steves sedimentary notes

8/2/2019 Steves Sedimentary Notes

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Bed:

- A layer of sedimentary rock that is clearly distinguishable from the layers above and below byvirtue of some discontinuity in rock type, internal structure, or texture

- Strata can be any thickness of bed greater that 1cm thick, if thinner it is called laminaeMethods used to analyze sedimentary rocks can be divided into:

- Lithofacies analysis- Provenance studies- Paleocurrent studies- Dating using fossils

Two types of weathering:

Physical Weathering:

- Frost and root action, glacial grinding. Abrasion in streams and beaches etc.- Produces mostly detrital grains (pebbles, sand) though some clay sized material produced by

crushing

- Dominates in temperate and arctic regionsChemical Weathering:

- Oxidation, hydration, etc. can occur in altered zone of bedrock- Occurs when sediment is in storage in the transportation systems- Produces mainly clays though some grains (quartz from granite) are set through the chemical

weathering of grains enclosing them

- Chemical weathering predominates in tropical areasErosion and Deposition:

Material is carried in three ways:

- Solution- Suspension- In bedload (rolling along bed, traction and saltation)

Denudation rate:

- Mm of the earths surface stripped away by erosion every 1000 years.Relief:

- Relief is a basin is usually measured as the distance from the lowest land to the highest land- Denudation increases with increasing relief- 5-10% of the earths surface that is mountainous supplies 80% of the detritus- Streams draining areas of high relief have the highest proportion of bedloads


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Climate and Vegetation:

- Plant roots in the soil serve 2 functions- They help decompose detritus and form clays through chemical erosion- They bind the soil impeding erosion. The above surface portions of plants also impede erosion

by acting as baffles to water flowing over them during floods and lowering velocity thus causing

deposition

Uplift or subsidence may be divided into three types:

- Orogenesis earth movements commonly found in mountain chains (collision induced)- Eperiogenic large scale, gentle upwarping and downwarping of the crust- Isostatic movements caused by changing load on the load of the upper mantle

Classification of Sandstones:

Siliclastic rocks:

- Rocks composed predominantly of silica) are classified according to grain size, in the generalclassification scheme

Sandstones (arenites):

- Composed of three constituents- Sand grains, matrix and cement

Sand grains may be either:

- Monomineralic grains consisting of only one mineral crystal- Polymineralic grains consisting of more than one mineral

Monomineralic sand grains can be divided into 2 types:- Light minerals minerals with specific gravities similar to quartz- Heavy minerals- minerals with specific gravities greater than quartz

Matrix:

- Generally composed of fine silt and clay and are of great importance in interpreting depositionalenvironments

- Large amount of clay indicates lack of reworking of the sediment by currents capable ofremoving it

- Clay is also formed by the post depositional break down of unstable mineral fragments- Sandstones containing detrital clay are termed immature

Greywacke:

- Laterally extensive beds composed of clay-rich sandstoneArkose:

- Coarse-grained clastic rock containing abundant feldspar.


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Orthoquartzite:

- A sandstone with at least 90% of the grains composed of quartzLithic:

-

Objective meaning rock fragments are present- Granular portion of the rock must contain less than 90% quartz grains and more rock fragments

than feldspar

Chemical Composition:

Eugeosynclines:

- Basins formed associated with island arcs and deep marine environments with extensivevolcanism.

Exogeosynclines:

- Clastic wedges formed by the erosion of mountain chainsTaphrogeosynclines:

- Small deep fault bounded basins on the continent.Groupings related to the source terrain which exists in each area:

- Eugeo volcanic (mafic) rich in plag- Taph continental crust- Exo mixture of sources but generally granite dominate over mafic volcanics and the sediments

are immature containing a lot of clays

Grain shape:

Roundness:

- How sharp are the corners and edges of the grainSphericity:

- The degree to which the grain approaches a spherical shapeForm:

- Descriptive terms used in describing the shape i.e. Elongate, tabular etc.Surface Texture:

- Very small scale texture of the grains surface.- Dull most grains are dull (earthy)- Polished a sheen on the surface created by abrasion- Frosted like frosted groups. Created by grain-to-grain impact.


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Roundness:

- Measured by comparing a standard set of images of known roundness with the sampleSphericity:

-

Can be measured by calculating the diameter of a sphere with the same volume as the particledivided by the volume of the circumscribed sphere.

Surface Texture:

- Surface textures are dependant on three things:- The way the grain breaks- Attrition of the surface produced by impact with other grains- Modification of the surface by chemical dissolution and reprecipitation. Impacts produce small

pits and grooves on the grain surface while solution and reprecipitation tend to produce a

smooth rounded surface because the sharp edges summer far more from chemical weathering

than the smooth surfaces do.

Interpretation of Grain Shape:

- Roundness and sphericity increase as sediment is abraded- Sediments close to the source should be more angular and less spherical than pebbles- Certain shapes of pebbles are thought typical of certain environments- Ventifacts (cobbles scoured and pitted by sandblasting) = desert- Flatiron shaped cobbles (triangular) are typical of cobbles transported by glaciers. The flat side

developing through dragging along the substrate

- An abundance of flat clasts may indicate a beach environment though other evidence should besearched for.

Sediment Transport: The Variables

- Both air and water are fluids- A fluid is a substance that is deformed by a shear force no matter how small the force applied is.- Rate at which a fluid deforms under shear stress is termed its dynamic viscosity (u, my).- Viscosity is temperature dependant- Viscosity of water increases with decreasing temperatures- Addition of sediment also increases viscosity- If enough sediment is added and a small shear force will not move the flow it is termed a

pseudo-plastic fluid.

- Dynamic viscosity is often transformed into kinematic viscosity (V, nu) by dividing dynamicviscosity by the fluid density.

- When describing the physics of fluids there are 2 primary features (density and dynamicviscosity) and one derived from (kinematic viscosity) which may be outlined.

