Summer SchoolGeosciences

Geology

Lecture 2 Stratigraphy

Stratigraphy

• Stratigraphy is the study of the stratified rocks, i.e. sediments and volcanics, particularly their sequence in time, the character of the rocks and the correlation of beds in different localities

Copyright Skinner and Porter, 1987

Stratigraphy

• Lithostratigraphy

• Chronostratigraphy

• Biostratigraphy

• Magnetostratigraphy

• Seismic stratigraphy } oil industry

• Sequence stratigraphy } relevance

• Event stratigraphy

Lithostratigraphy • Lithostratigraphy is the description,

definition and naming of rock units

• Rock units are described and defined using their overall lithological characteristics and by their relationship with other units

• Units are named according to their perceived rank in a formal lithological hierarchy

Lithological hierarchy

• Supergroup

• Group

• Formation

• Member

• Bed

Lithological hierarchy

• The formation is the basic unit of the lithological hierarchy and is commonly defined as the smallest mappable or traceable unit

• A formation has characteristics which distinguish it from other formations

• E.g. The Collieston Formation belongs to the Southern Highland Group of the Dalradian Supergroup

Unconformities

• The record of strata may be broken because land may be uplifted, sea level changes or rock may be worn away

• This break in the rock record is called an unconformity

Copyright Skinner and Porter, 1987

Copyright Skinner and Porter, 1987

Chronostratigraphy

• Chronostratigraphy is the arrangement of rocks into units based on their age, oldest first, youngest last

• Chronostratigraphic units were formed during a specific portion of geological time and their boundaries are time significant

Chronostratigraphy

• Eon – Archaean, Proterozoic, Phanerozoic

• Era – Phanerozoic is divided into 3 Eras, Paleaozoic, Mesozoic and the Cenozoic

• Periods e.g. Cenozoic is made up of the Tertiary and the Quaternary periods

• Epochs e.g. the Quaternary is made up of the Pleistocene and the Holocene epochs

Biostratigraphy

• Biostratigraphy is the use of fossils for stratigraphical correlation and the organisation of sedimentary strata into biostratigraphical units on the bases of their fossil content

• These units are termed biozones

Biostratigraphy

• Biostratigraphy provides a means by which a Chronostratigraphic (time) scale can be constructed and applied to lithological successions, because biological species only live for a certain amount of time

• Biozones are often named after one or more fossil taxa, usually species, which constitute the index fossils

Chronostratigraphy

• Magnetostratigraphy uses stratigraphical variations in the magnetic properties of rocks as a basis for geological correlation

• The two most useful properties are– The magnetic susceptibility– The direction of remnant magnetism

• The basic unit is the magnetozone

Stratigraphy

• Using stratigraphy and radioactive dating as well as astronomical data the generally accepted age for the Earth is

• 4,600 Ma (million years)

Geological Time• If we compress 4,600 Ma to 1 year (365 days)• 4,600 Ma + New Year’s Day• 3000 Ma Earliest life = Spring of the year• 10,000Bp Last glaciers in Britain = 11.59pm, 31st

December• Jesus Christ lived and died 14 seconds before

midnight, 31st December• 1st Test Match played <1 second before midnight,

31st December• Our own lives represent a fraction of the last second

of the year

Sedimentary Environments

Most sedimentary strata change character from one area to another as a result of:

• Changes in the way sediments are transported

• Changes in the intensity of the progress

• The conditions under which sediments accumulate

Sedimentary Environments

• A variety of depositional Sedimentary Environments may be found, for example, moving from a continent to the adjacent ocean basin

• Within each zone there will be distinctive sediments and organisms, which reflect that depositional environment

Copyright Skinner and Porter, 1987

Sedimentary Environments

• “ A distinctive group of characteristics within a sedimentary unit that differs, as a group, from those elsewhere in the same unit is a Sedimentary Facies”

Skinner and Porter (1987 p.161)

Reading

Press and Seiver 1982 Earth. Chapter 2 (relevant sections) “ The rock record and the geological timescale”

Skinner, B.J. and Porter, S.C.1987 Physical Geology, New York Chapter 5