The Proterozoic
Learning Objectives
Overview:
During the Early Proterozoic smaller cratonic elements welded together through accretionary tectonics to form Early North American continent, referred to as Laurentia. Laurentia continues to grow throughout Proterozoic through accretion. On passive margins sedimentation builds wide continental shelves, lots of carbonates as well. The atmosphere continues to change and evolve into a modern, oxygen based atmosphere. Cyanobacteria are abundant, eukaryotic life forms evolve into the complex Ediacaran fauna. The end of the Proterozoic is marked by the appearance of abundant shelled invertebrates.
Related Website:
The Proterozoic
About cyanobacteria
fossil record of cyanbacteria
The Proterozoic lasted from 2.5 billion to 544 million years ago. Since it is such a long period of time it is divided into:
Early or Paleo Proterozoic: 2.5 - 1.6 billion years ago
Beginning marked by appearance of red beds
End marked by appearance of eukaryotes
Middle or Meso Proterozoic: 1.6 - 1.0 billion years ago
Beginning marked by appearance of eukaryotes
End marked by widespread glaciation
Late or Neo Proterozoic: 1.0 billion to 544 million years ago
Beginning marked by widespread glaciation
End marked by appearance of abundant shelled invertebrates
During the Proterozoic there was more modern style of tectonics
Orogeny: a mountain building event, due to either compressional or tensional tectonics.
Early Proterozoic: 2.5 - 1.6 billion years ago
Wompay Orogeny: this is a typical example of modern Proterozoic tectonics and occurred many times and throughout the world during the Proterozoic.
The Western Slave province of Canada (see map in book) underwent a Wilson cycle. (A Wilson cycle is the opening of a continental rift, formation of a shallow sea followed by subduction and mountain building.) The Slave province underwent rifting which produced and inland sea. Sediments washed off the shield and into this opening sea producing typical shelf sediment sequences. Eventually the rift begins to close through subduction as a microplate from the west moves towards the area. A trench forms and the foredeep fills with sediment. This area undergoes folding and faulting as subduction continues. Granitic batholiths are emplaced and some volcanics break through the crust. As the sea continues to close, sedimentation along the edge of the sea changes from marine to delta and stream. The rift continues closing producing folding of the sediments and mountain building. This area may have been part of an aulacogen.
The Wompay orogeny was typical of others during this time. Through continued rifting/subduction cycles Laurentia grows in size.
Meso or Middle Proterozoic: 1.6 - 1.0 billion years ago
Keweenawan Sequence: 1.2 - 1 billion years ago, extensive mafic lava deposits indicate a continental rift was opening. If successful it would have divided early continent into two with a sea in between.
Greenville Province: a continental mass collides with Laurentia along its eastern edge. This causes deformation and mountain building down the eastern margin of today's North America, wrapping around through what is Texas today. The evidence for this is deformed sedimentary material.
Neo or Upper Proterozoic: 1.0 billion - 544 million years ago
This is a relatively quiet time geologically. In the east a rift opens that separates Laurentia from the continental plate that had collided with it causing the Greenville orogeny. Lava flows present followed by coarse sediments washing off of the continent into the Proto-Atlantic.
Deposits of red beds VERY common. It is believed the composition of the atmosphere is very much like today's. Extensive shallow seas evidenced by shallow water deposition in basins and shelfs. The epeiric seas allow for large populations of stromatolites, oxygen continually pumped into atmosphere.
The Upper Proterozoic was a time of widespread glaciation from 1 billion years ago - approximately 850-600 million years ago. Glaciation occurred in distinct events interrupted by interglacials. Evidence of this glaciation is worldwide, the Earth became very cold during this period.
The Proterozoic comes to a close at 544 million years ago and is marked by the appearance of abundant shelled organisms.
Life in the Proterozoic:
The earliest life consisted of prokaryotic organisms.
Prokaryotes:
Represented by Bacteria today
No nucleus: genetic material contained in a molecule within the cell rather than in the nucleus
Small in size, 1 - 10 microns
Asexual reproduction through binary fission
Eukaryotes:
All living organisms except bacteria
Cell has a nucleus and genetic material is continued on chromosomes on the nucleus
Large in size, greater than 10 microns
Sexual reproduction, shared genetic material encourages diversity
Animikie Group: 2.5 billion years ago.
Life in the Early Proterozoic characterized by cyanobacteria which pumped oxygen into the atmosphere producing the banded iron formations or red beds that are the marker for the beginning of the Proterozoic. Banded Iron Formations consist of red iron deposits banded or interspersed with dark cherts. The Animikie Group is important because it indicates the presence of
free oxygen in the atmosphere.
Gunflint Chert: 1.9 billion years ago.
Contain fossils of cyanobacteria AS WELL AS other prokaryotic life. Important because it shows that life is starting to diversify and evolve new forms.
Eukaryotic organisms appear in the fossil record 1.6 billion years ago. This is the marker that divides the Middle and Early Proterozoic. They are common by 900 million years. Eukaryotes reproduce sexually through shared genetic material, this
encourages DIVERSITY. Once a mechanism (sexual reproduction) existed for sharing of genetic material then organic evolution could proceed at a faster rate.
Ediacaran Fauna: ~ 560 million years ago
Multicellular, soft bodied organisms. Show evidence of more than one type of cell, cells organized into tissues and tissues into organs.
By the end of the Proterozoic, ~750 million years ago single celled organisms with a secreted shell began to appear.
The end of the Proterozoic (544 million years ago) is marked by the appearance of abundant shelled invertebrates.
