Sediments and sedimentary rocks

Assembled rocks

1. Precipitants and water-lain

fragments, low T and P,

Sedimentary.

2. Re-equilibrated materials, wide

range of T and P, Metamorphic.

3. Melted materials, high T,

Igneous. Me

ch

an

ica

l

Th

erm

alAssembling minerals

Surface (map or plan view) - cover much of the earth.

Depth (cross section) - thin veneer.

Easy to observe, and contain economic materials (including

fossil fuels).

Abundance

New York Bedrock

Extensive Paleozoic sediments

Map modified by T. Wayne Furr, after Branson and Johnson; WWW version by Jim Anderson.)

Thick sediments

Fragments of pre-existing rocks. The fragments

are produced by weathering and erosion.

Weathering - mechanical and chemical breakdown

of rocks and minerals.

Erosion - fragments are moved away from source

(downhill).

May operate together or separately.

Clastic

Physical: hardness, fracture, cleavage

Chemical: Resistance of bonds to chemical

attack

Si-O bonds very strong. Increased

polymerization means more resistant.

Tectosilicates are among the more resistant

components. Solubility of silica in water at STP

aids stability.

Weathering

Fast paths for water-rock interactions

Corner areas

Elephant rocks, Saint François Mountains, MO. Tor type

weathering of widely-spaced jointed granites.

Variable resistance

Resistant sandstone remnant on shale, Green River, WY.

Weathering

Press and Sevier, 1986

Weathering

Frost wedging

Water ice is more

voluminous than

liquid water.

Trapped water that

freezes splits the

rock. Or in the

case of Troy in

winter, the roads.

34

.7 m

23

.3 m

27

.5 m

20

.5 m

Mount Scott Granite Oxides

Diffusive alteration

Pristine igneous

rock oxides

become more

oxidized near

exposed surface.

Note alteration at

27.5m along

fractures in grains.

Dissolution

If a mineral is abundant in the crust

and resistant to chemical attack, it

is likely to be a major constituent of

clastic sedimentary rock.

Good clastic materials

Small and less dense

Phyllosilicates (cleavage)

These can be transported with lower

amounts of energy

Fast versus slow moving streams

Wind (loess)

Clay minerals

Chemical and mechanical breakdown of rocks results in

particles of increasingly smaller size.

Earth scientists have formal names for size ranges

Cobble > 10 mm

Gravel 1 mm – 10 mm

Size matters

0.001

0.01

0.1

1

10

100

1000

10000

Bould

er

Cobble

Peb

ble

Gra

nule

V. c

oarse

san

d

Coar

se s

and

Med

ium

san

d

Fine

sand

V. f

ine

sand

Coar

se s

ilt

Med

ium

silt

Fine

silt

V. f

ine

silt

Cla

y

Gra

in s

ize (

mm

)

Press and Sevier, 1986

Gravity driven

Clastic particles

transported by water

movement

Stream deposits

Unconsolidated sediments

reveal the clastic

processes at work in cut

bank adjacent to a small

stream. Channel

movement layers

conglomertic sediments on

top of bank sands. Sands

at top reworked by wind.

Big Bend NP, TX.

Streams over time

Peter Mozley,

NM Tech Website

Wind and water

Stratigraphic layering

The sediments are always products of

surface environments. These are a

snapshot into Earth’s past.

Dunes

Large particles of eroded rock, typically embedded in

finer particles (typically silicate)

Origin: High energy fluid transport

James Madison Univ. Sedimentology

Conglomerate

Sand sized particles (typically quartz, feldspar, or rock

fragments – typically silicate) from eroded rock

Origin: Moderate energy fluid transport

Sandstone

Silt sized particles (typically quartz and feldspar

framework silicate) from eroded rock

Origin: Low energy fluid transport

James Madison Univ. Sedimentology

Siltstone

Silt sized particles (typically clay – sheet silicate) from

eroded and highly weathered rock

Origin: Low energy fluid transport

James Madison Univ. Sedimentology

Q: What is the difference between

siltstone and shale?

Shale

Press and Sevier, 1986

(Compaction, Limited re-equilibration, and Cementation)

Continuing deposition, sediments buried (increase in P < 5

kbar and T < 100 oC).

Sediments are frequently porous (grainsize dependent), lots

of fluids.

Sedimentary minerals grow

Minerals grow between grains

Diagenesis

10% GROWTH

Pores

Preservation

Modern desiccation

cracks in mud

300 Ma desiccation

cracks in shale

Gastropoda and Pelecypoda,Cretaceous, New Jersey

Trilobite Ameura ,

Upper Pennsylvanian,

Kansas

KGS Website

Corals

Corals are great limestone

builders. They put a lot of time

and energy into building

skeletons made from

aragonite

The coral polyp only inhabits the

outer 1-2 cm of the structure.

The rest is just a framework to

increase reproduction and

feeding

Coccoliths

Tiny plankton with

calcareous shells.

