Lecture 9: Surface Processes:
chemical and physical weathering and
sedimentary rocks
• Questions
– What is the rock cycle? How do rocks get destroyed
and recycled at the surface of the Earth?
– At the other end of the transport system, how do
weathered and eroded materials end up making the
various kinds of sedimentary rocks?
– What can observations of the sedimentary record reveal
about the tectonics, petrology, and climate of both
depositional environments and upstream source
environments?
• Reading
– Grotzinger and Jordan, Chapters 5, 16, 18, 19
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Weathering and Sedimentation in the Rock Cycle
• Our geology so far has focused on internally-driven processes:
plate tectonics, magmatism, metamorphism, orogeny.
• The rest of geology is
driven by surface
processes: the
hydrologic cycle
(rainfall, streams,
ice), gravity, aqueous
chemistry.
• Weathering and erosion
are the processes that
form and transport form
sediment.
• Sedimentation, burial and
lithification are the
processes that transform
weathering products into
sedimentary rocks.
2
Weathering and Sedimentation in the Rock Cycle
• A more detailed view of the surface-driven parts of the rock cycle
shows the various steps between source rock and sedimentary
product
3
Weathering: decomposition of rocks
• There is a distinction between
weathering and erosion:
– Weathering converts exposed
rock to soil in place
– Erosion transports dissolved
or fragmented material from
the source area where
weathering is occurring to a
depositional environment .
– Most of the earth’s surface is
covered by exposure of
sediment or sedimentary rock,
by area.
– But the sediment layer is thin
in most places, with respect to
overall crustal thickness, so
sedimentary rock is a minor
volume fraction of the crust
(in part by definition: once
buried to the mid-crust,
sediments get cooked to
metasediments).
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Weathering: chemical and physical
• The destruction of rocks at the Earth’s
surface by weathering has two
fundamental modes of operation:
– Chemical weathering is dissolution or
alteration of the original minerals,
usually by reactions with aqueous
solutions
• Chemical weathering puts ions from
the source minerals into solution for
subsequent erosion by transport in
flowing water as dissolved load.
– Physical weathering is fragmentation
into progressively smaller particles,
from intact outcrop to boulders and on
down to mineral fragments and sand
grains.
• Physical weathering makes loose
pieces of rock available for downslope
movement by mass wasting or
transport in flowing water as suspended
or bed load.
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Chemical Weathering
• Chemical weathering is driven by
thermodynamic energy minimization, just
like chemical reactions at high
temperature.
– The system seeks the most stable
assemblage of phases.
– The differences are that (1) kinetics are
slow and metastability is common; (2) the
stable minerals under wet, ambient
conditions are different from those at high
T and P; (3) solubility in water and its
dependence on water chemistry (notably
pH) are major determinants in the stability
of minerals in weathering.
• A fresh rock made of olivine and
pyroxenes will end up as clays and iron
oxides, with other elements in solution
• A fresh rock made of feldspars and quartz
will end up as clays, hydroxides, and
quartz in most waters.
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Chemical Weathering
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Chemical Weathering
• The most common alteration product of feldspars is kaolinite, Al2Si2O5(OH)4,
which serves as a model for the formation of clays by weathering generally.
– The reactions of feldspars to kaolinite illustrate some of the basic trends:
•
•
•
•
K, Na, Ca are highly soluble and readily leached by chemical weathering.
Excess Si can be removed as silicic acid although quartz is relatively insoluble.
Al is extremely insoluble, and is essentially conserved as source rock is converted to clays.
Weathering is a hydration process, leaving H2O bound in the altered minerals.
– 2 KAlSi3O8 + 9 H2O + 2 H+ -> Al2Si2O5(OH)4 + 2 K+ + 4 H4SiO4
• Note the H+ on the left-hand side…only acidic water can drive this reaction
• Natural waters are acidic due to equilibrium of carbonic
acid with CO2 in the atmosphere
– CO2 (g) + H2O = H2CO3
– 2 KAlSi3O8 + 9 H2O + 2 H2CO3 ->
Al2Si2O5(OH)4 + 2 K+ + 4 H4SiO4 + 2HCO3–
– Alteration of rock transforms acidic rainwater into
neutral surface or ground water, with bicarbonate the
dominant species (relative to CO2 and CO32–).
