Chapter 9. Proterozoic: Dawn of a More Modern World

Transcription

1 Chapter 9 Proterozoic: Dawn of a More Modern World

2 Proterozoic Eon 2.5 billion years to 542 million years ago Comprises 42% of Earth history Divided into three eras: Paleoproterozoic Era ( by ago) Mesoproterozoic Era (1.6 to 1.0 by ago) Neoproterozoic Era (1.0 by ago to the beginning of the Paleozoic, 542 my ago)

3 The Beginning of Proterozoic Marks the: More modern style of plate tectonics More modern style of sedimentation More modern global climate with glaciations Establishment of the beginnings of an oxygen-rich atmosphere Emergence of eukaryotes

4 Precambrian Provinces in North America Precambrian provinces were welded (or sutured) together to form a large continent called Laurentia during Early Proterozoic.

5 Proterozoic Plate Tectonics During Late Proterozoic, the continents became assembled into a supercontinent called Rodinia.

6 Proterozoic Climate Proterozoic glaciations occurred during: Paleoproterozoic, about b.y. ago (Huronian glaciation) Neoproterozoic, m.y. ago (Varangian glaciation) was known as Snowball Earth.

7 Overview of the Precambrian

8 Overview of Proterozoic Events

9 Major Events of the Paleoproterozoic 1. Active plate tectonics 2. Major mountain building on all major continents 3. Earth's first glaciation 4. Widespread volcanism (continental flood basalts) 5. Rise in atmospheric oxygen (great oxidation event) 6. Accumulation of high concentrations of organic matter in sediments (Shunga event) 2000 m.y. ago, and generation of petroleum 7. Oldest known phosphorites and phosphate concretions

10 Paleoproterozoic Glaciation - First Ice Age? A Paleoproterozoic ice age is recorded in rocks north of Lake Huron in southern Canada (called the Huronian glaciation). Gowganda Formation. Age of Huronian glaciation = m.y. Apparent rapid onset of global glaciations from what had been relatively stable climatic conditions.

11 Evidence for glaciation includes: Mudstones with laminations or varves - fine laminations indicating seasonal deposition in lakes adjacent to ice sheets. Glacial dropstones (dropped from melting icebergs) in varved sedimentary rocks. Tillites or glacial diamictites (poorly sorted conglomerates of glacial debris). Scratched cobbles and boulders in tillite, due to abrasion as ice moved.

12 Banded iron formations and prokaryote fossils Extensive banded iron formations (BIF's) on the western shores of Lake Superior, indicate that photosynthesis was occurring and oxygen was being produced. Text Some BIF deposits are >1000 m thick, extend over 100 km. Rich iron deposits were foundation of steel industry in Great Lakes region (Illinois, Indiana, Ohio, Pennsylvania).

13 Banded iron formations and prokaryote fossils The Gunflint Chert, within the BIF sequence, contains fossil remains of prokaryotic organisms, including cyanobacteria. Age = 1.9 b.y.

14 Highlights of the Mesoproterozoic The Midcontinent rift, an abandoned oceanic rift in the Lake Superior region with massive basaltic lava flows. Copper mineralization in the Lake Superior region. Continental collisions producing the Grenville orogeny (Grenville Mountains) in eastern North America, suture of supercontinent. The assembly of continents to form the supercontinent, Rodinia.

15 Midcontinent Rift Large volumes of basaltic rock indicate presence of an old abandoned rift zone called the Midcontinent rift. Rift developed 1.2 b.y b.y. ago. Extended from Lake Superior to Kansas. Rifting ceased before the rift reached the edge of the craton, or the eastern U.S. would have drifted away from the rest of North America.

16 The Keweenawan Sequence The Keweenawan Sequence consists of: Clean quartz sandstones, Arkoses and Conglomerates Basaltic lava flows more than 25,000 ft thick (nearly 5 mi) with native copper Basaltic rock beneath the surface crystallized as the Duluth Gabbro, 8 mi thick and 100 mi wide. Native copper fills vesicles (gas bubbles) in the Keweenawan basalt, and joints and pore spaces in associated conglomerates. Native Americans mined the copper as early as 3000 BC. Copper was mined extensively from 1850 to 1950, but copper production ceased druing the 1970's.

17 Grenville Province and Grenville Orogeny Grenville rocks were originally sandstones and carbonate rocks. Grenville Province was the last Precambrian province to experience a major orogeny. Grenville orogeny = 1.2 b.y. to 1.0 b.y. ago

18 Grenville Province and Grenville Orogeny Grenville rocks were originally sandstones and carbonate rocks. Grenville Province was the last Precambrian province to experience a major orogeny. Grenville orogeny = 1.2 b.y. to 1.0 b.y. ago Orogeny occurred when Eastern North America (Laurentia) collided with western South America (Amazonia). Orogeny was associated with formation of the supercontinent, Rodinia. Later, during Paleozoic, Grenville rocks were metamorphosed and intruded during the three orogenies involved in the building of the Appalachians.

19 The supercontinent as it appeared about 1.1 b.y. ago. The Supercontinent, Rodinia

20 The Supercontinent, Rodinia Rodinia formed as the continents collided during the Grenville Orogeny. Rodinia persisted as a supercontinent for about 350 million years. It was surrounded by an ocean called Mirovia.

