minerals found in amygdules vary but calcite, epidote, feldspars and zeolites are most common.
  •  St. Croix River has spectacular basalt relief from the uplift of the faulted blocks and rift-valley horst.
  •  Glacial striations are found on some of the exposed basaltic rocks.
  •  Earlier increased discharges of the St. Croix River channel from glacial melting episodes allowed for large boulders to be transported.  The increased glacial discharge was turbulent enough to allow the boulders to erode underlying bedrock. 
  •  The boulders and turbulent motion of the river formed whirlpools as they carved potholes in to the underlying bedrock.  These potholes are now visible in detail due to a decrease in discharge and surface water elevation.  Some of these whirlpools are still evident in the St. Croix River.
  Copper Falls State Park (Mellen,  WI.)
  •  Keweenawan basalts are found to form the relief of  two rivers as they meet at a gorge.  Tyler's Forks, a branch of the Bad River meets up with the main channel to form 60-100 foot waterfall named Brownstone Falls.
  • Overlying the basalts are tilted sedimentary strata of the Oronto Group.  This group consists of the Copper Harbor Conglomerate, Nonesuch Shale and Freda Sandstone. The Oronto Group can be seen along the trails at Copper Falls State Park.
  •  The rivers and water nearby are heavily concentrated with iron carried in solution from the dissolution of mafic basalts and groundwater in the region.
  •  This is a picturesque park that early Native Americans revered.  I highly recommend visiting this location for camping or outdoor activities.

 
   Caledonia Mine  (SW of Maas City, MI.)
  •  Heated waters formed from contact of groundwater with underlying high temperature areas of the lower crust and associated Keweenawan Rift System.  The hydrothermal water allowed for dissolution of minerals. 
  •  Transportation of minerals in hydrothermal solutions to other areas was common through faults, fracture systems, or other permeable geologic pathways.  This enabled for precipitation of minerals at locations that have lower water temperatures.
  •  The mine contains precious silver and copper deposits.  These are native minerals and contain fewer impurities than other copper sulfides.  The mine is often requested for locating ideal copper samples.