Introductory Geosciences II (Geol-1122)
Outline of Important
Geologic Events in Earth History
BYA to 542 MYA) - 87% of Earth History
- A.Archean (4.6 BYA to 2.5 BYA)
Proto-continents were apparently small steep-sided island arcs
which were surrounded by deep ocean basins.
Evidence: The presence of abundant
greenstone belts with pillow structures and the general absence of widespread continental
- 2.Higher Crustal Temperatures
- the geothermal gradient
was apparently much higher.
Evidence: The presence of abundant
granulites (high-grade metamorphic granitic gneisses found only in Archean terrains)
and widespread volcanic/plutonic rocks indicates that the thermal
gradient and surface temperatures were higher.
- the atmosphere appears to have been anaerobic (depleted in oxygen).
Evidence: Absence of banded-iron formations and redbeds, and the
presence of abundant uranite and sulfides in Archean sediments
indicate that the atmosphere was relatively depleted in oxygen.
- 4.Early Life Forms
- a.Fossil Prokaryotes
- several types of cyanobacteria (photosynthesizers which live
in an anaerobic, reducing environment) and other bacterial forms
have been found fossilized in the stromatolites and associated
rocks (as old as 3.9 BYA).
- b.Stromatolites -
layered masses of Archean limestone and chert which are very similar
to procaryote blue-green algae mounds of modern times. These structures are found in rocks which are up to 3.5 BYA
- B.Proterozoic (2.5 BYA to 542 MYA)
- More-evolved continental landmasses were present at this time.
Evidence: There is much greater volume and diversity of rocks
types especially thick sequences of quartzite-carbonates-shale of this age. Large continental landmasses with extensive
depositional basins and orogenic belts had evolved by this time.
(example - Gondwana).
- 2.Lower Surface Temperatures
- the earth had apparently lost enough of its original heat of formation so that surface temperatures were substantially cooler.
Evidence: The absence of granulites
and the presence of widespread Proterozoic glacial deposits indicate
a lower heat flow.
- An oxygen-rich atmosphere had evolved by this time.
Evidence: The appearance of banded
iron formations then redbeds and absence of uranite- and sulfide-rich
sediments indicate the atmosphere had become enriched in oxygen.
- 4.Life Forms
- Stromatolites become very abundant. They were probably responsible for enrichment of atmospheric oxygen
and decrease in atmospheric carbon dioxide as they formed thick limestone deposits.
- the first appearance of eukaryotes is recorded around 1.8 BYA by the dramatic
increase in cell size and appearance of branching algae.
- c.Multicellular Organisms - branching algae is first found in the fossil record about 800 MYA.
- d.Edicarian Fossils -
a very diverse collection of Upper Proterozoic fossils indicating
that more complex soft bodied
animals existed at this time than was previously believed.
II.Paleozoic (542 to 251 MYA)
- A.Crustal Events
- By the beginning of the Cambrian, the continents had evolved
into landmasses which were similar to modern-day continents. These
contained: shield areas, platforms, mobile belts. They included
the landmasses of Laurentia, Siberia, China, Gondwana, Baltica
and possible others. There were several periods of mountain building
separated by periods of tectonic quiescence. There were also important
changes in sealevel.
- a.Taconic Orogeny
- In the Ordovician, the Iapetus (proto-Atlantic) Ocean was partially
destroyed beneath Laurentia causing intense folding and igneous activity. This
orogeny was the 1st episode of Appalachian Mt.building, forming the Taconic Mountains and depositing thick clastic sediment sediments in the inland seas.
- b.Acadian Orogeny
(the second major episode of Appalachian Mt. building). Iapetus
was destroyed as Baltica collided with Laurentia forming Laurasia. This was the strongest episode of mountain building which produced the high-grade metamorphic rocks of the modern-day Appalachian Mountains.
- c.Alleghanian (Appalachian)
Orogeny (3rd and last
episode of Appalachian Mt. building) Laurasia later collided with
Gondwana in the Permian-Pennsylvanian. This collision formed the supercontinent of
Pangaea and uplifted the metamorphic core of the Appalachian mountain belt.