Abundant enough to live

and die to make massive

limestone deposits like

Santa Elena Canyon, TX

Carbonate producing organisms die, but their

shells will only be preserved if they are deposited

above 4 km depth

Ice cores trap gas and dust. Temperature from

concentration of deuterium in ice.

375 Current

Atmosphere

CO2

Vostok, Antarctica Ice Core

Feely et al., 2001

Increased atmospheric CO2 means big changes in ocean

chemistry. This could be detrimental to much of marine

life.

Feely et al., 2001

Carbon isotopes show

the location of industrial

CO2 in the worlds

largest oceans. As we

know ocean mixing is a

slow process - the most

recent excess CO2

remains in the first 1 km

of the ocean.

By now you know that minerals can be recorders of the processes that

create and sustain them.

Carbonates - some powerful information

•Trace element incorporation (divalent cations, such as Sr in calcite)

•Isotopes of C and O (13C and 18O)

•These are influenced by temperature (directly or indirectly)

Because carbonate materials are incorporated into marine organisms,

carbonate minerals may record ocean climate history.

Details

18O is a stable isotope

influenced by global ice

content.

!

"18 =

18O16O

#

$ %

&

' ( sample

)18O16O

#

$ %

&

' ( s tandard

18O16O

#

$ %

&

' ( s tandard

*

+

, , , ,

-

.

/ / / /

01000

Standard for carbonates is a

belemenite fossil from the K

Peedee formation, SC

18O/16O = 0.0020672

H2O16 vs. H2O

18

For a volume of water, more 16O

water evaporates relative than

water with 18O.

Evaporated water from near the

equator is eventually transported

toward the poles through repeated

evaporation and precipitation.

The 18O/16O ratio will be lower in the

snow that falls on a glacier than it is

in the ocean from which the water

evaporated.

If global ice volume increase, the18O values of seawater become

larger as more 16O stored is locked

away in ice.

Analyzing the 18O/16O ratio of

a dated carbonate mineral

from a fossil or marine

precipitate gives you an

estimate of global ice - a

proxy for two parameters

1.Global climate trends

2.Sea level

Carbonate originsCarbonate is abundant at the earth’s surface, but may be

produced deeper in the earth where the activity of CO2 is

elevated.

Igneous: Carbonatites - rare magmatic systems that

produce carbonate materials

Hydrothermal: Alteration of mantle rocks and dissolution

in aqueous fluids

Ground/surface water: Chemogenic precipitation, karst

Metamorphic: Recrystallization and metasomatism

Sedimentary: Biogenic and reworking of biogenic

materials, clastic continental materials

Critters make minerals

Aragonite and calcite

In general the oceans are nearly saturated in CaCO3

Ca2+ + HCO3- = CaCO3 + H+

Temperature is important (latitude, depth)

Biogenic Sediments

Diagenesis

In sedimentary rocks,

the individual particles

need to be cemented

together. Carbonate

minerals precipitate out

of water to cement

grains.

Thin section under XP

shows calcite

overgrowth on micrite

grains.

0.3mm©Ryan Hanson

Fragmented or whole aquatic invertebrate hard parts

Origin: Bodies of water with suitable environments

Limestone

Fossiliferous limestone - pore space colored blue

Oolitic limestone

Ooids form through growth and/or

accumulation in dynamic carbonate

environments.

Gypsum CaSO4 2H2O

Anhydrite CaSO4

Barite BaSO4

Epsomite MgSO4 7H2O

Image from mineral.galleries.com

Sulfates

Geology!Volume 35, Number 4

Cave of Crystals in

the Naica mine,

Chihuahua, Mexico.

The giant faceted and

transparent single

crystals of gypsum

measure up to 11 m

in length. Garcia-Ruiz

et al. propose that

these crystals derived

from a self-feeding

mechanism driven by

a solution-mediated,

anhydrite-gypsum

phase transition.

Image from Klein and Hurlbut, 1985

Minerals precipitate due to

oversaturation of an evaporating fluid.

Some form in closed bodies of water,

with significant evaporation.

Gypsum, Anhydrite, Halite, Sylvite

Chemogenic Sediments

Sea water - crystallization

Sabkha

Arid near-marine

environments

may host

anhydrite and

gypsum deposits.

The mineral

precipitated is

largely a function

of proximity to

water

Salt Flat

Shallow seas, lakes

in closed basins

where evaporation

outpaces input.

Shallow mesozoic seas

covered modern-day

Colorado, leaving thick

deposits.

Lansing mine,

6 miles of

room and pillar

2,300 feet

below Lake

Cayuga.

300 Ma

deposit made

from a shallow

sea, now

buried deep.Daily Ithican Online

NY Salt

Salt domes

Salt is less dense

and more fluid than

surrounding rocks.

May move upwards

as diapirs.

Classic examples

are found along

Gulf Coast -

Louann SaltAmerican Scientist, Sept.-Oct. 1991, p.426

Gypsum expansion

Anhydrite

becomes

rehydrated -

forming

gypsum. The

expansion

produces

bowing of layers

in Triassic rocks

in Caprock

Canyon, Texas