Summary of Precambrian geologic events in Minnesota

This summary will be useful throughout days 12-15. You should refer to it frequently to keep track of the sequence of Precambrian geologic events

Summary of important events in Earth's Proterozoic History - helps to place Minnesota's geologic history into a broader perspective

Introduction

Early Proterozoic time (the Middle Precambrian) extends from 2.5 Ga to about 1.6 Ga and includes the development of Minnesota's iron formations which have so profoundly affected the entire socio-economic-political history of the state.

Rocks of this age are mainly found in a sedimentary basin in east-central and northeastern Minnesota, in the area of the Mesabi, Gunflint and Cuyuna iron ranges.

Five groups of rocks are present, listed here from oldest at the top to youngest at the bottom

• poorly preserved dolomite and quartz sandstone (will not be discussed further)
• quartz sandstones metamorphosed to quartzites
• iron formations
• dark-colored sandstones and mudstones (the latter metamorphosed to black slates)
• granite batholiths intruded into the older rocks

See the accompanying cross section for the geologic relationships between these rocks and the underlying Archean rocks.

Erosion interval between Algoman orogeny and Early Proterozoic deposition

• erosion by running water
• erosion by glaciers
  • dates on tillites bracketed by igneous rocks between 2.4 and 2.1 Ga
  • no tillites of this age are preserved in Minnesota, but are preserved in Wyoming, upper Michigan, and Ontario (see accompanying map), suggesting a single ice cap of large extent
  • if tillites of this age are present in the subsurface of east-central Minnesota, it would be very important, as conglomerates of this same age in Ontario, also derived from long weathering and erosion of the underlying Archean rocks, have important uranium deposits
  • the uranium-bearing conglomerates in Ontario were deposited under an atmosphere with no free oxygen, indicated by the fact that iron and uranium in the minerals of these rocks are in a reduced, not an oxidized, state

Quartz sandstones metamorphosed to quartzites

• The Pokegama Quartzite and associated mudstone lies atop the Archean rocks in the Mesabi Range. On the other ranges, this quartzite has different names.
• 300+ feet thick
• rests on a very low relief erosion surface
• Quartz grains in the Pokegama and equivalent rocks are well rounded and sorted, and no unstable minerals are present.
  • This is the result of prolonged intense weathering during the erosion interval following the Archean.
  • The mudstone interbedded with the sandstone is also the result of prolonged chemical weathering and breakdown of unstable silicate minerals to clay.
• Cross bedding and other sedimentary features suggests a tidal flat environment of deposition in an inland sea for these early Proterozoic quartzites

Iron Formations

Iron formations of about 2.0 Ga overlie the basal sandstone unit on all the ranges - the Biwabik Iron Formation on the Mesabi, the Gunflint Iron Formation on the Gunflint, and the Trommald Iron Formation on the Cuyuna.

