Chapter 49 Weathering Grains and Sediment Transport

Year 3 · Sedimentology · G332-L1

Measure grain size sorting roundness and porosity and connect aqueous wind ice and gravity transport to sedimentary evidence.

Illustrative poorly and well sorted grain-size populations. Original Geology Hub teaching diagram.

This independent lesson follows topics in the supplied University of Peshawar BS Geology curriculum labelled session 2004–05 onward. It is a learning resource, not an official university course or a statement of current degree requirements.

Learning objectives

Describe grain properties independently and quantitatively.
Connect transport mechanisms with testable depositional clues.
Calculate phi grain size and a simple pore-volume estimate.

Making and describing sediment

Sediments are produced by physical breakdown, chemical alteration, biological activity and precipitation. Weathering changes source material; erosion removes it; transport moves it; deposition stores it. These processes overlap, so a sediment can contain newly broken mineral grains, recycled older sediment and biological fragments. Mineral composition reflects both source and the changes experienced during travel and burial.

Grain size is measured using a stated diameter definition and method. The phi scale is φ = −log₂ d, where d is diameter in millimetres. Larger phi values represent smaller grains. Sorting describes the spread of grain sizes, roundness the smoothing of corners and sphericity the resemblance to a sphere. A well-rounded grain can be flat and therefore have low sphericity. Do not combine these independent properties into the vague phrase “well developed grains.”

Grain-size distributions can be shown as histograms, cumulative curves and summary statistics. State whether the sample was sieved, imaged or analysed by another method, because different methods may measure different effective sizes. A mean alone can conceal two distinct grain populations. Surface textures can provide clues to abrasion or chemical alteration but should be interpreted with a suitable comparison dataset.

Transport mechanisms

Water moves grains through traction along the bed, saltation and suspension. Flow strength, turbulence, grain density and grain size influence which modes operate. Turbidity currents are sediment-laden density flows that can travel downslope underwater; their deposits require more evidence than graded bedding alone. Wind efficiently transports some sand and finer particles, while glaciers can carry mixed sizes within, on and beneath the ice.

Gravity flows include debris flows, grain flows and fluidized or liquefied mixtures. They differ in how grains are supported and how the mixture behaves. A poorly sorted deposit may suggest a particular transport mechanism, but several processes can produce poor sorting. Combine grain fabric, bed geometry, matrix content and surrounding facies before selecting a model.

Pores and flow

Porosity is the fraction of a bulk volume occupied by voids. Permeability concerns the ability of connected pathways to transmit fluid. A sediment can have high porosity but low permeability if pores are poorly connected or pore throats are small. Compaction, cement and fractures can change those properties after deposition. Describe pore geometry rather than assuming that coarse grain size alone predicts a productive aquifer or reservoir.

Read a distribution rather than one average

A sediment sample can contain a mixture of grain populations. If half the grains are fine sand and half coarse gravel, a single intermediate mean may describe few actual particles. Plot the distribution before choosing a summary. A cumulative curve shows the proportion finer than each size and allows percentiles to be estimated. Sorting concerns the spread around the distribution, while skewness describes asymmetry and depends on the size scale and convention used. Record whether percentages refer to mass, number or volume because those are not interchangeable.

Separate the record of shape from composition

Roundness can increase through abrasion, but its rate depends on mineral durability, fractures, grain size and transport mechanism. A soft clast may change quickly while a resistant quartz grain preserves an earlier shape. Sphericity concerns overall geometry; a polished flat pebble can remain far from spherical. Surface pits or polish can raise hypotheses about mechanical or chemical modification, but inherited textures complicate a one-process interpretation. Compare particles of similar composition and size before attributing every difference to travel distance.

Explain how a flow supports sediment

Traction moves grains in contact with the bed, saltation involves repeated short hops, and suspension maintains particles within the moving fluid for longer periods. Turbulence can counteract settling, but settling speed also depends on density, shape and fluid properties. A density current moves because the mixture differs in density from surrounding fluid; a debris flow additionally involves grain interactions and matrix effects. A poorly sorted matrix-supported deposit is therefore a useful observation, but not a complete proof of one flow law. Look for bed contacts, internal fabric and changes along the deposit.

Calculate and interpret a packing change

An illustrative deposit contains a constant 70 cm³ of solid material. Initially it occupies 100 cm³, giving 30% porosity. After idealized compaction it occupies 90 cm³, leaving 20 cm³ pores and porosity 20/90 ≈ 22.2%. Porosity fell by about 7.8 percentage points, not by ten percentage points, because the bulk-volume denominator changed. Real compaction can also break or deform grains and expel fluid. Pore-throat narrowing may reduce permeability more strongly than the change in total pore volume suggests. A calculation should state which volumes are assumed conserved and should not turn a bulk porosity estimate into a flow prediction without connectivity evidence.

Worked activity

Convert 0.25 mm and 2 mm grains to phi units. Their values are +2 and −1 respectively. Now imagine a dry sample with 80 cm³ of solid grains occupying 100 cm³ of bulk volume. Its pore volume is 20 cm³, giving 20% porosity.

Interpretation: the calculation does not establish permeability. Two samples with the same porosity may have very different pore-throat connections. Draw one connected-pore model and one isolated-pore model to explain why fluid movement differs.

Self-check

Does higher phi mean coarser grains?
Answer
No. The negative logarithm makes higher phi values finer.
Are roundness and sphericity the same?
Answer
No. They describe corner shape and overall form respectively.
Does high porosity guarantee high permeability?
Answer
No. Connectivity and pore-throat geometry matter.

Chapter 49 multiple choice questions

Choose one best answer for each question. Open each answer panel after making your choice.

What phi size corresponds to a grain diameter of 0.125 mm?
1. +3
2. −3
3. +0.125
4. +8
Answer and explanation
A. +3 Since 0.125 = 2⁻³, φ = −log₂(0.125) = +3.
A flat pebble has smoothly rounded corners. Which description distinguishes the two properties?
1. High sphericity necessarily implies angular corners
2. High roundness with relatively low sphericity
3. Roundness and sphericity are identical
4. The pebble must be well sorted
Answer and explanation
B. High roundness with relatively low sphericity Roundness concerns corners; sphericity concerns overall resemblance to a sphere. Sorting describes a population’s size spread.
A sample contains 72 cm³ of solid grains in a bulk volume of 90 cm³. What is porosity?
1. 18%
2. 25%
3. 20%
4. 80%
Answer and explanation
C. 20% Pore volume is 90 − 72 = 18 cm³. Porosity is 18/90 × 100 = 20%.
Two sediments have equal porosity but markedly different permeability. Which explanation is consistent with the chapter?
1. Porosity must have been measured in different units
2. Equal porosity requires equal permeability
3. Permeability depends only on total pore volume
4. Their pore throats and connectivity differ
Answer and explanation
D. Their pore throats and connectivity differ The amount of void space does not uniquely determine the ability of connected pathways to transmit fluid.
A deposit is poorly sorted. What additional evidence best helps test a proposed debris-flow origin?
1. Matrix content, bed geometry, fabric and surrounding facies
2. Sorting alone, repeated as a numerical label
3. The display cabinet’s location
4. The assumption that all poorly sorted deposits share one origin
Answer and explanation
A. Matrix content, bed geometry, fabric and surrounding facies Poor sorting can arise through several processes. Multiple sedimentary observations are needed for a mechanism.

All 84 chapters

Search This Blog

Geology Hub