– Mg and Fe2+ are also readily leached, but Fe3+ is very
insoluble…the ultimate residue of alteration of mafic
rocks is hematite.
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Chemical Weathering
• Knowing the chemistry of reaction of minerals to kaolinite, it is possible to
reconstruct from the dissolved ions in stream water the amount of each source
mineral that reacted with the water.
• Questions: How do you do the correction for atmospheric input? Do the source
minerals in the Sierra Nevada all weather at equal rates?
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Chemical Weathering
• Some minerals are congruently soluble in acidic water, leaving no residue
– The most abundant is calcite: CaCO3 + H2CO3 = Ca2+ + 2HCO3– (the Tums reaction)
– Effects of dissolution (and precipitation) of calcite can be dramatic, to say the least.
Sinkhole
Karst terrain
Speleothems
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Rates of Chemical
Weathering
• Many factors affect the
rate at which a rock will
weather, as summarized
here.
• Some of these variables are
local (e.g., source rock),
some are global. These
include temperature and
pCO2, leading to the CO2weathering feedback cycle.
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Physical Weathering
• Anything that promotes disaggregration of a rock so that pieces can form soil or be
eroded away by wind, water, or gravity transport is physical weathering.
– The distinction between physical weathering and erosion is subtle, but think of physical
weathering as fragmenting the rock and erosion as carrying the fragments away; at
times these may be the same event, of course.
• Rocks that are jointed or faulted or have pre-existing weak zones are most easily
weathered.
– Few of the stresses associated with physical weathering are significant compared to the
tensile strength of intact rocks; something, has to start the process, either initial cracks
and weaknesses or chemical attack on mineral cohesion.
• Organisms, especially plants (think tree roots), are fond of breaking up rocks.
• Freeze-thaw, frost wedging, frost heave…the volume change between ice and
water is effective in widening cracks in rock in suitable climates.
• Physical abrasion by flowing air or water, or more often by rock particles already
mobilized by water or wind (think Fossil Falls).
• Tectonics…rocks caught in a fault zone are definitely undergoing physical
weathering.
• Etc.
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Weathering feedbacks: chemical and physical
• Physical weathering and
chemical weathering
generally proceed in
parallel in most
environments.
• Physical and chemical
weathering promote one
another:
– Formation of cracks by
physical weathering
increases reactive surface
area, promoting chemical
weathering.
– Chemical weathering
replaces intact
interlocking minerals
with weak clays or void
space, making the rock
easier to physically
disaggregate, promoting
physical weathering
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Weathering feedbacks:
more generally
• Weathering of both kinds plays
key roles in several feedbacks.
• Tectonics affects weathering
through slopes and elevations,
climate affects weathering
through temperatures (via
chemical kinetics and freezethaw), rainfall, pCO2, etc.
• Conversely, weathering and
erosion affect tectonics and
climate:
– Denudation by erosion must be
isostatically compensated and
so affect vertical motions of the
crust…
– Weathering controls water
chemistry, courses of streams
and groundwater, removes CO2
from the atmosphere, etc.
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Soil formation
• Chemically and physically weathered rock that is not
eroded or transported but remains in place becomes soil.
• A weathered surface develops a
stratified structure, with intact rock
at the bottom (or inside) and
maximum weathering at the top .
• Leachable ions are transported
downwards by groundwater flow,
possibly redeposited as water
chemistry adjusts towards
equilibrium with the developing
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soil profile.
Soil formation
• The mineralogy and thickness of soil layers depends on
source rock, climate (temperature and rainfall), and age.
• Which of these soil types would you rather farm?
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Erosion and Transport
• Between weathering and sedimentation, matter must be
transported from source to destination. This is erosion.
– We dealt with the landforms generated by erosion in the
geomorphology lecture; here our concern is with the effects of
transport on sedimentary rocks.
• Modes of transport:
– Gravity (short distances and steep slopes)
– Wind (small particles only)
– Glaciers
– Water
• Surface runoff carries dissolved, suspended, and bed loads
• Groundwater flow only carries dissolved load
– All these mechanisms carry products of physical
weathering and insoluble residues of chemical
weathering.
– Only water transport carries away leached soluble
products of chemical weathering.
