Chapter 8 Cryptozoic History

1. Chapter 8 • Cryptozoic History • The Cryptozoic Eon (the time before abundantly fossilized life) spans the interval between the origin of the earth 4.6 billion years ago to about 600 million years ago, or over 80 percent of the earth history. • Because Cryptozoic fossils are rare, correlation must be done by field criteria and isotopic dating, so our resolution of Cryptozoic events is relatively crude. • What did our planet look like 3 - 4 billions years ago?? • No living creatures, just a few mute bacteria only. • the planet was covered with countless volcanoes belching forth gas and steam all over the surface. • Collisions of giant meteorites with the earth's juvenile surface. • Continents, oceans and mountain belts would not be clearly differentiated as yet, and the scattered, small areas of land would be covered with barren rocky plains and sand dunes rather than forests or grasslands. Fig. 8.1.

2. Fig. 8.1 • Artists conception of the scenery during most of the first half billion years of the earths existence. • It was described as • follow:- • Mostly molten surface. • a few cooling crustal • fragments beginning to • form microcontinents. • Meteorite • bombardment was • intense. • The moon was almost • twice as close, exerting • enormous tidal pull on • the earths surface . • The atmosphere had no • free oxygen, but may have • been loaded with nitrogen, • methane, ammonia, • carbon dioxide and water.

3. General characters of cryptozoic rocks:- • - The importance of the Cryptozoic rocks comes from that it forms the major source of Copper, Iron and other metals like gold, silver, nickel, chromium and uranium. • - Perhaps half of the worlds metallic mineral resources come from the Cryptozoic rocks. • - The cryptozoic rocks characterized by lacking index fossils, although microscopic cyanobacterial fossils now show promise for dating. • Many of them are severely deformed, metamorphosed and deeply eroded. Fig. 8.2 • The chronology or dating of these rocks was established by isotopes. This point will be our next topic.

4. Fig. 8.2 Banded high-grade metamorphic rocks, typical of ancient shield regions, exposed by glaciation along Sonrde Stromfjord, Greenland. Some of the oldest dated rocks in the world (3.8 billion years) occur near here.

5. ARCHEAN EON Each continent contains pieces of the Earth’s oldest crust referred to as shields.

7. 8.1 Development of a Cryptozoic Chronology • 8.1.1 Sedgwick in Wales • Adam Sedgwick recognized clearly the relationships • between older, unfossiliferous rocks and fossiliferous • early Paleozoic ones in Wales. • Based on that Azoic ‘without life’ name was the first name • proposed for Precambrian time. After the fossils were • found the name Eozoic and Archeozoic ‘ancient life’ and • Cryptozoic ‘obscure life’ was proposed. • The Precambrian- Paleozoic boundary, how it can be • defined ??? • Sedgwick depend on that the Precambrian rocks are more • deformed and metamorphosed than the overlain • Paleozoic ones. • The presence of the unconformity between the two rock • sequences. • Traditionally the lowest stratigraphic appearance of Cambrian index fossils has defined that boundary, today this boundary can be defined by isotopes.

8. 8.1.2 The Canadian Shield: • Cryptozoic rocks first recognized in Britain, north • America and Scandinavia. Fig. 8.3 • The cryptozoic rocks on all continents are most widely • exposed in the stable continental Cratons or shield. • The shield concept comes out from the appearance of • these rocks on the geologic map, while the shields are • largely accidents of erosion where the stripping off of • later deposits has exposed the ancient basement. • The tectonic term Craton is more useful than shield • because it defines the overall relative structural stability • of a large portion of the earths crust through a long time • interval regardless of the age of the rocks exposed there • today.