- Discharge is a term derived from depth and is a measure of how much flowing water passes apoint during a specified time. (Volume / Time)


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Sediment Transport: Shear Stress, Laminar vs. Turbulent Flow, Bed roughness and the Boundary

Layer

- In laminar flow the stream lines are smooth and linear- In turbulent flow the streamlines are highly distorted- Laminar flow can be looked at as indefinitely thin layers of water sliding over one another

without mixing. (Low velocity and low depth)- Turbulent flow occurs when the laminar flow system breaks down and water particles from

one layer mix with another

- In Laminar flow the water in infinitely thin layers is all travelling with its own characteristicvelocity and not mixing with others

- Instantaneous velocity fluctuations are called turbulent fluctuating components of velocity- Transition from a laminar to turbulent flow is exhibited when the velocity is increased for

water flowing around a sphere suspended in it.

- As velocity increases a ring eddy starts to form- Resistance to flow can either be viscous or inertial- The balance between these which characterizes a flow (Reynolds Number)

Reynolds Number R= inertial force / viscous force

R= ULP/u = velocity x Length x density / viscosity = UL/V

- Dominance of viscous forces cause laminar flow and dominance of inertial forcescause turbulent flow

- Flow in open channels changes from laminar to turbulent flow between 2000 10000.

Boundary Layer

-

The portion of the flow where the velocity gradient exists is termed the boundary layer and theportion above this is the free or external layer.

- Divided into 3 layers: lowest layer in contact with the solid boundary is called the laminarsublayer

- Viscous forces dominate this layer- Overlying the laminar sublayer is the buffer layer- It is thicker than the laminar sublayer but much thinner than the entire boundary layer- Above the buffer layer is the fully turbulent portion of the boundary layer- Laminar sublayer and buffer layer may be destroyed if pebbles on bottom are large compared to

thickness

- Boundary layer may separate from substrate, occurs in depressions


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Froude Numbers

- Ratio between velocity and inertia F= U/ square root of gD(depth)- Gravity waves are unable to travel upstream at F greater than 1 as their velocity is less than the

velocity of the stream

- When a flow increases in F from below 1 to above 1 the depth gradually decreases and apparentturbulence lessen. This usually occurs when slope increases.

- Flows with F 1 are termed rapid, shooting or supercritical- As opposed to the smooth transition from subcritical to supercritical flow the reverse transition

is violent with the formation of a hydraulic jump (standing wave which represents an abrupt

increase in depth and turbulence).

- Similar to Re flow velocity ^2 / flow depth x gSediment Transport: Hiulstroms diagram and Shields Criterion

- Sediment on the bed will not move until the streams Velocity increases past a certain point- Two forces act on a sediment Shear stress and pressure- Hiulstroms plotted the relationship between the largest particle capable of being entrained- It graphically illustrates how the maximum diameter able to be entrained increases with

increasing velocity

- Relationship exists at sizes greater than 0.5mm- Grains of fine sand are too small and are entirely enclosed in the viscous sublayer- Thus pressure is greatly reduced

Shields Criterion:

- Shields developed a graph relating the initiation of grain movement to hydraulic parameters.- When a moving fluid interacts with sediment imparting some of its energy to the sediment 5

variables are important

- The size of the particle, its submerged specific weight, the shear stress acting on the bottom,viscosity and water density

- F (d, (Ys-Y), to, u, p)=0- Shields is the ratio of two forces shear stress/ weight of a single layer of grains- The above theories apply to rolling grains along a bed

Sedimentary Structures: Classification and Flow Regime

- The movement of sediment and water over a substrate tends to mold the substrate- Moulding can be caused by either erosion or deposition- Two main types of Primary Sedimentary Structures- Biogenic- Rootlets - internal- Burrows internal- Trails bedding- Tracks plane- Non-biogenic = current and wave ripples- Save wavesbedding


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- Dunesplane- Antidunes plane- Flutes- Cross-lamination- Cross-stratification- These can be divided into structures seen on a bedding plane surface of cross sectional view- Cross-stratification refers to layering found internally in a thicker bed which is at an angle to the

depositional slope of the area

- Cross stratification usually forms through the avalanching of sand down the lee slope or ripples,dunes, sand waves, bars, or small deltas.

- Occurs where deposition and erosion occur together- If deposition only is occurring the bedform would be preserved.- If erosion dominates nothing is preserved

Flow Regimes:

- The velocity and depth of the flow plus the size of sediment govern the bedform which willdevelop- With increasing velocity the bedforms which progressively develop in fine to coarse sand are

1. No movement2. Ripples (if the bed is a flat plane bed will separate the zone of no movement from ripple zone)3. Sand waves4. Dunes5. Upper plane bed6. Antidunes7. Chute and pool structures- Flows are usually decelerating during deposition- The bedforms which develop during increasing or decreasing flow velocity may be divided into

two groups

1. Lower flow Regime (lower plane bed, ripples and waves dunes). These bed forms are out ofphase with the waters surface F1

Ripples Sand Waves, Dunes and Antidunes:

- When looking at rocks the presence of ripples, sand waves, dunes and antidunes is usuallyindicated by cross-lamination, which is formed on the lee side of these bedforms.

- Ripples need the laminar sublayer to be intact in order to form (cannot exist when grainsize istoo large)

- Dunes to not need the presence of the laminar sublayer in order to form- Size to which dunes will grow is controlled primarily by water depth where as ripple height is

unrelated to D

- Ripples, sandwaves, dunes and some bars all grow by means of the same process- Sand rolls up the stoss slope, accumulates at the top, avalanches down the lee slope- As the ripple adds laminae it advances downstream.- In some instances sedimentation progresses so rapidly that the scouring of the stross side of the

ripple does not take place and the ripple when examined appears to have grown vertically


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- This is called ripple-drift cross lamination or climbing ripple cross-lamination and is usuallyvertically gradational with ordinary ripple cross lamination.

- A large amount of sand being rapidly deposited from suspension allows this bed form todevelop.

