Chapter 7 - Sedimentary Rocks

Created by

Kirsty Camba

Cards (60)

About 75% of land areas are covered by sediments and sedimentary rocks.
Rock cycle (sedimentary rocks)
chemical and mechanical weathering decompose and disintegrate rock.
gravity moves solid particles downslope
glaciers, rivers and wind transport sediment.
deposition of solid particles produces different features (glacial ridges, dunes, floodplains, deltas)
much sediment reaches the ocean floor
as sediments are buried, they become compacted and cemented into solid rock.
Rock cycle (sedimentary rocks)
when material dissolved in water precipitates, it is the source of features like reefs and deposits rich in shells.
soluble products of chemical weathering become dissolved in groundwater and streams.
Sediments have 2 sources:
accumulation of material that originates and transported as solid particles derived from both weathering - deposits are detrital sedimentary rocks.
soluble particle produced by chemical weathering. When these ions in solution are precipitate by either inorganic/biological process, the sediment is - chemical sedimentary rocks.
3rd category - organic sedimentary rocks, form from carbon rich remains of organisms (e.g. coal)
Detrital Sedimentary Rocks
clay minerals and quartz are the chief constituents.
clay minerals are the most abundant product of chemical weathering from silicates, especially feldspars.
clays are fine-grained with sheet-like crystalline structures like micas while quartz is also abundant due to its extreme durability and resistance to weathering.
other common minerals feldspars and micas.
their presence in sedimentary rocks indicates that erosion and deposition occured fast enough to preserve some of the primary minerals.
Particle size is the primary basis for distinguisihing various detrital sedimentary rocks.
Detrital Sedimentary Rock
Shale
consist of silt and clay size particles.
particles are so small that they can't be identified without great magnification.
its formation can be accounted to its tiny grains that indicate its deposition.
it occurs as a result of gradual settling from quiet, nonturbulent currents.
shale, mudstone or siltstone?
shale must exhibit the ability to split into thin layers along well-developed closely spaced plans (a property called fissility)
mudstone is used when rocks break into chunks or blocks.
siltstone is often grouped with shale but lacks fissility.
Detrital Sedimentary Rock 
Rock formed from the accumulation and consolidation of mineral or organic fragments derived from pre-existing rocks or organisms
Shale 
A fine-grained sedimentary rock that forms from the consolidation of clay and silt-sized particles
Shale formation
1. Silt and clay accumulate and form thin layers (laminae)
2. Particles in laminae are randomly oriented
3. Pore space - disordered arrangement that leaves a high percentage of open space
4. More layers of sediment pile up and compact the sediment below
5. Clay and silt particles take on a nearly parallel alignment and become tightly packed
6. Rearrangement of grains reduces pore space and forces water out
7. Since the grains are pressed together, solutions with cementing material are not allowed
Shales are described as weak since they are poorly cemented and not well lithified.
the inability of water to penetrate explains why this rock often form barriers to the subsurface movement of water and petroleum.
Detrital Sedimentary Rock
Shale
Gentle slopes
shales do not form prominent outcrops
shales crumble easily and forms a cover of soil that hides unweathered rock below.
some shales contain substantial quantities of oil and natural gas that can be obtained by a process called hydraulic fracturing.
e.g. Grand Canyon, where the greater slopes are inconspicuous and overgrown with vegetation.
in sharp contrast with sandstone and limestone that produce bold cliffs.
Detrital Sedimentary Rock
2. Sandstone - rock name for sand size grains that predominate.
Sorting
degree of similarity in particle size in a sedimentary rock.
can contribute to depositing current.
deposits of wind blown sand are better sorted than deposits crashed by wave activity
particles washed by waves are better than the ones in streams.
poor sorting usually result when transportation is hort and deposition is rapid.
e.g. Well-sorted > if all grains are same sized.
e.g. Poorly - sorted > if rock contains mixed large and small particles.
Detrital Sedimentary Rock
2. Sandstone
Particle shape
> shapes of sand grains
> streams, winds or waves move particles
> grains lose their sharp edges and become more rounded as they collide during transport.

