Proterozoic Rocks, Glacier NP Proterozoic sedimentary rocks in Glacier National Park, Montana The angular peaks, ridges and broad valleys were carved by Pleistocene and Recent glaciers

The Length of the Proterozoic the Proterozoic Eon alone, at 1.955 billion years long, accounts for 42.5% of all geologic time yet we review this long episode of Earth and life history in a single section

The Phanerozoic Yet the Phanerozoic, consisting of Paleozoic, Mesozoic, Cenozoic eras, lasted a comparatively brief 545 million years is the subject of the rest of the course

Disparity in Time Perhaps this disparity but we know far more between the coverage of the Proterozoic and the Phanerozoic seems disproportionate, but we know far more about Phanerozoic events than we do for either of the Precambrian eons

Archean-Proterozoic Boundary Geologist have rather arbitrarily placed the Archean-Proterozoic boundary at 2.5 billion years ago because it marks the approximate time of changes in the style of crustal evolution However, we must emphasize "approximate," because Archean-type crustal evolution was largely completed in South Africa nearly 3.0 billion years ago, whereas in North America the change took place from 2.95 to 2.45 billion years ago

Style of Crustal Evolution Archean crust-forming processes generated granite-gneiss complexes and greenstone belts that were shaped into cratons Although these same rock associations continued to form during the Proterozoic, they did so at a considerably reduced rate

Contrasting Metamorphism In addition, Archean and Proterozoic rocks contrast in metamorphism Many Archean rocks have been metamorphosed, although their degree of metamorphism varies and some are completely unaltered However, vast exposures of Proterozoic rocks show little or no effects of metamorphism, and in many areas they are separated from Archean rocks by a profound unconformity

Other Differences the Proterozoic is characterized In addition to changes in the style of crustal evolution, the Proterozoic is characterized by widespread rock assemblages that are rare or absent in the Archean, by a plate tectonic style essentially the same as that of the present by important evolution of the atmosphere and biosphere by the origin of some important mineral resources

Proterozoic Evolution of Oxygen-Dependent Organisms It was during the Proterozoic that oxygen-dependent organisms made their appearance and the first cells evolved that make up most organisms today

Evolution of Proterozoic Continents Archean cratons assembled during collisions of island arcs and minicontinents, providing the nuclei around which Proterozoic crust accreted, thereby forming much larger landmasses Proterozoic accretion at craton margins probably took place more rapidly than today because Earth possessed more radiogenic heat, but the process continues even now

Proterozoic Greenstone Belts Most greenstone belts formed during the Archean between 2.7 and 2.5 billion years ago They also continued to form during the Proterozoic and at least one is known from Cambrian-aged rocks in Australia They were not as common after the Archean, and differed in one important detail the near absence of ultramafic rocks which no doubt resulted from Earth's decreasing amount of radiogenic heat

Focus on Laurentia Our focus here is on the geologic evolution of Laurentia, a large landmass that consisted of what is now North America, Greenland, parts of northwestern Scotland, and perhaps some of the Baltic shield of Scandinavia

Early Proterozoic History of Laurentia Laurentia originated and underwent important growth between 2.0 and 1.8 billion years ago During this time, collisions among various plates formed several orogens, which are linear or arcuate deformation belts in which many of the rocks have been metamorphosed and intruded by magma thus forming plutons, especially batholiths

Proterozoic Evolution of Laurentia Archean cratons were sutured along deformation belts called orogens, thereby forming a larger landmass By 1.8 billion years ago, much of what is now Greenland, central Canada, and the north-central United States existed Laurentia grew along its southern margin by accretion

Craton-Forming Processes Examples of these craton-forming processes are recorded in rocks in the Thelon orogen in northwestern Canada where the Slave and Rae cratons collided,

Craton-Forming Processes the Trans Hudson orogen in Canada and the United States, where the Superior, Hearne, and Wyoming cratons were sutured The southern margin of Laurentia is the site of the Penokian orogen

Wilson Cycle Rocks of the Wopmay orogen A complete Wilson cycle, in northwestern Canada are important because they record the opening and closing of an ocean basin or what is called a Wilson cycle A complete Wilson cycle, named for the Canadian geologist J. Tuzo Wilson, involves fragmentation of a continent, opening followed by closing of an ocean basin, and finally reassembly of the continent

