Introduction to Sedimentary Rocks

Photo by W. W. Little

What is a Sedimentary Rock?

A sedimentary rock is a rock that formed through the accumulation of particles derived from preexisting rocks, including dissolved ions that recombine by chemical and biological processes.

Also called:• Siliciclastic (from silicate minerals)• Terrigenous (from the earth)• Detrital (disintegrated or weathered)• Epiclastic (from the surface)

Compositional ClassificationTerrigenous

Clastic sediment derived through extra-basinal weathering and erosion of subaerially-exposed rock bodies. E.g. gravel, sand, and mud.

AllochemicalReworked intrabasinal sediment produced through chemical or biochemical precipitation. E.g. bioclasts, ooids, and intraclasts.

OrthochemicalInsitu intrabasinal sediment produced through chemical or biochemical precipitation. E.g. micrite, phosphate, halite, gypsum, and chert.

T: TerrigenousA: AllochemicalIA: Impure allochemicalO: OrthochemicalIO: Impure orthochemical


Sediment Production

Sediment is produced by the weathering and erosion of preexisting rock bodies. Once formed, sediment is transported to a new area where it is deposited as layers that are subsequently lithified to form sedimentary rocks.


Weathering

Weathering is the breakdown or decomposition of a rock body.


Erosion refers to the removal (entrainment) of weathered material (sediment).

Erosion


Transport

Once sediment has been eroded, it is moved to a new location by water, wind, ice, or mass movement.


Deposition

Eventually, sediment is dropped to form a sedimentary deposit, in this case, a point bar.



Lithification

Through burial and processes related to groundwater, sediment is converted to rock.

Diagenesis

Diagenesis includes all changes (physical & chemical) that occur to sediment following deposition, including compaction, cementation, and dissolution. Lithification is part of the diagenetic process.


What is Weathering?

Weathering is the breakdown or decomposition of a rock body.

Physical vs. Chemical Weathering

Chemical weathering refers to the decomposition of rocks and minerals as chemical reactions alter them into new substances.

Physical weathering refers to the disintegration or disaggregation of rocks by physically breaking them apart

Photo by W. W. Little Photo by W. W. Little


Physical Weathering

Physical weathering breaks rock into smaller pieces but does not change its composition.


Chemical Weathering

Chemical weathering destroys or alters existing minerals, changing the rock’s composition and reducing its size.

Sediment Entrainment & Deposition

Hjulstrom & Sundborg showed a critical current velocity is required to both entrain and deposit sediment of a specific grain size for a fixed water depth. Sediment entrainment is also found to be dependent on sediment cohesion and consolidation and deposition is influenced by grain shape. The critical velocities for entrainment and deposition are not the same, particularly for fine-grained sediment.

Hjulstrom Diagram

Entrainment Forces

Two sets of opposing forces operate in determining whether or not a particle begins to move. Opposing transport are friction and gravity. For entrainment, these must be over come by lift (Bernouilli Effect) and drag (shear stress).

Reynolds Number

The Reynolds Number is used to distinguish between laminar and turbulent flow conditions. Turbulence occurs under conditions of high flow velocities, great depths, and low viscosity. Laminar flow takes place under the reverse. Wind and water flow are nearly always turbulent. Ice and mud are mostly laminar.

ul

ReRe = Reynolds Numberu = flow velocityl = flow depthv = kinematic viscosity (dynamic viscosity/fluid density)

Re > 2000 = turblent flowRe < 500 = laminar flow

Re = 500 to 2000 = mixed or transitional flow

Froude Number

The Froude Number is used to distinguish between subcritical flow (lower flow regime) and supercritical flow (upper flow regime), which determines whether or not the flow is too high to produce bedforms. Supercritical flow exceeds wave propagation velocities.

Fr = Froude Numberu = flow velocityd = flow depth√gd = celerity (wave velocity)

Fr > 1 = supercritical flowFr < 1 = subcritical flow

Fr = 1 = the “hydraulic jump”

gdu

Fr

Stokes Law

Stokes Law indicates the rate at which fine-grained sediment settles in a static fluid and appears to be valid only for spherical grains with a density similar to that of quartz. Theoretically, the larger or more dense the grain, the more rapidly it settles. Density and grain size also affect entrainment

Vg = settling velocityD = grain diameterPg = grain densityPf = fluid densityµ = dynamic viscosityμ 18

)ρ(ρgDv fg

2

g

Fluid Viscosity & Competence

Competence is a measure of a transport medium’s ability to entrain and carry particles of a given size. The greater the viscosity, the larger and denser the particles can be. For example, water has a greater competence than air and ice can transport larger particles than water. Likewise, a mudflow can move larger grains than pure water.

