Paleozoic Earth History Notes

Paleozoic Earth History

Chapter 20: Rocks of the Phanerozoic Eon

Overview of the Phanerozoic Eon

  • Characteristics of Phanerozoic Eon:
    • More accessible geological record
    • Less altered compared to previous eons
    • More fossiliferous, allowing better understanding of ancient life forms
  • Timeline:
    • 542 million years B.P. (Ma) - beginning of Paleozoic Era
    • Continues until present
  • Geologic Time:
    • 251 Ma represents a significant boundary in geological history

Continental Architecture

  • Key Geological Features:
    • Craton: Stable interior region characterized by Precambrian rocks.
    • Epeiric Seas: Shallow seas covering continents at various times.
    • Domes and Arches: Elevated geological structures.
    • Basins: Low-lying regions where sediments accumulate.
    • Mobile Belts: Active orogenic regions that include regions of mountain-building activity.
    • Examples of mobile belts:
      • Cordilleran
      • Appalachian
      • Franklin
      • Ouachita
      • Caledonian
  • Geological Map Highlights:
    • Tectonic provinces of North America outlined by cratons, mobile belts, basins, and domes.

Paleozoic Paleogeography

  • Continental Arrangement:
    • Breakup of the supercontinent Rodinia occurred by the end of the Proterozoic, leading to the formation of six major continents and smaller microcontinents.
    • Major continents composed of cratons that were stable and static throughout the Paleozoic.
  • Significant Geological Events:
    • Continents located near the equator with ice-free poles 510 Ma.
    • Notable continental collisions and movements:
    • Gondwana moving South (450 Ma)
    • Baltica and Laurentia colliding (410 Ma) leading to Laurasia formation
    • Formation of mountain ranges due to Laurasian and Gondwana convergence.
    • Gondwana's movement North and collision with Laurasia (350 Ma).

Formation of Pangaea

  • Key Developments:
    • Continuous collision between Gondwana and Laurasia leading to the formation of Pangaea (250 Ma), a supercontinent.
    • Climate effects of Pangaea:
    • Creation of widespread arid and semiarid regions.
    • Formation of mountain ranges contributed to rain shadows.
    • Resultant dry conditions experienced in North America and Europe.
  • Evaporites: Extensive deposits formed due to climatic conditions over Pangaea.

Sequence Stratigraphy in Paleozoic North America

  • Laurence Sloss (1963) divided sedimentary rocks into six sequences illustrating major transgression-regression phases bounded by unconformities.
  • Significance of Sea Level Changes:
    • Global sea-level changes influenced by climatic conditions and sea floor spreading events.

Major Sequences of the Paleozoic

  • Sauk Sequence:

    • Spanning the Late Proterozoic to Early Ordovician.
    • Recognized as the first major transgression onto North America.
    • Characteristics:
    • Dominance of epeiric seas covering continents with limited ice at poles and no vegetation, leading to rapid erosion and weathering.
    • Formation of transcontinental arch with large islands above sea level.
    • Sedimentary features included:
      • Carbonate deposits, stromatolites, and reefs evident in early Cambrian rocks.

  • Tapeats Sandstone (Grand Canyon example):

    • Composition:
    • Sand grains primarily deposited in nearshore environments.
    • Characteristics of clean, mature sandstones predominantly consisting of quartz (up to 99%);
      • Grains well-rounded and sorted.

  • Transition from Sandstones to Carbonates:

    • As the sea transgressed, land exposure diminished, leading to limestone deposition in warm, shallow, clear waters.
    • Formation of shell debris and oolites, indicating significant carbonate formation during transgressive events.

  • Sauk Sea Depth and Features:

    • Depth not exceeding 200 m due to the presence of stromatolites requiring sunlight for photosynthesis.
    • Evidence of shallow sea conditions noted by occurrence of mudcracks, suggesting exposure to atmosphere.

  • End of the Sauk Sea:

    • Regresed during Early Ordovician (505 Ma), exposing eroded limestones in tropical conditions.
    • Presence of a significant unconformity marking transition from the Sauk to Tippecanoe sequences.

  • Tippecanoe Sequence:

    • Major transgression onto craton, characterized by clean, well-sorted quartz sand deposited over craton, known as St. Peter Sandstone.
    • Geological Variants:
    • Carbonates followed by shallow seas, supporting diverse marine life including fossils like brachiopods, bryozoans, crinoids, cephalopods, corals, and algae.

  • Tippecanoe Reefs and Evaporites:

    • Formation of reefs created by organisms like Archaeocyathids, stromatoporoids, and corals thriving in restricted geographical areas (30°N and S).
    • Significant evaporite deposits notably in the Michigan Basin signifying drastic changes in water chemistry and salinity during periods of sediment deposition.

  • Kaskaskia Sequence:

    • Extending from Middle Devonian to Middle Mississippian, characterized by major unconformity with underlying Tippecanoe sequence.
    • Notable deposits included:
    • Clean quartz sands eroded from Appalachian region.
    • Range of fossiliferous limestones, carbonate deposits, black shales indicative of deeper marine environments with limited oxygen.

  • Mississippian Period:

    • Return to carbonate dominance in sediment composition, particularly in the Kaskaskia sequence following the regressive nature of the sequence.
    • Transition marked by increased detrital sediment from the Appalachians with resultant excellent petroleum reservoirs forming in sandstone deposits [Upper Mississippian Tar Springs Formation].

  • Absaroka Sequence:

    • Spanning Late Mississippian to Jurassic, showcasing significant unconformities leading to alternating sedimentary sequences of marine and nonmarine origins.
    • Formation of cyclothems reflecting advances and retreats of the sea influenced by glacial movements of Gondwana.
    • Relation to the Ancestral Rockies with significant geological upheaval noted from Precambrian basement rocks uplifting and eroding due to geological activity.

  • Evaporites and Reefs during Late Absaroka:

    • Evidence of significant evaporite formations during the Permian period, hinting towards thick deposits shaped by climatic conditions.
    • Reefs noted at locations such as the Guadalupe National Park playing vital roles in the sedimentary record.

Geological and Economic Implications

  • Paleozoic Mineral Resources include:
    • Common resources:
    • Sand, rock salt, gypsum/anhydrite (sheetrock), and limestone.
    • Metallic mineral resources:
    • Tin, copper, gold (notable depletion), lead, zinc, iron, petroleum, natural gas, and coal resources, providing significant economic implications from these geological formations.

Coal Types and Their Importance

  • Types of Coal in relation to total coal percentage includes:
    • Anthracite: 1%
    • Bituminous coal: 48%
    • Subbituminous coal: 34%
    • Lignite: 17%
    • Significance: Understanding the distribution and formation of various coal types is critical for resource management, energy production, and understanding past environmental conditions.