Revised 8 / 06 (Monroe 6th ed.)
The PreCambrian represents the first 4 billion years of earth history (approx. 85%)
Pre-Cambrian rocks make up the bulk of continental crustal rocks
Mostly obscured by a relatively thin veneer of Phanerozoic sedimentary, igneous, and metamorphic rocks
These "rafts" of Pre-Cambrian igneous/metamorphic rocks are called the continental 'cratons'
The exposed areas are called the 'shields'
These shields are only partially exposed on the continents
DIGRESS TO: shields vs. platforms
These limited outcrop areas form the basis for our knowledge of the Pre-Cambrian earth
Review of the generalized theories of the formation of the earth
Condensation - collapse of the nebula by gravitational attraction
Nucleation - the initial "crystallization" of solid matter as a result of the condensation
May have resulted in partial differentiation of an iron-nickel "core" material and a silicate "mantle" material (inhomogonous accretion)
Accumulation - the concentration, due to gravitational attraction, of the solid nucleated matter into the pre-earth
Most assume that this process resulted in a cold body
But a "profound" heating event must have occurred early in the earth's history
The source of the initial heat uncertain (isn't it all?):
Impacts of objects at high velocities
Friction from the movement of differentiating material
This heating resulted in the "final" differentiation of the iron/nickel core
Final? Seems to me that it could still be going on, at least to a limited extent
Certainly the majority of the present core must have differentiated relatively quickly
There's evidence of a magnetic field in the earliest rocks
The formation of the core was (is?) EXOTHERMIC
A large quantity of heat was liberated
Possibly enough to melt the "outer several hundred kilometers" of the earth's surface
Resulted in the formation of the "primordial crust"
What was the composition of this original crustal material
All possible composition have been suggested
Felsic, Mafic, Ultramafic, Anorthositic - define
Most assume a mafic to ultramafic composition was the most likely
This actually supports what I've been saying about the differentiation of the crust!
ie. That the initial earth was relatively un-differentiated
Felsic crustal material has only become available through time due to differentiation
"Possibly this first crust was rather unstable" for a number of reasons
Rapid convection within the mantle due to the extremely high temperatures
This upwelling of new mantle material and decent of the initial crustal materials would speed up the initial differentiation process of the crust
We're talking about the initial differentiation of relatively felsic rocks out of the original mafic to ultramafic source material
In any event...
None of this original crust has been positively identified
If any exists, it would probably be a part of the high-grade metamorphic cores of the continents
Or as xenoliths in Archean intrusives
Meteorite impacts probably had quite an effect on the earth's crust
Up to 3.9 billion years ago
This probably represents the last major stage of the earth's accumulation
Continued impact of large meteorites resulted in disruption of the original crust
Possibly instrumental in the formation of the earliest felsic continental rocks
It is assumed that from 25 to 30 "major" impacts occurred in the last 200 million years of the major bombardment phase
We're talking about some BIG chunks - "a few hundred to a few thousand kilometers in diameter"
How did these impacts result in the formation of the initial proto-continents?
Let's start with a meteorite 50 to 60 km in diameter (a small one!)
Upon impact, the meteorite would explode and create an impact crater up to 1000 km across
As well as penetrate into the earth for up to 100 km
The rocks around the crater would be "greatly heated"
The excavation of all this material would result in a decrease in pressure at depth and partial melting of the upper mantle in the immediate area below and lateral to the impact
Isostatic rebound and uplift would occur due to the loss of material
In addition, an upward readjustment of the local isotherms and increased volcanism would occur
This upwelling within the mantle is called a "mantle plume"
Describe the Hawaiian plume - is this the result of an impact in the Pacific 60 million years ago?
The impact basin would begin to fill with volcanic flows and volcaniclastics
The weight of this material would lead to basin subsidence
Partial re-melting of the volcanics at depth within the basin would lead to the formation of felsic to intermediate magmas
Remember how Bowen's Reaction Series (reversed) will result in the melting of the felsic minerals before the mafic minerals
These differentiated magmas would intrude into the overlying mafic volcanics
This sequence may have caused the initial formation of the continental sialic masses
Basically stir up the crust & allow further differentiation
Most of the exposed PreCambrian rocks consist of either metamorphic rocks or intermediate to felsic intrusives
Most represent the roots of ancient mountain systems
As do all metamorphic/intrusive terrains
Other portions of PreCambrian terrains include sections of relatively un-metamorphosed volcanics and sediments
These sedimentary sequences reflect the surface conditions at the time of formation and are important to our understanding of PreCambrian terrains and events
Many are "several thousands of meters thick"
PreCambrian rocks "contain no useful fossils"
Useful by what definition - maybe we just haven't deciphered them yet
This lack of fossil correlation makes an understanding the timing of PreCambrian events extremely difficult
There is a general division of PreCambrian rocks into two VERY broad categories
Archean - the earliest PreCambrian rocks
2.5 to 4.0 billion years old
Proterozoic - late PreCambrian
570 million to 2.5 billion years old
The rocks in most PreCambrian shield regions can be grouped into broad "structural provinces"
Individual provinces have distinctive structural characteristics
Separated by "sharp metamorphic or fault boundaries"
Archean sequences of relatively un-metamorphosed sedimentary rocks, and slightly metamorphosed interlayered volcanic flows and breccias
The term greenstone is derived from the chloritic alteration of the mafic minerals within the predominantly basaltic lavas
Many of the basalts exhibit pillow structures
Represent seafloor extrusion
Are often associated with extensive granitic intrusives
Common in the Canadian Shield
Occur in all PreCambrian shield areas in "huge elongate downwarps"
More or less define the "broad structural provinces"
Archean sedimentary rocks associated with the greenstone belts
Predominantly greywacke, with interbedded shale & conglomerate
Greywacke indicates rapid uplift and erosion of a nearby highland area
Not much highland area during the Archean?