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Erosion and Transport
• Certain modes of transport physically modify and
physically and chemically sort particles en route.
• Size sorting by surface water runoff flow:
Current of a given
velocity can generally
carry all noncohesive
particles smaller than a
critical size; since
current velocity drops
with decreasing slopes
from mountains to
lowlands, it follows that
sediments evolve from
poorly sorted and
coarse-grained near
source to well-sorted
and finer grained with
increasing transport
distance.
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Erosion and Transport
• Chemical sorting with
increasing transport
distance is like a
continuation of
chemical weathering:
most stable minerals are
transported the farthest.
• Textures of particles are
modified by abrasion
during wind or water
transport. Close to
source particles are
angular; far from
source particles are
rounded.
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Sedimentation
• Eventually transported particles and dissolved ions reach a place where they can
be permanently deposited and accumulated. This is sedimentation.
• The sedimentary rocks that result from this accumulation are controlled by and
record the sedimentary environment where they were deposited.
– We interpret ancient sedimentary rocks by comparison to modern
environments where we can observe ongoing sedimentary processes and
relate them to the composition, texture, and structure of the resulting rocks.
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Sedimentation
• Sediments and the
environments in which they
form are fundamentally divided
into clastic and chemical:
– Clastic sediments are made of
physically transported and
deposited particles (they may
later gain chemically grown
cement during diagenesis)
– Chemical sediments are grown
from solution, organically or
inorganically; biochemical
sediment more specifically
refers to minerals grown from
solution by organisms
• In some cases the relationship
between the environment and
the character of the sediment is
absolute and obvious (carbonate
in reefs, boulder-strewn till in
periglacial deposit, etc.); other
cases are more subtle.
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Diagenesis
• The process of modification of
newly deposited sediments into
sedimentary rocks is diagenesis or
lithification.
– Processes include:
• physical compaction by the pressure of
overburden, accompanied by expulsion of
pore waters
• Growth of new diagenetic minerals and
continued growth of chemical sediments
from pore waters.
• Dissolution of soluble elements of clastic
rocks.
• Recrystallization and remineralization as
water chemistry, pressure, and
temperature evolve.
• At the high-T and P end, diagenesis
merges smoothly into the low-T and P
end of metamorphism. The distinction is
arbitrary.
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Sedimentary Rocks
• The preserved end-result of weathering, erosion, transport,
sedimentation, and diagenesis is sedimentary rocks.
– Like sediments and sedimentary environments, the resulting rocks are
divided into clastic (or siliciclastic or volcaniclastic, etc.) and chemical (or
biochemical).
• Clastic rocks are classified by particle size (and sorting) and
composition.
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Sedimentary Rocks
• Chemical sediments are primarily classified, of course, by
mineralogical composition.
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Sedimentary rocks and environmental information
• How do sedimentary rocks preserve information about their
depositional environments?
– By composition, mineralogy and grain size, obviously, but also
through sedimentary structure
• Elements of sedimentary structure:
– Bedding
• Bed thickness, from finely laminated to massive
Burgess Shale:
fine
Vasquez formation: massive
30 m
30 cm
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Sedimentary structure
• Character of bedding, from simple horizontal laminae to cross-bedding,
ripples, soft-sediment deformation, or bioturbated.
• Cross-bedding indicates high and
unidirectional current velocity, often
winds in terrestrial settings, forming
sand dune lee-slopes.
• Ripple marks record back-and-forth action by waves in shallow water.
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Sedimentary Structure
• Mud cracks demonstrate
drying-out of a thin layer of
sediment fine enough to
have significant cohesion.
Definite proof of terrestrial
setting or very shallow water
marginal marine.
MODERN
ANCIENT
• What about this structure?
(Hint: it is not the surface of
the Moon)
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Sedimentary Structure
• Soft-sediment deformation indicates slumping or compression of layers
before complete lithification.
• Bioturbation is the vertical
mixing of sedimentary layers
by burrowing organisms.
Evidence of such activity
can be preserved on bedding
surfaces as trace fossils.
Indicative of water depth,
availability of nutrients and
oxygen, etc.
