Soils, Sediment, Weathering
and Sedimentary Rocks
Sedimentary Rocks
Rocks that form by the cementing of grains or fragments of pre-existing rocks, or by
the precipitation of minerals out of a solution
Form a veneer over basement rocks (igneous and metamorphic)
Sedimentary layers (cover) over the Vishnu schist (basement) in the Grand Canyon, AZ
Weathering and Erosion
• Sediment – loose fragments of rocks or
minerals broken off of bedrock,
minerals that precipitate directly out of
water, and shells of organisms.
• Deposition / Sedimentation – occurs
when sediment settles out as
winds/water current die down, or as
glaciers melt.
• Lithified – sediment that has been
cemented together by geologic
processes to form a rock
• Rocks are broken down and turned into
sediments by two main processes
– Physical Weathering
• Plumbers snake
– Chemical Weathering
• Liquid Drain-O
Highly weathered sandstone in Bryce Canyon N.P., UT
How do Sedimentary Rocks Form?
The following applies to Clastic/Detrital rocks; biochemical and chemical sed rocks are different
• 1) Erosion – mobilizes particles by weathering, most
commonly by rainfall & gravity.
• 2) Transportation – Occurs when currents of wind, water,
and moving ice (glaciers) transport particles to new locations
(downhill or downstream)
• 3) Deposition – sediment is deposited when the transporting
current slows to the point that it can no longer carry its load.
• 4) Burial – As layers of sediment accumulate, the layers
accumulate in sedimentary basins. Older, previously
deposited sediments are compacted. These layers remain at
depth until either erosion or tectonic processes act on them.
• 5) Diagenesis & Lithification – Refers to the physical and
chemical changes that lithify sediment into rock. Includes
pressure, heat and chemical reactions
The Sedimentary Stages of the Rock Cycle
Weathering
breaks down
rocks.
Erosion carries
away particles.
Transportation moves
particles downhill.
Glacier
Delta
Desert
Playa
lake
Sedimentary
rocks
Metamorphic
rocks
Plutons
Deposition occurs
when particles
settle out or
precipitate.
Burial occurs
as layers of
sediment
accumulate.
Diagenesis lithifies
the sediment to make
sedimentary rocks.
Detritus & Grain Size
• Physical weathering breaks rocks into chunks called detritus
• Detritus is classified by size (diameter)
» Boulder > 256 mm
Visible grain size
» Cobble - between 64 and 256 mm
Coarse-grained
» Pebble - between 2 and 64 mm
Medium-grained
» Sand - between 1/16 and 2 mm
Fine-grained
» Silt - between 1/256 and 1/16 mm
Fine-grained
» Mud < 1/256 mm
Microscopic grain size
Physical/Mechanical Weathering
Rocks can be physically weathered by:
• Jointing – Formation of cracks in rocks.
Joints – form in rocks due to, stretching, or cooling (contraction)
• Exfoliation joints – when deep rocks are exposed, the removal of
overburden causes sub-horizontal cracks to form causing the rock to
easily peel away like layers of an onion. Also called sheet joints or
unloading joints and are common in exposed batholiths.
Joints in the Field
• Below: Joints in sedimentary
rocks in Brazil. Near vertical
joints in sedimentary rocks are
common.
• Above: Exfoliation / Sheet /
Unloading joints in the
Sierra Nevada Batholith
(granite) in California.
Physical/Mechanical Weathering
• Frost wedging: water fills cracks,
freezes, expands, and forces
cracks to open causing them to
grow. Can lift large blocks.
• Root wedging: same as frost
wedging except that roots pry
open the cracks.
• Salt wedging: salt crystals form
when evaporating water flows
through rocks. The salt crystals
pry open the cracks
Results of Physical Weathering
• Eventually, mechanical weathering processes create
an apron or pile of debris at the margins of slopes
called talus
Chemical Weathering
Chemical weathering is typically strongest in warm wet climates
Types include:
• Dissolution:
– Primarily affects carbonates and salts…when a chemical reaction breaks down
minerals into new compounds
• E.g. CaCO3 (Calcite) + H2CO3 (carbonic acid)  Ca2+ (aq) + 2HCO-3 (bicarbonate)
• Hydrolysis:
– Water acts to ‘loosen’ chemical bonds to break down minerals. Works faster in
slightly acidic water
• E.g. H2O (acidic)  H+ + OH• E.g. H+ + KAlSi3O8 (K-feldspar)  Al2Si2O5(OH)4 (Kaolinite) + K+ (aq)
• Kaolinite is a clay mineral
• Oxidation:
– When an element loses an electron…commonly when it bonds with oxygen.
• E.g. 4Fe2+ + 3O2  2(Fe3+)2O3
(iron lost an electron and went up in charge)
• Hydration:
– Absorption of water into some minerals (mainly clays) causes them to expand
Dissolution
• Chemical weathering
processes act on
rock/mineral surfaces
– Results in rounding of
edges
Surface Area and Weathering
• All other things being equal, the ratio of
surface area to volume of a material
controls the rate of weathering
Weathering & Roundness
• Weathering tends to round off corners and leave things
more smooth in shape.
