shales sandstones and associated rocks chapter 4

Shales sandstones and associated rocks

Chapter 4

Pyroclastic versus Epiclastic

Clast – a particle, or grainEpiclastic rocks are those composed

of (nonvolcanic) particles of all sizes, clay to boulders

Pyro – fire, volcanicPyroclastic rocks are those composed

of eruptive volcanic rock particles

Ch 4.1 mineral and rock grains

textural and compositional termsTable 4.1

Grade or fraction

Friktion

> 0,06 mm

• Cohesion

< 0,06 mm

Grade size in mmBoulder over 200Cobbles60-200Gravel 2-60

pebbles 4-60granules 2-4

Sand 0,06-2Silt 0,002-

0,06Clay < 0,002

Cohesion

Surface charges and water that hold the sediment together

• clay and silt - charges are great compared to grain weight

• sand – charges weak compared to grain weight but capillary water important

? what two meanings has the word clay?

1) clay mineral2) grade size

To avoid genetic inferences other terms are used:

clay size - lutite, lutiteous, argillite, argillaceous

sand size – arenite, arenaceous

courser than sand size – rudite, rudaceous

Clay minerals

important but a lot of names based on composition

– some are expansive others not

Brucite - expansiveGibbsiteKaolinite – NOT expansiveMontmorillonite (also called Smectite) expansive (deposition env. near shore)Illite – expansive (deposition env. deep sea)Bentonite – expansive (formed by weathering of volcanic ash)

similar clay mineralschloritehalloysitevermiculite

expansive clay

Slide risk with clay

Clay – fresh deposited – slides on slopes as little as 1 degree!!!

Clay – fills in the bottoms of basins first

Glacial and post glacial clay in Sweden

Source of clay

• Glacial clay – rock flour produced by abrasion of rocks in the glacier

• Post glacial clay – “normal” clay – consists of clay minerals

• the product of weathering

• thus the clay rich rocks are not very effected by weathering

?? How is the age of a clay rich rock related to the % or expected occurrence of

swelling clays??

Answer – p. 85 Table 4.2

the older they are the less expansive the clay is

?? What could you say about the glacial clay

deposits from the Weichselian Glaciation?

?? Post glacial clay??

4.2 Lithification

• To make into a rock• lithic, litho = rock

4.2 Lithification

• consolidation – water squeezed out

• compaction – air squeezed out• densification – both consolidation

and compaction• diagenesis – both densification

and cementation

Lithification

Types of cement – Fig 4.5,

• Different strengths

• Weathering can “remove” cement

• quartz• iron oxide• calcite • dolomite• gypsum• halite• clay

Characteristics of cement

• quartz – strongest • iron oxide - strong• calcite - soluble, crystalline intergrowths• dolomite – soluble but less than calcite• gypsum – extremely soluble• halite – extremely soluble• clay - (not true cement) can be leached

by ground water

Classification Rock or Soil

an engineer would classify a sediment that was either loose and unconsolidated or hard and “rocklike” as a soil if it lacked a soil if it lacked cementcement

a geologist would classify it as a rock if it was pre-quaternary in agepre-quaternary in age

Cementation of clay

• movement of ground water is low• difficult to consolidate – squeeze

out water• difficult for cement to migrate into

voids

Cementation of clay

How can thin layers of sand in clays and silts enhance lithification?

Glacial clays are varved = winter layer and summer layer / some sand in summer layers

Consolidation of glacial clay

If the water is caught in the basin – then it will support the clay

If it is allowed to drain out – the clay will consolidate – resulting in subsidence of the ground surface

Consolidation of glacial clay

Problems when a “basin of clay” is punctured and water is allowed to escape

example: Stockholm area – Huddinge, slussen subway, and more!

Strength versus porosity

• ?? How is rock strength and porosity related in clay rocks??

• Can you draw a simplified curve showing this relationship??

• p. 87, 88 Figure 4.6.