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Sedimentary Structure
• Graded Bedding: sorting of particle sizes within beds
indicates time dependence and hence process of deposition
– An environment in which a episodes of high-energy transport give
way to periods of low-energy transport gives normal graded beds:
– Alluvial settings, with wandering
channels that fill up and become
overbank deposits
– Continental slopes with turbidity
currents
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Carbonate Rocks
• Most carbonate rocks are entirely
biochemical sediment, made up of the body
parts of calcite or aragonite-precipitating
organisms
– Deep-sea carbonate ooze is made of foram shells
– Reef carbonates are made of coral reefs (usually)
– Stromatolites are formed by carbonate
precipitation by microorganisms
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Tour of sedimentary environments
Let us go through each of the major categories of sedimentary
environment, keeping in mind the relationship between
observable processes in modern settings and the preserved
features in ancient examples, and the ways in which
observation of a sedimentary rock formation can be used to
infer the type of setting and detailed information about it.
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Sedimentary environments: Terrestrial
I. Fluvial (rivers and streams of all kinds and sizes)
a. Alluvial Fans
We saw alluvial fans on the field trip. They form where drainages exit
mountain fronts onto surrounding lowlands.
Individual fans may merge to form a piedmont slope (like Pasadena).
In arid regions like California,
sediment transport on
alluvial fans is dominated
by debris flows like
mudslides and landslides,
and by periodic stream
flows that divide the fan
into channel and overbank
deposits.
Sorting is poor, but increases
downstream; grain size
decreases downstream;
sediments are often
oxidized and poor in fossils
or organic matter.
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Sedimentary environments: Terrestrial
I. Fluvial
b. River systems
Rivers are classified into meandering or
braided, most often.
Braiding is favored by high sediment
load, steep gradients, variable stream
flow, and unstable poorly vegetated
banks.
Meandering is favored by the opposite.
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Sedimentary environments: Terrestrial
I. Fluvial
b. River systems
Meandering rivers develop in a fairly
regular pattern by channel migration,
leaving a predictable sequence of
cyclic, fining-upward sedimentary
deposits. Braided river deposits are
more chaotic leave somewhat random
deposits, since channels wander
randomly across the floodplain.
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Sedimentary environments: Terrestrial
• II. Desert environment
• Deserts basins are basically alluvial
fans, playas, and sand dunes. They
may be dominated by wind transport
or by fluvial transport restricted to
rare, seasonal storms and floods
• Alluvial fans are debris flow and
stream flow deposits (as above).
• Playas are dry or seasonal lake beds dominated by
evaporites or fine-grained and finely laminated
mudstones and siltstones.
• Sand dunes leave fascinating cross-bedded to massive
sandstone deposits.
• Sustained
deposition of
wind-blown dust
makes thick
deposits of loess.
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Sedimentary environments: Terrestrial
• III. Lacustrine (i.e., lakes)
• Lakes are special, compared to rivers and oceans, in several ways:
– Small size (no large waves), absence of tides, and low currents makes lakes very
low-energy sedimentary environments. Coarse sediments are limited to their
margins.
– Lakes generally keep all sediment that arrives from a large drainage area, so
sedimentation rates are high, often ten times higher than marine settings.
– Open lakes (with inlet and outlet streams) are usually fresh-water and generate
only clastic sediments. Closed basin lakes become saline and lead to chemicaldominated sedimentation. Many lake deposits show cyclic alternations between
closed and open conditions.
Annual variations in sediment supply
(especially if the lake freezes over
each winter) are often preserved in
low-energy lacustrine depositional
environments as countable annual
layers or varves.
Varves
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Sedimentary environments: Terrestrial
• IV. Glacial and peri-glacial
• We saw some of the typical valley glacier deposits on the field trip.
But there is more to the glacial environment than moraines and tills.
– Glaciers generate characteristic river deposits (frequently braided) and lake
deposits (frequently varved) when they terminate on land, and characteristic
marine deposits when they terminate in the ocean (dropstones). They move large
boulders, but they also generate huge amounts of very fine rock flour that ends
up as mud or loess.
Periglacial deposits, like
most sedimentary
sequences, have several
facies: a basal till
deposited in front of the
glacier is overlain by
moraines, lake sediments,
glacio-fluvial deposits,
and finally loess.
