صخور رسوبية

Sedimentary Rocks

Sedimentary rocks forming about 5% of size from16 km out of earth crust. Its importance more than its percent because it Cover about 75% of the continent forming thin parts of earth crust. Sedimentary rocks economically very important fore example the coal is sedimentary rocks , oil and gas find in the sedimentary in addition to the sources of iron , aluminum manganese sulfur phosphate and others .

Weathering and Transportation of sediments and formation of sedimentary rocks by mode of river water- deposition on the continent and on the ocean floor.
Marine deposits
Fluvial deposits


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SEDIMENTARY ROCKS: are the secondary rocks which are formed from the loose fragments or detrital or clastic sediments produced by weathering of older rocks.

IMPORTANCE OF SEDIMENTARY ROCK

“Present is the key to the past”Helps in knowing depositional environment viz. marine (ocean deposits), fluvial (river deposits), aeolian (wind deposits), glacial, estuarine, Lacustrine (lake deposits) etc.Helps in knowing the provenance (i.e. source area of the sediments); change in climatic conditions i.e. in knowing and understanding old climate=paleoclimate.

Sandstones Conglomerates Breccia Shale/mudstones

TYPES OF SEDIMENTARY ROCKS
Clastic rocks
Chemical & Organic rocks
Evaporitic rocks
These rocks are formed due to evaporation of saline water (sea water) eg. Gypsum, Halit (rock salt)
Carbonate rocks
Form basically from CaCO3 – both by chemical leaching and by organic source (biochemical) eg. Limestone; dolomite Organic rocks
Form due to decomposition of organic remains under temperature and pressure eg. Coal/Lignite etc.



formed from broken rock fragments weathered and eroded by river, glacier, wind and sea waves. These clastic sediments are found deposited on floodplains, beaches, in desert and on the sea floors.

CLASTIC ROCKS

Clastic rocks
solidify
Clastic rocks are classified on the basis of the grain size: conglomerate, sandstone, shale etc.

Grain Size of sedimentary rocks

Gravel >256-2 mm
Sand 2- 0.062 mm =2-1/16mm

Boulder: >256mm Cobble: 64-256 mm Pebble: 4-64 mm Granule: 2-4mm Fine gravel

Mudstone Shale
Siltstone
Sandstne
clay stone
Conglomerate ,Breccia



Clastic rocks mainly comprise broken fragment of older rock – they are also know as Terrigenous rocks

Any rock fragment (size is > 4 mm=Pebble)

Fine-gravel/ Granule (size <4mm)
Matrix: is the finer grains or material that surrounds the larger clasts. It consist of either clay, silt and sand.
Cement: dissolved substance that bounds the sediments. Calcareous siliceous

Texture: Grain Size

Grain size is used to describe the size of the individual mineral grains, rock fragments, or organic material that are cemented together to form a clastic or chemical sedimentary rock
Grain Size Categories
Grain Size Divisions
very coarse-grained
>
16 mm
coarse grained
>
2 mm < 16 mm
medium grained
>
0.25 mm < 2 mm
fine grained
>
0.032 mm < 0.25 mm
very fine-grained
>
0.0004 mm < 0.032 mm
cryptocrystalline
<
0.0004 mm (4 μm ) * Table of Contents

Texture: Sorting

Sorting is used to describe the grain size distribution or range of grain sizes in a rock. Poorly sorted rocks contain a variety of different sized grains. Poorly sorted rocks contain a wide range of grain sizes including fine, medium, and coarse. Well sorted rocks contain almost all grains of the same size. Moderately sorted rocks contain particles of relatively similar grain sizes. Moderately sorted rocks may contain fine and medium grains, or medium and coarse grains.

* Table of Contents

Poorly Sorted
Well Sorted

Texture: Rounding

Rounding is used to describe the relative shape of the grains. Classifications are describe as deviations from rounded or spheroidal grain shapes. Well rounded grains are smooth with rounded edges.. Moderately rounded grains are in-between the sharp, angular edges of a poorly rounded grain and the smooth, roundness of a well-rounded grain. Poorly rounded grains may be sharp or angular.
* Table of Contents
Well-rounded, spheroidal grains
Poorly–rounded, angular grains

When clastic fragments are cemented or undergo consolidation they are called CONGLOMERATES

Similarly when angular clasts undergo consolidation they are called BRECCIA



Sorting of the sediments also suggest the mode of deposition and transportation.
Long distance transport= well-rounded and well-sorted sediments, Short distance transport = poorly sorted angular grains. Also helps in knowing the energy conditions of the river.