- Difference between sandwaves and dunes is that dunes are high amplitude, low wavelength andsandwaves are low amplitude, high wavelength).

- The impact the curvature the crestline has on the internal structure of the bedform is tied inwith how the shape of the crestline modifies fluid flow over the bedform.

- More curving the crestline the greater the points where flow separation does not occur- More curved the crestline the more curved the laminae.-

Types of Ripple Lamination:

1. Ordinary ripple cross lamination2. Climbing ripple cross lamination3. Flaser Bedding4.

Wavy Bedding5. Lenticular Bedding

Antidunes:

- Grow upstream- Produced at high flow rates with surface of water in phase with the beds surface- May be composed of one of 3 types of bedding1. Continuous symmetrical low-angle laminae produced by non breaking waves2. Shingled, overlapping, low angle laminae produced by individual bursts of turbulence3. Upstream dipping laminae produced by upstream moving hydraulic jumps- Wavelength related to velocity through the formula U*2= gxL/2pie-

Can only form at Froude numbers above 8, that is close to and in the supercritical flow field notin the subcritical flow-field.

Sedimentary Structures: Scours, Tool Marks, and Flutes:

Scour

- Cannot take place unless the flow is competent to erode the sediment- Scour at any one location is equal to the rate of transport of sediment away from that area

minus the rate of sediment movement into the area.

- Usually occur where local flow velocity is higher than average- Results in the digging of a pit- Pits vary from 10s of metres to 10 mms across- May cause armouring of the bed by development of a pebble lag- Scouring in mud can proceed in two ways- Mud will have an upper layer near the surface where it is not cohesive because its not

compressed

- During erosion noncohesive mud near the surface is swept away by each individual particlebecoming entrained

- As the more cohesive mud is uncovered a greater velocity is needed to entrain it and it break offas flakes and chumps


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- This may form an intraformational conglomerate.- Intraformational conglomerate may also from due to undercut of mud banks

Tool Marks:

- Produced by objects such as branches being dragged across the bottom by the current- Best preserved when bottom is muddy

Grove Marks:

- Long, straight, gutter like troughs produced by something being dragged along the bottomChevron Marks:

- A ridge formed by a series of Vs closing in the down current direction.- Produced by something moving along very close to the bottom and the turbulence in effect

sucking up the bottom into a series of Vs.

Prod Marks:

- Asymmetrical, elongate semi conical to triangular depressions with a shallow pointed up currentpart and a deeper broad down-current part.

- Formed by something bouncing off the bottom at a high angleBounce Marks:

- Symmetrical depression tapering and flattening off in both the up current and down currentdirections. Produced by something bouncing off the bed at a low impact angle.

Skip and Roll Marks:

-

Skip marks are produced when a tool in saltation hits the bottom at regular intervals.- Roll marks are produced when tools roll along the bottomFlute Marks:

- Discontinuous elongated hollows- Upstream ends of the hollows are deep and steep, flare out and shallow down-current merging

into the sediment surface

- Generally found as moulds on the lower surface of a sandy layer being cut into a muddysubstrate

- Formed by:1. An eddy scouring a small groove in the mud2. Flow generation of a separation eddy3. Further scouring of the substrate by the separation eddy forming a flute4. Sedimentation of sand filling the depression. A highly viscous flow is needed for the generation

of flutes, thus they generally form in turbidites, mass flow deposits and soft sediment injections.


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Primary Current Lineation:

- Fine, mm scale ridges alternating with hollows on the surface of a horizontal sand or sandstonebed.

- Forms during flow regime plane bed flow- Common in beach and fluvial (river) deposits- Formed by very small spiral vortices travelling along next to the bed with the downward

component piling the sand into a ridge

Rill Marks:

- Rill marks are small (mm to cm scale), bifurcating, dendritic, erosional furrow cut by a system ofsmall subareal riverlets.

- Formed under the flow of a thin layer of water and common on muddy stream banks whereafter a rainfall the water runs down the bank and into the stream.

Waves and Wave Formed Structures

- Waves set up currents which are unidirectional for very short periods of time- Water assumes a circular motion as waves pass- When water becomes shallower than 1/6 the wavelength interreaction with the bottom

starts to cause:

1. A decrease in wavelength2. An increase in steepness3. Smaller waves between the large waves die out4. Orbitals change from circular to elliptical5. Change in the wave form from symmetrical to asymmetrical, steepening of the front, over

steepening, and breaking

- The above changes result in the establishment of zones as the shore is approached:1. Zone of deep water waves2. Zone of shoaling waves3. Breaker zone4. Surf zone5. Swash and backwash (beach face)- Orbital paths of water particles not circular thus wave motion produces a slow drift of water

onshore

- Water returned through long shore drift and rip currentsSediment movement by waves:

- Oscillatory movement of water under waves causes a corresponding movement of sedimentunderlying them and the formation of ripples

- As crest of the wave passes overhead the flow direction switches from offshore to onshore andthe flow of water over the ripple produces a separation eddy similar to ones formed in the lee of

a current ripple.

- When flow velocity slackens the eddy rises and carries upward sand in suspension- As wave trough passes an offshore surge is set up which carries the sediment in suspension over

the store side of the ripple


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- Coarser sediment moving as bedload thus forms onshore dipping ripple laminae while the finermaterial is gradually moved off sure in intermittent suspension clouds

- Wave ripple different than river currents- These types of ripples will only form seaward of the zone of breaking waves- How far out offshore this seaward zone of ripple formation occurs is proportional to wave

amplitude

- Change in the shape of the ripples occurs as the shore is approached in deeper water waveformed ripples are symmetrical and may have bimodal laminae dip directions.

- As shore is approached flow becomes increasingly unidirectional and the ripples becomeasymmetrical.

