sedimentary rocks, depositional environments and stratigraphy

Sedimentary Rocks,

Depositional

Environments and

Stratigraphy

The Nature of Sedimentary Rocks

• Sedimentary rocks are composed of:

– Fragments of other rocks (detrital or clastic)

– Chemical precipitates

– Organic matter or biochemically produced materials

Types of Sedimentary Rocks

Detrital Chemical

Biologic

Clastic

Texture Crystalline

Texture

The Nature of Sedimentary Rocks

• Sedimentary rocks are common at the Earth’s surface

– Cover ~75% of the continents

– Cover nearly all of the ocean floor

– Easily eroded

– Occur in distinct layers (strata)

The Nature of Sedimentary Rocks

• Layers are easily identified

– Majors layers (formations) easily recognized over large distances

– Smaller layers within a formation are separated by bedding planes

– Gradation in grain size, composition or physical features may vary

Sedimentary layers may extend for many miles

Identifying and correlating the layers is

Stratigraphy. More on that later.

What do Sedimentary Rocks Record?

•Source (Provenance) of sediment

•Erosion and Transport Agent

•Depositional Environment

•Paleogeography/Tectonic Setting

•Diagenesis (what happened after deposition)

Rock Identification is based on:

• Composition

What minerals make up the rock?

• Texture

What is the shape, size and orientation of the mineral grains that make up the rock?

Major Classes:Clastic Crystalline (chemical

and/or biochemical)Biologic (coal,

fossiliferous limestones, etc.)

These can easily be confused

Clastic Sedimentary Rocks

• Made of rock & mineral fragments or clasts

• Clasts are broken and worn particles transported by water, wind or ice

• Clastic rocks are subdivided by grain size

Clastic Sedimentary Rocks

• Grain size is controlled by:

– Size and mineralogy of grains in source rock

– Carrying capacity of transport process

– Weathering and erosion that occurs during transportation

– Energy of the depositional environment

Grain size ranges for classification of

common clastic sedimentary rocks

Clastic Sedimentary Rocks

• Common clastic sedimentary rocks

– Conglomerate

– Sandstone

– Mudrock or Shale

• Siltstone

• Claystone

Conglomerate

Sandstone

Shales

Shales

Shales erode very easily

and form slopes

Chemical/Biochemical Sedimentary Rocks

• Formed by a process that takes ions from solution to form a solid

– Chemical Sediments

• Precipitates from water by an inorganic process, e.g. evaporites

– Biochemical Sediments

• Formed during the growth of some organism

What about coal? Oil?

Chemical/Biochemical Sedimentary Rocks

• Subdivided by composition and mode of formation

• e.g., Limestone– Biochemical formation by algae, coral, etc.

– Direct chemical precipitate from warm sea water -oolites

– Chemical precipitate from springs and in caves

Chemical/Biochemical Sedimentary Rocks

• Common Chemical/Biochemical rocks:

– Dolostone - composed of dolomite

– Chert - microcrystalline quartz

• Various modes of formation

– Evaporites

• Rock salt - halite

• Gypsum

Limestones

Limestones

Limestones

Oolitic Limestone

Chalk(Coccolithophores)

Travertine(Limestone)

Dolostone

Chert (Flint, Jasper, Agate…)

Evaporites:Bonneville Salt Flats, Utah

Rock

GypsumRock Salt

Sedimentary Rocks on Earth

Shale Sandstone Siltstone LimestoneConglom.

Global Soils

• The Nature of Soil

• The Soil Water Balance

• Soil Development

• The Global Scope of Soils

• Global Climate Change and Agriculture

The Nature of Soil

Figure 10.1, p. 365

Soil is a mixture of

1) inorganic material

derived from

regolith weathered from

bedrock;

2) organics

derived from forest litter

and surface vegetation),

3) Liquids (water with

dissolved nutrients), and

4) atmospheric gases.

typical soil composition

45%

inorganic

5%

organic

25% water

25% air

The Nature of Soil

Figure 10.2, p. 366

Soil texture,

refers to the

proportions of

sand, silt and

clay found in

the soil

The Nature of Soil

Figure 10.3, p. 366

the variable proportions of sand, silt and clay found in

different soils facilitate the classification of soil texture

classes

The Nature of Soil

The negatively charged surfaces of soil colloids (mineral

particles < 0.00001 mm, organic humic and fulvic acids) attract

and store base ions (Ca2+, Mg2+, Na+, K+) as plant nutrients

Soil color, generated by soil forming processes, is the

most obvious soil property

dark brown to black indicates high humus content;

red or yellow indicates iron mineral abundance;

white flecks or spots indicate the presence of calcium

carbonate)

Soils of cool moist regions tend to be acidic (pH < 7),

[often low base status >> hard for plants to get some nutrients]

Soils of arid climates are alkaline (pH > 7)

[can be high base status >> easier for plants to get some nutrients]

unless it is too alkaline!

