ESCI 203 Sedimentary Rocks

Weathering

Exposure of pre-existing rocks to physical and chemical weather conditions; water, wind and ice.

Erosion

The movement of weathered rock from its original location.

Physical Weathering

Physical disintegration; results in exposure of ever increasing number of surface area to weathering.

Chemical Weathering

Changes in chemical composition; new mineral formation through addition or subtraction of elements or further chemical decomposition of minerals.

Dominating agent of physical and chemical weathering

Water; introduces elements and physically erodes.

Diagenesis

Processes used to lithify sediment. Deposition, burial, compaction, cementation.

What does weathering produce in water?

Sediment and ions

Ions in water can…

Used in cementing sediment, used as nutrients in life, or precipitated to form new rock.

Detrial

Clastic

How are clastic sedimentary rocks classified?

Grain size, composition, and shape (rounded, angular)

What environments create inorganic chemical sedimentary rock?

Ones where temperature, pressure, solute concentration, or dissolved gas change causes minerals to crystalize.

Formation of biochemical sedimentary rock

Living creatures use aqueous chemicals to create shells or other body parts from calcite or silica

What organisms create calcite shells?

A wide variety of sea life

What organisms create elaborate silica skeletons?

Radiolarian, single-celled microorganisms

Organic sedimentary rock

Remnants of organic material that has undergone lithification. Coal, oil, methane natural gas

Conglomerate

Poorly sorted, rounded clasts >2mm in diameter. Finer-grain sediments constitute the matrix.

Breccia

Poorly sorted, angular clasts >2mm in diameter. Finer-grain sediments constitute the matrix.

Quartz Sandstone

Variably sorted, variably rounded sand-sized (0.06-2mm) clasts of mostly quartz.

Arkose Sandstone

Variably sorted, mostly angular sand-sized (0.06-2mm) clasts of mostly feldspar. Typically pink (potassium feldspar) and white (plagioclase feldspar).

Graywacke

Sand-sized (0.06-2mm) quartz, feldspar, and small rock fragments set in a fine-grain clay matrix. A variety of sandstone often characterized by its dark grey colour.

Siltstone

Primarily composed of silt-sized particles (<0.06mm), and smaller pore space compared to sandstone. Often confused with Shale, but lacks fissility and lamination.

Fissility

Easy separation along bedding planes (layers).

Shale

Silt-sized particles (<0.06mm) embedded in clay matrix, characterized by breaks along parallel layering or bedding <1cm thick. Most common sedimentary rock.

Claystone

Clay particles that cannot be seen with the naked eye (<0.004mm). Non-fissile (cannot be broken into easy layers).

Fossiliferous Limestone

Ooltic Limestone

What creates beds?

Movement of Sediment; bedload transport, suspended particle accumulation, surface waves, flow regime

Bedload transport

Fluid interacts with beds of loose sand/sediment. It moves them if the fluid velocity overcomes the threshold velocity.

Traction Carpet

Movement of the upper sediment layers of a bed across one another, produced by the sheer stresses of fluid flow (drag and lift); they do not leave the bed, but instead roll, slide, and jostle along the bed.

Saltation Load

Sediment in bedload transport that leaves the bed and is moved directly by the fluid. Less dense sediments are winnowed.

Platy Grain

A sediment grain that is elongated in one direction, instead of being equal in all dimensions.

Threshold Velocity

Velocity needed to overcome gravity and inter-sediment friction.

Hjulström Curve

Describes the flow speeds (y-axis) at which sediment particles of a certain size (x-axis) will be eroded, transported, or deposited. The yellow highlight indicates sand sediment.

Why does the Hjulström Curve neglect grain friction/cohesion?

Grain cohesion is only relevant in clay particles; in larger particles, it is negligible.

What is suspension load?

The portion of sediment suspended by turbulence (eddies, boils) in fluids. Sediment accumulates when turbulence decreases significantly.

Settling Velocity

The rate at which particles settle, when buoyancy and drag created by fluid velocity caused by turbulence can no longer overcome gravity.

Surface Waves

Created by energy transfer from wind; water particles below the waves move in elliptic orbitals, and if those orbitals extend to the sediment floor, the sediment moves as well.

Wave Base

The depth to which elliptic movement generated by surface waves extends, typically ½ wavelength.

What does the Bouma Model describe?

Graded sediment bedding

Larger particles are found towards the bottom, and finer particles towards the top.

Lenticular Bedding

Alternating layers of mud and sand, associated with fluvial deposits (rivers, streams, flooding).

Channelized Bedding

Concave upward bedding planes indicating previous channel in which sediment settled.

Tabular Bedding

Parallel beds, can form in any environment in any stone.

Herringbone Crossbeds

Indicate alternating left/right paleoflow in rock beds. Indicates tidal flats.

Structures indicated by tabular beds

Energy of flow, braided or meandering rivers, but can be attributed to a variety of environments.

Trough Crossbeds

Concave-up “troughs” associated with channelized flow. Tidal, estuarial, fluvial, alluvial.

Alluvial

Deposits made by flowing water

Estuary

Enclosed coastal body of brackish water; transition zones between marine and freshwater environments.

Interference Ripples

Common on tidal flats, indicating ebb and flood flows.

Hummocky Cross-Stratification

Forms at storm wave-base (intense wind; increased wavelength and thus orbital penetration). Upper layers are typically reworked by fair-weather orbitals.