Wopmay Orogen Some of the rocks in Wopmay orogen are sandstone-carbonate-shale assemblages, a suite of rocks typical of passive continental margins that first become widespread during the Proterozoic

Early Proterozoic Rocks in Great Lakes Region Early Proterozoic sandstone-carbonate-shale assemblages are widespread near the Great Lakes

Outcrop of Sturgeon Quartzite The sandstones have a variety of sedimentary structures such as ripple marks and cross- beds Northern Michigan

Outcrop of Kona Dolomite Some of the carbonate rocks, now mostly dolostone, such as the Kona Dolomite, contain abundant bulbous structures known as stromatolites Northern Michigan

Penkean Orogen These rocks of northern Michigan have been only moderately deformed and are now part of the Penokean orogen

Accretion along Laurentia’s Southern Margin Following the initial episode of amalgamation of Archean cratons 2.0 to 1.8 billion years ago accretion took place along Laurentia's southern margin From 1.8 to 1.6 billion years ago, continental accretion continued in what is now the southwestern and central United States as successively younger belts were sutured to Laurentia, forming the Yavapai and Mazatzal-Pecos orogens

Southern Margin Accretion Laurentia grew along its southern margin by accretion of the Central Plains, Yavapai, and Mazatzal orogens Also notice that the Midcontinental Rift had formed in the Great Lakes region by this time

BIF, Red Beds, Glaciers This was also the time during which most of Earth’s banded iron formations (BIF) were deposited The first continental red beds sandstone and shale with oxidized iron were deposited about 1.8 billion years ago We will have more to say about BIF and red beds in the section on “The Evolving Atmosphere” In addition, some Early Proterozoic rocks and associated features provide excellent evidence for widespread glaciation

Early and Middle Proterozoic Igneous Activity During the interval from 1.8 to 1.1 billion years ago, extensive igneous activity took place that seems to be unrelated to orogenic activity Although quite widespread, this activity did not add to Laurentia’s size because magma was either intruded into or erupted onto already existing continental crust

Igneous Activity These igneous rocks are exposed in eastern Canada, extend across Greenland, and are also found in the Baltic shield of Scandinavia

Igneous Activity However, the igneous rocks are deeply buried by younger rocks in most areas The origin of these granitic and anorthosite plutons, Anorthosite is a plutonic rock composed almost entirely of plagioclase feldspars calderas and their fill, and vast sheets of rhyolite and ash flows are the subject of debate According to one hypothesis large-scale upwelling of magma beneath a Proterozoic supercontinent produced the rocks

Middle Proterozoic Orogeny and Rifting The only Middle Proterozoic event in Laurentia was the Grenville orogeny in the eastern part of the continent 1.3 to 1.0 billion years old Grenville rocks are well exposed in the present-day northern Appalachian Mountains as well as in eastern Canada, Greenland, and Scandinavia

Grenville Orogeny A final episode of Proterozoic accretion occurred during the Grenville orogeny

Grenville Orogeny Many geologists think the Grenville orogen resulted from closure of an ocean basin, the final stage in a Wilson cycle Others disagree and think intracontinental deformation or major shearing was responsible for deformation Whatever the cause of the Grenville orogeny, it was the final stage in the Proterozoic continental accretion of Laurentia

75% of North America By this final stage, about 75% The remaining 25% of present-day North America existed The remaining 25% accreted along its margins, particularly its eastern and western margins, during the Phanerozoic Eon

Midcontinent Rift Grenville deformation in Laurentia was accompanied by the origin of the Midcontinent rift, a long narrow continental trough bounded by faults, extending from the Lake Superior basin southwest into Kansas, and a southeasterly branch extends through Michigan into Ohio It cuts through Archean and Early Proterozoic rocks and terminates in the east against rocks of the Grenville orogen

Location of the Midcontinent Rift Rocks filling the rift are exposed around Lake Superior but are deeply buried elsewhere

Midcontinental Rift Most of the rift is buried beneath younger rocks except in the Lake Superior region where various igneous and sedimentary rocks are well exposed The central part of the rift contains numerous overlapping basalt lava flows forming a volcanic pile several kilometers thick In fact, the volume of volcanic rocks, between 300,000 and 1,000,000 km3, is comparable in volume although not areal extent to the great outpourings of lava during the Cenozoic

