Sedimentary RocksSedimentary Rocks

Geology 200Geology for Environmental ScientistsGeology for Environmental Scientists

Major Concepts

• Sedimentary rocks form by the processes of weathering, erosion, transportation, and g, , p ,deposition, or by chemical precipitation.

• Two main typesTwo main types– clastic: pieces of older rocks (clasts)

h i l i i d f d i l– chemical: precipitated or formed in place• Sedimentary structures are critical to

interpreting sedimentary rocks.

The Rock CycleThe Rock Cycle

Major Concepts• Major sedimentary environments:• Major sedimentary environments:

– fluvial and alluvialli– eolian

– glacial– delta– shoreline– organic reefs– shallow marine– deep marine

Major Concepts

Sedimentary rock layers are hierarchically grouped or subdividedhierarchically grouped or subdivided into both:

k i hi i– rock stratigraphic units– time stratigraphic unitsg pWe’ll cover this in the section on

t ti hi l tistratigraphic correlation.

Clastic Sedimentary Rocks

• Produced by weathering of rocks.• Breccia large angular grains• Breccia - large, angular grains• Conglomerate - large, rounded grains• Sandstone - sand sized grains• Siltstone silt sized grains• Siltstone - silt sized grains• Shale and Mudstone - silt and clay

sized grains

Grain Size

• Grain size can be an indicator of the energy of the environmentenergy of the environment.

• Generally speaking, higher energy i d i dwater or wind currents are required to

move larger grain sizes.

Grain Size cont’dGrain Size, cont dSize ranges: small to largeS e a ges: s a o a ge• boulders, >256 mm (>1 ft.)

bbl >64 (>3 i )• cobbles, >64mm (>3 in.)• pebbles, >4mm• granules, 2-4mm• sand 1/8-2 mmsand, 1/8 2 mm• silt, >1/256 mm

l 1/2 6• clay, <1/256 mm

Fig. 5.4. Grain size ranges for classification ofcommon clastic sedimentary rocks

Grain SortingGrain Sorting• Grain sorting can also be an indicatorGrain sorting can also be an indicator

of the energy of the environment.W ll d di d i d i• Well-sorted sediments are deposited in high energy environments. Currents sort the grains by size.

• Poorly-sorted sediments may indicatePoorly sorted sediments may indicate weak currents, or transport by glaciers.

Well sorted sand grains and an outcrop of g pwell-sorted sandstone.

Poorly sorted glacial deposit ranging from cobbles to mud-sized particlescobbles to mud-sized particles.

ConglomerateConglomerate

Conglomerate deposited as an alluvial f C t f Ut hfan, Cretaceous of Utah

SandstoneSandstone

Sh lShale

Chemical Sedimentary RocksChemical Sedimentary Rocks• Produced by chemical precipitation.• Evaporites - formed by evaporation of

seawaterseawater–Salt, NaCl–Gypsum, CaSO4

• Carbonates• Carbonates–Limestone, CaCO3

–Dolostone, CaMg(CO3)2

Gypsumyp

Limestone

F ilifFossiliferous Limestone

Carbonates• Typically, carbonates form in warm,

clear water free of clastic sediment.• Carbonate grainstone composed of• Carbonate grainstone - composed of

sand-sized grains from invertebrate k l t litskeletons or oolites.

• Carbonate mudstone - clay and silt-ysized grains from pellets and calcareous algae.calcareous algae.

Carbonate Grainstone:

OoliticOolitic Limestone

Oolite Shoals

Carbonate Mudstone: Micritic Limestone

Carbonate Mud Banks in Florida Bay

Investigating Carbonate Mud Banks in Fl id BFlorida Bay

Investigating Carbonate Mud Banks in Fl id BFlorida Bay

Coal

• Classified as a chemical rock by most textbooks But it is not a chemicaltextbooks. But it is not a chemical precipitate. Instead it forms from plant debris not consumed by bacteria anddebris not consumed by bacteria and other organisms. It is an organic rock.

• Takes about 10 ft. of peat to form 1 ft. of bituminous coal.o b u ous co .

Types of coal:

peat

lignite

bituminous

th itanthracite

Upper Freeport Coal, Preston County, WVUpper Freeport Coal, Preston County, WV

Examples of SedimentaryExamples of Sedimentary Structures

Sedimentary Structures

• Provide clues to depositional environments Some examples:environments. Some examples:

• Cross bedding - rivers, dunes, tidal h lchannels

• Graded bedding - storms and turbiditesg• Ripple marks - lower energy• Mud cracks - subaerial exposure

Bedding planes are primary sedimentary structuressedimentary structures.

Modern Sedimentary Structures: Mudcracks on a flood plainMudcracks on a flood plain.

Ancient Sedimentary Structures: mudcracks in rocks 1 billion years old.y

Modern Sedimentary Structures: Current ripples on a modern beachripples on a modern beach.

Beach in Ireland at low tide

Sedimentary Structures: Current ripples on an ancient beach, 400 MY old.,

The processes by which symmetrical p y yand current ripples are formed.

Geometry of sand dunes, both eolian and aquatic.

TOP

BOTTOM

Fig. 5.9. Large scale cross-bedding formed by eolian dune migration. Flow direction and top of bed indicated.

Wind blown dune deposits.p

Mississippian cross beds near Altoona, PA

Mississippian ppcross beds near Altoona, PA

Jurassic Navaho Sandstone, Zion National Park, wind-blown cross-beddingwind-blown cross-bedding.

