The Proterozoic. Neoproterozoic b.y Mesoproterozoic 1.6-1b.y. Paleoproterozoic b.y.

Transcription

1 The Proterozoic

2 The Proterozoic Neoproterozoic b.y Mesoproterozoic 1.6-1b.y. Paleoproterozoic b.y.

3 Laurentia: Proterozoic N. America During Proterozoic the six elements (or provinces) are welded together during orogenic episodes This period of continental welding ended by 1.7bya Extensive accretion of continental margin ( bya)

4 Plate tectonics in the Proterozoic Internal temperatures much higher than today. Rapid lateral motions - Vigorous subduction/orogeny - Extensive Rifting - Rapid sea-floor spreading

5 Paleoproterozic Events Wopmay Orogeny Western Margin of the Slave province Evidence of opening of an ocean basin Sedimentation along the new continental margin Closure of the ocean basin Wilson Cycle

6

7 Depositional Zones Eastern zone of ocean basin (next to Slave) Coastal Region Quartz sandstone Shallow marine environment Over time Sandstone metamorphosed to Quartzite. Also contained massive stromatolites (now dolomitic) Known as the Rocknest Formation Western Zone Continental Shelf deposits Deep water turbidites

8 Evidence of Wilson Cycle Numerous tensional faults Alluvial Fan and Fluvial deposits As the ocean widened the western edge of the province became passive margin with two parallel zones of deposition

9 Closure of the ocean basin As westward edge of Slave subducted the continental shelf buckled downward. As depth increased: Carbonate deposition ended Accumulation of deep water clastics began Subsequent collisional forces caused additional folding and faulting.

10 Aulacogens Aulacogens are inactive rifts of a radiating three-rift system that develops over areas of crust that are being uplifted Two arms fill with water to form oceans The third, failed, arm trends inland

11 Can you identify any modern Aulacogens on the world map?

12 Trans-Hudson Orogeny N and W side of Superior Province Records Wilson Cycle Joins Hearne and Wyoming to Superior Severe folding and metamorphism during closure of wilson cycle welded the Superior plate to the Wyoming and Hearne

13 Paleoproterozoic Ice Age Superior Province by Gowgonda Formation Notable for conglomerates and laminated mudstones Laminations represent repeated summer and winter sediment layers (called varves) In Gowgonda Formation varves alternate with tillites Formation lies above 2.6 by crystalline rock and is intruded by 2.1 by igneous rocks

14 Labrador Trough Curved track on east side of Superior Province Records yet another Wilson Cycle During closing phase eastern zone underwent intense folding, thrust faulting, and metamorphism- known as Hudsonian Orogeny Hudsonian Orogeny serves as the event that separates paleoproterozoic from mesoproterozoic

15 Keweenawan Sequence Rocks rest on crystalline or animikian strata Extend for hundreds of kilometers from Lake Superior region Consist of Quartz sandstone, arkoses, conglomerates, as well as basaltic volcanics. The lava flows are well known for their content of native copper.

16 Keweenawan Sequence In some areas lava flows accumulated to a thickness of several kilometers. Although this seems like a lot of lava, most of the magma remained underground forming the Duluth Gabbro (12kmX160km) Large amounts of mafic magma typically signals the presence of a rift Had the rift continued, the eastern US would have drifted away

17 The Grenville Province Exposures extend from Labrador to Lake Huron. However, region actually extends down eastern US and westward into Texas. Consist of carbonates and sandstones that have been deformed, metamorphosed, and intruded. Deformation occurred bya during the Grenville Orogeny. (the beginnings of the Appalachians)

18 Rodinia

19 Neoproterozoic Rodinia: assembled by 1.0 bya, Begins to split apart 750mya During this time the protopacific ocean (Panthalassa) was created west of N. America

20 Proterozoic Ice House Neoproterozoic rocks on all continents, except India and Siberia, show glacial striations. Tillites and varves can also be found Evidence is so prevalent, geologist refer to our planet during this time as Snowball Earth Formally this time is called the Varangian Glaciation

21 What could have cause this? Plate tectonics Most land masses located at low or mid lattitudes Land is much more reflective than water As glaciers formed land became even more reflective Atmospheric Loss of Carbon Dioxide During this time fungi, lichens and vascular plants were present.

22 Sediment Deposition During this period extensive layers of limestone were deposited. If Limestones are typically associated with relatively warm seas how could this be possible?

23 The Fossil Record Inherited Archean Life Photosynthetic cyanobacteria (oxygenated) anaerobic prokaryotes Stromatolites molecular fossils

24 Heliotropic Stromatolites Sine-wave growth form=yearly cycles Laminar structure=daily growth phase Bitten Springs Fm., Australia (850 my) Anabaria juvensis (Stromatolite) 435 laminae/cycle=435 days/year

25 Eukaryotes Record of large cells: b.y to present Steranes: molecular fossils Acritarchs (Planktonic Algae; Skiagia) Max age 1.6 b.y. Fossil sites; Russia, California, Australia Eukaryotic Protozoans: Vase-shaped fossils from Spitsbergen and Arizona Metazoans Australia: Ediacara Hills China: Doushantuo Formation (~570 m.y.) Russia: White Sea coast (~550 m.y.) Some may not have modern equivalents

26 Body and trace fossil Metazoans Cloudina: 3cm, tube dwelling worm Ediacaran trace fossils Cyclomedusa: discoidal (Jellyfish?) Tribrachidium: circular (unknown) Charniodiscus: Frond shape (Sea pen) Dicksonia: ovate-shape (Flatworm) Spriggia: Elongate (Annelid)

27 Fossil Expansions Mesoproterozoic (1.4 b.y. ago) Eukaryotes Neoproterozoic (0.6 b.y. ago) Metazoans Proterozoic Prokaryotes (Gunflint Fossils, 1.9b.y.) Gunflintia: unbranched filament (algae) Animikiea: fine filament (algae) Eostrion: Dawnstar (bacterium) kakabekia: plumose form (unknown)

28 Changing Environment Free Oxygen begins to Accumulate in atmosphere (2.0 b.y. ago) Oxygen increases sporadically as CO2 level decrease (1.0 b.y.) Oxygen rises to 3-10% of current level (0.6 b.y. ago) Wide variety of climates Hot, dry (evaporates) Tropical (algae reefs) Ice Ages (glacial deposits) Warm, tropical (Carbonates)