Sedimentary Rocks

Biblical Reference

When he makes all the altar stones to be like limestone crushed to pieces, no Asherah poles or incense altars will be left standing. Isaiah 27:9

Top 5 Reasons to Study Sedimentary Rocks

5) Sedimentary Rocks are the most common rocks.

4) Sedimentary Rock features help decipher Earth’s history.

3) Fossils in Sedimentary Rocks give knowledge of pre-existent life.

2) Some sedimentary rocks contain resources (water, petroleum, natural gas).

Top 5 (Cont’d)1) Sedimentary Rocks are the most interesting

of the three types of rocks!

Sedimentary RocksHow They are Made

Wind and water break down the earth

Bits of earth settle in lakes and rivers

Layers are formed and build up

Pressure and time turn the layers to rock

Types of Sedimentary Rocks

Gypsum

Sandstone

Shale

Limestone

Conglomerate

Sedimentary Rocks are the principal repository for Information about the Earth’s past environment.

SedimentarySediments – Pieces of solid material that have been deposited on the Earth’s surface by:

– Wind– Water– Ice– Gravity– Precipitation

Weathering• Surface is being worn away• Breaks rocks into smaller pieces• Produces Clastic (broken) rocks• Minerals remain chemically unchanged

Erosion & Transport• 4 Main agents of Erosion

– Wind Gravity– Moving Water Glaciers

• Rocks are then transported to a new location.• Surface material is removed (eroded).

Deposition• When transported sediments are deposited

on the ground or sink to the bottom of a body of water.

• Often these are sorted with larger grains on the bottom and smaller grains on the top.

• Transportation Agents determine the method of sorting.– Example: Sand dunes = fine sand

• Wind too weak to carry large grains

Lithification• Sediments deposited in low areas (valleys

or ocean basins).• As sediment piles up, increased pressure

& temperature lithifies the sediment into Sedimentary Rock.– From the Greek word, lithos, which means

stone.

• Two Stages:– Compaction– Cementation

Compaction• The weight of the overlying sediment forces

the grains closer together.

– Mud goes from 50% - 60% to 10% - 20% water

• Sand grains maintain open spaces, which can trap:– Groundwater– Oil– Natural Gas

Cementation• Mineral growth glues the sediment grains

together into solid rock.

From Sediment to Sedimentary Rock

Environmental Clues• Grain Size• Rippling• Rounding• Sorting• Bedding• Fossils• Color• Chemistry

Grain Size• Power of Transport Medium

– Water• Large Particles – Fast Moving• Small Particles – Slow Moving

– Wind• Can only move small grains (sand

dunes)

– Glaciers• Can carry all sizes equally – Unsorted

particles

– Landslides• Sediment moves downhill in a

jumbled mess

Rippling• Sediment is moved into small ridges by wind

or waves.– Often a sign of flooding– Can be symmetrical or asymmetrical

Rounding• When rocks break apart, the edges are sharp.

– As they bump into each other or encounter weathering, the edges become rounded.

– Harder rocks (quartz sand) round better than softer rocks (carbonate sand).

– Sediment that travels far experiences more rounding.

Sorting• Sediments are “sorted” by grain size

– Indicates:• Energy, Rate and Duration of Deposition• Mode of Transportation

• Well-Sorted– Quartz Sandstones– Shales

• Poorly-Sorted– Conglomerates– Breccia

Rounding & Sorting

well-rounded, poor-sorted gravel angular, poor-sorted gravel

Bedding• Horizontal Layering

– Layers can be between a few millimeters to more than a meter thick.

• Two Types:– Graded Bedding

• Particle Sizes become progressively larger toward the bottom of the layer

– Cross-Bedding• Inclined layers across a horizontal surface

Fossils• Fossils are not Sedimentary Structures

– Remains or traces of ancient organisms• Salt Water - Corals, Echinoderms

– Heavy-Shelled Clams: Shallow, Turbulent Water

– Thin-Shelled Clams: Deep, Calm Water

– Filter-Feeding Organisms: Clear Seawater

• Fresh Water: Insects, Amphibians

• Terrestrial: Leaves, Land Animals

Color & Chemistry• Red Beds - Often Terrestrial • Black Shale - Often Deep Water• Dark or Black – Organic Material• Different layers can have very different colors• Oxygen Poor - Often Deep Water• Evaporites – Arid Climates• Iron-Rich – Red, Brown or Pink

