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)
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