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 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 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 until approximately .
The Algoman orogeny occurred 2800 to , 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 until
Animikie Group The Animikie Basin, measuring , 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 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 .
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
The Penokean Mountain Range formed in the Penokean orogeny 1880 to 1830 million years ago, when an oceanic island arc called the Pembine–Wausau terrane collided with the southern margin of the Superior craton. From 1880 to 1850 million years ago, the region was volcanically active. This volcanism ceased 1850 million years ago when a fragment of Archean crust arrived from the South at the subduction zone. Collision of this Archean crust in the south with the Superior craton in the North caused a period of intense crustal shortening. Rocks of the Pembine–Wassau terrane were thrust up over and onto the Superior craton, forming a mountain range that covered all but the northernmost portion of Lake Superior, crossed parts of three US states (south-central Minnesota, northern Wisconsin, and the Upper Peninsula of Michigan), and continued to the southernmost tip of Ontario, Canada. Loading of the lithosphere by these thrust sheets caused it to flex downwards, forming a foreland basin at around 1850 million years ago in the south and 1835 million years ago in the north. The Rove Formation was deposited in the northern basin. In the southern basin, crustal thickening caused high-grade metamorphism of the sedimentary fill by 1830 million years ago. A series of post-orogenic plutons intruded into the overlying rocks 1830 million years ago,; these plutons mark the end of the Penokean orogeny.
Sudbury impact ejecta At the base of the Rove Formation, between the Rove and the underlying Gunflint Iron Formation, there is a lateral layer of shocked quartz and feldspar grains found within accretionary lapilli, accreted grain clusters and spherule masses. These pieces of debris indicate that the layer contains hypervelocity impact ejecta. Zircon geochronologic data shows that this layer formed 1878 to 1836 million years ago; the Sudbury Impact event occurred . Because of the closeness in dating and the nearness of the crater, the Sudbury Impact event is the likely source for the ejecta; these are the oldest ejecta linked to a specific event on Earth. In the Rove area this layer is about thick; this thin layer very likely represents the catastrophic events of a single day nearly .
Evidence indicates a diameter meteorite collided with Earth in the current-day vicinity of Sudbury, Ontario, Canada, about . The meteorite vaporized and created a wide crater (this is the second-largest impact depression on Earth). This impact is east of the Minnesota-Ontario border of the Rove Formation. Earthquakes shattered the ground hundreds of miles away and within seconds ejecta (cloud of ash, rock fragments, gases and droplets of molten rock) began to spread around the globe. It is estimated that at ground zero the earthquake would have registered 10.2 on the Richter scale. Seas covered the Rove Formation area and the Sudbury impact generated huge tsunamis.
To put the Sudbury meteorite impact in perspective, the Chicxulub impact on the Yucatán Peninsula occurred from an object perhaps 60% the size of the Sudbury impactor; the results of this impact caused the worldwide extinction of many species (including dinosaurs).
The Sudbury Impact would have had global ramifications; it is conjectured that this caused the end of the iron deposits. The impact fundamentally affected concentrations of dissolved oxygen in the sea; the accumulation of marine sediments (known as banded-iron formations) were almost instantaneously shut down. Banded-iron formations are massive deposits rich in iron oxides; they accumulated at several periods in the Earth's geologic past. One extended episode of banded-iron formation buildup suddenly ended about . In northeastern Minnesota these banded-iron formations lie immediately under the ejecta layer. Minnesota's Iron Range is composed of this layer of banded-iron formation.
Most of the impact layer in the Rove area consists of beccia, a mixture of rock fragments which ripped loose from the sea floor during the earthquakes. The tsunamis jumbled the loosened bedrock and ejecta together; over time this layer was buried by younger sediments, cemented together and fused by molten rock to form a solid layer. Ejecta from the Sudbury Impact was found in May 2007 on the Gunflint Trail in Cook County, Minnesota. Geologists Mark Jirsa and Paul Weiblen from the University of Minnesota took advantage of the burnt-over landscape resulting from the intense, hot Ham Lake fire to explore the newly exposed geology along the Gunflint Trail. Jirsa picked up some rocks which turned out to be ash and debris from the distant impact site; this is the farthest distance that Sudbury detrita has been found.
Midcontinent Rift
The Midcontinent Rift (also known as the Keweenawan Rift) began about ; it lasted for about 20 million years. After the Penokean Mountains had eroded away, the proto-North American continent nearly split in half along this rift zone. The bow-shaped rift extended from northeastern Kansas, through the southeastern corner of Nebraska, diagonally northeast through Iowa, through Minnesota along the current Minnesota-Wisconsin border, arced through the present-day Lake Superior basin and angled southeasterly through Michigan.
The Midcontinent Rift is the largest-known continental rift in the world. The rift began as a hot spot of basaltic magma underneath the Lake Superior region; it extruded layers of lava up to thick and extending up to on either side of the rift. The deposited lava along the North Shore of Lake Superior is thick.