Clastic Sedimentary Rocks

define Sediments

unconsolidated fragments of:

• pre-existing rocks & minerals

• shells

• mineral precipitates

• organic material from biochemical processes (in specific environments)

define strata

 distinct layers of sedimentary rock

• Rock characteristics determined by sediment source and processes they undergo prior to lithification

• Different minerals/sediment sources can stack up overtime and compact to form layers

sedimentary textures

arrangement of grains

• Clastic = grains held together by cement

  • Categorised by grain size

    • Shale = Fine grain sedimentary rocks -> mostly composed of clay

    • Siltstone = grains larger than shale but smaller than sand

    • Sandstone = grains same size as sand on beach

    • Conglomerate = grains are seen by naked eye and are rounded

    • Breccia = grains are seen by naked eye and are angular

  • Bedding = having a distinct top and bottom layer of sedimentary rock

    • Distinct composition

  • Strata = layer of sedimentary rock

• Crystalline = grains are interlocking from precipitating from sedimentary solution

Formation of Sedimentary rocks

1. Production of sediment

   1. Joints = natural cracks in rocks

   2. Rocks fall off cliffs at joints and break down into little blocks

   3. Collection of rock at the bottom of hill/cliff is called debris

   4. Overtime debris forms its own slopes

      1. Also called talus piles

      2. Overtime, moving fluids pick up clasts and transport them

2. Transportation

   1. Air, water, ice, rain

   2. Grains bash against each other

   3. Grains can undergo weathering and break apart

   4. Causes grains to decrease in size and become rounder

   5. Further grains travel = finer and rounder they are

      1. Travel far = clay

      2. Travels not that far = only pebbles, conglomerate, sandstone

3. Deposition

   1. Fluid moves fast = can pick up more things

      1. Sand, gravel, large boulders

   2. Fluid moves slow = pick up lighter things

      1. Clay, silt

   3. If different speeds of fluid occur and the sediment settles, we can get layers of different sedimentary rocks

      1. Oldest layer is at the bottom

      2. Newest layer is at the top

   4. Type of sedimentary rock depends on depositional environment

   5. Beach = fine grains are carried out + leaving only course sand

   6. Stream bed = flow is fast enough to carry away sand -> only left with course pebble = conglomerate if cemented

   7. Creek = flow is slow that clay and shale remains

Types of sedimentary rocks

Clastic

Biogenic

Chemically precipitated

clastic sed rocks

Made from grains of pre-existing rocks

• Classified by texture and composition

  • Clast (grain) size

  • Mineral composition? Any iron oxide?

  • Roundness: angular or spherical

  • Sorting: uniformity of grain size

Character of cement

• Coarse clastics

  • >2mm

  • Clearly visible to naked eye

  • Conglomerates = rounded clasts

    • Experienced more weathering during transport

    • Deposited further away from source

  • Breccia = angular clasts

    • Less weathering experienced before deposition

    • Deposited closer to source

• Medium grain clastics: Sandstones

  • Just visible to naked eye, might need some magnification

  • Mostly made of quartz

  • Some feldspar

  • Lithic sandstone = contains rock fragments

• Fine grain clastics: Mudstones

  • Cannot see with naked eye at all

  • Siltstones = silt -> gritty texture

  • Claystones = clay -> smooth texture

biogenic sed rocks

Made of organic remains

• E.g. undecayed plant matter (coal), shells, skeletal fragments of marine life (limestone)

chemically precipitated sed rocks

Made of minerals that precipitate directly from water solutions

• E.g. salt, gypsum

Have a crystalline structure

name sedimentary processes

1. Weathering and erosion

2. Transportation

3. Deposition

4. Lithification

types of weathering

Mechanical

Physical break down of rock

• No compositional change to material

  • Mineral composition stays the same

• Impacts causing rock to break

Chemical

Minerals react with water/oxygen to break it down

Chemical composition changes

types of mechanical weathering

Jointing & Sheeting

-> material breaking at joints

Thermal expansion

Animal activity

Abrasion

what is ointing & Sheeting

-> material breaking at joints

• Frost/Ice wedging

  • Due to penetration of water

  • When rock freezes, it expands due to water

• Root wedging

• Salt wedging

  • Salt crystalises when salt water seeps into rocks

  • Wedges minerals out

• Shrinking/swelling clays

  • As feldspars and ferromagnesium silicates interact with O2 in air + water = break down to make clay

