Introduction to Sedimentology
and Stratigraphy
UNIT -1
What are Sediments
 Sediments
are loose Earth materials (unconsolidated
materials) such as sand which are transported by the action
of water, wind, glacial ice and gravity.
 These materials accumulate on the land surface, (such as in
river and lake beds), and / or on the ocean floor and form
sedimentary rocks.
How are sediments formed?
 Sediments form by weathering
of rock.
 Then they are eroded from the
site of weathering and are
transported by wind, water,
ice and mass wasting.
 Finally
sediment settles out
and
accumulates
after
transport: This process is
known as deposition.
 Sedimentation
is a general
term for the processes of
weathering,
erosion,
transport, and deposition
Examples of Erosion
Classification of Sediments
 Sediments are commonly subdivided into three
major groups. They are
 Mechanical,
 Chemical and
 Organic
Formed by the breakdown
of pre-existing rocks and
minerals by the process of
weathering and erosion
Detrital
Mineral
Grains
Common: Quartz,
Felspar, Mica and
Heavy Minerals
Lithic
Fragements
Polymineral grains or
rock fragments
Biogenic
Particles
Shell Fragments,
woods, seeds and other
parts of land plants
Authigenic
Minerals
Minerals which grow
crystals in deposition
environment, eg
.Glauconite
Composition
Materials which form
within the environment of
transport and deposition
Mechanical Sediments
 Mechanical,
or
clastic,
sediments are derived from
the erosion of earlier
formed rocks on the earth's
surface or in the oceans.
 These are then carried by
streams, winds, or glaciers
to the site where they are
deposited.
 Streams deposit sediment in
floodplains or carry these
particles to the ocean,
where
they
may
be
deposited as a delta.
Mechanical Sediments
 Ocean
sediments,
especially in the form of
turbidites, are usually
deposited at the foot of
continental slopes.
 Glaciers carry sediment
frozen within the mass of
the ice and are capable
of carrying even huge
boulders (erratics).
Chemical sediments
 Chemical sediments are formed by chemical reactions in
seawater.
 This result in the precipitation of minute mineral
crystals.
 These crystals settle to the floor of the sea and ultimately
form a more or less chemically pure layer of sediment.
 Evaporation in shallow basins results in a sequence of
evaporite sediments, which include gypsum and rock
salt.
Organic sediments
 Organic sediments are formed as a result of plant or
animal actions; for example, peat and coal form by the
incomplete decay of vegetation and its later compaction.
 Deep-ocean sediment known as pelagic ooze consists of
the remains of microscopic organisms (mostly
foraminifera and diatoms) from the overlying waters as
well as minor amounts of windblown volcanic and
continental dust.
 Limestones are commonly formed by the aggregation of
calcite shells of animals.
The process involved in the formation of
Sedimentary Rocks
 WEATHERING (Weathering is the process that changes bedrock
to soil)
 EROSION (Erosion is the process that removes loose materials
from the place of weathering)
 TRANSPORTATION
 DEPOSITION
 DIAGENESIS
 COMPACTION
 CEMENTATION
Diagenesis
 Lithification
refers
to
processes that convert loose
sediment to hard rock.
 Diagenesis is any chemical,
physical, or biological change
undergone by a sediment
after its initial deposition
and during and after its
lithification.
 Two of the most important
processes involved during
diagenesis are compaction
and cementation
Imagine some rocky outcrops at the side of a valley high in the mountains.
Cracks in the rock fill with water when it rains.
When it is cold, the water changes to ice. As it forms, the ice expands and
makes new cracks in the rock.
After this happens a few times, pieces of rock are loosened and fall from
the outcrop. They slide downhill and eventually land up in the river
flowing along the valley floor.
Once in the river, the pieces of rock tend to slide, roll and bounce
downstream as they are forced along by the flow of water
As they travel downstream, they get broken up into smaller sand grains and
mud particles.
When the river flows into the sea, all the rock fragments fall to the sea bed as
the flow of water slows and stops.
