3.3 Sedimentary processes and depositional environments

Sedimentary Processes

Sedimentary processes control how particles move from their source to their final resting place on the seafloor. Erosion breaks rocks apart, transportation carries those fragments by wind, water, ice, or gravity, and deposition drops them when energy runs out. Understanding how these processes work in marine settings helps geologists reconstruct ancient ocean conditions and locate resources like hydrocarbons.

Describe the major processes of sediment transport

Three linked stages move material from source rock to sedimentary deposit:

Erosion breaks down existing rock into transportable particles.

• Physical weathering fragments rocks through mechanical forces like freeze-thaw cycles and wave abrasion along coastlines.
• Chemical weathering alters mineral composition. Limestone dissolving in slightly acidic rainwater is a classic example.
• Biological weathering results from organisms: plant roots prying apart rock, burrowing animals loosening sediment, and marine organisms boring into rock surfaces.

Transportation moves those particles to a new location.

• Wind carries sand and dust, building features like sand dunes and loess deposits.
• Water moves sediment through rivers, ocean currents, and turbidity currents. River systems deliver most terrigenous sediment to the ocean.
• Ice transports debris locked within glaciers, depositing moraines and glacial erratics when the ice melts.
• Gravity drives mass wasting events like landslides and submarine debris flows.

Deposition occurs when transport energy drops below what's needed to keep particles moving. Coarse particles settle first because they require more energy to stay in motion, while finer particles remain suspended longer and travel farther from the source.

Explain the concept of sediment sorting and its importance in sedimentary environments

Sediment sorting describes how well separated particles are by size, shape, and density after transport. The longer and more consistent the transport energy, the better sorted the deposit becomes.

Why sorting matters:

• It serves as a fingerprint of the depositional environment. A well-sorted deposit (uniform particle size) points to a consistent-energy setting like a beach, where waves repeatedly wash away fines and leave behind similarly sized sand grains. A poorly sorted deposit (mixed particle sizes) suggests variable or chaotic energy, like a glacial till or debris flow.
• Sorting directly controls porosity and permeability in sedimentary rocks. Well-sorted sediments have more uniform pore spaces, allowing fluids (water, oil, gas) to flow through more easily. Poorly sorted sediments have small grains filling gaps between large ones, reducing pore space and fluid flow.

Discuss the factors that influence sediment deposition rates

Deposition rate depends on both the particles themselves and the environment they're settling through.

Particle characteristics:

• Size — Larger particles settle faster than smaller ones.
• Shape — Flat or irregular shapes experience more drag and settle more slowly than spherical grains of the same mass.
• Density — Denser minerals (like quartz at ~2.65 g/cm³) sink faster than less dense particles of the same size.

Environmental factors:

• Current velocity — Faster-moving water or wind keeps particles suspended longer and carries them farther.
• Turbulence — Chaotic flow can keep even relatively large particles from settling.
• Water depth — In deeper water, particles take longer to reach the bottom and can be redistributed by intermediate currents along the way.

Stokes' Law gives the settling velocity for small, spherical particles in a still fluid:

$V = \frac{2}{9} \frac{(\rho_s - \rho_f)}{\mu} g r^2$

• $V$ = settling velocity
• $\rho_s$ = density of the particle
• $\rho_f$ = density of the fluid
• $\mu$ = fluid viscosity
• $g$ = gravitational acceleration
• $r$ = particle radius

The key takeaway: settling velocity increases with the square of the radius, so doubling a particle's size makes it settle four times faster. This is why grain size decreases so predictably with distance from shore.

Depositional Environments

Identify and describe the main types of marine depositional environments

Marine depositional environments span from the shoreline to the deepest ocean floor, each with distinct energy levels, sediment types, and biological influences.

Coastal environments sit at the land-ocean boundary where energy is high and sediment supply is often abundant.

• Beaches are dynamic zones of sand and gravel constantly reworked by waves and longshore currents.
• Estuaries are semi-enclosed areas where freshwater mixes with saltwater. Low current velocities allow fine-grained mud and silt to accumulate.
• Deltas form where rivers deposit their sediment load into a standing body of water (covered in detail below).

