The Paleozoic Earth

Question 1

Names of the three principal eras of the Phanerozoic, their periods, and the age of the boundaries between them.

Answer 1

Paleozoic Era (541–252Ma)
Cambrian, Ordovician, Silurian, Devonian, Carboniferous and Permian
Mesozoic Era (252–66Ma)
Triassic, Jurassic and Cretaceous
Cenozoic Era (66Ma –Present)
Paleogene, Neogene and Quarternary

Question 2

What defines the base of the Phanerozoic?

Answer 2

The Cambrian Explosion

Question 3

What proportion of Earth history does the Phanerozoic represent?

Answer 3

12%

Question 4

What type of events are used to define the boundaries between the eras and periods of the Phanerozoic?

Answer 4

Mass extinctions, major environmental and climatic changes and tectonic events

Question 5

What is the overall trend in the distribution of the continents through the Phanerozoic?

Answer 5

Paleozoic Era shows the formation of Pangea
Mesozoic Era shows the breakup of Pangea in the Late Triassic and continued into the Jurassic and Cretaceous periods, formation of the modern continents, such as North America, South America, Africa, and Eurasia
Cenozoic Era showed continued drift and modern configuration

Question 6

How did sea level change during the Phanerozoic relative to present day sea level? Could you draw the relative sea level curve? During which periods was sea level highest and lowest?

Answer 6

High during the Cambrian, Ordovician, and Jurassic.
Low during the late Carboniferous and Permian.
High during the Cretaceous, and then decreasing but more variable in the Cenozoic
Highest sea levels during the Cretaceous Period and the lowest sea levels could be seen in the Late Carboniferous to Permian

Question 7

What is the relationship between sea level, transgressions and regressions, and unconformities?

Answer 7

Sea level changes drive transgressions and regressions, which in turn influence the formation of unconformities in the geological record, representing shifts in Earth’s environmental and sedimentary conditions.

Question 8

What is an epicontinental sea? Why might an epicontinental sea form and retreat? What does this have to do with transgression and regression?

Answer 8

An epicontinental sea is a shallow sea that covers part of a continent, typically during periods of high sea levels. These seas form when sea levels rise (transgression) and flood continental areas. They retreat (regression) when sea levels fall due to factors like glaciations, tectonic uplift, or climate cooling, exposing previously submerged land.

Question 9

What are the general trends in the climate and sea level through the Paleozoic? What are the names of the major Palaeozoic continental masses and oceans and the approximate ages of their formation and amalgamation? Specifically, for each period of the Paleozoic what were the environmental conditions: ie where were the continents located (palaeogeography) and what was global temperature and sea level.

Answer 9

Cambrian to Ordovician: Sea levels were generally high, with many epicontinental seas. The climate was warm, with no polar ice caps. The Cambrian saw the rise of marine life, while the Ordovician was marked by the first significant glaciation, leading to a drop in sea level.
Devonian to Permian: Sea levels fluctuated due to tectonic events and the growth and retreat of ice sheets. The climate was initially warm but became more variable in the Carboniferous and Permian, with significant ice ages at the end of the Permian.

Early Paleozoic:
Continents were dispersed and surrounded by oceans. Laurentia (North America) and Gondwana (Southern Hemisphere) were the major masses. The Iapetus Ocean separated them.
Ordovician: First signs of continent collision and the start of Laurentia merging with parts of Gondwana.
Middle Paleozoic:
Devonian: Laurentia and Baltica formed Laurussia. The Appalachian Mountains were formed by the collision of Laurentia and Gondwana, in the formation of Pangaea, and sea levels remained high.
Late Paleozoic:
Carboniferous and Permian: Pangaea fully formed, amalgamating Laurentia, Gondwana, and Baltica. The Panthalassa Ocean surrounded it, and global temperatures were cooler with glaciations in the southern polar regions, especially in Gondwana.

Question 10

What major change took place on the Earth at the beginning of the Paleozoic? What is the Cambrian explosion?

Answer 10

A significant increase in the variety and complexity of life forms, especially in marine environments. During this period, most major animal phyla (including arthropods, mollusks, and vertebrates) first appeared.

Question 11

Does the unconformity at the base of the Cambrian indicate a major marine transgression or regression?

Answer 11

Transgression

Question 12

What led to a high concentration of metal ions in the oceans that were the source materials of biomineralization
in the Cambrian?