Midcontinental Rift Along the rift's margins In the vertical section coarse-grained sediments were deposited in large alluvial fans that grade into sandstone and shale with increasing distance from the sediment source In the vertical section Freda Sandstone overlies Cooper Harbor conglomerate, which overlies Portage Lake Volcanics

Cooper Harbor Conglomerate Michigan

Portage Lake Volcanics Michigan

Middle and Late Proterozoic Sedimentation Remember the Grenville orogeny took place 1.2 billion – 900 million years ago, the final episode of continental accretion in Laurentia until the Ordovician Period Nevertheless, important geologic events were taking place, such as sediment deposition in what is now the eastern United States and Canada, in the Death Valley region of California and Nevada, and in three huge basins in the west

Sedimentary Basins in the West Map showing the locations of sedimentary Basins in the western United States and Canada Belt Basin Uinta Basin Apache Basin

Sedimentary Rocks Middle to Late Proterozoic sedimentary rocks are exceptionally well exposed in the northern Rocky Mountains of Montana and Alberta, Canada Indeed, their colors, deformation features, and erosion by Pleistocene and recent glaciers have yielded some fantastic scenery Like the rocks in the Great Lakes region and the Grand Canyon, they are mostly sandstones, shales, and stromatolite-bearing carbonates

Proterozoic Mudrock Outcrop of red mudrock in Glacier National Park, Montana

Proterozoic Limestone Outcrop of limestone with stromatolites in Glacier National Park, Montana

Proterozoic Sandstone Proterozoic rocks of the Grand Canyon Super-group lie unconformably upon Archean rocks and in turn are overlain unconformably by Phanerozoic-age rocks The rocks, consisting mostly of sandstone, shale, and dolostone, were deposited in shallow-water marine and fluvial environments The presence of stromatolites and carbonaceous impression of algae in some of these rocks indicate probable marine deposition

Grand Canyon Super-group Proterozoic Sandstone of the Grand Canyon Super-group in the Grand Canyon Arizona

Style of Plate Tectonics The present style of plate tectonics involving opening and then closing ocean basins had almost certainly been established by the Early Proterozoic In fact, the oldest known complete ophiolite providing evidence for an ancient convergent plate boundary is the Jormua mafic-ultramafic complex in Finland It is about 1.96 billion years old, but nevertheless compares closely in detail with younger well-documented ophiolites

Jormua Complex, Finland Reconstruction of the highly deformed Jormua mafic-ultramafic complex in Finland This sequence of rock is the oldest known complete ophiolite at 1.96 billion years old

Jormua Complex, Finland Metamorphosed basaltic pillow lava 12 cm

Jormua Complex, Finland Metamorphosed gabbro between mafic dikes 65 cm

Proterozoic Supercontinents You already know that a continent is one of Earth's landmasses consisting of granitic crust with most of its surface above sea level A supercontinent consists of all or at least much of the present-day continents, so other than size it is the same as a continent The supercontinent Pangaea, which existed at the end of the Paleozoic Era, is familiar, but few people are aware of earlier supercontinents

Early Supercontinents Supercontinents may have existed as early as the Late Archean, but if so we have little evidence of them The first that geologists recognize with some certainty, known as Rodinia assembled between 1.3 and 1.0 billion years ago and then began fragmenting 750 million years ago

Early Supercontinent Possible configuration of the Late Proterozoic supercontinent Rodinia before it began fragmenting about 750 million years ago

Pannotia Rodinia's separate pieces reassembled and formed another supercontinent this one known as Pannotia about 650 million years ago judging by the Pan-African orogeny the large-scale deformation that took place in what are now the Southern Hemisphere continents Fragmentation was underway again, by the latest Proterozoic, about 550 million years ago, giving rise to the continental configuration that existed at the onset of the Phanerozoic Eon

Ancient Glaciers Very few times of widespread glacial activity have occurred during Earth history The most recent one during the Pleistocene 1.6 million to 10,000 years ago is certainly the best known, but we also have evidence for Pennsylvanian glaciers and two major episodes of Proterozoic glaciation