Types of Sedimentary Rocks

Relative Abundance of Sediments

Formation of Sedimentary Rocks

Clastic Rocks• Made of Fragmentary

Material • Deposited by

– Water (Most Common) – Wind – Glacial Action – Gravity

Biochemical Rocks• Evaporation • Precipitation • Biogenic

Sediments

Clastic Sedimentary Rocks• Most Common Sedimentary Rocks

• Greek Word klastos – “broken”

• Made up of broken pieces of pre-existing rocks– Could be Igneous or Metamorphic in origin

• Three types categorized by grain size:– Coarse-Grained– Medium-Grained– Fine-Grained

Coarse-Grained Clastic Rocks• Usually gravel-sized rock & mineral fragments

– Rarely boulder-sized (see below)

• Two Main Types:– Conglomerates– Breccias

Conglomerates• Gravel Transported by High Energy Water

– Mountain Streams– Flooding Rivers– Ocean Waves

• Well-Rounded• Lithification Compacts & Cements these

fragments into Rocks

Conglomerate = rounded gravel (>2 mm)

Breccias• Angular, Gravel-Sized Particles• Particles transported short distances from

their sources– Did not have time to become rounded

Breccia = angular gravel

Medium-Grained Clastic Rocks• Sand-sized sediments in streams, rivers,

deserts and beaches

• Sandstone– Ripple Marks and Cross-Bedding indicate

direction of current flow

– High Porosity – Open spaces allow liquid to travel• Valuable as underground reservoirs of oil, natural gas,

and water

Sandstone• Sand-size (1/16 mm – 2 mm particles)• Classified by Minerology

– Most Common: Quartz Sandstone– Arkose Sandstone = 25% Feldspars

• Feels Rough & Gritty

Fine-Grained Clastic Rocks• Consist of Silt or Clay-sized particles

• Typically found near swamps or ponds– Still or slow-moving water

• Poor Porosity which forms a barrier to the movement of groundwater

• Three Main Types– Siltstone– Claystone– Shale

Fine-Grained Clastic Rocks• Siltstone

– Made from Silt• 1/256 mm to 1/16 mm

– Three times stronger than most bricks

• Claystone– Made from Clay

• < 1/256 mm

• Shale– Made from Fissile Clay– Splits along closely spaced planes

Clastic rocks

Sediment Sizes and Clastic Rock Types

Sedimentary rocks made of silt- and clay-sized particles are collectively called mudrocks, and are the most abundant sedimentary rocks.

Rock Type Sediment

Shale Clay

Siltstone Silt

Sandstone Sand

Conglomerate Gravel

Chemical Sedimentary Rocks• Ions are taken into solution by the parent

weathering material• Evaporation in arid locations leaves chemically

enriched crystals, which solidify into rock• Other conditions for Evaporites:

– Fresh water supply is low– No connect to an open sea

• Usually are light-colored

Common Chemical Rocks

• Dolostone– Magnesium ions bonded to

CaCO3

• Flint– Formed from Silica in Sea Beds

• Rock Gypsum– Sulfate-based– Very soft– Used to construct the Egyptian

pyramids

Bio-Chemical Sedimentary Rocks• Formed from remains of once-living organisms• Many contain fossils

– Evidence of their biological origins– Falls of the Ohio fossil beds are made of

limestone

Common Bio-Chemical Rocks• Limestone

– Predominately CaCO3

– Formed from skeletal fragments of marine invertebrates

• Chert– Silica-based– Layers of microscopic shells

• Coal– Carbon-based– Compressed, altered plant

remains from bogs & swamps

Francis John Turner

A. cementation

B. crystallization

C. compaction

D. evaporation

During which process does the weight from layers of sediment force out fluids and decrease the space between grains?

Pop Quiz

A. chemical

B. biochemical

C. fragmented

D. clastic

Which types of rocks are sedimentary rocks made up of broken pieces of minerals and rock fragments?

Pop Quiz

A. gas

B. water

C. sediment

D. magma

Chemical rocks form when minerals crystallize directly from which of these?

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