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SEDIMENTARY

ROCKS

Processes, Environments, Structures and

Rocks

Sedimentary Processes and Rocks

Origins of Sedimentary Rocks

Sediment transport and texture

Sedimentary structures

Lithification

Classifying Sedimentary Rocks

Detrital/Clastic sedimentary rocks

Chemical/Biogenic sedimentary rocks

Interpreting Sedimentary Rocks

Depositional environments

Sedimentary facies

Sedimentary rocks and Plate Tectonics

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Most sediments are

deposited in the ocean

basins but they are also

deposited in other

environments such as

lakes, streams, desert

basins, etc.

Sediment is loose,

unconsolidated material

and is transformed into

sedimentary rock

through a process

known as lithification

(cementation and

compaction).

Sedimentary rocks

Preserve evidence of the surface processes

responsible for deposition

Contain fossils – the only record of past life

Can be interpreted for depositional

environment

Can be used to interpret past Plate Tectonic

History

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Sedimentary Rocks

Mechanical and chemical weathering produce the raw materials for

sedimentary rocks.

Sedimentary rocks only comprise ~5% of the Earth's crust — however,

they are much more important than this percentage would imply.

Sedimentary rocks occur in the portion of Earth where we live.

Important source of resources:

energy (coal, petroleum)

water (storage of groundwater)

minerals (gold, aluminum, lead/zinc,etc)

Origins of Sediment

Weathering results in two types of products:

1. solid particles (sand grains, clay minerals, etc.)

2. dissolved constituents in water

This results in two fundamentally different types of sedimentary rocks:

1. detrital (clastic) sedimentary rocks - formed from transported solid

particles (detritus or clasts).

2. chemical sedimentary rocks - formed by the precipitation of

dissolved substances by either inorganic or biologic processes

river rocks salt flat

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Detrital Sediment

Quartz and clay minerals are the dominant constituents of detrital

sediment.

quartz - very resistant to weathering.

clay - product of chemical weathering of silicate minerals - very

stable at Earth’s surface.

Feldspars, micas and other silicate minerals may occur in sediment that

has not traveled far.

Texture of Detrital Sediment

Sedimentary rocks are classified partly on

the basis of Texture :particle size and

shape.

Sizes of particles are related to mode of

transport and the environment of

deposition:

• Gravels are found in high energy

environments such as mountain streams

and some beaches.

• Sands are found in intermediate energy

environments such as beaches and river

deposits.

• Clays are found in quiet environments

such as lakes and the deep ocean.

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Transport and Texture

Type of transport, e.g., waves, stream flow, wind

(capacity, energy)

Distance traveled – angular or rounded (more or less

abrasion)

Tendency of medium to sort (well or poorly sorted)

Composition of original weathered rock (soft or hard)

Changes in the energy of medium in different

environments (fast/high energy to slow/low energy)

Sedimentary Rock Properties

Sedimentary Structures

Compaction and Cement (lithification)

Texture

Composition

Rock geometry (Sedimentary Facies)

Fossils

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Sedimentary Structures

Laminations and Bedding

Graded Bedding

Cross-Bedding

Ripple Marks

Mudcracks

Soft-sediment deformation

Biogenic structures

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Sedimentary structures

Laminations and Beds

Beds, Outcrop and Formations Death Valley

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Sedimentary Structures

Beds

Examining Recent Undersea Fan Deposits

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Sedimentary Structures Graded Bedding

Sedimentary Structures

Cross-bedding

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Dune Crossbeds

Valley of Fire State Park,

Nevada

Sedimentary Structures

Ripple Marks – Wind or water currents

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Sedimentary Structures

Mudcracks –Alternate wetting and drying

Mudcracks and Ripple Marks in a Vertical Wall

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Sedimentary Structures

Trace fossils and bioturbation

LITHIFICATION

Diagenesis

Compaction

Cementation

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CLASSIFICATION AND

IDENTIFICATION

Texture (Grain Size, Crystal

size)

Composition (Mineral

Composition)

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Detrital Sedimentary Rocks - Conglomerate and Breccia

These rocks consist of dominantly gravel-sized particles.