Shallow marine environments extend across the submerged continental margins.

• The continental shelf is the gently sloping (average ~0.1°) underwater extension of the continent, typically reaching depths of about 120–200 m. Sediments here are a mix of terrigenous sand and biogenic material.
• Coral reefs are biogenic carbonate structures that thrive in warm (>18°C), clear, shallow tropical waters.

Deep marine environments lie beyond the shelf break.

• The continental slope descends more steeply (~4°) from the shelf edge toward the deep ocean and is cut by submarine canyons that funnel sediment downslope.
• Abyssal plains are the flat expanses of the deep ocean floor (3,000–6,000 m depth), blanketed by very fine clay and biogenic ooze that accumulates extremely slowly.
• Mid-ocean ridges are volcanic mountain ranges where new crust forms; hydrothermal vents here deposit metallic sulfide minerals.

Polar environments contribute sediment through ice-related processes.

• Ice shelves and glaciers calve icebergs that carry embedded rock debris far out to sea. As the ice melts, this debris drops to the seafloor, producing ice-rafted debris — a distinctive poorly sorted deposit found in otherwise fine-grained deep-sea sediments.

Explain the characteristics of deltaic depositional systems

A delta forms where a river enters a slower-moving or stationary body of water and loses the energy needed to carry its sediment load. Over time, sediment accumulates and builds new land outward into the water body.

Delta types depend on which force dominates:

• River-dominated (elongate) — River flow overpowers waves and tides, pushing sediment outward in finger-like lobes. The Mississippi River delta is the textbook example, with its distinctive "bird's foot" shape.
• Wave-dominated (cuspate) — Strong wave action redistributes sediment along the coast, smoothing the delta front into a rounded or triangular shape (e.g., the Nile Delta).
• Tide-dominated (irregular) — Strong tidal currents create complex networks of channels and sand bars (e.g., the Ganges-Brahmaputra Delta).

Internal structure of a delta consists of three distinct bed types:

1. Topset beds — Horizontal layers deposited on the flat delta surface by the river.
2. Foreset beds — Steeply dipping layers at the advancing delta front, where sediment avalanches down the slope. These make up the bulk of the delta.
3. Bottomset beds — Gently sloping, fine-grained layers deposited beyond the delta front by the finest suspended particles.

Sediment gets finer with distance from the river mouth. Coarse sand and gravel drop out first near the channel, while silt and clay travel farther offshore before settling.

Discuss the formation and characteristics of turbidites

Turbidity currents are dense, sediment-laden flows that rush down submarine slopes, sometimes reaching speeds over 60 km/h. They can be triggered by earthquakes, storm waves disturbing shelf sediments, or simple overloading of sediment on the continental slope.

These flows deposit a characteristic layered sequence called the Bouma sequence, which records the progressive decrease in current energy from bottom to top:

1. Division A — Graded bedding: coarsest grains at the base, fining upward. Deposited during the highest-energy phase of the flow.
2. Division B — Parallel lamination: horizontal layers of sand deposited from the lower part of the suspended load.
3. Division C — Ripple cross-lamination: small-scale ripple structures formed as current velocity continues to decrease.
4. Division D — Upper parallel lamination: thin layers of fine silt settling from suspension.
5. Division E — Pelagic mud: the finest clay and biogenic particles that slowly settle from the water column after the current has passed.

Not every turbidite contains all five divisions. Thinner or more distal deposits may preserve only the upper, finer-grained portions.

Turbidites matter for two reasons: they record past submarine landslides (useful for reconstructing geologic history and assessing hazards), and their sandy layers can have good porosity, making them significant hydrocarbon reservoirs in oil and gas exploration.

Compare and contrast the sedimentary processes in shallow and deep marine environments

The fundamental contrast comes down to energy. Shallow environments sit above wave base, so waves and currents constantly sort and rework sediment, producing well-sorted, coarser deposits. Deep environments sit below wave base in relatively still water, so only the finest particles reach the seafloor through slow settling — unless a turbidity current delivers a sudden pulse of coarser material.