Transcript Field Sedimentology, Facies and Environments

Field Sedimentology, Facies
and Environments
UNIT - 5
FIELD SEDIMENTOLOGY
 A large part of modern sedimentology is the
interpretation of sediments and sedimentary rocks in
terms of processes of transport and deposition and
how they are distributed in space and time in
sedimentary environments.
 To carry out this sort of sedimentological analysis
some data are required and this is mainly collected
from exposures of rocks.
Field equipment
 Only a few tools are needed for field studies in
sedimentology and stratigraphy
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A notebook to record data is essential
A hand lens (10 magnification)
A compass – clinometer
A geological hammer.
For the collection of samples, small, strong, plastic bags and a
marker pen are necessary.
A small bottle containing dilute hydrochloric acid is very useful to
test for the presence of calcium carbonate in the field
Camera
A GPS (Global Positioning Satellite) receiver
Field studies
 The organization of a field program of sedimentary studies will
depend on the objectives of the project.
 When an area with sedimentary rock units is mapped the character
of the beds exposed in different places is described in terms given
below.
 To describe the lithology the Dunham classification can be used for
limestones, and the Pettijohn classification for sandstones .
 Other features to be noted are bed thicknesses, sedimentary
structures, fossils (both body and trace fossils)
 Rock color and any other characteristics such as weathering, degree
of consolidation and so on.
Graphic Sedimentary Log
 A sedimentary log
is
a
graphical
method
for
representing
a
series of beds of
sediments
or
sedimentary rocks
Examples of patterns and symbols used on graphic
sedimentary logs
PALAEOCURRENTS
 A palaeocurrent indicator is evidence for the
direction of flow at the time the sediment was
deposited, and may also be referred to as the
palaeoflow.
 Palaeoflow data are used in conjunction with facies
analysis and provenance studies
palaeogeographic reconstructions.
to
make
Palaeocurrent indicators
 Two
groups of palae-ocurrent indicators
sedimentary structures can be distinguished
in
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Unidirectional indicators are features that give the
direction of flow.
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Flow axis indicators are structures that provide
information about the axis of the current but do not
differentiate between upstream and downstream directions.
Unidirectional indicators
 Cross-lamination
is produced by
ripples migrating in the direction of the
flow of the current. The dip direction of
the cross-laminae is measured.
 Cross-bedding
is formed by the
migration of aeolian and subaqueous
dunes and the direction of dip of the
lee slope is approximately the direction
of flow.
 Clast
imbrication is formed when
discoid gravel clasts become oriented
in strong flows into a stable position
with one of the two longer axes dipping
upstream when viewed side-on.
Flow axis indicators
 Primary current lineations on bedding planes are measured
by determining the orientation of the lines of grains.
 Groove casts are elongate scours caused by the indentation of
a particle carried within a flow that give the flow axis.
 Elongate clast orientation may provide information if needle-
like minerals, elongate fossils such as belemnites, or pieces of
wood show a parallel alignment in the flow.
 Channel and scour margins can be used as indicators because
the cut bank of a channel lies parallel to the direction of flow.
Measuring Palaeocurrents
 The most commonly used features for determining
palaeoflow are cross stratification, at various scales.
 As many as possible data points should be collected to
carry out palaeocurrent analysis.
 The statistical validity of the mean will be improved with
more data
 If only a general trend of flow is required for the project
in hand, then fewer will be required.
Presentation and analysis of directional data
 Directional data are commonly collected and used in geology.
 Palaeocurrents are most frequently encountered in sedimentology.
 Palaeocurrent data are normally plotted on a rose diagram
COLLECTION OF ROCK SAMPLES
 Field studies only provide a portion of the information that
may be gleaned from sedimentary rocks, so it is routine to
collect samples for further analysis.
 Material may be required for palaeontological studies, to
determine the biostratigraphic age of the strata, or for
mineralogical and geochemical analyses.
 Thin-sections are used to investigate the texture and
composition of the rock in detail, or the sample may be
disaggregated to assess the heavy mineral content or
dissolved to undertake chemical analyses.
 The size and condition of the sample collected will depend
on the intended use of the material. For most purposes
pieces that are about 50mm across will be adequate.
COLLECTION OF ROCK SAMPLES
 It is good practice to collect samples that are ‘fresh’, i.e. with
the weathered surface removed.
 The orientation of the sample with respect to the bedding
should usually be recorded by marking an arrow on the
sample that is perpendicular to the bedding planes and points
in the direction of younging.
 Every sample should be given a unique identification number
at the time that it is collected in the field, and its location
recorded in the field notebook.
 Samples should always be placed individually in appropriate
bags – usually strong, sealable plastic bags.
Provenance studies
 Information about the source of sediment, or provenance of the
material, may be obtained from an examination of the clast types
present .
 Provenance studies are generally relatively easy to carry out in
coarser clastic sediments because a pebble or cobble may be
readily recognized as having been eroded from a particular
bedrock lithology.
 Many
rock types may have characteristic textures and
compositions that allow them to be identified with confidence.
 It is more difficult to determine the provenance where all the
clasts are sand-sized because many of the grains may be
individual minerals that could have come from a variety of
sources.
Provenance studies
 Quartz is often of little value in determining provenance. It
has been found that certain heavy minerals are very good
indicators of the origin of the sand.
 Provenance studies in sandstones are therefore often carried
out by separating the heavy minerals from the bulk of the
grains and identifying them individually.
 Clay mineral analysis is also sometimes used in provenance
studies because certain clay minerals are characteristically
formed by the weathering of particular bedrock types.