Answer 12

Increased weathering of continental rocks, enhanced volcanic activity and oxygenation of oceans

Question 13

What effect did the advent of mineralised hard parts have on the fossil record?

Answer 13

Increased fossil preservation and improved identification and classification

Question 14

What are the advantages of biomineralization?

Answer 14

Protection, support and structure, defense mechanisms, regulation of metabolic processes, energy processes and enhanced mobility

Question 15

When did the Great Ordovician Biodiversification events (GOBE) occur?

Answer 15

Ordovician Period, roughly between 485 and 443Ma

Question 16

How did GOBE differ to the Cambrian explosion?

Answer 16

While the Cambrian Explosion involved the appearance of new phyla, the GOBE involved the expansion and diversification of existing phyla, as well as the appearance of more complex ecosystems.

Question 17

What were the physical causes of the GOBE?

Answer 17

Changes in ocean chemistry, climate changes, increased oxygen levels, transgressions and tectonic activity

Question 18

What is thought to be the principal cause of the Late Ordovician mass extinction?

Answer 18

Formation of extensive ice sheets over southern Gondwana. The growth of ice sheets lowered global sea levels, reducing shallow marine habitats.
The cooling also disrupted ecosystems, causing stress on species adapted to warmer conditions, leading to significant extinction among marine organisms like brachiopods, corals, and trilobites.

Question 19

What was the depositional setting of reefs in the Silurian? How did the distribution of reefs in the Silurian differ to the distribution of reefs today and why? How have the major reef forming organisms changed through time?

Answer 19

Reefs primarily formed in shallow, warm marine environments, located along continental shelves and in epicontinental seas, where water was clear and rich in nutrients. While modern reefs are often found in the tropical belt, Silurian reefs were more widely distributed at lower latitudes, reflecting global warming after the Ordovician ice age.

In the Paleozoic, tabulate corals and stromatoporoids dominated, while in the Mesozoic and Cenozoic, modern scleractinian corals became the primary reef builders.

Question 20

When did the Caledonian Orogeny occur and why? What are the names of the continental masses and of the
ocean between them that was closed in the Caledonian orogeny? Make sure that you can locate the Calendonian orogen.

Answer 20

490- 390Ma (Silurian to Devonian).
Collision of Laurentia, Baltica and Avalonia.
Iapetus Ocean between them closed.
Formation of the Caledonides, across parts of Scotland, Ireland, Norway, and Eastern Greenland

Question 21

How did sea level fall through the Silurian-Devonian affect deposition on the land? What are red beds and what was their depositional setting?

Answer 21

Red beds are sedimentary rocks, often sandstones, shales, or conglomerates, with presence of iron oxides (like hematite). In the Silurian-Devonian, red beds were often formed in regions where continental interiors were exposed due to the drop in sea level causing regression, marking a shift from marine to terrestrial deposition.

Question 22

When is the oldest evidence for land plants? When did plants start evolving into something more than small, primitive forms? What age are the oldest forests? What impact did land vegetation have on the oxygen content of the atmosphere, the climate, and on rivers and animal life? If meandering rivers are seen Mars, does this mean vegetation also existed there at one time?

Answer 22

Late Silurian (420Ma), with spores.
Complex land plants, like vascular plants with roots and stems, evolved during the Devonian (400Ma).

The oldest forests date to the Early Devonian (390Ma), with development of trees.

Land vegetation contributed to the rise in oxygen levels in the atmosphere. It also influenced climate regulation by stabilizing soil, reducing erosion, and acting as a carbon sink. The development of plant life helped to shape rivers, promoting the creation of more defined channels.

The presence of meandering rivers does not indicate past vegetation. While ancient river channels suggest liquid water flowed, river formation could be due to other factors, such as water flow or sediment deposit without biological influence.

Question 23

When in the Devonian did one of the big-five mass extinction events occur and what were the physical causes?

Answer 23

360Ma, often referred to as the Frasnian-Famennian extinction. Climate change, anoxic oceans, sea level change and volcanic activity all contributed

Question 24

How did the climate at the poles and equator compare in the Carboniferous?