Diagenesis 1

Diagenesis collectively refers to the physical, chemical, and biological changes which may occur during the formation of sedimentary rocks. Recrystallization, compaction, cementation, and lithification, are all examples of diagenetic changes.
Recrystallization occurs when unstable minerals recrystallize to form more stable minerals. Recrystallization most often occurs during the formation of chemical sedimentary limestone rocks that previously contained aragonite a chemically unstable form of calcium carbonate (CaCO3).
Compaction occurs when sediments are progressively deposited on top of one another, and over time the weight of the accumulated sediments increases and compresses the buried sediments. Continued compression of buried sediments reduces pore-spaces and removes excess water, as a result the closely packed individual grains begin to slowly compact into a solid rock.

Cementation involves a chemical change whereby individual grains are cemented together as minerals are precipitated out of saturated solution that is percolating as a matrix between individual sediments. The accumulation of the precipitated minerals causes the grains to cement together. Cementation can occur in combination with the presence of other minerals, rock fragments, or organic constituents such as fossilized organisms.
Lithification occurs when unconsolidated sediments are cohesively bound to form a solid sedimentary rock. Compaction and/or cementation are generally the precursor to the lithification process.
Diagenesis 2

Cementation:

Cementing material is usually secondary Silica (Siliceous cement), Calcium carbonate (Carbonate cement), Iron rich (ferruginous cement)
Cement itself to some extent is the source of weakness in the sedimentary rocks Because cementing material and the clastic sediments are usually of different composition, leading to heterogeneity in their physical characteristics.

Sandstone

Sandstone rocks are composed almost entirely of sand-sized quartz grains (0.063 – 2 mm) cemented together through lithification.Sandstone rocks are generally classified as quartz sandstone, arkose (quartz with feldspars), or graywacke (quartz with feldspar, clay, and other coarse-grained mineral fragments).Sandstones comprise about 20% of all sedimentary rocks and are formed in a variety of different environments including fluvial (rivers), marine, coastal (oceans and beaches), aeolian (wind blown), and glacial (ice).The differences in texture, sorting, and rounding help geologists decipher the environmental conditions that formed the sandstone. * Table of Contents
Courtesy: Florida Department of Environmental Protection

Shale

Shale is a fine-grained, moderately to well-sorted rock formed by the compaction of well rounded silt-and clay-sized grains.Shales often contain fine laminations which helps impart fissility to the rock. Fissility is a term used to describe layered laminations formed by compression forces exerted over long-time periods.Shale usually contains about 50% silt, 35% clay, and 15% chemical materials, many shales may also contain organic plant materials and fossils. Shale is characterized by thinly, laminated layers, representing successive deposition of sediments.Shale accounts for about 50% of all sedimentary rocks deposited on the Earth’s surface.The sediments that form shale are most likely deposited very gradually in non-turbulent, environments such as a lakes, lagoons, flood plains, and deep-ocean basins. * Table of Contents

Siltstone

Siltstone is finer grained than sandstone, but coarser grained than mudstone, and it consists primarily of well-sorted, rounded grains ranging between 3.9 - 62.5 μm.Siltstone is similar to shale except that it lacks fissility. * Table of Contents

Mudstone

Mudstone consists of very silt-sized and clay-sized grains ( <0.0625 mm) and are often well consolidated with little pore space. Mudstones do not contain laminations or fissility, but they may contain bedding-plane features such as mud cracks or ripples. Mud cracks are formed by subaerial drying conditions. Ripples suggest gentle wave activity or water movement during deposition.
* Table of Contents
http://www.auckland.ac.nz/

Conglomerate

Conglomerates are poorly-sorted composites of a wide range of rounded grain sizes ranging from sand to cobbles (< 0.062 to > 2 mm). Conglomerates usually contain a framework of large grains held together by a matrix of sands, silt, and clay-sized particles. The combination of poorly-sorted, predominantly coarse, rounded grains suggests that conglomerates form in high-energy environments such as steep-gradient streams.
South Carolina Geological Survey
* Table of Contents

