Transcript Stratigraphic concepts and lithostratigraphy

Stratigraphic concepts and
lithostratigraphy
UNIT - 6
GEOLOGICAL TIME
 The passage of time since the formation of the Earth is
divided into geochronological units.
 These are divisions of time that may be referred to in
terms of years or by name.
 The abbreviations used for dates are ‘Ma’ for millions
of years before present and ‘ka’ for thousands of
years before present.
 The
time billlion of years before present is
abbreviated to ‘Ga’ (Gigayears).
Geological time units
 The geoogical time units have been divided into 5
groups. They include:

Eons: These are 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 542Ma, and
the Phanerozoic Eon from 542Ma up to the present.
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Geological time units
 Eras
are the
Phanerozoic:
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three time divisions
of the
The Palaeozoic Era up to 251 Ma,
The Mesozoic Era from then until 65.5Ma and
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.
Geological time units
 Periods are the basic unit of geological time and are the
most commonly used terms when referring to Earth history.
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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.
Geological time units

Epochs are the major divisions
of periods
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Ages/Stages: The smallest
commonly used divisions of
geological time are ages. They
are typically a few million years
in duration.

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 GEOLOGICAL COLUMN
AND TIME SCALE
The time is given in millions of years.
Eg: 505 stands for 505 million years.
STRATIGRAPHIC UNITS
 The International Stratigraphic Chart and the Geologic Time Scale
that it shows provides an overall framework within which we can
place all the rocks on Earth and the events that have taken place
since the planet formed.
 It is, however, of only limited relevance in determining the
stratigraphic relationships between rocks in the field.
 Strata do not have labels on them which immediately tell us that
they were deposited in a particular epoch or period, but they do
contain information that allows us to establish an order of
formation of units.
 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.
Sub-division of rock record
 Lithostratigraphy
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Study of the physical relationship among rock units;
No time connotation other than superposition
 Physical properties and stratigraphic position relative to other
lithostratigraphic units
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 Chronostratigraphy
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Integrated approach to establishing the time relationships
among geologic units
Sub-division of rock record
Integrated Approach to Establishing
the Time Relationships Among Geologic Units
 Chronostratigraphy:
 Biostratigraphy
 Study of the fossil record with emphasis on faunal succession to establish relative
time relationships
 The correlation web
 Magnetostratigraphy
 Study of the magnetic properties of rock units for the purpose of correlation
using magnetic polarity reversals
 Allostratigraphy
 Study of rock units defined by unconformities and other features generated by
base level change
 Geochronology
 Various techniques, especially isotope geochemistry, to establish the absolute age
of rock units
Methods of Stratigraphic sub-divison
Stratigraphic Correlation
LITHOSTRATIGRAPHY
 In lithostratigraphy rock units are considered in terms of
the lithological characteristics of the strata and
their relative stratigraphic positions.
 The relative stratigraphic positions of rock units can be
determined by considering geometric and physical
relationships that indicate which beds are older and
which ones are younger.
 The units can be classified into a hierarchical system of
members, formations and groups that provide a
basis for categorising and describing rocks in
lithostratigraphic terms.
Principles (Laws) of Stratigraphy
Principle of…
 Original
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Horizontality
Superposition
Lateral Continuity
Cross Cutting
Relationships
Inclusions
Faunal Succession
Stratigraphic relationships
The principle of original horizontality
 It is based on the fact that sediment usually accumulates in horizontal
layers.
 If sedimentary rocks lie at an angle, we can infer that tectonic forces
tilted them after they formed
Stratigraphic relationships
The principle of superposition
 It states that sedimentary rocks become younger from bottom
to top (as long as tectonic forces have not turned them upside
down).
 This is because younger layers of sediment always accumulate
on top of older layers. In the figure below the sedimentary
layers become progressively younger in the order E, D, C, B,
and A.
Principle of Lateral Continuity
 It states that if layers are deposited horizontally over the sea floor, then
they would be expected to be laterally continuous over some distance.
 Thus, if the strata are later uplifted and then cut by a canyon, we know
that the same strata would be expected to occur on both sides of the
canyon.
Stratigraphic relationships
The principle of crosscutting
relationships
