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Oceans and Continents
Chapter 07
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Chapter 07 Notes - Proterozoic Eon
I. Introduction
A. Proterozoic Eon began 2.5 billion years ago and ended only 544 million years ago.
1. comprises 42% of Earth history
2. designation of the beginning of the Proterozoic is somewhat arbitrary, a more modern style of
plate tectonics and sedimentation began to be prevalent
3. less altered and easier to study and interpret than rocks of the Archean
4. not as abundantly fossiliferous as many Phanerozoic strata
II. The Archean cratonic elements welded together to form a large supercontinent during the early
Proterozoic.
A. Laurentia - North American cratron
1. welding occurred along belts of crustal compression, mountain building, and metamorphism, called
an orogen
2. completed by about 1.7 billion years ago
3. continued to grow as a result of accretionary events through additions of crustal materials to
continental margins
a. Incorporation of small blocks of crustal material carried to subduction zones next to continents by
sea floor spreading
1. resulted in the formation of the Earth's crust very large cratons where wide marginal shelves and
epeiric seas existed
2. shallow water clastic sediments and carbonate deposits accumulated which is rare in the
Archean
3. characterized by substantial amounts of lateral plate motion and accompanying subduction,
rifting, and sea floor spreading
4. consist of sedimentary rocks deposited along the once passive margins of continents
5. Later compressed and welded onto the continent
III. Paleoproterozoic Events in North America - Proterozoic Plate Tectonics
A. Modern style of Plate Tectonics - Wilson cycle - describes the tectonic processes and associated
rock record for the opening and closing of an ocean basin
B. Wopmay orogen - (Figs. 7-1,2,3)
1. western edge of the Slave province was a passive continental margin
a. belt of deformation, igneous intrusion, and metamorphism
2. Initiation of the Wilson cycle
a. evidence of the opening of an ocean basin and sedimentation along continental margins
b. numerous normal faults
c. rift basins, which are represented by an accumulation of alluvial fan and fluvial deposits
d. Two aulacogens have been detected along the western side of the Slave province
1. Bathhurst aulacogen - trends southeastward from Coronation Gulf
2. Athapuscow aulacogen - extends northeastward from Great Slave Lake
e. basaltic lavas interlayered with sediments
f. sandstones (shallow) followed by dolomite (deeper)
3. Closure of Ocean basin
a. westward subduction of the leading edge of the Slave plate beneath a microplate approaching
from west
b. flysch (marine) followed by transitional sediments followed by Molasse (Marginal and
nonmarine deposition)
c. Igneous intrusives and volcanic to the west (volcanic arc similar to the volcanic arc in the
western Pacific today).
4. Metamorphism due to collision - folding, faulting, etc.
C. The Trans-Hudson Orogen (Fig 7-4)
1. Orogenic belt that separates the Superior province from provinces to the north and west
2. Welded the Superior plate to the Hearne and Wyoming plate lying to the west
3. Records a historical sequence of initial ritting, opening of an oceanic tract, and subsequent closure
along a subduction zone
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4. Is one representative for the suturing of the protocontinents that formed during the Archean into
the large continental shield that we refer to as the Canadian shield.
D. The Keweenawan Sequences (Fig 7-6)
1. Sequence of lava flows that extend from the Lake Superior region southward beneath a cover of
Phanerozoic rocks for hundreds of kilometers
2. Volcanism developed about 1.3 billion years ago
a. extended from Lake Superior southward beneath covering sedimentary rocks into Kansas
b. Rifting ceased before a separation could occur
3. Consist of clean quartz sandstones and conglomerates as well as basaltic volcanoes
a. lava flows are well known for their content of native copper
b. lavas accumulated to a thickness of several thousand meters
c. Duluth Gabbro: 12,000 meters thick, 160 km long mass
d. signals the presence of a rift zone along which a continent is going to break apart and admit an
ocean
E. The Grenville Province
1. The youngest middle Proterozoic region of the Canadian shield - in the eastern part of North
America (fig. 7-7,8)
a. Last to experience a major orogeny - deformation occurred 1.2-1.0 billion years ago
b. Extends from the Atlantic coast to Labrador to Lake Huron
c. continues beneath covering Phanerozoic rocks down the eastward side of the US and westward
into Texas
d. consist of carbonates and sandstones that have been metamorphosed and intruded by igneous
bodies
e. subjected to the destructive processes that accompanied the building of the Appalachian
Mountains during the Paleozoic
2. plate tectonic model - sedimentation along the passive eastern margin of Laurentia; followed by
mountain-building resulting from collision of another continental block against eastern Laurentia
3. One of the major orogenic events that led to the suturing of the supercontinent - Rodinia.
a. The most significant global event of late Proterozoic time was the gathering of Precambrian
continents to form a large supercontinent named Rodinia.
b. the great ocean that surrounded Rodinia has been named Mirovia
c. Collision occurred with the northern part of South America.
d. Breakup of Rodina during the Neoproterozoic and possible assembly of another
supercontinent near the end of the Proterozoic.
e. rifts developed - Iapetus Ocean formedwhich was the precursor ocean to the Atlantic. It lay to
the east of the Grenville province
IV. The Changing Proterozoic Environment (Fig 7-9)
A. Paleoproterozoic Ice Age
1. Huronian - rocks in the region north of Lake Huron consist mainly of coarse clastics eroded from
older crystalline rocks
2. includes the Gowganda Formation, a rock unit that is notable because of its conglomerates and
laminated mudstones of probable glacial origin - varves
3. intruded by igneous rocks that have been dated as 2.1 b.y. which set on older crystalline rocks that
are 2.6 b.y
4. rocks similar to the Gowganda are recognized in Finland, southern Africa, and India
5. represents the first episode of glaciation
B. Varanger Glaciation
1. Neoproterozic glaciation that extended down to the equator - may represent 4 glaciations
2. Snowball earth Hypothesis
3. May be directly related to (cause for) latest Proterozoic (Vendian) metazoans (Ediacaran).
C. Oxygen in the Atmosphere
1. 2 billion years ago the atmospheric oxygen began to accumulate because of increased plant
activity.
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Chapter 07
a. 10% of present levels of free oxygen achieved by the Cambrian Period
2. accumulation on land of considerable amounts of ferric iron oxide (3.5 to 1.9 billion years ago)
a. The Animikie Group
1. Surround the western shores of Lake Superior and is early Proterozoic age
2. World famous for the bonanza iron ores
3. record the presence of free oxygen in the Earth's atmosphere
4. oxygen-generating process of photosynthesis was in operation
5. banded iron formation, known by the acronym BIF
6. also known in Archean sequences - not nearly as extensive as those of the Proterozoic
7. stained terrestrial sediments a rust-red color, red beds after 1.9 Ma - considered a valid
indication of the advent of an oxygenic environment
Belt Supergroup of Montana, Idaho, and British Columbia
• > 12,000 m thick
• deposited in relatively shallow water along the passive margin of North America
Last event in the late Precambrian history of the western US
• development of a north-south trending passive margin
Precambrian rocks of the Grand Canyon region
Consist of two distinct units
Vishnu Schist
• lower and older unit
• complex body of metamorphosed sediments and gneisses
• intensely folded and invaded by granites
• granite intrusions cut into the Vishnu
• emplaced 1300-1400 million years ago
• part of a deformational event
Mazatzal orogeny
• Grand Canyon Series: upper unit
• late Proterozoic in age
• unconformably overlain by Paleozoic rocks
• correlates to the Belt Supergroup
• consists mostly of clastic rocks which accumulated in ~a trough-like basin that extended into the
craton
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