GEOL 238 Midterm 1

Sedimentology

The study of processes of formation, transport, and deposition of material that accumulates as sediment in continental and marine environments and eventually forms sedimentary rocks

3 resources provided by sedimentary rocks

1. Grow food on deposits formed by sedimentary processes

2. Contain most of the world’s fossil fuels

3. Obtain water from sedimentary deposits

Erosional systems

Areas of high relief where sediment is produced and transported downstream

Depositional systems

Areas of low relief where sediment deposition creates sedimentary archives

Weathering

Physical, chemical, and biological processes that act to break down rock

Erosion

Physical (or biological) removal of weathered sediments or other material

2 types of weathering (both types work simultaneously and enforce each other)

1. Physical weathering

2. Chemical weathering

Physical weathering

Physical forces breaking rocks into smaller pieces

Chemical weathering

Chemical transformation of rock into new compounds

4 methods of physical weathering

1. Unloading/Stress Release

2. Volume Changes

3. Biological Agents

4. Abrasion

4 methods of volume changes (physical weathering)

1. Insolation Weathering

2. Freeze-Thaw (Frost) Weathering

3. Salt Weathering

4. Wetting/Drying

Unloading/Stress Release

Overlying rock is eroded, compressional stress is reduced and rock unit “rebounds” upward

Sheeting occurs due to…

Process of unloading/stress release

How abrasion affects grain size (during erosion + transport)

During transport, grains bash against each other and become more rounded and break down

COARSE —> MEDIUM —> FINE

3 methods of chemical weathering

1. Simple Solution (“congruent dissolution”)

2. Hydrolysis (“incongruent dissolution”)

3. Oxidation and Reduction (Redox)

Oxidation

Extraction of electrons from a substance

Reduction

Addition of electrons to a substance

Oxidation and Reduction (Redox)

Loss of electron from an element in a mineral (often Fe or Mn), resulting in formation of oxides or hydroxides (if water present)

Simple Solution (“congruent dissolution”)

Mineral completely dissolves (ex. calcite, gypsum, halite)

• Occurs more quickly in acidic water

Main solid product of hydrolysis….

Clay minerals (such as kaolinite)

Hydrolysis

Hydrogen ion attacks and replaces other positive ions; which results in dissolved ions and solid products

• Occurs in pure water but is faster if water is acidic

• Common weathering type in silicate minerals

Mineral type that experiences simple solution

• Highly soluble minerals (calcite, gypsum, halite)

• Quartz

• Carbonate rocks

Mineral type that experiences hydrolysis

• Silicate minerals

Mineral type that experiences redox

• Iron sulfides

• Fe- and Mn-bearing silicates

Products of weathering feldspar….

Clay minerals + ions + SiO2

Products of weathering Fe-Mg minerals….

Clay minerals + ions + SiO2 + Fe-oxides

Product of weathering quartz…

Quartz grains

Product of weathering calcite…

Ions

3 types of products of weathering

1. Source-rock residues (chemically resistant minerals and rock fragments)

2. Secondary minerals (formed in situ)

3. Soluble constituents (released from source rocks)

How surface area affects weathering

Mechanical weathering breaks rock into smaller pieces, therefore more surface area for chemical weathering

Differential weathering

Weathering affects rocks/minerals differently based on the mineral hardness + planes of weakness

2 characteristics of more resistant rocks (i.e granite)

1. Hard minerals

2. Crystals interlocked

2 characteristics of less resistant rocks (i.e schist)

1. Soft minerals

2. Crystals platy

Most stable mineral is…

Quartz

Goldich’s Weathering Series

Reverse of Bowen’s reaction series; series of increasing resistance to chemical weathering from least resistant (olivine) to most resistant (quartz)

4 factors that affect nature of sediment

1. Source area composition

2. Amount of weathering

3. Climate

4. Transport processes

Simplified path of sediment travel

SOURCE (erosional systems) —> SINK (depositional systems)

2 major elements that affect stratigraphic record

1. Allogenic forcing

2. Autogenic forcing

Allogenic forcing

Controls external to the depositional system (ex. climate change, tectonics, sea-level)

Autogenic forcing

Controls internal to the system (ex. river avulsion, dune migration, delta lobe switching)

Compositional maturity

Degree to which sediment contains resistant grains (quartz and stable heavy minerals like magnetite, apatite, rutile, zircon)

3 reasons for high compositional maturity

1. Warm/humid source region

2. Very long transport distance

3. Source rock was already mature

Climates where chemical weathering is the strongest….

Warm, wet climates

Textural maturity

Depends on 3 things:

1. Removal of clay

2. Sorting of non-clay portion

3. Roundness of grains

Cause of increasing textural maturity….

More transport

4 factors that affect grain size and shape

1. Composition of source region

2. Transport distance

3. Transport processes

4. Local climate

3 techniques to determine sediment provenance

1. QFL (Quartz, Feldspar, Lithics) plot

2. Strontium isotopes (Sr-87/Sr-86)

3. Detrital zircons (U/Pb dating)

QFL plots

Determine tectonic setting of source regions; lithic fragments can include volcanic, metamorphic, sedimentary lithics

Strontium isotopes (Sr-87/Sr-86)

• Continental crust is enriched in Sr-87/Sr-86

• Primitive mantle and magma have low Sr-87/Sr-86

Ratio tracks continental influence on source rocks

Detrital zircons (U/Pb dating)

Detrital zircons using U/Pb dating can tell you age of source region as zircon crystals can survive multiple sedimentary cycles

Sedimentary basin

Depression capable of trapping sediment

• Tectonic forces control size, shape, and location of basins

• Dynamic entities

Accomodation

Space available for sediment to accumulate

How is permanent accomodation in basins created?

