(a) Discuss the primary processes involved in the formation of sedimentary rocks, emphasizing the role of diagenesis and…

(a) Primary processes in the formation of sedimentary rocks, with emphasis on diagenesis and lithification (20 marks)

The formation of a sedimentary rock follows a chain: weathering, erosion, transport, deposition, and diagenesis, which ends in lithification.

• Weathering: physical (frost, thermal and salt action) and chemical (hydrolysis, oxidation, solution) breakdown of source rock into detritus and dissolved ions.
• Erosion and transport: water, wind, ice and gravity move the material. Transport rounds and sorts grains, and it removes weaker minerals such as feldspar and mica, so the sediment becomes more mature.
• Deposition: settling of grains when energy falls, and precipitation of chemical or biochemical sediments such as limestone, evaporite and chert.

Diagenesis. Diagenesis covers all physical, chemical and biological changes after burial, at low temperature (below about 200°C) and pressure, short of metamorphism. It is divided into eodiagenesis (shallow, near the surface), mesodiagenesis (deep burial) and telodiagenesis (after uplift).

• Compaction: the weight of overburden squeezes out water and reduces porosity from about 50–60% in mud to below 10% at 3–4 km. Pressure solution at grain contacts forms sutured contacts and stylolites.
• Cementation: precipitation of calcite, silica, iron oxide or clay in pores binds the grains, using material from pressure solution or pore water.
• Recrystallisation and replacement: aragonite converts to calcite, and dolomitisation and silicification replace the original minerals.
• Dissolution: removal of unstable grains or cement creates secondary porosity.
• Authigenesis: new minerals grow in place, for example glauconite, pyrite and clays.
• Organic changes: peat matures to lignite and coal, and kerogen to oil and gas.

Lithification is the conversion of loose sediment into hard rock by compaction and cementation. Diagenesis thus turns sediment into rock and controls the final porosity, permeability and reservoir quality.

(b) Effect of primary processes on mineralogy and porosity (15 marks)

Mineralogical composition.

• Provenance sets the initial mineral supply: granite gives quartz and feldspar, basalt gives lithic grains.
• Climate and relief control weathering. In warm, humid, low-relief regions, chemical weathering removes feldspar and leaves quartz and clay. In cold or arid, steep regions, feldspar survives.
• Long transport abrades soft minerals, and reworking on beaches and dunes concentrates quartz. This gives compositional maturity, the sequence being arkose and lithic arenite (immature) to quartz arenite (mature).
• Sorting by energy separates heavy minerals, sand and mud.

Porosity.

• Porosity depends on sorting, packing, rounding and matrix. Cubic packing of equal spheres gives 47.6% porosity and rhombohedral packing gives 26%, independent of grain size.
• High-energy deposits, such as beach and dune sands, are well sorted and clean, so they have high porosity and permeability. Low-energy or rapidly deposited sediments, such as greywackes, are poorly sorted with matrix filling the pores, so they have low porosity.
• Later compaction and cementation reduce porosity.

Textural maturity therefore reflects clay content, sorting and rounding, and compositional maturity reflects the quartz content.

(c) Depositional environments and characteristic features (15 marks)

Classification. The environments are continental (fluvial, alluvial fan, lacustrine, aeolian, glacial), transitional (deltaic, estuarine, lagoonal, tidal flat, beach) and marine (shelf, reef, slope and deep sea).

Characteristic features.

• Fluvial: cross-bedded sandstone, channel conglomerate, fining-upward cycles, ripples, plant fossils.
• Alluvial fan: poorly sorted, angular conglomerate and breccia from debris flows.
• Lacustrine: fine laminated shale and varves, evaporites, freshwater fossils such as fish and ostracods.
• Aeolian: well-sorted, rounded, frosted grains, with large-scale cross-bedding.
• Glacial: tillite, unsorted with striated clasts, and dropstones in varved clays.
• Deltaic: coarsening-upward sequence with topset, foreset and bottomset beds, and coal.
• Beach: well-sorted sand with low-angle cross-bedding and swash marks.
• Tidal flat: herringbone cross-bedding, flaser bedding, mud cracks.
• Reef: massive limestone with corals and algae.
• Shelf: fossiliferous limestone and shale, with hummocky cross-stratification.
• Deep sea: turbidites showing graded bedding, Bouma sequence and flute casts, and siliceous or calcareous ooze.