Degree of rounding indicates time/distance involved in the transport.
> highly rounded grains - indicate a great deal of abrasion and transport has occurred.
> very angular grains - the materials were transported only a short distance before deposition, or carried by some medium other than water/land.
Detrital Sedimentary Rock
2. Sandstone
Transport affect Mineral composition
length of transport by turbulent air and water current also influence mineral composition.
> substantial weathering and long transport lead to the destruction of weaker and less stable minerals like feldspars and ferromagnesians (except quartz)
it can be inferred that a well-sorted, quartz rich sandstone consist of highly rounded grains and a result of great transport.
vice versa with sandstone containing feldspar and ferromagnesians.
Detrital Sedimentary Rock
2. Sandstone
Varieties
Quartz is the predominant mineral
> Quartz sandstone
2. Contains appreciable quantities of feldspar (25% or more)
> arkose
> contains quartz and mica
> mineral composition indicate that grains were derived from granitic source rocks.
Detrital Sedimentary Rock
2. Sandstone
Varieties
3. dark colored, abundant rock fragments and matrix, fine-grained material in which fragments are embedded.
> graywacke
> more than 15% of its volume is matrix
> poorly sorted and angular
> before it can be reworked and sorted, it was buried by additional layer of materials.
> turbidity currents - associated with submarine deposits made by sediment choked torrents.
Detrital Sedimentary Rock
3. Conglomerate & Breccia
Conglomerate
consists largely of gravels
large enough to be identified as distinctive rock types
poorly sorted and spaces between large gravel particles contain sand or mud.
gravels accumulate in a variety of environments and indicate steep slopes or turbulent currents.
coarse particles reflect the action of energetic mountain streams or strong wave activity along a rapidly eroding coast.
Detrital Sedimentary Rock
3. Conglomerate & Breccia
Breccia
large particles are angular than rounded
since large particles abrade and become rounded very rapidly during transport, the pebbles and cobbles in a breccia indicate that they did not travel far from the source area before they were deposited.
the particle sizes reveal the strength of the currents that transported them.
degree of rounding indicate how far the particles traveled.
Chemical Sedimentary Rocks
chemical sediments derive from ions that are carried in solution to lakes and seas.
some of this precipitates to form chemical sediments, forming rocks such as limestone, chert, and rock salt.
precipitation of material occur in 2 ways: organic & inorganic
Chemical Sedimentary Rocks
organic
> processes of water-dwelling organisms also form chemical sediments that are said to be biochemical origin.
> e.g. water-dwelling animals and plants extract dissolved mater to form shells and other hard parts. after the organisms die, their skeletons collect by the millions on the floor of a lake or oceans as a biochemical sediment.
inorganic
> processes such as evaporation and chemical activity can produce this.
> e.g. dripstones on caves, salt left behing as body of seawater evaporates.
Chemical Sedimentary Rocks
Limestone
- 10% of the total volume of all sedimentary rocks
- most abundant, composed of CaCO3
- formed by inorganic or biochemical process
- marine biochemical origin is the most common variety.
Carbonate Reefs
> corals are capable of creating large quantities of marine limestone
> secretes calcareous external skeleton
> capable of creating massive structures called reefs.
> Reefs - consist of coral colonies made up of great numbers of individuals that live side by side on a calcite structure secreted by animals.
Chemical Sedimentary Rocks
Limestone
Carbonate Reefs
> e.g. Great Barrier Reef, Australia
> 2600 km long, develop in the shallow, warm waters of tropics and subtropics equatoward of about 30 degrees latitude.
> Earth's first reef buildings were photosynthesizing bacteria that lived during Precambrian.
> Corals have been found in fossil reefs as 500 million yrs old
Chemical Sedimentary Rocks
Limestone
Coquina & Chalk
Coquina - biochemical limestone, coarse rock composed of poorly cemented shells and shell fragments.
Chalk - sot, porous rock made up of hard parts of microscopic marine organisms.
Chemical Sedimentary Rocks
Limestone
Inorganic Limestones - occur when chemical changes or high water temperatures cause CaCo3 to precipitate out of the water.

Travertine
> commonly seen in caves.
> when deposited in caves, groundwater is the source of CaCO3
> as water droplets become exposed to the air in a a cavern, some of the CO2 dissolved in the water escapes, causing CaCO3 to precipitate.
Chemical Sedimentary Rocks
Limestone
Inorganic Limestones