• the distribution of iron formations is indicated by the gravity and magnetic maps of Minnesota. The presence of iron results in a more dense and more magnetic bedrock, that can be sensed by doing surveys of the strength of the magnetic and gravity fields across a region.
• made of chert (a hard, cryptocrystalline siliceous sedimentary rock like flint) together with the iron oxides hematite, magnetite and other iron minerals. Cherty layers and metallic iron-oxide-rich layers are commonly interlayered to produce a banded iron formation. The cherty layers are light-colored, and the iron-rich layers are dark reddish colored.
• the Sudan Iron Formation on Minnesota's iron range is a wonderful example of a BIF (closeup of Sudan Iron Formation). This iron formation is actually of late Archean age, and is the first BIF to appear in the Minnesota geologic record. Even though some iron formations worldwide are of Archean age, the vast majority formed during the Early Proterozoic, indicating the appearance of a well-developed oxygen-bearing atmosphere at that time.
• iron minerals present in form of oolites.
  • In may of the iron formationt, the iron minerals are chemically deposited in concentric layers or rings around a central nucleus to form the subspherical grains called oolites.
  • Formation of oolites requires water agitated by some sort of a current, usually in a shallow sea or lake
  • When the chert is colored red by the iron minerals, the rock jasper is the result
• The Minnesota iron formations of Proterozoic age are correlative with similar formations of the same age and depositional environment in Wisconsin and Michigan, indicating a widespread basin in which all the Early Proterozoic rocks were formed. This basin was in the general area where modern Lake Superior is now located
• Iron formations in the Cuyuna Range are steeply tilted, but are only mildly deformed on the Mesabi and Gunflint Ranges, gently inclined toward present-day Lake Superior. This gentle tilt enabled open-pit mining on the latter ranges, whereas steep dips required deep mining in the former
• Iron formations of the same age are found in Labrador, western Australia, Russia, Venezuela, Brazil and Africa, suggesting a global atmospheric change richer in free oxygen at this time.
• Note that the timing of the widespread appearance of BIF's globally is in the Early Proterozoic
  • This event correlates with the widespread appearance of cyanobacteria (sometimes mistakenly referred to as algae) in the rock record. The cyanobacteria perform photosynthesis, which utilizes carbon dioxide and energy from the sun to produce glucose (a sugar) and free oxygen gas
  • Evidence of cyanobacteria is layering in the rocks called stromatolites. The stromatolites generally display wavy layering. These structures are produced when sticky mats of cyanobacteria trap sediment in shallow water, often forming mound-like features (the mound-like features are shown in ancient rocks at the top of this photo, and the mounds of cyanobacteria in the actual environment of deposition are shown at the bottom of this frame).
  • Modern cyanobacteria are relatively rare, and the niche they once occupied during the Precambrian has been occupied by true algae since Paleozoic time. One place where modern cyanobacteria are living is in Hamelin Pool at Shark Bay on the western coast of Australia.
  • The Gunflint cherts also have evidence of microscopic unicellular cyanobacteria, indicating an oxygen-producing biologic system had evolved by this time.
• Evidence for Archean composition of the atmosphere
  • The present atmosphere is greatly depleted in Ne, Xe, and Kr, which are inert gases that should be preserved in the atmosphere. This suggests that the Earth's initial atmosphere was lost early on either by boiling away during the magma ocean event or by being carried away by intense solar wind in the early solar system. The present atmosphere and hydrosphere of Earth developed from volcanic emissions over the last 3.5 billion years.
  • Uraninite (a uranium-bearing mineral) is only stable under oxygen poor (reducing) conditions; it is very soluble under oxygen-rich conditions, and is therefore, not generally found in younger sediments. The presence of grains of uraninite in Archean sedimentary rocks indicates an oxygen-poor Archean atmosphere.
  • Archean sediments are also unusually iron-poor. Iron is very soluble under reducing conditions and insoluble under oxygen-rich conditions, and therefore, behaves just the opposite of uraninite, dissolving in oxygen-poor water and precipitating in oxygen-rich water.
  • Banded iron formations appeared about 2.6 billion years ago and continued until about 1.8 billion years ago. These deposits of marine hematite and quartz (chert) represent precipitation of dissolved iron from sea water as the dissolved oxygen content of the water increased. After 1.8 billion years, banded iron formations are rare, but terrestrial red-beds are common for the first time, suggesting that iron is being oxidized and precipitated in soils and rocks on land in the source area of sediments instead of being dissolved and carried into the oceans in its unoxidized form.
• Sources of the iron and silica?
  • extensive weathering following uplift in the Algoman orogeny. With the advent of free oxygen in the environment, oxygen dissolved in the water could combine with the iron to precipitate the iron minerals, and with the silica to produce chert
  • Some geologists argue that volcanism was the source of the iron
  • Other geologists argue that some of the iron minerals could have formed after deposition and during burial of the sediment.
• Were the iron formations deposited across the entire basin, or restricted to the shallower margins where currents were present, as evidenced by the oolites? No agreement
• If iron formations are confined to the basin margins, what was deposited in the basin center?
  • mud from prolonged weathering and mud from volcanic ash
  • mud was the background sediment in which the iron was concentrated. It could be that iron was precipated very rapidly in agitated shallow-water environments around the basin margin, but not in the middle. The mud was therefore not masked in the basin center by the iron
  • evidence for this idea? each iron formation on each iron range is overlain by mudstone, now metamorphosed to slate
  • What was the environment of deposition of the muddy formations?
    • Interbedded dark-colored sandstones and siltstones are very poorly sorted both from the standpoint of grain size and minerals present. Such poor sorting is often evidence of rapid deposition by turbidity currents in a basin which was rapidly subsiding and therefore structurally unstable.
    • Thompson Formation Turbidites at Jay Cook State Park - light-colored sands and dark-colored muds
    • Graded bedding is also present, making turbidity current deposition a likely candidate.
    • Elongate markings on the underside of beds, produced by currents dragging some sort of tool along the bottom, indicate current directions toward the old basin center
• The history of the western part of the basin in which the Lake Superior Iron Formations and overlying dark-colored mudstones, sandstones and siltstones were deposited, is shown in the accompanying diagram.
• Could the volcanic arcs related to the basin represent development of a subduction zone, with an oceanic plate moving north and being subducted beneath a continental plate north of the basin? Perhaps.

