Chapter 9 Precambrian Earth and Life History The Proterozoic

1
Chapter 9 Precambrian Earth and Life History
The Proterozoic

• The Proterozoic 1.955 billion years, 42.5 of
  geologic time. Proterozoic beginning placed at
  2.5 b.y., the end of the Archean-style of crustal
  tectonics.

2
2

• Changes
• Proterozoic rocks are generally less
  meta-morphosed than Archean rocks. In some areas
  there is a long-term unconformity between
  Proterozoic and Archean rocks.
• Proterozoic plate tectonics similar to today.
• Proterozoic rock assemblages are different from
  Archean.
• Proterozoic atmosphere and biosphere more
  developed than Archean.
• Different mineral resources than Archean.
• Multi-celled organisms developed, as did
  Eucaryotes, i.e., cells with nuclei.

3
3

• Cratons assembled during Archean, accretion
  continued during Proterozoic larger continents.
• Greenstone belts less common in Proterozoic,
  ultramafic lavas almost unknown due to lower
  temperatures.
• Focus on Laurentia, the Proterozoic landmass that
  portions of which include North America,
  Greenland, NW Scotland, possible portions of
  Baltic Shield of Scandinavia.
• Important Laurentian growth 2.0 to 1.8 b.y.,
  collisions of smaller plates with craton leaving
  linear or arcuate deformation belts.

4
4
During Proterozoic, Laurentia grew to southeast
by accretion of other cratons. Collision zones
orogenic belts. Brown masses Archean age.
5
5

• Rocks of the Wopmay orogenic belt in NW Canada
  are important because they record the opening and
  closing of an ocean basin, i.e., 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,
• Final reassembly of the continent
• Some of the Wopmay rocks include are
  sandstone-carbonate-shale assemblages, a suite of
  rocks typical of passive continental margins that
  first become widespread during the Proterozoic.

6
6
Early Pro-terozoic rocks in the Great Lakes
region.
Sandstones with ripple marks and cross-beds
suggest shallow water deposition. Stromatolites
in dolostones suggest tidal flat conditions.
Banded Iron Formations suggest fluctuations.
7
7
Breccia dike gives hints of what might lie below
the Castner Marble.
Interlayered marble hornfels (green/gray) sea
level fluctuations /or influxes of clay or ash.
8
8
Middle Proterozoic Castner Marble, Franklin Mts.,
El Paso, TX Also see p. 156 Kona Dolomite,
Michigan.
Middle Proterozoic rocks in Franklin Mts.
Castner Marble, Mundy Breccia (pillow lavas), La
Noria Quartzite. Grand Canyon Supergroup (later
slides)- http//www.rockhounds.com/grand_hikes/geo
logy/supergroup_formations.shtml
9
9

• Proterozoic also marked first deposition of
  continental red beds and oldest evidence of
  glacial conditions. Grooving of rocks and
  poorly-sorted tillites provide evidence of
  glaciers (see next slide).
• Laurentian igneous activity from 1.8 to 1.1
  billion years ago, extensive igneous activity
  took place, apparently unrelated to orogenic
  activity, i.e., igneous activity is usually
  related to plate tectonic activity (rifting,
  subduction, etc.).
• Igneous activity did not add to Laurentias size
• because magma was either intruded into
• or erupted onto already existing continental
• crust.

10
10
Unconformity between tillites and grooved
bedrock.
Norway
11
11

• These Proterozoic igneous rocks include
• Granites, e.g., Red Bluff Granite and other
  granites in Franklin Mts. and central Texas
• Anorthosite plutons (composed almost entirely of
  plagioclase feldspars),
• Calderas and their associated vast sheets of
  rhyolite and ash flows, e.g., Thunderbird
  Rhyolite, Franklin Mts.
• According to one hypothesis - large-scale
  upwelling of magma beneath a Proterozoic
  supercontinent produced these rocks. In most
  areas, they are covered by younger rocks.

12
12

• 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. This
  is the final accretion to the continent during
  the Proterozoic.
• Disagreement over what Grenville Orogeny was
  Wilson Cycle closure? Intracontinental
  deformation? Major shearing?
• Grenville rocks are well exposed northern
  Appalachians eastern Canada, Greenland, and
  Scandinavia. Exposures of Grenville-age rocks in
  Georgia are SW of Jasper, between Canton and
  Cartersville, and in the Pine Mt. area (see next
  slide).

13
13

• Diagram from Univ. of Alabama 2001 FieldTrip
  web-page. Grenville basement shown in black.
  (Modified from Hatcher et al. (1990), Steltenpohl
  et al. (1995), and Hooper Hatcher (1988).)

14
14

• By the end of the Proterozoic 75 of
  present-day North America was present. Remainder
  accreted during Paleozoic Mesozoic.
• During time of Grenville deformation,
  Mid-Continent Rift was forming as a series of
  graben basins extending from the Lake Superior
  basin southwest into Kansas, with a southeasterly
  branch through Michigan into Ohio (next slide).
• 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
  over-lapping basalt lava flows forming a volcanic
  pile several kilometers thick.

15
15
Infilling of grabens with basalts clastics,
just as with Newark Supergroup
16
16

• Along the western margin of Laurentia, clastic
  sediments were deposited in three deep basins,
  the Belt Basin, the Uinta Basin, the Apache
  Basin.
• Belt Series rocks are well exposed in northern
  Rockies, Glacier Natl Park.