Marine Cross Beds in Ste. Genevieve Ls., Indiana

Tidal Cross Beds in Ste. Genevieve Ls., Indiana

Smaller cross beds deposited in aSmaller cross beds deposited in a river or stream.

A meandering stream with a sandy point bar on the inside curve, and a cut bank on the

outside curve. The point bar dips toward the p pstream channel.

St Ch lStream Channel

U d d d Fl id KUnderwater sand dunes, Florida Keys

Graded Bedding –formed byformed by storms or turbidite depositiondeposition

Another example of a graded bed

Sedimentary Environments or Environments of Deposition

Color of Sedimentary Rocksy• Color can be useful in the

interpretation of depositional environments.

• Black color - indicates deposition in the absence of oxygen in either thethe absence of oxygen in either the ocean, lakes, or swamps.

• Red color - indicates deposition in the presence of abundant oxygen in a p ygwarm, humid terrestrial environment.

Black Shale, Anoxic Environment

Sideling Hill Outcrop, I-68. What is the depositional environment of the black layer?depositional environment of the black layer?

Red Beds, Oxygenated Environment

Environments of Deposition

• 3 Major EnvironmentsC i l T i l• Continental or Terrestrial

• MarineMarine• Transitional

Continental or TerrestrialContinental or Terrestrial Environments

• Glacial - both Alpine and Continental• Alluvial Fans at the base of mountains• Alluvial Fans - at the base of mountains• Lakes• Rivers and Floodplains = Fluvial and

alluvial depositsalluvial deposits• Swamps• Deserts

M i E iMarine Environments• Continental Shelf - most common

marine environment in the rock recordmarine environment in the rock record• Shelf Edge• Slope• Ocean FloorOcean Floor

Transitional Environments

• Beaches• Barrier Islands• Barrier Islands• Tidal Flats• Lagoons and Bays• Estuaries• Estuaries• River Deltas

Fig. 5.13. Major sedimentary environments.

Alluvial Fan depositsAlluvial Fan deposits

Fluvial and floodplain deposits

What is the depositional environment?What is the depositional environment?

What is the depositional environment?What is the depositional environment?

Eolian depositsEolian deposits

A modern swamp in Louisiana

A strip coal mine in the western U.S. --an ancient swamp environment

A strip coal mine in the western U.S. --an ancient swamp environment

Beach depositsBeach deposits

Barrier Islands, bays, and y ,

continental shelf on the Newon the New Jersey coast.

Barrier Islands, bays, and continental shelf on the Gulf Coast: Mobile Bay

Seneca Rocks – Paleozoic barrier island deposits

2004 G200 class on top of Seneca Rocks

2005 G200 class on top of Seneca Rocks

Transitional or Marginal Marine Environments, Tid l I l M h d LTidal Inlets, Marshes, and Lagoons

Marginal Marine Environments, Tidal g ,Inlets between Barrier Islands

Marginal Marine Environments, a Barrier Island

Marginal Marine Environments, a prograding Spit on a Barrier Islandprograding Spit on a Barrier Island

Transitional Environment: The Mississippi RiverMississippi River

delta

Transitional Environment:

TheThe Mississippi River delta

Fig. 12.32. Historic flow of the Mississippi

The Nile Delta. Shaped like the p

Greek letter deltadelta.

Sedimentary FaciesSedimentary Facies• Facies general appearance or aspect• Facies - general appearance or aspect

of sedimentary rocks. Often d t f ticorrespond to formations.

• A reflection of the depositional penvironment.

• Lithofacies defined by lithologic• Lithofacies - defined by lithologic features

• Biofacies - defined by organic features

Sedimentary Facies

• Facies occur laterally adjacent to one another just as do their depositionalanother just as do their depositional environments.

diff f i h i il• Two different facies can have similar lithofacies but different biofacies, or vice-versa.

Sedimentary Facies

• Facies migrate laterally with changes in sea levelin sea level.

• Rising sea level = transgression. i f i li iMarine facies overlie nonmarine

facies.• Falling sea level = regression.

Nonmarine facies overlie marineNonmarine facies overlie marine facies.

Facies changes produced by Marine Transgression

Transgression, regression, and stratigraphic cycles.Figure 5 26Figure 5.26

Causes of Changes in Sea Level

• Changing ice volumes at the poles.• Thermal expansion or contraction of• Thermal expansion or contraction of

the oceanic crust.• Continental tectonics or subsidence.• Shoreline progradation from river andShoreline progradation from river and

deltaic sedimentation.

Pennsylvanian SedimentaryPennsylvanian Sedimentary Rocks in the Appalachians

P l i i t b d it WVPennsylvanian point bar deposits on WV Rt. 19 near Oak Hill.

coal seam point bar surfaces

A meandering stream with a sandy point bar on the inside curve, and a cut bank on the

outside curve. The point bar dips toward the p pstream channel.

Stream Deposits at Exit 7, I-68 pby Glenmark Centre

Depositional Model for Coal formation

Upper Freeport Coal, Preston County, WVUpper Freeport Coal, Preston County, WV

Upper Freeport Coal with overlying fluvial d d i d b d i isandstones deposited by meandering river.

Coal outcrop at the Morgantown MallCoal outcrop at the Morgantown Mall

An idealized cyclothemcyclothem sequence.

Rarely foundRarely found in West Vi i iVirginia

Pennsylvanian Cyclothems in Kentucky

6

45

34

2

1

A modern marine transgression is exposing Holocene peat deposits on the

Louisiana coastline.Louisiana coastline.

Change of the Louisiana Coastline

Middle Pennsylvanian, Kanawha Fm., Rt 19 Nicholas Co WVRt. 19, Nicholas Co., WV