- As waves begin to break lunate dunes replace ripples.- Further inshore in the surf zone the velocity has increased to the point where plain-bed

dominates

- Closer to shore is an inner area of the surf zone where offshore flowing water impedes velocityof the onshore flowing water creating turbulence and forming dunes

- On beach face itself plane bed again developsSymmetrical ripplesAsymmetrical RipplesLunate DunesPlanerLunate DunesPlaner

- If a regression or transgression occur these zones will be stacked on top of each other- Sometimes a bar or multiple bars will exist along the sore causing these zones to repeat and

setting up an along shore flow in area between bars.

Tide and Wind Formed Sedimentary Structures

- Tides produced by the gravitational attraction of the moon and the sun- Height of tide is governed by the geometry of the coastline- Long, funnel-shaped, bays tend to bunch up the water and produce higher tides- When sun and moon in line produce extreme high tides (spring tides)-

When sun and moon are 90 degrees apart tides are smaller than average (neap tides)- Macrotidal 4mmesotidal 2-4mmicrotidal


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Sedimentary Structures formed by wind:

- Harder time moving sediment then water- Turbulence is only able to suspend silt and clay sized particles- Saltation major way of sand transport- More developed in air because sand grains are not as shielded from sharp impact with each

other by water

- Thus a saltating sand grain in air in propelled more by the force transfer during impact thanturbulent eddying

- This is what causes the frosting of sand grains- Larger grains cannot be moved by saltation move by surface creep- If erosion is occurring and material is being moved out of an area a pebble-cobble lag will left

behind

- Wind ripples form through a different process than current ripples- Saltating grains cause surface creep.-

Eventually an uneven bed will develop with the stoss slopes more exposed and thus moretransport will occur on these slopes and ripples will grow

- Ripples tend to be composed of the coarser sand in a section as they are formed from bedload- Ripples have the coarser sand concentrated near their tops, thus the coarsen upward, the

opposite of subaqueous ripples

- With increasing flow velocity a plane bed will succeed ripples- Large bedforms are also common in deserts- Grow by same processes as dunes in water but are not limited in height due to flow depth as are

their subaqueous counterparts

- Internally dominated by large scale planar x-stratificationSediment Gravity Flows:

- 3 types of sediment gravity, movement (falls, slides and flows)- Rock falls form scree slopes next to highland areas or submerged slopes- Slides and slumps represent dislocation of a mass of sediment with this mass sliding downslope

along a shear plane

- Usually deformation and disruption of units only occurs close to the shear plane and where thebounding normal fault turns into an over thrust

- Sediment gravity flows develop if a slump, looses its internal cohesion and disaggregates intoindividual particles mixed with the water

- Once material is disaggregated it is also necessary to keep the sediment suspended in the watermass

- Accomplished by one of 4 processes:1. Turbidity currents sediment is supported by the upward component of turbulence2. Grain flows grains are supported by grain to grain collisions3. Liquefied sediment flows grains are supported by the upward movement of the fluid escaping

form between the grains. Shear stress and P exerted by the fluid keep the grains suspended

4. Debris flow here larger grains are supported by a highly viscous sediment water mixture whichalso has a high density. The matrix has enough strength to prevent the larger grains from

settling but it still able to flow. Flow is laminar


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Turbidity Currents:

-

Diffuse sediment laden clouds which flow down subaqueous slopes under the force of gravity- Different than the flows we have been talking about in that they have an upper bounding

surface which differs little in density form the overlying fluid.

- Thus mixing will occur along this boundary, with sediment laden water being lost to theoverlying lower density water

- Amount of mixing which will occur can be expressed by the densimetric Froude number- F = U/ sqr. Root g D (flow thickness)- If the number is considerably less than unity there will be very little mixing and the density

current can flow for miles without becoming overly mixed with the surrounding water and losing

its identity

- Some density currents flow continuously while some are spontaneous events.- These types have a head, body and tail- Sediments formed by turbidity flow are common In deep water successions- Consists of sandstone at the base of a bed and fine upwards- Bouma sequence top from bottommudhorizontal. Parallel laminated siltripple laminated

sand to silt horizontal, parallel laminated sandmassive, graded sand to granule at base

- Sometimes only portion of sequence present- Turbidites range from cms to ms thick and are interbedded with pelagic mud

Grain flows:

- Deposits rather rare due to high slope needed and probably formed in combination with otherprocesses

- Deposits are thick, massive sandstones with1. Scattered large clasts in a sand matrix2. Ungraded, sharp bedding planes3. Dish structures4. Scarcity of sole marks5. No lamination or X-lam. Typically formed by traction6. May be reverse graded

Liquefied sediment flows;

- Flow on slopes as low as 3 degrees as long as escaping pore fluid continues to support grains- Probably works in conjunction with grain flow mechanisms- Beds formed by this process are similar to grain flow deposits- Escape structures are common, as are deformation structures produced by dewatering

Debris Flows:

- Various types of debris flows exist which are named according to constituents- Concentrate boulders at their front and side, but this is the only form of sorting that occurs- Subareal debris flows produce poorly sorted beds 1 to several metres thick.


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- Subaqueous flows are similar and may turn into turbidites down slope once they have lost theirlarge material

Penecontemporaneous Deformation:

- Features present in the deformed bed which indicate that deformation occurred at the time ofdeposition or slightly afterwards

- Five main mechanisms may cause pene. Deformation1. Sinking of denser sediments overlying less dense sediments downward into the less dense units2. Liquefaction3. Gravity movement of sediment deposited on a slope4. Shear stress of a flow on recently deposited sediment5. Evaporation induced shrinkage-- First mechanism results in the formation of1. Load structures2. Loaded flutes and grooves3. Loaded ripples4. Ball and pillow structures-- Second mechanism results in the formation of1. If liquefaction of the bed at the surface occurs, a sediment gravity flow will develop or if there is

not enough of a slope the unit will simply sit there with the only effect being the destruction of

any internal structures. As the bed must have a high water content for this phenomenon to

occur and this means rapid deposition and unstable packing vs. the this type of destruction of

sedimentary structures is common in grain flows and liquefied sediment flows

2. If the liquefied sediment is buried beneath other sediments, it may inject these sediments andproduce sedimentary dykes and sills