Recognizing Glaciation How can we be sure that there were Proterozoic glaciers? After all, their most common deposit called tillite is simply a type of conglomerate that may look much like conglomerate that originated by other processes Tillite or tillite-like deposits are known from at least 300 Precambrian localities, and some of these are undoubtedly not glacial deposits

Glacial Evidence But the extensive geographic distribution of other conglomerates and their associated glacial features is distinctive, such as striated and polished bedrock

Proterozoic Glacial Evidence Bagganjarga tillite in Norway overlies striated bedrock surface on sandstone of the Veidnesbotn Formation

Geologists Convinced Geologists are now convinced based on this kind of evidence that widespread glaciation took place during the Early Proterozoic The occurrence of tillites of about the same age in Michigan, Wyoming, and Quebec indicates that North America may have had an Early Proterozoic ice sheet centered southwest of Hudson Bay

Early Proterozoic Glaciers Deposits in North America indicate that Laurentia had an extensive ice sheet centered southwest of Hudson Bay

One or More Glaciations? Tillites of about this age are also found in Australia and South Africa, but dating is not precise enough to determine if there was a single widespread glacial episode or a number of glacial events at different times in different areas One tillite in the Bruce Formation in Ontario, Canada may date from 2.7 billion years ago, thus making it Late Archean

Glaciers of the Late Proterozoic Tillites and other glacial features dating from between 900 and 600 million years ago are found on all continents except Antarctica Glaciation was not continuous during this entire time but was episodic with four major glacial episodes so far recognized

Late Proterozoic Glaciers The approximate distribution of Late Proterozoic glaciers

Most Extensive Glaciation in Earth History The map shows only approximate distribution of Late Proterozoic glaciers The actual extent of glaciers is unknown Not all the glaciers were present at the same time Despite these uncertainties, this Late Proterozoic glaciation was the most extensive in Earth history In fact, Late Proterozoic glaciers seem to have been present even in near-equatorial areas

The Evolving Atmosphere Geologists agree that the Archean atmosphere contained little or no free oxygen so the atmosphere was not strongly oxidizing as it is now Even though processes were underway that added free oxygen to the atmosphere, the amount present at the beginning of the Proterozoic was probably no more than 1% of that present now In fact, it might not have exceeded 10% of present levels even at the end of the Proterozoic

Cyanobacteria and Stromatolites Remember from our previous discussions that cyanobacteria, also known as blue-green algae, were present during the Archean, but stromatolites the structures they formed, did not become common until about 2.3 billion years ago, that is, during the Early Proterozoic These photosynthesizing organisms and to a lesser degree photochemical dissociation added free oxygen to the evolving atmosphere

Oxygen Versus Carbon Dioxide Earth's early atmosphere had abundant carbon dioxide More oxygen became available whereas the amount of carbon dioxide decreased Only a small amount of CO2 still exists in the atmosphere today It is one of the greenhouse gases partly responsible for global warming What evidence indicates that the atmosphere became oxidizing? Where is all that additional the carbon dioxide now?

Evidence from Rocks Much carbon dioxide is now tied up in various minerals and rocks especially the carbonate rocks limestone and dolostone, and in the biosphere For evidence that the Proterozoic atmosphere was evolving from a chemically reducing one to an oxidizing one we must discuss types of Proterozoic sedimentary rocks, in particular banded iron formations and red beds

Banded Iron Formations (BIF) Banded iron formations (BIFs), consist of alternating layers of iron-rich minerals and chert Some are found in Archean rocks, but about 92% of all BIFs formed during the interval from 2.5 to 2.0 billion years ago

Early Proterozoic Banded Iron Formation At this outcrop in Ishpeming, Michigan the rocks are alternating layers of red chert and silver- colored iron minerals

Typical BIF A more typical outcrop of BIF near Nagaunee, Michigan

BIFs and the Atmosphere How are these rocks related to the atmosphere? Their iron is in iron oxides, especially hematite (Fe2O3) and magnetite (Fe3O4) Iron combines with oxygen in an oxidizing atmosphere to from rustlike oxides that are not readily soluble in water If oxygen is absent in the atmosphere, though, iron easily dissolves so that large quantities accumulate in the world's oceans, which it undoubtedly did during the Archean