Soil structure refers to the way in which soil particles are grouped together

into peds

Soil water storage

capacity and

wilting point

according to soil

texture

The Soil Water Balance

Figure 10.7, p. 369

Finer grains

(clays mostly)

hold more water,

but don’t want to

share it with

plants

available water

=

water gain

- water loss

+ changes in storage

The Soil Water Balance

The Soil Water Budget

simplified soil water budget for a middle latitude, moist climate

Soil Development

soil horizons are distinctive horizontal layers that differ in physical and chemical composition, organic content, or structure

soil horizons are often distinguished by their colour

the display of horizons horizontally in a cross section is termed a soil profile

Soil Development

two types of soil horizons: organic

and mineral

organic horizons, designated

by the capital letter O, are formed

from accumulations of organic matter

derived from plants and animals

the upper Oi horizon contains

decomposing organic matter that is

recognizable as leaves or twigs

the lower Oa horizon contains

material that is broken down beyond

recognition by eye (humus)

Soil Development

two types of soil horizons: organic

and mineral

mineral horizons lie below the

organic horizons

Below the mineral horizon is the

bedrock with a weathered top

Figure 10.11, p. 372

Soil Development

the A horizon is the

uppermost mineral horizon

it is rich in organic matter,

consisting of numerous plant roots

and down-washed humus

from the organic horizons above

the E horizon: clay particles and

oxides of aluminum and iron are

removed from the E horizon by

downward-seeping water, leaving

behind pure grains of sand or

coarse silt

Figure 10.11, p. 372

Soil Development

the B horizon receives the clay

particles, aluminum, and

iron oxides, as well as organic

matter washed down from the

A and E horizons

It is made dense and hard by

the filling of natural spaces

with clays and oxides

Figure 10.11, p. 372

Soils and Climate

in cold climates, decomposition of organic

matter is slow and organic matter

accumulates

in warm climates, organic matter

decomposes rapidly and soil organic matter

is scarce

How does this compare to their growth rates?

Global Geography of Soils

IG4e_07_02a

IG4e_07_02b

The Global Geography of Soils

Group I: soils with well-developed horizons or with fully weathered minerals, resulting from long-continued adjustment to prevailing soil temperature and soil water conditions

Oxisols: very old, highly weathered soils of low latitudes, with a subsurface horizon of accumulation of mineral oxides and very low base status

Ultisols: soils of equatorial, tropical, and subtropical latitude zones, with a subsurface horizon of clay accumulation and low base status

Vertisols: Soils of subtropical and tropical zones with high clay content and high base status (develop deep, wide cracks when dry, and the soil blocks formed by cracking move with respect to each other)

The Global Geography of Soils

Group I: soils with well-developed horizons or with fully weathered minerals, resulting from long-continued adjustment to prevailing soil temperature and soil water conditions

Alfisols: soils of humid and subhumid climates with a subsurface horizon of clay accumulation and high base status; range from equatorial to subarctic latitude zones

Spodosols: soils of cold, moist climates, with a well-developed B horizon of illuviation and low base status

Mollisols: soils of semiarid and subhumid midlatitude grasslands, with a dark, humus-rich epipedon and very high base status

Aridisols: soils of dry climates, low in organic matter, and often having subsurface horizons of accumulation of carbonate

minerals or soluble salts

The Global Geography of Soils

Group II: soils with a large proportion of organic matter

Histosols: soils with a thick upper layer very rich in organic matter

Oxisols: very old, highly weathered soils of low latitudes (tropics and equatorial)

a subsurface horizon of mineral oxides and

very low base status

Rotten soils!How do these soils support tropical forests?

Ultisols: soils of equatorial, tropical, and subtropical latitude zones, with a subsurface horizon of clay accumulation and low base status

Tough living for a plant

Vertisols: Soils of subtropical and tropical zones with high clay content and high base status (develop deep, wide cracks when dry, and the soil blocks formed by cracking move with respect to each other)

Alfisols:

soils of humid and subhumid climates

a subsurface horizon of clay

high base status

Good soil

F. Spodosols

Spodosols: soils of cold, moist climates,

with a well-developed B horizon of illuviation

low base status

A pretty crummy soil, but specialized plants survive in it.

Mollisols:

soils of semiarid, subhumid midlatitude grasslands,

a dark, humus-rich layer and very high base status

Potentially productive

Aridisols:soils of dry climates,

low in organic matter,

often having subsurface accumulation of carbonateminerals (caliche) or soluble salts

Need a tough plant to thrive here

Histosols:

soils with a thick upper layer very rich in organic matter

Just step back and watch the corn grow