Tholeiitic basalts in the Precambrian of Wisconsin, USA.

The outcrop shown above (seen from the Minnesota side of the St. Croix River) is a basalt lava flow in the Clam Falls Volcanics of northwestern Wisconsin. It is equivalent to & the same age as the North Shore Volcanic Series of northeastern Minnesota and the Portage Lake Volcanic Series of northern Michigan's Keweenaw Peninsula (<a href="https://www.flickr.com/photos/jsjgeology/albums/72157632266738191">www.flickr.com/photos/jsjgeology/albums/72157632266738191</a>). The Clam Falls, North Shore, and Portage Lake successions are ~1.1 billion years old and represent basalt lava flows, plus minor sedimentary rocks, that filled up an ancient rift valley. This old rift is the Lake Superior segment of the Mid-Continent Rift System, a tear in the ancient North American paleocontinent of Laurentia (see: <a href="http://www.earth.northwestern.edu/people/seth/research/mcrfig1.jpg" rel="nofollow">www.earth.northwestern.edu/people/seth/research/mcrfig1.jpg</a>). Tectonic rifting started along this tear, exactly like the modern-day East African Rift Valley. Laurentia's Mid-Continent Rift System started and then stopped and was subsequently filled and buried. This ancient failed rift is now exposed on either side of Lake Superior in North America's Great Lakes.

The lava flow shown above consists of high-aluminum, high-iron tholeiitic basalts.

Description of the Clam Falls Volcanics in Runkel & Boerboom (2010): “A thick succesion of largely mafic volcanic rocks between the Pine Fault on the west and the Cottage Grove-Lake Owens Fault on the east in Wisconsin (Cannon and others, 2001). Outcrops near Taylors Falls consist of thick, coarse-grained, ophitic basalt flows with thick, fragmental flow tops, and thinner flows of fine-grained, intergranular basalt and porphyritic basalt. All the exposed flows contain abundant epidote and actinolite, which indicates that the flows were deeply buried (approximately 4.7 miles [7.5 kilometers]; Wirth and others, 1998) prior to uplift of the St. Croix Horst. Based on deep seismic profiles the thickness of the remaining volcanic rocks in the St. Croix Horst beneath the Clam Falls Volcanics is estimated to be approximately 5 miles (8 kilometers). The Clam Falls Volcanics underlie Paleozoic bedrock across a large expanse of southeastern Chisago County. They subcrop beneath unconsolidated Quaternary material in deep bedrock valleys, and are exposed as the uppermost bedrock in the Taylors Falls area.”

Stratigraphy: Clam Falls Volcanics (Dresser unit or Trap Rock Alley unit), Keweenawan Supergroup, late Mesoproterozoic, ~1.1 Ga

Locality: Interstate State Park, St. Croix Falls, western Polk County, northwestern Wisconsin, USA (vicinity of 47° 24' 00.09" North latitude, 92° 38' 59.88" West longitude)

Some info. from:

Boerboom et al. (2005) - Field trip 10, the western margin of the Keweenawan Midcontinent Rift System: geologic highlights of Archean, Paleoproterozoic, Mesoproterozoic, and Paleozoic bedrock in eastern Minnesota and northwestern Wisconsin. Minnesota Geological Survey Guidebook 21: 181-207.