- 3.Major Transgressions
- a.Late Cambrian Transgression
- covered much of N. America forming thick deposits of sandstone.
shale and limestone.
- b.Ordovician-Silurian Epicontinental
Seas - Seas remained high throughout the Upper Ordovician and Silurian. These shallow warm subtropical seas created the perfect environment for marine
life to diversify and increase in abundance. Weathering of the
Taconic Mts. also formed iron-rich deposits such as the Silurian
- c.Late Devonian-Mississippian
Epicontinental Seas - Inland seas existed across N.America for most of the Devonian and Mississippian. This was the last great episode of continental
flooding in N.American geologic history. Thick organic-rich, pyrite-
and uranium-bearing, black shales such as the Chattanooga Shale
were also deposited at this time in oxygen-depleted conditions.
During the Late Mississippian, there was a widespread regression
which resulted in the formation of an extensive unconformity.
- d.Upper Paleozoic Minor Transgressions
and Regressions - There were several minor fluctuations of sealevel which formed cyclic non-marine and marine deposits which were separated by coal beds (cyclothems).
- B.Paleozoic Life Forms
- General Trends
- At the Cambrian/Precambrian boundary, all but one phylum of
the marine invertebrates with hard parts appears. This rapid diversification
represents a break in the fossil record where no intermediate
stages are represented. The appearance of hard parts resulted
in better preservation and a more dramatic increase than actually
- 2.Development of Shells
- Abundant shelled trilobites and brachiopods appear in the Cambrian. The simultaneous appearance of predatory cephalopods could have caused selective pressures and favored expansion of the shelled organisms.
- 3.Changes in Plant Life
- The appearance of grazing marine invertebrates apparently resulted
in the rapid decrease of stromatolites. In the Silurian the appearance
of the first vascular plants was the first stage in the rapid
movement of life forms onto the land surface. Scale trees and
tree ferns formed thick dense forests. The appearance of seed-bearing
plants in the Upper Paleozoic then allowed plant life to move
into the semi-arid and arid areas.
- 4.Appearance of Vertebrates
- Fish, the most primitive vertebrate, appear in the Upper Cambrian. Earliest fish were
bottom-dwelling, mud-sucking jawless ostracoderms. Jawed fish
appear shortly after and rapidly diversify and increase in abundance.
Lungfish, specifically the Crossopterygians also appear to be
the ancestors of amphibians.
- 5.Amphibians and reptiles
also appear during the Paleozoic, but do not show substantial
diversity or abundance until the Mesozoic.
- 6.Permian Extinctions
- the disappearance of trilobites, rugose and tabulate corals
and several families of other marine invertebrates marks the upper
boundary of the Paleozoic.
III.Mesozoic Era - Time of Middle Life/Age of the Reptiles (251-66 MYA)
- A.Important Geologic Events
- 1.Failed Rifting of the Triassic
- The failed rifting of Pangaea in the early Triassic (first indication of extensional forces) resulted in the formation of Triassic Basins along the
eastern margin of North America.
- 2.Successful Rifting of Pangaea
- Jurassic and Cretaceous rifting resulted in formation of the
Atlantic Ocean and separate N. American, S. American, African
and Eurasian continents. Plate movement initiated during this
time has continued in the same directions for many of the lithospheric
- 3.Rocky Mountain Orogeny
- Compressional forces along the western margins of the North
and South American plates resulted from the extensional movement
along the Atlantic. This caused the collision and subduction of
the Pacific plate during the Rocky Mountain Orogeny which formed
most of the western mountain ranges on these two continents.
- 4.Cretaceous Seas
- During the Upper Cretaceous, marine water spread north- and
westward into the adjacent platforms. Sealevel never reached the
levels it did in the Paleozoic; however, it transgressed enough
that large deposits of chalk, sandstone, and shale formed in continent
- 5.Late Cretaceous Extinctions
- at the close of the Upper Cretaceous, many animal families disappeared.
The extinctions mark the end of this era.