– Angular detritus = not very weathered
– Rounded detritus = very weathered
Weathering and Bowen’s Reaction Series
Minerals that form early (high temp) in Bowen’s reaction series are least stable.
Exceptions are calcite and halite, which are highly susceptible to chemical weathering.
Typically, mafic minerals weather by oxidation, felsic minerals weather by hydrolysis,
carbonates weather by dissolution, and oxides don’t weather at all.
Follow’s Bowen’s reaction series!!
Dissolves
Differential Weathering
Differential weathering – a primary control on the shape of our physical
landscape.
Under the same set of climatic conditions, not all minerals and not all rocks will
weather at the same rate.
Some develop more joints and fractures, some undergo faster dissolution,
some remain more-or-less intact.
Weak layers weather more
quickly, leaving behind
more resistant layers.
This process occurs on small
scale – such as an outcrop
of rock
And at a large scale, such as
entire valleys and
mountain ranges.
e.g. El Capitán is the product of
differential erosion
(Show valley and ridge in GoogleEarth)
El Capitán – Guadalupe Mtns (SW Texas)
Soil
• “Soil consists of rock and sediment that has been modified
by physical and chemical interaction with organic material
and rainwater, over time, to produce a substrate that can
support the growth of plants.”
• Soil-forming processes require long periods of time.
• Soil may be easily destroyed by human activities.
• Soil is a crucial natural resource in need of protection.
Soil Thickness
• Weathering breaks rocks down into detritus, which forms regolith
• Regolith: any unconsolidated material that covers bedrock
– Soil is a regolith
• What controls the thickness of soil?
1- Age: The longer a surface has been exposed, the thicker the
soil
Soil Thickness
2- Slope: The steeper the slope the less soil will accumulate
bowl-shaped areas, such as basins will accumulate thick soils
Soil Thickness
3- Bedrock Type: Thick soils will form over bedrock that is
easily weathered, such as carbonates, evaporites, or muds.
The soil composition will partly reflect the composition of the bedorock
Soil Thickness
4- Climate: Thick soils will form in warm wet
climates; deserts will form little or no soil
Providence Canyon, SW Georgia
• Formed due to poor farming techniques in the 1800’s
– Trees and natural land cover was removed for agriculture
– Fast rates of soil erosion created deep gullies
– Up to 150 feet deep
Sedimentary Rock Types
• Detrital / Clastic – Cemented
fragments of pre-existing
rocks.
– E.g. sandstone, mudstone
Arkose
• Biochemical – Rocks made of
cemented shells of organisms
– E.g. coquina, limestone
Coquina
• Chemical – made from
minerals that precipitate out of
water solutions.
– E.g. travertine, various
evaporites
Evaporites
near a
salt lake
Clastic Sedimentary Rocks
• A clastic/detrital rock forms in five stages:
1- Weathering/Erosion
2- Transportation
3- Deposition
4- Burial
5- Lithification and Diagenesis
(compaction+cementation)
Grain size is reduced as sediment is transported
Classifying Clastic Sedimentary Rocks
Clastic/Detrital rocks are classified by:
1. Clast size
2.
3. Angularity and Sphericity
4.
5. Type of cement
Clast composition
Sorting
Common Clastic Rock Types
• You should know most of these from lab, but if not, then
make sure to know the basic characteristics of these types
of clastic rocks.
Biochemical and Organic Sedimentary Rocks
• Biochemical Limestone – A biochemical
or chemical rock made of calcite, which
is made of the remains of shells of
organisms that secrete calcite or aragonite
(a polymorph of CaCO3) shells.
– Fossiliferous limestone – lots of fossils!
– Micrite – made of calcite mud
– Chalk – made of plankton shells
• Biochemical Chert – made of
cryptocrystalline (microscopic crystals)
quartz, formed from the shells of
plankton that sat on the sea floor and
dissolved into a silica rich ooze.
Limestone
Folded Chert Beds
• Organic Rocks
– Coal, black shale, made of organics derived
from plants and animals
• We’ll talk about this stuff more in Chapter 14.
A Coal Seam
Chemical Sedimentary Rocks
• Evaporites – products of the evaporation of water
– Main minerals: gypsum, halite
• Travertine (chemical limestone) – limestone that precipitated out
of groundwater
– E.g. stalactites, stalagmites, also found at gysers & hot springs
• Dolomite – chemical alteration of limestone CaCO3 into dolomite
CaMg(CO3)2.
– Happens when Mg bearing groundwater reacts with limestone.
• Replacement Chert – When chert (SiO2) replaces minerals in a
rock.
