Outline of "Minnesota's Geology"

Pages 46-61

Late Proterozoic Geologic History

---

Introduction

The Late Proterozoic spanned the interval of time from 1.6 Ga to 0.6 Ga at the beginning of the Paleozoic Era. By the begining of the Paleozoic (600 million years before the present), organisms had evolved hard parts and began to appear in great numbers in the fossil record. Bedrock of this age is exposed mostly in northeastern and southwestern Minnesota, together with buried bedrock in a strip from St. Cloud south to Austin and Albert Lea.

Late Proterozoic rocks in Minnesota include, in order from oldest to youngest

• quartz sandstones
• basalt lava flows and related gabbro intrusions (dated about 1.1 Ga)
• postvolcanic sandstones

See the accompanying cross section for the geologic relationships between these rocks and the older Precambrian record.

The Early Proterozoic to Late Proterozoic erosion interval and deposition of quartz sandstones

• Penokean mountains eroded over several hundred million year interval
• formed a major unconformity marking the base of the Late Proterozoic record
• erosion of exposures of older rocks to the northwest and southwest also continued
• large volume of resistant quartz sand produced and deposited in various basins
  • Sioux quartzite (SW Minnesota and SE South Dakota) ~5,000' thick
  • Nopeming Quartzite (Duluth area) ~25' thick
  • Puckwunge Formation (NE Minesota) ~200' thick
• basal conglomerate in each of the sandstones has pebbles of quartz, chert and ironstones
• no fossils, as is the case with many Precambrian sedimentary rocks - makes interpretation difficult
  • impossible to correlate with other rocks and the time scale using index fossils
  • can't be dated radiometrically because they are siliciclastic sedimentary rocks
  • environment of deposition difficult to determine
• sedimentary structures not conclusive as to environment of formation
  • this is a big problem in working out history of many Precambrian sedimentary rock units - are they marine or non-marine???
  • cross bedding near top of units suggests tidal origin, which would indicate marine setting, at least for uppermost one-third
• similar sandstones of presumably similar age in Wisconsin (Baraboo Quartzite), Michigan and Ontario
• widespread distribution of sandstones suggests deposition as a sheet in a broad river (alluvial) plain or in a shallow sea, or in both environments, across large basin
  • if in a sea, the sea probably encroached from south to north across the eroded Penokean Mountains
  • if in an alluvial plain, the rivers probably flowed south from sources in the Penokean uplands
• constraints on ages of the sandstones
  • rhyolite in Sioux Quartzite dated at 1.47 Ga
  • rhyolite beneath Baraboo Quartzite dated at 1.76 Ga
  • lava flows above sandstones dated at 1.1 Ga
  • therefore, sandstones are somewhere between 1.76 and 1.1 Ga
• problem with the idea of a single sheet of sandstone - magnetic polarity stratigraphy of the various sandstone formations does not match, suggesting that they are of different ages. So, the exact nature of the processes and environments of deposition, together with the nature of the sedimentary basin or basins, is very much in doubt and is the subject of much study
• Other evidence of history
  • all quartz indicates removal of any unstable silicate minerals by prolonged weathering and erosion, and maybe even recycling of sediment. (Recycling means that the sand comes from an earlier sandstone, rather than from an igneous or metamorphic rock. The history of derivation, transport and deposition goes through a loop in the rock cycle, from exposure of sandstone to formation of new sandstone, to uplift and exposure of that sandstone, to formation of new sandstone, and so on.)
  • The fact that all the quartz grains are well-rounded also suggests recycling
  • quartz grains are often cemented together by crystals of quartz cement to form a sandstone. When the sandstone is uplifted and exposed at the surface, weathering occurs. Often the quartz grains are released from the source rock with coatings of the cement crystals around them. These crystals of cement that coat the grains will become rounded during transport and deposition. If new crystals of quartz cement form in the new burial environment, then the resulting sandstone will have two layers of cement crystals surrounding individual quartz grains. The fact that the first layer is rounded indicates that the sandstone has gone through at least two cycles.

Extensional forces attempt to rift North America

• analogies to plate tectonic theory and the modern rifting of Pangaea to form the Atlantic Ocean Basin
• rifting occurs at spreading centers with high heat flow, volcanism and shallow-focus earthquakes
• new ocean crust in the form of basaltic (mafic) lava flows formed at spreading center which eventually evolves into an ocean ridge
• were there rifting events prior to the breakup of Pangaea? Yes. One occurred when an earlier supercontinent rifted around 450 Ma. And other rifting event affected the mid-section of the North American continent around 1.1 - 1.2 Ga.
• The feature developed at this time is referred to as the Mid-continent rift zone.
• This rift zone is apparent on a map of the strength of the gravitational field for North America and for Minnesota. Because the rift is filled with denser basalt lava flows rich in iron-bearing silicate minerals, the gravitational field is stronger over the rift, producing the so-called mid-continent gravity high.
• A map of the strength of the magnetic field would also show this rift zone, because the mafic basalts with their iron-bearing silicates are magnetized and add their strength to the overall magnetic field.
• rifting of North America to form the mid-continent rift was aborted before a fully developed ocean could form. Note how narrow the rift is. Spread distances were only 30 to 50 miles.