3. Fluid escape also cause dish and pillar structures-- The third mechanism results in the formation of:1. Slumps and slidesalready covered so just briefly stress the importance of the contorted

laminal here. To distinguish from structures produced by diastrophism look for

1. Extremely plastic behaviour2. Disaggregation of granular material3. Striated folds4. Rolled up balls of plastic units5. Pull apart structures6. Erosive upper contact7. No cleavage-- The fourth mechanism results in the formation of:1. Cross-stratification with overturned upper portions-- The fifth mechanism results in the formation of:1. Mud cracks


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Biologic Structures (after Cambrian)

-

Burrows

both strait and curved may be filled with material from above bed, may have organicmaterial along walls

- Trails continuous markings along bedding surface- Tracks Footprints or bedding surface- Rootlets Fine network of single larger lines running vertically to subvertically through the rock

Diagenesis of Siliciclastics

- Diagenesis occurs because minerals stable at certain Ts and Ps are taken outside their stabilityfields

- Stability also dependant on Eh, Ph, surrounding chemicals and time- Usually occurs as minerals are buried in the earths crust- Increasing P and T with depth makes some minerals unstable- Minerals are then precipitated in the pore spaces causing lithification

Cement:

- Type of cement precipitated in the pore spaces dependant on:1. Chemical composition of the groundwater2. pH, Eh3. T, P- One cement may follow another cement with the previous cement being totally dissolved

before the later one is precipitated

- Silica commonly forms overgrowths in optical continuity with the quartz grains- Small mineral inclusions commonly define the boundary between the quartz grain and the over

growth. Some of the quartz is derived from pressure solution, but majority is deposited from

circulating meteoric water

- Calcium Carbonate CaC03 forms an extremely important type of cement. It is present inrelatively high amounts dissolved in groundwater

- Siderite cement may form by the replacement of Ca in CaCO3 by Fe. For this to happen rockmust have come in contact with a fluid rich in Fe.

- Hematite cement is developed primarily through the diagenetic alteration of hornblende,chlorite, biotite, ilmenite, and magnetite to ferric iron oxides. Extreme oxidizing conditions on

continents is conducive to this transformation thus subareally deposited rocks are usually red in

colour = redbeds

Deep Diagenetic Reactions

- When rocks are subjected to T of 200-300 degrees Celsius and P up to 1kbar metamorphismstarts to occur

- Quartz relatively stable at those T and Ps. May be an increase in crystal size if quartz is finegrained

- Feldspar K-feldspars undergo leaching of Potassium and silica causing sericitization. May bereplaced by carbonate. Both K-feld. And calcite feldspars are susceptible to albitization. The


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pore water in marine sand is derived from seawater and thus has a lot of Na. Turns feldspars

into plag.

- Rock Fragments Volcanic rock fragments disintegrate releasing Na, K, Al and Si. This maycause formation of zeolite minerals.

Classification of Carbonates:

- 10% of all sedimentary rocks are carbonates (composed of calcite and dolomite)- 50% of words oil exists in pools in carbonate rocks- Because of large biological influence in the formation of carbonates some constraints were

imposed

- Carbonates will form in the most organically productive zones. Main controls are:1. Climatemost carbonates form between 30S and 30N2. Light large number of plants form carbonates and these and the animals which feed on them

need light in order to live. Thus carbonates dominate shallow seas

3. Lack of siliciclastics A lot of detritus in the water column fouls up the tenticals filter feedersand fouls the gills of other organisms. Also, quartz sand will act like a ball mill grinding to dust

the much softer carbonate sand. Leads to limestones in the rock record containing less than 5%quartz.

Classification of Carbonates:

- Two main classification systems used in studying carbonate rocks Dunhams and Folks andWards.

- Dunhams is good for field work whereas Folk and Wards is good for microscope workDunhams:

Carbonate rocks divided into two main groups:

1. Those which owe their origin to strait sedimentary processes and generally bimodal mixtures ofmud plus sand

2. Those which owe their sedimentation to organic activity (framestone, boundstone, bafflestone).The first type is divided into subcategories depending on how close the larger clasts are to being

in grain support. (p 77)

Wards

Carbonate rocks have three main constituents:

1. Discrete carbonates in aggregates (allochems) (grains);2. Carbonate mud (micrite)3. Sparry cement (carbonate cement which precipitates in pore spaces after deposition). The firstpart of the rock name is based on the type of grains that are present. The second part of the

name is strait micrite or sparite depending on which one dominates. So an ossparite is a sparry

calcite cemented oolite and a pelmicrite is a rock composed of pellets and lime mud.

Carbonates Composition

Carbonate sand 8 types of grains which may be present in carbonate sand:


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1. Skeletal Particleswhole and broken skeletons of invertebrates and calcareous algae.2. Ooids sand-sized carbonate particles that have concentric rings of CaCO3 surrounding a

nucleus formed of a fragment of another particle. Thus, they resemble small onions. The

thickness of the coating varies. Ooids are generally only found where strong bottom currents

exist. The ooid coatings are formed by precipitation of CaCO3 (aragonite) on moving particles.

Continued periods of burial of the grain are needed for the formation of the rings. During burial

the new coating on the grain reacts with the pore water and becomes less susceptible to

dissolution. When the grain is exhumed and starts moving again carbonate rapidly precipitations

on it forming another ring. The rate of carbonate precipitation lessens with time. Thus the most

productive ooid growth occurs in environments where grains are constantly being buried and

exhumed.