Formation of BIFs The Archean atmosphere was deficient in free oxygen so that little oxygen was dissolved in seawater However, as photosynthesizing organisms increased in abundance, as indicated by stromatolites, free oxygen, released as a metabolic waste product into the oceans, caused the precipitation of iron oxides along with silica and thus created BIFs

Formation of BIFs One model accounting for the details Upwelling, of BIF precipitation involves a Precambrian ocean with an upper oxygenated layer overlying a large volume of oxygen-deficient water that contained reduced iron and silica Upwelling, that is transfer of water from depth to the surface, brought iron- and silica-rich waters onto the shallow continental shelves and resulting in widespread precipitation of BIFs

Formation of BIFs Depositional model for the origin of banded iron formation

Source of Iron and Silica A likely source of the iron and silica was submarine volcanism, similar to that now talking place at or near spreading ridges Huge quantities of dissolved minerals are also discharged at submarine hydrothermal vents In any case, the iron and silica combined with oxygen thus resulting in the precipitation of huge amounts of banded iron formation Precipitation continued until the iron in seawater was largely used up

Continental Red Beds Obviously continental red beds refers to red rocks on the continents, but more specifically it means red sandstone or shale colored by iron oxides, especially hematite (Fe2O3) Red mudrock in Glacier National Park, Montana

Red Beds Red beds first appear The onset of red bed deposition in the geologic records about 1.8 billion years ago, increase in abundance throughout the rest of the Proterozoic, and are quite common in rocks of Phanerozoic age The onset of red bed deposition coincides with the introduction of free oxygen into the Proterozoic atmosphere However, the atmosphere at that time may have had only 1% or perhaps 2% of present levels

Red Beds Is this percentage sufficient to account Probably not, for oxidized iron in sediment? Probably not, but no ozone (O3) layer existed in the upper atmosphere before free oxygen (O2) was present As photosynthesizing organisms released free oxygen into the atmosphere, ultraviolet radiation converted some of it to elemental oxygen (O) and ozone (O3), both of which oxidize minerals more effectively than O2

Red Beds Once an ozone layer became established, most ultraviolet radiation failed to penetrate to the surface, and O2 became the primary agent for oxidizing minerals

Important Events in Life History Archean fossils are not very common, and all of those known are varieties of bacteria and cyanobacteria (blue-green algae), although they undoubtedly existed in profusion Likewise, the Early Proterozoic fossil record has mostly bacteria and cyanobacteria Apparently little diversification had taken place; all organisms were single-celled prokaryotes, until about 2.1 billion years ago when more complex eukaryotic cells evolved

Gunflint Microfossils Even in well-known Early Proterozoic fossils assemblages, only fossils of bacteria are recognized Photomicrograph of spheroidal and filamentous microfossils from the Gunflint Chert of Ontario Canada

Prokaryote and Eukaryotes An organism made up of prokaryotic cells is called a prokaryote whereas those composed of eukaryotic cells are eukaryotes In fact, the distinction between prokaryotes and eukaryotes is the basis for the most profound distinction between all living things

Lack of Organic Diversity Actually, the lack of organic diversity during this early time in life history is not too surprising because prokaryotic cells reproduce asexually Most variation in sexually reproducing populations comes from the shuffling of genes, and their alleles, from generation to generation Mutations introduce new variation into a population, but their effects are limited in prokaryotes

Genetic Variation in Bacteria A beneficial mutation would spread rapidly in sexually reproducing organism, but have a limited impact in bacteria because they do not share their genes with other bacteria Bacteria usually reproduce by binary fission and give rise to two cells having the same genetic makeup Under some conditions, they engage in conjugation during which some genetic material is transferred

Sexual Reproduction Increased the Pace of Evolution Prior to the appearance of cells capable of sexual reproduction, evolution was a comparatively slow process, thus accounting for the low organic diversity This situation did not persist Sexually reproducing cells probably evolved by Early Proterozoic time, and the tempo of evolution increased

Eukaryotic Cells Evolve The appearance of eukaryotic cells marks a milestone in evolution comparable to the development of complex metabolic mechanisms such as photosynthesis during the Archean Where did these cells come from? How do they differ from their predecessors, the prokaryotic cells? All prokaryotes are single-celled, but most eukaryotes are multicelled, the notable exception being the protistans

