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Sediments are loose Earth materials such as sand that accumulate

By Cathy Austin,2014-04-08 21:21
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Sediments are loose Earth materials such as sand that accumulate

     SEDIMENTOLOGY (2011-2012)

    Lecture First

     Introduction to Sedimentology

Introduction

     Using such information's about sediments, may be possible to deduce the origin(s) of the sediment(s). Such reconstruction can be achieved by making inferences about the erosion, transportation and depositional history, by using the sediment characteristics (such as particle size, shape, roundness, etc.). Inferences (ت,,د?سلا) can also be made about the processes operating and, ultimately, deduction the depositional environment of the sediment.

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 are accumulate on the land surface, (such as in river and lake beds), and / or on the ocean floor which is eventually deposited as a layer of solid particles on the bed or bottom of a body of water or other liquid and latter forming sedimentary rocks. Sediments form by weathering of rock. Sediment settles out and accumulates after transport; this process is known as deposition. Sedimentation is a general term for the processes of erosion, transport, and deposition.

Classification of Sediments

    Sediments are commonly subdivided into three major groups: mechanical, chemical, and organic, (Fig-1).

    Mechanical or clastic (rock fragments) sediments: - These type of sediments are derived from the erosion of earlier formed rocks on the earth's surface or in the oceans. Then carried by streams, winds, or glaciers to the site where they are deposited. 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 (erratic). Chemical sediments (mineral deposits): Chemical sediment must crystallize from the solution, that is, it must be precipitated. So it is settling to the floor of the sea

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    and ultimately forms a more or less chemically pure layer of sediment. A common way for precipitation to occur is by evaporation. As water evaporates from the

    surface, it will become more concentrated until they begin to precipitate out of the water and accumulate on the bottom. Such as gypsum and rock salt.

    Organic sediments (organic matter) 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 largely of the remains of microscope 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. Organic matter is derived from the decaying remains of plants and animals. Minerals may also be forced to precipitate by the biological activity of certain

    organisms. For example, when algae remove carbon dioxide from water, this

    decreases the acidity of the water, promoting the precipitation of calcite. Some marine organisms use this reaction, or similar chemical reactions, to promote

    mineral precipitation and use the minerals to form their skeletons. This type is known as Biochemical sediments.

Fig -1 Schematic diagram illustrating the type of sediments.

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What is sedimentology?

    Sedimentology is the science that deals with the description (it is essentially a

    descriptive science), classification, and origin of sedimentary rock, and it is the study of sediments and sedimentation.

     Experience has shown that the skills of sedimentology are best learned and developed in the field.

    Definitions

     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.

     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 aims of sedimentology:-

    Sedimentology seeks objectively to describing sediments, and from careful observational analysis seeks to explain the origin of the sediment.In addition to this derive information on the depositional conditions which acted to deposit the rock unit and the relation of the individual rock units in a basin into a coherent understanding of the evolution of the sedimentary sequences and basins, and, thus, the earth’s geological history as a whole.

    Sedimentological conditions are recorded within the sediments as they are laid down; the form of the sediments at present reflects the events of the past and all events which affect the sediments, from the source of the sedimentary material to processes which affected upon them after diagenesis .

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The objective analysis

    The objective analysis of sediments requires standard methods if similar deposits from different locations are to be compared and contrasted.

    Some widely accepted descriptive techniques are as follow:

    a. Facies analysis

    This is a combination of a number of techniques of analysis that can be undertaken to reveal a depositional history.

    A facies is the sum total of all primary characteristics of a sedimentary rock from which the environment of its deposition may be induced an individual facies can be quite limited and closely related to the neighbouring facies. The most significant boundary is an unconformity.

    Different facies can be described according to traditional nomenclature. If sediment consists of particles of varying size (poorly sorted), then the sediment can be broadly described as either a breccia or a conglomerate.

    Therefore in order to make precise determinations of a sediment type other observations may need to be made and are often standard components of facies analysis (e.g. particle size). The following is a brief outline of procedures:

b. Particle size analysis.

    Particle size is generally classified according to the Wentworth scale. Larger particles are commonly (but not always) measured in the field. Sample size varies according to the nature of the study.

    c. Particle shape analysis.

    Particle shape, often expressed in terms of degree of roundness and it can be determined by a number of different techniques which have encountered in practical. d. Fabric analysis.

    Fabric analysis determines the preferred orientation of clasts within sediment. The orientation and dip of clasts is measured in the field using compass-clinometers. The simplest way of presenting and analyzing the data is diagrammatically (e.g. rose diagram) and a number of simple plotting programmes are available to assist producing the diagrams.

e. Clast lithological analysis.

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    Lithological analysis can be very important in determining the provenance of a sediment and sometimes process of deposition.

    Altogether, the description, sampling and analysis of sediments allow an interpretation of depositional environments.

    Sedimentological analysis is now applied in a wide range of environments in order to further our understanding of landscape processes and development. The application of sedimentological methods to geomorphological studies has greatly altered our understanding of earth surface processes and our understanding of landscape evolution over the last 20 years or so.

Methodology of Sedimentology:

    The methods employed by Sedimentologists to collect (gather) data and evidence on the nature and depositional conditions of sedimentary rocks which include: 1- 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.

    2-Sequence stratigraphy:

     -Describes the progression of rock units within a basin.

    3-Describing the lithology of the rock:

     - Petrology and petrography: measuring the textures; grain size; grain shape

     (sphericity, rounding….etc); sorting and composition of the sediment.

