Geological Time Scale and Principles of Stratigraphy

Posted on Nov 26, 2025 in Geology

Steno’s Fundamental Principles

Nicolas Steno proposed three main principles in stratigraphy:

  • The Principle of Original Horizontality of the Strata.
  • The Principle of Lateral Continuity of the Strata.
  • The Principle of Superposition of the Strata.

Principle of Cross-Cutting Relationships

An event or feature that cuts across layers of rock must be younger than the layers it affects.

Principle of Faunal Succession

The fossils within rock layers are from the time the rock was deposited. Therefore, we can recognize any geological period by its unique fossil content. Guide fossils are particularly useful for dating a stratum.

Guide Fossil: A guide fossil has several key characteristics:

  • Short Geological Time: Existed for a brief period, allowing for greater precision in dating.
  • Wide Geographical Distribution: Appears in several areas, allowing for correlation between rock layers.
  • Abundant in Rocks: Frequently found, making them easier to identify.

Complementary Dating Criteria

These criteria are useful for cases in which layers are folded vertically or inverted. We use sedimentary structures such as:

  • Internal Order: Graded bedding refers to the distribution of material sizes within a stratum (coarse at the bottom and fine at the top). It allows geologists to determine the original top and bottom of a stratum.
  • Top and Bottom Indicators: Some examples include ripples, cracks from drying, animal footprints, marks from currents, and bioturbation.
    • Cracks from Drying: These form when clays dry out. They are typically wide at the surface and narrow with depth.

Geological Time

Geological time is the period from the formation of the Earth to the present, covering approximately 4,600 million years (Ma).

The geological time scale is the ordered chronological scale of geological events. It is composed of eons, eras, periods, and epochs.

Origin of the Solar System and Earth’s Formation

The solar system formed from a nebula 4,600 million years ago. The Earth was formed at the same time.

  • The planet was initially molten.
  • Dense materials sank to form the core.
  • Intermediate-density materials formed the mantle.
  • Gases from volcanoes formed the early atmosphere.
  • The oceans were formed by condensing water vapor expelled from volcanoes.

The Precambrian Eon

Precambrian Eons

The Precambrian is formed by three eons (4,600 – 540 Ma):

  • Hadean
  • Archaean
  • Proterozoic

Key Precambrian Events

  • Appearance of life:
    • Stromatolites (colonies of bacteria) changed the atmosphere by producing O2 and removing CO2.
    • First eukaryotes appeared.
    • First multicellular organisms appeared.
  • Supercontinents: Vaalbara, Ur, Kenorland, Arctica, Atlantica, Columbia (Nuna), Rodinia, and Pannotia.
  • The largest glaciation in Earth’s history occurred.

The Paleozoic Era

Paleozoic Periods

The Paleozoic is formed by six periods (540 – 250 Ma):

  • Cambrian
  • Ordovician
  • Silurian
  • Devonian
  • Carboniferous
  • Permian

Key Paleozoic Events

  • Life expansion:
    • The Cambrian explosion: almost all major animal phyla emerged.
    • Trilobites appeared.
    • The first life forms colonized the continents (plants).
    • The origin of vertebrates.
    • The Great Permian Extinction event occurred.
  • Supercontinent: Pangaea formed.
  • Two major glaciations occurred.

The Mesozoic Era

Mesozoic Periods

The Mesozoic is formed by three periods (250 – 65 Ma):

  • Triassic
  • Jurassic
  • Cretaceous

Key Mesozoic Events

  • Diversification of life:
    • Maximum reptile development: ichthyosaurs, plesiosaurs, pterosaurs, and dinosaurs.
    • First mammals appeared.
    • First birds appeared.
    • First flowering plants appeared.
    • The Great Cretaceous Extinction event occurred.
  • Continental separation: The fragmentation of Pangaea began.

The Cenozoic Era

Cenozoic Periods

The Cenozoic is formed by three periods (65 Ma – present day):

  • Palaeogene
  • Neogene
  • Quaternary

Key Cenozoic Events

  • Appearance of current living beings:
    • Diversification of flowering plants.
    • Bird diversification.
    • Mammal diversification, including the appearance of hominids.
  • Formation of the current continents.
  • Elevation of the current great mountain ranges.
  • A major glaciation occurred in the Quaternary.