Animikie Group

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The Animikie Group is a group of supracrustal sedimentary and subordinate volcanic rocks up to 10,000 meters thick , which was deposited in the Animikie Basin in the course of the Paleoproterozoic in central and northeast Minnesota .

introduction

Geological map with the Animikie Group in Minnesota, the Marquette Supergroup in northern Wisconsin and Michigan, and the Huronian Supergroup in Ontario

After completion of the Algoman Orogeny at the output of Archean to 2.6 billion years BP spacious had erosion used that the neuentstandene mountain building again completely einebnete to 2.5 billion years BP. The former mountain suture, the Great Valley Tectonic Zone (abbreviated GVTZ) developed under the influence of crustal expansion between 2450 and 2100 million years BP into a rift zone . The sedimentation accompanying the rift process documents the transition from deposits in the shelf area to deep sea conditions . It can be divided into two major cycles by the invading magmatism. The first cycle was ended by the magmatism as it had spread on the surface. It was not until the oceanic crust was formed that the crustal area subsided gravitationally and the second sedimentation cycle was deposited. From 1900 million years BP the Minnesota River Valley sub-province approached from the south , the Animikie Basin was pushed together and the sedimentation tore off. The peak of convergence was reached around 1850 million years BP with the Penokean orogeny . The southern part of the basin was thereby deformed and metamorphosed , while the northern edge largely escaped metamorphic changes.

stratigraphy

The iron ore formations around Lake Superior

The rocks of the Animikie Group lie discordantly above the archaic basement in the north of the Animikie Basin. On the southern edge they also overlap discordantly the Cuyuna North Range Terrane , the Cuyuna South Range Terrane and the Moose Lake-Glen Township Terrane . In the southeast, the basin is in turn covered by the younger, Mesoproterozoic volcanic rocks of the Keweenawan Supergroup . The Duluth Gabbro intrusion divides the Animikie Basin into two parts, the Mesabi Range in the southwest and the Gunflint Range in the northeast. The Gunflint Range shows the following stratigraphic structure (from hanging to lying):

The Mesabi Range is structured as follows:

In addition to these formations of the Animikie Basin in the narrower sense, the older formations of the Mille Lacs Group , the North Range Group of the Cayuna North Range Terrane and the South Range Iron Formation of the Cayuna South Range Terrane are added in the eastern central part of Minnesota , as they are also important Deliver quantities of comparable iron ores.

Gunflint Range

Geological map of the extreme northeast of Minnesota. The Rove Formation (prv) is located immediately south of the Canadian border

The Gunflint Range begins in the extreme northeast of Minnesota and continues in a northeastern direction into Ontario. The 1099 million year old Duluth Gabbro separates it spatially from the Mesabi Range following further south-west. The sediments of the Gunflint Range start with a transgressive over the archaic bedrock based conglomerate to which the banded iron formation of the with 1.878 billion years BP dated Gunflint Iron Formation follow. The ejecta of the Sudbury Astroblem is a specialty. They cover the ore formation and could be dated to 1849 million years BP.

The clayey rove formation formed in deep water is deposited over the ejecta . The Gunflint Iron Formation forms a northeasterly trending band, a maximum of 8 kilometers wide, the thickness of which varies between 135 and 170 meters. Since the sediments of the Gunflint Range escaped metamorphosis, they are among the oldest, undeformed and non-metamorphic rocks in North America. The rove formation, for example, lies flat and was only penetrated much later by corridors and tunnels of the central continental rift system .

Mesabi Range

The Mesabi Range extends over 320 kilometers in a northeast-southwest direction. It can be up to 10 kilometers wide, but the average is 4 kilometers. Their thickness varies between 110 and 240 meters. Their sequence of layers begins with a base conglomerate, above which the Pokegama Quartzite , which is up to 153 meters (on average 60 meters) thick, follows. This formation, created in shallow water, consists of sandstones , siltstones and shale clays . The overlying Biwabik Iron Formation , which is 1900 to 1850 million years old, varies in thickness between 60 and 600 meters, with an average value of around 305 meters. It consists mainly of chert and contains up to 50% iron (mainly as hematite and secondary goethite ) and less than 10% silicon . They have been dismantled since the beginning of the 20th century . Their reserves are in the millions of tons. The formation is divided into four members:

  • Upper Slatey Member
  • Upper Cherty Member (with ore mining)
  • Lower Slatey Member
  • Lower Cherty Member (with ore mining)

On the eastern edge, the Biwabik Iron Formation is overprinted with contact metamorphosis by the Duluth Gabbro . It is followed by the 1850 million year old Virginia Formation , which, however, does not appear anywhere above ground, but is only known from drill cores. It is composed of claystones and underlies the Duluth gabbro in the area of Ely and Hoyt Lakes .

Cayuna North Range Terrane

Geological map of the Cuyuna Range

The North Range Group can be divided as follows (from hanging wall to horizontal):

Discordant above the North Range Group is the Thomson Formation deposited between 1880 and 1870 million years BP (minimum age 1770 million years BP).

The Cayuna North Range Terrane was subject to a regional metamorphosis during the Penokean Orogeny between 1870 and 1850 million years BP, which, for example, transformed the original greywacke , siltstone and clays of the overlying Thomson Formation into shale .

