Earth History - The Phanerozoic Study Notes

Earth History: The Phanerozoic

Overview of the Phanerozoic

  • The Phanerozoic Eon spans from 541 million years ago (Ma) to the present.

  • Significant event marking the beginning of the Phanerozoic: the appearance of diverse hard-shelled organisms.

    • Etymology: "Phaneros" means visible and "zoic" means life, indicating the era of abundant fossil records due to hard shells.

  • Duration of the Phanerozoic: Covers the most recent 541 million years of Earth's history.

Divisions of the Phanerozoic Eon

  • Paleozoic Era: 541 - 252 Ma, termed "Early Life".

  • Mesozoic Era: 252 - 66 Ma, termed "Middle Life".

  • Cenozoic Era: 66 Ma - Present, termed "New Life".

International Chronostratigraphic Chart

  • Chart establishes various geological time intervals based on significant stratigraphic markers and fossil records.

  • Terminology includes:

    • Eonothem/Eon: Major divisions of geological time.

    • Erathem/Era: Subdivisions within eons.

    • System/Period: More specific subdivisions.

    • Series/Epoch: Further divides periods into more granular units.

    • Stage/Age: The finest division, referencing specific age-dated formations.

Numerical Age Indications

  • Detailed attributions of ages are expressed in million years (Ma).

  • E.g., Pleistocene has an age of 0.774 Ma, and Upper Cretaceous displays ages such as 72.1 ± 0.2 Ma.

  • Distinct units are indicated for both ratified and non-ratified geological units, including placeholder units.

Geological Histories and Paleogeography

Intricacies of Rock Ages

  • Continental crust composition varies significantly across geological eras.

  • Younger Rocks: Associated with more recent mountain-building events, often obscuring older sediments.

  • Cratonic Preservation: Paleozoic rocks show better preservation on stable cratons compared to younger formations.

Paleogeographic Reconstructions

  • Emphasizes the abundance of preserved rock units, paleomagnetism data, and fossil records for an extensive time frame.

  • Correlations between geological intervals across separated oceanic regions benefit from fossil evidence.

Sea Level Changes

  • Significant fluctuations: Sea level has varied from a low of -200 m to a high of +500 m.

    • High Sea Level Events: Resulted in transgressions, which typically flooded continental interiors, promoting sediment deposition.

    • Low Sea Level Events: Led to regressions, exposing continental margins and initiating erosional processes.

Major Geological Events

Unconformities

  • Large-scale unconformities reflect historical sea level cycles influencing sedimentary layers.

    • Rising sea levels expanded inundation over time, while decreasing levels indicated erosive events extending outward from continental interiors.

    • The Cretaceous Interior Seaway is a notable example where sedimentary sequences demonstrate these patterns.

Mass Extinctions

  • Defined as rapid and widespread disappearances of a significant number of species from the fossil record.

    • Always Ongoing: Regular extinction occurs at a low rate (3-5% of genera every 10 Ma).

    • Mass Extinctions: Classified by event where 25-50% of genera vanish within shorter time frames.

    • The 'Big Five' Mass Extinctions:

    • End-Ordovician: 443 Ma

    • End-Devonian: 350 Ma

    • End-Permian: 252 Ma (most notorious)

    • End-Triassic: 200 Ma

    • End-Cretaceous: 66 Ma

    • Discussion of causes includes rapid climate change and other environmental shifts.

Early Paleozoic: Cambrian & Ordovician (551 - 443 Ma)

Tectonic Fragmentation

  • The period saw the development of continental fragments from the supercontinent Pannotia:

    • Gondwana: Included areas now known (South America, Africa, Antarctica, India, and Australia).

    • Laurentia: Corresponded to present-day North America and Greenland.

    • Baltica: Consisting of Europe.

    • Siberia: Representing today’s eastern Russia.

Environmental Conditions

  • Warm shallow seas dominated, promoting a prolific marine ecosystem, with North America situated near the equator.

  • Characterized by a period of rising sea levels, contributing to vast fossil accumulation in sedimentary rocks.

Cambrian Life Explosion

  • Marked by the emergence of hard-shelled organisms, leading to rapid diversification known as the Cambrian explosion:

    • Development of complex ecosystems becoming apparent, alongside significant predator-prey relationships in marine environments.

Ordovician Life and Events

  • Emergence of first vertebrates, jawless fish, crinoids, and early land plants:

    • Mass Extinction: At end of Ordovician due to climatic events such as glaciation.

Taconic Orogeny

  • Early Ordovician tectonic activity: A volcanic arc approached Laurentia, resulting in mountain building through subduction processes.

    • Characterized by the first Appalachian orogenic event impacting eastern North America.

Middle Paleozoic: Silurian & Devonian (443 - 359 Ma)

Climatic Conditions

  • Transition into a greenhouse climate fostering the growth of epicontinental seas and extensive reef complexes.

Silurian and Devonian Geography

  • Variability in sediment accumulation rates along different coasts:

    • Active tectonics on the east coast with passive sedimentation in the west, resulting in contrasting geological features.

Life Development

  • Rapid proliferation of fish species and land plants during the Silurian:

    • Introduction of Cooksonia, the first land plant, and arthropods like sea scorpions/preadaptive terrestrial taxa.

Acadian Orogeny

  • The arrival of the Avalonia microcontinent and its collision with Laurentia during Devonian times:

    • Related geological activity contributed to sediment deformation and resultant mountain-building phases in the Appalachians.

Late Paleozoic: Carboniferous & Permian (358 - 252 Ma)

Formation of Pangea

  • The collision of continental fragments leading to the formation of the supercontinent Pangea.

    • Climatic cool-down followed expansive greenhouse conditions.

Acadian Orogeny

  • Extensive geological shifts due to continental convergence and climactic changes across the areas.

Carboniferous Life

  • Rich ecosystems promoted in formerly tropical conditions leading to significant plant and animal diversification.

    • Development of early reptiles adapted to terrestrial life.

Permian Extinction

  • The most significant extinction event at 251.9 Ma, leading to the loss of a substantial portion of marine biodiversity, with causes linked to extreme climatic events and geological activities including volcanic eruptions of the Siberian Traps.

Summary of Geological Ages & Extensions

Detailed Age References

  • The temporal references indicate geological eras defined by notable events (mass extinctions, climatic shifts) keeping in mind the significant ecological aspects influenced by geological phenomena.

Open Questions

  • Ongoing debates on the timing and specific causes of extinction events throughout history represent critical areas for future research, inviting further exploration into climate dynamics and geological processes related to mass extinctions throughout the Phanerozoic.