9. Fig. 8.3 Cryptozoic and earliest Paleozoic isotopic-data provinces of basement rocks of continents.

10. Table 8.1 Copy to all !!!!

11. 8.2Interpretation of Crustal development from Sediments • The sources from which sedimentary rocks were derived are reflected by composition. • Composition can be modified by climatic condition or chemical changes after deposition. • 8.2.1Terrigenous Versus Non-terrigenous Sediments • There is two type of sediments: • Terrigenous clastic sediments contain chiefly silicate minerals, such as quartz, and are derived from erosion of older rocks in land areas. • Non- terrigenous sediments, formed with aqueous depositional environments where terrigenous material was not abundant, include chemically precipitated sediments, such as the evaporites and carbonate rocks composed of fossils skeletal debris or precipitated calcareous particles.

12. 8.2.2 Textural Maturity • Clastic textures reflect primarily:- • - Theratesof physical sedimentary processes. • - The intensities of physical sedimentary processes. • A. Grain size or coarseness reflects the power of • transporting agents, such as :- • - Wind which normally moves only sand and silt. • - Moving water can carry sand, silt and gravel. • - Mudflows and Glaciers can carry all and large blocks for along • distances because of their grater density and viscosity. • In general way, size tends to decrease with time and • distance of transport. Why??? • - decreasing of the carrying power with distance for most agents. • - reduction of particle size by abrasion. • Therange of sizes in a given clastic sediment, described • as size sorting, which depend on:- • - the total time of transport . • - constancy of physical energy of transportive agents.

13. Example:- - A sediment subjected to long and constant agitation (e.g., beach sand) tend to be well sorted, because there is maximum opportunity for the early dropping out of large particles and the removal or winnowing away of fine materials. So, the final result will be :- a. deposition of gravel near the source. b. well-sorted sand in another place, sea ward. c. well-sorted particles of fine silt and clay in a third place, more deep water. Generally, clastic sediments become finer as they are moved farther from their source. Fig. 4.9 and 4.12. Fig. 4.12 Fig. 4.9

14. B. Degree of rounding of sharp corners of fragments also is related to: a. intensity of abrasion. b. duration of abrasion. c. toughness of the materials themselves, i.e. the clasts. Generalization: with greater abrasion and winnowing by currents or waves, size of particles will be reduced, sorting of size will improve, and rounding will increase, Fig. 8.14. Fig. 8.14 Idealized development of textural maturity of sand through abrasion and separation or sorting of different sized grains.

15. Fig. 8.14 Idealized development of textural maturity of sand through abrasion and separation or sorting of different sized grains.

16. Fig. 8.16 A: Photograph taken through a polarizing microscope of Protrozoic sandstone. It shows:- An immature graywacke sst. Composed of a mixture of quartz, feldspar, and diverse rock fragment grains surrounded by a fine, dark clay, note the angularity and poor sorting of the grains.

17. Fig. 8.16 B: Photograph taken through a polarizing microscope of Protrozoic pure quartz sandstone. It shows:- Well –rounded quartz grains, lack of fine, dark material, this lead to say that the sandstone is compositionally and texturally mature. Fig. 8.16b

18. 8.2.3 Compositional Maturity • The mineral composition of a clastic sediment will change as its particles are subjected to:- • a. repeated physical crushing. • b. chemical destruction of the less stable minerals. • Rock fragments tend to be ground down rapidly to their separate mineral grains, and dark minerals, such as pyroxenes and amphiboles, suffer rapid chemical breakdown. • The more stable mineral will stay and reach the ultimate residue, such as quartz, feldspar, some mica and some heavy minerals such zircon, garnet, and magnetite. • So, the composition of the ultimate residue will depend on the:- • Resistance of the original components. • Abundance of the original components. • See Fig. 8.15.

19. Fig. 8.15 Typical changes through time of the mineral composition of sand that was derived from erosion of a granitic source. Less stable minerals are broken down both physically and chemically to leave a residue of most resistant mineral grains.

