CHAPTER 8: Cryptozoic (Precambrian) History

(4.6 b.y. to 600 m.y.)



1. The Cryptozoic Eon represents the first 80% of earth's history and a time before fossilized life became abundant. The Cryptozoic Eon is divided into the Hadean (4.6 - 3.8 b.y. ago), Archean (3.8 - 2.5 b.y. ago) and the Proterozoic (2.5 - 0.7 b.y. ago) Eras.

2. Rocks of the Archean Era (3.8 - 2.5 b.y.) consist mainly of metamorphic gneisses and granitic intrusions representing early protocontinents that assembled to produce larger continents (Precambrian cratons). Between protocontinents are belts of metamorphosed oceanic crust (greenstone belts) and sedimentary cover representing the margins of protocontinents that were deformed during collisions.

3. It wasn't until the Proterozoic Era (2.5 -0 6 b.y) that full-sized continents were assembled.


The Early Earth

1. Table 6.5: Following accretion from the solar nebula, the earth was heated by gravitational contraction in addition to rapid decay of abundant radioactive elements and numerous asteroid impacts. As the interior melted, dense iron and nickel migrated into the core leaving behind the silicate-rich portion which formed the mantle.

2. Much of the Earth's surface consisted of a magma ocean that slowly began to cool and solidify into crustal fragments called micro- or proto-continents consisting of basalt and komatiite. Over time, the magma ocean shrunk at the expense of growing microcontinents.

3. The oldest rocks recovered to date are only around 4 b.y. old, suggesting that the crust had not developed sufficiently to become permanent until about 3.9 b.y. ago.

4. By 3.8 b.y., the earth was no longer constantly bombarded by meteorites and its surface was cool enough for bodies of liquid water and primitive sedimentary rocks to form. The earth continued as a violent planet covered by countless volcanoes spewing immense clouds of gases and dust. Continents, oceans and mountain ranges were not yet clearly differentiated. Instead, the earth's surface contained scattered small land masses consisting of barren rocky plains and sand dunes devoid of vegetation.

5. Until 3.9 b.y. ago, the earths atmosphere was constantly churned and vaporized by extraterrestrial impacts. The early atmosphere consisted mainly of CO2, methane, ammonia and N2 but no free O2.

6. Over the next billion or so years, from 4 b.y. to 3 b.y. ago, the small protocontinents collided and assembled into larger continents so that by 3 b.y. ago, large continental masses had formed.

7. The assemblage of full-sized continents produced major changes in the earth's crust and plate tectonics began to operate by about 3 b.y. ago. Microtectonics may have operated on protocontinents before 3.0 b.y. ago.

8. During the Proterozoic Era (2.5 - 0.6 b.y. ago), plate tectonics was active, thick wedges of mature quartz sandstone replaced abundant immature graywacke on the continents, the oceans were filled with cyanobacteria and algae and oxygen began to accumulate in the atmosphere. Around 600 m.y. ago, life exploded on the planet.


Early Crustal Development

1. In 1983, a group of scientists found detrital zircons in Australia that yielded isotopic ages of 4.1 to 4.2 billion years, although the zircons were contained in much younger sedimentary rocks. These zircons are the oldest dated crustal minerals and place an upper limit on the time when the early continents stabilized. Furthermore, these zircons suggest that the 4.1 b.y. proto-crust from which they were derived was probably more silicic than basalt, possibly having a composition close to diorite or even granite.

2. In 1989, dates of 3.96 billion years were obtained from detrital zircons in the Acasta Gneiss in northwestern Canada. These rocks are currently the oldest known crustal fragments on the earth's surface.

3. The original crust was thin and composed of basalt and komatiite that may have been the crystallized products of an early magma ocean.

4. Later redistribution of lighter elements through weathering, erosion and igneous activity gradually transformed the original basaltic crust into a granitic composition.

5. Both differentiation and erosional processes result in rocks more silicic, which are then recycled to produce new crust slightly more granitic than the previous generation.

6. Igneous differentiation and erosion were major contributors to the gradual evolution of the early crust from basaltic and komatiitic compositions to more of a granitic composition.

7. Scientists are not sure what processes were responsible for crustal recycling in the early Archean, perhaps some primitive form of plate tectonics. All we know is that stable continental crust compositionally more silicic than basalt was present by about 3.9 billion years ago.


The Precambrian Rock Record

1. Figure 8.3: The Cryptozoic rocks on all continents are most widely exposed in the stable continental cratons and are usually highly deformed and metamorphosed.

