Lecture notes Sedimentology and Stratigraphy (CSM2183)

Sedimentology and Stratigraphy [2183]
W1

Weathering and soil formation




Lecture 1
Sedimentology→ study of the formation transportation and depositions of sedimentary
particles which accumulate in continental and marine environments. These become rocks
through lithification.

Stratigraphy→ studying strata to determine the order/ timing of events in earths history
through interpretation of sedimentary rocks and the processes which formed them.

1. Siliclastic Rocks- Allochthonous/ extrabasinal (from outside area of formation)
1. breccia
2. congliomeragte
3. sandstone
1. quartz arenite
2. lithic arenite
3. arkose
4. wacke
5. siltstone
6. mudstone/ shale
2. Carbonate Rocks- Autochthonous/ intrabasinal
1. limestones
1. boundstone
2. graistone
3. packstone
4. wackstone
5. mudstone
2. dolomites
1. dolo-grainstone
3. Chemical Rocks- Autochthonous/ intrabasinal
1. Ca and Mg carbonate
2. gypsum
3. anhydrite
4. halite
5. sylvite
6. carnatlite

Formation of Clastic Sedimentary Rock

1. uplift and exposure of bedrock
2. in situ weathering processes→ physical, chemical or biological

, 3. erosion→ attrition, abrasion
4. transport→ suspension, saltation, traction/ rolling
5. deposition→ rivers and flood plains, lakes, deserts, deltas and estuaries, shallow
clastic/ carbonate shelves, deep marine
6. burial
7. diagenesis and lithification→ grain repacking and compaction, dissolution and
neomorphism, pressure solution and cementation. These control the preservation and
enhancement or destruction of the sediment

Udden-Wentworth and Phi scale

Grain sizes

Grain size provides a preliminary indication of sediment transport and depositional
environment.

Matrix; generally clay minerals mixed with silt sized quartz and feldspar usually deposited
together with the grains.

Cements; post depositional diagenetic products filling in voids int eh framework.

Rock properties are controlled at time of deposition by factors like porosity and permeability.
The more poorly sorted a rock is the less porosity and permeability the rock has. Grain shape
and sorting are helpful in understanding the depositional environment of a rock.

Textural maturity is characteristic of depositional environment:

Classification of Carbonate Sedimentary rock

Dunham Classification

Folk Classification

Sedimentary basin; an area of the earths surface where sediments have accumulated to a
greater thickness than they have in adjacent areas.

Depocentre; the thickest part of a sedimentary basin- may migrate through time.

Types of Sedimentary Basins

Wilson Cycle- life and death pf supercontinents by the creation and destruction of mountain
belts, ocean rifts and subduction zones.

WEATHERING AND SOIL FORMATION

weathering→ the adjustment of physical, mineralogical and chemical properties of rocks in
response to environmental conditions at the earths surface.

,Physical and chemical weathering are inextricably linked, working in tandem to break down
rocks. Surface area; as rocks weather, their surface area increases providing more
opportunity for further weathering.

Physical weathering; the mechanical action of physical processes that break rocks down into
individual crystals/ smaller rock fragments.

 freeze-thaw weathering
 salt weathering
 exfoliation

Chemical weathering; primarily caused by meteoric water reacting with mineral grains in
rocks to form new minerals and soluble salts. This can occur anywhere but reactions are most
efficient in warm humid climates (higher temperatures increases kinetic energy).

 solution
 oxidation
 hydrolysis

What does the rate of chemical weathering depend on?

1. Mineral composition- for example ionically bonded minerals are affected by water yet
covalently bonded minerals are not.
2. Mineral structural integrity- lines of weakness suck as cleavage or twinning are weak
parts of the minerals structure.
3. Crystallinity- defects in crystal lattices
4. Chemistry of weathering environment- acidic/ alkaline, warm/ cool etc.

Biological weathering; organisms can cause and enhance physical and chemical weathering.

More stable minerals are generally less affected by weathering. These are those that form a
lower temperatures, closer to the surface and the most abundant- e.g. quartz is not largely
affected by weathering and is made of silica the most abundant on earths crust. Goldich’s
Weathering Series.

Erosion→ the removal of regolith material following weathering encompassing transport of
weathered material from the site of weathering.

Soil formation and development;

Soil profiles are dynamic systems- they become thicker over time as more bedrock is broken
up and more organic matter deposited. At the same time some soil is removed but at a slower
rate.o

How latitude creates weathering variations;



W2

, Sediment Transport and fluid flows

-Gravity driven erosion and transport processes

-water driven erosion and transport processes

-wind driven transport processes

-ice driven transport processes

-Fluid flow

Sedimentary Structures

-How different flow regimes and substrates lead to the formation of different bedforms,
which can subsequently be preserved as sedimentary structures

-Construcitonal structures (wind vs water)

 graded bedding and sorting
 ripples (wave vs current vs climbing) and cross lamination
 dunes and cross bedding
 imbrication

-Erosional sedimentary structures

 flute casts, tool marks, scour marks

-other sedimentary structures (mudstones)

 mud cracks, syneresis cracks

-Biological sedimentary structures

 ichnofossils




Lecture 2
Erosion and Transport

Transport under gravity:

 falls (dry)- rock breaks up during the movement and accumulates a chaotic mass at
the base of the slope. Triggers;
o earthquakes
o undercutting at the base