B.Changes in Life Forms
- 1.Plant Life - In the Early Mesozoic, seed-bearing plants were dominant, replacing the Paleozoic seedless plants. These were then replaced by flowering plants in upper Mesozoic
- 2.Invertebrates- Several important transitions occurred. Pelecypods became very diverse and abundant. Rudist bivalves and hexacorals (corals with
six-fold symmetry) were the important reef builders of the Cretaceous.
Ammonites became very diversified and abundant at this time, such
that they are often used as Mesozoic index fossils.
- 3.Vertebrates - The gradual evolutionary changes which occurred in reptiles resulted in the dominance of the land by dinosaurs during the
middle and upper Mesozoic. These forms apparently evolved from
mammal-like reptiles, which were also the probable ancestors of
the mammals. The first land mammals (mostly shrew-like mammals)
appeared during the upper Mesozoic.
II.Cenozoic Era - 'Age of the Mammals' (66 MYA to the recent)
- A.Important Geologic Events
- 1.Stabilization of Plate Movement
- Constant and widespread plate movement formed most of the present-day
ocean floor. Continents stood relatively high with only limited
- 2.Formation of Himalaya Mountains
- This orogeny resulted from the collision of India and Eurasia
- 3.Great Ice Ages - Four major advances covered more than 33% of all the land surface.
- B.Changes in Life Forms
- 1.Great Diversification and Increase in Abundance of Flowering Plants
- This created new environments and allowed for diversification
in land animal species.
- 2.Mammals and Birds - It was a time of great experimentation and diversification in the mammals and birds. Homo sapiens did not appear until about
34,000 years ago.
- 3.Reptiles - There were only limited survivors of the Late Cretaceous extinctions, including turtles, crocodile, snakes and lizards which were so
well suited for their environment such that they couldn't be driven
out of their niches.
III.Important Geologic Provinces of Georgia
- A.Appalachian Mountain Chain -
This is an extensive mountain chain which stretches from Maine
to Alabama (>3000 miles) and includes three separate geologic
provinces; Valley and Ridge, Blue Ridge and Piedmont. The mountain
belt was originally continuous with the Ouachita Mountains of
Arkansas and Oklahoma and the Caledonian Mountains of Europe.
- 1.Piedmont Province
- This is an area of low relief and gentle rolling hills which
is underlain by early Paleozoic to Precambrian, highly weathered,
metamorphic rocks. This belt extends from the northern border
of the Coastal Plain to the base of the Blue Ridge Mountains,
covering approximately 1/3 of the state. Many of the rocks were
apparently formed along an active continental margin or island-arc
environment. Several were formed as deep-seated intrusives (Stone
- 2.Blue Ridge Province -
This area displays the greatest topographic relief and elevations
in the state. It is underlain by a band of highly-metamorphosed
Precambrian and early Paleozoic sedimentary and igneous rocks.
These rocks were initially metamorphosed during the Taconic Orogeny,
and again deformed in the Acadian Orogeny. Most of the overlying
rocks have since been eroded as basement rocks were thrust upward
and westward along faults during the Appalachian (Alleghanian)
Orogeny, as Africa collided with North America.
- 3.Valley and Ridge Province- This area is characterized
by long, linear ridges separated by intervening valleys. Only
the northwestern corner of the state in included in this geologic
belt. The province consists of a sequence of relatively unmetamorphosed
Paleozoic sediments which accumulated along broad inland seas.
Much of this material was eroded form the Taconic and Acadian
mountain belts which formed to the east. The Appalachian Orogeny
resulted in extensive thrust faulting and folding of these sediments.
Today, resistant sandstones form ridges; whereas, more-soluble
limestones form valleys.
- B.Coastal Plain Province- This geologic province
is underlain by a thick, gently dipping, Cretaceous to recent
sequence of marine sediments (sandstones, shales and limestones)
which overlie metamorphic and igneous basement rocks. The area
is relatively flat with only minor relief. The sedimentary rocks
were deposited during minor transgression in the Cretaceous and
Cenozoic. A gradual drop in sealevel during this period, resulted
in the deposition of younger sediments toward the coast. Separated
from Piedmont by Fall Line.