Breccia

Breccia is a poorly-sorted composite of a wide range of grain sizes ranging from clays to gravels (< 0.062 to > 2 mm). Breccias usually contain a framework of gravel-sized grains held together by a matrix of sands, silts, and clay. Breccia is similar to a conglomerate except that it consists of angular grains, as opposed to rounded grains. The combination of poorly-sorted, predominantly coarse, angular grains suggests that breccias form from rapid deposition in high energy environments such as steep-gradient streams, glacial flood deposits, landslides, talus, alluvial fans, or in association with faulting.

South Carolina Geological Survey

* Table of Contents

Dolostone

Dolostone is composed of Dolomite, a calcium-magnesium carbonate mineral. Dolostone forms when magnesium in pore water replaces some of the calcium present in limestone. For this reason, dolostone is often preceded by the formation of limestone deposits. Dolostone forms very slowly and is rarely observed forming in modern environments. Dolostone abundance increases with age. There are more older than younger dolostones.

* Table of Contents

Evaporites

Evaporites are chemical deposits formed when restricted bodies of saline water evaporate, precipitating out a range of minerals. Evaporite deposits do not involve a single chemical precipitate, instead they consist of chlorides, sulfides, carbonates, and borates. Halite and gypsum are two common examples of mineral precipitates.


Utah’s Bonneville Salt Flats
* Table of Contents

Limestone

Limestone consists almost entirely of the mineral calcite (CaCO3) and can form by either inorganic or biochemical processes. Limestones form under a variety of environmental conditions and for this reason several types of limestone exist. Limestone accounts for about 10% of all sedimentary rocks, and of those, limestones with marine biochemical origin are the most common.
* Table of Contents

This example of limestone formed in a shallow, marine environment where dinosaurs once roamed the Earth. This set of tracks is from an Arancanthosaurus track in the Paluxy River in Dinosaur Valley State Park in Glen Rose, Texas.

Coral Reefs

Coral Reefs are limestone formations created by marine organisms.Corals are invertebrate animals which secrete a calcareous (calcite-rich) external skeleton . Over long periods of time coral colonies form massive reef formations. Some of which surround entire islands or extend along the shoreline for 100’s of miles.The Florida Keys were once an underwater coral reef rich in biodiversity of sea life. Today the Keys are lithified limestone deposits exposed above modern sea level. Living coral reefs exist offshore along the Atlantic Coast of the Keys. South Carolina Geological Survey
This is an example of a fossilized brain coral from the Key Largo Limestone formation in the Florida Keys. Interestingly snorkelers and divers can view living brain coral just 20 miles offshore from these fossilized coral reefs.
* Table of Contents

Oolite

Oolitic limestone is formed by the cementation of tiny spherical grains called ooids. Ooids form in warm, shallow marine environments. When small grains of shell roll back in forth in the current, They are coated with calcium carbonate precipitating out of the supersaturated marine water. Ooids exhibit growth rings from the accumulation of the calcium carbonate precipitates. The presence of algae and sea-grasses accelerates and increases the formation of ooids. Ooid formation and oolitic limestones cover vast areas of the Bahamas creating shoals and tidal flats.

* Table of Contents

Copyright Marli Miller, University of Oregon

Coal

Coal is made almost entirely of plant material and other organic deposits that have been buried for millions of years under elevated conditions of heat and pressure. Although the chemical composition of coal changes from its organic origins, it often retains fossilized imprints of plant leaves, bark, wood, and organisms that lived during the time the organic materials were deposited. It requires very specific environmental conditions for plant material to become coal. The organic material must be deposited in an anoxic (oxygen free) environment to prevent it from decomposing. Most coal beds originated in swampy, saturated, environments. Deposited organic material goes through four main phase of coal formation, which are related to increasing heat and pressure : 1. Peat 2. Lignite 3. Bituminous 4. Anthracite
* Table of Contents

