Earth History
GEOL 2110
The Cryptozoic (Precambrian)
SuperEon
Part 1: The Archean Eon
Major Concepts
• The oldest coherent terranes on Earth have ages of 3.8-3.0
Ga (Paleoarchean Era) and are composed dominantly of
granitic gneiss and minor sedimentary rocks. These
ancient gneiss terranes indicate that continental crust was
being created, though probably not by Plate Tectonics.
• Most Archean terranes were formed about 2.8-2.6 Ga
(Neoarchean Era) and are composed of volcanicsedimentary belts and granite batholiths. These “granitegreenstone terranes” are thought by most geologists to
have formed by processes akin to modern-day Plate
Tectonics and formed by the accretion of island arc and
granitic protocontinents.
• About 50% of the current volume of continental crust is
estimated to have formed in the Neoarchean Era.
The Precambrian Nucleus (Craton) of
North America
Craton– Precambrian
basement of the Continent
Shield– Exposed part of craton
Platform – Covered part “
Precambrian of Minnesota
Tells the Story of How the NA Craton was Made
Telling the Precambrian Story
Direct Evidence: OUTCROP
- Depth to
Bedrock
Outcrop
Telling the Precambrian Story
More Direct Evidence: DRILL CORE
Image from Dale Setterholm, MGS
Location of core hole
Drill core
1st vertical derivative of aeromagnetic data
Drill hole locations provided by Minnesota Department
of Natural Resources, Division of Lands and
Minerals, Hibbing 2002
Telling the Precambrian Story
Indirect Evidence: GEOPHYSICAL DATA
Telling the Precambrian Story
Supporting Evidence:
GEOCHRONOLOGY AND GEOCHEMISTRY
Telling the
Precambrian
Story
Geologic Paradigms
and Principles
Precambrian
Terranes of
Minnesota
Archean
Terranes
Terrane 1:
The
Minnesota
River Valley
Gneiss
Terrane
>3.0 Ga
THE
MINNESOTA
RIVER
VALLEY
Actually it’s
3,485,000,000 years
±10,000,000
but hey, close enough
GRANITIC (= continental)
GNEISS (= high grade metamorphic)
Amphibolite
(oceanic crust?)
Geologic
Structure of
Southwestern
Minnesota
Yellow Medicine
Shear Zone
Yellow
Medicine
Shear Zone
A suture zone
between the Morton
and Montevideo
Blocks?
Composed of garnet mafic
gneiss and amphibolite (ocean
crust in suture zone?)
3.8-3.5 Ga GRANITIC GNEISS
Early Continental Crust Formation by PT?
“Sagduction”
vs. Subduction
Areas of
>3.5 Ga
Rocks
Terrane 2:
The
Superior
Province
(GraniteGreenstone
Terrane)
2.7-2.5 Ga
GREENSTONE
Metamorphosed Basalt
Pillowed Structures indicative
of submarine eruption
Banded Iron Formation
Algoma (Soudan)-type
Interbedded with greenstone; composed of alternating bands of chert
(cryptocrystalline quartz SiO2) and hematite (Fe2O3)
Possible Origins
-hydrothermal deposits
-evaporite deposits
-organically-mediated
deposits
Jasper Chert
Hematite
Granular Chert
Deformation – soft sediment, not tectonic
GRAYWACKE
Immature clastic sediment “dirty sandstone”)
UP
Clay-rich
Sandy
Graded
Bedding
Soft-sediment Deformation
GRANITE
Remelted Crust
Large-scale Fault Zones
Mica schist in the Mud Creek Shear Zone
Vermilion Fault
Vermilion Fault
Archean Geology of the
Soudan Mine/Lake Vermilion Area
Felsic Gneiss
Felsic volcanics
Iron formation
Soudan
Volcanoclastics
Pillowed Greenstone
Granite
Graywacke
(~ 2.6 Ga)
The
Superior
Province
Multiple
Accreted
Volcanic Arc
Terranes
SUPERIOR PROVINCE TECTONIC MODEL
Accretion of Island Arcs, Oceanic Plateaus, and
Protocontinents
Neo-Archean
Tectonics
The Beginning of
Modern-day
Subduction
SUPERIOR PROVINCE
A major episode of continental
growth creating KENORALAND,
the core of North America
Kenoraland
Next Lecture
The Cryptozoic (Precambrian) SuperEon
Part 2: The Paleoproterozoic Era