20. 8.2.4 Stratification Stratification also provides important clues about depositional processes. For example, thin, horizontal lamination in very fine sediments is formed by slow settling of clay and silt particles from suspension within a transporting fluid. This type of sediments need a minimum of current agitation, therefore, a deep lake or sea bottom environment is the best proposed one for this type of sediments. A. Cross- stratification is the most common feature of sands and fine gravels deposited by either wind or water. Most cross-stratification is formed by moderately strong, turbulent traction currents that roll and bounce particles over a loose sediment surface corrugated by small-scale ripples or larger-scale dunes. Fig. 8.17 Grains carried to the crests of the ripple or dune roll or slide down the steeper side. Therefore, the resulting inclined cross-stratification reflects the steeper face of the migrated ripple or dune, and it dips toward the down current direction. Fig. 8.18.

21. Fig. 8.9

22. Fig. 8.17 Ripple marks in early Proterozoic quartzite. Ripples are exposed on three different stratification planes: those on the middle surface (just left of the center) are perpendicular in trend to the others, indicating a 90˚ shift of wave and current directions between the times of deposition of these strata.

23. Fig. 8.18: Origin of cross-stratification by the migration of ripples or dunes produced by vigorous bottom currents. Grains roll and bounce over the dune crest, coming to rest on the steeper faces. Successive inclined or cross-stratification forms as the steeper face migrates; each lamina is a buried fossil steeper face.

24. Fig. 8.19: Comparison of relative sorting of sand-grains sizes by different sedimentary processes. Sorting helps in determining the origin of an ancient sandstone; for example, note the great difference of sorting by surf and turbidity currents.

25. 8.3 The Cryptozoic Sedimentary and Volcanic Record • First of all, no record is known for the first 100 million • years of the earth history. • We know that the earths entire surface was volcanic and • suffered intense bombardment by large meteorites. • As heat began to dissipate and large meteorite impacts • ceased or stopped about 3.9 billion years ago, local crustal blocks • began to survive. • The oldest known rocks (2.5 - 4.0 billion years) are two • different assemblages:- • Greenstone belts, which are made up of mildly metamorphosed • volcanic rocks and associated with sediments. • “green stone” drives from dark, green-colored minerals produced by metamorphism of mafic igneous rocks. • Gneiss belts, high- grade metamorphic rocks. • The relation between the above two assemblages is not clear, one view is that they differ only in degrees of metamorphism and reconstitution. • The other explanations assume that the gneisses are remnants of incipient continents, whereas the greenstone belts remnants of oceans and volcanic arcs formed between continents.

26. 8.3.1 Early and Middle Proterozoic Sediment The Proterozoic sediments are two type: 1. Poorly sorted graywackes. 2. Light –colored, well-sorted pure quartz sandstones, this sand show cross-strata, ripple marks and well- rounded quartz or chert pebbles. 3. As a third component the limestone, which mainly contain wavy laminated structure called stromatolites formed by marine, bottom-dwelling cyanobacteria. Fig. 8.22 Stromatolitic reef structure from 1.6 billion year- old Proterozoic carbonate strata.

27. 8.4 The Cryptozoic Ocean and Atmosphere • Archean rocks bear evidence of anaerobic early conditions, that was supported geologically by many clues, such as :- • Many Archean sediments are dark-colored due to the presence of • unoxidized carbon, fine iron sulfide (FeS2) and iron carbonate (FeCO3) minerals. • 2. Archean rocks contain not only iron but also several other metals with affinities for oxygen, including manganese, copper, zinc, vanadium, and uranium, which present widely in their least oxidized states. • 3. Sulfur has an affinity for oxygen but it present only in its unoxidized or reduced state in the mineral pyrite (FeS2). • 4. Differences between sulfur isotopes in Archean and Proterozoic sediments were thought to reflect a contrast in atmospheric oxygen content. • The Archean chemical sinks consumed any available early oxygen, this was by oxidation of hydrogen (H2O) and carbon (CO2). • After these sinks were full saturated with oxygen, sulfur, iron and other metals begin to be oxidized, this was indicated by red color of oxidized iron and oxidized sulfur (CaSO4).