– E.g. petrified wood
Travertine at
Mammoth Hot
Springs, MT
Formation of
Evaporites
• Evaporites can form
anywhere where a large
body of water
evaporates
– E.g. the Great Salt Lake,
or the Dead Sea
• Evaporites can also
form when an inland
sea has limited
circulation
– E.g. the Persian Gulf, or
the Mediterranean Sea
Sedimentary Structures
Many sedimentary rocks have features/structures
that help indicate how they formed
• Bed – A single layer in a sedimentary rock with a
definable top and bottom
– Bedding plane – the boundary between two beds;
forms when depositional conditions change
• Strata – A group of several beds
– Bedding/Stratification – The overall arrangement
of strata
• Bioturbation – When burrowing organisms
and/or plant roots destroy bedding
Cross Bedding
• Cross Bedding: Internal laminations within a bed that are not
parallel to the larger bedding orientation. Usually caused by
deposition in a moving current of wind/water.
Note: bedding planes
and laminations are
not always parallel
http://www.ed.ac.uk/
Bedforms
• Cross bedding – When
internal laminations
(thin layers) are not
parallel to the boundary
of the main sedimentary
layer
• Ripple Marks – a type
of cross bedding that
forms due to deposition
in moving water.
Typically < a few cm
high
Asymmetric – one flow direction
Symmetric – two flow directions
Ripple Marks
asymmetric ripple marks in a beach
Symmetric ripple marks in 1.5Ga
quartzite
More Bedforms: Dunes
• Dunes - form and
grow just like
ripple marks
except that they are
deposited by wind
(usually) and can
be many meters
tall
• Since deposition
occurs on the slip
face dunes migrate
with time
• The inclination of
the cross beds can
be used to tell the
direction of wind
Ancient Dunes: Zion N.P.
Turbidity Currents and Turbidites
• Turbidity Current – An underwater moving cloud of water and sediment.
Moves downhill because it is more dense than water. Forms when
sediment on a slope in the ocean floor is disturbed (e.g. earthquake or
storm). Can carry large boulders, snap phone lines, move sunken ships.
• Turbidite – A rock formed by turbidity currents
Turbidites and Graded Beds
• Graded bedding –
when the grain size in
a single bed varies
consistently from
coarse grained at the
base to fine grained
at the top.
– This is a diagnostic
feature of turbidity
currents
Turbidites and Graded Beds
A typical turbidite with graded bedding
Bed Surface Markings
• Mud Cracks – when wet mud dries it contracts and forms
pseudo-hexagonal plates that curl up on their edges
• Scour Marks – when currents flow over a sedimentary
surface, the current may scour out small troughs, which
can be preserved
• Fossils – Evidence of past life including
shells, plant material, and footprints.
Scour Marks
Mud Cracks
Mud Cracks
Rain Spots
Mud Cracks
Formations and Groups
• Formation – A
particularly thick bed or
a sequence of alternating
rock types, sometimes
called a stratigraphic
formation
• Group – A group of
stratigraphic formations
• Sedimentary Basins – A
region where the surface
of the Earth has
subsided (gone down)
and leaves a space for
sediments to fill.
Stratigraphic formations in the Grand Canyon, AZ, note that
the sandstones and limestones form cliffs, whereas the shales
form eroded slopes
Sedimentary/Depositional Environments
• Where do sedimentary
rocks form?
Terrestrial Red Beds
– Marine Environments:
deposited under the ocean
– Terrestrial Environments:
deposited on land
• Red Beds - Exposure to air
can cause oxidation of iron
leading to a red color in
some terrestrial beds.
Marine Limestone
Terrestrial
Environments
• Glacial – mix of everything
boulders-mud. Very poorly
sorted. Deposits: Till
• Mountain Streams – coarse
grained conglomerates and
sandstones. Poorly sorted.
• Alluvial Fans – deposited at the
mountain front by mountain
streams in arid regions. Poor to
moderate sorting. Deposits:
Arkose, Sandstone, Conglomerate
• Sand Dunes – in arid regions,
wind blows sand and silt around.
Well sorted sandstones and
siltstones.
• Lakes – low energy environment
forms laminated lacustrine
deposits of mudstone and shale
Mountain Stream
Alluvial
Fan
Glacial Till
Terrestrial Environments
• Rivers – complex environments that deposit a
variety of sedimentary rocks, from mudstone to
conglomerates. In general, sorting increases
with distance traveled.
Also, the farther
the sediment
has traveled,
the greater percentage
of quartz will be
present. Rivers
deposit fluvial
sedimentary
rocks.
Marine Environments
• Beach – well sorted mature sandstones with well rounded grains,
ripple marks common
• Shallow Marine – well sorted siltstones and mudstones often
with lots of fossils. Can also produce limestones and other
carbonates if enough organisms are present. Carbonate
environments are usually in shallow warm water (reefs).
• Deep Marine – Fine grained mudstones, chalks if enough
plankton shells present. Only non-fine grained deposits are those
of turbidites.
Chalk, Up Close
The Chalk Cliffs, England
Carbonate
Environments
• Tropical carbonate environments
yield a diverse array of
sedimentary rocks.
Transgression
& Regression
• Changes in sea
level are common
throughout geologic
time.
• Linked to
Sedimentation
(Show transgression animation)
• Transgression – when
sea level rises, the coast
line moves inland
• Regression – when sea
level falls, the coast
line moves seaward.
– Not well preserved
due to erosion
• This means that a
regional unit may not
form laterally at the
same time