17
17
Outcrop of Proterozoic Belt Series red
mudrock in Glacier National Park
(Young geologists at play).
18
18
Mid - Late Proterozoic G. C. Supergroup,
features suggest shallow marine/fluvial
environments.
19
19
(Above) G.C. Supergroup, NE? part of
canyon. (Right) Foreground Early Proterozoic
Brahma Schist. Background M. Proterozoic
Hakatai Shale (of G.C. Supergroup), over-lain by
Cambrian Bright Angel Shale Muav Ls.
Brahma Schist
20
20
Ophiolite suites oceanic crust preserved in
areas of conti-nental collision. Jormua
mafic-ultramafic com-plex in Finland, about 1.96
billion years old, com-pares closely in detail
with younger well-documented ophiolites.
21
21
Metamorphosed pillow basalts Jormua, Finland.
22
22

• Proterozoic Supercontinents
• Continent a landmass of granitic crust with
  most of its surface above sea level
• A supercontinent consists of all or at least most
  components of the present-day continents, differs
  mainly in size.
• Rodinia - the first recognized supercontinent,
  assembled between 1.3 and 1.0 b.y. ago and then
  began fragmenting 750 m.y. ago.
• Pannotia assembled from parts of Rodinia 650
  m.y. ago, then began fragmenting 550 m.y. ago, at
  the end of the Proterozoic.

23
23
Rodinia final assembly during Grenville
Orogeny (1.3 to 1.0 b.y. ago), then fragmentation
began 750 m.y. ago. Re-assembly of some
continents as Pannotia caused the Pan-African
Orogeny, mainly affecting modern Southern
Hemi-sphere landmasses.
http//www.scotese.com/pcanima.htm
24
Proterozoic Glaciation
24

• Primary evidence tillite - a type of
  conglomerate
• Tillite or tillite-like deposits known from at
  least 300 Precambrian localities, some likely not
  glacial in origin. Distribution striations
  (slide 10) more evidence.
• Tillites similar to Norway in
• Michigan, Wyoming,
• Quebec suggest presence
• of Laurentia ice sheet.

Other tillites in South Africa Australia are of
uncertain age.
25
25

• Tillites and
• glacial features
• on all continents
• except Antarctica
• 900 to 600 m.y.
• ago, not contin-
• uous, but 4
• episodes.
• Greatest extent in
• Earth history, with
• some in apparent
• near- equitorial
• areas.

Ice sheets shown above may not have been present
at the same times.
26
26

• Proterozoic Atmosphere
• 1 of present O2 at beginning of Proterozoic,
• 10 of present O2 at end of Proterozoic.
• Cyanobacteria (blue-green algae) present at 3.0
  b.y., but their structures (stromatolites) not
  common until 2.3 b.y. ago. Increasing numbers
  more photosynthesis more free oxygen.
• With increasing oxygen atmosphere evolving from
  reducing to oxidizing.
• Evidence - Banded iron formations (BIFs) -
• Alternating layers of iron-rich minerals and
  chert
• 92 of all BIFs formed from 2.5 to 2.0 b.y. ago.

27
27
Early ProterozoicBIF Alternating layers of
chert and iron minerals.
Fe and Si from submarine volcanic vents,
dissolved in deep water. Currents bring Fe, Si
to shallow water O2 deposition of Fe and Si
sediments. O2 byproduct of blue-green algae.
Fe used up silica deposited Fe-poor layers.
28
28

• Continental Red Beds
• Red, Fe-oxide stained
• sandstones, siltstones,
• mudstones first
• appearance 1.8 b.y.,
• increase in abundance
• due to increasing O2.
• UV radiation converts
• some O2 to O3, which
• also oxidizes Fe.

Once stratospheric ozone became established,
blocking UV, O2 became primary agent of
oxidation.
29
29

• Proterozoic Life
• Archean Life bacteria and blue-green algae
  Procaryotes (p. 165), no nucleus nor organelles.
• First Eucaryotes appeared at 2.1 b.y., are more
  complex, larger than Procaryotes

Eucaryote appearance allowed more complex
organisms specialized organs.
30
30
Acritarchs thought to be cysts from planktonic
algae, appeared 1.4 b.y., present into Paleozoic.
Vase-shaped microfossils from G.C. Supergroup
likely algal cysts, also. Eurcaryote development
pp. 165 166.
31
31

• Late Proterozoic Animals Ediacaran fauna
• 1947 - Australian geologist, R.C. Sprigg,
  dis-covered impressions of soft-bodied animals in
  the Pound Quartzite in the Ediacara Hills of
  South Australia.
• Additional discoveries yielded apparent
  impressions of algae and several animals with no
  resemblance to existing organisms.
• Before these discoveries, geologists were
  perplexed by the apparent absence of
  fossil-bearing rocks predating the Phanerozoic.
• Present in all continents except Antarctica in
  rocks 670 to 545 m.y. old. Fossils rare because
  of the lack of hard parts.

32
32

• Ediacaran fauna of Australia see p. 168
• Tribrachidium heraldicum, a possible primitive
  echinoderm

Spriggina floundersi, a possible ancestor of
trilobites
33
33

• Three present-day phyla may be represented in the
  Ediacaran fauna
• jellyfish and sea pens (phylum Cnidaria),
• segmented worms (phylum Annelida),
• 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.