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?

• 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 following applies to Clastic/Detrital rocks; biochemical and chemical sed rocks are different

Sedimentary rocks

Metamorphic rocks

Plutons

Desert

Playa lake

Delta

Glacier

The Sedimentary Stages of the Rock Cycle

Weathering breaks down rocks.

Erosion carries away particles.

Transportation moves particles downhill.

Deposition occurs when particles settle out or precipitate.

Diagenesis lithifies the sediment to make sedimentary rocks.

Burial occurs as layers of sediment accumulate.

• Physical weathering breaks rocks into chunks called detritus

• Detritus is classified by size (diameter)

» Boulder > 256 mm

» Cobble - between 64 and 256 mm

» Pebble - between 2 and 64 mm

» Sand - between 1/16 and 2 mm

» Silt - between 1/256 and 1/16 mm

» Mud < 1/256 mm

Detritus & Grain Size

Fine-grained

Coarse-grained

Medium-grained

Fine-grained Microscopic grain size

Visible 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.

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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

• Biochemical – Rocks made of cemented shells of organisms

– E.g. coquina, limestone

• Chemical – made from minerals that precipitate out of water solutions.

– E.g. travertine, various evaporites

Arkose

Coquina

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. Clast composition

3. Angularity and Sphericity 4. Sorting

5. Type of cement

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.

• Organic Rocks – Coal, black shale, made of organics derived

from plants and animals • We’ll talk about this stuff more in Chapter 14.

Limestone

Folded Chert Beds

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.

http://www.ed.ac.uk/

Note: bedding planes

and laminations are

not always parallel

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

Symmetric ripple marks in 1.5Ga

quartzite

asymmetric ripple marks in a beach

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?

– 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 Red Beds

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 Glacial Till

Alluvial

Fan

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.

The Chalk Cliffs, England

Chalk, Up Close

Carbonate

Environments

• Tropical carbonate environments

yield a diverse array of

sedimentary rocks.

Transgression

& Regression

• 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

(Show transgression animation)

• Changes in sea

level are common

throughout geologic

time.

• Linked to

Sedimentation