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Mike Horne FGS

Historical geology

Stratigraphy

TERMS

Strata - layers of rock - sedimentary, plus lava flows and volcanic ash falls.
Stratigraphic units are defined by an event at the base (the top is the base of the next unit)
Lithostratigraphy - based on rock units. Should be named after places were good example is exposed.

Group

Formation

Member

Bed

The names of lithostratigraphic units have capital letters, even when they do not have the unit type used in their name: for example Cloughton Formation, Kimmeridge Clay, or Chalk (though clay and chalk do not have capital letters when we just writing about a rock type, they do when we are using them in the name of a stratigraphic unit).

Chronostratigarphy -

Chronostratigraphy - based on time
TIME UNITS	CHRONOSTRATIGRAPHIC UNITS
Era	Erathem
Period	System
Epoch	Series
Age	Stage
Time	Substage

SUBDIVISIONS :
Late	Upper
Middle/Mid	Middle
Early	Lower

Note - The Chronostratigraphic Unit refers to the rock deposited during a time unit
Example - Lower Jurassic rocks were deposited during Early Jurassic times.
DO NOT GET THIS WRONG - it is common error !

Biostratigraphy - based on the time span of fossils.

Unit is the Zone or Bio-zone. Named after a zone fossil. Base defined by the first or last appearance of a fossil. Good zone fossils must have wide geographical spread and short time span. Example - ammonites in the Jurassic - which swam in oceans and each species lasted about 1 million years before next one evolved.

Fossil names are usually printed in italics or are underlined when handwritten, but when the fossil species name is used as a biozone it is not. DO NOT GET THIS WRONG - it is common error !

Event stratigraphy -

Uses events, which cover large areas and define a geological 'instant' in time. Examples volcanic ash falls, sea level rise or fall, temperature change, change in magnetic field of the earth.

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