21 Rifting in Rodinia Rodinia began to rift and break up about 750 million years ago, forming the proto-pacific Ocean, Panthalassa, along the western side of North America. Around 570 million years ago, rifting began again, and South America began to separate from North America, forming the Iapetus Ocean (or proto-atlantic Ocean). The rift ran along what is now the Blue Ridge province. Basaltic lava flows formed the Catoctin Formation. As the Iapetus Ocean opened, sands and silts were deposited in the shelf areas.

22 Highlights of the Neoproterozoic Extensive continental glaciations Sediments deposited in basins and shelf areas along the eastern edge of the North American craton from erosion of Grenville Mts. Most of these rocks were deformed during Paleozoic orogenies.

23 Glacial deposits - Neoproterozoic This time is referred to as "snowball Earth" because glacial deposits are so widespread. Varangian glaciation (named after an area in Norway). Late Proterozoic ice age lasted about 240 m.y.

24 Plate Tectonics and Glaciation Continents were located around the equator about 600 to 700 m.y. ago. No tropical ocean. Heat lost by reflection from the rocks on the surface of the continents may have caused global cooling. (Land plants had not yet appeared.) As continental glaciers and ice caps formed, reflectivity of snow and ice caused further temperature decrease.

25 Atmospheric Gases and Glaciation Glaciation was associated with: Decrease in CO 2 and Increase in O 2. CO 2 causes the greenhouse effect and global warming. Decrease in CO 2 may have caused cooling. Decrease in CO 2 was probably caused by increase in the number of photosynthetic organisms (cyanobacteria, stromatolites).

26 Proterozoic Rocks South of Canadian Shield Extensive outcrops of Precambrian rocks are present in the Canadian Shield. Precambrian rocks are also present in other areas, including: Rocky Mountains Colorado Plateau (Grand Canyon)

27 Precambrian rocks of the Grand Canyon Vishnu Schist metasediments and gneisses, intruded by Zoroaster Granite about 1.4 b.y. to 1.3 b.y.a. during the Mazatzal orogeny. Grand Canyon Supergroup overlies unconformity. Neoproterozoic sandstones, siltstones, and shales.

28 Other forms of life appeared during Proterozoic 1. More diverse eukaryotes including acritarchs 2. Metazoans or multicellular animals with soft bodies 3. Metazoans with tiny calcium carbonate tubes or shells

29 Microfossils of the Gunflint Chert First definitive Precambrian fossils to be discovered (in 1953) were in the 1.9 b.y. old Gunflint Chert, NW of Lake Superior (Paleoproterozoic). The fossils are well-preserved, abundant and diverse and include: String-like filaments, Spherical cells, Filaments with cells separated by septae (Gunflintia), Finely separate forms resembling living algae (Animikiea) and Star-like forms resembling living iron- and magnesiumreducing bacteria (Eoastrion) Gunflint fossil organisms resemble photosynthetic organisms. The rock containing these organisms contains organic compounds that are regarded as the breakdown products of chlorophyll.

30 Microfossils of the Gunflint Chert A = Eoastrion ( = dawn star), probably iron- or magnesium-reducing bacteria B = Eosphaera, an organism or uncertain affinity, about 30 micrometers in diameter C = Animikiea (probably algae) D = Kakabekia, an organism or uncertain affinity

31 The Rise of Eukaryotes The appearance of eukaryotes is a major event during the history of life. Eukaryotes have the potential for sexual reproduction, which increases variation through genetic recombination. Genetic recombination provides greater possibilities for evolutionary change.

32 Eukaryotic cells can be differentiated from prokaryotic cells on the basis of size.

33 The First Metazoans (Multicellular Animals) Metazoans are multicellular animals with various types of cells organized into tissues and organs. Metazoans first appeared during Neoproterozoic Preserved as impressions of soft-bodied organisms in sandstones.

34 Metazoan fossils in Proterozoic rocks Ediacara fauna - Imprints of soft-bodied organisms, first found in Australia during the 1940s Metazoan eggs and embryos in uppermost Neoproterozoic Doushantuo Formation, South China Trace fossils of burrowing metazoans in rocks younger than the Varangian glaciation. Tiny shell-bearing fossils (small shelly fauna)

35 Discoidal Frondlike Elongate or ovate Types of Ediacara fossils

36 Ediacara fauna Because the Ediacara creatures are not really similar to animals that are living today, this has led to the suggestion that they be placed in a separate taxonomic category or new phylum. The name proposed for this new category is Vendoza (named after the Vendian, or the latest part of the Neoproterozoic in Russia).

37 Small Shelly Fauna: Origin of Hard Parts Small fossils with hard parts or shells appeared during Neoproterozoic. Cloudina, an organism with a small, tubular shell of calcium carbonate (CaCO 3 ). Resembles structures built by a tube-dwelling annelid worm. Earliest known organism with a CaCO 3 shell. Text Found in Namibia, Africa.

38 Precambrian Trace Fossils Trails, burrows, and other trace fossils are found in Upper Neoproterozoic rocks. In rocks deposited after Neoproterozoic Varangian glaciation. Mostly simple, shallow burrows. Trace fossils increase in diversity, complexity, and number in younger (Cambrian) rocks.

39 Review of Proterozoic Events