The Rove Formation is a sedimentary rock formation of Middle Precambrian age underlying the upper northeastern part of Cook County, Minnesota, United States, and extending into Ontario, Canada. It is the youngest of the many layers of sedimentary rocks which constitute the Animikie Group. Before the Rove sediments were laid down, during the Archean Eon, the Algoman orogeny added landmass along a border from South Dakota to the Lake Huron region; this boundary is the Great Lakes tectonic zone. Several million years later a thin layer of hypervelocity impact ejecta from the Sudbury impact event was deposited on the older, underlying, Gunflint Iron Formation, and the Rove was then deposited on top of the ejecta; it is estimated that at ground zero the earthquake generated by the meteor impact would have registered 10.2 on the Richter scale. During the Middle Precambrian a shallow inland sea covered much of the Lake Superior region and formed the Animikie Group of sedimentary rocks overlying 2700-million-year-old Archean rocks. After the Rove Formation sediments were deposited, the Penokean orogeny added more land mass by accretion that occurred from the south. A few hundred million years later the proto-North American continent nearly split in half along the Midcontinent Rift, which is a bow-shaped rift extending from northeast Kansas, arcing through the present-day Lake Superior Basin and then angling southeast through Michigan. Then came a period of advancing and retreating glaciers. The more resistant diabase sills and dikes remained, while the softer shales were bulldozed away by the glaciers. The north path of glaciation is transverse to the general trend on the valleys and ridges. As a result of erosion of sandstone and the erosion-resistant sills and dikes, the topography in Minnesota has repeated parallel hills and valleys. The tightly packed lakes in the narrow valleys are long and narrow, and they orient from the east to west. The cliffs in these narrow valleys are the habitat to several rare plants which prefer living in narrow cliff areas in a sub-Arctic climate. In Ontario the Rove Formation is overlain by a thick diabase cap.
Location
The Rove Formation is in the Arrowhead Region of northeastern Minnesota, U.S., and extends into Ontario, Canada. In Minnesota it occurs along the U.S.-Ontario border from Gunflint Lake to Pigeon Point (both in northeastern Cook County) and northward into Canada. Pigeon Point is the most eastern part of Minnesota; it is a diabase sill about 152 m (500 ft) thick. Both the north and south coasts of the point expose Rove slates under and over the sill. Within the sill, rates of cooling and gravity have created an interesting distribution of rock types. The visible formation is in Minnesota and contains many east–west oriented ridges and valleys. Many lakes in this 5 to 8 km (3 to 5 mi) wide band along the Canada–US border are in the elongated east–west valleys; included are Caribou, Clearwater, Crocodile, Daniels, Duncan, Dunn, Hungry Jack, Iron, Loon, Moose, Pine, Portage and South. In Ontario the Rove Formation is overlaid by a thick diabase sill.
Geologic history
Archean Eon
The Archean Eon lasted from 3,800 million years ago until approximately 2,500 million years ago.
The Algoman orogeny occurred 2800 to 2,500 million years ago, and it marks the end of the crust-building Archean Eon. There were several episodes of continental collision, compression and subduction which resulted in mountain building during this time. Orogenic events are characterized by extensive metaphorism, granitic extrusions and unconformities. The Algoman orogeny added landmass along a border from South Dakota to the Lake Huron region; this boundary is the Great Lakes tectonic zone (GLTZ). Northeast Minnesota has 2700-million-year-ago exposed rocks formed during volcanic activity that was in the form of seepage of lava from rifts in the sea floor. These lava flows began to rise up out of the ancient ocean to form the Superior Craton; the Superior craton later assembled into the Canadian Shield, which became part of the North American craton. The Superior province is the largest preserved fragment of Archean crust, and the Canadian shield is the nucleus of the North American craton.
Proterozoic Eon The Proterozoic Eon lasted from 2,500 million years ago until 570 million years ago
Animikie Group The Animikie Basin, measuring 700 km × 400 km (430 mi × 250 mi), is an elongated oval straddling the North Shore of Lake Superior, mainly in Minnesota. Approximately the northwestern two-thirds lies to the northwest of the shoreline; the southeastern third lies to the southeast of the shoreline (so is under Lake Superior's waters). During the Middle Precambrian a shallow inland sea covered much of the Lake Superior region and formed the Animikie Group, which are layers of sedimentary rocks which unconformably overlies 2700-million-year-old Archean rocks. This group contains both the Rove and Gunflint Iron formations. The Rove Formation is the youngest of the many Animikie layers; it consists of gently tilted fine-grained sediments. It is composed of greywackes and black shale, and contains lower concentrations of iron and taconite than the underlying Gunflint Iron Formation does. The Rove Formation consists of a lower argillite unit, a middle transition unit and an upper thin-bedded greywacke unit. The lower argillite unit is about 150 m (490 ft) thick; this layer contains fine-grained greywacke, and silty and graphitic argillites. Greywacke is a sedimentary rock composed of a mixture of poorly sorted grains of sand, silt and clay particles. Argillite is a fine-grained sedimentary rock composed primarily of clay particles; they are essentially lithified muds and oozes. Greywacke is abundant in the middle unit and dominates the upper unit. The complete thickness of the upper two units is about 900 m (3,000 ft). Gunter Faure and Jack Kovach, using Rb-Sr dating, determined the age to be 1635 ±24 million years old. The Resident Geologist Program, Geology of the Thunder Bay South District, reports an age of 1800 million years old.
Penokean orogeny
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