Over long periods of time, layers of pebbles, sand and mud build up on the
sea bed at river mouths.
Water circulates in the tiny spaces between the rock particles in these layers and
slowly deposits chemicals that cement the particles together. Sediments such as
sand and mud eventually become sedimentary rocks called sandstone and
mudstone.
Over millions of years, layer upon layer of different sedimentary rocks pile up
on top of each other to create rock formations hundreds of metres thick.
These formations often lie undisturbed for hundreds of millions of years.
Classification of Sedimentary Rocks
 Clastic sedimentary rocks are composed of fragments
of weathered rocks, called clasts, that have been
transported, deposited, and cemented together. Clastic
rocks make up more than 85 percent of all sedimentary
rocks.
 Chemical sedimentary rocks form from precipitation
within the environment of deposition, so their composition
depends on the chemical composition of the aqueous
solution from which they are deposited (e.g. lake, ocean,
groundwater) and chemical conditions (e.g. carbon dioxide,
oxygen, pH)
Classification of Sedimentary Rocks
 Biogenic
sedimentary rocks result from the
accumulation of skeletal remains of plants or animals. The
most important types are
 (1) made up of calcium carbonate (calcareous) e.g. limestone,
and shelly sandstone;
 (2) made up of silica (siliceous) e.g. chert; and
 (3) made up of plant remains (carbonaceous) e.g. coal.
Classification of Sedimentary Rocks
What is Sedimentary Geology
 Sedimentology is the science that deals with the description,
classification, and origin of sedimentary rock.
 It is the study of sediments and sedimentation.
 Sedimentology = the study of the processes of formation, transport
and deposition of material which accumulates as sediment in
continental and marine environments and eventually forms
sedimentary rocks
 Stratigraphy = the study of rocks to determine the order and timing
of events in Earth history
 Sedimentary geology » Sedimentology + Stratigraphy
What is Sedimentary Geology
 Stratigraphy focuses on the larger scale
strata and Earth history (when and
where were sediments/sedimentary
rocks formed?)


Larger temporal and spatial scales
The stratigraphic record is nearly always very
incomplete due to a limited preservation
potential, that decreases with increasing time
scales
 Sedimentology
focuses primarily on
facies and depositional environments
(how were sediments/sedimentary rocks
formed?)

Smaller temporal and spatial scales
The aim of Sedimentology
 Description of sediments
 Origin of sediments
 Understanding the depositional condition resulting in
the formation of a given sedimentary rock
 Evolution of the sedimentary basin and the sequence
 Information and understanding of the past depositional
and environmental conditions
Analysis of Sediments
 The analysis of sediments requires standard methods if
similar deposits from different locations are to be
compared and contrasted. The standard methodology
includes:
 Facies Analysis
 Particles Size Analysis
 Particle Shape Analysis
 Lithological Analysis
Methods in Sedimentology
 The methods employed by sedimentologists to collect data and evidence about
the nature and depositional conditions of sedimentary rocks include:
 Measuring and describing the outcrop and distribution of the rock
unit
 Describing the rock formation; thickness; lithology; outcrop; distribution
 Contact relationships with other formations.
 Mapping the distribution of the rock unit; or units
 Stratigraphy
 Describes the progression of rock units within a basin
 Describing the lithology of the rock
 Petrology and petrography: measuring the textures; grain size; grain shape;
sorting and composition of the sediment
 Analyzing the geochemistry of the rocks
 Isotope geochemistry, including uses of radiometric dating, to determine the age
of rocks
Methods in Sedimentology
 A basic understanding of sedimentary processes is essential for
many geological and geotechnical engineering applications.
 Stratigraphers
and Sedimentologists must apply their
knowledge of how sedimentary successions are deposited and
preserved to interpret the geologic history recorded in the
rocks.
 Sedimentology is primarily with the formation of sedimentary
rocks.
 As soon as these beds of rock are looked at in terms of their
temporal and special relationships the study becomes
stratigraphy.