3. Pisoliths Look like very large ooids. Form diagenetically in soil horizons.4. Peloids Round to oval grains composed of microcrystalline calcite. These grains can be

formed by various processes. The most important of these is by passage through an organism

guy. If this is the case, the grain is termed a (fecal) pellet. Fecal pellets are little mud balls and

are internally structure less. Another way to form peloids is by the micritization of skeletal grains

or ooids. These types of peloids are termed pelletoids. Usually some of the original structure can

be seen and the pelletoid is recognized on this ground5. Grapestone Formed from ooids or peloids bound together by organically rich carbonate. May

look like a bunch of grapes, but will not always, they are formed by shoals becoming inactive

due to raising water depth and organic material binding the grains together.

6. Algal accretionary grains Blue-green algae attach themselves to a grain and start to grow.Their sticky filiments collect fine carbonate grains and minute lamination is formed. Grains of

this type are called oncalites and may be up to 8cm in length. They resemble a biscuit.

Laminations are not continuous right around the oncolite, but die out on one side

7. Intraclasts Fragment of partially lithified carbonate sediment which are derived from near thesite of deposition of the sediment. Material derived from tidal flats and beaches often form

intraclasts.

8. Lithoclasts Rock fragments derived from older lithified limestone (carb. Pebbles)Carbonate Mud:

- Aragonite mud is very common in modern day carbonates environments. Donated to thesediment from 3 sources:

1. Mechanical or biological abrasion2. Direct inorganic precipitation3. Aragonite needles within the tissue of calcareous algae (most important)

Sedimentary Structures and Deep Sea Carbonates:

- Different specific gravity than quartz thus bedforms will develop at different velocities.- Indication of the transport of carbonate grains is provided by:1. Presence of x-strat2. Abrasion of grains3. Ooid coating on grains4. Size sorting of grains

In Situ accumulation:


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- Most carbonates deposited in situ or have not moved very far from their area of formation.- Ratio of sand sized material to mud sized material depends on the rate of production of both

these sizes at the site of deposition, and also if currents are removing or adding mud sized

material

- Recognition of in situ accumulation depends on lack of evidence of current activity- Lack of lamination is often caused by bioturbation and many not denote lack of current activity- Bioturbation is extremely common in carbonate sediments and is absent in Precambrian

carbonates as well as depositional environments that are too harsh for infauma to exist.

- Examples of harsh environments:1. Areas where bottom is instable such as ooid shoals2. Anoxic areas3. Playa lakes4. Supratidal areas5. Algal mats (tough resistant to burrowing) also grow in intertidal areas where periodic exposure

to sunlight and dessication limits burrowing and also limits grazing by metazoans)

Deep-Sea Carbonate Sediments:

- Three factors control carbonate accumulation here:1. The local productivity of planktonic organisms with carbonate shells2. Rate of sedimentation of detrital silica and silica shelled organisms3. Depth of water vs. the depth to the CCD (carbonate compensation depth)(depth at which the

water is under saturated with respect to CaCo3 and it dissolves).

- Foraminifera and coccoliths are the major CaCO3 producing microorganisms. These organismsdate back to the Jurassic. Both forams and coccoliths are composed of calcite and this is

important with reference to dissolution of carbonate with depth.

- Aragonite will dissolve before calcite does in the Atlantic aragonite starts to dissolve at 1000mdepths below surface whereas calcite starts to dissolve at 5000m depths. There is a zone where

some of the carbon dissolves but not all.

- Beginning of the zone is called the lysocline and in the Atlantic would be at 1000m.- Depth below which carbonate does not accumulate is termed the CCD

Freshwater Carbonates:

- Deposits identified from marine environments by the organisms present and associatedsubareal deposits.

- In some instances very saline lakes deposit carbonates directly by inorganic- Precipitation may produce a banding of massive aragonite, calcite and gypsum- Lakes around Thunder Bay accumulating marl, calcium carbonate and detrital silicate mud.

Carbonate Diagenesis- Cement vs. Neomorphism

- Diagenesis includes all processes that act on the sediment after deposition but before elevated Tand Ps create minerals and structures normally considered metamorphism

- Carbonates extremely susceptible to alteration by coming into contact with water with adifferent composition of dissolved material than the water from which the carbonate grains

were precipitated.

- Interpretation of diagenetic history of carbonates is an interpretation of pore water history.- Most processes acting on sed. Rock accomplish 1 of 3 following operations


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1. Dissolution produces void spaces in the rock by removal in solution of carbonate2. Cementation growth of new crystals into void space. Void space created by dissolution or be

primary porosity

3. Replacement simultaneous dissolution of existing minerals and the precipitation of a newmineral

Dissolution:

- High Mg calcite and aragonite only stable in surface seawater.- These minerals preferentially dissolved out of carbonate rock if rock is subject to fresh water

moving through it.

- If this occurs fossil fragments composed of high Mg calcite or aragonite will be dissolved leavingcavities with the shape of the fossil while low Mg calcite fossils will be left undisturbed

- Holes often filled with calcite cement- Ooids particularly susceptible to preferential dissolution as they are composed of aragonite.- To produce any noticeable amount of dissolution a large amount of fluid must be pumped

through the rock

- Large scale dissolution will form a collapse brecciaCementation:

- Mineralogy of cement depends on the composition of the fluid from which the crystals grew.- When crystals grow from seawater aragonite and sometimes high-Mg calcite form.- When cement grows from a freshwater solution low Mg calcite forms- Carbonate cementation occurs very soon after deposition- Example of this is beach-rock carbonate sand which is bound together by the growth of

aragonite needles between the grains.

- Cement forms when seawater splashed onto the high portion of the beach during storms,evaporates leaving behind an aragonite cement

Three types of cement exist:

1. Drusy elongate crystals rimming or filling voids. Long axis of the crystals are perpendicular tothe void wall

2. Blocky a mosaic of equidimensional crystals. Tend to fill the interior of void spaces3. Rim cements (syntaxial overgrowth) large crystals of calcite which have grown in optical

continuity with original single crystal grains

Replacement:

- Indicated by an internally well preserved grain being composed of a substance other than thatwhich originally formed it

- Presence of a well preserved internal structure could only by caused by molecule for moleculereplacement

- Calcite ooids are an example of thisNeomorphism:

- Type of replacement


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- Occurs when fine grained calcite mud recrystallizes to form calcite spar cement- Thus a muddy matrix maybe turned into a sparry matrix- A sparry calcite in carbonate rock may form from either the free growth of crystals in pore

spaces as cement or by the replacement-neomorphic process.