28. It was inferred that this saturation occurred sometime • after photosynthetic organisms appeared and began to • release O2 into the atmosphere. • Appearance of widespread, red-colored strata, called ‘red • beds’ among Proterozoic rocks seemed to confirm • a gradual accumulation of free O2. • An Alternative View • The geologic evidence which mentioned above cited or • indicated that the Archean condition was an aerobic, this • view is fit to the biochemists. • But some biologist argued that the free O2 should be at • the same time generated to the atmosphere since the • photosynthesis began, so they think that no need to one • billion years which was suggested by geologist for the • consumed O2by the chemical sinks.

29. A growing number of geologist have challenged the slow- accumulation of free oxygen on the following ground :- • The unoxidized carbon in Archean sediments may not be significantly different, either in total abundance or in isotopic makeup, from carbon in younger ones. • Free carbon as well as iron sulfide and iron carbonate minerals have been preserved in oxygen-poor muddy environments (such swamps) right up to the present day. • Unoxidized manganese, copper, zinc, vanadium, uranium and the like, although widely scattered, are not very common in Archean sediments and, so, like free carbon and iron sulfide, might be explained by local anaerobic conditions of deposition. • Some Archean red beds and oxidized sulfate evaporites have now been discovered, as well as some oxidized Archean soils. • Therefore, this group of geologist believe that • significant free O2 existed even in • the Archean atmosphere.

30. 8.5 Cryptozoic Climate Evidence about cryptozoic climates is scant, but there is nothing that indicates conditions different from later geologic time. Chemical arguments suggest more atmospheric CO2, a condition that should have caused a warmer average global temperature (greenhouse effect) and more acidic rains. Anyhow, there is some direct evidence of climatic extremes, such as:- A. Evaporite deposits and mud cracks attest to dray and probably hot conditions sufficient to completely evaporite sea-water locally. Fig. 8.31 B. Proterozoic wind deposits also are known, and they suggest large desert dune fields. C. Evidence of cold periods is even better known by glaciers deposits. لاىلاالالاش

31. Fig. 8.31 Mud cracks in red shale, rocks such as these, along with salt crystals, show that hot, dry conditions were common 1.8 billion years ago and that enough free oxygen was present in the atmosphere to turn the sediments rusty red. Fig. 8.31

32. 8.5.1 Early Proterozoic Glaciation As a strong indicator for cold conditions coming from sediments, were Gowganda Formation in Ontario show the following features of the sediments:- - massive rock type. - almost unstratified and unsorted large boulders, pebbles, sand, and fine clay surrounded by a fin matrix of dark material. - laminated mudstone that resembles Pleistocene glacial lake clays containing laminae interpreted as seasonal layers, some of these layers contain scattered pebbles. Fig. 8.32. - at several localities where the surface is exposed, fine, parallel scratches are visible, which strongly resemble striations made by glaciers. All of these features which had been seen in the Gowganda Formation are direct clues for glaciers deposits or moraine.

33. Fig. 8.32 Fig. 8.32 Laminated mudstone with scattered pebbles and sand grains dropped from above. Association with glacial tills suggests dropping of sandstones from drifting icebergs.

34. 8.5.1 Late Proterozoic Glaciation • The end of the Proterozoic was marked by a great global • glaciation event, the Varangian glaciation, which • produced continental glaciers at nearly equatorial • latitudes. Fig. 8.33 • The exact causes of this extraordinary glaciation are • controversial, but the planet nearly became a lifeless • frozen world, like Mars. • When the Varangian glaciers retreated, multicellular life • emerged for first time after nearly 3 billion years of • single-celled life on this planet.

35. Fig. 8.33 Global distribution of late Proterozoic (Varangian) glacial deposits (triangles), showing their occurrence in ancient equatorial regions. In some places, these glacial deposits are interbedded with marine limestone, further proving their low-latitude origin. Such evidence leads some scientists to suggest that the earth my have barely avoided freezing over completely in the Varangian. Fig. 8.33