Oolitic limestone
> composed of small spherical grains called ooids.
> ooids formed in very shallow marine waters as tiny seed particles (shell fragments) are moved back and forth with currents
> ooids are also formed by chemical precipitation of CaCO3 around a tiny nucleus.
Chemical Sedimentary Rocks
Limestone
Inorganic Limestones
Dolostone
> composed of calcium - magneisum carbonate mineral: dolomite
>[CaMg(CO3)2]
> limestone: fizz one acid is applied, dolostone: only react when acid is in powdered form.
> abundant in ancient sedimentary rock successions.
> produced when magnesium-rich water circulate thru limestone and replace some calcium ions with magnesium ions (a process called dolomitization)
Chemical Sedimentary Rocks
Limestone
Inorganic Limestones
Chert
> very compact and hard rocks made up of microcrystalline quartz.
> appears gray, brown or brownish grey.
> e.g. Flint - dark color due to organic matter
Jade - red from iron oxide
Petrified wood - made when silica rich material such as volcanic ash burn trees. as groundwater rich in dissolved silica from the ash penetrates the wood, silica precipitates and replaces the wood.
> has a conchoidal fracture
Chemical Sedimentary Rocks
Limestone
Inorganic Limestones
Chert
> occurs as layered deposits called bedded cherts or as spherical masses called nodules.
> mosy bedded cherts are originated from diatoms & radiolaria that produce hard parts from silica.
- these organisms extract silica from seawater; when they die, their hard parts may accumulate on seafloor.
- some occur in association with lava flows and layers of volcanic ash.
> chert nodules are known as secondary/replacement cherts.
- form when silica dissolves, migrates then chemically precipitates, replacing older material.
Chemical Sedimentary Rocks
Limestone
Inorganic Limestones
Evaporites
> many areas that are now dry land were shallow marine basins>
> seawater continually move into the bay to replace water lost by evaporations.
> the water of the bay becomes saturated, salt deposition began. these deposits are called evaporites.
> minerals common are halite (NaCl), gypsum (CaSO4 2H2O)
> less soluble minerals precipitate first, and more soluble minerals precipitate later as salinity increases.
Chemical Sedimentary Rocks
Limestone
Inorganic Limestones
Evaporites
> e.g. gypsum precipitates when about 80% of seawater as evaporated.
> e.g. halite crystallizes whn 90% of water has been removed.
> e.g. afterwards, potassium and magnesium salts precipitate.
> e.g. last formed is sylvite and it is mined as a source of potassium for fertilizers.

> as the water evaporates, dissolved minerals can precipitate on the surface forming a salt flat.
Organic Sedimentary Rock: Coal
coal is made of organic matter.
coal is the end product of large amounts of plant material being buried for millions of years.
swamp environment > burial > compaction > greater burial > compaction > metamorphism.
peat, lignite, bituminous, anthracite
Diagenesis & Lithification
Diagenesis - term for all chemical, physical and biological changes that take place after sediments are deposited during and after lithification to form sedimentary rocks.
Burial promotes diagenesis because as sediments are buried, they are subjected to increasing pressure and deep.
Diagenesis occurs within the upper few km of earth's crust, at temperature that are less than 150 to 200 degrees Celsius. Beyond this, metamorphism will occur.
Diagenesis & Lithification
e.g. diagenetic change is recrystallization (development of more stable minerals from less stable ones)
- aragonite is the less stable form of CaCO3.
- as burial takes place, aragonite recrystallizes to form calcite.
e.g. chemical alterations that takes place as plant matter is buried gradually in an oxygen-poor environment, evolving into peat then coal.

Lithification - processes by which unconsolidated sediments are transformed into solid sedimentary rocks.
basic processes include compaction and cementation
Lithification
Compaction
most common physical diagenetic change
the deeper a sediment is buried, the more it is compacted and the firmer it becomes.
as the grains are pressed closer, the more it is compacted and firmer it becomes.
as pore spaces decreases, much of the water that was trapped in the sediments is driven out.
Lithification
Compaction
e.g. when clays are buried beneath several thousands of meters of material, the volume of clay layer may be reduced to 40%.
because sands and other coarse sediments are less comprehensible, compaction is significant as a lithification process in fine-grained sedimentary rock (like shale)
Lithification
2. Cementation
most important, process in which ions carried in solution by groundwater crystallize in the spaces between sediment grains to form minerals that gradually cement the grains together.
this process reduces porosity of rock like compaction.
e.g. most common cements: calcite, silica, iron oxide.
Past Environments
Environment of deposition/sedimentary environment
> Geographic setting where sediment is accumulating.
> placed into 1 of 3 broad categories: Continental, Marine, Transitional (shoreline)
Sedimentary Facies
> Sedimentary layers can show the successive changes in environmental conditions that occurred at a place with the passage of time.
e.g. When sand is accumulating in a beach environment, finer muds are deposited in quiet offshore waters
e.g. In a zone where biological activity is high, deposits consist of calcite-rich remains
> Different parts of each layer possess a distinctive set of characteristics that reflect the conditions in a particular environment.
Past Environments

When a sedimentary layer is examined in cross section from one end to the other, each facies grades laterally into another that formed at the same time but exhibits different characteristics.