Distinguishing Cement and neomorphic fabric:

Cement

1. Spar is interparticle in current-transported grainstone2. Two or more generations of spar exist3. No relic structures found in the spar4. Micrite particles or envelopes not altered to spar5. Packstone micrite not changed by spar6. Spar lines and incompletely fills cavity7. Plain interfaces between spar crystals8. Crystal size increases away from initial sub-strate9. Crystal orientation perpendicular to initial substrate10.High percentage of enfacial junctions among triple junctions

Neomorphism

1. Contact between spar and original material sharp or diffuse2. Crystal size varies and is patchy from place to place3. Intercrystalline boundaries generally cured to wavy. Large crystals embay micrite4. Relics of micrite floating in three dimensions5. Spar replacing known shell structure6. % of enfacial junctions between 3 and 5%7. Syntaxial overgrowth into micrite

Dolomitization and Silicification:

- Dolomite refers to the mineral CaMg(CO3)2 and the rock composed mostly of this mineral- Most dolomite in the rock records can be demonstrated to be a replacement after limestone- Dolomite cross-cuts skeletal fragments and ooids- Dolomite replacement of Calcite takes place by simultaneous dissolution and precipitation- 2 criterion must hold in order for dolomite to replace CaCO31. the Mg/Ca activity ration of the water must be high enough to permit the reaction to take place2. enough fluid with a high Mg/Ca ratio must be able to be flushed through the rock- the theoretically calculated activity ratio of Mg/Ca needed to dolomitize limestone is

significantly lower than seawater

- Easiest way to increase the activity ratio is to precipitate aragonite and gypsum- This often happens in saline lagoons and in saline pore water on tidal flats- In both situations evaporates precipitate and the solution becomes capable of dolomitizing the

sediment


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- Dolomites may also form when Mg/Ca activity ratio is close to the theoretical dolomitizing valueof 1:1 and the solution is dilute

- Dolomitization occurs because segregation of ions to produce dolomite is accomplished morereadily if the crystallization rate is very slow and / or if the solution is dilute

- This occurs because of the precise Mg-Ca ordering needed to produce dolomite- This type of dolomitization is accomplished where marine waters filling pores are diluted with

freshwater from rain perculating down through small oceanic islands

Dedolomitization:

- The replacement of dolomite by calcite- Caused by fresh water dissolving gypsum, thus lowering its Mg/Ca activity ratio to the point it

can alter dolomite to calcite

- The diagnostic sign of the transition Is the presence of calcite rhombs which were oncedolomite

- If dolomite and calcite are both present in a rock the rock should be checked to ascertain if therock had once contained more dolomite and has in part been dedolomitized

Silicification:

- Involves the simultaneous solution of calcite and precipitation of chalcedony, chert ormicrocrystalline quartz

- Process favoured by relatively low pH, low T and saturation with respect to silica- May result in either the formation of structureless chert nodules or delicate replacement of

fossils by silica

Carbonate: Diagenetic Environments

- Three general environments where diagenesis occurs:1. On or just below the sea floor2. Subareal exposure in the vadose or shallow phreatic zone3. In the deep phreatic zone

Subsea Diagenesis:

- Main process which occurs on the seafloor is micritization- Occurs when boring bacteria or algae drill into the surface of a carbonate grain- If only surface is bored the grain is said to have a micritic envelope- If entire grain is bored a pelletoid is produced.- Micrite is very fine-grained cement which fills the little tubes bored into the organism- Submarine cementation is rare- Sometimes high Mg-calcite and aragonite cement form in shallow water.- These cements are microcrystalline and are probably aided in precipitation by the presence of

organic material

- Submarine cements sometimes also form in the internal pores of coral-algal rocks.Subareal Diagenesis:


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- Cement easily formed when carbonates are exposed to fresh water- Freshwater exposure may occur in either the vadose or phreatic zones- Cements formed in vadose zone generally concentrated in capillary sized pores and near grain

contact points as these are where thin films of water are present

- Vadose silt also present in the vadose zone.- Carbonate silt found in pore spaces and may be deposited on top of drusy cement- Silt will show parallel lamination of micro-X lamination- Silt particles freed during dissolution of above lying calcite carried down the void spaces by

perculating groundwater

- Pisoliths and laminated carbonate crusts may also be formed in the vadose environment due toprecipitation of calcite from saturated ground water.

- Groundwater becomes saturated due to dissolution of carbonate at a higher level- Thus carbonate crust development may be overlain by a dissolution collapse breccia

Deep subsurface diagenesis:

- Main process affecting carbonates is pressure solution, which causes dissolution andcementation of carbonate grains as well as formation of stylotites.- Carb. Rocks have 5% porosity vs. carbonate sediment 40-50%

- This means a great deal of reduction in porosity must occur- Half the volume of carbonate rocks is diagenetically added calcite cement.- When carbonate is subjected to lithostatic pressure it will sometimes dissolve- This may take the form of grains dissolving against other grains and/or the formation of irregular

zones of dissolution in the rock

- Stylotites often accentuated by a discontinuous sawtooth line of residual phyllosillicate clayminerals which were left behind when the zone of carbonate dissolved.

- Both processes free carbonate- This carbonate will precipitate in lower pressure areas forming a cement- Presence of a carbonate cement will usually indicate partial or total dissolution of nearly calcite

either through dissolution by freshwater in the vadose zone or by pressure solution

Evaporates:

- Form through evaporation of saline water leaving behind dissolved sediments- Primary evaporate minerals (Gypsum, anhydrite and halite)- Gypsum precipitates first then salt- In order to produce thick gypsum deposits we must constantly add new seawater to evaporate

and remove the old brine before it concentrates to the point it will precipitate NaCl.