Subsidence allows for long-term sediment accumulation in basins

Subsidence

Physical sinking of Earth’s crust

4 Mechanisms of subsidence

1. Crustal thinning

2. Mantle-lithosphere thickening

3. Sedimentary and volcanic loading

4. Tectonic loading

Isostasy

Isostatic compensation is an important part of sediment and volcanic loading— removing load from crust causing uplift (reverse of subsidence)

2 types of sedimentary basins

1. Rift-drift: lithospheric stretching/thermal effects

   1. Divergent and intraplate settings

   2. Some transform plate settings

2. Flexural: flexure of lithosphere from a load

   1. Convergent plate settings

   2. Some transform plate settings

Basins @ divergent plate boundaries

• Rift valley extends so far that basins transition to a passive margin (‘drift’ part)

• Sediment accumulates on continental margin=crust undergoes flexural subsidence

Basins @ convergent plate boundaries

• Foreland basins form parallel to mountain ranges

• Crustal flexure from weight of the orogenic (mountain building) wedge

  • Subsidence primarily driven by flexure of underlying plate from weight of overlying plate

  • Ex. forearc basins at volcanic arcs

Strike slip basins

Pull apart or fault overstep basins

Information gained from basins’ sedimentary strata

1. Initial basin opening

2. Denudation of mountain ranges

3. Changing sea level

4. Changing climatic patterns

3 types of sedimentary rocks

1. Siliciclastic

2. Carbonaceous

3. Chemical/biochemical

Formation of siliciclastic rocks

Made from clasts (fragments) of older rocks (e.g. sandstone, shale)

Formation of carbonaceous rocks

Accumulation and rapid burial of organic debris (e.g coal, oil shale)

• Must have 10-20% organic material

Formation of chemical/biochemical rocks

Precipitation of minerals from water (e.g limestone, evaporites)

• Inorganic: from natural processes (evaporation)

• Organic: from water-dwelling organisms

Framework grains

Large grains that generally form the most volumetrically important constituent of the rock

Matrix

Smaller grains that fill in the holes between framework grains (less than .03 mm in sandstone)

Cement

Crystals that precipitate in the spaces between grains

Porosity

Unfilled spaces between the grains, typically filled with liquid or gas

Carbonates

Class of sedimentary rocks composed primarily of carbonate minerals

4 Factors carbonates need to form

1. Water clarity

2. Sunlight

3. Nutrient levels

4. Salinity

Optimal carbonate habitat

Shallow water provides optimal growth conditions

• Carbonate sediment can form on the seafloor (benthic) or in the water column and settle to the seafloor

Evaporites

Composed of minerals that directly precipitate out of saline waters (contain halite, gypsum, anhydrite)

Chert

Rock composed dominantly of SiO2 that originates from accumulation of siliceous organisms (e.g. diatoms or radiolarians)

• Can also be diagenetic or directly precipitate (debated)

Iron-rich sedimentary rocks

Sedimentary rocks that contain at least 15% iron (rare but economically valuable)

2 kinds:

1. Iron formation: cherty iron-rich sediments (mainly precambrian)

2. Ironstone: noncherty non banded

   iron-rich rocks

Phosphorites

Rocks enriched in phosphorus (more than 15% P2 O5)

• Phosphorite nodules on present ocean floor

  • Upwelling of phosphorus rich waters from deep ocean and biologic incorporation of phosphate in body tissue seem to be important in formation

2 main ways to classify siliciclastic rocks

1. Grain size, shape and sorting

   1. Transport processes

   2. Transport distances

2. Grain composition

   1. Source area

   2. Transport processes

Wentworth grain size scale

Siliciclastic rock classification scheme (3 main categories)

1. Conglomerate/breccia

2. Sandstone

3. Mudstone (Siltstone, claystone, and shale)

Boundary between clay/silt

• 1/256 mm

• .0039 mm

• 8φ

Boundary between silt/sand

• 1/16 mm

• .00625 mm

• 4φ

Boundary between sand/gravel

• 2 mm

• -1φ

2 methods to estimate grain size

1. Sieving (particle passage through calibrated openings)

2. Particle size analyzers (Use laser diffraction or settling rates)

Sorting scheme

Angularity/roundness scheme

Sandstone classification scheme

Dott’s classification

Mudstone classification scheme

Depends on grain size + fissility

Conglomerate/breccia classification scheme

1. Clast angularity

• Rounded = conglomerate

• Angular = breccia

2. Clast-supported or floating in matrix?

• Clast-supported = orthoconglomerate

• Matrix-supported = paraconglomerate

Orthoconglomerate origin

Transported by moving water

Paraconglomerate origin

Transported by ice or mass flow

Breccia origin

Generated in place (fault breccia) or minimal transport from high relief source (e.g. scree slope)

Naming conglomerates

Composition + Grain size + Texture

i.e quartz pebble orthoconglomerate

What forces cause a grain to move?

Fluid force must be great enough to overcome gravity and frictional forces

Bed (aka boundary) shear stress

The differential force a grain feels from bottom to top (difference in velocity btwn top/bottom of grain creates a vertical pressure gradient)

• Turbulent flows = higher shear stress

Formula for bed shear stress

ρ = fluid density

g = gravity

h = flow depth

S = bed slope

Critical shear stress (τcr)

The threshold of particle entrainment (being picked up); shear
forces overcome resisting forces (gravity, friction)

Shear (aka Shields) stress

Balance of driving vs resisting forces that tells us whether a grain will move

Formula for shear (aka Shields) stress

ρf = fluid density

ρs = grain density

g = gravity

τb = bed shear stress

D = grain size

Shear stress increases with….

Increasing bed shear stress and velocity

Shear stress decreases with….

With increasing ρ and grain size