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stratigraphy summary - sedimentary systems (UU)
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Stratigrafie tentamen : 25/01/2021CHAPTER 19: : CONCEPTS AND LITHOSTRATIGRAPHY
The abbreviations used for dates are ‘Ma’ for millions of years before present and ‘ka’ for thousands
of years before present. The time thousands of millions of years before present is abbreviated to ‘Ga’
(Gigayears). The North American Stratigraphic Code suggests that to express an interval of time of
millions of years in the past abbreviations such as ‘my’, ‘m.y.’ or ‘m.yr’ could be used. This convention
has the advantage of distinguishing ‘dates’ from ‘intervals of time’ but it is not universally applied.
It has commonly been the practice to distinguish between geochronology, which is concerned with
geological time units and chronostratigraphy, which refers to material stratigraphic units.
Eons = the longest periods of time within the history of the Earth, which are now commonly divided
into three eons: the Archaean Eon up to 2.5 Ga, the Proterozoic Eon from 2.5 Ga to 542 Ma (together
these constitute the Precambrian), and the Phanerozoic Eon from 542 Ma up to the present.
Eras = the three time divisions of the Phanerozoic: the Palaeozoic Era up to 251 Ma, the Mesozoic
Era from then until 65.5 Ma and finally the Cenozoic Era up to the present. Precambrian eras have
also been defined, for example dividing the Proterozoic into the Palaeoproterozoic, the
Mesoproterozoic and the Neoproterozoic.
Periods/Systems = The basic unit of geological time is the period and these are the most commonly
used terms when referring to Earth history. The Mesozoic Era, for example, is divided into three
periods, the Triassic Period, the Jurassic Period and the Cretaceous Period. The term system is used
for the rocks deposited in this time, e.g. the Jurassic System.
Epochs/Series = Epochs are the major divisions of periods. Some have names , while others are simply
Early, (Mid-) and Late divisions of the period. The chronostratigraphic equivalent is the series, but it is
important to note that the terms Lower, Middle and Upper are used instead of Early, Middle and Late.
Ages/Stages = The smallest commonly used divisions of geological time are ages, and the
chronostratigraphic equivalent is the stage. They are typically a few million years in duration. For
example, the Oligocene Epoch is divided into the Rupelian and Chattian Ages (the Rupelian and
Chattian Stages of the Oligocene Series of rocks). Chrons are short periods of time that are sometimes
determined from palaeomagnetic information, but these units do not have widespread usage outside
of magnetostratigraphy.
The main systems had been established and partly divided into series and stages by the beginning of
the 20th century by using stratigraphic relations and biostratigraphic methods. Radiometric dating
has provided a time scale for the chronostratigraphic division of rocks. The published geological time
scales have been constructed by integrating information from biostratigraphy, magnetostratigraphy
and data from radiometric dating to determine the chronostratigraphy of rock units throughout the
Phanerozoic. Older versions of these stratigraphic charts show different ages for boundaries, and no
doubt future charts will also contain modifications to these dates. A unit of sedimentary rocks is
therefore never referred to as being, say, 160 Ma old unless there has been a direct radiometric
measurement made of that unit: instead it might be referred to as Oxfordian on the basis of its fossils,
and this will not change, whatever happens in future versions of these charts.
From the foregoing it should be clear that the Cambrian, for example, is not defined as the interval of
time between 542 Ma and 488.3 Ma, but those numbers are the ages that are currently thought to be
the times when the Cambrian Period started and ended. It is therefore necessary to have some other
,means of defining all of the divisions of the geological record, and the internationally accepted
approach is to use the ‘Global Standard Section and Point’ (GSSP) scheme, otherwise known as the
process of establishing ‘golden spikes’.