2. Figure 8.11: In North America, Cryptozoic rocks are exposed primarily in the eastern two-thirds of Canada, the U.S. margin of Lake Superior and most of Greenland. This vast extent of Cryptozoic rocks is called the Canadian Shield. Additional Precambrian rocks are exposed within the cores of mountain belts such as the Appalachians and Rocky Mountains.




1. Archean Terranes represent the oldest continental and oceanic crust on earth. Virtually all of this original crust has since been deformed and metamorphosed.


Archean Terranes consist of

(a) Regions of high metamorphic grade dominated by gneisses.

(b) Greenstone belts of low metamorphic grade


Gneissic Complexes

1. Archean gneissic complexes represent the highly metamorphosed and contorted relicts of original volcanic and sedimentary rocks that have undergone high-grade metamorphism and deformation. Archean gneisses are the highly deformed remnants of the early protocontinents.

2. Gneissic complexes also include graphite schist, quartzite, amphibolite and marbles that are the metamorphosed relicts of former sedimentary rocks.


Greenstone Belts

1. Figure 8.12: Greenstone Belts are elongated features composed of mildly metamorphosed volcanic (mostly komatiite and basalt) rocks and associated sediments. Greenstone belts were generally deposited in oceanic settings although the occurrence of associated andesitic and rhyolitic rocks suggests nearby volcanic arcs.

2. Greenstone belts are abundant in the Superior and Slave Provinces of the Precambrian Shield in Canada. Greenstone belts also occur in South Africa, western Australia and southern India.

3. Figure 8.12: Theories on the formation of greenstone belts are as follows:

(a) Greenstone belts may have formed in back-arc extensional basins within the interiors of proto-continents.

(b) Greenstone belts may also represent old oceanic crust between proto-continents near subduction zones. When the proto-continents collided, they collapsed the oceans filled with basalt and graywacke to form greenstone belts.


North America

1. Figure 8.11: Archaen microcontinents (pre-2600 Ma) on North America include the Superior, Nain, Slave, Rae and Hearne-Wyoming terranes.

2. Figure 8.6: Within the Great Lakes region of the Canadian Shield, at least four major mountain-building episodes (orogenies) are recognized (Saganagan, Algoman, Penokean and Grenville). Orogenic events are characterized by extensive metamorphism, granitic intrusions and unconformities.

3. Table 8.1: From studies of the rock record, isotopic dating, and recognition of orogenies, a chronology of Cryptozoic events was established.

4. Isotopic dating allowed geologists to divide the Canadian Shield into isotopic date provinces. The dates provided by these highly deformed rocks mainly represent orogenic events (high temperatures reset the isotopic clock) rather than the actual ages of the rocks. Many of the rocks have been metamorphosed more than once so that their ages generally record only the last resetting of isotopes.


Assemblage of Crustal Nuclei

1. Figure 8.13: Ancient crustal nuclei assembled into stable cratons surrounded by orogenic belts.


Growth of Crustal Nuclei Occurred Through

(a) Addition of igneous material through partial melting of underlying mantle

(b) Erosion of uplifted areas, followed by sediment deposition around periphery

(c) Accretion of arc terranes and microcontinents around periphery (collage tectonics)

(d) Collision with other nuclei


2. Orogenic belts represent areas of plate convergence, collisions and crustal uplift. Orogenic belts are possibly sites of former ocean basins. Orogenic belts may therefore contain deformed continental margin sediments, arc sediments, volcanic rocks and igneous intrusions

3. By 2 b.y. ago, several small Archean granitic continents were present.

4. By 1.5 b.y. ago, Siberia and possibly also Australia and Antarctica were fused to western North America.

5. Baltica (old Scandinavia nucleus) collided with North America about 1.2 b.y. ago.




Growth of Continents

Continental crust may have grown to about 30% of its present volume by 3 b.y. ago. Up until then, crust was composed primarily of basalt with minor felsic volcanic rocks and granodioritic intrisions. Crustal growth may have accelerated between 3 b.y. - 2.5 b.y. ago during a period of extensive intrusions of granitic and granodioritic plutons. Crustal growth continued at a more modest rate after 2.5 b.y. ago.


Proterozoic Supracrustals

1. By 2.5 b.y. ago, nearly full-sized continents had assembled following the agglomeration of the early protocontinents. The original basaltic crust of continents largely evolved into a more granitic type of composition after many cycles of weathering, erosion and igneous activity.