Lava flows of the Mid-Continent Rift

• Exposed near Lake Superior and south to Pine Valley ~70 miles south of Duluth. Exposures along the north shore of Lake Superior are especially spectacular, with narrow, deep stream valleys and waterfalls cut into the basalt lava flows.
• hundreds of single basalt lava flows, each varying in thickness up to several hundred feet thick, make up the North Shore Volcanic Group which overall is more than 25,000 feet thick.
• The 25,000 foot-thick group represents a composite thickness. Not all 25,000 feet were piled upon one another in a single place. Six different lava plateaus make up the group. Some of these lava plateaus are older, some are younger
• Individual flows can be traced for nearly 40 miles.
• Age relationships of flows
  • Oldest flows west of Duluth rest on basal Late Proterozoic sandstones, discussed above. These flows have pillow structures, and were erupted in submarine settings
  • Youngest flows located northeast near Tofte.
  • Flows become younger again northeast of Tofte to Grand Portage
• Radiometric ages
  • 1.1 Ga
  • U-238 - Pb-206 ages indicate a span of 20 Ma for the volcanism (1.14-1.12 Ma)
• flows on the north shore are generally tilted toward the south, and flows on the south shore, in Michigan, are generally tilted toward the north. A sag therefore existed at the site of present-day Lake Superior. This suggests that the source of lava was in the Superior Basin, and when the lava was removed from beneath the basin, the basin collapsed
• Erosion has exposed flow tops and cross sections of flows
  • conglomerate, sandstone and siltstone are often present between flows. These rocks contain fragments of the underlying flow on which they rest, indicating derivation from the lava flows.
  • cross-bedding in these siliciclastic sedimentary rocks indicates directions of transport south toward present-day Lake Superior, indicating that the sources of the lava were farther to the north
• Character of the flows
  • Most of the flows are basalt but there are some flows that are more sialic
  • all contain gas holes or vesicles which were later filled with mineral deposits.
    • Vesicles are most abundant near the top of each flow because the gas escaped upward
    • Some vesicles are filled with agate (made up of microcrystalline concentrically oriented colored bands of silica or quartz)
    • Others are filled with thomsonite which, like the Lake Superior agates, is a prized semi-precious gemstone.
  • copper deposits were also formed in the lava flow and the sediments between the flows
  • some lower flows cut by vertical dikes of basalt, which may represent the plumbing system through which the lava rose to the surface to form flows higher in the sequence
  • sills are also present in the flows and may be part of the Duluth Complex, discussed next.

Duluth Complex - Gabbro Intrusives

• Duluth to Ely to Pigeon Point
  • arc-shaped area
  • boundary of flows and older rocks
  • 10-mile thick sill complex
• compostions
  • range from ultramafic to mafic rocks - not a single gabbro body but a complex of intrusions
  • compositions like ocean crust and underlying mantle, indicating source in a spreading center
• textures
  • coarse-grained
  • cooled at great depth beneath lava flows
• cross cut continuous iron formations beneath the lava flows, making once continuous units into discontinuous bodies
• contact metamorphism occurs
  • for example, asbestos formed in Biwabik Iron Formation, making iron mining hazardous to health
  • lava flows baked at upper contact of intrusives
• small sills in the North Shore Volcanics also related to gabbro intrusive
• contain low-grade copper and nickel ores

Youngest Proterozoic Sedimentary Rocks

• Rift basin formation
  • more sagging because of the withdrawal of large amounts of magma to form the Duluth Complex formed this extensive narrow but elongate basin
  • lava flows beneath basin were depressed and tilted toward basin center
  • basin extended all the way to Kansas
  • smaller blocks within the basin subsided at different rates, so that some stood high and some stood low, making the geometry of the basin rather complex
• Sedimentation in basin
  • highlands at basin margin were eroded and sediment was carried into the basin
  • sedimentation in the basin continued for millions of years
  • thicknesses vary from aabout 1000' near the north shore of Lake Superior to more than 15,000' in northernmost Wisconsin.
  • Rock units
    • Solar Church Formation
      • nowhere exposed
      • studied in drill cores which were taken in southeastern Minnesota for studies of possible natural gas reservoirs in the rift basin sediments
      • 3000 feet thick
      • poorly sorted sandstones with lots of unstable silicate minerals together with quartz.
      • minor conglomerates, siltstones, mudstones and limestones are interbedded in the sequence
      • probably deposited in nonmarine environment, most likely meandering rivers and their associated floodplains.
      • 100 foot-thick zone of weathering at top indicates unconformity
    • Fond du Lac Sandstone
      • not everywhere present atop Solar Church
      • rifted blocks in the basin stood at different levels, so in some instances, Fond du Lac is absent where a block stood high during the time when it was deposited elsewhere in adjacent blocks which were dropped to lower elevations
      • ~300' thick up to 2000' thick in Minnesota (much greater thicknesses of equivalent units occur in Wisconsin, near the basin center)
      • where present, (for example west of Duluth in Jay Cooke State Park) it is a basal conglomerate with overlying interbedded sandstone, siltstone and shale
      • sedimentary structures such as mudcracks and mud chips embedded in the sandstone indicate deposition in river channels which eroded mud chips from their adjacent banks, and on muddy floodplains where exposure resulted in sun-dried mud with tell-tale cracks. Cross bedding indicates transport from basin margin to basin center
      • dates based on paleomagnetic reversals indicate ages of 950-1040 Ma
    • Hinckley Sandstone
      • relatively pure quartz
      • well-rounded and well-sorted
      • ~100" thick near basin margin (much greater thicknesses would be present in basin center, beyond Minnesota in Wisconsin)
      • may have formed at the margins of a sea or large lake in the rift, where waves and currents could destroy unstable silicates and concentrate the quartz