  • As clay wets and dries, it swells and shrinks

what is Thermal expansion

• Hot days = expansion

• Cold nights = contraction

• Change in size = grain boundaries open up

how does animal activity cause erosion

Digging burrows, walking = break apart rock

how is abrasion weathering

• Rocks rolling around and hitting each other

• Moving water causing rocks to hit each other

• Gravity causing rocks to roll down slope and hit each other

types of chemical weathering

Hydration

Hydrolysis

Oxidation

Dissolution

what is Hydration

Absorption of water into crystal structure of minerals

• Some minerals suck water in

• Causes minerals to swell

• Causes rock material to break up and crack

• Common in clays

what is hydrolysis

Minerals react directly with water and break down

• H+ or OH- ions replace cations or anions in the mineral structure to produce new minerals

  • Generally orthoclase, plagioclase, pyroxene, amphibole

  • Break down to form clays

• Chemical composition of clay impacted by hydrolysis of starting mineral

what is oxidation

Addition of atmospheric oxygen that attacks certain minerals in rock

i.e. rock rusting

• Near rock surfaces

• Causes it to rust

• Particularly in the presence of water

Common in pyrite or magnetite

• Pyrite + oxygen + water = Iron oxide + sulfuric acid

  • Sulfuric acid goes on to hydrolyses feldspars in the rock\

  • Iron oxide makes red colour

• Magnetite+ oxygen = hematite

  • Hematite -> hydrolyses ->  limonite

• Rocks with more pyrite/magnetite = more likely to be chemically weathered

what is dissolution

Rock material dissolved directly into solution

• Common for limestones and rock salts (e.g. calcite)

• Rain picks up atmospheric dioxide and makes calcium carbonate

• Calcium carbonate + carbonic acid ->

  • reacts to make calcium ions, water, CO2, bicarbonate, bicarbonate ion

  • Calcium ions + CO2 = make more carbonic acid

  • Acid rain (contains sulphur dioxide) accelerates this process

Factors that impact rates of weathering

Mineral composition

Differential erosion

Environment

Mineral composition

vs weathering

i.e. Different minerals weather at different rates

• Under the same conditions

  • Calcite dissolution in limestone will be faster than feldspar hydrolysis in granite

  • Quartz is resilient to weathering

  • rock with grains cemented by calcite will weather faster than a rock with grains cemented by quartz

• Minerals that weather by dissolution = easiest to weather

  • e.g., halite, gypsum, calcite

• Silicate minerals with lower silica to oxygen ratios = weather easier than silicates with higher ratios

  • Opposite of Bowens reaction series -> last rocks made (felsic) are more resistant to erosion

  • Low silica to O2 ratio -> silicates made of isolated silica tetrahedra or single chains

  • High ratio -> silica tetrahedra arranged sheets or frameworks

• By-product minerals of chemical weathering highly resistant to further chemical weathering

  • Still prone to physical weathering

  • E.g. clay minerals

differential erosion vs weathering

The process where softer rocks wear away faster than harder rocks

i.e. different rocks eroding at different rates

• Non-uniform weathering of rock

  • Lowering of ground surface by direct physical activity

  • Washing of water across system

  • Physical collapse of rock

• Softer/less resistant rocks weather faster than harder/resilient rocks

  • Soft -> shale or mudstone

  • Harder -> limestone or granite

• Uneven exposure can cause differential erosion in identical rock

• E.g. Grand Canyon

  • Soft layers form gentle slopes and ramps

  • Tough limestone layers form steep, vertical cliffs

• Wind and water carve away weak rock layers while leaving hard caps or arches intact

• Makes uneven surfaces

Environment vs weathering

Energy of system

High energy = more physical weathering

Climate

• Warm wet areas = lots of oxygen and water to subsurface = larger amount of weathering

  • lots of weathering in tropics = more water in atmosphere, hotter temperature accelerates weathering, weather favours bacteria that live in rocks = initiates more weathering

• Tropics

○ Thick soil, less parent material

○ Most extreme weathering

○ More water, warmer

• Desert

  • Mostly shattered rock rather than particularly weathered material

  • Mostly unweathered

• Temperate

  • Penetration of water and O2 in moderately wet conditions = moderately thick soil profile

  • Not much bacterial activity -> too cold

  • Mostly shattered/broken up rock rather than fine soil + clays

Exposure

How thoroughly rock is chemically weathered

• High exposure to water/O2 = more weathering occurring

• If water can peculate through rock = weathering can occur in subsurface of rock as well

• A weathered rock experiences more weathering than an unweathered rock

• Weathering = smaller rock fragments = more surface area exposed to weathering = faster weathering

how is sediment transported + what impacts how much of this

• Gravity

• Water (most significant) – riverine processes

• Wind – aeolian processes

• Ice – glacial processes

 

Viscosity/density and velocity of transportation system affect its ability to carry sediment (in terms of particle mass, size and sediment volume)

• Dictates volume and size of transported material

The Hjulstrom Diagram

Determine relationship between size of grains and velocity required to transport/deposit it

Erosion and transport curve

• Sand is easiest sediment to move

• Clay and silt stick together = hard to pick up by water/wind by light winds -> need faster fluids

