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
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 to make
palaeogeographic reconstructions.
Palaeocurrent indicators
Two groups of palae-ocurrent indicators in
sedimentary structures can be distinguished
Unidirectional indicators are features that give the
direction of flow.
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.
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.
