Chronostratigraphy

Any natural phenomena that obey a definite law of flow rate can be used to mark off certain intervals of time that can serve as a ‘clock’. The basic unit of time is second which. The other method that is commonly used by geologists is the subdividing time according to non-equal time intervals determined by a series of isolated events in a sequence.

• The time of an event is given with reference to another event that can be taken as ‘datum’.
• The datum for the geological time is commonly given in Ka (10³) years, Ma (10⁶) years and Ga (10⁹) years before present.
• The events are recorded in stratified rock sequences including the life evolution.

Geochronologic & Chronostratigraphic units

The rate of sedimentation, which determines the time duration during a phase of sedimentation is not uniform. Therefore the same thickness of rock formations of two different areas or of two different phases of sedimentation of the same area may not represent the same duration of time. That is why we need to have two time units one is based on time in digits another is based on rock depositional cycles integrated with time.

The “time” and “time-rock” units have been designated by two separate set of terms though they are directly comparable to one another and the definition of one depends upon the other. A time unit of the geological time, also known as geochronologic unit, is defined by a chronostratigraphic unit (time-rock unit).

———- Terminology ———-

Geochronologic / Time Unit	Examples	Chronostratigraphic / Time-rock Units	Examples
Era	Cenozoic	Erathem	Cenozoic
Period	Tertiary period	System	Tertiary period
Epoch	Miocene epoch	Series	Miocene
Age	Pannonian age	Stage	Pannonian age
Phase	Globorotalia tumuda flexuosa Phase	Zone	Globorotalia tumuda flexuosa Phase

• An Erathem is generally made up of three or more systems.
• A System is represented by succession of rocks deposited during a Period. Each system was identified by its typical fossil assemblage.
• A Series constitutes a part of system that was deposited during an Epoch. In marine basins, these epochs often correspond to a phase of marine transgression or regression.
• A Stage is a part of series, which was deposited during an age.
• A Zone is the smallest unit of the chronostratigraphic scale. A zone is characterized by an assemblage of fossils referred to as the “zonal assemblage”.