• Coarse gravel is too heavy to be picked up by light winds

• Deposition curve

• Easier to deposit heavier materials since they sink easily

• Harder to deposit light material because they float

As velocity of transport method increases, generally so does the size of material it can carry

• Large objects are deposited at high velocity

Types of transportation: gravity

Falling/rolling of soil

i.e. Landslides

• Rockfall/topple -> large wedge of rock collapses

  • Generates boulders, pebbles, cobbles, sand, rock fragments, silt

  • Some of this material will be washed into river systems and flow away

  • Source of sediment

Types of transportation: water

Dissolved

• Ions (cations + anions) dissolved into water

• Can precipitate out

Suspended

• Fine grain material -> silt and clay

• Due to turbidity

• Moves easily

Roll, slide, saltate

• Coarse grains, rock fragments, pebbles, boulders

Types of transportation: wind

Fine sand + course silts -> most easily erodible by wind and water

• Fine soils can be easily suspended in air low = forms dust clouds

• Coarser grains (like sand) roll or saltate near ground

• Clay is too sticky for wind to move along

• Loess deposit = thick deposits of sand/silt by wind

• Same pattern as water

what is deposition + how does it work

When sediment settles out of the transporting agent

• Associated with change in system energy or chemical gradient

  • Gravitational potential dissipates = rock is deposited on floor

• Ice melt

• Reduction of wind/flow velocity

  • Slowing down of water or wind

  • Fluid can't carry as heavy/large materials

• Change in slope gradient

  • Talus at base of slopes

  • Fine grains settle at flatter areas

  • Coarse grains settle along slopes

Sedimentary Structures

• Bedding and Stratification

  • Unconformities

• Cross-beds

• Ripples

• Dunes

• graded beds

• Bed surface markings

what is Bedding and Stratification in sed structures
+ how are they made

• Arranged in planar, horizontal beds

• Often laterally continuous for long distances

  • In same long layer = same age

  • Deeper layer = old

• Often consistent in composition, colour, and texture

 

How they are made

Bedding caused by changing conditions during deposition

i.e. we can group materials in repetition of rock types that reflect depositional environmental conditions

• Change in energy conditions = change in grain size deposited

• Change in environmental conditions

  • Changes in materials in different beds -> come from change in environmental depositional conditions

    • Silts of lake fall -> sands of river -> gravel by wind

  • Different material = different environment of deposited sediment

  • Change in sea level

  • Wetting/drying of system (tectonics or climate)

• Disturbance by organisms

types of erosional surfaces

Unconformity = distinctly long period of time where erosion/absence of deposition of new material

dis = Deposition stops + erosion continues = undulating surface with crooked boundary of old material at bottom

non = rock beneath is unlayered (cannot distinguish its age) while material overtop is layered

angular un = detect different orientation of planar surfaces from lower and upper layers

para = boundaries are parallel to two beds of material

what are
Cross-beds

Layer of sedimentary rock that is tilted at an angle compared to the main, horizontal layers around it

• Indicate direction of motion of wind/water current

how are cross beds made

1. Sediment bounces up gentle slop of dune/ripple

2. Piles up at peak of dune

3. Slips down steep dune face and deposited into the dip

4. Overtime these sediments pile up and reduce the dip

   1. If it is a dune/ripple it causes it to shift overtime in direction of wind

5. Slip face moves downstream

6. Added sediment forms incline cross strata

7. Bedded layer with cross strata forms cross-bed

where are cross beds made

• Rivers

• Tidal dominated settings

  • Enclosed coastal water bodies

  • Coast lines with tide >1m

  • Low water run off volume: tidal impact

• Shallow marine settings

• Aeolian deposit

what are ripples

Like mini dunes

• Reflect flow velocity and sediment

• Helps us determine type of environment materials were deposited

• Preserves information about flow conditions and direction

types of ripples

Asymmetric	Suggests unidirectional flow of transporting fluid -> wind/water Short, steep down- current slip face Long, gentle up-current ramp
Symmetric	Suggest wave oscillation of fluid -> moving back and forth of water or wind Sharp ridges and concave up troughs Back-and-forth swash Preserves

what are dune

Same concept as ripples but occur at a larger scale

• Aeolian dunes (>1-100’s m scale)

• Stream dunes (10’s cm scale)

what are graded beds

Rock which grades from coarse to fine grains as you through

Fining upwards / Normal grading

Fining downwards / Reverse grading

Fining upwards / Normal grading → graded beds

• More common

• Base is coarsest and top is finest

  • Base = coarse sand/gravel

  • Mid level = sand

  • Cap = fine silt/clay

• Abrupt change back to coarse base

• Made by

  • Submarine landslides -> cause coarse material to be deposited first while fine grains stay on top