    4- Analyzing the geochemistry of the rocks;

     -Isotope geochemistry, including uses of radiometric dating, to determine

     the age of the rock, and its relation to source regions.

     The applications of sedimentology extend too many other systems, including

    physical geology (especially geomorphology), structural geology, plate tectonic, and stratigraphy and soil science. A basic understanding of sedimentary processes is essential for many geological and geotechnical engineering applications. Stratigraphy and sedimentology are two of the main sub-disciplines of geology, often considered separately in the past but now increasingly combined in teaching,

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    academic research and economic application. It is often difficult to draw a sharp distinction between them. They can be considered together as a continuum of processes and products, in space (spatial) and time (temporal).

     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 may be concerned primarily with the formation of sedimentary rocks but as soon as these beds of rock are locked at in terms of their temporal and special relationships the study has become stratigraphy. Similarly, if the Stratigraphers wish to interpret layers of rocks in terms of environments of the past the research is sedimentological

PROCESSES AND PRODUCTS:

    The nature of the sedimentary material is very varied in: origin; size; shape and

    composition.

     The processes of rock cycle include:

     WETHERING

     EROSION

     TRANSPORTATION

     DEPOSITION (SEDIMENTATION)

     BURIAL

     DIAGENESIS

    Weathering

    Clastic and chemical sediments form during weathering of bedrock or pre-existing

    sediment by both physical and chemical processes. Organic sediments are also produced by a combination of physical and chemical weathering. Physical (or mechanical) weatheringthe disintegration of Earth materialsis generally caused

    by abrasion or fracturing, such as the striking of one pebble against another in a river or stream bed, or the cracking of a rock by expanding ice. Physical weathering produces clastic and organic sediment.

    Chemical weathering, or the decay and dissolution of Earth materials, is caused by a variety of processes. However, it results primarily from various interactions between water and rock material. Chemical weathering may alter the mineral content of a rock by either adding or removing certain chemical components. Some mineral by-products of chemical weathering are dissolved by water and transported below ground or to an ocean or lake in solution. Later, these dissolved minerals may precipitate out, forming deposits on the roof

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    of a cave (as stalactites), or the ocean floor. Chemical weathering produces clastic, chemical, and organic sediments.

    Erosion and transport

    Erosion and transport of sediments from the site of weathering are caused by one or more of the following agents: gravity, wind, water, or ice. When gravity acts alone to move a body of sediment or rock, this is known as mass wasting. When the forces of wind, water, or ice act to erode sediment, they always do so under the influence of gravity. So gravity can be considered as one of the important agents of erosion

    and transport

    Deposition

    When the velocity (force) of the transport medium is insufficient to move a clastic (or organic) sediment particle it is deposited. As you might expect, when velocity decreases in wind or water, larger sediments are deposited first, while the smaller sediments deposited last.

    However, when the transport medium's velocity increases again, these deposits will again be eroded and transported. Surprisingly, when compacted fine-grained clay deposits are subjected to stream erosion, they are nearly as difficult to erode as pebbles and boulders. Eventually the sediment will reach a final resting place where it remains long enough to be buried by other sediments. This is known as the sediment's depositional environment.

Major control

    There are many factors controls the accumulations of sediments. So the major

    controls on the sedimentary cycles are: tectonics; climate; worldwide (eustatic)

    changes in sea level; the evolution of environments with geological time and the

    effect of rare events. In addition to this the chemical sedimentation and biological

    effects also two other important factors.

    Tectonics is the large scale motions (both horizontal and vertical) of the Earth's crust. Tectonics is driven by forces within the interior of the earth, but has a large effect on sedimentation. These crustal movements largely determined which areas of the

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earth's crust undergo uplift and erosion, thus acting as sources of sediment, and

    which areas of the earth's crust undergo subsidence, thus acting as sedimentary

    basins. Rate of uplifted is closely balanced by rates of erosion.

    Climate plays a secondary but important role in controlling the rate of weathering and sediment production. The more humid the climates, the higher these rates (i.e. rate of weathering and sediment production) are.

    Tectonic and Climate together control the relative level of the sea. Change in sea level whether local or worldwide (eustatic), strongly influence sedimentation in shallow seas and along coastlines; throughout geological time sea-level changes also affect sedimentation in rivers by changing the base level below which a stream cannot erode its bed.

    Events that are rare by human standards but common on a geological time scale, such as earthquakes, volcanic eruptions, and storms produce widespread sediment deposits. Other examples: rapid drying up of large sea and collisions between the earth and large meteoric bodies.

    Deposition takes place when the rate of sediment movement decreases in the direction of sediment movement, deposition may be rapid or so slow.

    Chemical sedimentation: Chemical weathering dissolves rock materials and derives ions in solution to lakes and the ocean. The concentrations of ions in river and ocean water are quite different, showing that some ions must be removed by sedimentation. Comparison of the modern rate of delivery of ions to the ocean, with their concentration in the ocean, shows that some are removed very rapidly (residence times of only a few thousand years) whereas others, such as chlorine and sodium, are removed very slowly (residence times of hundreds of millions of years). Biological effects: Many so-called chemical types of sediment are actually

    produced by biochemical action. Depositional and diagenetic processes are often strongly affected by organic action. Plants in both terrestrial and marine environments tend to trap sediment, enhancing deposition and slowing erosion.

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