Sediments and Sedimentary Rocks © Houghton Mifflin 1998; N. Lindsley-Griffin, 1999. LAYERED ROCKS, PARIS CANYON, ARIZONA Geology Today Barbara W. Murck

Sediments and Sedimentary RocksSediments and Sedimentary Rocks

© Houghton Mifflin 1998; N. Lindsley-Griffin, 1999.

LAYERED ROCKS, PARIS CANYON, ARIZONA

Geology TodayBarbara W. Murck & Brian J. Skinner

Rock CycleWeathering,

Erosion, Deposition

Rock CycleWeathering,

Erosion, Deposition

SedimentsSediments unconsolidated

particles created by weathering of rock

by chemical and mechanical means

J. R. Griffin, 1999

Rock CycleDeposition,

Compaction, Lithification

Rock CycleDeposition,

Compaction, Lithification

Sedimentary rockSedimentary rockRock formed from

weathered products of pre-existing rocks,

plus or minus fossils that have been

transported, deposited and lithified

J. R. Griffin, 1999

SedimentsSediments

N. Lindsley-Griffin, 1999.

Bedding is the best clue that a rock is sedimentary.Also termed strata, layering. (Fig. 8.2, p. 219)

Capital Reef National Park, Utah

Lithification - loose sediment changes to sedimentary rock: grains in matrix, cement.

COMPACTION - pore space decreases, water forced out

CEMENTATION - dissolved ions precipitate between grains

RECRYSTALLIZATION - less stable minerals change to more stable forms

(Figs. 8.4B, p. 221; 8.9, p. 225)

LithificationLithification



The SIZE of the particle

transported depends on the density and speed of the transporting medium and the slope angle....

Clastic SedimentsClastic Sediments

Buffalo, WY


Sorting - the range

in clast sizes.

Poorly sorted = great size variation.

Well-sorted = grains all about the same size.


Fig. 8.4, p. 221


Sphericity - how equidimensional are the grains?Round vs. angular - how sharp are the corners?


Fig. 8.4, p. 221



Size of clast or fragment:gravel (pea size and larger) -- CONGLOMERATE

sand (pin head) -- SANDSTONE

silt (grain of table salt) -- SILTSTONE

clay (particle of flour) -- SHALE, CLAYSTONE

Mud is a mixture of clay and silt -- MUDSTONE

Fig. 8.3, p. 220

Houghton Mifflin 1998; N. Lindsley-Griffin, 1999

BRECCIAAngular fragments of rock in finer-grained matrix

CONGLOMERATERounded pebbles of rock in finer-grained matrix


These are lithic clasts


Clast size, sorting, roundness, and sphericity suggest:

how far it traveled

nature of transporting medium

how sediment was deposited


Glacial Till, Matanuska Glacier, AK (Fig. 8.4A, p. 221)

Quartz sand, St. Peter Sandstone, WI (Fig. 8.4B, p. 221)


COMPOSITION of the clastic particle depends on source:

white coral sand, Bora Bora

green olivine sand, Hawaii



COMPOSITION of the clastic particle depends on

strength of particle vs. distance traveled

basalt weathers quickly, is found only close to its source

quartz travels long distances


Basalt sand, Hawaii Quartz sand, Oregon

SAND can be any

composition...

SAND can be any

composition...

• but most is quartz because it is: – durable– chemically stable


Chemical SedimentsChemical Sediments


Precipitated from dissolved matter in sea or lake water:

- through activities of plants and animals (but not their remains)

- through evaporation of water containing dissolved ions

Banded Iron Formation, western Australia (Fig. 8.10, p. 225)

Evaporites form where dissolved ions are concentrated by evaporation



Evaporites:Least soluble carbonates (limestones) precipitate first

Sulfates (anhydrite, gypsum) precipitate next

Most soluble halides (rock salt) precipitate last


Great Salt Lake, Utah

Manganese nodules (manganese oxides) precipitate directly from sea water in the deep

ocean



Composed of the remains of plants and animals.Bioclastic sediments consist of broken clasts of remains.

Coquina - shells and shell fragments (Fig. 8.8, p. 225)

Biogenic SedimentsBiogenic Sediments



Coral reefs, where most limestones form, require certain conditions:

• Shallow water where light penetrates

• Warm water - tropical or temperate

• Little land-derived detritus



Coral reefs support a complex ecosystem that develops around the coral framework.

• Algae form a symbiotic relationship with corals



Sediment in the lagoon (quiet water behind reef) is biogenic calcareous mud which will form limestone


Deep ocean biogenic sediments are mostly microscopic fossil shells:

• Calcareous (foraminifers, nannofossils) - CHALK

• Siliceous (radiolarians, diatoms, sponges) - CHERT


Clastic sediments clastic rocks = siliciclastic rocks

conglomerate, breccia,

sandstone, siltstone,

mudstone, shale, claystoneChemical sediments chemical sedimentary rocks

gypsum, rock salt, phosphorite

banded iron formation

(a few limestones)

Biogenic sediments biogenic rocks

limestone, dolostone = carbonate rocks

chert

peat, coal


Sedimentary Rock NamesSedimentary Rock Names


Graded bedding: coarse grains at sharp base.

Grains gradually become smaller upwards.

Sedimentary StructuresSedimentary Structures

(Fig. 8.6, p. 223)



Turbidites form in deep ocean.

- rhythmic layering

- graded bedding

(Fig. 8.17, p. 237)


Ripple marks on bedding surfaces:

sediments were deposited in water, usually shallow, with waves or currents.

Sharp crests point upwards, rounded troughs point downwards.

(Fig. 8.11, p. 230)




Mudcracks:sediments on drying mud flats or lake bottoms.Cracks polygonal, narrow to “V” downwards. (Fig. 8.11, p. 230)

Interpreted from composition, texture, structures

Alluvium (left): sorted layers, rounding, mixed clasts

Lahar (above): no layers, angular, volcanic clasts

Sedimentary EnvironmentsSedimentary

Environments


Turbidites (right): beds graded from sand up to clay size

Beach (below): steep cross bedding, very well sorted

quartz sand, fossil seashells

Sedimentary EnvironmentsSedimentary Environments




Loess, an eolian sediment:

uniform silt size

massive - no structures

deposited by wind

common all over Nebraska

- both field trips

(Fig. 8.15, p. 235)



Eolian cross bedding, formed in sand dunes, preserves the steep front face (downwind)

gentle back face (upwind)

Fig. 8.7, p. 224



Varves record annual cycles (usually glacial lakes):

light silt deposited in warm months,

dark clay-rich layers in winter months.

Fig. 8.5, p. 222



Fig. 8.14, p. 233

Documents

Sediments and Sedimentary Rocks © Houghton Mifflin 1998; N. Lindsley-Griffin, 1999. LAYERED ROCKS, PARIS CANYON, ARIZONA Geology Today Barbara W. Murck