 It states that a rock must first
exist before anything can happen
to it.
 The
figure
below
shows
sedimentary rocks intruded by
three granite dikes.
 Dike B cuts dike C, and dike A
cuts dike B, so dike C is older
than B, and dike A is the
youngest. The sedimentary rocks
must be older than all of the
dikes.
Stratigraphic relationships
Principle of Inclusions
•
The Law of Inclusions states
that if a rock body (Rock B)
contained fragments of another
rock body (Rock A), it must be
younger than the fragments of
rock it contained.
•
The intruding rock (Rock A)
must have been there first to
provide the fragments.
Stratigraphic relationships
Principle of Faunal Succession
• Law of Faunal Succession staes
that fossils occur in a definite,
invariable sequence in the
geologic record.
• As you can see in the image
below the fossil remains of living
things are present in the rock
layers at definite intervals, and
exist within a discrete period of
time.
• In this instance, using the Law of
Superposition, would the age
Rock Unit A be older or younger
than the age of Rock Unit B?
Stratigraphic relationships
UNCONFORMITIES
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Layers of sedimentary rocks are conformable if they were
deposited without interruption.
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An unconformity represents an interruption in deposition,
usually of long duration.
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During the interval when no sediment was deposited, some
rock layers may have been eroded
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Thus, an unconformity represents a long time interval for
which no geologic record exists in that place. The lost record
may involve hundreds of millions of years
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There are several types of unconformities
Stratigraphic relationships
Stratigraphic relationships
Nonconformity
 In this case sedimentary rocks lie on igneous or metamorphic rocks
Stratigraphic relationships
Angular unconformity
 In this case tectonic activity tilted older sedimentary rock layers
before younger sediment accumulated
Stratigraphic relationships
Disconformity
 In this case the sedimentary layers above and below the unconformity
are parallel
 Geologists identify disconformities by determining the ages of rocks
using methods based on fossils and absolute dating
Way-up indicators in sedimentary rocks
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The folding and faulting of strata during mountain building can rotate whole successions of
beds (formed as horizontal or nearly horizontal layers) through any angle, resulting in beds
that may be vertical or completely overturned.
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In any analysis of deformed strata, it is essential to know the direction of younging,
that is, the direction through the layers towards younger rocks.
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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.
Lithostratigraphic units
 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 it
lithological characteristics and by its stratigraphic position.
 A formation should have some degree of lithological homogeneity
and its defining characteristics may include mineralogical
composition, texture, primary sedimentary structures and
fossil content in addition to the lithological composition.
Description of lithostratigraphic units
 The
formation
is
the
fundamental
lithostratigraphic unit and it is usual to follow a
certain procedure in geological literature when
describing a formation
 Members and groups are usually described in a
similar way.
Description of lithostratigraphic units
Lithology and characteristics
 The field characteristics of the rock, for example, an oolitic
grainstone, interbedded coarse siltstone and claystone, a basaltic
lithic tuff, and so on form the first part of the description.
 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.
Description of lithostratigraphic units
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
 Where the boundary is not a sharp change from one
formation to another, but is gradational, an arbitrary
boundary must be placed within the transition.
Description of lithostratigraphic units
Type section
 A
type section is the location where the lithological
characteristics are clear and, if possible, where the lower and
upper boundaries of the formation can be seen.
 The type section will normally be presented as a graphic
sedimentary log and this will form the stratotype.
 It must be precisely located (grid reference and/or GPS
location) to make it possible for any other geologist to visit the
type section and see the boundaries and the lithological
characteristics described.
Description of lithostratigraphic units
Thickness and extent
 The thickness is measured in the type section, but variations in the
thickness seen at other localities are also noted.
 The limits of the geographical area over which the unit is recognized
should also be determined.
 The variability of rock types within an area will be the main
constraint on the number and thickness of lithostratigraphic units
that can be described and defined.
 Quality and quantity of exposure also play a role, as finer
subdivision is possible in areas of good exposure.