Outline of "Minnesota's Geology"

Pages 34-45

Early Proterozoic Geologic History

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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. (mesabi is "giant" in the Chippewa language; Gunflint is after Gunflint Lake, which early explorers named for the occurrence of flint along its shores; and Cuyuna is named for "Cuy" from Cuyler Adams, the surveyor who found iron ore in this area, and "Una" for his faithful dog.

Five groups of rocks are present, 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
  • perhaps Earth's first glacial event is recorded in glacial deposits (called till), converted to rock (called tillite) near the beginning of the Proterozoic
  • 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, formed during a long interval of wearing down of the land
• 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.

• made of chert (a hard, cryptocrystalline siliceous sedimentary rock like flint) together with the iron oxides hematite, magnetite and other iron minerals
• iron minerals present in form of oolites.
  • 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, as well as the underlying sandstones and the overlying dark-colored sandstones and mudstones, 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.
  • This event correlates with the appearance of marine algae in the rock record. The algae perform photosynthesis, which utilizes carbon dioxide and energy from the sun to produce glucose (a sugar) and free oxygen gas
  • Evidence of algae is layering in the rocks called stromatolites. These structures are produced when sticky mats of algae trap sediment in shallow water, often forming mound-like features.
  • The Gunflint cherts also have evidence of microscopic unicellular algae, indicating an oxygen-producing biologic system had evolved by this time.
• Sources of the iron and silica?
  • Iron and silica were dissolved in the water of the basin in large amounts, by the 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, rather than prolonged weathering of Archean rocks, as volcanic ash is found in some of the iron formations
  • 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 exists on this question.
• 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 - e.g., the Rove and correlative Thompson Formations on the Gunflint Range and the Virginia Formation on the Mesabi Range. The Thompson Formation is well exposed near Cloquet, Thompson, and Jay Cooke State Park.
  • 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.
    • 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 basin in which the Lake Superior Iron Formations and overlying dark-colored mudstones, sandstones and siltstones were deposited, is shown in the accompanying diagram. Note that this diagram is for the western part of the basin, in Minnesota. Farther to the east and south, in Wisconsin and Michigan, the basin was more complicated and tectonically unstable.
• 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. And perhaps this same setting continued all the way northeast to Labrador, where similar iron formations are present. But this is conjecture at this point.

Penokean Mountain Building

• dated at 1.9-1.8 Ga
• folding and metamorphism of rocks near the center of the basin produced broad east-west oriented folds in the Thompson Formation. The tilted beds are especially obvious in Jay Cooke State Park. Foliation defined by platy silicate minerals called micas is also oriented east-west, indicating a south-north direction of compression, perpendicular to the layering.
• 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
• 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.