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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
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