  • Turbidity currents

  • Diminishing velocity after storms in rivers

  • Settling out of suspended sediment -> fine settles out to the top while heaviest sinks

Fining downwards / Reverse grading→ graded beds

• Less common

• Opposite of fining upwards where base is finest sediment and cap is coarsest sediment

• Due to granular convection -> keeps heaviest particles at top

• Generally made by aeolian ripples

what are Bed surface markings

Formed on bed surface while sediment is still soft

• Mud/desiccation cracks

• Morphology depends on rate of drying

  • Fast drying = widely spaced and irregular shape

  • Slow = closely spaced and regular shape

how are bed surface markings made

How it is made

1. Wet muddy sediment dries

2. Sediment shrinks and cracks -> forms hexagons

3. Curls up at edges

4. Crack again with filled sediment in edge

where are bed surface markings made

Water leaves and causes sediment to be wet and soft

• Abandoned river channels

• Flood plain muds

• Dried ponds

Terrestrial sed environments

Glacial environments

Alluvial fans

Aeolian environments

Mountain streams

Lacustrine/lake environments

Riverine deposits

glacial sed environment rock types

• Till -> glacial transport

• Angular talus deposits from ice wedging

• Sedimentary rock types: tillites

  • Generally coarse grain -> breccia

  • Not a lot of weathering going on

alluvial fans

• Where stream gradient drops in water flow

  • Mix of sediments

  • Deposited in fan shape

• Sediment dump

  • Lots of coarse and heavy grains

• Can be preserved If dries

• Cana be dissected by streams post rainfall

• Sedimentary rock types:

  • Arkoses, conglomerates

  • Variable energy environment so generally poorly sorted

aeolian environments

e.g. desert

• Bare ground -> strong wind pull sand along sand

• Dunes, loesses

• Sedimentary rock types: well sorted cross-bedded sandstones, siltstones

mountain streams

• Steep -> have high (potential) energy

• Braided gravel channels

• Sedimentary rock types: conglomerates

  • Due to coarse rock fragments made by toppling or sliding landslides

  • High energy environment

lake environments

• Still/slow moving water -> low energy system

  • Fine grain material

  • Muds, clays, silts

• Floods or overflow from other faster moving bodies of water introduce coarse sediment

  • Coarse sediment will then settle along shore

  • Fine clays in centre of lake

• Sedimentary rock types: laminated shales

  • Silt + clay

  • Low energy environment, so grain size is fine

riverine deposits

• Faster moving water -> higher energy than lakes

• Fine gravels, sands and coarse silts move along river channel

• Fine sediments move in flow + settle on flood plains

  • River can overflow

  • Mud, silt clay trapped along flood plain as flood recedes

  • Creates laminated deposit

• In channel sedimentary rock types: rounded sandstones, siltstones

  • Potentially crossbedding

  • Higher energy = can carry sand and silt

  • Sometimes conglomerates

• Flood plain sedimentary rock types: mudstones with mud cracks

Marine Environments

Tidal flats

Shallow marine

Deltas

Transitional marine deposits/ Continental slope

Deep marine deposits

tidal flats

• Low energy

• Dominated by tides and currents

• Sedimentary rock types: finer sandstones and mudstones

shallow marine environment

Shallow marine clastic

• Energy decreases as you move away from beach

• Sedimentary rock types: fine grained, well sorted siltstones and claystones

  • Can have marine fossils inside

 

Shallow marine carbonates

• Organically made sedimentary rocks

• Carbonates made from shells

• Sedimentary rocks: fossiliferous limestone/micrite

 

Coastal deposits

• High energy -> due to waves

  • E.g. beaches, lagoons

  • Low energy along tidal flats and back lagoons

• Lagoon sedimentary rock types:

  • fine-grained clastic rocks

  • Can contain organics

• Beach sedimentary rock types:

  • well sorted, mature sandstones

  • Sometimes conglomerates if higher energy waves

delta

• Where river meets sea

• Complex deposits

  • Sands deposited during large floods

  • Reworking of landscape by tidal currents

  • Storm surges can rework delta front and deposit different sediments

• Sedimentary rock types: variety formed

  • Sandstone and mudstone (shale)

Transitional marine deposits

/ Continental slope

• Between continental shelf and abyssal plain

• Steep slopes -> submarine canyons = underwater landslides

• Sedimentary rocks: graded clastic due to turbid currents

  • Coarse grains near steeper part of slope

    • Gravity/currents cause coarse sediment to be deposited first

  • Fine sediment deposited last at flat part

deep marine environment

• More than 200 m below the water surface

• Small water current -> only fine materials transported here

• Sedimentary rock types: fine red clays, cherts, some biogenic sandstones

  • Red = iron oxide -> oxidisation