Eukaryotes Most eukaryotes reproduce sexually, in marked contrast to prokaryotes, and nearly all are aerobic, that is, they depend on free oxygen to carry out their metabolic processes Accordingly, they could not have evolved before at least some free oxygen was present in the atmosphere

Prokaryotic Cell Prokaryotic cells do not have a cell nucleus do not have organelles are smaller and not nearly as complex as eukaryotic cells

Eukaryotic Cell Eukaryotic cells have a cell nucleus containing the genetic material and organelles such as mitochondria and plastids, as well as chloroplasts in plant cells

Eukaryotic Fossil Cells The Negaunee Iron Formation in Michigan which is 2.1 billion years old has yielded fossils now generally accepted as the oldest known eukaryotic cells Even though the Bitter Springs Formation of Australia is much younger --1 billion yrs old it has some remarkable fossils of single-celled eukaryotes that show evidence of meiosis and mitosis, processes carried out only by eukaryotic cells

Evidence for Eukaryotes Prokaryotic cells are mostly rather simple spherical or platelike structures Eukaryotic cells are larger much more complex have a well-defined, membrane-bounded cell nucleus, which is lacking in prokaryotes have several internal structures called organelles such as plastids and mitochondria their organizational complexity is much greater than it is for prokaryotes

Acritarchs Other organisms that were almost certainly eukaryotes are the acritarchs that first appeared about 1.4 billion years ago they were very common by Late Proterozoic time and were probably cysts of planktonic (floating) algae

Acritarchs These common Late Proterozoic microfossils are probably from eukaryotic organisms Acritarchs are very likely the cysts of algae

Late Proterozoic Microfossil Numerous microfossils of organisms with vase-shaped skeletons have been found in Late Proterozoic rocks in the Grand Canyon These too have tentatively been identified as cysts of some kind of algae

Endosymbiosis and the Origin of Eukaryotic Cells Eukaryotic cells probably formed from several prokaryotic cells that entered into a symbiotic relationship Symbiosis, involving a prolonged association of two or more dissimilar organisms, is quite common today In many cases both symbionts benefit from the association as occurs in lichens, once thought to be plants but actually symbiotic fungi and algae

Endosymbiosis In a symbiotic relationship, This may have been the case each symbiont must be capable of metabolism and reproduction, but in some cases one symbiont cannot live independently This may have been the case with Proterozoic symbiotic prokaryotes that became increasingly interdependent until the unit could exist only as a whole In this relationship one symbiont lived within the other, which is a special type of symbiosis called endosymbiosis

Evidence for Endosymbiosis Supporting evidence for endosymbiosis comes from studies of living eukaryotic cells containing internal structures called organelles, such as mitochondria and plastics, which contain their own genetic material In addition, prokaryotic cells synthesize proteins as a single system, whereas eukaryotic cells are a combination of protein-synthesizing systems

Organelles Capable of Protein Synthesis That is, some of the organelles within eukaryotic cells are capable of protein synthesis These organelles with their own genetic material and protein-synthesizing capabilities are thought to have been free-living bacteria that entered into a symbiotic relationship, eventually giving rise to eukaryotic cells

Multicelled Organisms Obviously multicelled organisms are made up of many cells, perhaps billions, as opposed to a single cell as in prokaryotes In addition, multicelled organisms have cells specialized to perform specific functions such as respiration, food gathering, and reproduction

Dawn of Multicelled Organisms We know from the fossil record that multicelled organisms were present during the Proterozoic, but we do not know exactly when they appeared What seem to be some kind of multicelled algae appear in the 2.1-billion-year-old fossils from the Negaunee Iron Formation in Michigan as carbonaceous filaments from 1.8 billion-year-old rocks in China as somewhat younger carbonaceous impressions of filaments and spherical forms

Multicelled Algae? Carbonaceous impressions in Proterozoic rocks, Montana These may be impressions of multicelled algae Skip next slide

The Multicelled Advantage? Is there any particular advantage to being multicelled? For something on the order of 1.5 billion years all organisms were single-celled and life seems to have thrived In fact, single-celled organisms are quite good at what they do but what they do is very limited