So short transport distances
Banded Iron Formations
Basically an iron-rich chert deposited during times of "relative volcanic and tectonic quiescence"
We'll discuss these in detail later
Features common to greenstone belts include:
DIAGRAM: generalized X-section (look familiar?)
All appear to be relatively thick
Ranging from 6,000 to 18,000 meters
Similar sedimentary features
Dominated by greywackes which were formed by turbidity currents
The amount of sedimentary material increases up-section
Bases are composed of flows and intrusives of mafic to ultramafic composition
Becoming more felsic (and clastic) up-section
Ultramafic to mafic flows and sills at the base
Intermediate flows and volcaniclastics in the middle
Felsic volcaniclastics at the top
Young greenstone belts (<3 billion years old) commonly repeat this progression several times
Younger greenstone belts also seem to have a greater amount of continental (sialic) debris associated with them
Indicates that the amount of felsic crustal material increased with time (?)
All contacts are faulted
Structurally emplaced assemblages
No idea what they rest on (ie. sima or sial)
Does all this greenstone stuff sound familiar?
It seems to me that similar suites of Phanerozoic rocks are associated with ophiolites
Refer to Back-arc model - handout
Back to the small proto-continents which were formed by meteorite impacts in the early Archean
These mini-rafts probably "drifted" around on the underlying material looking for something similar to accrete to (mate with?)
There must have been many localized "spreading centers" in which ophiolitic types of rocks could have been formed
Therefore, I feel that the majority of the greenstone belts were probably deposited not on sial or sima, per se, but directly on the upper mantle
Again, similar to Phanerozoic ophiolite sites
This is assuming, of coarse, that plate tectonics was active in the Archean!
Obviously, I think it was
Remember, I'm the guy who proposed that the moon went through a minor period of active plate motions early in its history
So how does plate tectonics relate to the Pre-Cambrian formation of the earth?
The heat flow from the earth was probably greater in the early Archean
This probably resulted in "vigorous" plastic flow in the upper mantle
Also, the amount of felsic 'scum' was far less than today
Crustal differentiation hadn't been going on for all that long
Therefore, any felsic plates would have been relatively small and thin
And there may have been a whole herd of them (possibly as a result of the meteorite impacts discussed above)
Much of the early history of the Pre-Cambrian may have been dominated by the 'accumulation' of these minor platelets into larger plates
How does plate tectonics relate to the development of the greenstone belts?
Within individual greenstone belts, the oldest (lowest) rocks are compositionally similar to MORB's
The younger (stratigraphically higher) units are similar to island arc type volcanics
This is similar to the Rogue Volcanics problem in Josephine County
And again indicates that greenstone belts help define the margins of pre-Cambrian continental blocks
These units generally surround the greenstone belts and form the basic cores of the sialic cratons
The granitics were intruded into the greenstone belts during the latest stages of their development
Emplaced as "huge batholiths"
Therefore, it can be assumed that they came from below and essentially underlie the greenstone belts as well as surrounding them
These forms the actual stable basement complex of the cratons
Can exceed 40 km in thickness
Must have differentiated from the upper mantle and/or be the product of re-worked mafic crust
Mafic and ultramafic rocks have "a proportionally minor but volumetrically huge quantity of felsic minerals (chiefly quartz and feldspar)"
Felsic minerals have a lower density, and melt at lower temperatures, than do mafic minerals
Partial melting of mafic rocks can provide a source for felsic magmas
Theory - Could eclogite be what is left after the lighter felsic minerals have been removed from the descending basaltic crust?
The metamorphic halo surrounding Archean granitic plutons is substantially smaller than those associated with Phanerozoic plutons
Indicates a shallower depth of burial and more rapid dissipation of the heat
Shouldn't this mean that the Archean granitics are generally finer-grained?
As we discussed... The continental masses probably began in the early Archean as small proto-continents
The eventual enlargement of the proto-continents could have occurred in two ways
Suturing of individual proto-continents through random collisions
Peripheral accretion of newly-formed sialic material
The actual enlargement is probably a combination of both processes
Let's talk "peripheral accretion" - Platform Sequences and Geosynclinal Belts
The continental cratons became "stabilized" at the close of the Archean
Following a "worldwide episode of igneous activity" which marks the Archean/Proterozoic boundary
This had a great effect on the types of sedimentary rocks which were formed
The Archean was dominated by thick sequences of deep-water marine greywacke and shale
Proterozoic sedimentary rocks resemble Phanerozoic sediments
Except for the lack of fossils
This includes both marine and continental sediments, as well as deposits associated with transitional environments
Beaches, tidal flats, deltas, etc.