2. Figure 8.15: Sedimentary rocks of the Archean had consisted largely of graywacke (Fig. 8.16a). The term 'graywacke' refers to an immature sandstone containing abundant mafic minerals like hornblende, biotite and even pyroxene derived from erosion of the Archean crust. As time went on, graywackes were subjected to reworking by weathering processes so that mafic minerals were gradually destroyed, leaving behind more resistant quartz and feldspar in the sedimentary record by early Proterozoic time.

3. There is evidence that stable continental shelves became established as early as later Archean time as evidenced by the 3 b.y. old Pongola Supergroup of South Africa. This rock sequence consists of a broad accumulation of flat-lying sediments and volcanic rocks deposited over the South African craton.

4. Many Archaen rocks, however, reflect unstable tectonic environments.

5. During the Proterozoic, poorly sorted graywackes were largely replaced by more light-colored, well sorted quartz sandstones, indicating that vast cratons with extensive passive margins had developed by then.

6. Early Proterozoic sediments generally differed from Archean sediments in being...

(a) texturally and compositionally more mature (Figure 8.14)

(b) better sorted (Figure 8.19)

(c) possessing features like ripple marks and cross-bedding, indicating transport of sand by wind and water (Figures 8.9 & 8.18).

(d) possessing graded bedding indicating deposition from sediment-laden turbidity currents.

(e) Proterozoic sediments are more likely associated with non-terrigenous, chemical sedimentary rocks such as carbonate and evaporite.

7. Figure 8.7: The Proterozoic also contains abundant iron formations composed of bands of metallic iron and chert. The chert was either precipitated directly from seawater. Alternatively, the original grains may have been replaced through precipitated of silica from solutions percolating slowly through the sediment. The banded iron-chert deposits were mostly deposited during the Late Archean and Early Proterozoic and possibly reflect increasing oxygen in the atmosphere.

8. The Proterozoic rock record includes abundant limestone and associated stromatolites (wavy, laminated structures formed by marine cyanobacteria) as seen in Figure 8.22. The presence of limestone suggests deposition on a stable continental shelf in warm shallow water near a tectonically stable landmass shedding little or no clastic sediment.


Major Proterozoic Orogenies in North America

1. The vast amounts of early Proterozoic quartz rich sediments indicates that the North American craton was already large, contained a great deal of granitic rock and had undergone significant erosion by 2 b.y. ago.

2. Several major orogenic events occurred in and around the Canadian Shield between 2 b.y. and 1 b.y. ago.

3. Figure 8.23: In the Great Lakes region about 2 b.y. ago, Early Proterozoic strata of the Animikean Group accumulated along the margins of an ancient stable craton located just to the north (present coordinates). The Animikean Group thickened from north to south in a direction away from the stable craton. This broad continental shelf persisted until about 1.9 b.y. ago when the Penokean Orogeny caused deformation and emplacement of igneous rocks to form the Penokean Mountains. The Penokean Orogeny possibly resulted from collision of the continental margin with a microcontinent just south of the present Great Lakes region.

4. Figures 8.11 & 8.13: The Trans-Hudson Orogenic belt formed around 1.9-1.8 b.y. ago due to closing of an ocean basin and collision between two land masses.

5. Figures 8.11 & 8.24: The Wopmay orogeny of northwestern Canada took place around 1.85 b.y. ago as a result of collision between a continental margin and microcontinent.

6. Following upheaval of the Penokean and Wopmay belts into mountain ranges, both were eventually eroded down to near sea level and subsequently covered by late Proterozoic strata.

7. Figure 8.11: A major orogenic event termed the Grenville Orogeny occurred along the southeastern margin of North America about 1 b.y. ago possibly due to collision of North America with another continent or microcontinent (Figure 8.13).

8. Figure 8.27: A major rifting event also occurred about 1 b.y. ago. The major rift valley that formed extended from the Lake Superior region southwestward into Kansas and produced extensive outpourings of basaltic lava that today comprises the Keweenawan System. (Figure 8.26). This ancient rift system is exposed as major basalt flows in the Lake Superior region and parts of Minnesota. Further south, the Keweenawan Rift System lies buried beneath younger strata but can be traced into Kansas as a band of greater-than-average gravity intensity known as the "midcontinent gravity high.


Cryptozoic Oceans and Atmosphere

1. The geologic record indicates that the early Archean atmosphere was anaerobic (devoid of free O2) as evidenced by the presence of unoxidized C, FeS2 and FeCO3 in many Archean sedimentary rocks. In addition, many Archean rocks contain Mn, Cu, Zn, V and U in their least oxidized state.