Earlier, if strata from two different places
contain the same fossils, they are
considered to be from the same period –
this is the basis of biostratigraphic
correlation. The GSSP scheme takes the
‘type area’ concept further by defining the
base of a period or epoch as a particular
point, in a particular succession of strata, in
a particular place. A ‘golden spike’ is
metaphorically hammered into the rocks at
that point, and all beds above it are defined
as belonging to one epoch/period and all
below it to another. All other beds of
similar age around the world are then
correlated with the strata that contain the
‘golden spike’, using any of the correlation
techniques that are described in this and
the following chapters (lithostratigraphy,
biostratigraphy, magnetostratigraphy, and
so on). The base of the Devonian, for
example, is defined by a golden spike in a
succession of marine strata in the Czech
Republic at a point where a certain
graptolite is found in higher beds, but not
in the lower beds. The procedure of
defining GSSPs cannot easily be applied in
older rocks because it is essentially a
biostratigraphic approach. The scarcity of
stratigraphically useful fossils in
Precambrian strata means that only one
pre-Phanerozoic system has been defined
so far: this is the Ediacaran Period/System,
the youngest part of the Neoproterozoic
Era. Other Precambrian boundaries have
been ascribed with ages, a Global Standard
Stratigraphic Age, or ‘GSSA’.
There are a number of different
approaches that can be used, each based
on different aspects of the rocks, and each
of which is of some value individually, but
are most profitably used in combinations.
- Lithostratigraphic units: s lithological characteristics and its stratigraphic position relative to
other bodies of rock
- Biostratigraphic unit: defined and characterised by its fossil content
, - Chronostratigraphic unit: where the age of the rock can be directly or indirectly determined.
Chronostratigraphic units have upper and lower boundaries that are each isochronous surfaces,
that is, a surface that formed at one time.
- Magnetostratigraphic unit: exhibits magnetic properties that are different to adjacent bodies of
rock in the stratigraphic succession
- Allostratigraphic units: bodies of rock can be defined by their position relative to unconformities
or other correlatable surfaces → sequence stratigraphy
Lithostratigraphy: In lithostratigraphy rock units are considered in terms of the lithological
characteristics of the strata and their relative stratigraphic positions.
Stratigraphic relationships
- Superposition = unless deformed the top layer is always younger than what lies under it
- Unconformities = a break in sedimentation and where there is erosion of the underlying strata
- Disconformity = marks a break in sedimentation and some erosion, but without any deformation
of the underlying strata.
- Cross-cutting relationships = Any unit that has boundaries that cut across other strata must be
younger than the rocks it cuts.
- Included fragments = The fragments
in a clastic rock must be made up of
a rock that is older than the strata in
which they are found
- Way-up indicators in sedimentary
rocks = The direction of younging
can be determined by small-scale
features that indicate the way-up of
the beds or by using other
stratigraphic techniques to
determine the order of formation.
There is a hierarchical framework of terms used for lithostratigraphic units, and from largest to
smallest these are: ‘Supergroup’, ‘Group’, ‘Formation’, ‘Member’ and ‘Bed’. The basic unit of
lithostratigraphic division of rocks is the formation, which is a body of material that can be identified
by its lithological characteristics and by its stratigraphic position. A formation may be, and often is,
a diachronous unit, that is, a deposit with the same lithological properties that was formed at
different times in different places. A formation may be divided into smaller units in order to provide
more detail of the distribution of lithologies. The term member is used for rock units that have
limited lateral extent and are consistently related to a particular formation. Individual beds or sets
of beds may be named if they are very distinctive by virtue of their lithology or fossil content. Where
two or more formations are found associated with each other and share certain characteristics they
are considered to form a group.
Description of lithostratigraphic units
- Lithology and characteristics Although a formation will normally consist mainly of one lithology,
combinations of two or more lithologies will often constitute a formation as interbedded or
interfingering units. Sedimentary structures (ripple cross-laminations, normal grading, etc.),
petrography (often determined from thin-section analysis) and fossil content (both body and
trace fossils) should also be noted
- Definition of top and base These are the criteria that are used to distinguish beds of this unit
from those of underlying and overlying units. This is most commonly a change in lithology from,
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