Chert

Chert represent a group of hard rocks made from micro- and cryptocrystalline silica (SiO2). Chert can develop as a nodules inside other rocks or as rock layers. Most cherts are hypothesized to originate from silica derived from one of three sources: solution in water, biochemical sediments, or lava flows and volcanic ash.Silicate materials can be precipitated out of a solution in marine waters, or produced as a byproduct of water dwelling organisms. Diatoms and radiolarians extract it from their surroundings and use it to grow silica-rich skeletons. When these organisms die and settle to the bottom, their skeletons provide the silica source for the chert to develop. Large beds of chert have been found to develop in association with lava flows and volcanic ash. It is thought that the chert is the produced by the decomposition of volcanic ash. Chert occurs in a variety of forms including flint, jasper, and agate.Chert is a very hard rock that generally breaks along conchoidal fractures, this characteristic makes it possible to carve sharp-pointed edges onto the rock. Native American’s used chert to create arrowheads that were attached to primitive spears, arrows, and knives. * Table of Contents

Chert

Flint is the most common form of chert. It is often a dark, glassy, colored rock that forms as nodules embedded in limestone. The dark color of the chert comes from the organic matter it contains. Jasper is a red variety of chert that gets its color from iron oxide. Agate is a banded form of chert that may contain several different colors layered throughout the rock.

Jasper

Agate forming inside a coral
Florida Department of Environmental Protection, Florida Geological Survey


* Table of Contents

Sedimentary Structures

, SCGS

Cross-Bedding

Copyright @Marli Miller
Ripple Marks
Copyright @ Bruce Molnia, Terra Photographics
Mud Cracks
Courtesy NASA Visible Earth
Ripples and Mudcracks
Copyright @Michael Collier
Copyright @ Bruce Molnia, USGS
Bedding Planes
Laminations
Copyright @ Bruce Molnia, USGS
* Table of Contents



SANDSTONE: Made up of sand grains dominantly of Quartz and Feldspars, where quartz is highly resistive to weathering Cementation plays similar role in this rock as seen in conglomerate However, Siliceous cement are best and highly desirable for CE purposes, also the ferruginous sst.

Different type of Sst. (based on their composition)

Quartz Sst.
Arkose (Feldspar)
Graywacke/lithic arenite

SHALE

Shales are clastic rocks, made up mainly fine silt/clay They are most abundant sedimentary rocks, accounts for about 80% of them Often contain fossils Mostly hydrous aluminum silicate in composition = from weathered feldspars Deposition takes place under low fluvial regime or under weak water current. Eg. Offshore or in Lagoon
Shale-fissile
Shales are made of fine well sorted silt and clayey sediments, where normally one can expect high porosity and permeability.
But due to surface tension phenomenon of water and extremely fine intergranular space shales are impermeable, hence does not yield water when tapped


When shales are saturated with water – under pressure they are likely to produce slippery foundation for any structure- therefore not suitable for CE structureseg. Lafayette dam of US constructed on argillaceous rocks sunk by 20 feet. Srisailam Dam in Andhra Pradesh (One of the 12th largest Hydroelectric Projects in the India) faced similar problem, however, precautions were take by grouting to stop the seepage along the weak zones.Because of its impermeable and porous nature it acts as cap rocks in the occurrence of Oil and Gas. CE IMPORTANCE

EVAPORITIC ROCKS

These rocks are formed within the a depositional basin from chemical substances dissolved in the seawater or lake water.
Gypsum
Halite
(NaCl)
CaSO4.2H20

Economic importance of Evaporites

SALT: other then daily use of salt for cooking, it is used For production of Paper, Soap Detergents Antiseptics As chemical for dyeing etc.
GYPSUM: is used for plaster and in manufacturing construction materials.

CARBONATE ROCKS

used as a building stone and in the manufacture of lime, carbon dioxide, and cement. Massive and compact lst. Are competent to support CE-structure However, if it occur in huge thickness then it is not advisable, because of its typical CAVING character.
Chalk

Sedimentary structures

Bedding is most imp. Feature of a sed. Rock Beds are usually > 1 cm Laminae < 1 cm Orientation of bedding helps in knowing the paleo-current direction of the old rivers

GRADED BEDDING

Fine gravelly lithounit
Medium-coarse sandy lithounit (cross stratified)

Laminated layers of fine silt and clay

Cross-stratified sst. Paleo-flow from right to left

Typical cross stratification in

Sand Dunes

Ripple marks

Foot prints
Biogenic structures
Snake Trails

Mud cracks