EPS 101 Spring 2003– Adrian Brearley - Lectures 20-21

The Rock record and Geologic Time

The great English geologist, William Smith, who made the first geological map and laid the foundation for modern geological maps.

Key concepts

Relative ages dating – the concept of using geological relationships between rocks to determine the relative ages of one rock to another, i.e. an igneous intrusion cutting into a sequence of sedimentary rocks must mean that the intrusion is younger than the sediments. Provides no information as to exactly how old any given rock is.

Absolute dating – dating of the absolute age of a rock, i.e. 3 billion year old, using the decay of naturally occurring radioactive isotopes such as Sr and Nd.

Stratigraphic record

Principle of original horizontality – principle that sediments are deposited in essentially horizontal beds. Tilting or folding of sedimentary beds means that they have been deformed by tectonic stresses.

Principle of superposition – principle that each layer of sedimentary rock in an undisturbed sequence is younger than the one beneath it and older than the bed above it. Sequence of sediments is therefore a vertical time line from the bottom of the sequence to the top.

Stratigraphic sequence– sequence of rocks, which from bottom to top is a chronological record of the geological history of the region. Stratigraphic sequence may include a wide variety of sedimentary rocks of different origins that formed in different environments.

Geologic time – time line spanned by the stratigraphic sequence: entire period of time that elapsed during the formation of the stratigraphic sequence.

Sequence stratigraphy – type of stratigraphy where the sequences of sedimentary rocks are the basic units of interest. A sequence is a set of sedimentary beds (i.e. an alluvial sedimentary sequence) bounded above and below by unconformities. Unconformities define fluctuations in sea level that allowed erosion to take place.

Fossil record

Paleontology – The study of ancient life forms on the Earth, preserved as fossils in rocks (the fossil record).

Faunal succession – the observation that in moving through a stratigraphic sequence, the characteristic fossils that are present in each sedimentary layer change from one layer to the next, in a consistent way.

Unconformities

Unconformity – a surface between two layers of rocks that were not laid down in an unbroken sequence. An unconformity represents a period of time where the lower sequence of rocks may have been unlifted and eroded or buried deeply and metamorphosed and then uplifted. An unconformity represents a period o geologic time when the geologic record has not been preserved.

Types of unconformity

Disconformity – type of unconformity where the upper set of sedimentary beds overlies an erosional surface developed on undeformed, still horizontal sedimentary beds.

Nonconformity - type of unconformity where the upper sedimentary beds overlie metamorphic or igneous rocks.

Angular unconformity - type of unconformity where the upper horizontal beds are not parallel to the beds below the unconformity. The lower beds may have been tilted or deformed. Erosional surface.

The geologic time scale

The relative timescale of the Earth’s geologic history based on relative ages of sedimentary rocks, the presence of unconformities that date tectonic episodes and cross cutting relationships between igneous rocks, faults and sedimentary rocks.

Divisions of geologic time: eons, eras, periods and epochs.

Archean eon – the oldest and longest eon ranging from the oldest earth rocks known (4 billion years – b.y.), to rocks 2.5. billion years old. Basic structure of Earth, core, mantle crust formed. Primitive unicellular organisms form.

Proterozoic eon – 2.5 billion to 544 million years – surface and interior of the Earth had evolved to basically the structure that they have today. Atmosphere was very oxygen poor at beginning of Proterozoic, but became more oxygen rich toward the end of the eon. More advanced life forms occur progressively through this eon and are preserved as fossils.

Phanerozoic eon – the last 544 million years of Earth history; the best understood. Life evolves rapidly into many complex forms.

Phanerozoic (visible life) eras

Paleozoic era (earlier life) – 544 – 245 million years ago.

Mesozoic era (middle life – 245-65 million years ago.

Cenozoic era (recent life) - 65 million years to present day.

Periods – further subdivision of eras, based on the geological characteristics of geographical locations throughout the world where the formations are best displayed, or were first described.

Epochs – further subdivision of periods, used for the Cenozoic era.

Absolute Geological Time Scale.

Geological time scale based on the precise measurement of the age of rocks using the decay of radioactive elements.

Radioactivity – process where elements spontaneously breakdown over time to form new atoms of a new elements and emit radiation (energy) in the process.

Radiometric dating – using the constant decay of radioactive elements in the earth to determine the absolute age of rocks of different ages.

Half-life – the time required for one half of the original radioactive atoms of an element to undergo radioactive decay. The half lives of elements used for radioactive elements range from thousands to billions of years. E.g. Uranium 238 half life = 4.5 billion years, Rubidium 87, 47 billion years.