Welcome to the World of Sedimentary Rocks!
Welcome! In this chapter, we are going to explore sedimentary rocks. These rocks are like the Earth’s diary; they record everything from ancient deserts and tropical seas to the movement of giant glaciers. Whether you are aiming for top marks or just getting your head around the basics, these notes will help you understand how rocks are broken down, transported, deposited, and lithified into solid stone.
1. How Sediments are Made: Weathering and Erosion
Before we have a rock, we need sediment (bits of sand, mud, or pebbles). This happens through weathering (the breakdown of rocks in situ) and erosion (the breakdown and removal/transport of rock material).
The Three Types of Weathering
• Mechanical (Physical) Weathering: The physical disintegration of rock without chemical change. An example is freeze-thaw (frost shattering), where water fills rock fractures, freezes, expands by approximately 9%, and wedges the rock apart.
• Chemical Weathering: Alteration of the chemical composition of minerals. Slightly acidic rainwater decomposes silicate minerals like feldspar into soft clay minerals.
• Biological Weathering: Mechanical or chemical breakdown caused by living organisms, such as tree roots expanding within joints or burrowing animals destabilising substrate.
Special Sediments
Not all sediments are broken rock fragments. Evaporites (such as halite and gypsum) precipitate directly from hypersaline bodies of water as water evaporates. Carbonate sediments derive predominantly from the calcium carbonate skeletons, shells, and remains of marine organisms.
2. The Journey: Transport and Maturity
Once liberated, sediments are transported by wind, water, or glacial ice. As sediment undergoes sustained transport, its characteristics evolve:
Sedimentary Maturity
• Textural Maturity: Grains become progressively more rounded (losing angular edges through abrasion) and better sorted (separated into uniform size classes by the transport medium).
• Compositional Maturity: Less stable, easily weathered minerals (like feldspar and ferromagnesian minerals) break down, leaving a high concentration of resistant minerals, primarily quartz.
The Hjulström Curve
The Hjulström relationship illustrates the critical water velocity required to erode (entrain), transport, and deposit grains of various sizes:
• Erosion: Coarse clasts require high velocities to be picked up. However, very fine clay particles also require high entrainment velocities due to cohesive electrostatic forces between clay platelets.
• Transport: Once entrained, particles remain in transport at lower velocities than those needed to initiate movement.
• Deposition: When water velocity falls below the critical settling velocity, grains settle out of suspension or bedload.
3. Measuring Grains & the Wentworth Scale
Grain sizes are classified using the logarithmic Wentworth scale and the statistical phi (\(\phi\)) scale, where \(\phi = -\log_2(d)\) (with diameter \(d\) in millimetres). Grain sizes are measured via laboratory sieve analysis and settling tubes:
• Boulder: \(>256\text{ mm}\)
• Cobble: \(64\text{ to }256\text{ mm}\)
• Pebble: \(4\text{ to }64\text{ mm}\)
• Granule: \(2\text{ to }4\text{ mm}\)
• Sand: \(0.0625\text{ to }2\text{ mm}\)
• Silt: \(0.0039\text{ to }0.0625\text{ mm}\)
• Clay: \(<0.0039\text{ mm}\)
4. Classifying Siliciclastic Rocks
Siliciclastic rocks are composed of silicate mineral and rock fragments (clasts). They are classified based on grain size and compositional maturity:
Coarse Clastics (Gravel-Grade)
• Conglomerate: Contains rounded to sub-rounded gravel clasts (indicating significant aqueous transport).
• Breccia: Contains sharp, angular gravel clasts (indicating minimal transport near the source area).
• Clast-supported (ortho-) conglomerates/breccias have clasts touching one another, whereas matrix-supported (para-) varieties have clasts floating in a finer matrix.
Sand-Grade Rocks (Sandstones)
• Orthoquartzite: Highly mature sandstone containing \(>90\text{–}95\%\) quartz grains. Grains are typically well-sorted and well-rounded.
• Arkose: Immature sandstone containing significant feldspar (typically \(>25\%\)), reflecting rapid erosion of granitic/gneissic source rock in arid or high-relief settings.
• Greywacke: Immature sandstone characterised by poorly sorted, angular grains set within an abundant (\(>15\%\)) fine-grained clay matrix, typically deposited by turbidity currents.
Fine Clastics (Mudrocks)
• Siltstone, Mudstone, and Shale: Composed of silt and clay. Shale displays distinct fissility (splitting easily along thin laminations).
5. Classifying Carbonate Rocks (Limestones)
Carbonates are composed of calcium carbonate (\(\text{CaCO}_3\)), primarily as calcite or aragonite. Carbonates consist of three fundamental components:
1. Allochems (Grains): Carbonate particles including ooids (concentrically coated spherical grains), bioclasts (fossil shell debris), peloids (faecal pellets), and intraclasts (reworked fragments of semi-consolidated carbonate mud).
2. Micrite: Microcrystalline lime mud matrix (\(<0.004\text{ mm}\)).
3. Sparite: Coarser, crystalline calcite cement filling open pore spaces.
The Dunham Classification (1962)
Dunham classifies limestones by depositional texture and grain support:
• Mudstone: Mud-supported fabric containing \(<10\%\) grains.
• Wackestone: Mud-supported fabric containing \(>10\%\) grains.
• Packstone: Grain-supported fabric containing fine lime mud matrix in intergranular spaces.
• Grainstone: Grain-supported fabric devoid of lime mud matrix (pore spaces cemented by sparite).
6. From Sediment to Rock: Diagenesis and Lithification
Diagenesis encompasses all physical, chemical, and biological changes undergone by sediment post-deposition at low temperatures and pressures:
1. Compaction: Overburden pressure from overlying strata expels pore fluids and repacks grains, substantially reducing primary porosity.
2. Cementation: Precipitation of authigenic minerals from pore waters into void spaces. Common cementing agents include silica (quartz), calcite, and hematite / iron oxides (giving red beds their color).
Porosity vs. Permeability
• Porosity: The percentage of a rock's total volume composed of pore spaces (voids).
• Permeability: The capacity of a rock to transmit fluids through interconnected pore networks.
7. Sedimentary Structures: Environmental Indicators
Sedimentary structures preserve hydrodynamic conditions, palaeocurrent directions, and way-up (stratigraphic geopetal) orientations:
• Cross-bedding: Inclined laminations deposited by migrating ripples or dunes. Asymptotic lower bounding surfaces and truncated upper sets serve as excellent way-up and palaeocurrent indicators.
• Ripple Marks: Symmetrical (wave/oscillatory) ripples indicate bi-directional marine or lacustrine wave action; asymmetrical (current) ripples indicate unidirectional water or wind flow.
• Graded Bedding: Progressive vertical decrease in grain size within a single bed from coarse base to fine top, formed by decelerating turbidity currents (a reliable way-up indicator).
• Desiccation (Mud) Cracks: V-shaped polygonal fractures in fine mud that taper downwards, recording subaerial exposure and drying.
• Salt Pseudomorphs: Cubic casts preserving the form of halite crystals that dissolved and were replaced by clastic sediment.
• Pebble Imbrication: Shingled alignment of flattened clasts dipping systematically upstream, recording the direction of palaeoflow.
Final Summary
Sedimentary rocks represent an integrated pathway from weathering and erosion through transport, deposition, and diagenesis. By analysing grain-size distributions, mineralogical maturity, carbonate textures, and sedimentary structures, geologists accurately reconstruct ancient depositional environments and Earth's dynamic history.