Grain flow: cohesionless sediment movement

Debris flow: viscous sediment movement

Liquefied flow: over-pressured interstitial fluid movement

Density/turbidity flow: slurry movement driven by differential density

Sediment Transport MechanismsFluid flow

Traction: grain rolling/sliding along substrate

Saltation: grain hopping along substrate

Suspension: ‘permanent’ grain entrainment

Solution: chemical transport as ions

Gravity flow

Types of Fluid Flow

Waves (water): wind-generated oscillatory motions of water. Height dependent upon wind strength, duration, and fetch. Wave ripples tend to be symmetrical.

Swash (water): generated by breaking waves. Water becomes turbulent, then laminar as sediment is carried onto and then away from a beach.

Marine currents (water): generated by temperature and salinity (thermohaline) contrasts in combination with the Coriolis Effect. Transport mud in suspension and rework sand along deep sea floor.

Gravity: slope failure due to earth’s gravitational pull. Enhanced by high relief and high water content. Can occur in both subaerial and subaqueous environments.

Chennelized (water): water derived through subaerial precipitation moves down-hill under the influence of gravity. Depending upon discharge, flow velocity, and nature of sediment, a wide variety of bedforms and a full range of ripple size can develop. Ripples tend to be highly asymmetrical.

Wind: generated by atmospheric pressure gradients. Transports fine-grained sediment – up to medium sand by traction/saltation and mud by suspension. Ripples of all sizes form and are highly asymmetrical.

Ice: Down-slope movement of ice under the influence of gravity. Highly competent. Creates very poorly-sorted deposits with few sedimentary structures.

Tides (water): generated by Lunar and Solar gravitational attraction. Tidal cycles are semi-diurnal, diurnal, and annual (neap and spring). Tidal ripples tend to be symmetrical.

Compaction: Caused by overburden pressure from sediment burial. Decreases rock volume by expelling fluids and forcing grains closer together. Particularly important in clays and organic materials. Reduces porosity. Extreme compaction causes pressure solution and the development of stylolites.

Cementation (authigenic mineralization): Different minerals are stable under different combinations of temperature and pressure; therefore, minerals that dissolve under one set of conditions may precipitate under another and vice versa. Relative dating principles applied to changes in mineralogy and to precipitation/dissolution features can be used to reconstruct burial history.

Lithification/Diagenesis

Chemical(mostly evaporites)(aka orthochemical)

Sedimentary Rock Classification

Sedimentary rocks are first divided on the basis of the major set of processes responsible for their formation.

Clastic(mostly siliciclastics)

(aka terrigenous)

Biochemical(mostly carbonates)(aka allochemical)

Siliciclastic Classification

Siliciclastic sedimentary rocks form through the physical accumulation of particles (clasts) and are subdivided on the basis of particle (grain) size.

Uden-Wentworth Scale

The Wentworth Scale is the standard classification for clastic sediment and rocks. Subdivisions are measured in both millimeters and in Phi units (log base 2). Class sizes closely reflect natural variation.

Conglomerate

Conglomerate is a siliciclastic sedimentary rock made up of at least 30% gravel size particles.

Breccia

Breccia is a conglomerate in which most clasts are distinctively angular.

Sandstone

Sandstone is a siliciclastic sedimentary rock made up of sand size particles.

Dott Classification

Siltstone

Siltstone is a siliciclastic sedimentary rock made up of silt size particles.

Claystone

Claystone is a siliciclastic sedimentary rock made up of clay size particles.


Coal

Coal is a rock made up primarily of plant debris and is found in association with some siliciclastic rocks.

Composition

Composition (provenance) tells us where the sediment came from. The study of rock composition is referred to as petrography.

Provenance

Provenance studies focus on composition are designed to determine the nature of a sediment source area, particularly source rock, tectonic setting, and climate.

Example from North America

Sandstones from North America show a northward increase in feldspar content for three reasons:

• Source rock: Much more granite is exposed in the northern part of the continent. Bedrock in southern regions consists mostly of Paleozoic sedimentary units.

• Climate: A colder, drier climate in north favors physical weathering; whereas, warmer, wetter conditions in south enhance chemical weathering.

• Transport distance: Lack of glaciation in south has resulted in longer, better-developed fluvial systems than in the north.

Texture

Texture (size, shape, & arrangement of particles) can tell us how the sediment was carried (water, wind, or ice) and how far it traveled.

Sedimentary Rock Components

Sedimentary rocks are made up of three main components, framework grains (called allochems in carbonate rocks), matrix, and cement.

Framework Grains (allochems)

Matrix

Cement

Framework Grains

Framework grains are the larger clasts that are used to define the rock.

