GEOL1006S Week 4: Siliciclastic Rocks and Carbonate Deposition

A comprehensive set of questions and answers covering the siliciclastic rock cycle, sediment transport, depositional environments, diagenesis, and carbonate deposition from the Week 4 lecture notes.

What are the main stages of the siliciclastic rock cycle?

Weathering; transport/erosion; deposition; diagenesis (including lithification).

What is weathering?

The breakdown of existing rocks into smaller pieces or ions; it can be chemical or physical.

Give an example of a chemical weathering process.

Hydrolysis (e.g., feldspar weathering to clay minerals) or dissolution of calcite; oxidation also discussed.

What is hydrolysis in chemical weathering?

Water reacts with minerals (e.g., feldspar to clays), weakening the rock.

What happens during dissolution in weathering?

minerals dissolve into solution (e.g., calcite dissolving in water), transporting material.

Why does acid in rainwater speed weathering?

Acidic water increases chemical reactions, accelerating weathering processes.

How does life contribute to weathering?

Organisms (e.g., lichens) produce acids that dissolve rocks and release nutrients.

Name two physical weathering processes.

Salt weathering and freeze–thaw (ice wedging); plant root growth also contributes.

What is salt weathering?

Evaporating saltwater leaves salt crystals in pores, which exert pressure and crack rock.

What is freeze–thaw (ice wedging) weathering?

Water freezes in cracks, expands, and widens cracks, breaking rock over time.

How do plant roots weather rocks?

Roots grow into cracks, applying pressure and widening openings until rock breaks.

What are bed load and suspended load?

Bed load rolls or bounces along the bed; suspended load remains carried in the flow.

What is saltation?

Particle hopping along the bed as part of bedload transport.

What is dissolution as a transport mechanism?

Ions dissolve in water and can later re-precipitate to form minerals elsewhere.

What is a unidirectional current?

Flow of water in a single direction, such as rivers; deeper ocean currents are quasi-steady.

What drives thermohaline circulation?

Deep ocean currents produced by density differences from salinity and temperature.

How can ripple cross-laminae inform us about ancient flows?

Ripples migrate in the direction of flow; cross-laminae preserve paleo-flow direction when rocks are deposited.

What is imbrication in larger clasts used for?

Stacking of clasts to resist flow; indicates paleo-flow direction.

What does ‘aeolian’ mean in sedimentology?

Sediments transported by wind; forms wind-ripples and dunes, with inverse grading.

What is a wave compared to a current in sediment transport?

A wave is oscillatory with no net transport; a current has a sustained direction of flow.

What is the difference between shallow and deep marine deposition?

Shallow marine includes beaches, deltas, and shallow seas; deep marine involves turbidity currents and fine sediments off the shelf.

What is a delta and what types can it have?

A sediment deposit where a river meets the sea; river-dominated, tide-dominated, and wave-dominated deltas.

What is a submarine fan and turbidity current?

A mass of coarse sediment transported down the continental rise by turbidity currents; deposits are turbidity sequences.

What is a turbidite sequence?

Coarse sand at the base, finer sands/silt upward, with progressively finer mud towards the top.

What are diagenesis and lithification?

Processes that transform unconsolidated sediment into rock after deposition, including compaction and cementation.

When do rocks enter metamorphism rather than diagenesis?

Typically at temperatures >~250°C and depths >~30 km, where original fabric/minerals are destroyed.

What is porosity reduction during diagenesis?

Pore spaces are filled by cement, grains rearrange under pressure, reducing porosity.

What is the difference between clasts and matrix?

Clasts are larger particles; matrix is fine-grained material that surrounds clasts.

What grain-size ranges define sandstone, according to the Wentworth scale?

Sandstone: 63 μm – 2 mm; gravel >2 mm; mud <63 μm.

What is the phi scale?

A logarithmic transformation of the Wentworth scale: φ = -log2(d), giving a scale where smaller grain size has larger φ values.

What is textural maturity in siliciclastic rocks?

Grains become smaller, more rounded/spherical, and better sorted with transport.

What is compositional maturity?

Increase in the proportion of resistant minerals (e.g., quartz) with transport; less weathered minerals decay.

What is the Q–F–L ternary diagram used for?

Classifies sandstones by quartz (Q), feldspar (F), and lithic (L) content and texture (clast vs matrix).

What is an arenite, wacke, and mudstone?

Arenite: sandstone with <15% mud; wacke: 15–75% mud; mudstone: mostly mud.

What is a detrital clast vs a lithic clast?

Detrital clasts are mineral grains like quartz, feldspar, mica; lithics are fragments of pre-existing rocks.

What are carbonate grains split into?

Skeletal (biominerals) and non-skeletal; simple vs complex biominerals; ooids and stromatolites as non-skeletal or microbial precipitates.

What are ooids and stromatolites?

Ooids are coated grains with concentric carbonate layers formed in supersaturated warm waters; stromatolites are layered carbonate built by microbial mats.

What are pelagic carbonates and chalk?

Pelagic carbonates form from shells of free-floating organisms (foraminifera, coccolithophores, pteropods); lithified as chalk.

What is the carbonate compensation depth (CCD)?

Depth where carbonate dissolution balances precipitation; below CCD (around 3 km) carbonate dissolves and is not preserved.

What are ramps and shelves in carbonate deposition?

Shallow marine carbonate geometries; ramps are gentle slopes, shelves are flat platforms that terminate on a slope.

What are reef types and why are reefs important?

Reefs can be fringing, barrier, or patch; built by coral and other organisms; major sites of carbonate deposition in warm, clear waters.

What facies are used to describe reefs in rocks?

Fore-reef, reef crest, and back-reef facies; preserved as part of reef complexes.

What happens to carbonate grains during diagenesis?

Cements fill pore spaces; dissolution and re-precipitation can alter original grains while preserving some shapes.

What makes carbonate deposition differ from siliciclastic deposition?

Carbonates form mainly by chemical/biological precipitation with limited transport; energy is not the primary control.

Where is the largest modern reef and what is notable about it?

Great Barrier Reef, Australia; barrier reef system along the edge of the continental shelf.

What are the three major components to describe siliciclastic rocks for communication?

Clasts and matrix; grain size and sorting; grain shape and distribution.

Why is ecology of depositional environments important for sedimentary rocks?

Grain size, sorting, composition, and cementation trace the energy, source distance, and transport history, shaping depositional environments.