The porosity decreases with time and depth of burialThus the lower the porosity the stronger the rock

4.3 Description of some epiclatic rocks

Rocks with grains coarser than 2mm

• Conglomerate or Rudite• Breccia (fault breccia)• Tillite

Conglomerate or Rudite –

Conglomerate or Rudite – more than 30% rounded particles larger than 2mm

• deposition environments: rivers, mouth of streams, beaches, and colluvium

Conglomerate or Rudite –

• physical character – bimodal, open work, imbricate structure Fig. 4.8, clast or matrix supported structure

Packning av partiklar

Conglomerate or Rudite –

• Imbricate structure

Conglomerate or Rudite –

• not common but due to resistance to weathering they often stand out in the landscape as ridges

Breccia

• more than 30% more than 30% angularangular particles larger particles larger than 2mmthan 2mm

• deposition environments - tallus and scree = rock fall, movement, glaciers, volcanic activity, landslides, meteorite impacts

talus

landslide

Glacial breccia

meteorite impact

Fault breccia

• Fault breccia, fault gouge, mylonite– sheets of crushed material in a fault or

fault zone• course angular rock fragments – breccia• fine clay, pulverized rock from intense

grinding – mylonite

• Very important with respect to permeability of “hard rocks”

Fault breccia / gouge

Permeability

• along grain boundaries

• along faults, fractures and joints

Tillite

• unstratified, unsorted soil deposited from glacial ice

• depositional environment – glaciated areas

• physical character – highly variable thickness both laterally and vertically and extremely variable grain-size distribution (boulder clay, gravel rich till)

tillite

tillite

Rocks with sand-size grains (0.06 to 2 mm)

Sandstone and Arenaceous rocks

sandstone often suggests that the grains are composed of quartz and feldspar

arenites often are “sandstones” with grains other than quartz and feldspar (kalkarenite)

Texture

• Texture refers to

– Kornstorleksfördelning

– Sortering

– Kornform

– Packning

– Geometry of beds

Grain size

Includes several qualities:• mean grain size• predominant grain size• range of grain size

• Grain-size distribution• Sorting

grain form plays a role!

grain size

• range of paticles

• sorting • % of

different fractions

Texture – packing of grains

• grain – grain• matrix supported

grain form - maturity

• the longer time a particle is transported

• the better rounded it will become

question in notes on homepage

Explain the concept of maturity or immaturity of sandstones.

Give an example (name) of both a mature and immature sandstone.

What is the main physical difference between these two.

Layering or bedding

Thickness defined• very thick > 100 cm• thick 30-100• medium 10-30• thin 1-10• very thin< 1 cm

Nature of bedding

describes the partings within a bed• massive >100 cm• blocky 30-100 • slabby 10-30• flaggy 1-10• laminated < 1 cm

geometry of bedding

planarcrossbedde

dtroughwedge

geometry of bedding

• planar• cross bedded

troughwedge

geometry of bedding

• planar• cross bedded

troughwedge

Example:

• Thick massive bed – the bed is between 30 and 100 cm thick and there is NO internal layering

• Thin laminated bed – the bed is between 1 and 10 cm and there are thin <1 cm internal layering

Describe these

Classification after Pettijohn p96

3 criterion:• % detrital matrix

(arenite < 15% > graywacke)• % quartz versus feldspar and rock

fragments• % rock fraagments versus %

feldspar

classification summarizedP96 <15% detritus mtr. / clean quartz f eldspar

versus rock f ragment

Arkose <75% F>R

lithic sandstone F<R

orthoquatzite >95%

sub arkose or f eldspatic sandstone >75%<95% F>R

protoquartzite >75%<95% F<R

> 15% detritus mtr. / dirty

sub graywacke < 75% F<R

f eldspatic graywacke F>R

lithic graywacke F<R

Main rock types

• Arkose – friable, pinkish or grey, porous formed from weathering of granite

• Orthoquatzite – strong grains of quartz, rock strength dependent upon cement type SiO2 or CaCO3

• Graywacke – not friable due to large quantity of matrix, often graded, sorting poor, thus low porosity

Organic matter in sandstone

• coal – formed from plant debris• oil and gas – formed from

decomposition of sea organisms, foraminifera and diatoms

Organic matter

• Positive if we want to explore for gas and oil reserves

• sandstone is permeable and allows the formation of concentrations of gas and oil

• requires a “trap”

Organic matter

• Negative – • gas is highly explosive• tunneling and underground works

risk explosions!!!