The Multicelled Advantage? For example, single celled organisms can not grow very large, because as size increases proportionately less of a cell is exposed to the external environment in relation to its volume and the proportion of surface area decreases Transferring materials from the exterior to the interior becomes less efficient

The Multicelled Advantage? Also, multicelled organisms live longer, since cells can be replaced and more offspring can be produced Cells have increased functional efficiency when they are specialized into organs with specific capabilities

Late Proterozoic Animals Biologists set forth criteria such as method of reproduction and type of metabolism to allow us to easily distinguish between animals and plants Or so it would seem, but some present-day organisms blur this distinction and the same is true for some Proterozoic fossils Nevertheless, the first relatively controversy-free fossils of animals come from the Ediacaran fauna of Australia and similar faunas of similar age elsewhere

The Ediacaran Fauna In 1947, an Australian geologist, R.C. Sprigg, in the Pound Quartzite in the Ediacara Hills of South Australia Additional discoveries by others turned up what appeared to be discovered impressions of soft-bodied animals impressions of algae and several animals many bearing no resemblance to any existing now Before these discoveries, geologists were perplexed by the apparent absence of fossil-bearing rocks predating the Phanerozoic

Ediacaran Fauna The Ediacaran fauna of Australia Tribrachidium heraldicum, a possible primitive echinoderm Spriggina floundersi, a possible ancestor of trilobites

Ediacaran Fauna Pavancorina minchami Restoration of the Ediacaran Environment

Ediacaran Fauna Geologists had assumed that the fossils so common in Cambrian rocks must have had a long previous history but had little evidence to support this conclusion The discovery of Ediacaran fossils and subsequent discoveries have not answered all questions about pre-Phanerozoic animals, but they have certainly increased our knowledge about this chapter in the history of life

Represented Phyla Three present-day phyla may be represented in the Ediacaran fauna: jellyfish and sea pens (phylum Cnidaria), segmented worms (phylum Annelida), and primitive members of the phylum Arthropoda (the phylum with insects, spiders crabs, and others) One Ediacaran fossil, Spriggina, has been cited as a possible ancestor of trilobites Another might be a primitive member of the phylum Echinodermata

Distinct Evolutionary Group However, some scientists think these Ediacaran animals represent an early evolutionary group quite distinct from the ancestry of today’s invertebrate animals Ediacara-type faunas are known from all continents except Antarctica, --were widespread between 545 and 670 million years ago but their fossils are rare Their scarcity should not be surprising, though, because all lacked durable skeletons

Other Proterozoic Animal Fossils Although scarce, a few animal fossils older than those of the Ediacaran fauna are known A jellyfish-like impression is present in rocks 2000 m below the Ediacara Hills Pound Quartzite, Burrows, in many areas, presumably made by worms, occur in rocks at least 700 million years old Wormlike and algae fossils come from 700 to 900 million-year-old rocks in China but the identity and age of these "fossils" has been questioned

Wormlike Fossils from China Wormlike fossils from Late Proterozoic rocks in China

Soft Bodies All known Proterozoic animals were soft-bodied, but there is some evidence that the earliest stages in the origin of skeletons was underway Even some Ediacaran animals may have had a chitinous carapace and others appear to have had areas of calcium carbonate The odd creature known as Kimberella from the latest Proterozoic of Russia had a tough outer covering similar to that of some present-day marine invertebrates

Latest Proterozoic Kimberella Kimberella, an animal from latest Proterozoic rocks in Russia Exactly what Kimberella was remains uncertain Some think it was a sluglike creature whereas others think it was more like a mollusk

Durable Skeletons Latest Proterozoic fossils of minute scraps of shell-like material and small tooth like denticles and spicules, presumably from sponges indicate that several animals with skeletons or at least partial skeletons existed However, more durable skeletons of silica, calcium carbonate, and chitin (a complex organic substance) did not appear in abundance until the beginning of the Phanerozoic Eon 545 million years ago

Proterozoic Mineral Resources Most of the world's iron ore comes from Proterozoic banded iron formations Canada and the United States have large deposits of these rocks in the Lake Superior region and in eastern Canada Thus, both countries rank among the ten leading nations in iron ore production