The Mahnomen formation at the base consists of two members, the top of which is iron-bearing. The subsequent Trommald Formation , 14 to 150 meters thick , is the principal iron formation of the Cayuna North Range. It was created by chemical precipitation and is made up of iron oxides (hematite), iron (II) carbonate , iron silicates and associated manganese oxides . The deposit environment of the iron formation was an eastward trench, deepening to the south, with significant hydrothermal activity. The Rabbit Lake Formation of the hanging wall , which was formed under Euxinian conditions , is a gray-wacke-slate-clay sequence with lens-like iron inclusions. In its lower section, it has layers of volcanic origin.

Cuyuna South Range Terrane

The Cuyuna South Range Terrane is separated from the North Range Terrane by a significant tectonic fault (thrust), the Serpent Lake Discontinuity . It contains the South Range Iron Formation , which has been stripped of its original stratification. The Mille Lacs Group is tectonically separated from the iron formation.

Mille Lacs Group

The Mille Lacs Group is tectonostratigraphically the deepest unit. For example, it is separated by a discordance from the overlying North Range Group, on whose northwestern edge it comes to the surface, or by a tectonic contact from the South Range Iron Formation. It also lies beneath the Moose Lake-Glen Township Terrane and should also be found underground south of the Malmo Discontinuity . The rocks of the Mille Lacs Group represent a large part of the fold and thrust belt formed by the Penokean Orogeny. They were strongly deformed tectonically (in contrast to the rocks of the Thomson Formation with only one folding phase they are characterized by two folding phases) and metamorphosed. Because of the deformations is not a continuous sequence can be created for the Mille Lacs Group, however, it generally consists of Ortho quartzites, limestones and dolomites ( Trout Lake Dolomite ), sulfide-containing, carbon-rich shales, iron formations and mafic pillow lava ( GlenTownship formation , Denham formation ).

Dating

Vulcanites at the base of the Mille Lacs Group yielded strontium - neodymium age of 2197 ± 39 million years BP.

However, the ages of the Animikie Group in the narrower sense generally range between 2125 and 1821 million years BP, with the 2125 million years BP representing a maximum age of the Pokegama Quartzite, which was established on the basis of the northwest-trending Kabetogama gang swarm. The minimum age for the Pokegama Quartzite is 1930 ± 25 million years BP. A layer of ash in the upper section of the Gunflint Iron Formation could be dated to 1878 ± 2 million years BP and the aforementioned ejection position of the Sudbury Astroblem on the upper edge of the same formation was 1849 million years BP. The age of 1821 million years BP refers to a layer of ash within the Rove Formation (70 meters above its base). The youngest known age of 1780 million years BP for the Animikie Group comes from a sandstone layer at the upper limit of the Rove Formation (400 meters above its base) and documents continued sedimentation for the northern edge of the basin even after the Penokean Orogeny had subsided (end of around 1830 million years ago BP).

See also

Individual evidence

  1. ^ Southwick, DL and Morey, GB: Tectonic imbrication and fore-deep development of the Penokean orogen, east-central Minnesota - an interpretation based on regional geophysics and the results of test-drilling . In: US Geol. Surv. Bull. Band 1904 , 1991, pp. 17 .
  2. ↑ Fralick, PW, Kissin, SA and Davis, DW: The age and provenance of the Gunflint lapilli tuff (para.) . In: 44th Inst. Lake Superior Geol. 1998, p. 66-67 .
  3. ^ Davis, Donald W .: Sub-million-year age resolution of Precambrian igneous events by thermal extraction-thermal ionization mass spectrometer Pb dating of zircon: Application to crystallization of the Sudbury impact melt sheet . In: Geology (Geological Society of America) . tape 36 (5) , 2008, pp. 383-386 .
  4. ↑ Melcher, F. et al .: Hyrdrothermal systems in manganese-rich iron formation of the Cuyuna North range, Minnesota: geochemical and mineralogical study of the Gloria drill core . In: Minn. Geol. Surv. Rept. Invest. tape 46 , 1996, pp. 59 .
  5. ↑ Beck, JW: Implications for early Proterozoic tectonics and the origin of continental flood basalts, based on combined trace element and neodymium / strontium isotopic studies of mafic igneous rocks of the Penokean Lake Superior Belt, Minnesota, Wisconsin and Michigan (doctoral thesis) . University of Minnesota, Minneapolis, MN 1988, pp. 262 .
  6. ^ Hemming, SR, McLennan, SM, Hanson, GN and Krogstad, KM: Pb isotope systematics in quartz [abs] . In: Eos . tape 71 , no. 17 , 1990, pp. 654-655 .
  7. ^ Kissin, SA, Vallini, DA, Addison, WD and Brumpton, GR: New zircon ages from the Gunflint and Rove Formations, northwestern Ontario [abs.] In: Institute on Lake Superior Geology, 49th Annual Meeting, Iron Mountain, Mich. , Proceedings . tape 49 , no. 1 , 2003, p. 43-44 .
  8. ↑ Schulz, KJ and Cannon, WF: The Penokean orogeny in the Lake Superior region . In: Precambrian Research . tape 157 , 2007, p. 4-25 .