Grain Contacts

The nature of grain contacts is influenced by mineralogy/rock type and can be an indicator of burial depth

Point

Sutured

Convavo-convex

Long

Long

Grain Form

Grain form refers to the relative length of three perpendicular axes and is a weathering characteristic that is strongly controlled by mineralogy, rock type, fractures, bedding planes, etc. Grain form impacts both entrainment and settling.

Equant

PlatyProlate

Surface Texture

Surface texture is a weathering characteristic that is strongly influenced by mineralogy, rock type, and weathering history.

Matrix

Matrix is composed of smaller particles that occupy some of the space between framework grains. It can be deposited together with the framework grains or physically infiltrated later.

Cement

Cement (authigenic mineralization) is chemically precipitated between framework grains and matrix particles and is useful for establishing burial history.

Common Authigenic Minerals

Carbonate cement (calcite, dolomite, ankerite, siderite)

Clay minerals (kaolinite, illite)

Quartz

Feldspar (albite) and zeolite are also found in small amounts.

Heavy (trace) Minerals

• Tourmaline

• Apatite• Garnet• Zircon• Rutile• Magnetite• Cassiterit

e• Monazite• Rarely

gold

Heavy minerals typically have specific gravities between 3.0 and 5.2. They are mostly found within quartz-rich sandstones but rarely make up more than 1% of the rock. Heavy minerals are useful in provenance studies.

Porosity

Porosity (Ø) is empty space within a rock and can be primary or secondary in nature. Some of the factors that influence porosity include grain size (mean, median, and sorting), shape (rounding & sphericity), composition, compaction, degree of cementation, dissolution.

Grain shape can greatly influence packing.

Primary porosity in sandstone.

Secondary porosity in sandstone.

Permeability

Permeability (K) is a measure of how well fluid can migrate through a rock and is dependent upon the same factors as porosity.

Sedimentary Textures

Sedimentary texture refers to the size, shape, and arrangement of clasts and provides information pertaining to the nature and distance of sediment transport.

Roundness

Roundness is a measure of surface irregularity and refers to whether or not the constituent grains have sharp corners.

Sphericity

Sphericity indicates how closely the grains approach a state where all three dimensions (length, width, and height) are the same.

Mathematical Determination

Roundness vs. Sphericity

The degree of roundness and sphericity for grains in sediment or a sedimentary rock are usually determined by comparing grain shapes to published charts that are based on statistical studies. The degree of rounding and sphericity both tend to increase with transport distance and with time. They are also impacted by grain composition.

Sorting

Sorting indicates how uniform or diverse particles are in size. Well-sorted refers to sediment or rocks where all particles are about the same size; whereas, poorly-sorted samples have a variety of grain sizes. Overall, sorting is better in finer-grained samples and in sediment that has been transported a long distance. Source material and transport medium are also important.

Well-sorted Sandstone Poorly-sorted Conglomerate

Sorting Classification

• Skewness: a measure of whether or not a particular grain size is favored. Distributions can be multi-modal.

• Kurtosis: A measure of the flatness or peakedness of the size distribution curve.

Sediment Maturity

As particles are transported from a source area to a basin, they become overall smaller in size, better rounded, more spherical, and better sorted. This is referred to as textural maturity. Concurrent with this is a change in composition as easily weathered minerals decrease in abundance, leading to a compositionally mature deposit.

Sorting SphericityRoundness

Textural Maturity

Sediment becomes finer-grained, more rounded, more spherical, and better sorted with downstream transport.

Demir, 2003 Demir, 2003

Gomez et al. 2001

The Bowen Reaction Series demonstrates the order in which minerals crystallize from a melt. An approximate reverse of the Bowen series seems to occur under atmospheric conditions, with quartz being the most stable and mafic minerals the least.

Compositional Maturity

Maturity Classification

There are four stages of sediment maturity, depending upon (in order of decreasing importance) clay content, sorting, and rounding.

Sediment Maturity & Environment

Sediment maturity can be tied loosely to depositional environment, reflecting distance of transport and energy conditions of the system.


Sedimentary Structures

Sedimentary structures form as sediment is deposited and are indicators of past depositional conditions, such as flow velocity, flow depth, flow direction, and the transporting agent. They can also provide significant clues as to paleoclimate.

Depositional Structures

Depositional structures form as sediment settles from movement during or following fluid flow.

Ripples Cross-bedding

Graded bedding Biological structures

Photo by W. W. Little Photo by W. W. Little

Photo by W. W. LittlePhoto by W. W. Little

Erosional Structures

Erosional structures are in two forms: diastems that represent short-term pauses between depositional events and form a normal part of a depositional system or unconformities that represent long-term cessation of deposition.

Fluvial or tidal channels Cut-and-fill structures

Flute casts UnconformitiesPhoto by W. W. Little Photo by W. W. Little


Post-depositional Structures

Following deposition, sediment can be altered by number of processes, such as dessication, rainfall, sediment flow, loading, and burrowing.