 Analysis of mud and mudrocks can also be used to determine
the average chemical composition of large continental areas.
Interpreting Past depositional Environment
 Sediments accumulate in a wide range of settings that
can be defined in terms of their geomorphology, such as
rivers, lakes, coasts, shallow seas, and so on.
 The physical, chemical and biological processes that
shape and characterize those environments are well
known.
 A
fundamental part of sedimentology is the
interpretation of sedimentary rocks in terms of the
transport and depositional processes and then
determining the environment in which they were
deposited.
The concept of Sedimentary Facies
 The term ‘facies’ is widely used in geology, particularly in the study
of sedimentology in which sedimentary facies refers to the sum of
the characteristics of a sedimentary unit.
 Every depositional environment puts its own distinctive imprint on
the sediment, making a particular facies. Thus, a facies is a distinct
kind of rock for that area or environment
 These
characteristics include the dimensions, sedimentary
structures, grain sizes and types, colour and biogenic content of the
sedimentary rock.
 An example would be ‘cross-bedded medium sandstone’: this would
be a rock consisting mainly of sand grains of medium grade,
exhibiting cross-bedding as the primary sedimentary structure.
The concept of Sedimentary Facies
 A
=
Sandstone
facies
(beach
environment)
B = Shale facies (offshore marine environment)
C = Limestone facies (far from sources of terrigenous
input)
 Each depositional environment grades laterally into
other environments. We call this facies change when
dealing with the rock record.
The concept of Sedimentary Facies
 Transgressions and Regressions
 Transgression = sea level rise
 Regression - sea level drop
 Fluctuations in sea level are caused by things such as:
 Changes in the size of the polar ice caps, due to climatic changes
 Melting of ice caps leads to sea level rise (transgression) - it has been calculated that complete
melting of the Antarctic Ice Sheet would cause a sea level rise of 60 - 70 meters (200 feet).
 Growth of ice caps leads to drop in sea level (regression) - calculations show that sea level was as
much as 100 meters (300 feet) lower than at present at the height of the last Ice Age glaciation.
Much of the Continental Shelf area would have been exposed and dry.
 Rate of sea floor spreading - during times of rapid sea floor spreading and
submarine volcanism, the ocean ridge system is enlarged by the addition of lava,
displacing water onto the edges of the continents (transgression).
 Localized subsidence or uplift of the land - In the 8000 - 10,000 years since
the melting of the last glacial ice sheet over North America, parts of Canada have
risen due to isostatic uplift by up to 300 meters.
The concept of Sedimentary Facies
 The principle that facies that occur in conformable
vertical succession of strata also occur in laterally
adjacent environments is known as Walther's law
of correlation of facies.
Transgressive sequence
Deeper water facies overlie shallow
water facies.
A "deepening upward" sequence.
The concept of Sedimentary Facies
Regressive sequence
Shallow water facies overlie deeper
water facies.
A "shallowing upward"
sequence.
The concept of Sedimentary Facies
 If the facies description is confined to the physical
and chemical characteristics of a rock this is referred
to as the lithofacies
 In cases where the observations concentrate on the
fauna and flora present, this is termed as biofacies.
 A study that focuses on the trace fossils in the rock
would be a description of the ichnofacies
Facies Analysis
 The facies concept is not just a convenient means of describing rocks and grouping
sedimentary rocks seen in the field, it also forms the basis for facies analysis.
 By interpreting the sediment in terms of the physical, chemical and ecological
conditions at the time of deposition it becomes possible to reconstruct
palaeoenvironments, i.e. environments of the past.
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So, from the presence of symmetrical ripple structures in a fine sandstone it can be
deduced that the bed was formed under shallow water with wind over the surface of
the water creating waves that stirred the sand to form symmetrical wave ripples.
 The ‘shallow water’ interpretation is made because wave ripples do not form in deep
water but the presence of ripples alone does not indicate whether the water was in a
lake, lagoon or shallow-marine shelf environment.
 The facies should therefore be referred to as ‘symmetrically rippled sandstone’ or
perhaps ‘wave rippled sandstone’, but not ‘lacustrine sandstone’ because further
information is required before that interpretation can be made.
Principal sedimentary environment
Facies Analysis
Reconstructing paleoenvironment in space and
time
 One of the objectives of sedimentological studies is to try to
create a reconstruction of what an area would have looked like
at the time of deposition of a particular stratigraphic unit.
 The first prerequisite of any palaeoenvironmental analysis is a
stratigraphic framework, that is, a means of determining
which strata are of approximately the same age in different
areas, which are older and which are younger.
 Once it is established that the rocks that we know are of
approximately the same age across an area, we can
reconstruct the paleo-environment using facies analysis,
paleo-current studies and provenance studies.
 Over thousands and millions of years of geological time,
climate changes, plates move, mountains rise and the global
sea level changes.
Reconstructing paleoenvironment in space and
time
 The record of all these events is contained within sedimentary rocks,
because the changes will affect environments that will in turn determine
the character of the sedimentary rocks deposited.
 Palaeoenvironmental reconstructions therefore provide a series of pictures
of the Earth’s surface that we can then interpret in terms of large- and
small-scale events.
 When palaeoenvironmental analysis is combined with stratigraphy in this
way, the field of study is known as basin analysis and is concerned with the
behaviour of the Earth’s crust and its interaction with the atmosphere and
hydrosphere.
 As stated above, one of the objectives of facies analysis is to determine the
environment of deposition of successions of rocks in the sedimentary
record. A general assumption is made that the range of sedimentary
environments which exist today have existed in the past.