Answer 24

The equator was a hot, humid climate, with tropical rainforests and swamps widespread. The climate supported evolution of large, diverse ecosystems dominated by ferns, lycophytes, and the early ancestors of gymnosperms.
In contrast, the poles experienced cooler conditions, but the south pole (part of Gondwana) was covered by glaciers during the Carboniferous.

Question 25

When did the Gondwanan glaciations occur?

Answer 25

Late Carboniferous to early Permian periods, 360 and 260Ma

Question 26

What caused the cyclicity observed in Mid- to Late Carboniferous successions?

Answer 26

Primarily due to glacial-interglacial cycles related to the Gondwanan glaciations. These cycles were driven by orbital variations, which influenced Earth’s climate and sea levels. During glacial periods, sea levels fell, and continental glaciation expanded, while during interglacial periods, sea levels rose, and glacial ice receded.

Question 27

What two economically important units were deposited at equatorial latitudes in the early and late Carboniferous respectively? What were their depositional settings?

Answer 27

Early Carboniferous, the economically important unit deposited at equatorial latitudes was limestone. The depositional setting was predominantly shallow, warm marine environments that supported extensive reef and carbonate platform formation. In the late Carboniferous, the economically important unit was coal. This was primarily deposited in tropical swamp environments during glacial-interglacial cycles, especially in low-lying regions around the equator

Question 28

When did the supercontinent Pangaea form? What is the Variscan orogeny and when did it occur? What are the names of the two large continental masses and of the ocean between them that was closed in the Variscan orogeny?

Answer 28

Late Paleozoic Era, with the final amalgamation occurring around 300Ma, by the end of the Carboniferous.

The Variscan Orogeny was a major mountain-building event that occurred from the late Devonian to the early Permian (380–290Ma). It was contributing to the formation of Pangaea.

Laurentia and Gondwana. The ocean that was closed during this event was the Ocean of Iapetus. The collision between these continents led to the formation of the Variscan Mountains.

Question 29

Locate the Variscan orogen, Panthalassic ocean, Paleo-Tethys ocean, Tethys Ocean,
Gondwana, Laurasia, Siberia and Cimmerian Terrane on a map of the world.

Answer 29

Variscan Orogen: primarily stretches across Europe, especially in areas like the British Isles, France, Spain, and Germany.

Panthalassic Ocean: the vast global ocean surrounding Pangaea. It encircled Pangaea on all sides, covering what would become the Pacific Ocean today.

Paleo-Tethys Ocean: existed between Laurasia and Gondwana. It was located roughly between Laurentia and Siberia in the north and Gondwana in the south.

Tethys Ocean: started forming earlier in the Paleozoic and continued through the Mesozoic. It existed between Laurasia to the north and Gondwana to the south, with parts of it becoming the Mediterranean Sea today.

Gondwana: a southern supercontinent that included modern-day South America, Africa, India, Antarctica, and Australia. During the late Paleozoic, it lay mostly over the southern hemisphere.

Laurasia: a northern supercontinent, made up of North America, Europe, and Asia (excluding India). It was positioned in the northern hemisphere.

Siberia: the Siberian Craton was a large landmass located to the north of Laurasia, roughly in the region of modern-day Russia, stretching eastward toward Arctic regions.

Cimmerian Terrane: a series of landmasses, including parts of Turkey, Iran, and Tibet, which were located between Laurasia and Gondwana. These areas were part of the complex tectonic processes of the time, and their collision contributed to the formation of the Tethys Ocean.

Question 30

What effect did the formation of Pangaea have on global sea level and climate in the Permian? How did the depositional setting change in the Permian from the Carboniferous at low to mid latitudes?

Answer 30

Lower global sea levels due to the reduction in shallow seas and the closing of ocean basins. The interior of Pangaea, far from the moderating effects of the ocean, experienced a more arid climate with desert conditions.
Depositionally, low to mid-latitude regions transitioned from tropical swamps and coal-forming environments in the Carboniferous, to continental and desert settings in the Permian. With reduced sea levels, evaporites, red beds, and desert sandstones became more common, replacing the coal swamps of the previous period.

Question 31

What economically important group of minerals were deposited in the Zechstein Basin, and when were they
deposited? What is the main process by which these minerals form?

Answer 31

The economically important group of minerals are evaporites, particularly salt (halite), gypsum, and potash. These minerals were deposited during the Late Permian, 260Ma, in a restricted marine basin.

The main process by which these minerals form is evaporation.