Mudcracks & rain imprints Distorted/contorted bedding

Load casts Bioturbation



Diagenetic Structures

Upon burial, sediment can be further altered by compaction, cementation, dissolution, and mineral replacement.

Nodules & Concretions

Disolution Pressure solution

Mineral replacement(irregular) (spherical)

Fossils

Fossils are indicators of ancient environments and can give clues regarding past climates.



Clay Minerals

Clay minerals have properties that make them both relatively slow to erode and to settle. A strong electrostatic charge increases cohesion, inhibiting erosion. A platy shape and low density lead to low settling rates; although, the electrostatic charge can cause flocculation and more rapid settling.

Chemical and Biochemical Rocks

Chemical and biochemical sedimentary rocks are composed of single-mineral precipitates.

Calcite

Limestone

The Carbonate Factory

Whereas siliciclastic sediment is produced primarily outside of the depositional basin and then transported to the basin, most carbonate sediment originates within the basin itself (primarily by biological processes) and undergoes little, if any, significant movement.

Aragonite

High-Mg Calcite

Calcite

Dolomite

Carbonate Classification

Carbonate sedimentary rocks form through the chemical or biochemical combination of Ca and Mg with the carbonate anion and are subdivided on the basis of mineral composition.

Increasing Mg content

Slide modified from Gahn, 2006

LIMESTONE DOLOSTONE

Limestone

Limestone is a biochemical rock composed of the mineral calcite or high-Mg calcite.

Dolostone

Dolostone is a biochemical rock composed of the mineral dolomite.

Dunham Classification

Folk Classification

Carbonate Rock Components

Like siliciclastics, carbonate sedimentary rocks are made up of three main components, framework grains (allochems), matrix (mud or micrite), and cement.

Allochems

MatrixCement

Carbonate Mud (Micrite)

Carbonate mud has two main sources, calcareous green algae and fecal material.

Penicillus Halimeda Carbonate Mud


Allochems

Allochems are grains larger than mud size and can include fossils, ooids, pellets, intraclasts, etc.

Carbonate Sediment Production

It has been said that “Carbonate sediments are born, not made.” (James, 1984), as the vast majority of carbonate sediment is of organic origin.


Biogenic Carbonate Mineralogy• Aragonite

– bivalves, gastropods, cephalopods– calcareous green algae– modern reef corals

• Calcite– Brachiopods– Planktonic brachiopods

• High-Mg calcite– Echinoderms– Benthonic foraminfera– Coralline red algae

Most Aragonite and high-Mg are altered to calcite upon burial. Slide modified from Gahn, 2006

Carbonate Organisms Have Changed Through Time

The organisms responsible for biogenic production of carbonate sediment have changed through geologic history.

Chert

Chert is a form of quartz that occurs in both siliciclastic and carbonate rocks, usually in the form of discontinuous beds, lenses, or nodules. It’s origin can be either biologic (mostly siliciclastics) or diagenetic (mostly carbonates).

Evaporites

Evaporites form through the inorganic precipitation of various minerals, mostly through high evaporation rates in environments with restricted circulation. They are composed of a single mineral and often have a crystalline texture.


Evaporitic Sequence

Carbonates (mostly dolomite)• Form when original seawater volume is reduced by

50%

Ca Sulfates (gypsum and anhydrite)• Form when original seawater volume is reduced to

20%

Chlorides (halite, sylvite, etc.)• Na salts form when the original seawater volume is

reduced to 10%• Mg and K salts form when the original seawater volume

is reduced to 5%

No modern equivalent exists for thick evaporite successions found in the ancient rock record. However, through laboratory experiments, a similar pattern has been produced.

Ironstones and Iron Formations

Iron-rich sedimentary deposits, such as the Banded Iron Formation (BIF), formed commonly in the Pre-cambrian, prior to the advent of an oxygenated atmosphere. Such deposits are no longer produced.


Depositional Environments

Putting it all together, we can use the features of sedimentary rocks to interpret the past geological history of an area.

Bouma Sequence

Some sedimentologic patterns are highly diagnostic of a specific depositional setting, such as the “Bouma Sequence” that is indicative of deposition by turbidites.

Genetic “Classification”

Some geologists “classify” rocks based on genetic origin. This is not a classification in the sense of a rock type, it is an interpretation and should not be used in the same manner as generic terms.

“Tillite” “Turbidite”

“Tempestite” “Tidalite”

Common Clastic Environments

Each depositional environment has a unique set of characteristics that distinguish it from all other sedimentary environments.

Common Carbonate Environments

Introduction to Sedimentary Rocks

Documents

Transcript of Introduction to Sedimentary Rocks