Penokean Mountain Building

• rifting along the margins of the Archean provinces in the Canadian Shield ~2.2 Ga
• passive margin sedimentation of Animike Group, including BIF's
• subsequent suturing of island arc and gneiss terrane ~1.9-1.8 Ga
• line of suture is the Great Lakes Tectonic Zone, which bounds gneisses to the south and Greenstones to the north
• compression came from south, as indicated by the orientation of the thrust faults in cross section
• folding and metamorphism along the axis of the basin during suturing produced broad E-W-trending folds ~1.9-1.8 Ga
• rocks N of basin were little deformed, further indication that compressional forces came from the south
• metamorphic grade also decreases toward the north showing south to north compression
• intrusion of granite (including those at St. Cloud) intruded the older rocks ~1.8 Ga
• The bedrock map of Minnesota shows the boundary between the Middle Archean Gneiss Terrane to the south (orange color) and the Greenstone Belt to the north (darker green color).
• the plate tectonic cycle of rifting, passive margin sedimentation, collision, and intrusion of granites is called a Wilson Cycle

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• folding and metamorphism of rocks near the center of the basin produced broad east-west oriented folds in the Thompson Formation.
• (closeup of anticline at Jay Cook State Park)
• Tilted rocks of Thompson Formation at Jay Cook State Park
• Virginia and Rove Formations on the north side of the basin were barely tilted and metamorphosed, indicating that the forces producing the mountain building were centered south of the basin and died out in a northerly direction
• another indication that the deforming force came from the south is that the slates of the Thompson Formation become schists and the minerals present indicate progressively higher temperatures and pressures of formation as one goes farther southward
• Early Proterozoic granites were intruded into the iron formations and associated sediments during the Penokean mountain building event. These graintes include the St. Cloud Granite, which is quarried in the St. Cloud area as a valuable building stone.
• Morey and Sims of the Minnesota Geologic Survey have suggested an explanation for the increasing degree of deformation and metamorphism toward the south.
  • Early Proterozoic rocks to the south of the Superior Basin were deposited on a basement of ancient Archean gneisses
  • Rocks of the same age to the north were deposited on a basement of greenstone and granite.
  • The gneissic basement may have behaved in a more plastic manner and melted more readily than the basement to the north, resulting in more deformation and metamorphism toward the south.
  • The boundary between the two types of basement is proposed to be a fault zone, called the Great Lakes Tectonic zone. This zone is still active, as most of Minnesota's modern earthquakes occur along its trace.