2. The first life forms to appear during the early Archean were photosynthetic cyanobacteria and algae. At that time, the atmosphere was relatively rich in CO2. The photosynthetic organisms began to release O2 into the atmosphere.

3. Figure 8.28: Scientists postulate that the oxygen scavengers, such as H2 and CO2, consumed any available free O2 in the atmosphere. Only after these oxygen sinks were largely used up could elements such as S, Fe and Mn become oxidized (see reactions in Box 8.3).

4. Atmospheric oxygen may have reached half its present concentration by 2 b.y. ago. Banded iron formations, sedimentary rocks consisting of inter-layered chert and Fe oxides, appeared in the geologic record 2.5 b.y. ago. This suggests that oxygen concentrations in the atmosphere had reached levels that caused dissolved Fe2+ in seawater to become oxidized to Fe3+ and precipitate as banded iron formations (Fig. 8.7).

5. The mutual association of iron and chert in banded iron formations was puzzling to scientists. Iron is soluable in acid and precipitates in alkaline solutions whereas silica does just the opposite. Some scientists reasoned that the higher concentrations of atmospheric CO2 during the early Proterozoic produced acid rain which, in turn, dissolved iron. The early Proterozoic oceans, being anaerobic and relatively acidic, therefore carried abundant Fe2+ [Fe(OH)2] in solution. In localized areas of cyanobacterial colonies, however, O2 was abundant enough to oxidize Fe2+ toFe 3+ and precipitate Fe2O3, FeCO3 and chert to form banded iron formations.

6. By 2.2 b.y. ago, extensive deposits of red beds (Fe2O3) and anhydrite (CaSO4) appeared in the geologic record, suggesting that atmospheric oxygen had increased to the point that Fe and S were being readily oxidized everywhere.


An Alternate View

1. Some scientists dispute the slow-accumulation theory for oxygen and claim that once photosynthesis began in the early Archean, abundant free O2 should have formed almost instantaneously rather than slowly accumulate over the course of a billion years.

2. They argue that the presence of reduced C, Fe and other metals in Archean rocks is not evidence for an anaerobic atmosphere because reduced elements continue to be deposited today in localized oxygen-poor environments such as swamps. In addition, red beds and sulfate deposits have recently been discovered in Archean rocks, suggesting that free oxygen was available during the Archean.

3. Proponents of early accumulation of oxygen argue that banded-iron formations were the result of chemical changes after deposition due to percolating water or decay of organic matter. Textural evidence suggests that many banded iron formations were originally deposited as limestones that were later replaced by Fe and Si that precipitated from percolating solutions.

4. Overall, some banded iron formations appear to represent original deposits formed in anaerobic environments whereas others indicate formation through post-depositional alteration. The debate goes on.


Early Proterozoic Glaciation

1) Evidence of glaciation have been found in rocks as old as Early Proterozoic in age. In Ontario, Canada, glacial deposits of the Proterozoic Gowganda Formation (Fig. 8.32) overlie a major unconformity, suggesting that continent-wide glaciation occurred in Canada around 2.2 b.y. ago.

2) Figure 8.33: A major glaciation event is recorded in the Late Proterozoic rock record where glacial deposits that are found on virtually all the continents. Scientists speculate that a major worldwide glaciation event, the Varangian glaciation, took place around 700-800 m.y. ago resulting in massive ice sheets covering entire continents and icebergs dispersed throughout the oceans. The occurrence of glacial tillite over an exceptionally broad latitudinal range suggests that the planet nearly became a frozen, lifeless world during the Late Proterozoic.

3) Some workers suggest that the Varangian glaciation event resulted from the earth's orbit becoming highly elliptical whereas others attribute the glaciation event to extreme tilting of the earth's axis.

4) Another theory points to plate reconstructions which places most of the continents in low and middle latitudes during the late Proterozoic. The abundance of continents around the equatorial region caused much of the solar radiation to be reflected back into space. The resulting global cooling caused ice caps to form over the polar oceans causing even more solar energy to be reflected. Eventually the earth became cold enough that glaciers even formed near the equator.

5) The worldwide ice cover, however, inhibited silicate weathering and photosynthesis, processes that remove CO2 from the atmosphere. Suppression of these CO2 sinks caused the atmospheric CO2 concentration to gradually increase. Eventually, the abundant atmospheric CO2 caused global warming which, in turn, melted the glaciers.

6) Soon after Varangian glaciation ended and the planet warmed, multicellular organisms appeared.