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Sedimentary environments: Marginal Marine
• I. Deltaic environment: Deltas form wherever rivers empty into
oceans or lakes. Much of the clastic load carried to the mouth of the
river is deposited in a restricted area at or near the coast, forming a
delta.
– Because deltas prograde outwards, they build deposits with reverse grading,
coarsening upwards as the delta moves past a given location.
– The forces affecting sedimentation in a delta are fluvial, tidal, and waves, and
different deltas display effects of dominance by different forces.
The Mississippi delta is
fluvial-dominated:
Both tides and waves are
weak in the Gulf of
Mexico, so distribution of
sediment is dominated by
the river itself, which forms
long, relatively stable
channels (life span ~1000
years) with levees; each
channel narrows upwards
until it pinches off.
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Sedimentary environments: Marginal Marine
• I. Deltaic environment
• Flow at the mouth of a fluvial-dominated delta is controlled by the relative density
of river outflow and ambient sea-water. Depending on river sediment load and
temperature (and on ocean salinity and temperature), the flow may be hyperpycnal
(river outflow denser), or hypopycnal (river outflow less dense).
– Hyperpycnal flow leads to turbidite deposits from sediment-rich flows along the
bottom. Hypopycnal flow leads to uniform, well-sorted sediments since in this
case settling is controlled by flocculation of fine particles.
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Sedimentary environments:
Marginal Marine
• The Ganges-Brahmaputra
delta is tide-dominated
– Although the river outflow is
higher and more sediment-laden
than the Mississippi, the tidal
range is large (about 4 meters).
This type of delta breaks up
into sand bars and channels
oriented parallel to the tidal
inflow-outflow direction. There
is a large, intermittently
exposed, tidal flat.
• The Sao Francisco river in Brazil
is wave-dominated
– Wave-energy here is 100 times that
at the Mississippi. Sediments
reaching the mouth of the river are
rapidly reworked and redistributed
by longshore currents to build
beaches, barriers, and lagoons,
similar to stretches of coast where no
river is present.
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Sedimentary environments: Marginal Marine
II. Beach-barrier
environment
Any continental margin where
there is not a river mouth is
likely to form a beach with a
single shoreface or a beachbarrier island-lagoon system
• A beach produces a
distinctively ordered set of
recognizable facies, from
dune sands through the
surf zone, breaker zone
and into deeper water.
• A barrier complex has a
lagoon and often a swamp
deposit behind the barrier.
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II. Beach-barrier
environment
Sedimentary environments: Marginal Marine
If a simple beach is prograding,
i.e. building out to sea and
depositing near-shore facies
on top of distal facies, it
might produce a stratigraphic
column like this, coarsening
upwards and hence clearly
distinct from any river
floodplain or continental
slope deposit.
Keep in mind the relationship
between the lateral
succession of environments
at any constant time across a
beach and the vertical
succession of sediments
shown in a column like this
one.
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Sedimentary environments: Marginal Marine
III. Estuarine environment
• An estuary is a partly enclosed body of water at the mouth of a river.
It may be part of a delta; it may be the lagoon behind a barrier-island. Generally,
estuaries must have a connection to the open ocean at least at high tide. They are
environments of mixing between seawater and freshwater. Example: San Francisco
Bay
IV. Tidal flats
• A wide, flat area of land between low-tide level and high-tide level is
a tidal flat. These are common environments for deposition of carbonates and
evaporites. They may be associated with deltas, beaches, or estuaries
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Sedimentary environments: Marine
I. Neritic environments
• This term refers to depths below wave-base and low tide, and above
the shelf-slope break.
– At times of sea level highstand, when shallow seas cover the continental
platforms, the neritic environment may encompass a significant fraction of the
earth’s area.
– The neritic environment is where carbonate reefs are built.
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II. Oceanic environments
Sedimentary environments: Marine
• Continental slope deposits are characterized by turbidites, cyclic fining-
upward sedimentary sequences that form by turbidity flows of suspended sediment
down the moderately steep slopes of the continental slope.
• Deep sea (abyssal) deposits
There is a clear
regional pattern
with areas
dominated by
chemical sediment
(carbonate ooze or
siliceous ooze) or
by a very slow
accumulation of
fine clastic
particles (pelagic
clay).
We will develop
the ocean
chemistry and
geology to
understand this
pattern...
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