Proterozoic sedimentary rocks can be grouped into 2 broad categories
The stable internal portions of the cratons
The elevations of the cratons appear to have been generally close to sea level since the beginning of the Proterozoic
They were covered by shallow "epicontinental" (or epieric) seas during times of subsidence
Other periods are marked by emergence and erosion
Because of these periodic relative fluctuations, Proterozoic sedimentary sequences are characterized by numerous unconformities
Thick sedimentary sequences in excess of 10,000 meters
Nearly continuous sedimentation, so unconformities are rare
The stratigraphic record is essentially complete
Geosynclines commonly form on the trailing edges of continental plates
Can be broken into two distinctive types of sedimentary environments
Shallow-water sediments deposited on the continental shelf
Include limestone, dolomite, shale, and sandstone
These form in relatively stable settings which are free from igneous activity
This means that there shouldn't be any volcanics associated with them
Turbidites (greywacke) and pelagic sediments (shale) deposited on the continental slope and abyssal plain
Carbonates are rare (limestone and dolomite)
Volcanic flows and clastics may be present
Eugeosynclines sound just like the types of sedimentary units common to the Archean
Maybe the only difference in the Proterozoic is the addition of the 'stable' continental land masses and their associated sedimentary units
Commonly, a change in the relative plate motions has resulted in the compression of these geosynclinal sedimentary belts
Results in large-scale folding and thrusting, and ultimately mountain building
Partial melting of the base of the geosynclinal sediments results in the formation and emplacement of granitic magma
These active regions along the plate margins are called "Mobile Belts"
The Appalachian Mountains are an excellent example of a compressed Paleozoic mobile belt
Note: the terms Geosyncline, Miogeosyncline, and Eugeosyncline seem to have fallen out of favor with many geologists, and that is fine with me - they are, after all, only words representing earth processes and environments, which continue to exist no matter what we call them.
Banded Iron Formations and the increase of free oxygen
Refer to separate discussion
Oxygen Metabolism and Evolution of the Metazoa
Oxygen metabolism is the way in which the higher forms of animals and plants break down compounds to produce energy
This is an aerobic process
Many primitive "bacteria" are anaerobic, and don't use oxygen to produce energy
This is a fairly inefficient process and is not suitable for complex organisms
Ex. glucose - anaerobic fermentation produces 57 kilocalories/mole
Oxidation of glucose produces 637 kilocalories/mole
Important Note: As we all know, the fermentation process is critical in the production of certain life-sustaining beverages, so let's not get too uppity about this matter!
Oxygen metabolism is required for complex, multi-celled organisms
Eucaryotic cells as opposed to prokaryotic cells
Some modern single-celled organisms can do it both ways (in humans this is illegal!)
Will normally metabolize aerobically, but when oxygen levels drop they can use anaerobic processes
Example : Yeast
It's uncertain when eucaryotic cells developed
Data indicate that there was sufficient free oxygen by 1.4 BYA to support widespread aerobic metabolism
The First Fossils
The oldest "convincing" fossils are bacteria and algae from cherts 3.2 billion years old
The first "abundant" remains are Stromatolites (a form of blue-green algae)
Some workers use stromatolite shapes to establish broad divisions in the upper Proterozoic
Biotic Change and the End of PreCambrian Time
A major expansion of life forms occurred 600 million years age
Marks the PreCambrian / Cambrian boundary
True animal fossils began to appear in the latest portions of the Proterozoic
These earliest organisms were fairly thin because of the need to disperse oxygen by diffusion through cell walls
Only later were "fatter" organisms possible due to the evolution of respiration systems
These were soft bodied organisms without hard calcium-based shells
The end of the PreCambrian (and start of the Phanerozoic) is "traditionally" when hard calcium-based shells began to appear
The first Cambrian fossils indicate a rapid evolution into complex organisms
The reason for the "sudden" appearance of abundant fossils in the Cambrian is unknown
Remember, life had existed for 3 billion years already
Why the sudden expansion of life forms?
Was it a "truly abrupt evolutionary proliferation"
Or was it that the development of hard shells allowed pre-existing life forms to be preserved?
The book seems to feel that it is unlikely that there were abundant soft bodied organisms in the PreCambrian
They would have at least left traces of burrows and trails
There is evidence of these traces increasing 700 million years ago
The book supports "a burst of metazoan evolution" at the close of the PreCambrian
What caused this rapid diversification of life forms?
The book breaks the possibilities into 2 broad categories
Ex. - the development of sexual reproduction ( as opposed to asexual 'division')
Allows a greater potential for evolutionary changes (and mutations)
Physical changes in the ecosystems
The increase of free oxygen levels in the atmosphere
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