Conglomerate - composed of rounded particles

indicating some degree of transport (e.g., mountain

stream, beaches).

Breccia - composed of angular fragments

indicating that they did not travel far from their

source (e.g., landslide, alluvial fan).

Gravel Bar to Conglomerate

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From Alluvial Fans to Breccia

Conglomerate and Sandstone in

Outcrop

Ancient Stream Deposits, Split

Mountain, California

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Detrital Sedimentary Rocks — Sandstone

Arenite (Quartz Sandstone) is composed of

sand-sized quartz particles. Quartz is the

dominant mineral in sandstone - due to its

durability at the Earth’s surface. A well-

sorted pure quartz sandstone with rounded

grains indicates that it may be the result of a

great deal of transport - maybe over several

cycles.

Arkose is a type of sandstone that contains

considerable amounts of feldspar. It

probably was deposited close to the source

of sediment. Likely source was weathering

of granitic rocks.

Detrital Sedimentary Rocks – Sandstone (cont.)

Greywacke is a sandstone composed of

sand sized grains of quartz and lithic

fragments, usually of volcanic rock, with

a matrix of clay. It was probably

deposited on an active marine margin as

a part of a deep sea fan or turbidite

sequence. Likely source of sediment is

volcanic mountains.

photomicrograph

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Air Photo of Underwater Sand Patterns

Gulf of California

Sandstone in Outcrop, Domengine Formation, California

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Sandstone in Outcrop, Coldwater Sandstone Formation,

Santa Ynez Mountain Range, Santa Barbara, CA

Detrital Sedimentary Rocks - Siltstone and Shale

• Mudstone is a general term for fine-grained

sedimentary rocks, usually without fine layering.

• Siltstone is composed of predominantly silt-sized

particles and is often difficult to distinguish from

shale.

• Shale is a sedimentary rock consisting of silt- and

clay-sized particles that commonly consists of

fine layers of clay minerals and can split into thin

layers

• Shale and siltstone commonly contain fossils of

plant remains and other organisms.

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Shale is formed from silt and clay deposited in relatively still

waters of lakes and the ocean.

Flathead Lake, Montana Russian River entering Pacific

Sandstone and Shale

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Silty Shale in Outcrop

Ancient Playa Lake

Bed Shales

Zabriske Point

Death Valley National Park

California

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Ancient Lake Bed Mudstone and Shale, Bryce Canyon Natl. Park

Chemical Sedimentary Rocks

— derived from material that is carried in solution to lakes and seas.

Precipitation from water occurs in two ways:

1. Inorganic processes such as evaporation can produce chemical

sediments. (ex. evaporites, dripstone, travertine, etc.)

2. Biologic processes (forming shells) results in chemical sediment.

Organisms die and collect on the seafloor (ex. coral, chalk, etc.).

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There is a wide variety of

chemical sedimentary rocks.

Note that there is a mineral

composition that is characteristic

for each type of chemical

sedimentary rock.

Chemical Sedimentary Rocks – Inorganic

Precipitates and Evaporites

Inorganic Limestone composed of calcite

precipitated from water without the influence of

living organisms, e.g., lithographic limestone,

travertine

Rock Salt and Rock Gypsum – composed of Halite

and Gypsum crystalized during evaporation of

water in arid conditions.

Travertine

Rock salt

Rock gypsum

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Bedded Limestone, Arkansas

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Rock Salt, an Evaporite

Owen’s Lake, California

Interior of Salt Mine

Chemical Sedimentary Rocks –

Inorganic Precipitates and Evaporites

Dolostone – similar to calcitic

limestone but made of calcium

magnesium carbonate

Inorganic chert –

microcrystaline quartz

precipitated from

supersaturated solutions

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Chemical Sedimentary Rocks –

Chert Chert is a chemical sedimentary rock

composed of microcrystalline silica (SiO2).

chert nodules (flint) are found especially in

limestones (chalk) - inorganic precipitation of

biogenic silica

Flint Nodules on a

Beach in Southern

England

dark gray due to

inclusions of organic

matter

Chemical-Biogenic

Sedimentary Rocks —

Biogenic Limestone

Coquina - composed of poorly cemented

coarse shell fragments

Chalk - made up of the shells of microscopic

marine organisms (foraminifera, calcareous

nannoplankton)

Chalk cliffs of Normandy

Foraminifera

coquina

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Source of Coquina

Massive Biogenic Limestone, Carlsbad

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Layered or bedded cherts are derived

from accumulation of silica-rich shells of

radiolaria (microscopic sea critters).