Sedimentary Rx < 0,02 mm

• Shale• Mudstone• Mud rocks

>50% of sedimentary rocks are fine grained

>0,02

• Formation in– still water

• lakes• deep seas• swamps• flood

plains

Grain size and composition

• Normally a mixture of silt and clay• % of these can be determined by a

chemical analysis • silt – usually composed of silica

and lacks alumina• clay – usually composed of alumina

Names – numerous, p99

Distinctly different compositions and names:

• marl – calcarious rich clay• diotomite – silica fossils of diatoms• chert or flint – recrystalized diatomite• alum shale – contains FeS2 and mineral

alum (hydrous potassium alumina sulfate)

Names

less distinct difference between rock types

Shale and Argillite vs

Mudstone and Claystone

Names

• shale and argillite – fissil, fissility

• mudstone and claystone – lack fissility

Fissility is a textural term – a tendency to break apart along closely spaced sets of joints in “dice” like cubes

Fissility

Fissility

Fissility is limited in size< 10 cmFlaggy or blockyis the same quality but > 10 cm

Fissil and Flaggy

More Names of Rocks

• Slate – slightly metamorphic

• Phyllite – more metamorphic and with visible mica

More Names of Rocks

Volcanic origin:• tuffaceous mudstone – ash rich

mudstone• tuff – volcanic ash

Age relationship

• Shale – Palaeozoic

• Mudstone - Tertiary

Engineering classification

Rock – cemented shale non rock – compacted shale

strength like concrete engineering soil

no deterioration (no slaking) deteriorates (slaks) looses strength when wet

lithifi ed with cement landslide problems

Slaking

• deterioration and breakdown of a rock after exposure by excavation

• cracking and heaving• most common in expansive clays• size of chunks varies• dissolves in water• proportional to permeability

Slaking test

• Clay wet• Clay dry• Clay low fired• Clay high fired

Deformation structures

• slickensides – polished surfaces in mudstones which is believed to be due to shearing due to volume changes associated with wetting and drying

Deformation structures

• shale mylonite – sheared and crushed mudstone

Deformation structures

• bedding plane mylonite – shear along bedding planes due to folding

Sedimentary Facies

Facies = environment of deposition

Rocks that are common and are associated with unique environments are given “facies” names

Flysch or turbidite facies

rock description• rhythmically bedded thin beds of

shale alternating with graywacke environment of deposition• deposited in sub marine by

submarine landslides from the continental shelf down to the deep ocean basin

Flysch or turbidite faciesrhythmically bedded thin beds of shale alternating with graywacke

Flysch or turbidite facies

rhythmically bedded thin beds of shale alternating with graywacke

Cyclothemic deposits – Molasse Facies

• Repeated sequence of, from the bottom up, sandstone, clay, coal, limestone (sometimes), and shale.

• Deltaic environment with a oscillating relative sea level change.

environment for molasse

Deltaic environment with a oscillating relative sea level change

Molasse

shale limestone

(sometimes)

coal, clay, sandstone

Molasse

shale limestone

(sometimes)

coal, clay, sandstone

•

Molasse

Aerial view of the Eocene-Oligocene Indus Molasse Group, India.

This sequence is interpreted as the deposits of a paleo-Indus River and shows that the river started to flow soon after initial collision and uplift of southern Tibet (Clift et al., 2001).

Accretionary wedge deposit – melange Facies

• Disturbed beds of shale, graywacke, sandstone, which are sheared and folded.

• Melange is French for mixture – and that is what this is – a big mix of rocks in different structures.

• (Fig. 4.22) forms along an accretionary wedge where a continental plate and oceanic plate collide.

accretionary wedge

wedge – thrust faults

Melange

This is the Dunnage Melange near Gander, Newfoundland, which marks where North America and part of Europe collided during the formation of the Appalachians.

(French for mixture)

Melange

black shale melange

(French for mixture)