Early Paleozoic Era

• The Early Paleozoic era is characterized by the closure of the Iapetus Ocean and the Caledonian Orogeny as well as the reconstruction of plate motions.

Chronostratigraphic Chart Highlights

• The Early Paleozoic encompasses the Cambrian, Ordovician, and Silurian periods.
• The Cambrian Period began 541 million years ago.
• The Ordovician Period started approximately 485.4 million years ago.
• The Silurian Period commenced around 443.8 million years ago but ended 419.2 million years ago.

Continental Drift and Climate

• The latitude of British and Irish crustal fragments has changed over time throughout the Phanerozoic Eon.
• The fragments were part of different continents at different times.
• Continents: Gondwana, Pangaea, Laurasia and Laurentia
• Throughout the Phanerozoic, global changes occurred across continents due to plate tectonics

Geological History of the British Isles

• Scotland and NW Ireland have distinct pre-Devonian geological histories compared to southern Ireland, England, and Wales.
• In the Early Paleozoic, the British Isles were formed from parts of two continents are Laurentia and Avalonia.
• The Iapetus Ocean separated Laurentia and Avalonia

Early Palaeozoic Palaeogeography

• Contrasting geological histories occur in different areas due to the differing sequences of rocks preserved.
• Deposition reveals different climatic conditions.
• The fossil record shows geographically distinct fossil assemblages.
• Palaeolatitudes derived from rock magnetism studies show a range of values.

Wilson Cycle

• The Wilson Cycle describes the opening and closing of ocean basins due to plate tectonics.
• The Wilson Cycle begins with rifting within a continent, leading to continental splitting.
• As the oceanic ridge spreads, passive margin cooling and sediment accumulation proceed with seafloor spreading.
• An oceanic plate subducts under a continental plate when convergence begins creating a volcanic chain at the active margin.
• Material is added to the continent through terrane accretion via sedimentary accretionary wedges or subducting plate fragments.
• An orogeny then thickens the crust and builds mountains as two continents collide, forming a supercontinent.
• The continent erodes and thins the crust and the process may begin again.

Earth's Magnetic Field

• The Earth has a magnetic field that flows from the South Pole to the North Pole.
• Magnetic north doesn't currently align with geographic north; it's offset by about 11°.
• Secular variation: Offset from the spin axis.
• Averaged over thousands of years, magnetic north coincides with geographic north.
• The inclination of the magnetic field varies with latitude.
• Field lines point vertically up at the south pole.
• Field lines point horizontally at the equator.
• Field lines point vertically down at the north pole.
• Field lines have intermediate inclinations at intermediate latitudes.

Magnetostratigraphy

• Sedimentary rocks record the Earth's magnetic field through the alignment of magnetic minerals as sediment accumulates and lithifies.
• In sedimentary rocks magnetic minerals make up a tiny proportion of the rock (< 0.1%).
• As the magnetic grains sink through the water they align themselves with the Earth's magnetic field.
• Lithification: the process in which the magnetic grains become locked and preserve a record of the magnetic field at the time of the rock’s formation.

Changing Latitudes

• The Early Cambrian, Late Cambrian, Early Ordovician, Middle Ordovician and Late Ordovician all show the relative latitudes of Avalonia and Laurentia
• These are measured using paleomagnetism
• Continental positions and palaeolatitudes for various periods are reconstructed using palaeomagnetic data, showing the movements of Avalonia and Laurentia.
• By the Late Silurian and Early Devonian periods, key fish faunas were established, further illustrating biogeographical patterns based on continental configurations.
• Early Devonian: Also shown is the major continental land areas which are green.