Chemical Sedimentary Rocks – Biogenic Chert

radiolaria

Coal

— very different

from other rocks

because it is

composed of organic

matter.

Stages in coal

formation:

1. accumulation of

plant remains in

swamp.

2. material does not

decay because of

lack of oxygen

(reducing

environment).

3. as material is

buried by

successive layers it

begins to transform

under increased

temperature and

pressure

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peat — soft brown

material

(recognizable plant

matter)

lignite — soft

brown coal

bituminous coal —

common low-grade

coal (90% reduction

in volume)

anthracite — rare

high grade coal

(metamorphic?)

Coal in Outcrop

Lignite beds in Arkansas Bituminous Coal, Colorado

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Interpreting Sedimentary Rocks

Depositional Environments

Sedimentary Facies

Sedimentary Rocks and Plate Tectonics

Depositional Environments

Continental Rivers and streams

Lakes

Deserts

Caves

Glaciers

Transitional Coastal deposits

Turbidity Currents

Marine Shallow Marine (Shelf)

Deep Marine

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Depositional Environments

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Depositional Environments

Continental – Fluvial (Streams and Rivers)

Ancient Stream Deposits, Split

Mountain, California

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Continental Environments

Desert – Sand dunes

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Continental Environment – Desert Dunes

Dune sandstone

Continental Environments - Desert

Alluvial fan

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From Alluvial Fans to Breccia

Depositional Environments

Glacial

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Glacial Till

Glacial till consists of rounded to angular fragments that are not

sorted by size and not laid down in horizontal beds.

South African Tillite, source where Indian Ocean is now

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Glacial Lakes - Varves

Transitional Environments

Where the land meets the sea

Deltas

Continental margins

Tidal flats

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Transitional Environments - Deltas

Colorado River Delta

Transitional Environments - Delta

Flathead Lake, Montana

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Sediments in Deltas

Delta topset and Foreset beds

Glacial Lake Deposit

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Barrier Islands

Barrier Island

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Depositional Environments

Marine

Detrital Marine Environments

Carbonate Marine Environments

Evaporite Environments

Marine depositional environments

Detrital Marine Environments

Continental Shelf

Continental Slope

Continental rise

Deep Ocean

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Detrital Marine Environments

Turbidites on Continental Slope, Rise

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Deep Marine Environment

Many forms of Planktonic life fill the deep marine sediments.

Distribution of deep marine sediments

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Lateral Relationships – Sedimentary

Rocks

Principle of Lateral Continuity – Sedimentary rock

layers extend outward in all directions until they

terminate.

Terminations

Edge of depositional basin

Later erosion of portion of basin

Truncation by faults

Pinching out

Lateral gradation

Sedimentary Facies

Different adjacent environments produce

different rocks and fossil assemblages

Marine Transgressions and Regressions

Transgression – rise in sea level or sinking of

land

Regression- lowering of sea level or rise of

land

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Walther’s Law

The facies seen in a conformable vertical

sequence will also replace one another

laterally

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Transgression / Regression

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Reasons for Sea Level Change

Amount of water frozen in glaciers

Rates of Sea Floor Spreading

Rapid spreading, high heat flow causes ridges to rise and displace water onto the continents causing universal transgression

Slower spreading, lower heat flow, causes ridges to subside, increased volume of ocean basins causes universal regression.

Sedimentary Rocks and Plate

Boundaries

Convergent-Subduction

Greywacke sandstone

Turbidite conglomerates

Limestone

Shale

Convergent-Collision/ Transform Continental/ Divergent Continental

Arkose sandstone

Continental conglomerates

Evaporites

Fluvial shales

Divergent Marine

Chert

Limestone

Shale

Passive Margins

Quartz sandstone

Shale

Limestone