Iron Mine The Empire Mine at Palmer, Michigan where iron ore from the Early Proterozoic Negaunee Iron Formation is mined

Nickel In the Sudbury mining district in Ontario, Canada, nickel and platinum are extracted from Proterozoic rocks Nickel is essential for the production of nickel alloys such as stainless steel and Monel metal (nickel plus copper), which are valued for their strength and resistance to corrosion and heat The United States must import more than 50% of all nickel used mostly from the Sudbury mining district

Sudbury Basin Besides its economic importance, the Sudbury Basin, an elliptical area measuring more than 59 by 27 km, is interesting from the geological perspective One hypothesis for the concentration of ores is that they were mobilized from metal-rich rocks beneath the basin following a high-velocity meteorite impact

Platinum and Chromium Some platinum for jewelry, surgical instruments, and chemical and electrical equipment is exported to the United States from Canada, but the major exporter is South Africa The Bushveld Complex of South Africa is a layered igneous complex containing both platinum and chromite the only ore of chromium, United States imports much of the chromium from South Africa It is used mostly in stainless steel

Oil and Gas Economically recoverable oil and gas have been discovered in Proterozoic rocks in China and Siberia, arousing some interest in the Midcontinent rift as a potential source of hydrocarbons So far, land has been leased for exploration, and numerous geophysical studies have been done However, even though some rocks within the rift are know to contain petroleum, no producing oil or gas wells are operating

Proterozoic Pegmatites A number of Proterozoic pegmatites are important economically The Dunton pegmatite in Maine, whose age is generally considered to be Late Proterozoic, has yielded magnificent gem-quality specimens of tourmaline and other minerals Other pegmatites are mined for gemstones as well as for tin, industrial minerals, such as feldspars, micas, and quartz and minerals containing such elements as cesium, rubidium, lithium, and beryllium

Proterozoic Pegmatites Geologists have identified more than 20,000 pegmatites in the country rocks adjacent to the Harney Peak Granite in the Black Hills of South Dakota These pegmatites formed ~ 1.7 billion years ago when the granite was emplaced as a complex of dikes and sills A few have been mined for gemstones, tin, lithium, micas, and some of the world's largest known mineral crystals were discovered in these pegmatites

Summary The crust-forming processes that yielded Archean granite-gneiss complexes and greenstone belts continued into the Proterozoic but at a considerably reduced rate Archean and Proterozoic greenstone belts differed in detail Early Proterozoic collisions between Archean cratons formed larger cratons that served as nuclei around which Proterozoic crust accreted

Summary One such landmass was Laurentia Important events consisting mostly of North America and Greenland Important events in the evolution of Laurentia were Early Proterozoic amalgamation of cratons followed by Middle Proterozoic igneous activity, the Grenville orogeny, and the Midcontinent rift Ophiolite sequences marking convergent plate boundaries are first well documented from the Early Proterozoic, indicating that a plate tectonic style similar to that operating now had been established

Summary Sandstone-carbonate-shale assemblages deposited on passive continental margins are known from the Archean but they are very common by Proterozoic time The supercontinent Rodinia assembled between 1.3 and 1.0 billion years ago, fragmented, and then reassembled to form Pannotia about 650 million years ago Glaciers were widespread during both the Early and Late Proterozoic

Summary Photosynthesis continued to release free oxygen into the atmosphere which became increasingly oxygen rich through the Proterozoic Fully 92% of Earth's iron ore deposits in banded iron formations were deposited between 2.5 and 2.0 billion years ago Widespread continental red beds dating from 1.8 billion years ago indicate that Earth's atmosphere had enough free oxygen for oxidation of iron compounds

Summary Most of the known Proterozoic organisms are single-celled prokaryotes (bacteria) When eukaryotic cells first appeared is uncertain, but they may have been present by 2.1 billion years ago Endosymbiosis is a widely accepted theory for their origin The oldest known multicelled organisms are probably algae, some of which may date back to the Early Proterozoic

Summary Well-documented multicelled animals are found in several Late Proterozoic localities Animals were widespread at this time, but because all lacked durable skeletons their fossils are not common Most of the world's iron ore produced is from Proterozoic banded iron formations Other important resources include nickel and platinum