Chapter 7
Stratigraphy
Jenø Nagy and Knut Bjørlykke
7.1
Introduction and Concepts
Stratigraphy is the study of the succession and timerelated architecture of rock strata. Stratification is not
limited to sedimentary deposit, but is also found in
igneous rocks, particularly volcanic developments,
and in certain plutonic rocks. All bedded rocks can be
treated stratigraphically, i.e. to establish age relations
between beds. However, the term “stratigraphy” is used
primarily in the context of sedimentary research.
Stratigraphy involves the study, subdivision and
documentation of sedimentary successions, and on
this basis to interpret the geological history they represent. In order to reconstruct an environment or special events in the geological past, regionally or even
globally, it is necessary to characterise and correlate
sedimentary horizons from different areas. It is important to establish which sedimentary units were deposited at the same time or by the same or similar
sedimentological or biological processes, even if they
are not quite contemporary. Correlation is generally a
question of what is possible to achieve with the available amount and quality of data.
We usually have no possibility of determining definitely which beds were deposited at the same time, but
attempt to use all the information available in the
rocks. This information falls into five main categories:
(1) Rock composition and structures resulting from
sedimentological processes. (2) Fossil content, which
is a result of biological, environmental and ecological
evolution throughout geological history. (3) Content
of radioactive fission products in minerals or rocks
which may be used for age dating. (4) Magnetic
properties of rock strata. (5) Geochemical features of
sediments. (6) Geophysical expression. These correlation methods are so different that it has been found
useful to work with three forms of stratigraphy which
can be used in parallel:
1. Lithostratigraphy: Classification of sedimentary
rock types on the basis of their composition,
appearance and sedimentary structures.
2. Biostratigraphy: Classification of sedimentary
rocks according to their fossil content.
3. Chronostratigraphy: Classification of rocks on the
basis of geological time. Geochronology is the
actual subdivision of geological time.
The first two are thus based on rock relationships
which can be described, and are often collectively
referred to as rock stratigraphy.
Despite radiometric dating, geological time is not
absolute. Even this dating method gives different ages,
depending on which half-life is used for calculating
radioactive decay, and is encumbered with several
other uncertainty factors. Chronostratigraphy is therefore a theoretical and abstract concept which describes
a time scale we cannot measure exactly. Rock stratigraphy, on the other hand, takes the rocks themselves as
its starting point and is based on boundaries which are
identified as potentially suitable for correlation across
greater or lesser areas. For rules for stratigraphic
nomenclature, see stratigraphy.org. The different
types of stratigraphic units are listed below in hierarchic arrangement. See also Stratigraphy.com.
J. Nagy (*) K. Bjørlykke
Department of Geosciences, University of Oslo, Oslo, Norway
e-mail: jeno.nagy@geo.uio.no; knut.bjorlykke@geo.uio.no
K. Bjørlykke (ed.), Petroleum Geoscience: From Sedimentary Environments to Rock Physics,
DOI 10.1007/978-3-642-34132-8_7, # Springer-Verlag Berlin Heidelberg 2015
231
Stratigraphy
Jenø Nagy and Knut Bjørlykke
7.1
Introduction and Concepts
Stratigraphy is the study of the succession and timerelated architecture of rock strata. Stratification is not
limited to sedimentary deposit, but is also found in
igneous rocks, particularly volcanic developments,
and in certain plutonic rocks. All bedded rocks can be
treated stratigraphically, i.e. to establish age relations
between beds. However, the term “stratigraphy” is used
primarily in the context of sedimentary research.
Stratigraphy involves the study, subdivision and
documentation of sedimentary successions, and on
this basis to interpret the geological history they represent. In order to reconstruct an environment or special events in the geological past, regionally or even
globally, it is necessary to characterise and correlate
sedimentary horizons from different areas. It is important to establish which sedimentary units were deposited at the same time or by the same or similar
sedimentological or biological processes, even if they
are not quite contemporary. Correlation is generally a
question of what is possible to achieve with the available amount and quality of data.
We usually have no possibility of determining definitely which beds were deposited at the same time, but
attempt to use all the information available in the
rocks. This information falls into five main categories:
(1) Rock composition and structures resulting from
sedimentological processes. (2) Fossil content, which
is a result of biological, environmental and ecological
evolution throughout geological history. (3) Content
of radioactive fission products in minerals or rocks
which may be used for age dating. (4) Magnetic
properties of rock strata. (5) Geochemical features of
sediments. (6) Geophysical expression. These correlation methods are so different that it has been found
useful to work with three forms of stratigraphy which
can be used in parallel:
1. Lithostratigraphy: Classification of sedimentary
rock types on the basis of their composition,
appearance and sedimentary structures.
2. Biostratigraphy: Classification of sedimentary
rocks according to their fossil content.
3. Chronostratigraphy: Classification of rocks on the
basis of geological time. Geochronology is the
actual subdivision of geological time.
The first two are thus based on rock relationships
which can be described, and are often collectively
referred to as rock stratigraphy.
Despite radiometric dating, geological time is not
absolute. Even this dating method gives different ages,
depending on which half-life is used for calculating
radioactive decay, and is encumbered with several
other uncertainty factors. Chronostratigraphy is therefore a theoretical and abstract concept which describes
a time scale we cannot measure exactly. Rock stratigraphy, on the other hand, takes the rocks themselves as
its starting point and is based on boundaries which are
identified as potentially suitable for correlation across
greater or lesser areas. For rules for stratigraphic
nomenclature, see stratigraphy.org. The different
types of stratigraphic units are listed below in hierarchic arrangement. See also Stratigraphy.com.
J. Nagy (*) K. Bjørlykke
Department of Geosciences, University of Oslo, Oslo, Norway
e-mail: jeno.nagy@geo.uio.no; knut.bjorlykke@geo.uio.no
K. Bjørlykke (ed.), Petroleum Geoscience: From Sedimentary Environments to Rock Physics,
DOI 10.1007/978-3-642-34132-8_7, # Springer-Verlag Berlin Heidelberg 2015
231