- Three mechanism for doing this1. A water body joined to the ocean by a restricted strait2. Seepage-reflux3. Evaporative pumping- Evaporites can be looked at as forming in 3 depositional settings1. Large standing bodies of waterproduced during initial rifting when large isolated basin are

produced between continental blocks

- This is why salt diapers are common in continental margin succession- Water is periodically fed in from the main ocean mass through tortuously connected channels- Deposits in these basins consist of interlaminated gypsum and shale, limestone or dolomite.- Layers are thin an extremely laterally continuous


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2. Sabkhas intertidal flats accumulates evaporates through evaporative pumping.- Gypsum grows as bladed crystals and popcorn shaped masses in the clay, limestone or dolomite

sediment

- Force of crystallization forces the parallel laminated, fine-grained sediment apart and if theprocess is carried to completion chicken wire structure develops

3. Playa lakes lakes formed by internal drainage from areas surrounded by rocks containingelements capable of being dissolved and transported by water will usually be saline

- If lakes evaporate they will deposit a layer of evaporate minerals.- These minerals are usually mostly anhydrite, gypsum, and halite- Evaporates in these deposits are commonly interbedded with mudcracked shale

Phosphates, Chert and Iron Formation:

Phosphates:

- Bedded phosphate rock sometimes occurs in marine successions- If abundant can be mined for fertilizer, uranium, vanadium and REE- Phosphate occurs as a mixture of Flurapatite, chlorapatite and hydroxyapatite.- Phosphorites in the rock record contain grains analogous to those you would expect to find in

limestone and represent shelf deposits

Inorganic precipitation:

- They form in warm water and on the W sides of continents where the prevailing winds blow thesurface water offshore creating an upwelling of bottom waters.

- These bottom waters are rich in phosphate and may precipitate as waters upwellReplacement of CaCO3:

- The deposits that contain bioclasts and ooids partially replaced by apatite are clearly ofsecondary origin

- In these types of deposits replacement has probably occurred slightly below the sedimentsurface

Chert:

- Composed of microcrystalline to cryptocrystalline quartz- Palaeozoic to recent varieties usually form the sediments directly overlying the volcanic and

intrusive portions of ophiolite successions

- Found to be made up of sponge spicules and radiolarian plus diatom tests and were formed byrainout of the material on the bottom

- They are layered and interbedded with pyriteferious black shales- These deposits form in the deep oceans- Sometimes chert layers show grading of radiolarian tests indicating turbiditic slumping- Origin of Precambrian cherts unknown- Majority believe they were produced through inorganic precipitation form seawater

Iron Formation:

- Two main groups of iron rich rocks1. IronstonesPalaeozoic


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2. Iron FormationPrecambrian- Iron stones generally composed of hematite ooids and form in shallow water- Believed to be early replacement after CaCO3

Iron formations are common in Precambrian successions. 4 different facies:

1. Oxidehematite, magnetite2. Carbonate siderite, ankerite3. Silicate greenalite, stilpnomeline, granerite4. Sulphide pyrite

Iron formations can further be divided into 2 types:

Superior associated with shelf clastics

- Thinly interbedded iron minerals and chert bands- Bands have great lateral continuity- Iron is probably an original precipitate not associated with volcanic exhalation- These Iron Formations dominate the proterzoic, ex. Gunflint

Algoman found in Archean greenstone belts

- Finely interbedded Fe-mineral rich layers and chert not laterally continuous and oftenassociated with volcanics

- Thought to be result of volcanic exhalation into an ocean already saturated with respect to FeCoal and Petroleum:

Coal:

- Forms from accumulation of plant debris- In order for coal to relatively pure siliclastic material must not be able to get into the area where

coal is accumulating

- If clays and silts are transported to coal area coals will have a high ash content- If large amounts of silt and sands are transported in during flood events sandstone partings will

form between coal beds

- Coal made up of mostly carbon, oxygen and hydrogen- 4 types of coal distinguishable in hand specimen1. Vitrain a coherent and uniform, whole, brilliant, glossy and vitreous2. Clarain smooth surface when broken at right angles to bedding plane3. Durain hard, appears granular to the naked eye4. Fusain occurs as patches or wedges, powdery, falls apart easily- Coals are easily modified by increasing T (not P). As T increases:1. Content of C increases2. Content of H2 decreases3. Amount of volatiles decreases4. Darkness, lustre and reflectivity increases


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- Therefor coal reflectivity is often used to estimate burial depthCoal generally accumulates in one of 2 settings:

1. Around lakes and on floodplains. Generally the coals here are thick laterally discontinuous andare low in S

2. On delta tops and along strandlines. Generally the coals here are thin laterally continuous arehigh in sulphur (coals often interbedded with lacustrine and floodplain shales and sandstones)

Oil:

- Forms from the bodies of microorganisms which accumulate in marine and sometimes inlacustrine settings

- Unlike coal distinct beds of organic material do not develop, rather he microorganism arescattered throughout fine-grained sediment (black shale).

- Variables which effect the extent and rate of petroleum genesis are1. Concentration of organic matter in source rock2. The geologic time interval3. Temperature4. Extent of abiogenic oxidation before, during and immediately following deposition

1. Concentration of organic mattermarine source rocks contain only a few % organics and mostof these are left behind even after petroleum has formed

2. Time and Temperature Amount of time for which temperature is maintained is important. Iftemperature increases too quickly or goes too high the petroleum will breakdown first into

natural gas then H2S and CO2 plus residual hydrocarbon gunk. If temp. is too low and the time

not long enough the organic material will not form petroleum (liquid hydrocarbons form 65-150

degrees and gaseous hydrocarbons up to 200 degrees)

3. Abiogenic oxidation adds oxygen to organic matter. Detrimental to formation of petroleumas it destroys lipids which are probably the major building block of petroleum.


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steves sedimentary notes

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